Chapter hvac engineering Fundamentals: Part 1 Introduction

316
Chapter Nine
Figure 9.28
Evaporative cooling (sprays).
Figure 9.29
Psychrometric chart for evaporative cooling.
If the entering wet-bulb temperature
is less than the entering dry-
t
wb
1
bulb temperature
then some of the water will be evaporated. Be-
t
,
db
1
cause the process is essentially adiabatic, the heat required for evap-
oration must come from the airstream, thereby lowering its dry-bulb
(sensible) temperature. Then the leaving dry-bulb temperature
t
db
2
will be less than
The wet-bulb temperature will remain constant,
t
.
db
1
⫽
, and the process can be shown on the psychrometric chart
t
t
wb
wb
2
1
as a constant wet-bulb process (Fig. 9.29). If the system were 100 per-
cent efficient, then
, would equal
and point 2 would be on the
t
t
db
wb
2
2
saturation curve. No evaporative cooler is 100 percent effective.
Ef-
fectiveness
(eff) is defined as the ratio of the change of the dry-bulb
Equipment: Part 1
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