Air, completely saturated at 60 degrees F enters the heating
coil at 10,000 cubic feet per minute.
It leaves the heating coil at 110 degrees F. Find the amount of heat that needs to be supplied to bring this
change to the condition of the air.
Solution:
Let us use the Psychrometric Chart suitable for High
Temperatures ranges (Figure 5.11a).
Locate the Point 1 representing completely saturated air at
60 degrees F.
Draw a horizontal line to read a humidity value (W1)
of 0.0112 pounds of water per pound of dry air.
Move along the wet bulb line and extend it further to read
the enthalpy of saturation from the slanted scale as 26.5 BTUs per pound.
Please note that humid volume is plotted as curves on the
Psychrometric Chart at angles with negative slope. These values are 13, 13.5, 14, and 14.5 cubic feet per pound of
dry air. For case in hand, its value is
13.33 cubic feet per pound of dry air.
The completely saturated air is heated to 110 degrees
F. This heating process is represented
by horizontal line. Point 2 has a dew
point of 60 and dry-bulb of 110 degrees F and is located by drawing a
horizontal line from Point 1 and extending it to a vertical line of 110 degrees
F. Its properties can be read as:
Humidity remains unchanged at 0.0112 pounds per pound of dry
air.
Enthalpy of humid air can be calculated by noting that
enthalpy deviation as minus 0.33 BTUs per pound and Enthalpy at Saturation as
38.6 BTUs, thus giving humid enthalpy of 38.27 BTUs per pound dry air.
Mass flow rate of air can be calculated from the volumetric
flow rate and specific volume information at entering state. This value is found to be 10,000 divided by
13.33 times 60 equal 45,011.2 pound per hour.
Heat requirement can be found by multiplying this mass flow
rate with enthalpy change per pound of dry air. It is equal to 529,782 BTUs per hour.