Civil Engineering Reference
In-Depth Information
Modulating valve
Coil
p
Coil
Bypass pipe
DPV
Controller
M
Variable speed
pumps
Chiller
Chiller
Chiller
Figure 9.5
Configuration of variable primary-only pumping system.
This system has two significant disadvantages: the complexity of the
bypass valve control and the complexity of the chiller sequencing control.
Both result in a high risk of system failure unless careful engineering consid-
erations are taken into account.
9.4 Chiller performance and optimal control
9.4.1 Chiller energy performance
The energy consumed by a chiller itself is basically used to transfer the
refrigerant from low pressure (evaporating temperature) to high pressure
(condensing temperature) as illustrated in Figure 9.6. The condensing and
evaporating temperatures establish the refrigerant pressure and thus the
energy needed per unit cooling load (see Figure 9.7).
Considering the efficiency of a chiller alone, the condensing temperature
should be maintained as low as possible and the evaporating temperature
should be maintained as high as possible to minimize refrigerant head which
means less compressor energy consumption. Theoretical improvements of
chiller COP by raising evaporating temperature and lowering condensing
temperature for typical refrigerants under a typical air-conditioning situation
are illustrated in Table 9.1. A decrease of 1 K in condensing temperature or
an increase of 1 K in evaporating temperature can result in an increase of
about 3 per cent in chiller COP.
In reality, the actual effect is influenced by various losses, and the saving
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