What pressure to unload?

[Drew1]

Generally I work on commecial refrigeration systems, but have been asked to commision a shop AC system. It is a cooling only system running on 134a. The compressor has 2 unloaders on the head and the evap has 2 texv and 2 solenoid valves. Therefore by there being 2 sol valves I understand this to a 2 stage cooling system. When 1 sol valve is de energised the suction pressure drops away. To lift the suction to prevent the coil from icing up we need to unload the compressor. At what pressure settings do I unload the compressor?

[ozone drone]

To keep ice from forming on the evap ... keep it above 32 degrees.
which is about 28 psi for 134a...... set 1st stage to unload at 28 and the second about 5-8 psi higher....or 33 psi. Remember that unloaders are mostly an energy saving device that reduces system capacity under low load conditions and increases capacity as the load rises. If you set the differential too narrow they will constantly load and unload...

[duke of earl]

2 unloaders makes for 3 stage-1st stage 33%, 2nd stage 66%, 3rd stage fully loaded. Depending on how wide a load swing will determine deadband for unloaders. Pressure unloaders work better than other methods such as leaving air temp, return air temp, etc. The key is to prevent going below freezing (around 28 degrees). With 134-a, normal suction pressure would run about 34 psig. Say the evap load drops and suction falls to 30 psig you would want to unload a stage. This unloading should somewhat balance the compressor to the load. If the load drops off more and the suction pressure hits the target 30 psig unloader setting than the 2nd unloader will kick in and try to balance the compressor to the load. You will need to keep suction pressure above 25psig to prevent freezeup. You can add a coil freeze cycle control to disable the compressor if the load goes below all stages of mechanical unloading and freezing could occur. Hope this helps.

[Airmechanical]

most important thing is oil pressure, when your unloading project is finished

double check your oil pressure, cause unloading will lower velocity

you may be required to trap refrigerant piping!



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[Drew1]

Thanks guys. That does help.