Control and optimization of central chilling systems - Intelligent Buildings and Building Automation - page 199

load increases, the flow in the secondary system will exceed that in the pri-
mary system and cause a reverse flow direction in the common pipe, then
another chiller and primary chilled water pump will be sequenced online
by the sequence controller. A chiller is sequenced off when the water in the
bypass pipe flows from supply to return and the flow exceeds the design
flow of one chiller. Similarly, the flow direction and the value in the bypass
pipe provides good reference information on building load only, and chiller
sequence control based solely on the flow in the bypass pipe hardly provides
precise and reliable control in practical applications.

9.7.4 Direct power-based sequence control

The simplest indicator of the simultaneous chiller cooling load is the percent-
age of full-load amperage of the compressor motors, not because it is such a
straightforward measure of the chiller load but because it is generally avail-
able on the chiller control panel and a reliable measurement. Correlating this
percentage of full-load amperage to the chiller cooling load is complicated
as it relates to the influence of the power factor, part-load chiller efficiency
and the capacity change due to the change of condenser and evaporator
water temperatures. The percentage of full-load motor amperage tends to
over-predict the actual load because the actual operation conditions may be
more favourable than those of the design. For example, at 60 per cent of
full-load amps the real chiller load might be only 55 per cent and at 30 per
cent of full-load amps the real chiller load might be only 20 per cent. But the
controller would think the instantaneous cooling load to be 60 per cent and
30 per cent, respectively. Thus the overestimation may sequence an additional
chiller on earlier than it should be and hence more energy is consumed. This
direct power-based sequence control recommends that the lead chiller be
driven close to 100 per cent load before another chiller is brought online.
In recent years, the cost of power (kW) meters is much lower and direct
power measurement is used as the regular instrumentation of many building
automation systems. That means the power can be measured directly and
accurately, which can be used for chiller sequence and other optimal con-
trols more precisely. The indirect measurement ( Q in ) of total cooling load of
multiple ( n ) chillers can be computed using an inverse model of individual
chillers as shown in Equation (9-5):

n

= ∑ 1

Qf P = ( , T , T ) (9-5)

in i com i , cd i , ev i ,

i

where f i (·) is the i th chiller inverse model, P com,i is the i th chiller power con-
sumption, T cd,i is the i th chiller condensing temperature, and T ev,i is the i th
chiller evaporating temperature. Having the measured total cooling load, the
number of chillers required can be determined by the total cooling load-based
sequence control method as described in the following section.