Handbook of air conditioning and refrigeration / Shan K

7.56 CHAPTER SEVEN
loops. However, the pressure characteristics of the supply and return mains of the distribution loop at
reduced flows make it difficult to satisfy various load profiles in different buildings during part-load
operation. Having a variable-speed building pump for each building also saves more pump energy.
Therefore, the use of variable-speed pumps for both distribution and building loops is preferable.
Control of Variable-Speed Distribution Pump. Two types of controls can be used to modulate a
variable-speed distribution pump to transport the required chilled water volume flow to various
buildings:
● A differential-pressure transmitter may be located near the farthest end of the distribution supply
main, as shown in Fig. 7.22a. Theoretically, the head of the building pump should extract the exact
required amount of chilled water corresponding to the sum of the coil loads in the building
loop, force it through the coils, and discharge it to the distribution return main. Therefore, a set
point for the pressure differential of about 5 ft (1.5 m) may be appropriate. This type of control is
widely used.
● A DDC system measures the total water flow that returns from each building by means of
flowmeters and modulates the variable-speed distribution pump to supply exactly the required
amount to various building loops. This type of control is more precise but more expensive.
Building Entrance. Chilled water is usually supplied directly from the distribution supply main
to the building supply main. A pressure throttling valve may be used to offset the excess pressure
differential �H s,r along the distribution loop.
Although using a heat exchanger at the building entrance entirely isolates the chilled water in
the distribution loop from the chilled water in the building loop, a temperature increase of about 3
to 7°F (1.7 to 3.9°C) is required for a chilled water heat exchanger. Because chilled water has a supply
and return temperature differential of only about 15 to 20°F (8.3 to 11.1°C), a heat exchanger is
seldom used at a building or zone entrance for a chilled water system. Because a hot water system
has a greater supply and return temperature differential, a heat exchanger is sometimes used at the
building entrance for a hot water system.
A chilled water building loop may be divided into various zones based on different height levels
within the building. In such an arrangement, the coils in the lower floors of the building loop will
not suffer a high static pressure because the low-level water loop is often isolated from the highlevel
water loop by means of a heat exchanger at the zone entrance.
If a building requires a chilled water supply temperature higher than that given by the distribution
supply main, a crossover bridge with a temperature control valve can be arranged for this purpose,
as shown in Fig. 7.22c.
The return temperature from the coils in a building loop is affected by the cleanliness of the coil,
including air-side cleanliness, and the control system in the building loop. The building’s variablespeed
pump is often controlled by the pressure-differential transmitter located at the end of the
building supply main, as shown in Fig. 7.22a and c.
Plant-Distributed Building Loop
A plant-distributed building loop water system has nearly the same configuration as a plant-distribution-building
loop system except that there is no distribution pump in the distribution loop. Constant-speed
chiller/boiler pumps in the plant loop supply water to the beginning of the supply main
of the distribution loop, point S, and extract water from the end of the return main of the distribution
loop, point R. Various variable-speed building pumps in the building loops extract water from
the distribution main point S. They also overcome the pressure loss of the distribution supply main
piping up to the building entrance, the pressure loss of the building loop �p bg including building
supply and return mains, coils, two-way control valve, and fittings; and the pressure loss of the distribution
return main piping from the building outlet to point R.