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desA
27-01-2010, 04:15 PM
I have a small heat-pump on test, in my lab. The unit has a capillary tube installed, together with inlet filter/strainer.

At various points during the heat-up phase, noises like machine-gun fire, can be heard - from time to time. I traced the main noise to the junction between capillary tube exit, & evaporator inlet. Checking the sight glass shows a fair amount of small bubbles coming up from the condenser coil.

At times, the system goes completely quiet again. The noise seems to be associated with the bubbles, as is to be expected.

This is the first time I've worked with a capillary tube expansion device, on a heat-pump.

Does anyone have any thoughts on how to reduce the expansion noise?

chemi-cool
27-01-2010, 04:50 PM
Noise from cup tube expansion are normal, they are far from a machine gun noise;)

What refrigerant are you using?
Is it a small 10 to 30 grams drier filter?

How can you tell the amount of refrigerant? Is there a sight glass?

It sounds to me that you don't have enough refrigerant or condenser is too small.

Mark II
27-01-2010, 11:54 PM
Des,

I always tell my customers .."When it stops making a noise , then you know you have a problem."

lowcool
27-01-2010, 11:56 PM
maybe go up a size or two

desA
28-01-2010, 01:53 AM
Capillary tube setup.

http://i47.tinypic.com/28iawjq.jpg

I'd value comments & suggestions for improvement.

desA
28-01-2010, 02:02 AM
Noise from cup tube expansion are normal, they are far from a machine gun noise;)

Sound-bite: sssshhhh....ssshhh... ka-ping... ping... ping... ka-ping... sssshhhh.... ka-ping... :D


What refrigerant are you using?R-134a


Is it a small 10 to 30 grams drier filter?See copper drier in pic below, at entrance to capillary. A larger filter-drier exists upstream.


How can you tell the amount of refrigerant? Is there a sight glass?There is a sight glass. At the moment, a fair amount of bubbles seem to come through - mainly clouds of small size, during operation - the clouds come & go.


It sounds to me that you don't have enough refrigerant or condenser is too small.I'll run SH trials today to check refrigerant mass (roughly). I do think that the condenser will be under-size, based on the system design. I'll check the performance theoretically, then report back on this.

desA
28-01-2010, 02:04 AM
Des,

I always tell my customers .."When it stops making a noise , then you know you have a problem."

Hahaha... one sure check, it seems. :eek:

Why is a domestic refrigerator so quiet then?

Gary
28-01-2010, 02:06 AM
The drier outlet should be pointed downward.

desA
28-01-2010, 02:07 AM
maybe go up a size or two

This is what I am thinking, I must admit.

To my 'ear' (big screwdriver on cap tube region), it sounds like the bubbles make their way to the capillary tube entrance, then form liquid-vapour bubbles-slugs, which then accelerate through the capillary tube - at high velocity.

desA
28-01-2010, 02:08 AM
The drier outlet should be pointed downward.

Thanks so much, Gary.

Which drier? The small one?

Gary
28-01-2010, 02:10 AM
Both... the strainer and the drier.

desA
28-01-2010, 02:17 AM
Both... the strainer and the drier.

Thanks, Gary.

So, basically, to form a liquid seal at the bottom, to ensure continuous liquid feed into cap tube?

Would this be enough to catch all/most of the bubble clouds, would you think? Remember, I can see clouds of vapour passing through the vertical sight glass, which sits not too far from condenser exit - before both filter & strainer.

Gary
28-01-2010, 02:23 AM
Thanks, Gary.

So, basically, to form a liquid seal at the bottom, to ensure continuous liquid feed into cap tube?

Would this be enough to catch all/most of the bubble clouds, would you think? Remember, I can see clouds of vapour passing through the vertical sight glass, which sits not too far from condenser exit - before both filter & strainer.

Usually cap tube systems do not have a sight glass installed... because it tends to confuse people.

desA
28-01-2010, 02:28 AM
Usually cap tube systems do not have a sight glass installed... because it tends to confuse people.

Can you explain further?

Gary
28-01-2010, 02:33 AM
Unlike a TXV, the cap tube doesn't necessarily have solid liquid at it's inlet. Depending on the cap tube sizing and system balance, some work with lots of subcooling and some very little... and even this changes with conditions.

desA
28-01-2010, 02:37 AM
Unlike a TXV, the cap tube doesn't necessarily have solid liquid at it's inlet. Depending on the cap tube sizing and system balance, some work with lots of subcooling and some very little.

Interesting.

With this kind of condenser - external coil around water storage tank - if the coil were of correct size (surface area), I would expect some level of sub-cooling to be present, at least. The reason for this would be that the temperature gradient in the storage tank (cold base, hot top), would tend to allow some level of liquid sub-cooling.

lowcool
28-01-2010, 03:38 AM
when i was taught about charging with a capillary and sightglass fitted it was normal practice to have a small stream of bubbles,no bubbles=system was overcharged

desA
28-01-2010, 03:52 AM
when i was taught about charging with a capillary and sightglass fitted it was normal practice to have a small stream of bubbles,no bubbles=system was overcharged

Thanks, lowcool.

What size bubbles are we talking about - roughly?
Do these bubbles come as a continuous flow, or are there spurts (clouds)of bubbles, followed by quieter patches, then spurts again?
What 'should' the system sound/feel like?

mad fridgie
28-01-2010, 04:40 AM
Firstly sorry for not replying earlier.
First fix. Place ear muffs over ears, I think that should solve the problem.
Second drier upside down excess vapour entering. (well done gary, have a banana)
you can not use a slight glass for charge analise (only moisture indication) Reason charge migrates from evap to cond and back again.
A cap system is basically a flooded system.

desA
28-01-2010, 04:52 AM
Firstly sorry for not replying earlier.
First fix. Place ear muffs over ears, I think that should solve the problem.

Engineers & our ears... small noises like that bother me. :D


Second drier upside down excess vapour entering. Let's expand on this, after piping corrected to set inlet strainer downwards. Does the length of the cap inlet filter make a difference?


you can not use a slight glass for charge analise (only moisture indication) Reason charge migrates from evap to cond and back again.
A cap system is basically a flooded system.Now, would that apply if the system were only operating in 'batch' mode, where it cycles cold to hot, stops, repeats - or. in 'semi-continuous/quasi-steady' mode, where the heating cycle can be reasonably consistent over long periods of water usage?

In the first case, I see migration inducing a transient operating state. In the second, the charge will tend to balance at the operating point, during the quasi-steady operation.

mad fridgie
28-01-2010, 05:02 AM
I would have just poking in your are attemting to achieve a liquid seal.
One would expect some sort of equalibrium to reached, When and where it sits ?????????? I would say you just about always have some liquid in the evap, what level comes out/or is in the condensor changes. is more likely to be variable.

mad fridgie
28-01-2010, 05:03 AM
PS, I do not like capillaries on forced draft evaps, Acceptable on static coils.

desA
28-01-2010, 05:08 AM
I would have just poking in your are attemting to achieve a liquid seal.

:D


One would expect some sort of equalibrium to reached, When and where it sits ?????????? I would say you just about always have some liquid in the evap, what level comes out/or is in the condensor changes. is more likely to be variable.

Fair comments. Thanks very much for that.

I figured this much, & will be watching the SH ultra-closely. ;)

Chef
28-01-2010, 05:09 AM
desA - Whats the conditions at the condenser and evaporator, temperatures and pressures whilst in steady state and what are the dimensions of the cap tube.
The capacity of the compressor is also very important, the volumetric flowrate at rated RPM is the best number to have.

Once the operating point of your tube is established the cause of the noise can be determined.

Chef

desA
28-01-2010, 05:10 AM
PS, I do not like capillaries on forced draft evaps, Acceptable on static coils.

Me neither - they scare the pants off me, to be honest. :(

I far prefer to see a TXV in place.

I suspect for this unit, cost is a big factor. That said, these capillary tubes don't seem to be too cheap. I'd rather save elsewhere & go to a small TXV.

mad fridgie
28-01-2010, 05:13 AM
desA - Whats the conditions at the condenser and evaporator, temperatures and pressures whilst in steady state and what are the dimensions of the cap tube.
The capacity of the compressor is also very important, the volumetric flowrate at rated RPM is the best number to have.

Once the operating point of your tube is established the cause of the noise can be determined.

Chef
Well here is the man to talk to, I bow my head, and leave the room backwards on my knees

desA
28-01-2010, 05:14 AM
desA - Whats the conditions at the condenser and evaporator, temperatures and pressures whilst in steady state and what are the dimensions of the cap tube.
The capacity of the compressor is also very important, the volumetric flowrate at rated RPM is the best number to have.

Once the operating point of your tube is established the cause of the noise can be determined.

Chef

Hi Chef,
Thanks for weighing in here. You are our resident 'cap tube fundi'. :)
Once I've finished a few more runs, to settle the system, I'm going to have to hack into the lines to install a hp service port. After this, I'll be able to get definitive data, for us to work with.

One difficulty with a heat-pump is that evap & condenser operating points move over the course of the cycle. This will be interesting.

Chef
28-01-2010, 05:28 AM
One difficulty with a heat-pump is that evap & condenser operating points move over the course of the cycle. This will be interesting.

With a fridge system the condensor stays roughly at the same temp and the evap slowly drops in temp. This is opposite in a heat pump where the evap is roughly constant and condenser increases in temp.

Sometimes this is a benefit to a cap tube as it stabilises the tube conditions over a wider range of operating conditions. It means once the tube is correct dimensions it will operate at best efficiency over a wide range.

The bubbles are quite normal and gives a very good indication of what state your tube entrance conditions are.

Chef

desA
28-01-2010, 06:11 AM
Thanks, Chef. Excellent points.


With a fridge system the condensor stays roughly at the same temp and the evap slowly drops in temp.

Very true. An a/c follows a similar pattern.


This is opposite in a heat pump where the evap is roughly constant and condenser increases in temp.For a heat-pump, the evaporator Te,sat does actually drift upwards slightly, over the course of a heating cycle, although this drift is small compared to the Tc,sat movement.


Sometimes this is a benefit to a cap tube as it stabilises the tube conditions over a wider range of operating conditions. It means once the tube is correct dimensions it will operate at best efficiency over a wide range.Good point.


The bubbles are quite normal and gives a very good indication of what state your tube entrance conditions are.
Now, I'd like to get your thoughts on what the bubbles should look like. This will surely be an important system indicator? In other words, large lazy bubbles, or intermittent clouds of tiny bubbles.

Chef
28-01-2010, 07:11 AM
Now, I'd like to get your thoughts on what the bubbles should look like. This will surely be an important system indicator? In other words, large lazy bubbles, or intermittent clouds of tiny bubbles.

Without the full system details it is not easy to predict and your systems geometry will play some part in it but in general the following would be a good guide.

A If you have SC of around 4 to 6C and above it will be all liquid with maybe a few very small bubbles.

B With an SC of 1 to 4C there will be more bubbles and if the piping were perfect it should be steady streams roughly constant.

C As the SC falls to 0C then the bubbles will be larger are more of them with even slugs of gas passing temporarily followed by bubbles entrained in liquid.

In both conditions B and C above when the more gas bubbles enter the tube its resistance increases, the flowrate falls a little and the condensor pressure rises - just a little - but enough to increase the SC by a fraction and thus produce a stream of enhanced liquid flow. This in turn eases the pressure drop along the tube, increases the flow and drops the condensor pressure just a fraction, lowers the SC and more bubbles pass. The cycle may repeat itself.

D The SC is at 0C and condensor pressure continues to rise. In this case more and more gas will pass through the tube to balance its flow rate to the higher condensor pressure, X values from 5% to 10% may occur and the sight glass if Horizontal will show a clear liquid but will not fill the sight glass ie it will have liquid on the bottom and gas on the top.

E The condensor pressure rises even further the value of X increases to up about 40% (max). From 10% up to the 40% the pressure drop is high in the tube and flow rate is low because of all the entrained gas, the outlet of the tube will likely be sonic and now the evap pressure will start to fall as the compressor sucks more gas than the tube and evap can provide, the evap pressure drops and the system finds a new balance point.

Note - there is only one set of evap and condenser conditions that provide the perfect cap tube conditions for its particular length.
When the condensor is at lower temps than perfect there will be more SC than desired and when the condensor is at higher temps than perfect the tube will operate in the X region with gas entrainement. There is nothing you can do about this except optimise the perfect point so there is some SC excess and some small X during the whole cycle.

Post a picture of the sight glass.

Chef

desA
28-01-2010, 08:08 AM
Thanks so much for a stunning review. I've added in what I'd estimate the flow regimes to be, judging from your descriptions. If you could amend these as you see fit, it would be greatly appreciated.


Without the full system details it is not easy to predict and your systems geometry will play some part in it but in general the following would be a good guide.

Agree. Very true.


A If you have SC of around 4 to 6C and above it will be all liquid with maybe a few very small bubbles.Ok. Single phase liquid.


B With an SC of 1 to 4C there will be more bubbles and if the piping were perfect it should be steady streams roughly constant.Ok. Bubbly flow.
Can you expand on the point - "if the piping were perfect"?


C As the SC falls to 0C then the bubbles will be larger are more of them with even slugs of gas passing temporarily followed by bubbles entrained in liquid.Ok. Slug flow.


In both conditions B and C above when the more gas bubbles enter the tube its resistance increases, the flowrate falls a little and the condensor pressure rises - just a little - but enough to increase the SC by a fraction and thus produce a stream of enhanced liquid flow. This in turn eases the pressure drop along the tube, increases the flow and drops the condensor pressure just a fraction, lowers the SC and more bubbles pass. The cycle may repeat itself.Ok.


D The SC is at 0C and condensor pressure continues to rise. In this case more and more gas will pass through the tube to balance its flow rate to the higher condensor pressure, X values from 5% to 10% may occur and the sight glass if Horizontal will show a clear liquid but will not fill the sight glass ie it will have liquid on the bottom and gas on the top.Ok. Annular flow.


E The condensor pressure rises even further the value of X increases to up about 40% (max). Ok. Annular - semi-annular.


From 10% up to the 40% the pressure drop is high in the tube and flow rate is low because of all the entrained gas, the outlet of the tube will likely be sonic and now the evap pressure will start to fall as the compressor sucks more gas than the tube and evap can provide, the evap pressure drops and the system finds a new balance point.Ok. Sonic choke.


Note - there is only one set of evap and condenser conditions that provide the perfect cap tube conditions for its particular length.
When the condensor is at lower temps than perfect there will be more SC than desired and when the condensor is at higher temps than perfect the tube will operate in the X region with gas entrainement. There is nothing you can do about this except optimise the perfect point so there is some SC excess and some small X during the whole cycle.This is the crux of the design, isn't it? Where would you aim to set this balance point in a heat-pump cycle - cold start / mid-range / hot? Bearing in mind that most of the time a heat-pump of this nature will be operating in the mid-to-hot range as it 'tops up' heat in the storage tank.

Thanks so much, Chef for an absolutely stunning post. It is incredibly helpful & very much appreciated.


Post a picture of the sight glass.
I've been trying to snap decent pics. With my camera, it is easier said than done... Will need to practice. :D

mad fridgie
28-01-2010, 08:20 AM
I have had a closer look at your pic, (but without knowing all components)
I suspect that your present cap, is too long and too small diameter. You may struggle to even get a descent starting point.

desA
28-01-2010, 08:45 AM
I have had a closer look at your pic, (but without knowing all components)
I suspect that your present cap, is too long and too small diameter. You may struggle to even get a descent starting point.

Thanks for that. :)

Where would you go, from the present cap tube, and why?

Chef
28-01-2010, 08:46 AM
Can you expand on the point - "if the piping were perfect"? :D

First MAD has a point, it does look like it is somewhere too long but only your data will allow this to be confirmed.

Having the 2 driers feeding from the bottom to the top might allow them to act as accumulators and upset the natural balance of charge dissipation as the system heats up.

When it is subcooled these will be full of liquid and so deplete the evap from what could be more liquid and more performance.

When it is in X regime these will be part gas and part liquid and act like a bubble pipe - no reason for that but means your charge will not be perfect.

If they are small compared to the evap size and condenser size dont worry but they may have affect on the last 10 to 15% of the performance?

Chef

desA
28-01-2010, 09:00 AM
First MAD has a point, it does look like it is somewhere too long but only your data will allow this to be confirmed.

This machine has been designed & built by others. I bartered it for design time on a project. The plan is to debug it as far as possible & at the same time trim costs all the way down - if possible. So, it is a fantastic learning tool. :)
I will get into the specific retrofitting side once we get some clear direction from this thread. I have the Danfoss cap tube program & can purchase a second alternative. I'd then like to design/select the optimum cap tube to suit the system. This seems like it will be a challenge.

Filter-drier & inline strainer
On the current machine the liquid line goes - tank condenser - sight glass (vertical upflow) - filter-drier (horizontal) - inline strainer (vertical upflow).


Having the 2 driers feeding from the bottom to the top might allow them to act as accumulators and upset the natural balance of charge dissipation as the system heats up.Did you mean bottom-to-top, or top-to-bottom? :confused:


When it is subcooled these will be full of liquid and so deplete the evap from what could be more liquid and more performance. Mmhhh... good point. Would this require a tad more refrigerant to make up for the inventory hold-up?


When it is in X regime these will be part gas and part liquid and act like a bubble pipe - no reason for that but means your charge will not be perfect.Could you perhaps expand this a little further?


If they are small compared to the evap size and condenser size dont worry but they may have affect on the last 10 to 15% of the performance? Very interesting viewpoint. In this particular machine, I consider the evap performance to be marginal. Could you expand this point a little further?

Thanks again... excellent food for thought.

mad fridgie
28-01-2010, 09:54 AM
The problem you are going to have is choosing adesign point for your cap. the nature of the hot water heating beast is that it is always in a wide range of flux (compared to say a fridge which generally is more stable once the cabinet is at the required temperature)
maybe you should look at a single pass water heating system, controled by head pressure, this will give you a constant to work with, so then you only have to optimised around SST.
I do not think you should use your sight glass at all to judge charge. The only benefit (excluding moisture) is to show that you have flow. (Gauges would show the same)

Chef
28-01-2010, 10:08 AM
When the system has too much accumulator volume after the condenser it will steal liquid from the evap.

When the system goes in to X mode then the evap will be full of liquid (as full as it can be without flood back) so this is the max charge you should use.

But when in SC mode there is more liquid in the condenser/accumulator than you need so less in the evap and lower performance.

The cap tube is just a simple pipe and has no control function whatsoever - it is the changes in condenser pressure and SC and the changes in evap density from pressure and temp that provide the system balance and so it these items you need to be watching. They will change all the time as the system parameters alter and will work to match the compressor flow so its a combination of four parts all the time to get an understanding of whats going on.

Without real data from the system it is difficult to run a simulation and see whats happening.

Oh and by the way - as discussed on many threads before be very careful using Dancap as its results are 'unique'.

Chef

Chef
28-01-2010, 10:15 AM
I do not think you should use your sight glass at all to judge charge. The only benefit (excluding moisture) is to show that you have flow. (Gauges would show the same)

Your quite right that selecting charge on a sight glass in this system would be almost useless. But it does show what condition the refrigerant is at as it enters the tube and for research and cap size changes it can be very useful.

It would be great if one could buy a glass tube - say 300mm long - about the same diameter as the compressor discharge tube to see the refrigerant condition. What a tool that would be.

Chef

desA
28-01-2010, 10:18 AM
The problem you are going to have is choosing adesign point for your cap. the nature of the hot water heating beast is that it is always in a wide range of flux (compared to say a fridge which generally is more stable once the cabinet is at the required temperature)

I think that this is the key, here. The varying heat-loads, on both the evaporator side & condenser side, keep the system in relative dis-equilibrium.


maybe you should look at a single pass water heating system, controled by head pressure, this will give you a constant to work with, so then you only have to optimised around SST.A very good idea. This kind of device ends up with far less in the way of variables. It's a way of thinking, more than anything else.


I do not think you should use your sight glass at all to judge charge. The only benefit (excluding moisture) is to show that you have flow. (Gauges would show the same)Not to judge charge, but more of an overall indication of the instantaneous state of the system. Charge optimisation has always to be done together with a number of other parameters - even then, things change on the day... :D
These RHVAC systems are a lot more sensitive than many folks would have us believe.

nike123
28-01-2010, 01:36 PM
Maybe this article (http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V1Y-4M6SGC0-1&_user=10&_coverDate=04%2F30%2F2007&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1184015083&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=f29d304ecfc2458d55f4783f16e3cd05) could have some interesting information.

desA
28-01-2010, 01:52 PM
Some further feedback from today's runs:

1. Leveled the heat-pump mounting plate - evap more level;
2. Adjusted system to various quasi-steady settings:
2.1 Full sight-glass
2.2 Capillary noise minimal
3. Change load to next quasi-steady point:
3.1 Bubble appeared until steady - then disappeared;
3.2 Capillary noise minimal.

Moved right up the heating range to ~ 57'C.

4. Quasi-steady at 57'C
4.1 Capillary sounds like fast flow;
4.2 No intermittent noise.

5. Set up to tank heat-up cycle
5.1 Capillary noise - pop, ping, but more constant

6. Added mastik dampers to cap ends
6.1 Quietened noise a lot - more constant - sounds like plug flow.

An interesting day.

desA
28-01-2010, 01:57 PM
Maybe this article (http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V1Y-4M6SGC0-1&_user=10&_coverDate=04%2F30%2F2007&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1184015083&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=f29d304ecfc2458d55f4783f16e3cd05) could have some interesting information.

Interesting article. Does anyone have access to this paper through a University library?

ScienceDirect, as usual, want an enormous amount (USD 31.50) for a paper - an incredibly greedy company is Elsevier, considering that the authors pay to have their paper published.

chemi-cool
28-01-2010, 02:28 PM
Very interesting discussion.

I have learnt a few new things today.

I may add one thing concerning the noise.

1. would it be quieter if a few smaller diameter cup tubes will be instead of one large diameter?

2. Because the refrigerant is R 134a, in a short time it most likely will block the cup tube, my suggestion is to use a small 1\4 drier, size 052 and braze the cup tube to the 1\4 tube on the outlet.

And Des, how about some work today?:D

desA
28-01-2010, 03:58 PM
When the system has too much accumulator volume after the condenser it will steal liquid from the evap.

Fair comment.


When the system goes in to X mode then the evap will be full of liquid (as full as it can be without flood back) so this is the max charge you should use.

Where in a heating cycle would you expect 'X' mode to occur? Start-up, hot?

When you say 'full' of liquid - are you saying that all the charge will migrate to the evap? This unit has a fair vertical leg on the suction line, before entering the compressor. I'd assume that the maximum evap liquid level allowed would then correspond to a fair portion of this height? Where would you determine the 'critical charge'?

Surely, the tank condenser could be expected to retain some refrigerant?


But when in SC mode there is more liquid in the condenser/accumulator than you need so less in the evap and lower performance.

So, the whole balancing act is realistically determined by how much liquid that cab safely be 'buffered' in the evaporator?


The cap tube is just a simple pipe and has no control function whatsoever - it is the changes in condenser pressure and SC and the changes in evap density from pressure and temp that provide the system balance and so it these items you need to be watching.

Fair comment. Yes, correct.


They will change all the time as the system parameters alter and will work to match the compressor flow so its a combination of four parts all the time to get an understanding of whats going on.

Your paper from the Indian university explained some of this pretty well, I thought. Good stuff.


Without real data from the system it is difficult to run a simulation and see whats happening.

Agreed. I'm looking at tapping into an existing port on the high side, to at least get the initial data, so we can begin making sense of where we are. I'm probably going to have to order in a number of additional thermocouples, as my lot is still in my large test heat-pump. :confused:


Oh and by the way - as discussed on many threads before be very careful using Dancap as its results are 'unique'.

I did wonder about that. I have another program on hand, but will also write up a program to calculate this. You wouldn't just happen to have written one in EES, would you - one that can fit into the Academic version I happen to have on hand. :D

desA
28-01-2010, 04:19 PM
I may add one thing concerning the noise.
1. would it be quieter if a few smaller diameter cup tubes will be instead of one large diameter?

Interesting thought. I wonder if anyone has thoughts on this?


2. Because the refrigerant is R 134a, in a short time it most likely will block the cup tube, my suggestion is to use a small 1\4 drier, size 052 and braze the cup tube to the 1\4 tube on the outlet.

Isn't that what we have at the start of this capillary? Is R-134a worse than other refrigerants for blockage?

http://i47.tinypic.com/28iawjq.jpg


And Des, how about some work today?:D

The R&D is part of my work... :D :D

Gary
28-01-2010, 04:22 PM
When you say 'full' of liquid - are you saying that all the charge will migrate to the evap? This unit has a fair vertical leg on the suction line, before entering the compressor. I'd assume that the maximum evap liquid level allowed would then correspond to a fair portion of this height? Where would you determine the 'critical charge'?

Surely, the tank condenser could be expected to retain some refrigerant?


During the off cycle, virtually all of the refrigerant charge is going to migrate to the evaporator, because it is cooler than the condenser.

That vertical leg is a very good thing.

desA
28-01-2010, 04:38 PM
A further thought.

What 'noises' could one expect if the refrigerant were (slightly) over-charged?

For instance, I noticed that on many occasions, with a full sight-glass, during quasi-steady operation, noises did, on occasion occur. It made me realise that the bubbles, when they showed up, were not the main cause of the capillary noise, but rather some other effect.

desA
28-01-2010, 04:41 PM
During the off cycle, virtually all of the refrigerant charge is going to migrate to the evaporator, because it is cooler than the condenser.

True, you are correct - especially for this type of tank condenser.


That vertical leg is a very good thing.

What kind of vertical height would you recommend - relative to evaporator height? Is there some kind of rule-of-thumb?

chemi-cool
28-01-2010, 08:13 PM
Hi Des,

I am changing on a weekly base blocked driers on small cup tube systems such as air driers, beverage cabinets, water coolers, domestic fridges and freezers.

This refrigerant is too good when it comes to cleaning.

By using regular 1\4 drier, the problem disappear.

lowcool
29-01-2010, 02:35 AM
has anyone used that 1234 whatever refrigerant it is yet.
maybe you could step us into the future des by taking that path.
i know more r&d = more work

desA
29-01-2010, 04:21 AM
Hi Des,

I am changing on a weekly base blocked driers on small cup tube systems such as air driers, beverage cabinets, water coolers, domestic fridges and freezers.

This refrigerant is too good when it comes to cleaning.

By using regular 1\4 drier, the problem disappear.

Thanks very much, chemi-cool. Your experience is very valuable.

Do you perhaps have a link, or picture of the 'regular 1/4" drier' you're referring to?

desA
29-01-2010, 04:33 AM
has anyone used that 1234 whatever refrigerant it is yet.
maybe you could step us into the future des by taking that path.
i know more r&d = more work

Interesting point. I've been following that refrigerant for some time & would love to test it as a drop-in on some of these test machines. I'm not sure that it is commercially available yet.

nike123
29-01-2010, 08:35 AM
Thanks very much, chemi-cool. Your experience is very valuable.

Do you perhaps have a link, or picture of the 'regular 1/4" drier' you're referring to?


I think that he mean something like Danfoss DCL/DML 032S

http://rc.danfoss.com/TechnicalInfo/literature/manuals/01/PDE00B322.pdf

chemi-cool
29-01-2010, 10:26 AM
Thanks Nik,

Thats the one, DCL 052's

desA
29-01-2010, 11:51 AM
I think that he mean something like Danfoss DCL/DML 032S

http://rc.danfoss.com/TechnicalInfo/literature/manuals/01/PDE00B322.pdf

Thanks so much, Nike. :)

desA
30-01-2010, 11:52 AM
To extend the discussion a little further.

I calculated what I estimated to be 'critical charge' for this system, (evacuated, N2)x3, charged in steps until this 'critical charge', observing system response along this charge-addition process. I've set up LP & HP gauges, & a temp sensor (still rudimentary at this point).

Well, very interesting so far. General qualitative observations. There is none of the pinging, popping, machine-gun noise - only a rushing sound as would be expected from a high-velocity flow, in a thin tube. The compressor is also running slightly warmer to the open hand (don't laugh), than previously. Te,sat has dropped a few degrees.

I'm going to make a judgement call at this stage, subject to additional feedback from the experimental stage. It looks like the system could very well have been over-charged, as received. Let's see as testing develops.

chemi-cool
30-01-2010, 12:50 PM
Des, Can you set a video camera and let us see in real time how it goes?

After all, by now we are part of your experiment...:D

desA
30-01-2010, 01:04 PM
Des, Can you set a video camera and let us see in real time how it goes?

After all, by now we are part of your experiment...:D

Hahaha... That's an incredibly interesting concept. Let me think a bit more on that. I have colleagues who've done this in their factories & labs. Could be very interesting...
:D

desA
30-01-2010, 01:08 PM
I'd like to ask for views on determining the 'critical charge' for a cap-tube system. I've listed my logic below. This is very much open to discussion.

What I have done so far, is as follows (based on all liquid residing in evap in off-duty condition):
1. Evaporator internal tube volume = Ve [L];
2. Suction pipe max height - 90% tubes liquid-full => Vc=0.9*Ve [L]
3. R134a liquid density at 30'C ~ 1187.51 kg/m3 (data program);
4. Critical mass of cold refrigerant in evap - xxx g

I'm now up to this charge limit, having previously charged in steps of 20g, from a lower starting point.

What other items can/should be included in this critical charge calculation? In other words, are there a few more grams I could legitimately squeeze into the system, without endangering the compressor - suction line - portion of, compressor ?sump?.

I have kept an eye on the evap superheat, throughout. It is currently in the region of 7-7.5K.

Obviously, I'll put up test results as the experimental side progresses. I also have a reliable CapTube program on hand.

desA
30-01-2010, 02:35 PM
Maybe this article (http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V1Y-4M6SGC0-1&_user=10&_coverDate=04%2F30%2F2007&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1184015083&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=f29d304ecfc2458d55f4783f16e3cd05) could have some interesting information.

Thanks Nike. My curiosity overcame my miserly tendencies. I downloaded the paper a few minutes ago. I'll study it over the next few days.

There is always something useful in these papers.

Chef
31-01-2010, 05:14 AM
What I have done so far, is as follows (based on all liquid residing in evap in off-duty condition):
1. Evaporator internal tube volume = Ve [L];
2. Suction pipe max height - 90% tubes liquid-full => Vc=0.9*Ve [L]
3. R134a liquid density at 30'C ~ 1187.51 kg/m3 (data program);
4. Critical mass of cold refrigerant in evap - xxx g

I have kept an eye on the evap superheat, throughout. It is currently in the region of 7-7.5K.

Obviously, I'll put up test results as the experimental side progresses. I also have a reliable CapTube program on hand.

What you will be interested in is the correct charge for optimum COP and best operating conditions over the full range. If you calc the charge in the OFF condition it may be way off, but it will serve as a maximum value.

When you calc the charge in operation you will need to know in what phase the cap tube is operating in. For instance if it will always have SC then there will be more liquid in the condenser but if it runs mainly with X then there will be far less liquid in the condenser.

Choose your operating cycle on a PH diagram, get the value of X (or SC) for h3 and X for h4.
Lets assume X=0.0 at h3,
then using a simple linear condensation through the condenser Vol liquid = 0.5*Vc
It may be more accurate to use a logarithmic formulae for the condensate and most certainly in the case for SC.

Once X is 0.1 or 0.2 then the volume of liquid falls dramatically and can be found using
vol liquid =(Vc-X*Vgas*Vc)/(1-X) where Vgas is specific gas volume and Vc is condenser volume. Again doing it logarithmically and in finite elements down the condenser would be better. You can see it falls to below 10% of the value at X=0 very quickly.

The evap can similarly be treated and also for the piping but only you can determine this as you have the geometry.

As for the SH - is this SH out of the evap or into the compressor, it is important as cap systems can have SH as low as 1C at design conditions at evap exit.

Any cap dimensions, pressures and temps yet?

Chef

desA
31-01-2010, 09:47 AM
Thanks very much, Chef.


What you will be interested in is the correct charge for optimum COP and best operating conditions over the full range. If you calc the charge in the OFF condition it may be way off, but it will serve as a maximum value.

The issue here, is the definition of critical charge, as I see it. If we are saying that the charge may never exceed a certain holding volume in the evaporator, in the cold condition, then this may very well occur long before the system optimum COP, or maximum power (different points), can occur.

In other words, the logic may be something like this:
1. Where system refrigerant volume is small, evap internal volume dominates;
2. Where system refrigerant volume is large, evap internal volume not dominant;

Cold condition (off-cycle):
1. Charge migrates to evap:
1.1 Storage critical charge - liquid flood-back control critical.
2. Charge migrates to evap:
2.1 Evap has ample bufer storage - liquid flood-back not critical.

Operating condition:
1. Cold-condition charge dominates - operating conditons just have to fit in.
2. Operating charge important - operational critical charge, to prevent liquid floodback.

As I see it, there are two very different scenarios here, which seem to be loosely lumped under the concept of 'critical charge'.

desA
31-01-2010, 09:54 AM
Further to the determination of operational optimum charge

For a larger system volume, where the evap internal volume exceeds the total system charge, the system optimum for either COP, or max condenser power (two different scenarios), can really only come by a huge amount of testing, where the charge level is gradually increased & cycle performance figures closely recorded.

Typically, this can take some 20-30 rounds of tests, to hone in on the optimum charge range. The final balance of COP, to Qc,max can turn in a fairly short difference in charge.

desA
31-01-2010, 10:51 AM
When you calc the charge in operation you will need to know in what phase the cap tube is operating in. For instance if it will always have SC then there will be more liquid in the condenser but if it runs mainly with X then there will be far less liquid in the condenser.

With this kind of coil condenser, the coil winds its way from top, to bottom, with a vertical up-leg for the outlet fluid. Judging from the sight-glass, with its clouds of bubbles, the SC would appear to be in the range 0-4K. Since the temp gradient over such a tank is some 10K different from bottom-to-top, an amount of liquid sub-cooling, could be expected - subject to adequate coil surface area (never a sure thing).


Choose your operating cycle on a PH diagram, get the value of X (or SC) for h3 and X for h4.
Lets assume X=0.0 at h3,
then using a simple linear condensation through the condenser Vol liquid = 0.5*Vc

Ok, that's an elegant way to look at it. Decent rule-of-thumb. Thanks for that.


It may be more accurate to use a logarithmic formulae for the condensate and most certainly in the case for SC.

Can I ask you to perhaps expand on this point a little further?


Once X is 0.1 or 0.2 then the volume of liquid falls dramatically and can be found using
vol liquid =(Vc-X*Vgas*Vc)/(1-X) where Vgas is specific gas volume and Vc is condenser volume.

Ok. Understood. I had to re-read a few times. Condenser.


Again doing it logarithmically and in finite elements down the condenser would be better. You can see it falls to below 10% of the value at X=0 very quickly.

If you explain the logarithmic part? dTlm? Which temperatures? The finite element part I understand.


The evap can similarly be treated and also for the piping but only you can determine this as you have the geometry.

Good point.


As for the SH - is this SH out of the evap or into the compressor, it is important as cap systems can have SH as low as 1C at design conditions at evap exit.

SH at evap discharge & SH' at compressor inlet. Surely with an SH ~ 1K at evap discharge, the evap would be very close to flooding - with some droplet carryover? This could be a little tricky. In my system, the operating SH today, at mid cycle is ~ 6.5K at evap discharge. Observing the evap tube hairpins also gives a reasonable 'feel' for the amount of the core involved in super-heating. Currently this is in the range of around 24-26.5% of core tube count.


Any cap dimensions, pressures and temps yet?

I've got the pressure & temps parts moving & should have a set of results tomorrow. I've been running rough trials to check on the evap behaviour in regards to flooding. I'll have to go off & get a decent micrometer to measure the cap tube diameter.

desA
01-02-2010, 03:21 AM
Does all the refrigerant migrate to the evaporator during the off-cycle - with a heated condenser?

How do we know this to be true?

This is the principle I'm trying to establish in term of the definition of the term 'storage critical charge' as distinct from 'operational critical charge'.

Chef
01-02-2010, 04:50 AM
DesA

This is a better equation to calc the charge.

Charge (Kg)=Vc*g[1+(1-X)/X] / [1+(g/L)(1-X)/X]
where Vc is vol of condenser m3
g is gas density Kg/m3
L is liquid density Kg/m3

I do it every 1% along the condenser and get X from heat transfer equations for the input conditions using an equation solver. You wont need that complexity so it would be easy to put in excel and set the value of X from .01 to 1 in 100 steps and then sum up the results. Easier this way.

If you want to take into account SC and de-superheating then you could set a few segments at each end to 0 and 1 and see the changes easily.

You can even 'shape' the curve of X along the condenser from linear to exponential or whatever you like by changing X to alter slower the entrance and faster at the exit.

Hope this works better for you.

Chef

Gary
01-02-2010, 04:50 AM
Does all the refrigerant migrate to the evaporator during the off-cycle - with a heated condenser?

How do we know this to be true?

This is the principle I'm trying to establish in term of the definition of the term 'storage critical charge' as distinct from 'operational critical charge'.


If we take two half full refrigerant containers and connect them with a hose, then warm one of the containers, the vapor will condense in the cooler container and evaporate in the warmer container.

This will continue until the cooler container is full of liquid and/or the warmer container has only vapor.

At this point the pressures will equalize and will correspond to the temperature of the cooler container.

desA
01-02-2010, 05:09 AM
If we take two half full refrigerant containers and connect them with a hose, then warm one of the containers, the vapor will condense in the cooler container and evaporate in the warmer container.

This will continue until the cooler container is full of liquid and/or the warmer container has only vapor.

At this point the pressures will equalize and will correspond to the temperature of the cooler container.

An excellent analogy.

Now lets bring the interconnecting pipework into the picture. Logically, any piping exposed to ambient air should be at similar temperature to the idle evaporator. It is logical then, that condensation must also take place in the liquid line, filter-drier (150~ 200g), suction line, compressor sump.

In other words, the 'storage critical charge' may need to logically be extended beyond that of the evaporator itself.

desA
01-02-2010, 05:12 AM
DesA

This is a better equation to calc the charge.

Charge (Kg)=Vc*g[1+(1-X)/X] / [1+(g/L)(1-X)/X]
where Vc is vol of condenser m3
g is gas density Kg/m3
L is liquid density Kg/m3

I do it every 1% along the condenser and get X from heat transfer equations for the input conditions using an equation solver. You wont need that complexity so it would be easy to put in excel and set the value of X from .01 to 1 in 100 steps and then sum up the results. Easier this way.

If you want to take into account SC and de-superheating then you could set a few segments at each end to 0 and 1 and see the changes easily.

You can even 'shape' the curve of X along the condenser from linear to exponential or whatever you like by changing X to alter slower the entrance and faster at the exit.

Hope this works better for you.

Chef

Thanks very much, Chef. Much obliged.

I'll program that up & we can use it for discussion purposes. :)

Chef
01-02-2010, 05:13 AM
SH at evap discharge & SH' at compressor inlet. Surely with an SH ~ 1K at evap discharge, the evap would be very close to flooding - with some droplet carryover? This could be a little tricky. In my system, the operating SH today, at mid cycle is ~ 6.5K at evap discharge. Observing the evap tube hairpins also gives a reasonable 'feel' for the amount of the core involved in super-heating. Currently this is in the range of around 24-26.5% of core tube count.

Usually a cap system has a suction accumulator and depending on its size you can push the SH down to gain extra evap area for cooling. This is especially true if the tube is sized to run in X at the most common operating point as very little extra liquid can be pushed into the evap as the condenser is already starting to be very low. But if you have SC and a SH of 1C then any small change in condenser conditions could push more liquid into the evap and possibly cause floodback. In the case of running in SC then some SH would be safer.

Maximum COP is obtained with an SC of about 1C or 2C and SH of just a few C.

Chef

mad fridgie
01-02-2010, 07:34 AM
Usually a cap system has a suction accumulator and depending on its size you can push the SH down to gain extra evap area for cooling. This is especially true if the tube is sized to run in X at the most common operating point as very little extra liquid can be pushed into the evap as the condenser is already starting to be very low. But if you have SC and a SH of 1C then any small change in condenser conditions could push more liquid into the evap and possibly cause floodback. In the case of running in SC then some SH would be safer.

Maximum COP is obtained with an SC of about 1C or 2C and SH of just a few C.

Chef
I am not quite sure if the bottom statement is quite true, (refrigerant dependent) if a liquid/suction heat exchanger is included there will be an increase in net cooling effect. thus increasing the COP. But considering this a heat pump, cooling is not required. "Shut up mad!"
The goal is to keep the suction pressure high, i am not sure that you can achieve the desired results using a cap, with out haveing other methods of controls, which would likely cost more than a TXV

mad fridgie
01-02-2010, 07:41 AM
If you are trying to compare against the cheaper end air cond/heat pumps that use caps, be aware that they are designed to meet a very specific standard. Acheive an end results becomes easy.
The reality is with these units, as soon as you deviate from design performance and efficiency dive out of the window, compared to systems that have a modulating expansion device.
With ASHP, there does not seem to be such a standard to work to.

Chef
01-02-2010, 08:14 AM
I am not quite sure if the bottom statement is quite true, (refrigerant dependent) if a liquid/suction heat exchanger is included there will be an increase in net cooling effect. thus increasing the COP. But considering this a heat pump, cooling is not required. "Shut up mad!"
The goal is to keep the suction pressure high, i am not sure that you can achieve the desired results using a cap, with out haveing other methods of controls, which would likely cost more than a TXV

Hi Mad

Its just that when you plot it out on a PH diagram it is those parameters which give the best COP - it might not be the best point to design for.

Its desA who is doing the design we are just answering his questions and he may be getting paid for it so we can all expect a large (post dated) cheque in the post soon. ??

Not sure why you think a cap is not good for heat pumps? The range of temperature change on the evap is small and also quite small on the condenser and its always within very clear bounds so a cap system duly designed would operate over the range without much change so would be mostly running at its optimum.
In a fridge system the cap sees very large changes in its operating mode so is more difficult to select the correct tube size and its much more of a compromise.

Chef

desA
01-02-2010, 08:43 AM
Its desA who is doing the design we are just answering his questions and he may be getting paid for it so we can all expect a large (post dated) cheque in the post soon. ??

I'll answer the remaining posts above, but this did catch my eye & I thought I'd address it head on.

I will state, for the record, that I am not being paid one bean for this research - it is all purely at my own expense. This is an interest & the reason I've put it up on RE so openly, is that we can all learn from the debate.

There are parts of the heat-pump that will not be shown, purely, because that would not be professional practice, nor fair to the original machine builder.

With this kind of thread being put out in the public domain, it should hopefully teach us all something about how these systems really work & allow us to debate in a healthy way. It is the way of the Open Age.

desA
01-02-2010, 08:48 AM
Usually a cap system has a suction accumulator and depending on its size you can push the SH down to gain extra evap area for cooling. This is especially true if the tube is sized to run in X at the most common operating point as very little extra liquid can be pushed into the evap as the condenser is already starting to be very low. But if you have SC and a SH of 1C then any small change in condenser conditions could push more liquid into the evap and possibly cause floodback. In the case of running in SC then some SH would be safer.

Surely, though, most designers would want to see some level of SC in the condenser, to create a liquid seal & ensure full condensation takes place (asuming condenser to be of adequate size in the first place)?


Maximum COP is obtained with an SC of about 1C or 2C and SH of just a few C.

Could I ask you to please refer this to a log(p)-h or T-s chart, so that we can debate why this should be so?

mad fridgie
01-02-2010, 08:53 AM
Hi Mad

Its just that when you plot it out on a PH diagram it is those parameters which give the best COP - it might not be the best point to design for.

Its desA who is doing the design we are just answering his questions and he may be getting paid for it so we can all expect a large (post dated) cheque in the post soon. ??

Not sure why you think a cap is not good for heat pumps? The range of temperature change on the evap is small and also quite small on the condenser and its always within very clear bounds so a cap system duly designed would operate over the range without much change so would be mostly running at its optimum.
In a fridge system the cap sees very large changes in its operating mode so is more difficult to select the correct tube size and its much more of a compromise.

Chef
On a Ph diagram yes it presumes a constant on the compressor, so the angle would change slightly (related to comp performance) some where in RE this was described in detail.
With a ASHP the process variables are ambient -10 to 45C and water temps from 10 to 70C.
(unlikely to have ambient at 45C and water at 10) So if we can agree that cap is a fixed pressure drop, then if the water is cold (non controled head pressure) then suction will plumet giving poor COP. these changes are very frequent and are over long periods.
With a fridge you are generally dealing with air load, which is a short term load, normal on and offs are with a small working band, yes ambient change, but ambient is low then infiltration load is low, so long term energy use becomes less important.

mad fridgie
01-02-2010, 09:02 AM
I do noy not like caps! Or could it be that I do not know how to design a system truely correct with a Cap? So do not get decent performance!! As TXV does give alittle flexability.
Could be!!!

desA
01-02-2010, 09:11 AM
With a ASHP the process variables are ambient -10 to 45C and water temps from 10 to 70C.
(unlikely to have ambient at 45C and water at 10) So if we can agree that cap is a fixed pressure drop, then if the water is cold (non controled head pressure) then suction will plumet giving poor COP. these changes are very frequent and are over long periods.


This is the inherant difficulty in designing an AWHP. Both ends, source & sink, are in a state of flux. It makes designing over an operating range extremely challenging.

Chef
01-02-2010, 09:16 AM
So if we can agree that cap is a fixed pressure drop, then if the water is cold (non controled head pressure) then suction will plumet giving poor COP. these changes are very frequent and are over long periods.
.

Thats the point about a cat tube in a heat pump - it is not a fixed pressure drop - quite the opposite.

When the water is 10C there will be a lot of SC available and so even if the pressure is less the flow through the cap tube will increase over the value assumed if it was just related to Dx and Sx. As the temperature of the water rises the SC is less but the pressure is higher so the flow is roughly the same and when its hot then lets assume we get to no SC and much higher pressure which gives a similar flow. So in this arrangement a cap might be good in a heat pump.

Take the point about ambient rangeing from -10 to 45C would cause problems in the evap area but are these temps really common?

Chef

desA
01-02-2010, 09:16 AM
Not sure why you think a cap is not good for heat pumps? The range of temperature change on the evap is small and also quite small on the condenser and its always within very clear bounds so a cap system duly designed would operate over the range without much change so would be mostly running at its optimum.


This is precisely why I started this thread - so that we could all have a bash at knocking out much of the speculation & possible mis-conceptions we have come across.

I've personally been scared stiff of them, based on what I'd heard. Seeing a running system in action, with it stroking repeatably across the water temp range 28-60'C, is quite intriguing.

Let's learn what we can about cap tubes & heat-pumps.

desA
01-02-2010, 09:19 AM
Thats the point about a cat tube in a heat pump - it is not a fixed pressure drop - quite the opposite.

When the water is 10C there will be a lot of SC available and so even if the pressure is less the flow through the cap tube will increase over the value assumed if it was just related to Dx and Sx. As the temperature of the water rises the SC is less but the pressure is higher so the flow is roughly the same and when its hot then lets assume we get to no SC and much higher pressure which gives a similar flow. So in this arrangement a cap might be good in a heat pump.

Excellent points.


Take the point about ambient rangeing from -10 to 45C would cause problems in the evap area but are these temps really common?

Seasonal variation - yes - daily variation - unlikely, except in a desert. Add into the mix the RH%... :D

desA
01-02-2010, 09:21 AM
Current cap tube dimensions:

OD = 2.4 -2.45mm
L = 2246 mm = 2.246 m

These are from measurements of the existing system.

Chef
01-02-2010, 09:41 AM
Surely, though, most designers would want to see some level of SC in the condenser, to create a liquid seal & ensure full condensation takes place (asuming condenser to be of adequate size in the first place)?

Could I ask you to please refer this to a log(p)-h or T-s chart, so that we can debate why this should be so?

Thats very true for TXV and would be ideal for a cap also but as they only have a single point at which they perform 'perfect' it means any system load or temperature changes means the SC will increase and so COP will fall or it will move into X and the COP will again fall. You can't design a cap tube to have the same SC over its operating range - its just the nature of the tube.

But you can size the condenser and tube to give the results as close to the ones you are looking for.

It is SC and X that control the device and so if you can control these 2 or at least understand how they work it will be a lot easier to design a system.

As for the PH diagram! I may be able to put it in words till I get to plotting it.
The system we will assume is simply compressor, condenser, cap tube and evaporator.
Higher SC values mean higher discharge pressure which is bad for COP.
Minimal SH means highest evap pressure which is good for COP.

Oh and about the cheque - just kidding as thats why I said post dated - like jan 2030 or similar! Maybe the humour got lost in translation.

Chef

mad fridgie
01-02-2010, 09:48 AM
Thats the point about a cat tube in a heat pump - it is not a fixed pressure drop - quite the opposite.

When the water is 10C there will be a lot of SC available and so even if the pressure is less the flow through the cap tube will increase over the value assumed if it was just related to Dx and Sx. As the temperature of the water rises the SC is less but the pressure is higher so the flow is roughly the same and when its hot then lets assume we get to no SC and much higher pressure which gives a similar flow. So in this arrangement a cap might be good in a heat pump.

Take the point about ambient rangeing from -10 to 45C would cause problems in the evap area but are these temps really common?

Chef
Would you actually have a lot of sub-cooling, or just a low condensing pressure. To get the high level of liquid sub-cooling then the liquid must be backed up in the condenser, which must leave the evap short of refrigerant, I agree that with increased sub-cooling, the pressure drop will reduce, so would increase flow, but would you not then loose the liquid which is backed up. thus loosing the sub-cooling benefit.
Some form of equalbrium will be reached, I still believe that the suction will trend down proportianally to liquid pressure on a critically charged system on a cap. (greater than that of a modulating valve, and non critcal charge)

Chef
01-02-2010, 10:06 AM
Some form of equalbrium will be reached, I still believe that the suction will trend down proportianally to liquid pressure on a critically charged system on a cap. (greater than that of a modulating valve, and non critcal charge)

Mad - you may not like cap tubes but you understand them very well.

Yes the evap will get shorter on liquid and be lower pressure and the system will fall off the COP obtainable from a TXV.

But h3 moves to the left on the PH diagram and so does h4 so the amount of heat pumped is still good and maybe even better than at design point.

Its not the best solution but it is the one desA is dealing with.

I dont particularly like caps either because of this SC and X problem but just got caught up in them.

Chef

mad fridgie
01-02-2010, 10:26 AM
Taking in all the information, and allowing for the variables, if a cap is to be used (what ever sized is picked)
I believe the most important issue is the size of the suction accumulator. If a relatively large one is installed, we could effectively over charge the system, allowing for increased sub-cooling at lower pressure ratios, and low SH at higher compression ratios.
At the lower ratios SH has less importance in relation to compressor discharge.

mad fridgie
01-02-2010, 11:49 AM
Mad - you may not like cap tubes but you understand them very well.

Yes the evap will get shorter on liquid and be lower pressure and the system will fall off the COP obtainable from a TXV.

But h3 moves to the left on the PH diagram and so does h4 so the amount of heat pumped is still good and maybe even better than at design point.

Its not the best solution but it is the one desA is dealing with.

I dont particularly like caps either because of this SC and X problem but just got caught up in them.

Chef
You are have struck on a really good point, one I have pondering on for a while (not real work) and a light has just switched on.
Why do we bother controlling heat pressure (cooling applications) if a piece of refrigeration is just allowed to reach equalbrium would it be cheaper to run.
(only suitable for refrigeration not related to producrs that are moisture sensitive)
Even with TXV a certain pd is required, why do we hold the SCT high, just to keep the SST high.
Umm I might just have a little play. Thanks chef!

desA
02-02-2010, 03:26 AM
As for the PH diagram! I may be able to put it in words till I get to plotting it.
The system we will assume is simply compressor, condenser, cap tube and evaporator.
Higher SC values mean higher discharge pressure which is bad for COP.
Minimal SH means highest evap pressure which is good for COP.

Now, I think what is important to note here, is that the optimum COP & maximum Q'cond do not generally coincide.

For the evaporator, the SH to Te,sat relationship is levered off T,cross & Ta,out. SH will naturally decrease across a heating cycle.

For the condenser, sub-cooling is added to the condensing heat-transfer. So, a small increase in Tc,at with increased sub-cooling may actually be beneficial, up to the point at which SC begins to swamp the condensing area. After this point, the trade would be non-beneficial.


Oh and about the cheque - just kidding as thats why I said post dated - like jan 2030 or similar! Maybe the humour got lost in translation.

No problem - thought it good to set the record straight, though. :)

desA
02-02-2010, 04:51 AM
http://i46.tinypic.com/33ok6dj.png

Results of yesterday's run.

Tc,exit = exit from tank coil.

mad fridgie
02-02-2010, 05:34 AM
They are really stable, I presume that when you say Te sup, that is actual temp and not superheat over Te sat.

mad fridgie
02-02-2010, 05:37 AM
You are using a tank with a coil round it, so you are getting lots of sub-cooiling or coil pressure drop.
can you add service port to the cond outlet pipe.
If this is pressure drop, (not sub-cooling) then stability would be more expected.

Gary
02-02-2010, 08:01 AM
Presumably the liquid is being subcooled by the bottom turns of the coil. In order for Tc,exit to be cooled to 40C, the water at the bottom of the tank must be less than 40C. Seems there is considerable stratification inside the tank.

desA
02-02-2010, 08:17 AM
They are really stable, I presume that when you say Te sup, that is actual temp and not superheat over Te sat.

Te,sup = measured directly at evap exit.
SH1 = Te,sup - Te,sat

Tc,suc = suction line measured 150mm before compressor - on pipe OD.

desA
02-02-2010, 08:19 AM
You are using a tank with a coil round it, so you are getting lots of sub-cooiling or coil pressure drop.
can you add service port to the cond outlet pipe.
If this is pressure drop, (not sub-cooling) then stability would be more expected.

The coil exit temp is fairly consistent across the whole heating cycle, moves ~2K from start to finish. Measured in temp pocket on outside of pipe.

desA
02-02-2010, 08:23 AM
Presumably the liquid is being subcooled by the bottom turns of the coil. In order for Tc,exit to be cooled to 40C, the water at the bottom of the tank must be less than 40C. Seems there is considerable stratification inside the tank.

Typically, these coils wind their way down from the top of the tank. The exit line then runs vertically upwards from the base of the tank, in the insulation barrier. There would be some temp loss to atmosphere, I'd expect.

Measurements on these tanks typically shows at least 10K temp stratification from top to bottom.

Gary
02-02-2010, 08:38 AM
Typically, these coils wind their way down from the top of the tank. The exit line then runs vertically upwards from the base of the tank, in the insulation barrier. There would be some temp loss to atmosphere, I'd expect.

Measurements on these tanks typically shows at least 10K temp stratification from top to bottom.


I was thinking the vertical run crosses over the warmer tubes, and if anything would gain temp.

Possibly you could measure the temp of the bottom of the tank? Maybe open the drain and measure the temp of the water coming out?

Gary
02-02-2010, 08:58 AM
The coil exit temp is fairly consistent across the whole heating cycle, moves ~2K from start to finish. Measured in temp pocket on outside of pipe.

Note also that it coincides with the Te,sat drift.

desA
02-02-2010, 10:22 AM
I was thinking the vertical run crosses over the warmer tubes, and if anything would gain temp.

Knowing the way these tanks are made, the coil may, or may not be touching along its length. I wouldn't rely on it.


Possibly you could measure the temp of the bottom of the tank? Maybe open the drain and measure the temp of the water coming out?

Ok, good idea. I'll modify the current piping arrangment, so that I can bleed off at the base during the test. It'll probably take a few days, as I'll be out of the lab during te last few days of this week.

desA
03-02-2010, 12:26 PM
Updated cap tube dimensions:

ID = 1.397 mm
L = 2246 mm = 2.246 m

These are from measurements of the existing system.

I have run a cap tube estimation program for this diameter cap tube. At mid range it estimates a required length of 1.2023 m & at hot condition, a length of 1.7517 m.

It would seem that MF & Chef were probably correct in estimating that the current cap tube is too long.

I have back-checked the standard cap tube length for the compressor used. This length (at dia) (2.246m) corresponds to the rated compressor condition of Te,sat = 7.2'C, Tc,sat=54.4'C, Tc,suct=35'C, SC=5K.

What is normal procedure for reducing cap tube length?

I'll also reset the liquid piping arrangement to flow downwards into the cap strainer.

mad fridgie
03-02-2010, 06:48 PM
If you are breaking into the system,
Install pressue point at condensor outlet(cap inlet) and at cap outlet (evap inlet)

desA
04-02-2010, 02:16 AM
If you are breaking into the system,
Install pressue point at condensor outlet(cap inlet) and at cap outlet (evap inlet)

Thanks for that, MF. Good idea. Then we can get a good handle on exactly what is going on across the cap tube, as well as across the condenser coil & evap.

Chef
04-02-2010, 11:11 AM
If you post the compressor volumetric flowrate we can then begin to see what the cap is doing and if the original length is right or your new numbers are right.

There are of course a number of tests you can do before breaking the system to see how close your original cap is.

Chef

desA
04-02-2010, 11:57 AM
If you post the compressor volumetric flowrate we can then begin to see what the cap is doing and if the original length is right or your new numbers are right.

I'll write up the compressor details tonight. Excellent point.


There are of course a number of tests you can do before breaking the system to see how close your original cap is.

I agree - absolutely. I would like to push the current arrangement, as it stands, to its absolute limit. Once we've reached the end of what this can deliver, we can modify the piping. The final cut is so terribly terminal, isn't it? :D

desA
04-02-2010, 04:21 PM
Compressor details

Rated performance - at 220V, 50Hz
Capacity.... : 910 W
Input power.: 386 W
COP.......... : 2.36
Current.......: 1.80 A

Rating condition
Evaporating temp : 7.2'C
Condensing temp.: 54.4'C
Ambient temp.....: 35'C
Return gas temp.: 35'C
Liquid temp........: 46.2'C

Compressor & motor data
Bore, stroke.......: 25 mm / 18mm
Displacement......: 8.83 ml/rev


Nothing is mentioned about the rotational speed.

In Coolpack at rated conditions, a thermodynamc balance is obtained at Vs=1.25 m3/h.

Chef
05-02-2010, 01:08 AM
Could either 1450 or 2900 RPM and the latter seems close your 1.25

Chef

mad fridgie
05-02-2010, 01:36 AM
Could either 1450 or 2900 RPM and the latter seems close your 1.25

Chef
I also expect high speed, but at high speed there is normally motor slipage, I would work on 2800 RPM
1.48M3H( also checked) i suspect volumetric efficiency is down at the stated compression ratios, hence the lower than expected performance. or even more slipage

desA
05-02-2010, 02:20 AM
The thermodynamic balance at manufacturer's rated specs, providing Vs=1.25 m3/h (Coolpack - note, COP's also correspond at this balance point), would imply a slippage of around 18.67% against maximum possible delivery.

Perhaps the manufacturers are just erring on the side of caution?

Chef
05-02-2010, 12:36 PM
The thermodynamic balance at manufacturer's rated specs, providing Vs=1.25 m3/h (Coolpack - note, COP's also correspond at this balance point), would imply a slippage of around 18.67% against maximum possible delivery.

It seems quite a lot 18% for slippage - maybe some motor experts can chip in with whats more common?

Still even at 2900RPM which is the basic industry standard as I believe it and some volumetric efficiency and the fact we do not know exactly how Coolpack calculates it I would propose using 1.48m3Hr until further clarification.

At least we can begin to run some models and see where things lay with a common data base but of course its your call.

Chef

desA
05-02-2010, 01:26 PM
It seems quite a lot 18% for slippage - maybe some motor experts can chip in with whats more common?

Still even at 2900RPM which is the basic industry standard as I believe it and some volumetric efficiency and the fact we do not know exactly how Coolpack calculates it I would propose using 1.48m3Hr until further clarification.

At least we can begin to run some models and see where things lay with a common data base but of course its your call.

Chef

That's fine. We can work with that & see what comes out of the calcs.

chemi-cool
05-02-2010, 03:10 PM
2900 rpm is after slippage calculation, without it it would be 3000.

The same with 1450 rpm, should be 1500.

Chef
06-02-2010, 05:12 AM
I have run a cap tube estimation program for this diameter cap tube. At mid range it estimates a required length of 1.2023 m & at hot condition, a length of 1.7517 m.

It would seem that MF & Chef were probably correct in estimating that the current cap tube is too long.

I have back-checked the standard cap tube length for the compressor used. This length (at dia) (2.246m) corresponds to the rated compressor condition of Te,sat = 7.2'C, Tc,sat=54.4'C, Tc,suct=35'C, SC=5K.

What is normal procedure for reducing cap tube length?

I'll also reset the liquid piping arrangement to flow downwards into the cap strainer.

You have back checked the tube length is 2.246m with discharge pressure (Pdis) of about 14.7bar.

Then you calculate for your high temp condition which we assume is 62C condensing and a Pdis of about 17.7bar. In this instance you arrive at 1.75m length.

This is very strange as the tube should be longer with the higher pressure Pdis=17.7 - maybe in the region of 3.7m

Something seems to be seriously wrong here as you cant increase the pressure ratio across the cap and also make it shorter.

It would be a nice idea to braze your new trial cap tube into stub ends of pipe and fit it to the system with flare fittings, certainly make it easier and cleaner to change the cap easily as the R&D progresses.

Also the sight glass is going to be the most valuable piece of kit you have as its the only way to know if you start getting into X territory.

But before you cut the tube why not do some parametric studies, ie get it up to 60C and place some ice on the evap, the suction pressure will fall, the mass flow will fall and the sight glass might start showing more bubbles. The more ice, the more bubbles, plot several evap temps against the bubble count (you will have to invent something here) and do it all again at 50C and 40C.

Do similar sets of measurements without ice and even some heating but working on SC.

Now plot it all out as mass flow versus SC and X for the various delta pressures and you will have the ultimate design graph.

Chef

desA
06-02-2010, 06:29 AM
You have back checked the tube length is 2.246m with discharge pressure (Pdis) of about 14.7bar.

Then you calculate for your high temp condition which we assume is 62C condensing and a Pdis of about 17.7bar. In this instance you arrive at 1.75m length.

This is very strange as the tube should be longer with the higher pressure Pdis=17.7 - maybe in the region of 3.7m

From my estimates, the original cap tube was sized at the compressor's rated condition & is, in fact, too long for the in-service application. The cap estimator I use, requests:
Te,sat; SH; Tc,sat; SC; Q'e or m'g, as well as lp piping & evap dP.


Something seems to be seriously wrong here as you cant increase the pressure ratio across the cap and also make it shorter.

The original cap tube length appears to be incorrect. I calculate the cap tube estimate at each operating point across the heating range, then compare with the original tube - in all cases, so far, these estimates are much lower.


It would be a nice idea to braze your new trial cap tube into stub ends of pipe and fit it to the system with flare fittings, certainly make it easier and cleaner to change the cap easily as the R&D progresses.

A good tip. Thanks for this - I will do this.


Also the sight glass is going to be the most valuable piece of kit you have as its the only way to know if you start getting into X territory.

Judging from the current way the system is running, I sincerely doubt that we will get into X territory. :)


But before you cut the tube why not do some parametric studies, ie get it up to 60C and place some ice on the evap, the suction pressure will fall, the mass flow will fall and the sight glass might start showing more bubbles. The more ice, the more bubbles, plot several evap temps against the bubble count (you will have to invent something here) and do it all again at 50C and 40C.

I've been doing a range of parametric studies, to determine system response to mass charge, to internal heat-transfer augmentation etc. Today, funnily-enough, is the ice-day, but in a different part of the heat-pump anatomy :D

I'll give the evap-ice a whirl as well - why not? Good idea. :D


Do similar sets of measurements without ice and even some heating but working on SC.

Now plot it all out as mass flow versus SC and X for the various delta pressures and you will have the ultimate design graph.


Good stuff... I'll press onwards & hopefully have some useful results to contribute to the thread.

Oddly-enough, I've been able to damp out the cap tube noise almost completely. There is no more machine-gun sound.

Chef
06-02-2010, 07:14 AM
I make my point again. You checked the compressor design conditions and came up with a tube legnth of 2.2m but then your program calls for 1.7m at a higher pressure - up to 17.7 from 14.7bar. This is not right and something is wrong.

All my results show tubes around 3+ metres for the high temp range to get the SC you want. ACC cubigel and Ashrae all have similar results.

If you put in a 1.2m tube you probably will be in X but you will no doubt find out when you bolt it on. You have already had bubbles in the sightglass so you may already be closer than you think

You say you need these input parameters:-
Te,sat; SH; Tc,sat; SC; Q'e or m'g, as well as lp piping & evap dP.
Not really sure why SH would do anything to affect the cap tube size, its based solely on Tcsat, SC(or X) and Tesat. Only if your running a simulation of the whole system will SH show up but then you will need condenser and evap volumes, surface areas, U, charge, Ta, load, etc.

And for the piping and evap Dp - for very detailed analysis it has some merit but its influence is very small indeed. I think you have bigger problems than +/- 0.5%

Chef

mad fridgie
06-02-2010, 07:26 AM
There does seem to be some sort of error,
When did your calcs on the original cap, did you have a very high level of sub-cooling, as your earlier tests indicated, When you have revistited at the higher discharge pressure, did you have only small amount of sub-cooling, this could indicate the need for a shorter cap?

desA
06-02-2010, 01:45 PM
http://i45.tinypic.com/28gz2i9.png

Simulation of thermodynamic equilibrium at compressor rated conditions. This balances completely with compressor rated specifications, right down to COP.

I will, below, establish the credibility of the cap-tube estimation tools.

desA
06-02-2010, 01:48 PM
http://i45.tinypic.com/2psnh2v.png

Capillary tube estimate at rated conditions. Measured/estimated captube length 2.2462 m (some internal projections not estimated. In other words - bang on).

Please note, maximum allowable superheat in cap-tube estimator is 20K. Actual superheat at compressor suction, is 27.8K.

desA
06-02-2010, 01:51 PM
http://i48.tinypic.com/23tp01e.png

DanCap simulation. Suggested length even shorter.

Gary
06-02-2010, 03:38 PM
Any conclusions based upon the original data are invalid, because the strainer is upside down.

Since there is no liquid seal at the cap tube inlet, given a normal charge the superheat would be very high. Extra refrigerant has been added to bring down the superheat, which causes the subcooling to be high, which in turn drives up the Tc,sat, which is masked by the fact that you are measuring the water temp at the top of a static tank with substantial stratification.

On a cap tube system, strainer/drier positioning is not some minor detail to be skimmed over. It changes everything.

desA
06-02-2010, 03:59 PM
Thanks, Gary.

I'm not sure why the machine builders decided to install the cap strainer the way they have. It goes contrary to every local refrigerator I've seen. I have a feeling that the unit this one may have been modelled off would have had the exact-same orientiation. It is a famous brand name, of long-standing.

Now, the odd thing with this type of heat-pump is that, due to the poor nature of the refrigerant-to-water heat-transfer, Tc,sat ends up rising very slowly, over the duration of the heating cycle. It almost seems to 'fall into itself' as it were. This is despite very large subcooling values.

It does not respond as you would expect a high-performance condenser to respond. It is very different, I've found.

This 'falling in on itself' is an oddity that, ironically, in the long run, contributes to a very flat heating curve & fairly even COP curve.

Gary
06-02-2010, 04:31 PM
http://i46.tinypic.com/33ok6dj.png

Results of yesterday's run.

Tc,exit = exit from tank coil.

If we assume that Tw is in error and that the TD is in fact constant, then the TD remains at 18K and the actual Tw at the end of the run is 44C (18K below Tc,sat), what does that do for your 'falling in on itself' theory?

desA
06-02-2010, 04:57 PM
TD for a condenser does not remain constant through any heat-pump cycle. It tends to decrease to a greater, or lesser degree, depending on the type of condenser used.

For these tests, they are quasi-static, in the sense that a small amount of water is passed through the system, in order to slow down the heatup rate - in order to allow data capture (manual), without temps moving off too far.

So, in this sense, the tank is not a static mass of water - it is being adjusted as the incoming water feeds in from the bottom. So, at worst, you could estimate the 'average' water temp in the tank to be (Tw,in + T,top)/2.

So, given that scenario, at the hottest reading Tw,av ~ (28+60)/2 = 44'C... So, yes this is remotely possible.

Practically, though, I would expect the average water temp to be a fair bit higher than this. as the coil does move all the way to the bottom. I'd at a rough guess actually expect something closer to 50'C, or a little higher, judging from the time it takes to get the tank contents to drop off the highs, when new fresh water is pushed through after a run.

Basically, the main reason I see for the 'falling in on itself' observation, is that the condenser effectiveness drops off dramatically over the cycle & the condenser just does not push back as expected.

I'm still trying to figure out how to get a decent represenative average tank temperature. It would take a fair bit of lower bleed-off to get to internal water during a test & this would modify the test results.

Gary
06-02-2010, 05:22 PM
The condenser may coil all the way to the bottom of the tank, but the bottom of the coil is filled with liquid because the strainer is upside down and the system is overcharged as a result. The condenser is far less effective than it should be.

Gary
07-02-2010, 03:26 AM
For these tests, they are quasi-static, in the sense that a small amount of water is passed through the system, in order to slow down the heatup rate - in order to allow data capture (manual), without temps moving off too far.

So, in this sense, the tank is not a static mass of water - it is being adjusted as the incoming water feeds in from the bottom. So, at worst, you could estimate the 'average' water temp in the tank to be (Tw,in + T,top)/2.

So, given that scenario, at the hottest reading Tw,av ~ (28+60)/2 = 44'C... So, yes this is remotely possible.


Replacing warm water at the top with cold water at the bottom increases the dT between top and bottom, lowering the average temp. Average temp of 44C is not a remote possibility. It is a strong probability.

desA
07-02-2010, 05:14 AM
The condenser may coil all the way to the bottom of the tank, but the bottom of the coil is filled with liquid because the strainer is upside down and the system is overcharged as a result.

I'd agree with the system being grossly-overcharged. Can you perhaps explain further why this can be attributed to the strainer orientation?


The condenser is far less effective than it should be.

To be sure - it is absolutely awful. :eek:

How would you improve the coil in this case?

desA
07-02-2010, 05:20 AM
Replacing warm water at the top with cold water at the bottom increases the dT between top and bottom, lowering the average temp. Average temp of 44C is not a remote possibility. It is a strong probability.

I'm not a betting man... :D

I'll have to work out a way to measure the internal contents.

Gary
07-02-2010, 05:33 AM
I'd agree with the system being grossly-overcharged. Can you perhaps explain further why this can be attributed to the strainer orientation?


The overcharge is there to compensate for the lack of liquid seal at the cap tube inlet. Without the overcharge the evap would be starved and the superheat would be high.

If the strainer were oriented properly, there would be a liquid seal at the cap tube inlet, the evap would be fed properly and the overcharge would not be needed.

desA
07-02-2010, 05:38 AM
Originally Posted by desA
I'd agree with the system being grossly-overcharged. Can you perhaps explain further why this can be attributed to the strainer orientation?

desA:
The overcharge is there to compensate for the lack of liquid seal at the cap tube inlet. Without the overcharge the evap would be starved and the superheat would be high.

If the strainer were oriented properly, there would be a liquid seal at the cap tube inlet, the evap would be fed properly and the overcharge would not be needed.

Thanks for the detailed explanation. This makes sense, but I don't think that it the full solution.

Now, what if the system were actually manufactured in imbalance. In other words, if the evap, compressor & condenser are mis-matched? How would this affect the system charge? How would the condenser system shape affect required system charge?

Gary
07-02-2010, 06:13 AM
Thanks for the detailed explanation. This makes sense, but I don't think that it the full solution.

Now, what if the system were actually manufactured in imbalance. In other words, if the evap, compressor & condenser are mis-matched? How would this affect the system charge? How would the condenser system shape affect required system charge?

Some matches are better than others, but there are no mis-matches... and the system will seek and find a point of balance, however disadvantageous that balance point may be.

Let's fix the obvious problem and then see what we have.

desA
07-02-2010, 06:30 AM
Let's fix the obvious problem and then see what we have.

Are we 100% certain that this 'obvious problem' is the real underlying reason for the system operating the way it currently does?

Are there examples of any other cap-tube systems operating successfully with strainers in the inverted position? What of the systems which do not have strainers (other than blockages)?

I'm asking this in order to make sure that we fully evaluate all possibilities, before moving down one path.

Chef
07-02-2010, 06:53 AM
If you have a lot of SC then it wont really matter which way the filter is oriented as the condenser has quite a bit of liquid in it, but once the SC gets small then having the filter run downhill as Gary is suggesting is probably essential. You can just cut it and change it - but once you cut you may as well change all things that need to be done and without the parametrics you cant decide what else needs to be changed.

Chef

Gary
07-02-2010, 07:00 AM
Are we 100% certain that this 'obvious problem' is the real underlying reason for the system operating the way it currently does?

Are there examples of any other cap-tube systems operating successfully with strainers in the inverted position? What of the systems which do not have strainers (other than blockages)?

I'm asking this in order to make sure that we fully evaluate all possibilities, before moving down one path.

Is it a problem? Yes.

Is it the only problem? Probably not.

desA
07-02-2010, 07:10 AM
If you have a lot of SC then it wont really matter which way the filter is oriented as the condenser has quite a bit of liquid in it, but once the SC gets small then having the filter run downhill as Gary is suggesting is probably essential.

These were my thoughts. Practically, for the current tank/coil setup, with its long vertical dicharge leg, I sincerely doubt that we will see X territory. Intermitant clouds of small bubbles seem to be the norm.

My reasoning is that at lower mass charge, the system response is very obviously under-charge. The compressor also runs far hotter, to the touch, at low charges. As the charge increases, the compressor cools off to a decent running condition.

So, under the current set of system components, a large mass charge is required to make the system settle into a reasonable operating regime.


You can just cut it and change it - but once you cut you may as well change all things that need to be done and without the parametrics you cant decide what else needs to be changed.

Agreed. The parameteric studies are incredibly useful in fully characterising the system response. When finally, system changes are introduced, they will be in a careful manner. We can then learn from the response of each change.

Gary
07-02-2010, 07:14 AM
If you have a lot of SC then it wont really matter which way the filter is oriented as the condenser has quite a bit of liquid in it, but once the SC gets small then having the filter run downhill as Gary is suggesting is probably essential. You can just cut it and change it - but once you cut you may as well change all things that need to be done and without the parametrics you cant decide what else needs to be changed.

Chef

Exactly so. The high SC compensates for the incorrect strainer orientation.

Be sure to change the drier orientation, too.

desA
07-02-2010, 07:57 AM
Be sure to change the drier orientation, too.

I've been mulling this one over for some time, having installed this orientation in another test heat-pump. I have to say that it gave me some very, very odd results, that were not overly simple to explain away, other than the vertical drier orientation. I am now not too convinced of its effectiveness, I have to say. Sporlan advocate this, but I've not seen it in other literature (probably is there, & I've missed it).

Care has to be taken in some instances, I expect, with the drier position relative to the condenser discharge & type. It can present issues within the condenser itself in terms of liquid-clearing.

For a vertical heat-pump, the clearing issue should not affect the condenser as it already has to push the liquid upwards a fair amount.

I remember Chef's concern regarding the possibility of this filter-drier accumulating too much refrigerant & that this could create a problem under critical charge conditions. Perhaps we could discuss this further?

mad fridgie
07-02-2010, 08:28 AM
How do you know that there is sub cooling.
How do you know the pressure drop across the cap.
Presumption is OK for design we need to be taking actual measurements at all point within the systerm.
Also interesting how 2 different software programs give 2 different answers.

Chef
07-02-2010, 08:39 AM
The dryer on Danfoss units is top in and bottom out (on the component breakdown I have for this model anyway) so I see no reason to not set it downwards.

The main issue seems to be with the riser on the exit of the condenser and also the temp drop down the condenser will always want to generate quite a lot of SC and hence hold up some liquid. Not sure if trying to get rid of this effect with charge manipulation is a good idea rather than looking at the condenser itself?

Changing the cap can obviously change the SC but if your condenser is not right then may your just shifting the problem elsewhere.

I think some more details on the condenser and the temps you plan to get will help.

Chef

mad fridgie
07-02-2010, 08:51 AM
I have had a closer look at the 2 programs, the danfoss has no allowance for sub cooling, so a shorter cap is required, why not re enter detail in first program with no sub cooling

desA
07-02-2010, 08:57 AM
The dryer on Danfoss units is top in and bottom out (on the component breakdown I have for this model anyway) so I see no reason to not set it downwards.

If you could perhaps put up a picture of the Danfoss standard arrangement for this dryer - relative to the condenser outlet, I'd be most grateful.


The main issue seems to be with the riser on the exit of the condenser and also the temp drop down the condenser will always want to generate quite a lot of SC and hence hold up some liquid. Not sure if trying to get rid of this effect with charge manipulation is a good idea rather than looking at the condenser itself?

It will not help to dramatically reduce charge on this machine, under its current condenser configuration, in order to try & reduce SC. The geometry & condenser design constraints are the main design bottleneck, in my view.


Changing the cap can obviously change the SC but if your condenser is not right then may your just shifting the problem elsewhere.

Yes, agreed, but...

This style of condenser is widely used by a few 'famous name' brands in the industry. Tuning the cap tube to suit 'what is', would be an early target. Some manfacturers seem to very happy with this type of design & they've been pretty successful with it & I'm sure that they won't welcome change. :D


I think some more details on the condenser and the temps you plan to get will help.


This will be the next step. I've already begun experimenting on how to improve this.

desA
07-02-2010, 09:04 AM
I have had a closer look at the 2 programs, the danfoss has no allowance for sub cooling, so a shorter cap is required, why not re enter detail in first program with no sub cooling

http://i50.tinypic.com/2i24dqq.png

There we are. :)

mad fridgie
07-02-2010, 09:11 AM
http://i50.tinypic.com/2i24dqq.png

There we are. :)
I am I being a pain in the a***. Duty is differnt

desA
07-02-2010, 09:17 AM
Yes, it is - by design.

At the same mass flow, with zero SC, the heat-transfer will automatically balance in this software. It's smart... :D

mad fridgie
07-02-2010, 09:20 AM
so which program can you trust?

desA
07-02-2010, 09:31 AM
so which program can you trust?

That is the question. The one that you guys use? :D

Chef
07-02-2010, 10:42 AM
Yes, it is - by design.

At the same mass flow, with zero SC, the heat-transfer will automatically balance in this software. It's smart... :D

So with 8.3C of SC you get 2.2581m
with 0C of SC you get 2.326m

Just 68mm to reduce all that SC to two phase flow.

That seems 'an interesting result'.

Chef

Chef
07-02-2010, 10:48 AM
Here is the picture you asked for about the top to bottom drier.

3343

desA
07-02-2010, 10:58 AM
Thanks Chef.

desA
07-02-2010, 11:00 AM
So with 8.3C of SC you get 2.2581m
with 0C of SC you get 2.326m

Just 68mm to reduce all that SC to two phase flow.

That seems 'an interesting result'.

Chef

Ok, fair-enough then. I'm always open to learning new things - cap tubes are new to me, at least. If you'd like to post the results you have to hand, then we can compare notes. What software are you using?

As these programs always say - this is a recommended starting point. If we find that the program is substantially in error, then I will contact the developer - a colleague & have him investigate.

Chef
07-02-2010, 11:52 AM
Ok, then. If you'd like to post the results you have to hand, then we can compare notes.

As these programs always say - this is a recommended starting point. If we find that the program is substantially in error, then I will contact the developer - a colleague & have him investigate.

I do not mind providing you with 'a recommended starting point' for your system.

However I do not believe that I should be involved in helping the developer of the program you are using. If the code is wrong its got nothing to do with me and I have no intention to provide details on how to fix it.

As you know I have developed a cap tube sizing program that includes system simulation. Asking me to present my results to demonstrate a possible flaw in someone else's program so you can forward them on to another company is, quite honestly, disappointing.

This forum is a public domain and they may even be reading this thread - I have no problem with that.

My comments on both the Danfoss program and the one you are using are normally limited to 'interesting results' or 'use with due diligence'. I will try to stay in this area in future.

Chef

My reply was based on your original post before you edited it.

desA
07-02-2010, 12:45 PM
Hi Chef. Wow. My thoughts most certainly did not involve industrial espionage of your cap-tube program. Far from it.

I was not aware that you had a cap tube program so dear to your heart - I knew you'd worked with EES & so forth. I'd say that if it will present a problem, then we shut this whole thread, here & now.

You can find the simulation software at www.coolit.com

It's not worth the hassle, honestly. I'm done. I'll program the cap tube simulator myself - it's not that difficult, actually. I have the logic in my library.

:eek:

Gary
07-02-2010, 06:26 PM
As I see it, there is a time for calculation & prediction and there is a time for measurement & analysis.

The time for calculation & prediction is before the system is built

The time for measurement & analysis is after the system is built.

I think we should re-orient the strainer and drier, adjust the charge and then see what the measurements tell us needs to be done next.