Page 263 - HVAC Pump Handbook

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Configuring an HVAC Water System

260 The HVAC World

necessarily the most efficient point to change the number of operat-
ing pumps. This curve demonstrates conclusively that the old tradi-
tion of just running one pump until it cannot pump any more is a
wasteful practice.
It is obvious that the system head curves and areas described ear-
lier in this chapter are affected by this variation in pump fitting
losses. How do designers incorporate pump fitting losses into their
calculations for a proposed system? It is difficult unless a computer
program is available for system friction loss calculation that includes
such fitting loss calculations. The energy evaluations that will be
reviewed in various chapters account for pump fitting losses. These
evaluations separates the pump fitting losses from system losses to
achieve a reasonable procedure for estimating system performance
and sequencing of pumps on multiple-pump systems.
System flow and head evaluation are very important matters, since
efficient pump selection depends on them. The preceding discussion
demonstrates that it can be a complicated subject. Any time devoted
to such an evaluation is well worth its cost.
One important factor that emerges from true system evaluation is
the indication of losses that are caused by energy wasters such as bal-
ance valves, pressure-reducing valves, and the like. Their losses
become pronounced under this type of careful scrutiny. It should be
noted that most of the HVAC water systems that are reviewed in this
book seldom use these devices.
The advent of computer software has enabled the designer to
achieve better part-load evaluation as well as a closer estimate of the
true diversity of the system under examination. Many different hypo-
thetical loads can be inserted as data in the computer by the designer,
who can then select the design that most closely fits the economic
parameters of the proposed system.

9.4 Static Pressure
The static pressure of an HVAC system is the system pressure that
avoids (1) drying out the tops of the water system or (2) imposing too
high a system pressure on the water system. Basically, the static pres-
sure for most systems consists of the height of the building plus a
cushion at the top of the building. Usually this cushion is around 5 to
2
10 lb/in or 10 to 20 ft. With a 200-ft-tall building and a cushion of 20
ft on top of the building, the static pressure would be 200 20, or 220
ft minimum (95.2 psig at 60°F). When the water system is totally
within a building, the actual elevation above sea level of the building
can be ignored.

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