R22 condensing unit Automatic Pumpdown

[kurt s]

I have a system that ive been working on its a large R22 condensing unit with 2 semi hermetic compressors. It has automatic pumpdown on it, and ive been noticing the compressors cycling on and off ALOT. sometimes every minute or two.

What exactly is the automatic pumpdown for? and why is it doing this cycling so often?

i shut the automatic pump down switch off for now so the compressors dont get destroyed from going on and off so often, They are pretty large semi hermetic compressors.

[chemi-cool]

The term automatic pump down is new to me, I guess what you mean is that the liquid SV shot when the right temp is achieved.
Since refrigerant migrates to the colder spot, pump down keeps all the refrigerant in the condenser and receiver.
It protects the compressors from liquid flush back, low starting currant and keeping the oil in the compressor sump.

If the compressors go on and off, i would check the suction valves, low pressure control settings, SV for leaks.
What you are describing is a fault in the system.

[kurt s]

chemi-cool thanks for the reply, after doing some reading i am thinking solenoid valve leaking....

What are the chances of slugging the compressor or creating a larger problem if this isnt fixed soon? i know its hard to say just looking for opinions.
thanks

[sedgy]

hi kirts, yes you are right, the comp,s will not last forever < short cycling < the system needs pumping down , inspecting the sol valve for dirt under the seal< not the one in the sea < new dryer compressor test , how many days till you dock ?

[aramis]

If you have a timer you could delay the start of the compressor long enough not to short cycle and short enough not to flood the low side with refrigerant.

[Brian_UK]

You could also modify the wiring to include a 'non-recycling pumpdown control'.

See the attached link, scroll down to the circuit and click on the numbered buttons to see the circuit working.

http://www.refrigerationbasics.com/1.../controls5.htm

[kurt s]

the compressors are cycling every 30 seconds .. i had to shut them off tell i can change the solenoid valve next week..

is it possible refrigerant could be leaking back through compressor into low side?? and end up finding out solenoid was ok? solenoid is located just before the evap. coil

or is solenoid my only fear?

thanks for the help

[aramis]

You can hear a solenoid leaking even in a noisy environment if you stick your ear to a tube touching the exit of the solenoid (and plug your other ear).

Yes pressures do equalize through a reciprocating compressor, slowly if the compressor is OK, faster if it has problems.

Front seat the compressors suction valve to discard the compressor. Don't forget to open it later.

Pump down is used to prevent refrigerant migration to the compressor crankcase.

If your compressor has a crankcase heater or the compressor is hoter than the evaporator and it doesn't stay off for long hours then you can eliminate pump down completely and fix it when you reach port.

You may have more issues with the boat pitching though.

If it must stay off for long hours then it would be better to start it manually watching the sight glas and if it foams stop it wait a couple of minutes and do it again until it doesn't foam.

[padraic]

If you have a timer you could delay the start of the compressor long enough not to short cycle and short enough not to flood the low side with refrigerant.

as aramis said fit a delay timer... that will insure enough duty there to run longer and not short cycle

[Brian_UK]

as aramis said fit a delay timer... that will insure enough duty there to run longer and not short cycle

Timers are a good idea but will not prevent flood back to the compressor if there is leakage through the valves somewhere.

[aramis]

Timers are a good idea but will not prevent flood back to the compressor if there is leakage through the valves somewhere.

Pump down does not prevent flood back. Unless we use different terms floodback only happens during the running cycle.

The the time you keep the compressor off must not be long enough to have a significant amount of refrigerant in the suction side. The thermostat (or system usual on signal) should override the timer.

[Peter_1]

You can hear a solenoid leaking even in a noisy environment if you stick your ear to a tube touching the exit of the solenoid (and plug your other ear).

You can hear it even better if you point a screwdriver on the tube and put the other end to your ear and plug the other ear.

[Peter_1]

Front seat the compressors suction valve to discard the compressor. Don't forget to open it later.

Pump down, measure LP and monitor pressure. Frontseat HP and monitor LP. If rising stops then you have leaking discharge valves. Is there an oil separator in your circuit?

[kurt s]

i installed a new time delay relay, the old one wasnt working, it was set at 8 min. but cycling every 25 seconds ---- now just need to change solenoid valve and should be good to go.