Compressor question

[AzRob]

Another A/C guy looking for refrig. info. When the suction service valve is front seated with unit running, is it expected the unit will pump down and the compressor go into a vacuum? (if lp doesnt kick in) If suction pressure goes to 10 psi rapidly from operating temp and sits there, does not go into a vacuum, is the compressor gone?

Am working on a walk in cooler and cant get a clear sight glass. Thought I would pump down the system and check out the txv screen but compressor did not go below 10psi....closed off valves and examined txv valve screen, it didnt have one, so no pluggage.

After blowing out evap and txv with nitrogen (no crud exited) after putting loop into vac, opened valves and NOW have clear sight glass with following temps on guages: suction 47, head 108. Ambient 105 and box temp 72. R-22 system. No SH or SC as thermometer is on fritz. Any help appreciated!

Project is an old McDonalds Bohn cooler/freezer combo. Box was donated to a church, the freezer "install" went off almost without a hitch.

[fridge doctor]

Am a bit confused at the order of play so to speak... To pump the system down, attach at least a suction guage to the service valve. Do this by winding the valve fully open (back), attach guage and then crack valve stem forward one turn or so. Next, fully close (Forward or clockwise) the liquid valve on the receiver. Observe your guage and be prepared to manually force the LP (if fitted). Do not allow system to go into vac. Kill power and close the suction valve. Done.

If you want to just check if the comp is pumping properly, close the suction valve only (with guage attached as described above). Close it completely and observe. You should see better than 20 inches vac, and it should hold when you stop the compressor, if it doesn't then the compressor valves are leaking, and depending upon how quickly it loses the vac will be the determining factor as to whether you need to replace it or not.

Reading the rest, it looks like the TX valve is goosed.

Regards, Trevor

[tbirdtbird]

"it didnt have one, so no pluggage."
Are you sure? I just had to unsweat a valve,disassemble, and blow 125 psi air thru a bunch of times every which way to clear it. Had no screen either.now works fine.

[AzRob]

"it didnt have one, so no pluggage." ---- My original meaning was "If it does'nt have a screen, the screen can't be plugged." but I hear you. I did blow nitrogen through the valve both ways, it was free and clear from the start but I did it anyway.

"If you want to just check if the comp is pumping properly, close the suction valve only (with guage attached as described above). Close it completely and observe. You should see better than 20 inches vac, and it should hold when you stop the compressor, if it doesn't then the compressor valves are leaking, and depending upon how quickly it loses the vac will be the determining factor as to whether you need to replace it or not."

Thats how I checked it (and pump down was done how you explained it) When suction valve is closed, guage goes down to 10psi fairly quickly and sits at 10psi. Compressor didnt go into a vacuum, didnt go below 10psi. I ran the compressor for 30 seconds after the guage stopped at 10psi, does it take longer to go past 10 and into a vacuum? Guage slowed at 15psi then sat at 10psi if that means anything. Thanks

[monkey spanners]

If its a scroll compressor it won't got to a vacuum (depending on temp range of comp) also the suction pressure will rise after the compressor stops as the gas in the scrolls leaks back. This is normal for scrolls.

Jon

[taz24]

When the suction service valve is front seated with unit running, is it expected the unit will pump down and the compressor go into a vacuum?

Yes

(if lp doesnt kick in) If suction pressure goes to 10 psi rapidly from operating temp and sits there, does not go into a vacuum, is the compressor gone?

Not always.
The valves could be faulty or damaged and if the system is a semi hermetic then access to the valves and possible repair.

Am working on a walk in cooler and cant get a clear sight glass. Thought I would pump down the system and check out the txv screen but compressor did not go below 10psi....closed off valves and examined txv valve screen, it didnt have one, so no pluggage.

Confusion!!
If your sight glass is not clear then it is not the valve or the evap that is blocked.
A blocked valve or evap would show a clear sight glass.

Any help appreciated!

System has faulty comp that is not pumping.
Back pressure is high so evap temp is high hence no cooling effect.
Check the comp.
taz

[AzRob]

When suction service valve is closed at compressor (to test it) could a condition exist other than a compressor problem that would not allow the compressor to pump down into a vacuum? ie if there was a flooding back of liquid ***** back towards the compressor or an overcharge condition.
I guess I'm thinking if liquid is occupying space in the piping/receiver/condensor it shouldnt be, what vapor the compressor is pushing toward the evap meets resistance sooner causing the compressor to not pull down into a vacuum.

[tbirdtbird]

"I guess I'm thinking if liquid is occupying space in the piping/receiver/condensor it shouldnt be, what vapor the compressor is pushing toward the evap meets resistance sooner causing the compressor to not pull down into a vacuum."

Ummm...no. Reread Taz..he usually has the answer.

[AzRob]

I had indicated that when suction valve is closed the compressor only pumps down to 10psi. From the replies this indicates the compressor is shot. (sealed hermetic unit)
My question was a theory question. Re-worded it could read "Is the compressor designed to pump down into a vacuum regardles of any problem in the system. Problems could be mixed *****s, floodback, restrictions, low amperage to compressor from bad wiring connections or a contactor on its way out...etc" Can one walk up to a unit regardles of cycle or condition of system and get a reliable check of the compressor by pumping it down via suction service valve. Im assuming the compressor wont pull a vacuum if there is no ***** in the system, but I dont know.....

[paul_h]

Yes, if its a piston compressor, nt matter whats in the system, if the suction service valve is shut, you will get a vacuum if the compressor is good and the suction valves in the compressor seal properly.
If you don't get a vacuum, it's because on an internal compressor fault, nothing else can cause that fault.

[paul_h]

Sorry thats wrong.
Forward seating of the valve will allow you to read the pressure in the comp and it will stop suction gas returning to the comp.
This is the most reliable way to do a pump test on a running compressor.

taz.

yeah I know.

[taz24]

I had indicated that when suction valve is closed the compressor only pumps down to 10psi. From the replies this indicates the compressor is shot. (sealed hermetic unit)
My question was a theory question.

In a hermetic compressor if the comp does not pull a vacuum (the deeper the better) then it is more than likley US (unserviceable).
If the comp pulls a vac but when the comp is turned off the vac is lost in a short period of time (5 to 15 secs) then it indicates that the system has a major fault (hermetic comps will equalise in pressure but if it happens unduly quick, thats the problem).
taz.

[Dan]

I had indicated that when suction valve is closed the Compressor only pumps down to 10psi. From the replies this indicates the Compressor is shot. (sealed hermetic unit)
My question was a theory question.

When it comes to "tin cans" or hermetic compressors, I have seen many that will not pull into a vacuum yet have been running adequately for 2 or so years once this was observed. It surprised me as well as the technician who observed it. That was Gary. We replaced one compressor and it behaved the same way, yet it refrigerated fine and is still running.

[AzRob]

Yes, if its a piston compressor, nt matter whats in the system, if the suction service valve is shut, you will get a vacuum if the compressor is good and the suction valves in the compressor seal properly.
If you don't get a vacuum, it's because on an internal compressor fault, nothing else can cause that fault.

Ahhh okay. Exactly what I wanted to know. Darn good point on the suction valve too. Thank you.

[AzRob]

Question for taz or ??

Back Pressure is high so evap temp is high hence no cooling effect.

I'm abit confused here. I thought evap temp for this cooler was around 20 degrees (to obtain 37 degree box) and the back pressure is only 108 degrees (bout a 220 psi). Ball park back pressure of ambient plus 25-35 is 130-140 degrees (head of 296-337psi) What am I missing?

[taz24]

Question for taz or ??



I'm abit confused here.

Back pressure is a term used for suction pressure or the low side (the cold pipe).
Discharge pressure, somtimes head pressure, delivery pressure or high side are terms for the other side(the hot pipe).

Sorry for the confussion

taz.

[AzRob]

Interchaning terminology sure can lead to confusion! Thanks for pointing it out.

Dan, thanks for reply about the compressor. Tech at Heatcraft said the same thing. Am gonna pull the R22 out and run 408a as the txv is for R502 as is the distributor orifice...

[akrefer]

what horsepower, refrigerant, electrical are you