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HVAC Fundamentals: Psychrometrics
W ( lb w ⁄ lb a )
( 1093 – 0.556t wb ( °F)
)W s ( lb w ⁄ lb a ) – 0.24( t–
t wb ) ( °F)
= -------------------------------------------------------------------------------------------------------------------------------------------- 1093 + 0.444t ( °F) – t wb ( °F)
,
(4.9)
where t and t wb are in °F, and W s is determined by inserting
the saturation pressure of water vapor at the wetbulb
temperature into Equation 4.8.
In summary, to determine moist air properties, the
dry-bulb temperature and the humidity ratio are needed.
The humidity ratio can be determined from one of three
indicators: wet-bulb temperature, relative humidity, or
dew-point temperature.
Equations for Moist Air Properties
The thermodynamic properties of moist air can be
determined from the dry-bulb temperature and humidity
ratio. These include the enthalpy, specific volume (or its
inverse, density), and specific heat. In the US, the current
convention is to set base values at 0°F and compute the
values at other temperatures. At 0°F, h w = 1061 Btu/lb
and h a = 0 Btu/lb. The specific heat of air is 0.24 Btu/lb⋅°F
and water vapor is 0.444 Btu/lb⋅°F. For moist air at drybulb
temperature (t) and humidity ratio (W),
h ( Btu ⁄ lb a ) = 0.24t ( °F) + W ( lb w ⁄ lb a )( 1061 + 0.44t ( °F)
) .
(4.10)
The specific heat of moist air is
c p ( Btu ⁄ lb a ⋅ °F) = 0.24 + 0.444W ( lb w ⁄ lb a ) .
(4.11)
The specific volume of moist air is
υ ( ft 3 ⁄ lb) = 0.37059t ( °F)
+ 459.67[ 1 + 1.6078W ( lb w ⁄ lb a )] ⁄ p ( psia)
.
(4.12)
THE PSYCHROMETRIC CHART AND
COMPUTER CODE
Many of the properties of moist air discussed in the
previous section can be estimated from a psychrometric
chart, a plot of dry-bulb temperature (horizontal axis)
and humidity ratio (vertical axis). A psychrometric chart
for sea level is shown in Figure 4.1. Dry-bulb temperatures
on the chart range from the freeze point of water
(32°F) to 120°F. Humidity ratios range from dry air
(W = 0) to 0.028 lb w /lb a (196 grains/lb a ). Lines of constant
humidity ratio are horizontal on the graph. Lines of
constant temperature exhibit a slight tilt to the left of
vertical, with the degree of tilt increasing with lower
temperature. As shown in the figure, the left top portion
of the plot is terminated at the saturation line, which represents
both the 100% relative humidity curve and the
plot for dew-point temperature. Lines of constant
enthalpy appear as straight lines that slope down as the
temperature increases. Lines of constant wet-bulb temperature
are nearly parallel to lines of constant enthalpy.
Lines of constant specific volume also slope down as
temperature increases but at a much greater angle compared
to enthalpy and wet-bulb lines. Lines of constant
relative humidity curve up as temperature increases.
This is a result of the fact that lines of constant RH are
formed by plotting a set of points that are a fractional
Figure 4.1
Psychometric chart for sea level.
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