Hussmann Ingersoll Rand, User Manual

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Mechanical Design 15
Application limits with part-load operation
With unloaders, compressor temperatures increase due to reduced refrigerant flow, reduced
motor cooling, and electrical and mechanical losses. Therefore, the application ranges of
compressors with unloaders are somewhat restricted. Please refer to the application envelopes
when selecting compressors with unloaders.
Control Differential with Unloaders
The control differential for starting and stopping the compressors must be adjusted so that it is
greater than the differential for activating the unloaders. It is also recommended that the
compressor start/stop cycling be limited by means of a time delay relay or by software timers in
an electronic control system.
Additional Important Notes About Unloaders
Do not use unloaders with Inject-Cool compressors. Do not use unloaders with I-Cool
compressors if the suction strainer is in the SL(B) position. Unloaders may be used with I-Cool
compressors if the suction strainer is in the SL(A) position.
For compressor models IR4C0770 through IR4C1385, the discharge pressure control should not
be connected to the same cylinder head that has the unloader.
Crankcase Heaters
Depending on system type and refrigerant charge, excessive refrigerant can dissolve in the
crankcase oil during extended shut-off periods. This may lead to increased oil carryover, liquid
slugging, and reduced lubricity during the next compressor start. Crankcase heaters reduce the
dilution of compressor lubricant in these situations.
Heaters are particularly important under the following conditions:
x
When oil temperature (crankcase) drops below roughly 30°F for extended shut off periods
x
When the ambient temperatures is less than 50
q
F at the compressor.
x
Crankcase heaters are especially important for compressors with oil pumps.
Crankcase heaters may not be needed under the following exceptions:
x
The system has a very small refrigerant charge – less than roughly 1.5 times the oil charge by
mass
x
Systems approved in long-term tests.
x
Pump down systems where temperature at the compressor location is greater than 50
q
F.
x
Systems with short shut-off periods during which cooling of the compressor cannot occur, for
example, automatic sequence switching-over in parallel systems.