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Configuring an HVAC Water System

Configuring an HVAC Water System 249

analysis, as well as during the piping and control design. Computer-
assisted evaluation allows the designer to study the many load varia-
tions that can occur on the prospective water system.
Chapter 3 provided the means to calculate pipe friction in HVAC pip-
ing systems. This chapter will use those data to demonstrate how the
head changes in a water system as the flow varies in that system. It
should be remembered that most hot and chilled water systems are
actually a number of small systems using common supply pipes. Each
heating or cooling coil is a water system in itself, since each coil has a
different total friction head. These friction loads for a specific coil consist
of the supply main loss, branch main loss, and loss through the coil itself
with its piping and control-valve losses. If the designer recognizes this
individuality of every coil and seeks to design the water system with
recognition of this fact, a simpler and more efficient system will result.
Traditionally, the pump head required for a water system has been
shown as a curve that results when the system head in feet is plotted
against the system flow in gallons per minute. Therefore, this curve
has been named the system head curve. Through many years of work
with HVAC water systems, it has been demonstrated that the head
requirements for many water systems cannot be represented by this
simple parabolic curve. Instead, the head varies through a broad area
that may be difficult to calculate. Before tackling this knotty problem,
the elements of a system head curve or area should be reviewed.

9.3.1 System head curve components
A system head curve consists of plotting the flow in the system, in gal-
lons per minute, horizontally and the system head vertically, in feet of
head. The system head must be broken down into variable head and
constant head. Variable head is the friction head of the water distribu-
tion system. Constant head can be divided into either static head or
constant friction head. Static head is simply raising water from one
level to another. Typical of this is the height of a cooling tower.
Constant friction head can be the loss across a heating or cooling coil,
its control valve, and connecting piping, as shown in Fig. 9.3a. This
constant head is typified by the differential pressure maintained
across a heating or cooling coil and its appurtenances at a constant
value by a differential pressure transmitter. This is the signal used to
control the system to which the coil is connected. Constant friction
head occurs in many systems with two-way temperature-control
valves on the coils; it is not found in old systems that used three-way
temperature-control valves. Constant friction head is plotted vertically
similar to static head of a cooling tower. A typical system head curve is
that for a variable-volume system with two-way temperature control
valves or a system head curve for a cooling tower installation.

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