Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

Most air-conditioning processes can be modeled as steady-flow processes,

and thus the mass balancerelation m

.

inm

.

outcan be expressed for dry air

and wateras

Mass balance for dry air: (14–16)

Mass balance for water: (14–17)

Disregarding the kinetic and potential energy changes, the steady-flow

energy balancerelation E

.

inE

.

outcan be expressed in this case as

(14–18)

The work term usually consists of the fan work input,which is small rela-

tive to the other terms in the energy balance relation. Next we examine

some commonly encountered processes in air-conditioning.

Simple Heating and Cooling (V
constant)

Many residential heating systems consist of a stove, a heat pump, or an elec-

tric resistance heater. The air in these systems is heated by circulating it

through a duct that contains the tubing for the hot gases or the electric resis-

tance wires, as shown in Fig. 14–21. The amount of moisture in the air

remains constant during this process since no moisture is added to or

removed from the air. That is, the specific humidity of the air remains con-

stant (vconstant) during a heating (or cooling) process with no humidifi-

cation or dehumidification. Such a heating process proceeds in the direction

of increasing dry-bulb temperature following a line of constant specific

humidity on the psychrometric chart, which appears as a horizontal line.

Notice that the relative humidity of air decreases during a heating process

even if the specific humidity vremains constant. This is because the relative

humidity is the ratio of the moisture content to the moisture capacity of air

at the same temperature, and moisture capacity increases with temperature.

Therefore, the relative humidity of heated air may be well below comfort-

able levels, causing dry skin, respiratory difficulties, and an increase in

static electricity.

A cooling process at constant specific humidity is similar to the heating

process discussed above, except the dry-bulb temperature decreases and the

relative humidity increases during such a process, as shown in Fig. 14–22.

Cooling can be accomplished by passing the air over some coils through

which a refrigerant or chilled water flows.

The conservation of mass equations for a heating or cooling process that

involves no humidification or dehumidification reduce to m

.

a 1 m

.

a 2 m

.

a for

dry air and v 1 v 2 for water. Neglecting any fan work that may be present,

the conservation of energy equation in this case reduces to

where h 1 and h 2 are enthalpies per unit mass of dry air at the inlet and the

exit of the heating or cooling section, respectively.

Q

#

m

#

a^1 h 2 
h 12 ¬or¬qh 2 
h 1

Q

#

inW

#

ina

in

m

#

hQ

#

outW

#

outa

out

m

#

h

(^) a
in
m#wa
out
m#w¬or¬a
in
m#a va
out
m#a v
(^) a
in
m#aa
out
m#a¬¬ 1 kg>s 2
730 | Thermodynamics
t 2 = t 1
Heating coils
Heat
Air T 2
T 1 , t 1 , f 1
f 2 < f 1
FIGURE 14–21
During simple heating, specific
humidity remains constant, but relative
humidity decreases.
(^21)
12 °C30°C
v = constant
Cooling
f 2 = 80% f 1 = 30%
FIGURE 14–22
During simple cooling, specific
humidity remains constant, but relative
humidity increases.
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