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27-01-2010, 03:15 PM #1
Expansion noise - capillary tube into evaporator
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?
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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27-01-2010, 03:50 PM #2
Re: Expansion noise - capillary tube into evaporator
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.
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27-01-2010, 10:54 PM #3
Re: Expansion noise - capillary tube into evaporator
Des,
I always tell my customers .."When it stops making a noise , then you know you have a problem."
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27-01-2010, 10:56 PM #4
Re: Expansion noise - capillary tube into evaporator
maybe go up a size or two
mmm to beer or not to beer...........lets drink breakfast
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28-01-2010, 12:53 AM #5
Re: Expansion noise - capillary tube into evaporator
Capillary tube setup.
I'd value comments & suggestions for improvement.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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28-01-2010, 01:02 AM #6
Re: Expansion noise - capillary tube into evaporator
Sound-bite: sssshhhh....ssshhh... ka-ping... ping... ping... ka-ping... sssshhhh.... ka-ping...
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.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
Rarefied Technologies ( SE Asia )
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28-01-2010, 01:04 AM #7
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28-01-2010, 01:06 AM #8
Re: Expansion noise - capillary tube into evaporator
The drier outlet should be pointed downward.
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28-01-2010, 01:07 AM #9
Re: Expansion noise - capillary tube into evaporator
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.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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28-01-2010, 01:08 AM #10
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28-01-2010, 01:10 AM #11
Re: Expansion noise - capillary tube into evaporator
Both... the strainer and the drier.
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28-01-2010, 01:17 AM #12
Re: Expansion noise - capillary tube into evaporator
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.Last edited by desA; 28-01-2010 at 01:20 AM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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28-01-2010, 01:23 AM #13
Re: Expansion noise - capillary tube into evaporator
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28-01-2010, 01:28 AM #14
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28-01-2010, 01:33 AM #15
Re: Expansion noise - capillary tube into evaporator
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.
Last edited by Gary; 28-01-2010 at 01:38 AM.
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28-01-2010, 01:37 AM #16
Re: Expansion noise - capillary tube into evaporator
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.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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28-01-2010, 02:38 AM #17
Re: Expansion noise - capillary tube into evaporator
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
mmm to beer or not to beer...........lets drink breakfast
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28-01-2010, 02:52 AM #18
Re: Expansion noise - capillary tube into evaporator
Last edited by desA; 28-01-2010 at 03:02 AM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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28-01-2010, 03:40 AM #19
Re: Expansion noise - capillary tube into evaporator
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.
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28-01-2010, 03:52 AM #20
Re: Expansion noise - capillary tube into evaporator
Engineers & our ears... small noises like that bother me.
Second drier upside down excess vapour entering.
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.
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.Last edited by desA; 28-01-2010 at 03:59 AM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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28-01-2010, 04:02 AM #21
Re: Expansion noise - capillary tube into evaporator
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.
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28-01-2010, 04:03 AM #22
Re: Expansion noise - capillary tube into evaporator
PS, I do not like capillaries on forced draft evaps, Acceptable on static coils.
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28-01-2010, 04:08 AM #23
Re: Expansion noise - capillary tube into evaporator
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.
I figured this much, & will be watching the SH ultra-closely.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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28-01-2010, 04:09 AM #24
Re: Expansion noise - capillary tube into evaporator
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
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28-01-2010, 04:10 AM #25
Re: Expansion noise - capillary tube into evaporator
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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28-01-2010, 04:13 AM #26
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28-01-2010, 04:14 AM #27
Re: Expansion noise - capillary tube into evaporator
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.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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28-01-2010, 04:28 AM #28
Re: Expansion noise - capillary tube into evaporator
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
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28-01-2010, 05:11 AM #29
Re: Expansion noise - capillary tube into evaporator
Thanks, Chef. Excellent points.
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.
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.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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28-01-2010, 06:11 AM #30
Re: Expansion noise - capillary tube into evaporator
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
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28-01-2010, 07:08 AM #31
Re: Expansion noise - capillary tube into evaporator
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.
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.
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.
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.
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.
Thanks so much, Chef for an absolutely stunning post. It is incredibly helpful & very much appreciated.
Post a picture of the sight glass.Last edited by desA; 28-01-2010 at 07:10 AM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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28-01-2010, 07:20 AM #32
Re: Expansion noise - capillary tube into evaporator
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.
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28-01-2010, 07:45 AM #33
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28-01-2010, 07:46 AM #34
Re: Expansion noise - capillary tube into evaporator
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
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28-01-2010, 08:00 AM #35
Re: Expansion noise - capillary tube into evaporator
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.
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?
Thanks again... excellent food for thought.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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28-01-2010, 08:54 AM #36
Re: Expansion noise - capillary tube into evaporator
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)
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28-01-2010, 09:08 AM #37
Re: Expansion noise - capillary tube into evaporator
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
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28-01-2010, 09:15 AM #38
Re: Expansion noise - capillary tube into evaporator
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
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28-01-2010, 09:18 AM #39
Re: Expansion noise - capillary tube into evaporator
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.
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)
These RHVAC systems are a lot more sensitive than many folks would have us believe.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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28-01-2010, 12:36 PM #40
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Re: Expansion noise - capillary tube into evaporator
Maybe this article could have some interesting information.
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28-01-2010, 12:52 PM #41
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.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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28-01-2010, 12:57 PM #42
Re: Expansion noise - capillary tube into evaporator
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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28-01-2010, 01:28 PM #43
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?
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28-01-2010, 02:58 PM #44
Re: Expansion noise - capillary tube into evaporator
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.
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.
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'.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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28-01-2010, 03:19 PM #45
Re: Expansion noise - capillary tube into evaporator
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.
And Des, how about some work today?Last edited by desA; 28-01-2010 at 03:35 PM.
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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28-01-2010, 03:22 PM #46
Re: Expansion noise - capillary tube into evaporator
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28-01-2010, 03:38 PM #47
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.Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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28-01-2010, 03:41 PM #48
Re: Expansion noise - capillary tube into evaporator
Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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28-01-2010, 07:13 PM #49
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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29-01-2010, 01:35 AM #50
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 workmmm to beer or not to beer...........lets drink breakfast
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