Unloader Set points

[KeithCA]

Bohn JLD6000L6DPP. 0deg. Box. 404a.
Short Cycling 8 times an hour.
Unloader setpoints are within a few PSI. System is grossly oversized. without putting in hot gas BYP. Any suggestions for unloader Pressure setpoints.
Thanks

[Steve Wright]

Does the unit shorcycle when compressor is completely unloaded? What type and size compressor? What is the Evaporator temp?

[KeithCA]

2 30 hp compressors copeland discus. each with unloader.
unload sequence is start loaded 1 & 2 then unload 1, 2, then stop 1,2.

Box is maintaining temp. Basically just looking for some ballpark setpoints that I can stage the compressors without flooding the evap.

[US Iceman]

Hi Keith,

unload sequence is start loaded 1 & 2 then unload 1, 2, then stop 1,2.[/

Is the first compressor starting loaded, then the second compressor starts fully loaded? Then the compressors begin to unload? Then shut off? Is this right?

I would suggest the compressors start with the minimum step of capacity available. If the suction pressure increases, load the second step of capacity on the same compressor. Continue this progression for the second compressor, if the suction pressure continues to increase.

When the suction pressure starts to decrease, reverse the order noted above.

It sounds like the system is ramping up quickly to keep the suction pressure set point constant, but in doing so, the compressors are short cycling.

This response needs to be slowed down. This can be done with delay timers, or larger pressure differentials for the set-points with the unloaders.

First question; What is the tolerance on the box temperature range? The suction pressure can float a few psi with very little effect on storage temperatures (if the box is relatively full). The mass of stored product limits the temperature excursions.

It is hard to guess at much more with the information provided.

Regards,
US Iceman

[Steve Wright]

you could try what US Iceman said and increase the cut in laod point pressures a little at a time.

What is cycling the compressors Temp or Pressure? Could you give us a little more detail on how Compressors are controlled?