Environmental Engineering Reference
In-Depth Information
q
h
T
2
Condenser
2
4
Pressure
reducer
4
w
Compressor
Liquid
Vapor
3
5
Liquid + Vapor
1
Evaporator
1
5
s
q
c
Figure 3.8
The vapor compression cycle for refrigeration begins with an isentropic compression
(
1
→
2
)
of
the vaporized fluid leaving the evaporator, followed by a constant-pressure cooling in the condenser
(
2
→
4
)
to form a saturated liquid
(
4
)
. The liquid leaving the condenser undergoes an adiabatic pressure
decline
(
4
→
5
)
, entering the evaporator as a cold liquid-vapor mixture
(
5
)
, whereupon it absorbs heat from
the refrigerated space
(
5
→
1
)
. (In the
T
-
s
diagram on the left, the area beneath the dashed line delineates
the conditions where both vapor and liquid refrigerant coexist, in contrast to vapor only to the right and
liquid to the left.)
exchanger, condenses the vapor to liquid form
by transferring heat to the atmosphere
or other environmental sink. The liquid refrigerant leaving the condenser
(
2
→
4
)
passes through a
small-diameter capillary tube, undergoing a viscous pressure drop to enter the evaporator at a
lower pressure
(
4
)
. In this adiabatic, constant-enthalpy process, the fluid temperature decreases
and some of the liquid changes to the vapor form. The liquid-vapor mixture then passes through
the evaporator, a heat exchanger that absorbs heat from the refrigerated space while changing the
liquid portion of the refrigerant to a vapor, completing the cycle.
In heat engine cycles that produce work from the combustion of fuel, the thermodynamic
efficiency measures the ratio of the output (work) to the fuel input (heat). The second law of
thermodynamics assures that the output is always less than the input, so that the thermodynamic
efficiency is less than 100%. For refrigerators and air conditioners, however, the desired output
(heat removed from the refrigerated space) is not necessarily less than the input (compressor work).
Nevertheless, we may form the ratio of output to input, which is called the
coefficient of performance
(
COP
), using this as a figure of merit for the performance of these devices.
The coefficient of performance of the vapor compression cycle may be determined in terms
of the changes in the thermodynamic states of the refrigerant fluid, such as those illustrated in
Figure 3.8. The work
(
5
)
w
required to compress a unit mass of refrigerant equals its change in
enthalpy
h
2
−
h
1
. The heat
q
c
absorbed by the refrigerant from the refrigerated space is equal to
its change in enthalpy
h
1
−
h
5
, which also equals
h
1
−
h
4
because the process 4
→
5 is one of
unchanging enthalpy. As a consequence, the coefficient of performance is
q
c
w
h
1
−
h
4
h
1
−
h
4
COP
≡
=
h
1
=
(3.45)
h
2
−
(
h
2
−
h
4
)
−
(
h
1
−
h
4
)
The coefficient of performance is greatest when the temperature difference between the refrigerated
space and the environment is least. It decreases monotonically as this temperature difference
