Handbook of air conditioning and refrigeration / Shan K

11.24 CHAPTER ELEVEN
11.5 CAPACITY AND SAFETY CONTROLS
Capacity Control
Refrigeration systems for comfort air conditioning or industrial applications need capacity control
to meet refrigeration load variations during operation. In vapor compression refrigeration systems,
control of the refrigerant flowing through the compressor is widely used as the primary refrigeration
capacity control. For reciprocating refrigeration systems using DX coils as evaporators, row or
intertwined face control of DX coil capacity is often employed in conjunction with compressor capacity
control for better performance. In medium-size and large systems, two separate refrigerant
circuits are often used so that refrigerant flow in one can be cut off when the refrigeration load
drops below 50 percent of design load.
There are four basic methods of controlling the capacity of reciprocating refrigeration systems
by reducing the refrigerant flow rate during part-load operation.
On/Off Control. On/off or start/stop control is widely used to control the refrigerant flow and
the capacity of the reciprocating compressors in residential air conditioners. For reciprocating
refrigeration systems equipped with multiple compressors, as used in supermarkets, on/off control
is often the suitable choice. On/off control is simple and inexpensive, but it produces a greater
variation in the space air temperature and relative humidity maintained by the refrigeration system
than other types of control do. In addition, cyclic operation may lead to a start/stop cyclic loss for
the evaporator and condenser, which are located indoors and outdoors separately. Cyclic loss is
discussed in the next chapter. On/off control also results in increased wear and tear on compressor
components and shortens service life.
Cylinder Unloader. For multiple-cylinder reciprocating compressors, a cylinder unloader is often
used for capacity control. Cylinder unloaders reduce the capacity of compressors by bypassing the
compressed gas to the suction chamber, blocking the suction or discharge valve, or closing the
suction valve late or early. Recently developed cylinder unloaders include a solenoid valve that
unloads the cylinder(s) in response to any one of the following:
● Variation of discharge air temperature in a packaged unit
● Change in return chilled water temperature in a water chiller
● Changes in suction pressure in the evaporator
Figure 11.13 shows a typical cylinder unloader. When a temperature sensor in a packaged unit
detects a drop in the discharge air temperature, the controller energizes the solenoid, draws the
plunger away from the connecting port, allows the compressed gas to flow through a side passage,
pushes the piston and valve stem of the unloader away from the solenoid, and opens the unloader
valve. The compressed gas from the cylinder then flows to the suction side of the compressor. The
two cylinders under the same head space are unloaded.
When the discharge air temperature rises, the plunger of the solenoid covers the connecting port.
The pressure of the compressed gas exerted on the disk closes the unloader valve. Consequently, the
increased pressure in the space above the cylinder opens the check valve and permits the discharge
gas to enter the discharge line. The two cylinders are now loaded.
A cylinder unloader is a form of step control. Typically, the capacity of an eight-cylinder reciprocating
compressor can be controlled in four steps: 100 percent, 75 percent, 50 percent, and 25
percent.
Speed Modulation. Recent developments in solid-state electronics may allow adjustablefrequency
ac invertor-driven variable-speed motors to be used for capacity control in reciprocating
compressors. Commercial variable-speed air conditioners with a capacity of few tons of refrigeration
were available in Japan in the 1980s.