Several designs of capacity control systems are in
use. One of the most common is shown in figure 10-6.
The capacity control system consists of a power element
and its link for each controlled cylinder, a step control
hydraulic relay, and a capacity control valve.
The systems components are all integrally attached
to the compressor. The suction or crankcase pressure of
the refrigeration plant is sensed by the capacity control
valve to control the system. In other words, a change in
the refrigeration load on the plant will cause a change in
suction pressure. This change in suction pressure will
then cause the capacity control system to react
according to whether the suction pressure increased or
decreased. The working fluid of the system is
compressor oil pump pressure. Compressor oil pump
pressure is metered into the system through an orifice.
Once the oil passes the orifice, it becomes the system
control oil and does work.
Locate the following components on figure 10-6,
and refer to them as you read the next two paragraphs.
Compressor oil pump pressure tap-off
Control oil strainer
Hydraulic relay piston
Unloader power element
Unloader power element piston
Capacity control valve
Crankcase (suction) pressure sensing point
The following functions take place when the
compressor is started with a warm load on the
Compressor oil (A) is pumped through the control
oil strainer (B) into the hydraulic relay (C). There the oil
flow to the unloader power elements is controlled in
steps by the movement of the hydraulic relay piston (D).
As soon as pump oil pressure reaches a power element
(E), the piston (F) rises, the lifting fork (G) pivots, and
the unloader sleeve (H) lowers, permitting the suction
valve (1) to seat. The system is governed by suction
pressure, which actuates the capacity control valve (J).
This valve controls the movement of the hydraulic relay
piston by metering the oil bleed from the control oil side
of the hydraulic relay back to the crankcase.
Suction pressure increases or decreases according
to increases or decreases in the refrigeration load
requirements of the plant. After the temperature
pulldown period with a subsequent decrease in suction
pressure, the capacity control valve moves to increase
the control oil bleed to the crankcase from the hydraulic
relay. There is a resulting decrease in control oil pressure
within the hydraulic relay. This decrease allows the
piston to be moved by spring action. This action
successively closes oil ports and prevents compressor
oil pump pressure from reaching the unloader power
elements. As oil pressure leaves a power element, the
suction valve rises and that cylinder unloads. With an
increase in suction pressure, this process is reversed, and
the controlled cylinders will load in succession. The
loading process is detailed in steps 1 through 7 in figure
The compressor discharges the high-pressure,
high-temperature refrigerant vapor to the condenser,
where it flows around the tubes through which seawater
is being pumped. As the vapor gives up its superheat
(sensible heat) to the seawater, the temperature of the
vapor drops to the condensing point. The refrigerant,
now in liquid form, is subcooled slightly below its
condensing point. This is done at the existing pressure
to ensure that it will not flash into vapor.
A water-cooled condenser for an R- 12 refrigeration
system is shown in figure 10-7. Circulating water is
obtained through a branch connection from the fire main
or by means of an individual pump taking suction from
the sea. The purge connection (fig. 10-7) is on the
refrigerant side. It is used to remove air and other
noncondensable gases that are lighter than the R-12
Most condensers used for naval refrigeration plants
are of the water-cooled type. However, some small units
have air-cooled condensers. These consist of tubing
with external fins to increase the heat transfer surface.
Most air-cooled condensers have fans to ensure positive
circulation of air around the condenser tubes.
The receiver (fig. 10-8) acts as a temporary storage
space and surge tank for the liquid refrigerant. The
receiver also serves as a vapor seal to keep vapor out of
the liquid line to the expansion valve. Receivers are
constructed for either horizontal or vertical installation.