Chapter hvac engineering Fundamentals: Part 1 Introduction

19.7
Effects of Altitude
The tables and the chart of Fig. 19.1 are based on a standard atmo-
spheric pressure of 29.92 inHg. The partial pressure of water vapor is
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Engineering Fundamentals: Part 4
483
Figure 19.10
Using the protractor.
a function of temperature only, while the total atmospheric pressure
decreases with altitude. The rule of thumb is that the standard chart
and tables are sufficiently accurate up to about 2000 ft above sea level.
At higher elevations, new tables and charts are needed.
2
High-altitude charts are available from several sources. ASHRAE
publishes charts for 5000 and 7500 ft. The U.S. Bureau of Mines pub-
lishes a
composite
chart for various elevations below sea level, down
to 10,000 ft.
The general effect of increasing altitude is to
expand
the chart (Fig.
19.11). That is, for a uniform grid of enthalpy and humidity ratio, as
the altitude increases (and atmospheric pressure decreases), the lines
defining the other properties change as follows:
1. Dry-bulb temperature lines are unchanged.
2. Wet-bulb temperature lines expand up and to the right.
3. Relative-humidity lines, including saturation, expand up and to the
left.
4. Volume lines expand up and to the right.
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484
Chapter Nineteen
Figure 19.11
Effects of altitude.
5. For a given combination of dry-bulb and wet-bulb temperatures,
the change in relative humidity is very small and for most air con-
ditioning processes can be neglected.
19.8
Summary
This discussion of psychrometrics has been very brief. The subject is
very important to the HVAC designer, and further study of Ref. 1 and
other sources is recommended. Every set of HVAC design calculations
should include one or more psychrometric charts, reflecting the antic-
ipated performance of the system being designed.
References
1. ASHRAE Handbook,
2001 Fundamentals,
Chap. 6, ‘‘Psychrometrics.’’
2. R. W. Haines, ‘‘How to Construct High-Altitude Psychrometric Charts,’’
Heating
/
Piping
/
Air Conditioning,
October 1961, p. 144.
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485
Chapter
20
Engineering Fundamentals: