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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
Chef
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.
Quote:
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
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Re: Expansion noise - capillary tube into evaporator
Current cap tube dimensions:
OD = 2.4 -2.45mm
L = 2246 mm = 2.246 m
These are from measurements of the existing system.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
desA
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
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
Chef
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)
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
mad fridgie
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
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Re: Expansion noise - capillary tube into evaporator
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.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
Chef
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!
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
Chef
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.
Quote:
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. :)
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Re: Expansion noise - capillary tube into evaporator
http://i46.tinypic.com/33ok6dj.png
Results of yesterday's run.
Tc,exit = exit from tank coil.
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Re: Expansion noise - capillary tube into evaporator
They are really stable, I presume that when you say Te sup, that is actual temp and not superheat over Te sat.
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Re: Expansion noise - capillary tube into evaporator
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.
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Re: Expansion noise - capillary tube into evaporator
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.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
mad fridgie
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.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
mad fridgie
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.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
Gary
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.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
desA
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?
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
desA
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.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
Gary
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.
Quote:
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.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
desA
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.
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Re: Expansion noise - capillary tube into evaporator
If you are breaking into the system,
Install pressue point at condensor outlet(cap inlet) and at cap outlet (evap inlet)
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
mad fridgie
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.
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Re: Expansion noise - capillary tube into evaporator
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
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
Chef
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.
Quote:
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
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Re: Expansion noise - capillary tube into evaporator
Quote:
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.
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Re: Expansion noise - capillary tube into evaporator
Could either 1450 or 2900 RPM and the latter seems close your 1.25
Chef
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
Chef
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
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Re: Expansion noise - capillary tube into evaporator
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?
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
desA
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
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
Chef
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.
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Re: Expansion noise - capillary tube into evaporator
2900 rpm is after slippage calculation, without it it would be 3000.
The same with 1450 rpm, should be 1500.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
desA
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
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
Chef
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.
Quote:
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.
Quote:
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.
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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. :)
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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
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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.
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Re: Expansion noise - capillary tube into evaporator
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
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Re: Expansion noise - capillary tube into evaporator
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?
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Re: Expansion noise - capillary tube into evaporator
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.
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Re: Expansion noise - capillary tube into evaporator
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.
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Re: Expansion noise - capillary tube into evaporator
http://i48.tinypic.com/23tp01e.png
DanCap simulation. Suggested length even shorter.
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Re: Expansion noise - capillary tube into evaporator
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.
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Re: Expansion noise - capillary tube into evaporator
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.
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Re: Expansion noise - capillary tube into evaporator
Quote:
Originally Posted by
desA
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?