Results 51 to 91 of 91
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19-05-2012, 12:42 PM #51
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19-05-2012, 12:54 PM #52
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20-05-2012, 04:15 AM #53
Re: Starved Evaporator Troubleshooting?
Last edited by Gary; 20-05-2012 at 04:22 AM.
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21-05-2012, 12:32 AM #54
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21-05-2012, 02:10 AM #55
Re: Starved Evaporator Troubleshooting?
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.Last edited by Uni; 21-05-2012 at 02:18 AM.
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21-05-2012, 03:25 AM #56
Re: Starved Evaporator Troubleshooting?
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.Well, did anybody ever dream of calling Aramis a coward? No, certainly not!
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21-05-2012, 07:01 AM #57
Re: Starved Evaporator Troubleshooting?
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?Last edited by Gary; 21-05-2012 at 07:03 AM.
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21-05-2012, 11:05 PM #58
Re: Starved Evaporator Troubleshooting?
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 outletLast edited by Uni; 22-05-2012 at 05:06 AM. Reason: Update information
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22-05-2012, 12:16 AM #59
Re: Starved Evaporator Troubleshooting?
Both the equalizer line and the TXV bulb must be on the suction line between the reversing valve and the compressor.
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22-05-2012, 04:46 AM #60
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22-05-2012, 06:00 AM #61
Re: Starved Evaporator Troubleshooting?
Drop the outdoor fan down to low speed and take new readings.
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22-05-2012, 07:14 AM #62
Re: Starved Evaporator Troubleshooting?
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.
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22-05-2012, 07:18 AM #63
Re: Starved Evaporator Troubleshooting?
(deleted double post)
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23-05-2012, 01:15 AM #64
Re: Starved Evaporator Troubleshooting?
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?
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23-05-2012, 01:47 AM #65
Re: Starved Evaporator Troubleshooting?
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23-05-2012, 02:19 AM #66
Re: Starved Evaporator Troubleshooting?
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?
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23-05-2012, 02:49 AM #67
Re: Starved Evaporator Troubleshooting?
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23-05-2012, 03:31 AM #68
Re: Starved Evaporator Troubleshooting?
by Testing
Actually, at 0°C outdoor, Tc = 31°C, Te = -42.5°C, SH = 40K.
That's why I am so desperate.
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23-05-2012, 03:35 AM #69
Re: Starved Evaporator Troubleshooting?
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23-05-2012, 03:57 AM #70
Re: Starved Evaporator Troubleshooting?
Yes, I'm very confused.
How it should be then?
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23-05-2012, 04:20 AM #71
Re: Starved Evaporator Troubleshooting?
Change the outdoor fan to low speed.
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23-05-2012, 04:50 AM #72
Re: Starved Evaporator Troubleshooting?
Then adjust the TXV stem back to where it was.
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23-05-2012, 04:52 AM #73
Re: Starved Evaporator Troubleshooting?
Then keep charging until the subcooling is 10-15F/5.5-8.5K.
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23-05-2012, 02:41 PM #74
Re: Starved Evaporator Troubleshooting?
... 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.Last edited by Gary; 23-05-2012 at 03:11 PM.
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23-05-2012, 02:59 PM #75
Re: Starved Evaporator Troubleshooting?
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.
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23-05-2012, 03:31 PM #76
Re: Starved Evaporator Troubleshooting?
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 outletLast edited by Gary; 23-05-2012 at 03:34 PM.
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23-05-2012, 11:25 PM #77
Re: Starved Evaporator Troubleshooting?
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.
Thanks again for your knowledge & patience. I'm confused and will not give up asking until my mind is cleared up.Last edited by Uni; 24-05-2012 at 02:56 AM. Reason: Update information
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24-05-2012, 12:05 AM #78
Re: Starved Evaporator Troubleshooting?
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24-05-2012, 02:58 AM #79
Re: Starved Evaporator Troubleshooting?
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.
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24-05-2012, 04:26 AM #80
Re: Starved Evaporator Troubleshooting?
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24-05-2012, 04:42 AM #81
Re: Starved Evaporator Troubleshooting?
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?
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24-05-2012, 05:08 AM #82
Re: Starved Evaporator Troubleshooting?
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.
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24-05-2012, 06:34 AM #83
Re: Starved Evaporator Troubleshooting?
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.
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.
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.
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?
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.
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.
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24-05-2012, 07:03 AM #84
Re: Starved Evaporator Troubleshooting?
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.
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24-05-2012, 07:50 AM #85
Re: Starved Evaporator Troubleshooting?
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 Adios!To the optimist, the glass is half full. To the pessimist, the glass is half empty.
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25-05-2012, 12:30 AM #86
Re: Starved Evaporator Troubleshooting?
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)
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25-05-2012, 12:49 AM #87
Re: Starved Evaporator Troubleshooting?
has the indoor coil got very small distributor legs.
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25-05-2012, 12:53 AM #88
Re: Starved Evaporator Troubleshooting?
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.
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25-05-2012, 01:00 AM #89
Re: Starved Evaporator Troubleshooting?
Did you get the instrument calibration problem sorted out?
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25-05-2012, 01:52 AM #90
Re: Starved Evaporator Troubleshooting?
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.
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25-05-2012, 03:20 AM #91
Re: Starved Evaporator Troubleshooting?
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?
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.
I put it back in place this time.
Hi guys, for a charge determination test using subcool (TXV system), is ambient temperature important?