View Full Version : Starved Evaporator Troubleshooting?
Hi everyone,
I've got a reverse cycle A/C system which has an oversize condenser.
In cooling it's great thanks to the big condenser. But in heating mode when the condenser become the evaporator, there comes problems.
I have very low Evaporating temp (-28C), very low Suction pressure, very high SH (30K, although 2-TXV is set to minimum SH, wide open)
What are the different ways to increase the evaporating temperature?
The indoor side if fixed, I can not change a thing there.
Things can be changed including outdoor airflow (which is currently quite high already, 670m3/h on high), outdoor coil size and tube configuration (can make it bigger or smaller or whatever tube arrangement), charge amount can also be change slightly (may have adverse effect because of relatively small indoor coil), can't fit a bigger TXV which is already oversize.
Any comment is greatly appreciated.
Thank you
cool_tech
09-05-2012, 10:53 AM
Hey mate, Is there a blockage anywhere drier, txv? What are your condenser fans doing when in heating mode are they running.
desA, the indoor HX is quite small so I was hesitated to increase the charge amount. May push a little bit further and see if it sustains high load condition.
cool_tech, small-non serviced system so no drier used, TXV was brandnew & it works perfectly in cooling mode. Cond Fan was high.
Brian_UK
09-05-2012, 11:12 PM
Fit outdoor fan control to maintain your pressures.
aramis
10-05-2012, 12:17 AM
Are you sure the oversized condenser is designed to work as an evap? Can you post pictures?
Does it initially work fine and after a while the pressure drops? If so it could be logging oil.
Brian_UK, do you mean higher airflow higher evap pressure/temp? My fan is a two speeds one and I switched between them.
aramis, this is a custom made system. That's also why I mentioned the whole outdoor setup can be changed.
What happen to the temps & pressures if I fit in a smaller outdoor coil (1/3 thinner) & maintain high, or even higher airflow? The supply side (Comp+ Indoor + TXV) is unchanged, if we just changed the demand, will there be any positive changes?
If the pipe between TXV & outdoor coil inlet is un-insulated, any bad effect?
It's quite hard to understand and predict the correlation between all the balance forces.
Magoo
10-05-2012, 04:03 AM
Uni.
it is short of gas, you can theorize till the cows come home from what ever direction. The system is short of gas. As per DesA post.
Thanks for posting Magoo, I'll push it to the upper limit today and will report back the result.
Auckland, I'll come one day.
mikeref
10-05-2012, 09:53 AM
"UNI", you said the a/c works "well" in cooling mode. What it the temperature difference across the evap coil?
Air on vs air off. If it's in the range of 12 degrees c or better, in cooling mode, on medium fan speed, (with the room temperature above 25 C) then i might not be interested in the gas charge.
Running in reverse cycle, with bugger all pressure, i would look at your expansion valve feed, as well as supply liquid temperature. Is the expansion valve and distributor heavily iced up? (Yes?)
Do you have the correct size orifice in the valve? Not looking for you to fit a bigger valve, but maybe you have too small an orifice in the valve to make it work properly, in reverse cycle.;)
mikeref, Td approx 12-14C at 27C amb., high fan spd.
I am not so sure how to change to orifice in the valve. It was factory fitted. The next size is ways to big for other mode & conditions. What about shortenning the spring? It's quite easy to open the assembly and refit it.
Also, have just tried more charge, several steps up, not much improvement to the parameters if not the same.
mikeref
12-05-2012, 06:32 AM
need a picture, or link to show me what valve you have out there on the condenser
You should have two TXV's, one at the outdoor coil and one at the indoor coil. Each of these should have a bypass line with a check valve in it. The arrow on the check valve should be pointed away from the coil. It sounds to me like a problem with the indoor check valve.
mark957
13-05-2012, 10:35 AM
hello all
From what I think, the gas charge should be lower in operation in the heat pump, since the capacitor is larger.
so I think that the expansion valve is not properly fed by the liquid causes of obstruction on the line.
however it is possible to insert a reserve of liquid to ensure the two cycles of operation.
In heat mode, the indoor coil is the condenser. The liquid from the indoor coil flows through the indoor check valve to the outdoor TXV. If the indoor check valve is stuck in the closed position, the liquid must flow backwards through the indoor TXV, severely restricting the flow.
You should also be aware that there are TXV's available which have the bypass/check valve built in. Scroll down to the bottom in this PDF:
http://www.emersonclimate.com/Documents/FlowControls/PDF%20resources/2006FC-174R1TXVre-article.pdf
Gary & mikeref, I used the two (one) ways TXV as described in Emerson's article. Two different TXV & check valve would be too bulky & could bring along more assembly issues.
Based on the datasheet, flow rate for both directions are almost identical.
I am gonna eliminate a whole row (1/3) of the outdoor coil today & see what happen. I expect this would re-balance the supply & demand in heatpump mode, reduce the pressure loss, and also further increase airflow which also support higher saturation temp, etc,.
The adverse effect is higher condensing temp in Cooling mode.
What do you reckon, everyone?
P/s:
Thanks for all of your comments & interests so far. I find this forum of exceptional quality.
Have a good new week, everyone.
Gary & mikeref, I used the two (one) ways TXV as described in Emerson's article.
I'm not sure what you mean. Are you saying both of your TXV's have the built-in check valves? Or are you saying that you are only using one TXV?
I am gonna eliminate a whole row (1/3) of the outdoor coil today & see what happen.
That would be a big mistake. Coil size is NOT the problem.
I am using one two-ways-TXV, or Two (one) way TXV as the manufacturer called it, same concept as in the article, Gary. One TXV with internal checkvalve used for both heating & cooling modes.
What do you reckon about the proper solution?
You should have a two way TXV at each coil.
Did you mean two traditional TXV & check valve? One for Heating, one for cooling?
Each of the coils needs a TXV and a check valve ***OR*** each of the coils needs a TXV with a built-in check valve.
http://www.refrigerationbasics.com/1024x768/heat_pumps1.htm
mad fridgie
14-05-2012, 05:06 AM
Do you have a Bi-flow expansion valve.
It is a single valve that allows expansion in either direction.
I simple question when you said it was working in cooling was it cold out side?
Gary, I have one indoor coil + one outdoor coil + one "whatever we call it" - two ways expansion valve in between + a reversing valve.
Yes, mad fridge. It's a single valve that allows expansion in either direction . A single valve for both heating & cooling modes. If we look at Gary's picture, the only thermo bulb will be installed downstream between the Reversing valve & the suction tube. No check valve used because this TXV is two ways expansion. Gas flow rate for both direction is quite identical for any specific balancing pressure.
As per my first post, It was at about 7C outdoor temperature and the unit was in heating mode.
P/s: the term I used before, two-(one)-way-TXV, is from the manufacturer manual. This could have made confusion.
Something like this?
http://www.emersonclimate.com/Documents/FlowControls/pdf/Contractor%20Tip%20Cards/Tip%20Card%2018.pdf
mad fridgie
14-05-2012, 09:35 AM
You said it was 7C when on heating but said "cooling was great" it would be great if the ambient was at 7C.
Add some more refrigerant.
Yep, Gary.
Mad fridgie, 35C outdoor for cooling mate. You're not gonna cool down if it's 7C, aren't you?!
mad fridgie
14-05-2012, 11:17 PM
Yep, Gary.
Mad fridgie, 35C outdoor for cooling mate. You're not gonna cool down if it's 7C, aren't you?!
Struggling here how can it be 7C and 35C at the same time. Yes it will cool down if it is 7C outdoor ambient, very well indeed to well actually..
To clarify,
Roughly it's a reverse cycle A/C system, so when we test the cool mode, we'll need 27C indoor/35C outdoor. In heating mode, we'll need 20C indoor/7C outdoor. No matter how you control the climate condition, it's international standard, or AS/NZS if we're Down Under. And, we can not do all tests at the same time, for sure.
This' also an A/C system with heat mode as stated above, it's not for a cold room.
Also as stated above, I'm very happy with the cooling mode. And if you put more gas in, it will affect the cooling mode. Don't forget we have to tune the cooling cycle for AU climate, like 48-50C outdoor.
mad fridgie
15-05-2012, 12:51 AM
To clarify,
Roughly it's a reverse cycle A/C system, so when we test the cool mode, we'll need 27C indoor/35C outdoor. In heating mode, we'll need 20C indoor/7C outdoor. No matter how you control the climate condition, it's international standard, or AS/NZS if we're Down Under. And, we can not do all tests at the same time, for sure.
This' also an A/C system with heat mode as stated above, it's not for a cold room.
Best explain who you are and what you are doing. I was under the impression you were just a service guy having a few problems.
The standards you talking about refer to performance, you can run/test any system under any number of conditions to find out if it working or not.
I believe the questions & solutions is what matters here. That will be helpful for all & is the purpose of this forum, I think. Our impression about others is not.
Please get back to what matters.
What is the subcooling at the TXV inlet?
mad fridgie
15-05-2012, 02:39 AM
So is this machine out in the field at someones house or shop or factory. Did it work and now it does not.
or are you trying to test/develop a new system.
It is important to how the questions are answered.
Almost no subcool, Gary. That's one reason why I was hesitated to increase charge.
This is a home made machine & never worked before I built & tested it. That's why I mentioned
Things can be changed including outdoor airflow (which is currently quite high already, 670m3/h on high), outdoor coil size and tube configuration (can make it bigger or smaller or whatever tube arrangement), charge amount can also be change slightly (may have adverse effect because of relatively small indoor coil), can't fit a bigger TXV which is already oversize.
In heating mode, charge it until the subcooling is 10-15F.
I have very low Evaporating temp (-28C), very low Suction pressure, very high SH (30K, although 2-TXV is set to minimum SH, wide open)
Almost no subcool, Gary. That's one reason why I was hesitated to increase charge.
To increase condenser SC, you need to add gas. To increase evap Tsat & reduce SH, you need to add gas.Your charge-determination trials need to be performed for BOTH modes, then compared.
If, in one mode, there is a huge difference, you may need to find a place (buffer) to store the excess charge. Perhaps a receiver? There may be pro's & con's against this mind.
In my view, the equation that needs to be in your head:
Evap heat [kW] + compressor input [kW] = condenser heat [kW]
If you move out of balance too far, in either mode, the system will be rather difficult to fine-tune.
Currently in heating mode,
Evap heat [kW] + compressor input [kW] >>> condenser heat [kW] (physically very small)
Yep, I'll do the charge determination test again. Currently for both modes, in high load conditions, I've got approx 57C condensing temp, can afford a bit more.
Thanks, everyone.
aramis
17-05-2012, 05:20 PM
Uni Try to post pictures showing the collectors of the big condenser and the evaporator too.
One thing you have wrong is Evapheat+Compressor input IS ALWAYS APPROXIMATE EQUAL TO Condenser heat. Never greater than.
Contrary to what many people think, large condensers do not condense because they need a certain velocity for the correct heat transfer to occur, thus the null subcooling at the outlet.
In a normal condenser this is solved by reducing the number of circuits like you try to do but this condenser must also work as an evaporator so the collector design becomes a mayor issue.
Bi-flow valves are used only on compact systems where the separation of refrigerant phases does not occur before the distributor or you will have a malfunctioning distributor which will cause maldistribution in the evaporator so it will never work!
Gents,
Just added up to 35% more gas yesterday in 50g steps and it yield not so impressive different. If you experienced the charge determination test, major parameters will eventually (another 20-30% more gas?) shoot up after a flat region I was observing. However, I just could not justify that much charge amount for such a little changes in a small domestic system.
aramis,
I meant outdoor coil (evap. now) + comp >>> indoor coil capacity (cond. now). please kindly correct me if I'm still wrong. Also, I agree with you about potential issues so I do not have any type of collectors or fancy distributors, just direct inlet & outlet tubing. This clarifies the concerns with bi-flow valve Vs distributor as well.
I'll try a smaller outdoor coil in heating mode today & see the effects.
Thanks again for all current and forthcoming suggestions.
Cheers,
Gents,
Just added up to 35% more gas yesterday in 50g steps and it yield not so impressive different. If you experienced the charge determination test, major parameters will eventually (another 20-30% more gas?) shoot up after a flat region I was observing. However, I just could not justify that much charge amount for such a little changes in a small domestic system.
What was the subcooling? Pressures?
Also, I agree with you about potential issues so I do not have any type of collectors or fancy distributors, just direct inlet & outlet tubing.
Is the liquid line feeding into the bottom on both coils?
aramis
18-05-2012, 01:55 AM
Gents,
I meant outdoor coil (evap. now) + comp >>> indoor coil capacity (cond. now). please kindly correct me if I'm still wrong. Also, I agree with you about potential issues so I do not have any type of collectors or fancy distributors, just direct inlet & outlet tubing. This clarifies the concerns with bi-flow valve Vs distributor as well.
Sorry to sound nerdy but you refer to the condenser’s nominal capacity. The actual capacity should be about equal, like desA said. Only about because the compressor is giving off energy as vibration, noise and some heat to ambient directly and also depends on insulation.
If the pressure drop of the evaporator in heating mode is high this will fool the bi-flow valve thinking it has a high superheat and tend to close. It works like an internaly equalized TXV in heating.
You should try to measure what Gary is asking in the heating cycle and I would add both pressures (high, low) plus temperature of refrigerant at the inlet of the evaporator (between the valve and the evaporator insulating the probe) this way we can estimate the evaporator’s pressure drop. This measurements must be as simultaneous as you possibly can.
Also check that there is not noticeable pressure drop (or temperature drop) in the reversing valve in the suction tubes (in and out) just to make sure it is opening the suction correctly.
If you are not measuring your pressures at the compressor valves, please indicate where exactly.
What was the subcooling? Pressures?
SC 1K (almost no changes), at +35% charge Pd was up 0.07MPa, Ps was up 0.05MPa.
Is the liquid line feeding into the bottom on both coils?
The liquid line feeds into the bottom on both coils.
Keep charging.
If the TXV were restricted, there would be a liquid back up and high subcooling.
Low subcooling + high superheat = low charge
Got your point, Gary. But it's a lot of cost doubling up the charge.
Can we explain it this way?
If
- TXV is max open (should be, I dipped it into a bow of hot water to check) ,
- and the indoor coil (cond) is small (not enough to make any sufficient SC),
- and the outdoor coil (evap) is too big, even though the valve is max open, still not feeding enough.
So the results are:
- Indoor coil does reject lots of heat (Td >= 25C), but No SC or minimum SC, there may be not fully liquid leaving the coil (?),
- Valve not restricted, feeding as much as it could,
- Starving outdoor coil created low suction pressure, high SH, then inturn, could not support very higher head pressure.
(Td >= 25C)...
Are you saying the saturated condensing temperature is 25K higher than the coil air on temp?
It was Air off temp - air on temp over the indoor coil in heating (ambient 24C outdoor/30C indoor)
p/s: Have good weekend, everyone.
It was Air off temp - air on temp over the indoor coil in heating (ambient 24C outdoor/30C indoor)
That would be the dT, not the Td... and it is way too high, indicating insufficient air flow.
Okay... let's re-think this:
The oversized outdoor coil is an advantage in cool mode. It is also an advantage in heat mode at very low outdoor temperature. The downside would be high compressor inlet superheat in heat mode in warm weather... possibly resulting in high discharge temperature.
I'm thinking we should put it in cool mode, adjust the TXV and the refrigerant charge... and generally fine tune it.
Then we can deal with the heat mode, paying particular attention to the discharge temperature.
aramis
19-05-2012, 12:42 PM
Okay... let's re-think this:
... It is also an advantage in heat mode at very low outdoor temperature. ...
Not if it may trap oil.
What diameter is the outdoor coil tube?
aramis
19-05-2012, 12:54 PM
Okay... let's re-think this:
... It is also an advantage in heat mode at very low outdoor temperature. ...
Not if the outdoor coil could trap oil.
What diameter is the outdoor coil tube and the compressor capacity?
To clarify,
Roughly it's a reverse cycle A/C system, so when we test the cool mode, we'll need 27C indoor/35C outdoor. In heating mode, we'll need 20C indoor/7C outdoor. No matter how you control the climate condition, it's international standard, or AS/NZS if we're Down Under. And, we can not do all tests at the same time, for sure.
Are you testing the system under the standard conditions for the heat mode?
It was Air off temp - air on temp over the indoor coil in heating (ambient 24C outdoor/30C indoor)
Ahhh... I see you are not. The outdoor temp is much too high for testing the heat mode.
Are you testing the system under the standard conditions for the heat mode?
Ahhh... I see you are not. The outdoor temp is much too high for testing the heat mode.
Yep, if the max heat mode setting > 24C, say 30C, then the system still need to heat up in that condition until indoor return air reaches the set point.
Just tested with the new condenser (2/3 original size), same air flow rate, same charge and everything else.
The results are the same, I mean most if not all parameters (both low side & high side) are unchanged.
Total outdoor coil's tube length is approx 12m (includes all bend& returns), 3/8" tube. Pressure drop (outdoor coil inlet to suction) approx 0.1MPa.
aramis
21-05-2012, 03:25 AM
Just tested with the new condenser (2/3 original size), same air flow rate, same charge and everything else.
The results are the same, I mean most if not all parameters (both low side & high side) are unchanged.
Total outdoor coil's tube length is approx 12m (includes all bend& returns), 3/8" tube. Pressure drop (outdoor coil inlet to suction) approx 0.1MPa.
The rule to use an internally equalized TXV is pressure drop not higher than 0.014 MPa in the coil, so when heating the Bi-flow valve will sense the wrong pressure and think it has high superheat and close.
You must change to two externally equalized TXV with check valves like Gary mentioned.
If you want to prove this, change the equalization to the external coil (if you can) and should experience good behavior in heating the inner coil and bad in cooling it.
Here's what we need:
Evap air in temp
Evap air out temp
Low side pressure
Suction line temp at TXV bulb
Cond air in temp
Cond air out temp
High side pressure
Liquid line temp at TXV
Is the TXV externally equalized?
If so, is the equalizer line attached to the common suction line?
http://www.hvacfun.com/images/a-images/a-everything-you-need-to-know-about-txv/everything-you-need-to know-about-tx-valves-figure-3.jpg
The sensing elements are set up like this one, further down the suction line, I believe it's external balancing, external temperature sensing type of valve.
The difference is this one can expand in both directions as I mentioned before. And this is probably the issue. It may not efficiently span the whole (big) operating range in both modes.
Mode: heating (Evap = outdoor coil, cond = indoor coil)
Evap air in temp: 24C
Evap air out temp: N/A, I'd say about 8C-15C
Low side pressure: 0.6MPa (Suction)
Suction line temp at TXV bulb: 13C
Cond air in temp: 30C
Cond air out temp: 55C
High side pressure: 3.5MPa (discharge)
Liquid line temp at TXV: N/A, 54C at Cond outlet
Both the equalizer line and the TXV bulb must be on the suction line between the reversing valve and the compressor.
Both the equalizer line and the TXV bulb must be on the suction line between the reversing valve and the compressor. They are, Gary.
Drop the outdoor fan down to low speed and take new readings.
I'm beginning to suspect that it isn't just the outdoor coil that is grossly oversized, but the compressor as well.
Even with a relatively low suction pressure, the compressor is delivering more heat to the indoor coil than it can reasonably handle. If the outdoor coil were fully loaded, the high side pressure would skyrocket.
In order to keep the high side pressure to a reasonable level, we need to drop the suction pressure even lower.
I'm beginning to suspect that it isn't just the outdoor coil that is grossly oversized, but the compressor as well.
Even with a relatively low suction pressure, the compressor is delivering more heat to the indoor coil than it can reasonably handle. If the outdoor coil were fully loaded, the high side pressure would skyrocket.
In order to keep the high side pressure to a reasonable level, we need to drop the suction pressure even lower.
I am desperately trying to do the opposite. I can afford higher Head pressure, but must bring low side pressure up.
Garry, you are right about the indoor coil, it's undersized. So what if the coil could not handle that much of hot gas and there is a mixture of gas & liquid exits the coil (we have very low subcool, another sign), then feeds to the TXV? Is that what reduce the TXV capacity and cause low suction pressure down stream?
i am desperately trying to do the opposite. I can afford higher head pressure, but must bring low side pressure up.
why???????
why???????
With the current system configuration, imagine testing the unit in heating mode at 0°C/0°C indoor/outdoor, I'd have -35°C (or even worst) evaporating temperature and this is way out of the safe operating range of the compressor.
So I must bring this up and accept the fact that in high heat load condition (24°C/30°C), I'd have high head pressure (but still under the compressor safe operating range).
Do you have any thought on the other question, Gary?
Garry, you are right about the indoor coil, it's undersized. So what if the coil could not handle that much of hot gas and there is a mixture of gas & liquid exits the coil (we have very low subcool, another sign), then feeds to the TXV? Is that what reduce the TXV capacity and cause low suction pressure down stream?
With the current system configuration, imagine testing the unit in heating mode at 0°C/0°C indoor/outdoor, I'd have -35°C (or even worst) evaporating temperature and this is way out of the safe operating range of the compressor.
What makes you think the evaporating temperature would be that low?
by Testing :)
Actually, at 0°C outdoor, Tc = 31°C, Te = -42.5°C, SH = 40K.
That's why I am so desperate.
Do you have any thought on the other question, Gary?
Yes... I think you are confused.
Yes, I'm very confused.
How it should be then?
Change the outdoor fan to low speed.
Then adjust the TXV stem back to where it was.
Then keep charging until the subcooling is 10-15F/5.5-8.5K.
by Testing :)
Actually, at 0°C outdoor, Tc = 31°C, Te = -42.5°C, SH = 40K.
... with little or no subcooling.
A TXV controls the flow of subcooled liquid into the evaporator. If there is no subcooled liquid, it has nothing to control... so it goes wide open (regardless of adjustment) and stays there.
Why does it have little or no subcooling and low evap temp? Because there is not enough refrigerant in the system.
Low subcooling means there is not enough refrigerant in the high side.
High superheat means there is not enough refrigerant in the low side.
Not enough high side refrigerant + not enough low side refrigerant = not enough refrigerant in the system.
When you tested the cool mode, presumably you had the TXV adjusted properly. Once a TXV is adjusted properly, it will never need to be adjusted again... ever.
But when you switched over to heat mode, you opened up the TXV adjustment. Big mistake. Any problems with the heat mode are not going to be solved by TXV adjustment.
You need to get the TXV adjusted properly. And then you need to leave it alone... forever.
You said it worked "great" in the cool mode. That is a highly subjective statement and it doesn't tell me anything. The following tells me everything I need to know:
Evap air in temp
Evap air out temp
Saturated suction temp (evap temp)
Suction line temp at TXV bulb
Cond air in temp
Cond air out temp
Saturated condensing temp (cond temp)
Liquid line temp near the condenser outlet
You said it worked "great" in the cool mode. That is a highly subjective statement and it doesn't tell me anything. The following tells me everything I need to know:
Evap air in temp 27
Evap air out temp 14
Saturated suction temp (evap temp) -3.5
Suction line temp at TXV bulb 10.5
Cond air in temp 35
Cond air out temp 55
Saturated condensing temp (cond temp) 44
Liquid line temp near the condenser outlet 50 (pick up hot air temp? Liquid line running along the cond air off stream, approx 400mm length)
The reason I adjusted the TXV in heating mode is to work out the configuration that works in heating. I used a check valve & 2 captubes before for cooling & heating modes on a similar system. If after all adjustment the conclusion is the TXV is not suitable for heating mode (although it's good for cooling), then I have to get back to captube. Again, hopefully you're correct & I won't have to play with those little tubes.
We know that bubble or flash gas in the liquid line cause the malfunction of TXV, what happens there, Gary?
And you reckon the two cases are different?
A TXV controls the flow of subcooled liquid into the evaporator. If there is no subcooled liquid, it has nothing to control... so it goes wide open (regardless of adjustment) and stays there.
p/s:
Thanks again for your knowledge & patience. I'm confused and will not give up asking until my mind is cleared up.
Evap air in temp 30
Evap air out temp
Saturated suction temp (evap temp) 4
Suction line temp at TXV bulb 16
Cond air in temp 50
Cond air out temp
Saturated condensing temp (cond temp) 56.5
Liquid line temp near the condenser outlet
There are some important numbers missing here, but I can tell you that it was NOT working properly in cool mode, either.
Maybe we should go back to cool mode and get it working right... and then deal with the heat mode.
I have just updated the information.
Today I fitted two sight glasses, one before & one after TXV, so we can see what happens during both modes.
I have just updated the information.
Today I fitted two sight glasses, one before & one after TXV, so we can see what happens during both modes.
Evap air in temp 27
Evap air out temp 14
Saturated suction temp (evap temp) -3.5
Suction line temp at TXV bulb 10.5
Cond air in temp 35
Cond air out temp 55
Saturated condensing temp (cond temp) 44
Liquid line temp near the condenser outlet 50 (pick up hot air temp? Liquid line running along the cond air off stream, approx 400mm length)
Since the refrigerant in the condenser is heating the air as it flows through, it is not possible for the cond air out temp to be higher than the saturated condensing temp (SCT).
It is also not possible for the liquid line temp to be higher than the SCT.
The SCT is wrong.
First air flow:
Evap air in temp 27
Evap air out temp 14
27 - 14 = 13K evap delta-T. Ideally, the dT should be as close as possible to 11K. High dT indicates insufficient air flow. If it is not possible to increase the air flow, 13K is within acceptable limits.
Cond air in temp 35
Cond air out temp 55
55 - 35 = 20K cond dT. The cond dT should be no more than 16K. High cond dT indicates insufficient airflow. Is the condenser fan on high speed?
Then subcooling:
Since SCT must be higher than air off temp, we can guesstimate that it is a few degrees higher, say 58C. The subcooling then must be at least 58 - 50 = 8K. Normal SC is 5.5-8.5K. Although the SC may be higher, indicating other problems, we know that it is at least high enough to assure liquid supply to the TXV inlet, which makes it possible to adjust superheat.
Then the superheat:
Saturated suction temp (evap temp) -3.5
Suction line temp at TXV bulb 10.5
10.5 - -3.5 = 14K SH. Normal SH for the air conditioning range is 6.5-8.5K, so the TXV needs to be opened a little.
Since the refrigerant in the condenser is heating the air as it flows through, it is not possible for the cond air out temp to be higher than the saturated condensing temp (SCT).
It is also not possible for the liquid line temp to be higher than the SCT.
The SCT is wrong.
The pressure gauges came with factory's calibration certs (traceable) and still within one first year in use. Anyway, I'll calibrate them all again.
My suspect is the Cond Air off probe. All other probes are fixed & insulated but that’s the one I move arround a lot to check different temps. </SPAN>
27 - 14 = 13K evap delta-T. Ideally, the dT should be as close as possible to 11K. High dT indicates insufficient air flow. If it is not possible to increase the air flow, 13K is within acceptable limits.
My indoor space is very confined so that's the most of indoor airflow I could do.
55 - 35 = 20K cond dT. The cond dT should be no more than 16K. High cond dT indicates insufficient airflow. Is the condenser fan on high speed?
Yes, it was Indoor fan high, outdoor fan high.
0.5 - -3.5 = 14K SH. Normal SH for the air conditioning range is 6.5-8.5K, so the TXV needs to be opened a little.
The TXV can be adjusted within +/- 02 turns. This result & previous heating results are with -2 turns (max open).
One day I tried one size bigger and got very high head pressure & even lower suction pressure (balancing forces not right?). I gave up on that one.
Smaller TXV gave lower head pressure & lower suction pressure at the same adjustment, charge amount & test condition.
The pressure gauges came with factory's calibration certs (traceable) and still within one first year in use. Anyway, I'll calibrate them all again.
My suspect is the Cond Air off probe. All other probes are fixed & insulated but that’s the one I move arround a lot to check different temps. </SPAN>
If the SCT is right and the air off is wrong, that would mean the cond dT is much lower, indicating plenty of air flow. It would also mean the liquid line temp must be wrong and we may not have enough subcooled liquid to adjust the superheat. Was the sight glass clear?
Obviously, we need accurate readings for accurate analysis.
mikeref
24-05-2012, 07:50 AM
Uni,going back to post #1, and post #62 as a reference.
Is the compressor sized/ matched to the indoor coil, or the outdoor coil? (Or neither)
Reason for asking: As you add more gas, your high pressure increases, in heat mode.
Temp drop across the evaporator (In cool mode) was 13K. That could be achieved with the suction pressure at 400 Kpa (58 psi), or 280 Kpa (42 psi) if the compressor is oversized.
Discharge temperature leaving the compressor will be be significantly higher.
I will now return to my cave incase the Jedi should choose to use mind control:D Adios!
Hi guys, for a charge determination test using subcool (TXV system), is ambient temperature important?
I am gonna confirm the charge amount as per Gary's suggestion.
Is the compressor sized/ matched to the indoor coil, or the outdoor coil? (Or neither)
It's a tough question for me. I mean if talking about norminal capacity, in cooling, it's the right size (or slightly oversized) for the indoor HX, and of cource the outdoor should be as big as you can for efficiency, and the outdsoor HX in this case, is providing sufficient heat exchange capacity.
mad fridgie
25-05-2012, 12:49 AM
has the indoor coil got very small distributor legs.
Hi guys, for a charge determination test using subcool (TXV system), is ambient temperature important?
Yes... the liquid line is cooled by it's surroundings. It's temperature cannot be lower than the surrounding air. If the liquid line is in a hot area, such as the discharge air flow, you will not get an accurate reading.
I generally move the sensor downstream to an area that is subject to ambient air. Then I charge to 5.5-8.5K subcooling or clear sight glass... whichever comes first.
Did you get the instrument calibration problem sorted out?
My suspect is the Cond Air off probe. All other probes are fixed & insulated but that’s the one I move arround a lot to check different temps
Hold the probe in the air on flow stream. If it shows the same temp as the air on probe, then it is probably not the problem.
Tape an accurate temp probe to the side of a refrigerant jug. Attach your gauge to the jug. The saturation temp should agree with the jug surface temp.
has the indoor coil got very small distributor legs.
All inlet & outlet of both coil are 3/8" direct tubes. I had to make careful configuration to have acceptable distribution.
Did you get the instrument calibration problem sorted out?
Unfortunately it would take a week at least. I send them out to licensed body.
Hold the probe in the air on flow stream. If it shows the same temp as the air on probe, then it is probably not the problem.
The probe accuracy might not be a problem, the concern is that I physically moved it around too much so when I reviewed the data, I am not sure where it was at that point. My bad.
I put it back in place this time.
Hi guys, for a charge determination test using subcool (TXV system), is ambient temperature important?
I meant to ask it's a common practice to use that method for charging. Something like charge it up until the bubble just finishes, or a certain SC. However, what can be seen in the sight glass in a hot 35°C day is what we see at 25°C ambient? is the method valid in most conditions?
Powered by vBulletin® Version 4.2.1 Copyright © 2024 vBulletin Solutions, Inc. All rights reserved.