Bally Refrigerated Boxes BVC Generation C – Direct Drive Air-Cooled – Vertical Air - Manual (Page 23)

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CONDENSER SELECTION
During a condenser selection process, the
application engineer should choose a condenser
which is large enough to reject all of the heat added
to the refrigerant during the refrigerating cycle. When
the condenser is sized to equal the total heat
of rejection (THR) at design conditions, enough heat
will be rejected to maintain the required condensing
temperature. This will ensure that sufficient
refrigeration capacity will be maintained at the
evaporator during the warm summer period when it
is needed the most.
If a condenser is undersized, the condensing
temperature (CT) will be driven upwards. This
naturally occurs as the system seeks its new
balance point. As the CT increases, the operating
temperature difference (TD)
o F the condenser also
increases. Even though the capacity of the
condenser increases with the higher TD, the
refrigerating capacity of the compressor will
decrease due to the higher condensing temperature.
An undersized condenser may perform
satisfactorily when ambient temperatures are
below design, but the overall system capacity will
not be high enough during the warmer periods.
Oversizing a condenser increases project costs and
can also lead to undesirable operating conditions.
Low ambient control devices such as pressure
regulators and fan cycling switches operate to
maintain a sufficient pressure in the condenser
during low ambient periods.
On systems utilizing a receiver and flooding type of
head pressure control, more refrigerant will be
required to flood the condenser in order to achieve
the desired condensing pressure.
Consider an air conditioning system with an
oversized condenser which is only used during the
summer time and does not have any type of head
pressure control. This particular system may
experience problems due to a lack of subcooling.
Since the condenser was oversized the amount of
natural subcooling available is less.
The maximum
amount of natural subcooling possible is the differ-
ence between the condensing temperature and the
ambient temperature. If this amount of subcooling is
not enough to offset the pressure losses in the liquid
line, then flashing is certain to occur.
Flashing produces vapor at the metering device
which was designed to meter 100% liquid. One cure
for this is to apply head pressure control devices to
the system that will increase the head pressure and
ensure adequate liquid subcooling.
UNDERSIZED
CONDENSER
PROPERLY SELECTED
CONDENSER
+95°F +95°F
+30°F -15°F