Starved Evaporator Troubleshooting?

[Uni]

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

[desA]

SOG? (Short of gas)

[cool_tech]

Hey mate, Is there a blockage anywhere drier, txv? What are your condenser fans doing when in heating mode are they running.

[Uni]

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]

Fit outdoor fan control to maintain your pressures.

[aramis]

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.

[Uni]

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]

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.

[Uni]

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]

"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.

[Uni]

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]

need a picture, or link to show me what valve you have out there on the condenser

[Gary]

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]

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.

[Gary]

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.

[Gary]

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/Docume...re-article.pdf

[Uni]

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]

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.

[Uni]

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?

[Gary]

You should have a two way TXV at each coil.

[Uni]

Did you mean two traditional TXV & check valve? One for Heating, one for cooling?

[Gary]

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.

[Gary]

http://www.refrigerationbasics.com/1...eat_pumps1.htm

[mad fridgie]

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?

[Uni]

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.

[Gary]

Something like this?

http://www.emersonclimate.com/Docume...0Card%2018.pdf

[mad fridgie]

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.

[Uni]

Yep, Gary.

Mad fridgie, 35C outdoor for cooling mate. You're not gonna cool down if it's 7C, aren't you?!

[mad fridgie]

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..

[Uni]

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.

[Uni]

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]

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.

[Uni]

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.

[Gary]

What is the subcooling at the TXV inlet?

[mad fridgie]

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.

[Uni]

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.

[Gary]

In heating mode, charge it until the subcooling is 10-15F.

[desA]

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.

[Uni]

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]

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!

[Uni]

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,

[Gary]

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]

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.

[Uni]

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.

[Gary]

Keep charging.

If the TXV were restricted, there would be a liquid back up and high subcooling.

Low subcooling + high superheat = low charge

[Uni]

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.

[Gary]

(Td >= 25C)...

Are you saying the saturated condensing temperature is 25K higher than the coil air on temp?

[Uni]

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.

[Gary]

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.

[Gary]

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.