Page 229 - HVAC Pump Handbook

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The Use of Water in HVAC Systems

226 The HVAC World

sized with a low velocity in the tubes and low friction loss, laminar
flow develops and excessive water flow results. The designer must
balance the selection of coils between high water pressure loss and
laminar flow. The flow of water in the coil should not pass into the
laminar range at any known load on the coil.
How well we use hot or chilled water will determine the efficiency
of operation of the entire water system. Hot and chilled water systems
are described together here, because there is so much similarity
between hot water and chilled water coils in their use and in their
connections.
Hot and chilled water coil connections must be studied in detail to
ensure that the supply water is being used efficiently. There are a
number of ways to connect these coils, and many are being advocated
in the HVAC industry. Some of them are being sold to solve a particu-
lar system problem.
The system problems that are being addressed with these connec-
tions are:

1. Laminar flow in coils
2. Dirty coils—water side, air side, or both
3. Improperly sized coil control valves or valve actuators
4. Poorly constructed control valves

8.3.1 Three-way valves for hot and chilled water coils
When three-way valves were used predominantly on these coils, the
systems were constant flow, and there was little concern for energy
savings. The three-way control valve wastes energy by bypassing the
supply water around the coil, as shown in Fig. 8.5a. On variable-
volume chilled water systems, this has a negative effect on the
chillers, since the return temperature to the chillers is reduced. The
chillers reach rated water flow long before they achieve design load in
tons. This forces more than one chiller to operate when the cooling
load is less than the capacity of one chiller.

8.3.2 Two-way valve with circulator
Recognizing the deficiency of the three-way valve, efforts have been
made to control the return water temperature by replacing the three-
way valve with a two-way valve using a circulator on the coil (see
Fig. 8.5b). Laminar flow was eliminated with this arrangement, but
dirty coils could still pass water through the control valve below
design temperature on chilled water systems. The significant defect of
this arrangement is the continuous operation of the coil pump whenever

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