liquid not reaching TXVs in old rack system

Hello to everyone. Nice to be here and posting for the same time.
I´m having some trouble trying to make the liquid refrigerant reaching the TXVs. Liquid and suction lines to the evaporators are a little long: 20 - 25 meters with 7 meter vertical risers. The liquid receiver is half full, so I don´t believe there is a lack of refrigerant. I changed the cores of the filter drier which is located next to the receiver, just in case they were plugged. There is a differential pressure regulating valve downstream from the drier, which throttles the liquid flow, but it only activates during defrost. The OPR valve which keeps the receiver at a minimum pressure seems to be working all right: there is a gauge in the receiver which reads the pressure and it doesn´t get below 180 psig.
There are 14 different refrigeration circuits, and none of them seems to be getting enough liquid. The rack contains 7 compressors, but only two of them work at any time since suction pressure gets too low if any more compressors are switched on. I checked a few of the TXVs filter mesh and all came out clean.
Do you think that it would be any helpful if I try to raise the reciever pressure a little bit in an attempt to overcome piping loses ? Someone told me about a hermetic type of liquid refrigerant pump used to presurize the liquid line and force the liquid into the evaporators, but I have no knowledge about this device. I would appreciate any comments. Thanks!!

What refrigerant is in the system?

Has the system worked properly in the past?

Have you installed a sight glass at the furthest TXV inlet to see if liquid is getting there?

hi Gary and thank you for your reply.
1. Refrigerant: R-22
2. according to the owner it worked all right until a few weeks ago.
3. there is a sight glass located in the liquid line of one of the most distant evaporators: there are no bubbling, it´s just clear.

Quote:

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Originally Posted by friocold2008

hi Gary and thank you for your reply.
1. Refrigerant: R-22
2. according to the owner it worked all right until a few weeks ago.
3. there is a sight glass located in the liquid line of one of the most distant evaporators: there are no bubbling, it´s just clear.

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What changed a few weeks ago? Was something worked on? Adjustments made? Did the weather change?

To Tropicold: Sorry for answering Gary the questions he asked, but I know that you probably will not be able to get back to the shop in time to log into this website, and as you know we should have had this rack fixed a few days ago. I didn´t mean to be rude.

To Gary:
tropicold and I work for the same hvac service company and attend this parallel rack together. That is why I know the details of this case.There is some more information that might be useful to you:
1) I guess this rack is of the floating head type: there is a condenser flooding valve.
2) Fan cycling is pressure controlled: setpoint: 200 psig, throttling range: 10 psig; there are 10 fans.
3) Compressors are as follows: (1) 7.5 HP, (1) 15 HP, (2) 22 HP, (3) 27 HP; all low temperature Copelands
4) Suction pressure setpoint: 17 psig
5) There is a log left by the previous operator that indicates that ORP setting is 160 psig and IPR setting is 185 psig.
6) As tropicold said before, as far as we know, everything was working fine up to two weeks ago. We haven´t changed any of the setpoints, but rather looked for the obvious: refrigerant leak, plugged filter cores or TXVs mesh, liquid line differential pressure valve shut, etc.

Down here in Panama, weather stays more or less the same all year around. We are going through a transition from rainy to dry season which have some effect in the humidity, but ambient temperatures remain in the 90´s.

With clear sightglass at the TXV it seems liquid is getting there.

I'm wondering if the liquid pressure reducing valve may be stuck in the activated position.

Quote:

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Originally Posted by tropicold

There is a differential pressure regulating valve downstream from the drier, which throttles the liquid flow, but it only activates during defrost. . I would appreciate any comments. Thanks!!

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What is the make and type of the differential valve.

Dependant on type some work be energising to reduce the pressure and some energise to increase the pressure.

The A7 and A8 valve.

So if it is the type that needs to be energised to open the valve and increase the pressure, if the coil burns out the valve fails in the restricted flow and that could cause a pressure drop.

If it was the other type of valve and the coil fails, then all the cases ice up because the defrost cannot work.

It might not be the problem but it is worth looking at.

Cheers taz.

Good afternoon fellows, and thank you Gary and Taz for your responses.
Friocold and I did as you both suggested and checked the pressure differential regulating valve located in the main liquid line, downstream from the filter drier. It´s a SPORLAN made OLDR. According to the manufacturer´s data, when de-energized, it is suppose to be 100% open. When one of the circuits begins defrosting, it is energized and enters differential mode. We pulled it open and discovered some debris inside: there is a little strainer which had broken into small pieces. We cleaned it and put it back in operation. After about 1 hour, liquid receiver level had dropped to the lowest sightglass (there are four of them in this vertical receiver). As it stayed this low for a while, we added added refrigerant until the liquid reached the next sightglass. Box temperatures improved a lot: from about -6 deg C to about -12 deg C. But suction manifold superheat stays high.
This rack has a serviceable compressor suction filter at each compressor. I don´t know when was the last time they were replaced.I guess it should be a good idea to take a look at them. Are they really needed, or can I just remove them and leave the shell empty ?
Thank you again for you help!!!

Thank you for your most helpful comments. What seems so difficult for us to figure out, seems so easy for you!!!
As tropicold wrote, box temperatures got much better, but compressor superheat stays high and suction pressure is below normal.

In another single compressor system I worked before which had the filter/drier plugged and the liquid was not reaching the TXVs, I realized that due to low refrigerant velocity, oil was getting trapped in the piping. And this was causing pressure drop in the suction side.

Trying to relate this experience to the current situation, is it possible that in this rack, as the main liquid solenoid valve was apparently stuck, causing a reduced flow of refrigerant, oil is now trapped in the evaporators, suction manifold and suction filters, creating low suction pressure and high superheat?

It's a possibility, check you are now defrosting correctly, your oil seperator is working and your levels are correct.
Another thing to check are your evaporator superheats, make sure they are not too wide as this would also cause your symptoms!

I would do nothing for a few days. Give the oil time to work its way back and allow everything to balance out.

Sounds like a rack that needs a lot of TLC.

The first thing that comes to my mind is plugged filters. Drier cores? Case liquid line filters?

Now, Celsius readings throw me a bit, but -12 C looks like low temp work to me. Low Temp R-22 has a few 'issues'. After a few years of running, expecially running with high suction superheat, you wind up with oil breakdown and gummed up TXVs.

I have taken the TXVs apart on this type of system and cleaned them with PVC cleaner. There winds up being a black crud built up around the pin in the valve.

is the pipework underground if so everything is probably frozen.maybe lift your condensing pressure to 270 psi or so and monitor oil return,have you had any smashed compressors yet?

Quote:

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Originally Posted by friocold2008

1) I guess this rack is of the floating head type: there is a condenser flooding valve.

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This has nothing to do with the problem at hand, but having a flooding valve on the dropleg does not make it a floating head system.

A floating head system involves rack controllers and condenser strategies involving outdoor dry and wet bulb temps along with the dropleg temps.

A condenser flooding valve (usually a Sporlan A-8) maintains a minimum condensing pressure for low ambient purposes. It's set in conjunction with the receiver pressurization valve and condenser fan setpoints.

Hope this helps you in the future.

I think jpsmith1cm is on the money,,,R22 does form residue over time and blocks tx valves,,,also your head pressure needs to be at least 250psi for the txvalves to operate correctly on R22.Clean out the strainers.

Have you checked the furtherist TX vav superheat, with a semi clear sight glass at TXV, and low performance, the TXV could have "gone to lunch "and need replacing, they do that after years of service, as in wear out.

We haven't heard anything for awhile, so I'm assuming that all is well.

Quote:

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Originally Posted by lawrence1

I think jpsmith1cm is on the money,,,R22 does form residue over time and blocks tx valves,,,also your head pressure needs to be at least 250psi for the txvalves to operate correctly on R22.Clean out the strainers.

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Thank you.

I'll disagree with you on the 250# thing.

I run R-22 between 130 and 160. I'd be screaming if one of my 22 racks was running a 250# head pressure, even on a 95*F day.

4) Suction pressure setpoint: 17 psig[/quote]

if the suction is at 17 psig thats giving you an evaporating temp ov about -23 C . thats not cold enuff to get the rooms/ cases down to temp . it cud just be down to the transducer or dead band switch be set to high or reading wrong ?

Hi JPSmith,
your figures intrigue me, at ambient of 95'F/ 35'C and a probable approach of 10'C, you are condensing at 25'C average.
magoo