Civil Engineering Reference
In-Depth Information
9.7.5 Total cooling load-based sequence control
In the above sequence control schemes, measurements of cooling load demand
of a building are performed by monitoring some indicators of cooling load
(i.e. the chilled water return temperature and the water flow in the bypass
pipe, etc.) and therefore are indirect measurements of the cooling load. The
sequence control based on the chilled water return temperature relies on the
assumption that the chilled water in the secondary loop is constant and well
controlled between each step, which actually is affected by the uncertainty of
flow control. The sequence control based on the flow in the decoupled bypass
pipe relies on the assumption that the
T
of the chilled water to and from
the building is constant. However, these assumptions are hardly maintained
in practice. For example,
Δ
T
tends to decrease below the design level for a
variety of reasons, which results in more partially loaded chillers online. In
addition, the above two control methods are performed according to some
pre-set level of temperature or water flow rate regardless of the operating
characteristics of the online chillers under different operating conditions.
In order to operate the chilled water plant in an efficient way, a direct
way of measuring the cooling load is often adopted nowadays. The instan-
taneous cooling load can be determined through measuring the total flow
rate of chilled water in the secondary loop and the difference between the
chilled water supply and return temperatures in the secondary loop. The
product of the two signals is then proportional to the total building load
(
Q
b
=
Δ
ρ
w
Cp
w
M
w
Δ
T
), which should be offset by the chilled water plant.
Chilled water
return pipe
M
T
Central
chiller plant
T
Chilled water
supply pipe
T
Q
Sequence
controller
Sequencer
Switch on/off signals for
chiller/pump
Figure 9.20
Schematics of chiller sequence control based on total cooling load.
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