Control and optimization of central chilling systems - Intelligent Buildings and Building Automation - page 197

9.7.2 Temperature-based sequence control

As mentioned earlier, the chilled water distribution network is often divided
into two circulation loops: the primary loop and the secondary loop. There
is a set of chilled water pumps in each loop and the two loops are hydrauli-
cally decoupled by a bypass pipe (Figure 9.18). The pumps ensure that the
particular differential pressure in each loop is sufficient to deliver the required
water flow.
For temperature-based sequence control, a water temperature sensor is
installed at the main chilled water return pipe, downstream of the bypass
line, which transmits the temperature signal to the sequence controller. The
chilled water supply temperature is controlled at a constant set-point by the
chiller internal control. When the chilled water return temperature exceeds
the upper limit of the control band, an additional chiller will be sequenced
online. When the chilled water return temperature drops blow the lower limit
of the control band, the running chillers will be switched off, one by one.
When the chilled water return temperature stays within the control band, no
further change to the status of on and off stages will occur.
In chiller systems, the chilled water pumps are normally switched on and
off together with their interlocked chillers so that the chilled water flow rate
in the central chilled water plant varies in steps according to the number
of chillers online. The temperature-based sequence control relies on the
proper operation of the differential pressure bypass control which makes
sure that the chilled water return temperature is a reasonable indicator of

Modulating valve

Coil

Coil

Chilled
water return

T

Bypass

Chilled
water
supply

Constant speed pumps

Chiller

Chiller

Sequence
controller

Chiller

Figure 9.18 Chiller sequence control based on the chilled water return temperature.