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4 HVAC Fundamentals:
Psychrometrics
The term moist air is used to emphasize the importance
of both dry air and water vapor in the practice of
HVAC design. Psychrometrics deals with the thermodynamics
of moist air. Although air is made up of a variety
of components, the properties of moist air can be adequately
addressed by considering only the two primary
components, dry air and water vapor. The amount of
moisture in air is very small, and properties are presented
per unit mass of dry air.
FUNDAMENTALS OF MOIST AIR
An important step in the analysis and design of
HVAC systems is to determine the properties of air in
order to provide indoor air quality (IAQ) that promotes
occupant comfort and health. Analysis must include
consideration of the mixture of dry air and varying levels
of moisture. This section reviews fundamental ideal
gas concepts and introduces moist air terminology,
equations, and tools from the ASHRAE Handbook—
Fundamentals (ASHRAE 2005). This chapter summarizes
the equations used to develop the fundamental
graphical tool for moist air analysis and design, the psychrometric
chart. The use of one or more of three
options is allowed:
1. Analysis and design using the psychrometric chart.
2. Analysis and design using packaged psychrometric
software available from vendors or manufacturers.
3. Analysis and design using the psychrometric
spreadsheet (PsychProcess.xls) on the CD that
accompanies this text. This program is in open code
and can be modified to suit additional applications.
Dry air is primarily nitrogen (~78%), oxygen
(~21%), argon (~1%), carbon dioxide (300–400 ppm), 1
and traces of other gases. It has a molecular weight
1. The concentration of carbon dioxide (CO 2 ) in parts per million
(ppm) in outdoor air is important since it has been
widely used in combination with indoor CO 2 concentration
as an indicator of adequate ventilation air.
(MW) of 28.96 lb/lb⋅mole, and the gas constant for air
(R a ) can be determined from the universal gas constant
(R).
R 1545 ( ft ⋅ lb
R a ---------
f ⁄ lb mole ⋅ °R)
= = --------------------------------------------------------------
MW 28.96 ( lb m ⁄ lb mole )
= 53.34 ft ⋅ lbf ⁄ lb m ⋅ °R
(4.1)
The density (ρ) of dry air or its inverse specific volume
(υ = 1/ρ) can be calculated from ideal gas relationships.
At the specified standard conditions of 60°F
(520°R) and sea level atmospheric pressure
(p = 14.696 psia), the specific volume of dry air (υ a ) is
1 R
υ a -- a T 53.34 ( ft ⋅ lbf ⁄ lb
---------
m ⋅ °R) × 520°F
= = = ------------------------------------------------------------------------------
ρ p
14.696 ( lb f ⁄ in. 2 ) × 144 in.2 -------
ft 2
=
13.1 ft 3 ⁄ lb m
(4.2)
The specific volume can be corrected by inserting
an atmospheric pressure corrected for altitude (Z, in feet
above sea level) into Equation 4.2 (ASHRAE 2005).
p (psia) = 14.696 (1 – 6.8753 × 10 –6 Z) 5.2559 (4.3)
It is convenient to express the amount of moisture
in air in terms of the humidity ratio (W), which is the
mass of water vapor (M w ) per mass of dry air (M a ). Current
practice in the US is to use the units of pound mass
of water (lb w ) to pound mass of air (lb a ). Some documents
continue the use of grains per pound mass of air,
where 7000 grains = 1.0 pound mass.
W
M w lb
------- w
lb
------- w
= ≡ ≡ 7000 ( grains ⁄ lb
M a lb w ) × -------
a
lb a
(4.4)
The amount of water vapor required to saturate air
increases with temperature. A widely used term is relative
humidity (RH), which is the mole fraction (or percent)
of water vapor (x w ) present in the air relative to the