Control and optimization of central chilling systems - Intelligent Buildings and Building Automation - page 198

the simultaneous cooling demand load. However, the situation in practical
operation is very complex, and chiller sequence control based solely on the
chilled water return temperature cannot provide precise and reliable control
in practical applications.

9.7.3 Bypass fl ow-based sequence control

There is normally a set of pumps in the primary and secondary chilled water
loops respectively. The two loops are hydraulically decoupled by a bypass
pipe, which forms the common part of the two loops. A schematic diagram
of such a piping network is shown in Figure 9.19.
The chilled water flow rate in the secondary loop is somehow proportional
to the total cooling load demand of a building. The flow rate in the primary
loop, however, will only vary in steps with the number of operating chillers.
The decouple bypass pipe allows any flow difference between the two loops
to flow through it, thus preventing flow rate fluctuation in one loop from
affecting that of the other loop.
This strategy sequences chillers (and associated pumps in the primary
loop) attempting to keep the primary system flow larger than the secondary
system flow. In this way, the chilled water supply temperature in the second-
ary system is equal to the chilled water supply temperature in the primary
system. Flow is often sensed by a flow meter in the common bypass pipe to
indicate the surplus or deficit flow rate and the flow direction is sensed by a
flow direction detection switch as shown in Figure 9.19. When the cooling

Modulating valve

Coil

Coil

Pumps
surplus flow deficit flow

S

M

Bypass pipe

Flow switch

Constant speed pumps

Chiller

Chiller

Sequence
controller

Chiller

Figure 9.19 Chiller sequence control based on the flow in the bypass pipe.