Handbook of air conditioning and refrigeration / Shan K

System Characteristics
WATER SYSTEMS
Chemical Feeding. Improper chemical feeding causes operating problems. A water treatment program
with underfed chemicals results in an ineffective treatment, whereas an overfed program not
only increases the operating cost but also may cause environmental problems. Generally, a continuous
feeding of very small amounts of chemicals often provides effective and economical water
treatment.
Closed water systems, including hot, chilled, and dual-temperature systems, can be characterized as
follows:
Constant or Variable Flow. A constant-flow water system is a system for which the volume flow
at any cross-sectional plane in the supply or return mains remains constant during the operating
period. Three-way mixing valves are used to modulate the water flow rates to the coils. In a variable-flow
system, all or part of the volume flow varies when the system load changes during the operating
period. Two-way valves are used to modulate the water flow rates to the coils or terminals.
Direct Return or Reverse Return. In a direct-return water system, the various branch piping circuits,
such as ABGBA and ABCFGBA, are not equal in length (see Fig. 7.9a). Careful balance is often
required to establish the design flow rates for a building loop when a direct-return distribution
loop is used as described in later sections. In a reverse-return system, the piping lengths for each
branch circuit, including the main and branch pipes, are almost equal (see Fig. 7.9b).
FIGURE 7.9 Direct-return and reverse-return water systems. (a) Direct-return; (b) reverse-return
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