Chapter 3 Moist Air Properties and Conditioning Processes

..M.. = i = 1150.4 BtuJJbm
t.w \I'
3-5 ClaSSIc Moi ,;, Air Processes 63
where iw is read from Table A-la. A parallel line is drawn from state 2 as shown in
Fig. 3-7. State X is determi ned by the intersection on lines 1 - X and X - 2. The heal
Iransfer rate is then given by
where
In = 0(60) = 1600 60 = 7296Ibma/hr
" vI J3.16
and i I and it' read from Chart 1 tI, are L6.8 and 29. 2 BtuJlbma, respectivel y. Then
if = 7296(29.2 -16.8) -::::: 90,500Btu/hr
The mass flow rate of the waler vapor is given by
,ill' = ,j1a(W2 - W I )
where W 2 and W I are read from Chart La as 0.0193 and O.00221bmvllbma. respectively.
Then
,hI. = 7296(0.0 193 - 0.0022) = 1251bmv I hr
Adiabatic Mixing of Two Streams of Moist Air
The mixing of airstreams is quite common in air-conditioning systems. The mixing
usually occurs under steady. adiabatic flow conditions. Figure 3-9 illustrates the mixing
of two airstreams. An energy balance gi ves
The milss balance on lhe dry air is
and the mass balance 01) the water vapor is
Iha l l{) + nZa2 W 2 = li1a3 W]
Combining Eqs. 3-41. 3-42. and 3-43 and eliminating m ll3 yields
i ) - i,
----- = W2 - IV3 = m al
",::, - IVI III a2
Figure 3-9 Schematic of the adiabatic lllixing of lWO airstreams.
(3-4 1 )
(3-42)
(3-43)
(3-44)

3-6 Space Air Conditioning- Design Condition, 71
In Example 3-9 the outdoor air was hot and humid. This is not always the caf>C.
and statc 0 (outdoor ai r) can be almost anywhere on Chart la , For example, the sou(h­
\vcstern part of the United Stales is hot and dry during the summer, and evaporative
cooling can ofte n be used to advantage under these conditions. A simple system of
this type is shown in Fig. 3-15. The dry outdoor air flows through an adiabatic spray
chamber and is cooled and humidified. An energy balance on the spray chamber will
show that the enthalpies io and i j are equal: therefore. the process is as shown in Fig.
3-16. Ideally the cooling process terminates at the space condition line. The air then
flows through the space and is exhausted. Large quantities of ai r are required , and this
system is nO! satisfactory \vhere the outdoor relative humidity is high. If Wo is too
high. the process 0-1 cannot intersect the condition line.
Evaporat ive cool ing can be combined with a conventional system as shown in Fig ,
3- 17 when outdoor conditions are suitable. There arc a l1umber of possibilities. First,
Cond itioned
space
Figure 3-1 5 A simple evaporativc cooling system,
Figurt> 3·16 Psychromctric diagram for the cvaporative cooling system of Fig , 3-1 S,
Exhaust
®
Evaporative qc
cooler
Figure 3-17 Combination evaporatlve and regular cooli ng system
@

3-6 Space Air Condllloning-Desig1J Conditions 73
225.000
SHF = ----- = 0.80
225,000 + 56,250
The state of the supply air lies 01) a line drawn through state point 3 parallel 10 (he
SHF :::: 0.8 line on the protractor of Char! la. Figure 3-20 shows th is construction.
State 2 is located at 120 F dry bulb and the intersection of this line. An energy balance
on the space gives
or
'i = Jil a2 (i Z - i3)
From Chart la, i2 :::: 42 Btu/lbma. i3 = 28. 2 Btu/lbma, and
. q 281,250 20400 1b /1
m aZ = -.--. = ::::, nla If
12 - l3 42 - 28.2
From Chart la, v 2 = 14.89 ft 3 /lbma, and
01 :::: 20.400 x 14.89 = 5060 cfrn
- 60
To find the conditions at state 1. the mixing process must be considered. A mass balance
on the mixing section yields
or
m (l4 = m a2 - lIloO
r;l :::: 0 0 and Vo :::: 12.54 ft 3 1lbma
(I Q v o
Figure 3·20 Psychrornelric processes for Example 3- 10.