CONDENSERS AND
EVAPORATORS
ME115
REFRIGERATION
ENGINEERING
WEEK 5
2023-2024/2T
1/20/2024
Prepared by:
Engr. Manuel B. Rustria
January 20, 2024
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Objectives
 Classify the types of condensers and evaporators.
 Evaluate overall heat transfer coefficient for condensers and evaporators.
 Evaluate heat transfer and pressure drop for liquid in tubes and liquid in
shell.
 Evaluate heat transfer and pressure drop using extended heat transfer
surfaces (fins).
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CONDENSERS
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CONDENSERS
 The previous sections presented tools for computing heat-transfer
coefficients and pressure drops of the fluid exchanging heat with the
refrigerant in a condenser or evaporator.
 For the condenser the fluid to which heat is rejected is usually either air or
water.
 Air-cooled condensers are shown in Fig. 12-2 and a shell-and-tube
condenser in Fig. 12-1.
 Another type of water-cooled condenser has cleanable tubes (Fig. 12-11).
 When the condenser is water-cooled, the water is sent in a cooling tower
(Chap. 19) for ultimate rejection of the heat to the atmosphere.
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CONDENSERS
Figure 12-11
Water-cooled condenser
with cleanable tubes
https://www.hphirc.com/ENG/Prodotti/3/c
ondensatori-ispezionabiliad-acqua-controcorrente
.
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CONDENSERS
 Some years ago air-cooled condensers were used only in small refrigeration
systems (less than 100 kW refrigerating capacity), but now individual aircooled condensers are manufactured in sizes matching refrigeration
capacities of hundreds of kilowatts.
 The water-cooled condenser is favored over the air-cooled condenser
where there is a long distance betweem the compressor and the point
where heat is to be rejected.
 Most designers prefer to convey water rather than refrigerant in long lines.
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CONDENSERS
 In centrifugal-compressor systems large pipes are needed for the lowdensity refrigerants, so that the compressor is close-coupled to the
condenser.
 Water-cooled condensers therefore predominate in centrifugal-compressor
system.
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REQUIRED CONDENSING CAPACITY
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REQUIRED CONDENSING CAPACITY
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REQUIRED CONDENSING CAPACITY
1.6
Figure 12-12
Typical values of the ratio of the
heat-rejected at the condenser to
the refrigerating capacity for
refrigerants 12 and 22
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Heat-rejection ratio
1.5
1.4
1.3
-10°C
evaporator
0°C
10°C
1.2
1.1
1.0 20Condensing
30
Temperature,
°C 60
40
50
10
REQUIRED CONDENSING CAPACITY
 Theoretical calculations of the condenser heat rejection can be made from
the standard vapor-compression cycle, but they do not take into
consideration the additional heat added by inefficiencies in the compressor.
 A graph of typical values of heat-rejection ratios is shown in Fig. 12-12.
 When the motor driving the compressor is hermetically sealed, some of the
heat associated with inefficiencies of the electric motor is added to the
refrigerant stream and must ultimately be removed at the condenser.
 The heat-rejection ratios of thermetically sealed compressors are usually
slightly higher than those of the open-type compressor.
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FOULING FACTOR
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12
FOULING FACTOR
12-21
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FOULING FACTOR
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EVAPORATORS
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EVAPORATORS
Figure 12-17
Air-cooling evaporator.
The device on the left
and is a refrigerant
distributor to feed the
several circuits
uniformly.
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EVAPORATORS
Figure 12-18 A liquid chilling evaporator in which refrigerant boils inside finned tubes.
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EVAPORATORS
 In most refrigerating evaporators the refrigerant boils in the tubes and cools
the fluid that passes over the outside of the tubes.
 Evaporators that boil refrigerant in the tubes are often called directexpansion evaporators, and Fig. 12-17 shows an air-cooling evaporator and
Fig. 12-18 a liquid cooler.
 The tubes in the liquid chiller in Fig. 12-18 have fins inside the tubes in
order to increase the conductance on the refrigerant side.
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EVAPORATORS
Figure 12-19 (a) Air-conditioning evaporator with refrigerant leaving in a superheated state,
(b)1/20/2024
liquid-recirculation evaporator with liquid refrigerant carried out of the evaporator.
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EVAPORATORS
 Direct-expansion evaporators used for air-conditioning applications are
usually fed by an expansion valve that regulates the flow of liquid so that
the refrigerant vapor leaves the evaporator with some superheat, as shown
in Fig. 12-19a.
 Another concept is the liquid-recirculation or liquid overfeed evaporator Fig.
12-19b, in which excess liquid at low pressure and temperature is pumped
to the evaporator.
 Some liquid boils in the evaporator, and the remainder floods out of the
outlet.
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EVAPORATORS
 The liquid from the evaporator is separated out, and the vapor flows on to
the compressor.
 Low-temperature industrial refrigeration system often use this type of
evaporator, which has the advantage of wetting all the interior surfaces of
the evaporator and maintaining a high coefficient of heat transfer.
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EVAPORATORS
 While refrigerant boils inside the tubes of most commercial evaporators, in
one important class of liquid-chilling evaporator the refrigerant boils outside
the tubes.
 This type of evaporator is standard in centrifugal-compressor applications.
 Sometimes such an evaporator is used in conjunction with reciprocating
compressors, but in such applications provision must be made for returning
oil to the compressor.
 In the evaporators where refrigerant boils in tubes, the velocity of the
refrigerant vapor is maintained high enough to carry oil back to the
compressor.
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BOILING IN THE SHELL
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BOILING IN THE SHELL
10
B
q/A ∙ W/m2
10
Figure 12-20
Heat-transfer coefficient
for pool boiling og water
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C
10
10
A
10
0.1
1
10
100
Temperature difference, K
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BOILING IN THE SHELL
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BOILING IN THE SHELL
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BOILING IN THE SHELL
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Figure 12-21
Heat-transfer coefficient: for refrigerants 12
and 22 boiling outside of tube bundles
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Heat-transfer coefficient, W/m2 ∙ K
BOILING IN THE SHELL
2000
1500
1000
800
600
400
200
500
1000
2000
4000 6000
Heat flux, W/m2
10,000
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BOILING IN THE SHELL
 The rate of evaporation can increase to a peak, point B, whose so much
vapor covers the metal surface that the liquid can no longer intimately
contact the metal.
 A further increase in the temperature difference decreases the rate of heat
transfer.
 The graph in Fig. 12-20 is useful in predicting the trends for heat-transfer
coefficients for boiling outside tube bundles.
 Hoffmann summarized the work of several investigators to provide the band
shown in Fig. 12-21.
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BOILING INSIDE TUBES
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BOILING INSIDE TUBES
 When refrigerant boils inside the tubes, the heat-transfer coefficient
changes progressively as the refrigerant flows through the tube.
 The refrigerant enters the evaporator tube with a low fraction of vapor.
 As the refrigerant proceeds through the tube, the fraction of vapor
increases, intensifying the agitation and increasing the heat-transfer
coefficient.
 When the refrigerant is nearly all vaporized, the coefficient drops off to the
magnitude applicable to vapor transferring heat by forced convection.
 Figure 12-22 shows local coefficients throughout a tube for three different
levels of temperature.
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Coefficient of heat-transfer, W/m2 ∙ K
BOILING INSIDE TUBES
Figure 12-22
Heat-transfer coefficient of
Refrigerant 22 boiling tubes. Curve
1 at 10°C. curve 2 at 3°C, and curve 3
at 2.8°C temperature of evaporation.
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2000
1500
1000
500
Superheat
00
0.2
0
1
0.4
0.6
0.8
1.0
Fraction of liquid vaporized
2
3
4
Distance along tube, m
5
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BOILING INSIDE TUBES
 The heat-transfer coefficient is highest for the high evaporating
temperature, probably because at high evaporating temperatures and
pressures the vapor density is high, permitting a greater fraction of the
metal to be wetted with liquid.
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PROBLEMS
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12-7
PROBLEMS
What is the UA value of a direct-expansion finned coil evaporator having
the following areas: refrigerant side, 15 m2; air-side prime, 13.5 m2; and
air-side extended 144 m2? The refrigerant-side heat transfer coefficient
is 1300 W/m2 ∙ K, and air-side coefficient is 48 W/m2 ∙ K. The fin
effectiveness is 0.64. Ans. 4027 W/K.
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12-8
PROBLEMS
A refrigerant 22 system having a refrigerating capacity of 55 kW operates
with an evaporating remperature of 5°C and rejects heat to a watercooled condenser. The compressor is hermetically sealed. The
condenser has a U value of 450 W/m2 ∙ K and a heat-transfer area of 18
m2 and receives a flow rate of cooling water of 3.2 kg/s at a temperature
of 30°C. What is the condensing temperature? Ans. 41.2°C
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12-9
PROBLEMS
Calculate the mean condensing heat-transfer coefficient when
refrigerant 12 condenses on the outside of the horizontal tubes in a
shell-and-tube condenser. The outside diameter of the tubes is 19 mm,
and in the vertical rows of tubes there are, respectively, two, three, four,
three, and two tubes. The refrigerant is condensing at a temperature of
52°C, and the temperature of the tubes is 44°C. Ans. 1066 W/m2 ∙ K
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12-10
PROBLEMS
A condenser manufacturer guarantees the U value under operating
conditions to be 990 W/m2 ∙ K based on the water-side area. In order to
allow for fouling of the tubes, what is the U value required when the
condenser leaves the factory? Ans. 1200 W/m2 ∙ K
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12-11
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PROBLEMS
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12-12
PROBLEMS
a) A Wilson plot is to be constructed for a finned air-cooled condenser
by varying the rate of airflow. What should the abscissa of the plot
be?
b) A Wilson plot is to be constructed for a shell-and-tube water chiller
in which refrigerant evaporates in the tubes. The rate of water flow
is to be varied for the Wilson plot. What should the abscissa of the
plot be?
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12-13
PROBLEMS
The following values were measured on an ammonia condenser:
Water flowed inside the tubes, and the tubes were 51 mm OD and had a
conductivity of 60 W/m2 ∙ K. Using a Wilson plot, determine the
condensing coefficient. Ans. 8600 W/m2 ∙ K
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12-14
PROBLEMS
Develop Eq. (12-23) from Eq. (12-22)
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12-15
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PROBLEMS
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12-16
PROBLEMS
Section 12-19 makes the
statement that on a graph of
performance of a waterchilling evaporator with the
coordinates of Fig. 12=23 a
curve for a given entering
water temperature is a
straight line if the heattransfer are constant. Prove
this statement.
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Figure 12-23
Evaporator performance curve.
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REFERENCES
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REFERENCES
• Stoecker, W. F., Jones, J. W. (1982). Refrigeration and Air Conditioning, 2nd
ed., McGraw-Hill, Inc.
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Righteousness exalts a nation,
but sin is a reproach to any people.
(Prov. 14:34, NKJV)
END
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