superheat, latent heat of vaporization, and some of its
Every mechanical refrigeration system operates at
two different pressure levels. The dividing line is
shown in figure 6-14. The line passes through the
discharge valves of the compressor on one end and
through the orifice of the metering device or expansion
valve on the other.
The high-pressure side of the refrigeration system
comprises all the components that operate at or above
condensing pressure. These components are the
discharge side of the compressor, the condenser, the
receiver, and all interconnected tubing up to the
metering device or expansion valve.
The low-pressure side of a refrigeration system
consists of all the components that operate at or below
evaporating pressure. These components comprise the
low-pressure side of the expansion valve, the
evaporator, and all the interconnecting tubing up to
and including the low side of the compressor.
Refrigeration mechanics call the pressure on the
The refrigerant low-pressure vapor drawn from the
high side discharge pressure, head pressure, or
evaporator by the compressor through the suction line,
high-side pressure. On the low side, the pressure is
in turn, is compressed by the compressor to a
called suction pressure or low-side pressure.
vapor, which is forced into the
The refrigeration cycle of a mechanical
refrigeration system may be explained by using figure
6-14. The pumping action of the compressor (1) draws
vapor drawn from the evaporator (2). This action
reduces the pressure in the evaporator, causing the
liquid particles to evaporate. As the liquid particles
evaporate, the evaporator is cooled. Both the liquid
and vapor refrigerant tend to extract heat from the
warmer objects in the insulated refrigerator cabinet.
The ability of the liquid to absorb heat as it vaporizes is
very high in comparison to that of the vapor. As the
liquid refrigerant is vaporized, the low-pressure vapor
is drawn into the suction line by the suction action of
the compressor (1). The evaporation of the liquid
refrigerant would soon remove the entire refrigerant
from the evaporator if it were not replaced. The
replacement of the liquid refrigerant is usually
controlled by a metering device or expansion valve (3).
This device acts as a restrictor to the flow of the liquid
refrigerant in the liquid line. Its function is to change
the high-pressure, subcooled liquid refrigerant to
low-pressure, low-temperature liquid particles, which
will continue the cycle by absorbing heat.
Figure 6-14.Refrigeration cycle.