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TRG-TRC010-EN 37
period five
Application Considerations
notes
Condensing Temperature Control
To achieve stable compressor unloading over a wide range of conditions, a
reduction in condensing pressure (condenser relief) must accompany a
reduction in load.
The starting point (A) is the full-load operating point. As the chiller load
decreases, the flow rate of refrigerant vapor through the compressor also
decreases. In turn, the pressure difference between the evaporator and the
condenser moves the operating point downward toward B.
If the condenser pressure had been controlled to a constant value instead, the
compressor would have unloaded along a nearly constant pressure line toward
C. This would result in a greatly reduced range of operation.
Condenser relief is, however, only beneficial to a certain point. ALL chillers
require a minimum pressure difference between the evaporator and condenser
to ensure proper management of oil and refrigerant. This minimum pressure
difference depends on the chiller’s design and controls. The most common
method of maintaining this pressure difference at various load conditions is to
control the condensing temperature by varying the temperature or flow rate of
water through the condenser. By controlling condensing temperature, most
centrifugal water chillers can start and operate over a wide range of conditions.
Controlling condensing temperature: (1) maintains chiller efficiency, (2)
maintains the required pressure differential between the evaporator and
condenser for controlled flow through the refrigerant metering system, and
(3) prevents the pressure imbalance that could cause oil loss problems.
Condensing Temperature Control
25
25
75
75
10
10
14
14
63
63
pressure difference
pressure difference
refrigerant flow rate
refrigerant flow rate
s
u
r
g
e
s
u
r
g
e
90
90
36
36
51
51
%
$
&
vane position
vane position
(degrees)
(degrees)
unloading line
unloading line
Figure 52
trgtrc010_book.book Page 37 Wednesday, September 1, 1999 10:42 AM