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CHILLER HEAT RECOVERY SYSTEM
A chiller heat recovery system uses heat rejected from a
chiller to satisfy a simultaneous heating load. To accomplish
this the heating water design temperature becomes the
condenser water design temperature for the chiller. A typical
simultaneous cooling and heating load includes an interior zone
cooling load and a perimeter zone heating load. When the
cooling load is greater than the heating load, the excess heat is
dissipated in a cooling tower. The control strategy is to use as
much heat as needed from the condenser and to reject excess
heat to the cooling tower when necessary.
Figure 32 shows a heat recovery system and control sequence
for a cooling system with one chiller and a double bundle
condenser where chiller capacity is controlled from the chilled
water supply temperature. The double bundle condenser uses heat
recovery water or cooling tower water or both to cool refrigerant.
Cooling tower water cannot be mixed with heat recovery water
so the two systems must be isolated. In a single chiller system,
the control system checks the water temperature coming out of
the heat recovery bundles (T3). If it is warmer than the water
returning from the heating loads (T2), valve V1 opens to circulate
condenser water through the system. A hot water generator (boiler
or converter) provides additional heat as required. If the heat
recovery system is not cooling the condenser sufficiently, the
cooling tower is used to dissipate excess heat. In multiple chiller
systems, the heat recovery chiller is controlled by the heating
load (T1). The other chillers provide additional cooling as required.
FOR MULTIPLE CHILLERS
THIS IS REPLACED
BY CAPACITY
CONTROL OF HEAT
REJECTION CHILLER
COOLING
TOWER
DOUBLE
BUNDLE
CONDENSER
T3
V1
HEAT
V2
SOURCE
HOT WATER
GENERATOR
Fig. 32. Heat Pump Cycle Controls and Sequence.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS
TEMPERATURE
AT T1 ( C)
ACTION
43
OFF
IF T3 > T2,
THEN T1 CONTROLS V1
(CONDENSER HEAT)
ON
40.5
OPEN
OA DAMPER
COOLING
CLOSED
38
CLOSED
V2 CONTROLLED
BY T1
OPEN
35
REFRIGERANT CIRCUIT
(REMOVING HEAT FROM
INTERIOR ZONES)
FOR MULTIPLE CHILLERS
THIS IS REPLACED BY
CAPACITY CONTROL OF
T
HEAT REJECTION CHILLER
T2
SHUT OFF OA
COOLING ON
FAN UNITS
T1
LOAD
C4088
If chilled water reset is used, stop the reset action when the
cooling tower is off. This provides recovery system heat to a lower
outdoor temperature before it is necessary to use fuel for heating.
FREE COOLING-TOWER COOLING
When the condenser water temperature from an evaporative
cooling tower is equal to or lower than chilled water load
requirements, mechanical refrigeration is not needed. During
these times "free cooling" from the evaporative cooling tower
is available without having to supply energy to the compressor.
Figures 33, 34, and 35 show three methods of providing tower
cooling to the chilled water circuit.
In refrigerant vapor migration (Fig. 33) two refrigerant paths
with migration valves are added between the condenser and
the evaporator. The paths bypass the compressor and allow
gravity flow of liquid refrigerant from condenser to evaporator
and vapor flow from evaporator to condenser. Refrigerant
vaporizes in the evaporator and migrates to the condenser where
it is condensed by cooling tower water. The liquid then flows
back to the evaporator by gravity. The chiller must be designed
so the flow paths are unrestricted and the evaporator is lower
than the condenser. The bypass valves are normally included
as a chiller package option by the manufacturer.
COOLING
TOWER
CONDENSER
TOWER
WATER
HEATER
PUMP
(OPTIONAL)
CONDENSER
COMPRESSOR
EVAPORATOR
CHILLED
WATER
PUMP
Fig. 33. Free Cooling Using
Refrigerant Vapor Migration.
323
REFRIGERANT
MIGRATION
VALVES
LOAD
C2697
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