Handbook of air conditioning and refrigeration / Shan K

For water-cooled centrifugal chillers with hermetic compressors, manufacturers’ products that
are currently available have a cooling capacity of 150 to 2000 tons (527 to 7032 kW). For watercooled
centrifugal chillers with open compressors, the cooling capacity varies from 150 to 10,000
tons (527 to 35,160 kW).
For hermetic compressors using vaporized liquid refrigerant to cool the motor, no additional
motor cooling system is required. Leakage of refrigerant from the bearing seals is also less than that
in the open compressor. Its main drawback is that 2 to 4 percent of the liquid refrigerant must be
used to cool the motor. Its cooling capacity is reduced, and its energy use is increased.
Unfortunately, the IPLV in ARI Standard 550/590-98 is developed to represent the average partload
efficiency of of a single chiller. Most of the chillers are used in multichiller operation. Actual
multichiller operation is different from the single-chiller operation.
13.3 CENTRIFUGAL CHILLERS INCORPORATING
HEAT RECOVERY
System Description
Operating Modes
REFRIGERATION SYSTEMS: CENTRIFUGAL 13.9
Many large commercial applications often require heating in the perimeter zone but cooling in the
interior zone during the winter. If the building is considered as a whole, a heat recovery system can
be used to transfer the internal heat from the interior zone to the perimeter zone to offset the winter
heating load there.
A typical centrifugal chiller incorporating a heat recovery system using a double-bundle condenser
is presented in Fig. 13.5. In a double-bundle condenser, water tubes are classified as tower
bundles and heating bundles. Heat rejected in the condenser may be either discharged into the
atmosphere through the tower bundle and cooling tower or used for heating in the coils of the perimeter
zones through the heating bundle. The tower bundle may be enclosed in the same shell with the
heating bundle, as shown in Fig. 13.5b, but separate plates are needed to divide the water circuits.
A storage tank stores the excessive hot water from the heating bundle when the building is occupied.
Hot water is then used for heating in the perimeter zone when the building is either occupied
or unoccupied. A supplementary heater using, e.g., electric power may be used to raise the temperature
of the hot water from the heating bundle, if necessary. A tower bypass line is always used to
maintain the condenser water entering the condenser at a required value.
Instead of a double-bundle condenser, an auxiliary condenser plus a standard condenser can be
used, as shown in Fig. 13.6. Hot gaseous refrigerant is forced to either the standard or the auxiliary
condenser after it is discharged from the compressor. When heating is required in the perimeter
zone, the auxiliary condenser pump is energized, and the tower bypass three-way valve bypasses
part of or all the condenser water returning to the standard condenser. Because the water returning
from the perimeter zone and entering the auxiliary condenser is colder than that entering the standard
condenser due to the heating load in the perimeter zone, the auxiliary condenser tube bundle
has a lower temperature and attracts the hot gas from the compressor. The auxiliary condenser recovers
as much heat as it can allow. The remaining rejected heat goes to the standard condenser and
is rejected to the atmosphere through the cooling tower. A balance of heat rejection between these
two condensers automatically holds.
A hot water bypass should be included if an auxiliary condenser is used, as shown in Fig. 13.5.
Such a bypass ensures that water will not circulate through the auxiliary condenser when the chiller
is deenergized. In Fig. 13.5a, the hot water pump P2 is energized only when the compressor of the
centrifugal chiller is in operation.
Depending on whether heating is required in the perimeter zone, the operating modes of a centrifugal
chiller that is incorporated with a heat recovery system in a building with perimeter and interior