Page 459 - Mechanical Engineers' Handbook (Volume 4)

448 Refrigeration

Capacity modulation is accomplished by slide valves that are used to provide a variable
suction bypass or delayed suction port closing, reducing the volume of refrigerant actually
compressed. Continuously variable capacity control is most common, but stepped capacity
control is offered in some manufacturers’ machines. Variable discharge porting is available
on a few machines to allow control of the built-in volume ratio during operation.
Oil is used in screw compressors to seal the extensive clearance spaces between the
rotors, to cool the machines, to provide lubrication, and to serve as hydraulic fluid for the
capacity controls. An oil separator is required for the compressor discharge flow to remove
the oil from the high-pressure refrigerant so that performance of system heat exchangers will
not be penalized and the oil can be returned for reinjection in the compressor.
Screw compressors can be direct driven at two-pole motor speeds (50 or 60 Hz). Their
rotary motion makes these machines smooth running and quiet. Reliability is high when the
machines are applied properly. Screw compressors are compact so they can be changed out
readily for replacement or maintenance. Today, the efficiency of the best screw compressors
matches that of reciprocating compressors at full load. Figure 11 shows the efficiency of a
single-screw compressor as a function of pressure ratio and volume ratio (Vi). High isentropic
and volumetric efficiencies can be achieved with screw compressors because there are no
suction or discharge valves and small clearance volumes. Screw compressors have been used
with a wide variety of refrigerants, including halocarbons, ammonia, and hydrocarbons.
Centrifugal Compressors
The centrifugal compressor is preferred whenever the gas volume is high enough to allow
its use, because it offers better control, simpler hookup, minimum lubrication problems, and
lower maintenance. Single-impeller designs are directly connected to high-speed drives or

Figure 11 Typical performance of a single-screw compressor. (Reprinted by permission from 2000
ASHRAE Handbook of HVAC Systems and Equipment. American Society of Heating, Refrigerating,
and Air-Conditioning Engineers, Inc., www.ashrae.org.)