Starved Evaporator Troubleshooting?

[aramis]

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]

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?

[Gary]

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.

[Uni]

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.

[Uni]

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]

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.

[Gary]

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?

[Uni]

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

[Gary]

Both the equalizer line and the TXV bulb must be on the suction line between the reversing valve and the compressor.

[Uni]

Both the equalizer line and the TXV bulb must be on the suction line between the reversing valve and the compressor.

They are, Gary.

[Gary]

Drop the outdoor fan down to low speed and take new readings.

[Gary]

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.

[Gary]

(deleted double post)

[Uni]

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?

[Gary]

i am desperately trying to do the opposite. I can afford higher head pressure, but must bring low side pressure up.

why???????

[Uni]

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?

[Gary]

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?

[Uni]

by Testing

Actually, at 0°C outdoor, Tc = 31°C, Te = -42.5°C, SH = 40K.

That's why I am so desperate.

[Gary]

Do you have any thought on the other question, Gary?

Yes... I think you are confused.

[Uni]

Yes, I'm very confused.
How it should be then?

[Gary]

Change the outdoor fan to low speed.

[Gary]

Then adjust the TXV stem back to where it was.

[Gary]

Then keep charging until the subcooling is 10-15F/5.5-8.5K.

[Gary]

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.

[Gary]

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.

[Gary]

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

[Uni]

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.

[Gary]

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.

[Uni]

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.

[Gary]

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.

[Gary]

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?

[Gary]

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.

[Uni]

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.

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.

[Gary]

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.

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]

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!

[Uni]

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]

has the indoor coil got very small distributor legs.

[Gary]

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.

[Gary]

Did you get the instrument calibration problem sorted out?

[Gary]

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.

[Uni]

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?