Page 549 - Mechanical Engineers' Handbook (Volume 4)

538 Indoor Environmental Control

˙
Q i i
SHF sensible A 2 (14)
˙
Q i i 2
1
SHF defines the slope for the process line (dashed line). The inner scale of the protractor in
the upper left corner of the psychrometric chart gives SHF values. The line connecting the
center of the protractor and SHF value is parallel to the process line in the chart.
Another typical process is heating and humidification for cold and dry climates. This
process needs a heating coil and a humidifier as presented in Fig. 5. The heat exchanger
heats up the air sensibly, while the humidifier adds moisture in either vapor or liquid form
without heat exchange with the surroundings. The first law and the conservation of mass
equations for this process are
˙
˙ mi Q ˙mi ˙mi (15)
a 1
a 2
ww
˙ m ˙m (W W ) (16)
1
a
w
2
When the moisture added is liquid at the wet bulb temperature of the incoming air
stream 1, the humidification process coincides with the constant enthalpy line (i i ).
A
2
Adiabatic humidification of the incoming air stream could be achieved with steam or
liquid at an arbitrary temperature. Figure 6 schematically shows the adiabatic humidification
without heating process. The outgoing air stream could have different states represented as
states 2, 3, or 4 in the example in Fig. 6. The constant enthalpy process 1-2, is achieved
with a spray of water at the wet bulb temperature (T WB,1 ). The air state 3 can be reached by
adding saturated steam at the dry bulb temperature of the incoming air (T ). In general, the
1
leaving air could be humidified and cooled if the water enthalpy is between the enthalpy for
T WB,1 and the saturated liquid at T , which would result in a process somewhere in between
1
process 1-2 and process 1-3 on the psychrometric chart. Cooling happens when the water
droplets fully evaporate by taking energy from the incoming air stream under the assumption
of adiabatic process. The leaving air could be humidified and heated, such as in process 1-
4, if the added steam has enthalpy greater than the saturated steam at T . Except for processes
1
1-2 and 1-3, the exact slope of an arbitrary adiabatic humidification process could be deter-
˙
mined from the following equation, which is derived from Eqs. (15) and (16) (Q 0):
i i i 1
2
i (17)
W W W waterorsteam
2 1
The value of the ratio i/ W can be calculated and used in the protractor to determine the
slope of the humidification process, in the same way as SHF factor was used. This ratio is
plotted on the outer scale of the protractor.
Q
1 2
W 1 m a W 2
T 1 T 2 i=const.
i i 2
1 2 ∆W
1
m w ∆T A
i
w
Figure 5 Heating and adiabatic humidification for cold and dry climates.