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Re: Expansion noise - capillary tube into evaporator
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.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
nike123
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.
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Re: Expansion noise - capillary tube into evaporator
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
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
Chef
When the system has too much accumulator volume after the condenser it will steal liquid from the evap.
Fair comment.
Quote:
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?
Quote:
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?
Quote:
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.
Quote:
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.
Quote:
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:
Quote:
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
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
chemi-cool
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?
Quote:
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
Quote:
And Des, how about some work today?:D
The R&D is part of my work... :D :D
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
desA
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.
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Re: Expansion noise - capillary tube into evaporator
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.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
Gary
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.
Quote:
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?
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Re: Expansion noise - capillary tube into evaporator
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.
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Re: Expansion noise - capillary tube into evaporator
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
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
chemi-cool
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?
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
lowcool
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.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
desA
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/.../PDE00B322.pdf
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Re: Expansion noise - capillary tube into evaporator
Thanks Nik,
Thats the one, DCL 052's
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
nike123
Thanks so much, Nike. :)
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Re: Expansion noise - capillary tube into evaporator
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.
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Re: Expansion noise - capillary tube into evaporator
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
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
chemi-cool
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
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Re: Expansion noise - capillary tube into evaporator
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.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
nike123
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.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
desA
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
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Re: Expansion noise - capillary tube into evaporator
Thanks very much, Chef.
Quote:
Originally Posted by
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'.
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Re: Expansion noise - capillary tube into evaporator
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.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
Chef
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).
Quote:
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.
Quote:
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?
Quote:
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.
Quote:
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.
Quote:
The evap can similarly be treated and also for the piping but only you can determine this as you have the geometry.
Good point.
Quote:
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.
Quote:
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.
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Re: Expansion noise - capillary tube into evaporator
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'.
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Re: Expansion noise - capillary tube into evaporator
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
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
desA
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.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
Gary
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.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
Chef
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. :)
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
desA
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
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
Chef
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
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Re: Expansion noise - capillary tube into evaporator
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.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
mad fridgie
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
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Re: Expansion noise - capillary tube into evaporator
Quote:
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.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
Chef
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)?
Quote:
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?
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
Chef
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.
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Re: Expansion noise - capillary tube into evaporator
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!!!
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
mad fridgie
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.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
mad fridgie
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
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
Chef
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.