Find the moist air properties of air at dry-bulb temperature
of 100 degrees F and wet-bulb of 80 degrees F.
Pressure is atmospheric or 29.921 inches of mercury.
Solution:
Let us use psychrometric chart for normal temperatures that
cover our temperature range.
Locate the state point having dry bulb 100 F (vertical line)
and wet-bulb 80 degrees (a line at an angle with negative slope) such that when
extended to the saturation curve meets the dry-bulb of 80 degrees F.
Draw a horizontal line to read a humidity value (W) of
0.0175 pounds of water per pound of dry air.
Move along the wet bulb line till you reach the saturation
temperature curve. Extend it further to
read the enthalpy of saturation from the slanted scale as 43.6 BTUs per
pound. Now, we need to correct by
applying the Enthalpy Deviation from the saturation. Note that these correction curves are broken curves running top
to bottom. Enthalpy deviation is zero
when the air completely saturated.
These corrective values are up to negative 0.3 BTU per pound of dry
air. For our situation, this deviation
can be read as –0.23 BTU per pound. So,
enthalpy of the moist air is 43.6 plus the correction (negative 0.23) equals
43.37 BTU per pound.
Draw a horizontal line to the left to reach the saturation
curve. At the saturation curve, both
dry- and wet-bulb temperatures have same value. In this case, it turns out be 73.2 degrees F.
Relative humidity curves are drawn on the psychrometric
curve. These curves are almost parallel
to the saturation curve. For our case,
the relative humidity can be interpolated as 42 percent.
Volume: Humid volume
is plotted on the psychrometric curve (at angles with negative slope). These values are 12.5, 13, 13.5, 14, and
14.5 cubic feet per pound of dry air.
For our case, this value is 14.5 cubic feet per pound of dry air.