17
1.7 Subcoolers
The subcooler in these systems is controlled through the
use of a single TXV (thermal expansion valve) that is fed
from a 'branch' off of the main liquid line. A small amount
of liquid refrigerant is expanded through the TXV (into the
subcooler / economizer) to cool the remaining liquid
refrigerant. The suction gas from that work is typically
superheated ~ 25°F (TXV adjustment) then flows from
the subcooler into the interstage section of the 06CC
model compressor, providing some (or all) of the de-
superheating needed for the refrigerant gas entering the
motor compartment. A normally closed liquid line sole-
noid valve must be installed prior to the subcooler TXV.
The solenoid valve must be controlled to close when all
of the compressors are OFF.
The subcooler must be connected in a 'parallel-flow' con-
figuration to reduce the potential for excessively super-
heated suction gas returning from the subcooler to the
interstage connection of the compressor. Highly super-
heated gas entering the interstage can cause TXVs to
operate in an unstable manner. Variation in condensing
pressures (as seen in air-cooled systems) will affect inter-
stage pressures in the system and may result in varying
liquid temperatures leaving the subcooler.
1.8 Subcooler Selection
Two-stage systems have the inherent benefit of being
able to utilize interstage subcooling and de-superheating
through the use of a subcooler. The application of a liquid
subcooler is strongly recommended for all Carlyle
®
Com-
pound Cooling 06CC models. Providing liquid subcooling
from a second compressor group is not needed and not
recommended with 06CC model compressors. Shown in
Fig. 7 is a diagram of a subcooler cycle. The liquid refrig-
erant exiting the condenser is routed to the subcooler
and sub-cooled for use in the evaporator(s). To accom-
plish this task, a small quantity of the same liquid refriger-
ant is taken from the main liquid line (prior to entering the
subcooler). This liquid is expanded into the subcooler to
reduce the overall liquid temperature of the system. This
subcooling method allows for the work to be performed at
the higher interstage pressure, which is more efficient,
resulting in increased compressor capacity and EER
(energy efficiency ratio). A tap off the main liquid line is
directly expanded across the subcooler from condensing
pressure to interstage pressure. The subcooling is done
at interstage pressure where the refrigerant can be com-
pressed more efficiently, therefore increasing the com-
pressor capacity and energy efficiency ratio.
Solenoid
Valve
TXV
Subcooler
TXV
Bulb
Subcooler Liquid
to Evaporate
To Compressor Interstage
(Economizer Connection)
Main
Condenser
Line
Fig. 7 — Subcooler Cycle
