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Brandon
27-04-2003, 08:58 AM
I've got a quick question on a system I've been working on. The problem I am having is after the tstat is satisfied and the solinoid valve closes, the system pumps down and shuts off on low pressure. That is great but seconds after this process occurs my suction pressure rises quickly and short cycling occurs. Why the rapid pressure gain? I know that there should be some pressure rise but not that quick. Anybody have any thoughts on this condition? Thanks for the input




P.S. I've got a full sight glass at the compressor after the filter drier. The system has a short line set and the refrigerant is R-22. Superheat at the compressor is 38F. Also it seems every morning the evap is iced up. After removing the ice the system operates as described above.

Andy
27-04-2003, 11:27 AM
Hi Brandon,
possible
causes are a passing liquid line solioide valve (dirt mechanical damage) or worn compressor valve plates.
1/ strip and check, or just renew liquid line solinoide valve.
2/ pump test compressor by lowering/shorting out L.P. cutout to see what level of vacumn the compressor will pull.
Another possibility is the L.P. switch is set up incorrectly. Herefishy will tell you more about this, but basically you do not need to set the switch much lower than say 30psig, coming back in at say 50 to 55 psig.
What is the settings on the L.P cutout?:confused:
Regards. Andy:)

Brandon
27-04-2003, 10:27 PM
I think the compressor is to blame. The valves in the compressor are probably damaged. When I started working on this system, I found it low on refrigerant. I would have to say in the past it had probably been "topped off" with liquid in the low side which we all know does wonders to the compressor. Thanks for the input

Gary
27-04-2003, 10:45 PM
Grab the suction line and shut off the compressor. If there is a rush of hot gas coming from the direction of the compressor, the discharge valves are leaking. Not a positive test, but may indicate badly leaking valves. If the system is holding temperature during operation, the problem is more likely to be the solenoid valve. But again, not a positive test.

Under stable operation, measure the following:

Low side:

Evap air in temp
Evap air out temp
SST (saturated suction temp) or pressure
Suction line temp at coil outlet
Suction line temp at comp inlet

High side:

Cond air in temp
Cond air out temp
SCT (saturated condensing temp) or pressure
Liquid line temp at cond outlet
Liquid line temp at TXV inlet

herefishy
30-04-2003, 02:54 PM
I wonder if this may be a scroll compressor?

Perhaps with a short lineset to the evaporator?

Brandon
01-05-2003, 03:56 AM
No....its not a scroll. Its a 1/2 hp recip. 230 1ph. I have not seen any walk in coolers with a scroll compressor, but then again I have only been in the trade 6 years.

Gary
01-05-2003, 08:46 AM
Without a full set of temperature measurements, all we can do is guess. Guessing games are fun, but not very professional.

Robearbam
01-05-2003, 12:35 PM
Andy,

Correct me if I'm wrong here. I was told never ever jump out the L.P. switch on a compressor and let it draw a vacuum. I always thought the reason was if there is a very minute leak somewhere it could draw air into the system. ???:eek:

herefishy
01-05-2003, 02:09 PM
Actually, I was told by Copeland Engineering that the reason for wanting not to run a compressor in a vacuum in such a manner, is due to the possibility of the fusetite arcing to the compressor shell (ground).

Admittedly this is a rare occurence, nonetheless a possibility and as such the reasoning. :)

Dan
02-05-2003, 03:56 AM
That is great but seconds after this process occurs my suction pressure rises quickly and short cycling occurs.

Depending upon the length of the line-set and low pressure control settings and the location of the liquid line solenoid, this could be okay. I didn't say that all that well, so it is time for a new paragraph.

If the liquid line solenoid is at the compressor unit, there is a lot of liquid in the liquid line that can still feed through the expansion valve, causing suction pressure to rise when the compressor is stopped. It is not uncommon to to see periodic cycling of the compressor as it continues to pump down under these circumstances.




I've got a full sight glass at the compressor after the filter drier. The system has a short line set and the refrigerant is R-22. Superheat at the compressor is 38F. Also it seems every morning the evap is iced up. After removing the ice the system operates as described above.

Whoa there! Icing up every morning? You are not defrosting? The following advice from Andy comes to mind:


causes are a passing liquid line solioide valve (dirt mechanical damage) or worn compressor valve plates.

Forget the valve plate. Your liquid line solenoid is not closing.

Okay. Now it is okay to think about the valve plate.

But first things first.:)

Andy
02-05-2003, 06:36 PM
Hi Robearbam,
shorting out an L.P can be done, but it is a good idea to establish if you have any lowside leaks first. If you were working on a NH3 blast freezer it would be running in a vaccum anyway.
I have worked on quite a few poorly designed R22 low temp plant over the years that had to run in a vaccumn to acheive temperature. Most leaks are not on the low side, why would the lowside leak, after all it is under no great pressure.
Shut the suction severice valve and run the compressor in a vac, a few second won't pull in that much air.
I would be more worried about marginal lubrication during the test, rather than air getting in.
Regards. Andy:D

Robearbam
05-05-2003, 11:43 AM
Hey Guys,
I guess I should have specified that which I work on is older open drive stuff dependent on drive shaft seals and the ever so infamous refrigerant know as 12. When I was a kid this stuff was old. I do remember some of the older mechs then, telling me to pump it down at the end of the season but never let it go below zero PSI. Never let anyone pull a vacuum with a compressor. (Gas back then for this was around .79 cents a pound.) Guess I should have questioned it more at the time but, I didn't, so....now....I'm in the process of learning why....right here. :)