Carrier 5H - Manual (Page 6)

AUTOMATIC PUMPDOWN CONTROL (Fig. 2) -
Pumpdown control is the most effective means of
compressor control in keeping liquid refrigerant
out of the crankcase on system shutdown.
In the basic pumpdown control sequence the
thermostat controls the liquid line solenoid valve
to stop or start the flow of refrigerant to the evap­
orator as required.
The pumpdown control system permits com­
pressor cycling if a system malfunction allows the
low side pressure to rise. Although this cycling
is sometimes considered objectionable by the cus­
tomer, it does promptly point out the need for
maintenance attention and provides positive pro­
tection against liquid refrigerant accumulating
in the compressor crankcase.
Pumpdown control should not be used with dry
expansion coolers as it may cause frost pinching
or freeze-up.
SINGLE PUMPOUT CONTROL - Pumpout control
is not as positive as pumpdown control in keeping
liquid refrigerant out of the crankcase. However,
it is usually satisfactory when used with crankcase
heaters if pumpdown is not acceptable.
Single pumpout control is similar to pumpdown
control, except in the following ways: a pumpout
relay is added, a normally open compressor auxil­
iary contact is necessary, and energizing of crank­
case heaters is definitely required at the end of
each operating cycle.
With single pumpout control, when the thermo­
stat is satisfied, the compressor pumps down once
and stops. It starts again only when the thermostat
calls for cooling. In pumpdown control, the com­
pressor cycles solely on the low pressure switch,
regardless of the demands of the thermostat.
Pumpout control should not be used with dry
expansion coolers as it may cause frost pinching
or freeze-up.
Fig. 2 - Automatic Pumpdown Control
Fig. 3 - Single Pumpout Control
SECTION
PAGE
DATE
5F,H-1XA
6
10-66
S U P E R S E D E S
SECTION 5F,H-1X
PAGES 1-42
DATE 11-63