Pump Down Cut Off Pressure

[jwasir]

Hi,

I am interested to know that what should be the correct setting for cut off pressure for pump down cycle.

As per Copeland bulletin, it should be just below the lowest suction pressure of the system.

If we set pump down pressure to say 50 PSI and cut in at 70PSI, compressor cycles number of times before it stops.

50PSI in the system is enough to boil out and raise it back to 70PSI after a little while, this just keep the compressor in cycling mode.

Too much cycling can also cause oil shortage in the compressor.

Also, is the cut off pressure different for Scroll compressors v/s semi hermetic?

[Brian_UK]

Personally I would set the cut out pressure at around 25psi in your sort of example.

I would also ensure that the system has a one-shot pump down cycle to prevent compressor restarts if the pressure rises above the cut in pressure unless there is a start command.

[wilks]

Hi,

I am interested to know that what should be the correct setting for cut off pressure for pump down cycle.

As per Copeland bulletin, it should be just below the lowest suction pressure of the system.

If we set pump down pressure to say 50 PSI and cut in at 70PSI, compressor cycles number of times before it stops.

50PSI in the system is enough to boil out and raise it back to 70PSI after a little while, this just keep the compressor in cycling mode.

Too much cycling can also cause oil shortage in the compressor.

Also, is the cut off pressure different for Scroll compressors v/s semi hermetic?

Weve had loads of problems with scrolls pumping down , converted them in the end to none pump down.Let me know how you get on please , id be very interested to know

[maher wahbe]

you have to set at 25psi and defferetionl should be at least 10psi to aviod compressor restart. the setting also debend for refrigeration type which you are using

[philfridge]

Hi you need to lower the cut out pressure then . Does the system have start delays timers on the compressor/s as this would stop the short cycling

[Taffy]

Well done Phil, Compressors need protecting, 10mins/6 starts an hour, if there are no timers recommend to your boss or your customer its a good option. Then as long as your LP is set on a lower setting than your running suction pressure, your fine an dandy. If you have a system that is critical ie "close control" you need then to look at offloading/capacity control devices in place otherwise that close control will be lost.

[GXMPLX]

Personally I would not lower the LP cutout below the operation envelope of the compressor! This does cause short cycling and Phill/Taffy are right that this should be corrected with a timer. Te problem with LP cutout being too low is that it causes high discharge temperatures even for a short while. This burns a little oil in the discharge of orbital compressors or valve plates and starts a chain of chemical reactions in the system. If the filters can cope with this depends on how often and how much acids you are producing every time you stop the system ... but it will fail!