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low water flow, sensed by low-flow switch;
high compressor discharge pressure, sensed by high-pressure switch;
high motor bearing or winding temperature, detected by high-temperature
switch;
low oil pressure.
Another interlock system guarantees that the following components are
operating upon starting the compressor: water pump, oil pump and water
to oil cooler. The suction vane usually has an interlock to be sure that it is
completely closed when the compressor stops.
9.3 Chillers and central chilling system confi gurations
9.3.1 Chillers using different heat-rejection methods
According to the actual environmental constraints of the buildings, three meth-
ods are normally adopted by chillers for their condenser heat rejection:
water-cooled system;
air-cooled system;
cooling tower system.
When a site is near a sea or a lake, the sea water or lake water can be used.
The limited fresh water resources and government restrictions on the use of
cooling towers in the past have resulted in Hong Kong using sea water cooling
as one of the popular heat-rejection methods for chilling systems in buildings.
Direct sea water cooling and indirect sea water cooling are two options. In
indirect sea water-cooled systems, the sea water cools the condenser indirectly
by passing through a sea water heat exchanger where the condenser cooling
water is cooled. In direct sea water-cooling systems, the sea water cools the
condenser directly by passing through the pipelines of the condenser. In this
case, efficiency is generally high as an intermediate heat exchanger is omitted.
However, the condenser itself should be made of special materials.
Air-cooled heat-rejection systems are used widely in small and middle-sized
chiller systems and heat pump systems, especially in transportation refrigera-
tion systems. In Hong Kong, it has been the normal system utilized when a
building is remote from the seafront. An air-cooled condenser is an air-to-
refrigerant heat exchanger, where air absorbs the heat from the refrigerant
directly without any mass transfer between them. The air-cooled condenser
is preferred in areas where the available water source is limited.
Usually, fans are used to enforce the heat transfer between the air and
the refrigerant in the condenser. The air flow rate through the condenser
affects chiller performance while the air temperature is not controllable. The
increase of air flow rate (fan speed) will reduce the condensing temperature
of chillers and therefore reduce the power consumption of chillers.
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