Capillary Tube.The capillary tube consists of a
long tube of small diameter. It acts as a constant
throttle on the refrigerant. The length and diameter of
the tube are important; any restrictions cause trouble in
the system. It feeds refrigerant to the evaporator as fast
as it is produced by the condenser. When the quantity
of refrigerant in the system is correct or the charge is
balanced, the flow of refrigerant from the condenser to
the evaporator stops when the compressor unit stops.
When the condensing unit is running, the operating
characteristics of the capillary tube equipped
evaporator are the same as if it were equipped with a
The capillary tube is best suited for household
boxes, such as freezers and window air-conditioners,
where the refrigeration load is reasonably constant and
small horsepower motors are used.
The four basic or major components of a
refrigeration system just described are enough for a
refrigeration unit to function. However, additional
devices, such as the receiver already described, make
for a smoother and more controlled cycle. Some of the
accessory devices used on a refrigeration unit are
described in this section. Before proceeding, take a
close look at figure 6-25 that shows one type of
refrigeration system with additional devices installed.
Some of the devices and their functions are explained
t o h e l p y o u u n d e r s t a n d i n s t a l l a t i o n a n d
troubleshooting of a refrigeration unit.
RELIEF VALVE.A refrigeration system is a
sealed system in which pressures vary. Excessive
pressures can cause a component of the system to
explode. The National Refrigeration Code makes the
installation of a relief valve mandatory. A
spring-loaded relief valve is most often used and it is
installed in the compressor discharge line between the
compressor discharge connection and the discharge
line stop valve to protect the high-pressure side of the
system. No valves can be installed between the
compressor and the relief valve. The discharge from
the relief valve is led to the compressor suction line.
DISCHARGE PRESSURE GAUGE AND
THERMOMETER. A discharge pressure gauge
and thermometer are installed in the compressor
discharge line (liquid line) to show the pressure and
temperature of the compressed refrigerant gas. The
temperature indicated on the gauge is always higher
than that corresponding to the pressure when the
compressor is operating.
COMPRESSOR MOTOR CONTROLS.The
starting and stopping of the compressor motor is
usually controlled by either a pressure-actuated or
temperature-actuated motor control. The operation of
the pressure motor control depends on the relationship
Figure 6-25.A basic refrigeration system.