ClimaCool UCR 30, 50 and 70 tons Manual - Manual (Page 24)

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24
High and Low Ambient Stability
The design of air conditioning systems, utilizing air cooled
condensing units, involves two main problems that must be
solved if the system is to operate reliably and economically
high ambient and low ambient operation. If the condensing
unit is properly sized, it will operate satisfactorily during
extremely high ambient temperatures. However, some units
will be required to operate at ambient temperatures below
their design dry bulb temperature during most of the year;
the solution to low ambient operation is more complex.
Without good head pressure control during low ambient
operation, the system can experience both running cycle
and off -cycle problems. Since the pressure diff erential
across the thermostatic expansion valve port aff ects the
rate of refrigerant fl ow, low head pressure generally causes
insuffi
cient refrigerant to be fed to the evaporator. Failure
to have suffi cient head pressure will result in low suction
pressure and/or iced evaporator coils. The primary off -
cycle problem is refrigerant migration to the condenser.
Insuffi
cient fl ow through the TXV will cause a low suction
pressure.
The typical method of maintaining normal head pressure
in a refrigeration system during periods of low ambient
temperature is to restrict liquid fl ow from the condenser
to the receiver, and at the same time divert hot gas to the
inlet of the receiver. This backs liquid refrigerant up into
the condenser reducing its capacity which in turn increases
the condensing pressure. At the same time the hot gas
raises liquid pressure in the receiver, allowing the system to
operate normally.
LAC-10 Valve Operation
The valve designation LAC stands for Low Ambient Control.
The LAC-10 is a three-way modulating valve that responds
to receiver pressure. As shown in Figure 1, the receiver
pressure acts under the diaphragm. As the receiver pressure
drops below the valve setting, the seat moves away from
the discharge port allowing discharge gas to bypass the
condenser. This discharge gas warms the liquid in the receiver
and raises the pressure to the valve setting. At the same time
discharge gas is bypassing the condenser, liquid fl ow from the
condenser is restricted, which allows liquid to back up in the
condenser. Flooding the condenser reduces the area available
for condensing thus raising the condensing pressure. During
summer conditions, the seat closes the discharge port due
to high pressure in the receiver. Therefore, there is full liquid
fl ow from the condenser to the receiver.
Refrigerant Migration
During an off cycle there is a potential for refrigerant to
migrate from the warm receiver to the cold condenser.
An auxiliary check valve should be used in the liquid line
between the LAC valves and the receiver to prevent this from
occurring. See Figure 2.
Piping Suggestions
The piping schematic shown in Figure 2 is only to illustrate the
general location of the head pressure control valves in the
system, refer to the Refrigeration Circuit Diagram (See page
51) for more information. Note that the schematic shown on
Figure 2 refl ects the use of two LAC-10 valves required per
circuit on 50 ton and above models, only one LAC-10 valve is
required per circuit on 30 ton and below models.
Head Pressure Control Valve Operation (LAC-10)
Figure 1
Figure 2