short cycling 404a freezer

Paul S_3

Hey everyone .... im having trouble with the condensing unit of a r404a walk in freezer.... this is for a good friend of mine who owns a grocery store.... do not work on a whole lot of refrigeration so bear with me.... i found the temp in freezer at 35f , my friend told me that the evap coil was completely frozen .....before i got there he shut the system down to defrost.....when i got there coil was half defrosted. ...i went to condensing unit tried to put system manually in defrost by turning the dial....it did not go into defrost when it hit the pin.... i had to turn dial multiple times then it finally went into defrost.....i condemned defrost timer installed a brand new one he had....correct me if im wrong but it should go into defrost every time the pin hits the lever manually or automatic????.....while in defrost i checked amperage and voltage to drain pan and defrost coil heaters to verify that the system is in defrost and heaters are working. ..... my main issue is when the system goes into defrost or the thermostat in the box satisfies and it goes into a pump down the condensing unit short cycles multiple times ..... The control that short cycles the condensing unit is the all range control set to 5 psi cut out / 25 psi cut in.....when it goes into pumpdown compressor will shut off at 5 psi (closer to 0 psi) then when compressor is off pressure will rise almost immediately to 25 psi (the cut in psi ) then compressor will kick back on....are the valves in compressor bad ???? Not holding a pumpdown..or is there something else that is leaking pressure to lo side..compressor was replaced a year and a half ago by another company. ... also no inverted trap at top of suction riser and no p trap at bottom of riser.....i posted a pic of inside condensing unit .....another thing i would like to know is what is the valve and what does it do??? that is in front of of compressor with a copper line leading to liquid line and has a capillary tube to top of compressor....it seems to be freezing at that valve when running thank you Paul S

JUGHNE

I would look for the liquid line solenoid valve, (controlled by t-stat and defrost timer) usually in or at the evap coil. It may not be closing completely and allowing liquid to flow constantly at a small rate causing constant freezing in the coil and repeated pump downs.
You could check the compressor valves by shutting off the liquid line valve at the condensing unit, unit should pump down and but suction side might rise maybe once or twice to repeat pump down but should hold at low setting.
Liquid injection valve is to cool compressor while operating.

That particular injection valve I am not familiar with, some have a separate solenoid valve just for that. But any pump down system would have a LL solenoid for the system.

Techman

That depends. The DTC (defrost time clock) might/should have a wire connected to the X pin. The X pin is for getting rid of eXtra defrost time, or for eXiting the defrost cycle.So, if DTC is set for a (1-2-3-4 defrost times a day))45min defrost and the frost is gone after 15min, a sensor sends a signal to the X terminal which terminates the remaining defrost time. So by you advancing the clock to the next defrost peroid does not mean there will be another defrost cycle. Turn off power ,test for "no power"at the DTC ,remove and tape the X wire, reapply power and now there should be another defrost UNLESS the HI TEMP sensor attached to the evap coil is not open.On the "bad"DTC look at the "arms" on the back of the DTC and watch them open/close.

Then depending, by front seating the King Valve,to try to manually pumpdown the system to check the "valves" in the comp, this system might be a "DNPD" (DoesNotPumpDown) system. That means there is not enough room in the CondUnit to hold all of the liquid from the KingValve all the way to the evap and all the way back to the CU. I tag ALL of my systems that DNPD w/ DNPD.Try front seating the SuctionServiceValve at the comp and see if the comp short cycles. Where is the LLSV in the system?

The comp looks like a SCROLL comp, and there are no valves in the SCROLL comp.They have a SCROLL in there(look at Copeland web site for pictures of a Scroll).

There should be an isolation valve for the Comp Cooling Device( LIquid Injector)

Check the pressures across the SuctionLineFilterDrier,check the SightGlassMoistureIndicator for color,if any other color than DRY change out that goofy LiquidLineFilterDrier, it is too small and its brazed in place ,go for a SAE (flare) 303-HH LLFD and change the SGMI. If there was acid in the system ,the acid "might" have coated the moisture tablet rendering it useless. Put in a SAE SGMI,also.

Paul S_3

I removed the wire from the X terminal , re applied power and it still did not go into defrost......the LLSV is at the evaporator coil in walk in box below condensing unit about 15 feet....so to check compressor valves i should front seat king valve to manually pump down system....if this is a DNPD system i should close the suction service valve and see if it holds if it does not compressor valves are bad ??....or solenoid valve could be leaking through? If it does hold then there is not enough room to store all that extra refrigerant....what would i do in that situation? ?? It is a scroll compressor. ...and why should there be a isolation valve for the compressor cooling device. ...and can that Ieak refrigerant to low side not allowing a pumpdown? Thanks Paul S

unclejohn

Two or three restarts after he LL valve closes is not that uncommon. You could try a pinch off tool on the injector valve and see if that stops the short cycle on pump down. there should be a 3 wire control on the evap that powers the X terminal at the TC and delays the evap fans. With the x wire removed it should go into defrost and you should read amps on all heaters. Bad heaters will cause your problem for sure. I remember a call I ran at a Deli and told the old Italian guy he needed new heaters for his freezer and he said, I quote " Boy I want that box cold not hot ".

Paul S_3

Thanks for reply uncle john....It turned out to be a faulty defrost timer....even with the "x" terminal wire removed it would not go into defrost every time. ....my main concern is the short cycling of the compressor...This is a good friend of mine I would rather fix this than replace a compressor which I would be doing for free ....on a pump down it will go on and off about five times then it will finally stop ....would this damage the compressor? If its not uncommon Thanks Paul S

Techman

If the comp cycles 4-5 times and stays off then I would think there is nothing wrong ,mechanically speaking. Fine tune the LP control differential .