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A.Mortezania
03-04-2017, 07:40 PM
Dear Friends,

When we pump down a cycle, after a while pressure of suction increases, Would it be possible to let me know :

Increasing suction pressure is as result of leakage on discharge valve or it is as result of leakage on liquid line solenoid valve ?
It means, Discharge valve can not 100% close the route or liquid line solenoid valve can not 100% close liquid line ?

Which one is true ?

Sincerely yours.

The Viking
03-04-2017, 08:52 PM
Most likely neither.

If we assume that you mean that the pressure rises after the compressor stopped and that it doesn't reach saturated pressure levels.
Your refrigerant cycle does not only carry refrigerant, there will also be oil with the refrigerant. This oil will bind with refrigerant and the amount of refrigerant mixed with the oil depends on the pressure, when we lower the pressure the refrigerant in the oil will start to boil off (or be released) and this will slowly increase the pressure.

:cool:

FaultCode
03-04-2017, 10:53 PM
The pressure will also increase due to temperature change upwards.

Tycho
04-04-2017, 05:05 PM
it can depend on a few things.

Length of the suction line, design of the suction line, evaporator temperature.

Let's say there is a room thermostat controlling the solenoid valve, when the temperature in the room is reached, it closes the solenoid, when the temperature in the room is reached, it means that all the products in the room has reached, or are very close to the same temperature as the evaporator temperature.

So there is not much heat load on the evaporator, but there may still be liquid refrigerant left in the evaporator after the first pump down, and the tiny heat load will in time cause the refrigerant to evaporate and increase the pressure, causing the LP-switch to cut in and start the compressor.

Or it could be a long suction line, with a pressure drop, or as The Viking said, oil residue containing refrigerant in the evaporator/suction pipe that evaporates and causes a increase in pressure.

in both cases, the compressor will start and stop a few times before it finally has emptied the system and there is not enough refrigerant left to evaporate.

To diagnose a leaking solenoid valve, check if there is constant ice/freezing on the pipe between the solenoid and the thermostatic expansion valve when the compressor pumps down.

On the systems we have delivered, we like to put a 5 minute timer relay in the pump down function, so the compressor doesn't cut in and out too much during a pump down cycle, to protect the compressor from running for a few seconds and not build up oil pressure, and to give the start contactor a longer life.

Hope this Helps :)

A.Mortezania
05-04-2017, 07:01 PM
Thanks friends for your replies.

Sincerely yours.

FaultCode
05-04-2017, 08:48 PM
Animated description of a non-recycling pump down circuit......

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

Grizzly
05-04-2017, 09:34 PM
Animated description of a non-recycling pump down circuit......

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

Interesting thread!
Grizzly