Handbook of air conditioning and refrigeration / Shan K

7.40 CHAPTER SEVEN
Variable Flow for Saving Energy
ASHRAE/IESNA Standard 90.1-1999 specifies that water systems having a total pump system
power exceeding 10 hp (7.5 kW) shall include control valves to modulate or step open and close as
a function of load and shall be designed for variable flow for energy savings. Water systems should
be able to reduce system flow to 50 percent of design flow or less. Individual pumps serving variable
flow systems having a pump head exceeding 100 ft (30 m) and motors exceeding 50 hp (37
kW) shall have controls and devices (such as variable speed control) that will reduce pump motor
demand of no more than 30 percent of design wattage at 50 percent of design water flow.
Water Systems in Commercial Buildings
The following types of water systems are currently used in commercial buildings in the United States:
● Plant-through-building loop: bypass throttling flow
● Plant-through-building loop: distributed pumping
● Plant-building loop
● Plant-distributed pumping loop
● Plant-distribution-building loop
● Plant-distributed building loop
● Multiple sources-distributed building loop
Since the 1960s, one of the old, energy-inefficient water systems, a constant-flow system using
three-way valves which has constant flow in its chiller/boiler and its supply and return mains, is
gradually being replaced by a water system that is equipped with cheaper and more effective twoway
valves and uses energy-efficient variable flow for distribution. Constant-flow systems using
three-way valves are not discussed here.
7.10 PLANT-THROUGH-BUILDING LOOP
Bypass Throttling Flow
In a plant-through-building loop system, water is transported only by plant (chiller/boiler) pump(s)
or by distributed pump(s). Plant-through-building loops can be classified into three categories: bypass
throttling flow, distributed pumping, and variable flow.
A plant-through-building loop water system using bypass throttling flow is one of the older
chilled/hot water systems that has been adopted in commercial buildings since the use of two-way
control valves. For each chiller/boiler, a corresponding plant constant-speed water pump is
equipped as shown in Fig. 7.9a. The chilled or hot water is supplied to the coils and terminals
through the supply and return mains and branches, and is then returned to the chiller/boiler. There is
a crossover bridge that connects the supply and return mains at junctions P and Q. A bypass twoway
control valve is often installed on the crossover bridge. A pressure-differential transmitter and
a pressure relief valve are used to maintain a set pressure differential across the supply and return
mains by modulating the opening of the bypass two-way control valve when the system pressure