Page 264 - HVAC Pump Handbook

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Configuring an HVAC Water System

Configuring an HVAC Water System 261

Such is not the case with a campus-type installation consisting of sev-
eral buildings. With this application, actual elevations of the buildings
must be included. The elevations of the tops of all the buildings must
be recorded as well as the elevation of the expansion tank at the oper-
ating level in the central plant. Static pressure is the elevation at the
top of the highest building minus the elevation at the operating level
of the central energy plant plus the required cushion at the top of the
tallest building. That is,
Z Z Z p
c
b
Static pressure psig (9.4)
(144/ )
where Z elevation of the top of the tallest building
b
Z elevation of the operating floor of the central energy plant
p
Z cushion required on top of that building in ft of head
c
specific weight of the water at the operating temperature
For example, assume that the top of the tallest building is 650 ft,
the desired cushion on that building is 10 ft, the operating level of the
central energy plant is 420 ft, and the system is a hot water system
operating at 180°F (60.57 lb/ft ). Thus,
3
650 10 420
Static pressure 101 psig
(144/60.57)
Distributed pumping, which relies on static pressure for movement
of the water in the system, requires special calculation of the static
pressure. The remote building whose height, cushion, and distribu-
tion loss are the greatest determines the static pressure. Equation 9.4
becomes
Z Z Z H d
b
p
c
Static pressure (9.5)
(144/ )
where H distribution loss in feet H O from central plant to the
d 2
building that determines the static pressure
If the building in the preceding example were the building that
determines the static pressure for the system and the friction loss
between it and the central energy plant was 50 ft at design flow, then
the static pressure would be

650 10 420 50
122 psig
(144/60.57)

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