REFRIGERATION SYSTEM
We will present the R-12 system as though it had only one evaporator, one compressor, and one condenser. A refrigeration system may (and usually does) include more than one evaporator, and it may include an additional compressor and condenser units.
COMPRESSORS
Many different types and sizes of compressors are used in refrigeration and air-conditioning systems. They vary from the small hermetic units used in drinking fountains and refrigerators to the large centrifugal units used for air conditioning.
One of the most common compressors on modem ships is a high-speed unit with a variable capacity. This compressor is a multicylinder, reciprocating design with an automatic device built into the compressor to control its output. This automatic capacity control provides for continuous compressor operation under normal load conditions. The capacity of the compressor is controlled by unloading and loading the cylinders. This is a very desirable design feature of the unit. If the compressor had to be started under a load, or with all cylinders working, a much greater amount of torque would be required, and it would be necessary to have a much larger drive motor. Also, if the compressor ran at constant capacity or output, it would reach the low-temperature or low-pressure limits or be constantly starting and stopping, thereby putting excessive work on the unit.
Unloading of the cylinders in the compressor is accomplished by lifting the suction valves off their seats and holding them open. This method of capacity control unloads the cylinders completely but still allows the compressor to work at as little as 25 percent of its rated capacity .
Unloader Mechanism
When the compressor is not in operation, the unloader mechanism is in the unloaded position as shown in figure 5-1. The mechanism is operated by oil pressure from the capacity control valve. The oil pressure pushes the unloader spring against the unloader piston. This action moves the unloader rod to the left, thereby rotating the cam rings. As the cam rings are rotated, the lifting pins are forced upward, raising the suction valve off its seat. The suction valve is held in this position until the compressor is started and oil pressure of approximately 30 psi is reached. At that time, the oil pressure from the capacity control valve pushes the unloader piston back to the right against the unloader spring. The motion transmitted through the pushrod rotates the cam ring. This lowers the lifting pins and allows the suction valve to close or operate normally and the cylinder to become loaded (fig. 5-2). On most compressors the unloader is connected to the cylinders in pairs.
Capacity Control Valve
The capacity control valve (fig. 5-3) is located in the compressor crankcase cover. The valve is actuated by oil pressure from the main oil pump. It admits or relieves oil to or from the individual unloader power elements,
—Unloader mechanism in the unloaded position.
.—Unloader mechanism in the loaded position.
depending on suction or crankcase pressure. Figure 5-3 shows the compressor at rest. The two cylinders equipped with the unloader element are unloaded and will remain unloaded until the compressor is started and the oil pressure reaches normal operating pressure.
The high-pressure oil from the pump enters chamber A of the capacity control valve. It then passes through an orifice in the top of the piston to chamber B, forcing the piston to the end of its stroke against spring A. When the piston of the valve is forced against spring
—Capacity control system.
A, the circular grooves that form chamber A are put in communication with the unloader connections. This admits high-pressure oil to the unloader cylinder and actuates the unloader mechanism.
A capacity control regulating valve controls oil pressure from the capacity control valve. It is connected to the crankcase and has an oil-connecting line to chamber B of the capacity control valve. As the crankcase or suction pressure pulls down slightly below the setting of the regulating valve, the regulator opens and relieves oil pressure from chamber B of the capacity control valve. This permits spring A to push the capacity control piston one step toward chamber B, uncovering the unloader connection nearest the end of the capacity control valve. This relieves oil pressure from the power element and allows the power element spring to rotate the cam rings and unload the cylinder.
If the suction pressure continues to drop, the regulator will relieve more oil pressure and unload more cylinders. If the heat load increases, the suction pressure will increase, causing the regulating valve to close and load more cylinders.
