A stream of outdoor air is adiabatically mixed with a
recirculated air. Outdoor air is
flowing at 2,500 cubic feet per minute and is at a temperature of 40 degrees
Fahrenheit dry-bulb and 35 degrees F wet-bulb.
The recirculated air is flowing at 7,500 cubic feet per minute and is at
a temperature of 75 degrees Fahrenheit dry-bulb and has a relative humidity of
50 percent. What is the final state of
mixed air, that is, its dry bulb and wet bulb temperatures.
Solution:
Let us use the Psychrometric Chart suitable for Normal
Temperatures ranges.
Locate the Point 1 representing outdoor air at 40 degrees
dry-bulb and 35 degrees wet-bulb.
Please note that humid volume value 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 12.65 cubic feet per pound of dry air. Mass flow rate of outdoor air can be
calculated from the volumetric flow rate and specific volume information at
entering stage. This value is found to
be 2,000 divided by 12.65 equal 197.5 pounds per minute.
Locate the Point 2 representing recirculated air at 75
degrees dry-bulb and 50 percent humidity.
Humid volume (V2) is 13.68 cubic feet per pound
of dry air. Mass flow rate of
circulated air is found to be 7,500 divided by 13.68 equal 548 pounds per
minute.
Once we have located these two points on the psychrometric
chart, we can locate Point 3, representing the mixed air. Keep in mind that both mass and energy
balances are followed. We could proceed
in two ways.
Firstly, we could calculate the humidity of mixed air, W3,
from the air and water balance. For
that we need to obtain, humidity values for both outdoor air and recirculated
air. Once we have calculated W3,
we can located this point the line 1-2 and read dry- and wet-bulb temperatures.
Secondly, calculate the ratio ma1 over ma2. This ratio is 197.5 divided by 548 equals
0.36. This value equals to the ratio of
length of Line 3-2 and Line 1-3. The
length of the line segment 1-3 over line 1-2 is found to be 1 over 1.36 equals
0.735. Use a ruler to located State 3
having a length 73.5 percent of the total length of line 1-2. Dry- and wet-bulb temperatures are found to
be 65.9 and 56.6 degrees Fahrenheit.