Page 19 - HVAC Pump Handbook

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Physical Data for HVAC System Design

16 The Basic Tools

The atmospheric pressure of outdoor air varies with the altitude of
the installation of HVAC equipment and must be recognized in the
rating of most HVAC equipment. Table 2.1 describes the variation of
atmospheric pressure with altitude. This table lists atmospheric pres-
sure in feet of water as well as pounds per square inch. For water tem-
perature in the range of 32 to 85°F, the feet of head can be used directly
in the net positive suction head (NPSH) and cavitation equations
found in Chap. 6 on pump performance. For precise calculations and
2
higher-temperature waters, the atmospheric pressure in lb/in ab-
solute must be corrected for the specific volume of water at the operat-
ing temperature. See Eq. 6.10, which corrects the atmospheric pres-
sure in feet of water to the actual operating temperature of the water.

2.3 Thermal Equivalents
There are some basic thermal and power equivalents that should be
summarized for HVAC water system design. This book is based on
1 Btu (British thermal unit) being equal to 778.0 ft lb (foot pounds).
Other sources list 1 Btu as equal to 778.0 to 778.26 ft lb, which re-
sults in different thermal equivalents. For example, the ASHRAE
Systems and Equipment Handbook lists 1 Btu as equal to 778.17 ft lb,
while Keenan and Keyes’s Thermodynamic Properties of Steam de-
fines 1 Btu as 778.26 ft lb. The following thermal and power equiva-
lents will be found in this book:
1 Btu (British thermal unit) 778.0 ft lb

1 brake horsepower, bhp 33,000 ft lb/min

1 brake horsepower hour, bhph 2545 Btu/h
0.746 kWh (kilowatthour)

1 kWh 1.341 bhp

3412.0 Btu/h

2.4 Water Data
Water is not as susceptible to varying atmospheric conditions as is air,
but its temperature and quality must be measured. Standard water
temperature can be stated as 32, 39.2 (point of maximum density), or
60°F. It is not very important which of these temperatures is used for
HVAC pump calculations, since water has a density near 1.0 and a
viscosity around 1.5 cSt (centistokes) at all these temperatures.

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