Page 455 - Mechanical Engineers' Handbook (Volume 4)

444 Refrigeration

Vapor pressure is an important consideration for coolants that will be used in open
systems, especially where it may be allowed to warm to room temperature between periods
of operation. It may be necessary to pressurize such systems during periods of moderate
temperature operation. For example, at 0 C the vapor pressure of R-11 is 39.9 kPa (299 mm
Hg); that of a 22% solution of calcium chloride is only 0.49 kPa (3.7 mm Hg). The cost of
vapor losses, the toxicity of the escaping vapors, and their flammability should be carefully
considered in the design of the semi-closed or open system.
Environmental effects are important in the consideration of trichlorofluoromethane (R-
11) and other chlorofluorocarbons. This is a refrigerant with a high ozone-depletion potential
and halocarbon global-warming potential. The environmental effect of each of the coolants
should be reviewed before seriously considering the use of it in a system.
Energy requirements of brine systems may be greater because of the power required to
circulate the brine and because of the extra heat-transfer process, which necessitates the
maintenance of a lower evaporator temperature.

7 SYSTEM COMPONENTS

There are four major components in any refrigeration system: compressor, condenser, evap-
orator, and expansion device. Each is discussed below.

7.1 Compressors

Both positive displacement and centrifugal compressors are used in refrigeration applications.
With positive displacement compressors, the pressure of the vapor entering the compressor
is increased by decreasing the volume of the compression chamber. Reciprocating, rotary,
scroll, and screw compressors are examples of positive displacement compressors. Centrif-
ugal compressors utilize centrifugal forces to increase the pressure of the refrigerant vapor.
Refrigeration compressors can be used alone or in parallel and series combinations. Features
of several of the compressors are described later in this section.
Compressors usually have a variety of protection devices for handling adverse condi-
tions. These include high-pressure controls, high-temperature controls, low-pressure protec-
tion, time delay, low voltage and phase loss, and suction line strainer. 18 High-pressure
controls are required by Underwriters Laboratories. These can include a high-pressure cutoff
or a relief valve. High-temperature devices are designed to protect against overheating and
lubrication breakdown. Low-pressure protection is provided to protect the compressor against
extremely low pressures, which may cause insufficient lubricant return, freeze-up, or too
high a pressure ratio. Time delays are required to prevent damage to the compressor motor
from rapid startup after a shutdown. A suction line strainer is used to remove dirt and other
particles that may be in the refrigerant line. The specific protection devices will depend on
the application and size of the compressor.

Reciprocating Compressors
High-speed, single-stage reciprocating compressors with displacements up to 0.283–0.472
3
M /sec (600–1000 cfm) generally are limited to a pressure ratio of about 9. The reciprocating
compressor is basically a constant-volume variable-head machine. It handles various dis-
charge pressures with relatively small changes in inlet-volume flow rate as shown by the
heavy line in Fig. 9.
Reciprocating compressors can also be found in an integral two-stage configuration. 18
These can use R-22 or ammonia and can achieve low temperatures from 29 to 62 C.
These compressors will consist of multiple cylinders, with the cylinders divided so that the