Modern Engineering Thermodynamics

12.7 Air Conditioning 425
WHAT ENVIRONMENTAL CONDITIONS MAKE PEOPLE COMFORTABLE?
Humans are essentially isothermal open systems with complex temperature-regulating mechanisms. Body temperature (98.6°F,
37.0°C) is normally above the surrounding environmental temperature so that the excess heat generated by the irreversibilities
inside the body can be removed by normal convection, conduction, and radiation heat transfer mechanisms. During periods of
physical stress or high environmental temperature, the body produces a surface layer of water, called perspiration, whose evaporation
into the atmosphere helps cool the body. This is one of the body’s primary temperature-regulating mechanisms. When the
relative humidity of the surrounding atmosphere is high, the evaporation of body perspiration is low and the body automatically
tries to minimize its internal heat generation, resulting in the person’s feeling lethargic and becoming inactive. Because the sensation
of human comfort is so subjective, attempts to define a “comfortable” atmosphere have met with only limited success. Tests
have shown that a relative humidity below 15% produces a dried (or parched) condition of the membranes in the mouth, nose,
and lungs and an increased susceptibility to disease germs. However, a relative humidity above 70% causes an accumulation of
moisture in the clothing and a general “sticky” or “muggy” feeling. For best health and comfort conditions, it has been found that
the relative humidity should range from 40 to 50% during cold winter weather and from 50 to 60% during warm summer
weather.
EXAMPLE 12.9
Desert air at 110.°F and 10.0% relative humidity is to be cooled and humidified by using evaporative cooling only. Determine
the minimum outlet mixture temperature and its relative humidity.
Solution
The minimum outlet temperature associated with the evaporation process is the wet bulb temperature corresponding to a dry
bulb temperature of 110.°F and 10.0% relative humidity. From Chart D.5, we find that this is approximately
ðT DB Þ min
= T WB = 69°F
and, of course, the relative humidity at this new dry bulb temperature is 100%.
Exercises
21. Determine the minimum outlet dry bulb temperature that could be realized through evaporation of the liquid water in
part b of Example 12.7. Answer: (T DB ) min = T WB (25.0°C, 20.0% relative humidity) = 13°C.
22. Determine the minimum outlet dry bulb temperature that could be realized using the evaporation of liquid water into
air with an inlet dry bulb temperature of 20.0°C and an inlet relative humidity of 50.0%. Answer: (T DB ) min = T WB (20.0°C
and 50.0% relative humidity) = 14°C.
23. Suppose we wanted to produce air with a dry bulb temperature of 60.0°F and a relative humidity of 100.% simply by
allowing liquid water to evaporate into the air. What is the maximum dry bulb temperature of the inlet air? Note: In this
case, (T DB ) max occurs when ϕ = 0% (i.e., when ω = 0). Answer: (T DB ) max = 107°F.
Hot, humid air can be easily cooled and dehumidified by cooling it to below its dew point (saturation)
temperature, condensing out some of the water, then reheating the remaining air–water vapor mixture to the
desired temperature. This is illustrated in Figure 12.7. The water in the cooling section condenses at various
temperatures, but it is assumed to exit the system at temperature T 2 in Figure 12.7.
Cooling coils
Electrical work
input
Cooling and
condensation 1–2
Hot
humid
air
1 2 3
Cool
conditioned
air
φ 1
1
Reheating
φ
2–3
3 < φ 1
p w
φ = 100%
2 3
ω
Condensate
discharge
(a)
Fan
Heater
T DB
T WB3 < T DB1
(b)
FIGURE 12.7
Dehumidification by cooling, condensing, and reheating again.