Civil Engineering Reference
In-Depth Information
Four types of chillers are normally used in centralized chilling systems:
reciprocating chillers, rotary-screw chillers, centrifugal chillers and absorp-
tion chillers, where the first three types belong to the category of vapour
compression chillers, which are discussed in this chapter. The energy-saving
potential of central chilling systems depends on operating conditions, space
constraints and the willingness of the building owners to pay for energy-
saving features and measures.
9.1.2 Basic working principles
A vapour compression chiller removes an amount of heat (
Q
e
) from the
chilled water and invests an amount of work (
W
) to deliver a quantity of
heat (
Q
c
) to the environment.
In the ideal pressure-enthalpy cycle of the chiller, there are two isothermal
and two adiabatic processes as shown in Figure 9.1. As the refrigerant goes
through the expansion valve, the high pressure sub-cooled refrigerant liquid
becomes the liquid-vapour mixture at a low pressure and low temperature.
As it goes through the evaporator, this becomes a superheated low-pressure
vapour stream. Then, in the compressor, the pressure and the temperature of
the refrigerant vapour are increased. Finally, this high-temperature vapour
is condensed at a high pressure. The adiabatic processes take place in the
expansion valve and the compressor. The isothermal processes take place in
the evaporator and the condenser. The isothermal processes in this cycle are
also isobaric since they occur at constant pressures. The refrigerant carries
the heat from a low to a high temperature level.
The coefficient of performance (
COP
) of a chiller is defined as the ratio
between the heat removed from the process (
Q
e
) and the work (
W
) required
to achieve this heat removal, as shown in Equation (9-1). The COP is
Refrigerant rejects
heat to environment
Q
c
Pressure
e
Condenser
P
c
Compressor
Valve
Refrigerant absorbs
heat from load
Evaporator
P
e
Q
e
Enthalpy
Figure 9.1
Schematic of the ideal refrigeration cycle.
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