CHAPTER
CONDENSERS
7.1 TYPES OF CONDENSERS USED
IN INDUSTRIAL REFRIGERATION
The three main types of condensers used in general refrigeration systems are:
air-cooled
water-cooled
evaporative
All of these serve the industrial refrigeration field as well. In comparison to
the air-conditioning industry, however, a lower percentage of air-cooled
condensers and a higher percentage of evaporative condensers are operating in
industrial refrigeration plants. In industrial refrigeration practice, it is common
to connect the evaporative condensers in parallel—a concept not normally used
in air conditioning.
The three types of condensers are shown schematically in Fig. 7.1a, 7.1b,
and 7.1c. The air-cooled condenser in Fig. 7.1a condenses refrigerant vapor by
rejecting heat to ambient air blown over the finned condenser coil with the aid
of a fan, usually a propeller type.
Most all water-cooled condensers (Fig. 7.1b) condense refrigerant in the shell and
on the outside of tubes through which water passes. The condenser cooling water
picks up heat in passing through the condenser and this warm water is cooled by
circulating through a cooling tower (Section 7.6). While the shell-and-tube
construction predominates for water-cooled condensers, plate-type condensers,
sister of the plate-type evaporator explained in Sec. 6.31, are now appearing.
The evaporative condenser of Fig. 7.1c might be considered a cooling tower,
with the condenser tubes washed by the water spray. Ultimately, the heat
rejected from the refrigeration plant is discharged to ambient air, except where
the condenser is cooled by water from a well, lake, or stream.
This chapter first explores the condensing process outside and inside tubes.
Next, the overall performance of water-cooled condensers and the translation
of performance to noncatalog ratings is examined. An explanation of the
performance of cooling towers, the constant companions of water-cooled
condensers, is given. Because of their prevalence in industrial refrigeration
plants, the emphasis of this chapter is on the performance, selection, application,
and operation of evaporative condensers.
7.2 THE CONDENSING PROCESS
Nearly a century ago, heat-transfer pioneer, Willhelm Nusselt, proposed a model
to predict the magnitude of a condensing coefficient for a special geometric
situation1. Nusselt envisioned the condensation of vapor on a cold vertical plate,
Fig. 7.2, as a process where vapor condenses on the plate and the condensate
drains downward, with the condensate film becoming progressively thicker as
it descends. The local condensing coefficient is taken to be the conductance
through the condensate film—the conductivity of the liquid divided by the film
thickness at that point. Nusselt developed the expression for the mean
condensing coefficient as
The immediate question is where, if at all, does condensation occur on a
vertical plate in industrial practice? Actually, a very old condenser design
oriented the tubes vertically and water flowed by gravity down the inside of the
tubes to ease their cleaning. The refrigerant in the shell condensed on the outside
of the vertical tubes.
A slight modification of Eq. 7.1 applies to the widely used horizontal shelland-
tube condenser, Fig. 7.1b. The product of the number of tubes in a vertical
FIGURE 7.2
Condensation of a vapor on a cold vertical surface
row multiplied by the diameter of the tubes replaces the vertical length of the
plane L. White2 found by experimental tests that the coefficient is 0.63 and Goto3
measured 0.65, so the equation for N tubes of diameter D in a vertical row is
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Before leaving the condensing equations, an interesting comparison of the
condensing coefficients of various refrigerants can be made. As Table 7.1 shows,
the condensing coefficients of ammonia condensing on the outside of tubes far
surpasses the coefficients of the other refrigerants shown. Experimental tests
also show ammonia to have a higher condensing coefficient—five times that of
the halocarbons in one study4
7.3 CONDENSATION INSIDE TUBES
In air-cooled and evaporative condensers, the refrigerant condenses inside tubes.
The mechanism of condensation is complex and the flow regimes continue to
change as the refrigerant passes through the tube5. Even though the state of
the refrigerant is superheated vapor on entering the tube, condensation begins
immediately and a spray regime develops. Later on the flow converts to annular
then stratified with the liquid flowing along the bottom of the tube. Near the
end of the condenser tube the flow regime is characterized as slug or plug
TABLE 7.1
Condensing coefficients on the outside of tubes for several refrigerants. The condensing
temperature is 30°C (86°F) and there are six 25-mm (1-in) tubes in a vertical row
Figure 7.3 shows relative values6 of the condensing coefficient throughout a
tube. At the entrance to the tube with its superheated vapor content the
coefficient is low, which is typical of convection heat transfer with a gas. The
coefficient increases once surface condensation begins and is usually at its
highest value during annular flow. As more and more condensed liquid flows
with the vapor, the surface available for condensation decreases. Near the end
of the condenser tube the coefficient drops quite low, because the process has
approached that of convection heat transfer to a liquid.
The low heat-transfer coefficient near the end of the condenser tube when
all or most of the vapor has condensed is pertinent to the plant operator. The
reason is that backing liquid into an air-cooled or evaporative condenser shifts
some heat-transfer area into the liquid subcooling mode which exhibits low
heat-transfer coefficients.
TABLE 7.1
Condensing coefficients on the outside of tubes for several refrigerants. The condensing
temperature is 30°C (86°F) and there are six 25-mm (1-in) tubes in a vertical row
7.4 HEAT-REJECTION RATIO
The heat-rejection ratio (HRR) is defined as the ratio of the rate of heat rejected
at the condenser to that absorbed at the evaporator.
The designer and operator of the refrigeration system will usually
characterize plant size by the refrigeration capacity. This capacity can be
translated to a condenser capacity through the condenser-to-evaporator heat
rejection ratio (HRR). The HRR is a function of the evaporating and condensing
temperatures, but is also influenced by the compressor type and any
supplementary cooling arrangements. The standard procedure for computing
the HRR from catalog data of the compressor is to propose that the heat rejected
at the condenser is composed of two contributions—the refrigerating capacity
and the thermal equivalent of the power supplied to the compnessor. The
standard equation for computing the HRR is, therefore,
(7.3)
where all the energy flow rates are expressed in the same units.
Figure 7.4 shows HRRs as functions of the evaporating and condensing
temperatures. Changes of either of these temperatures affect both the
refrigerating capacity and the power requirement of the compressor. The ideal
HRR can be derived from knowledge of the Carnot cycle (Section 2.17), in which
the ratio of area under the condensing line to that under the refrigeration line
represents the HRR,
(7.4)
where the temperatures T are in absolute, thus °C+273.1 (°F+459.7). Equation
7.4 assumes a 100% efficiency of the cycle and the compressor, and an improved
expression that can be used when compressor catalog data are not readily
available is
(7.5)
Example 7.1. Estimate the HRR when the condensing temperature is
35°C (95°F) and the evaporating temperature is -10°C (14°F).
Solution. On the absolute scale the evaporating temperature is 263.1 K
(473.6 R) and the condensing temperature is 308.1 K (554.6 R). The
estimated HRR is
Equation 7.3 is correct except for heat losses to the ambient or supplementary
transfers of heat to other devices. The curves in Fig. 7.4 apply to open-type
compressors, and the HRR will be higher for hermetic compressors servicing
small halocarbon systems, because some of the motor heat enters the refrigerant
stream. Also the HRR will be lowered if a reciprocating compressor uses watercooled
heads where the heat is rejected to a separate cooler or in a screw
compressor where oil is cooled by a separate water or antifreeze circuit.
7.5 PERFORMANCE OF AIR AND
WATER-COOLED CONDENSERS
Manufacturers of condensers provide performance data directed toward selecting
equipment. By applying some fundamentals of heat transfer, a user can
frequently translate catalog data to nondesign conditions. The strategy in
extending catalog data to nondesign conditions is usually to compute the UA
value (the product of the overall heat transfer coefficient and the heat-transfer
area) and for situations where the UA remains essentially constant, apply this
UA value to the new set of operating conditions. The temperature profiles are
somewhat complex because of desuperheating and subcooling, as shown in Fig.
7.5a, but to approximate, assume the condensing temperature prevails
throughout the condenser, as shown in Fig. 7.5b.
In the desuperheating section, the actual temperature difference between
the refrigerant and cooling water is higher than the ideal, but this error is at
least partially compensated for by the fact that the actual heat-transfer
coefficient for the convection process is less than during condensation. Real
condensers are rarely circuited strictly for counterflow or parallel flow. When
one fluid is at a constant temperature, however, the flow pattern is immaterial,
and an equation comparable to the one for evaporators, Eq. 6.11, applies
Example 7.2. The catalog for a Vilter 0.2 m×2.13 m (8 in×7 ft) R-22
condenser specifies a condensing capacity that accommodates a
refrigeration load of 204 kW (58.1 tons) at the evaporator when the
evaporating temperature is 4.4°C (40°F), the condensing temperature is
40.6°C (105°F), and a 9.8 L/s (156 gpm) flow rate of cooling water enters
at 29.4°C (85°F).
What condensing temperature would prevail if the cooling water
flow rate and its entering temperature remain constant, but the
refrigeration capacity is half of the catalog value?
Solution. The rate of heat transfer q at the condenser with the original
refrigeration load was:
and at an evaporating temperature of 4.4°C (40°F) and a condensing
temperature of 40.6°C (105°F), Fig. 7.4 shows a heat rejection ratio of
1.24, so q equals 253 kW (863,000 Btu/hr). The mass flow rate of cooling
water is 9.8 kg/s (1300 lb/min), so the outlet water temperature to is
This UA value should remain essentially unchanged as the
condensing capacity varies, so long as the water flow rate remains
constant. At the half-load condition, the LMTD will be half the original
value, 7.69÷2 or 3.85°C (6.92°F), and the condenser cooling water
experiences half its original rise in temperature, so its new value is
29.4+3.1=32.5°C (90.5°F). The equation for the LMTD can be solved for
the new condensing temperature,
so the new tc=35.0°C (95°F), which is a reduction from the original tc of
40.6° (105°F).
If greater precision is desired, perform an iteration which uses a
revised HRR based on the new condensing temperature.
Example 7.1 illustrates a situation where the U-value remains constant from
one condition to another. If the rate of water flow changes, the heat-transfer
coefficient on the water side will also change so that the U-value no longer
remains constant. It is advisable to consult the manufacturer in such a case.
The tubes of water-cooled condensers are subject to fouling caused by
impurities in the water. Some measurements7 made with suspended solids in
cooling tower water indicated that the fouling factor (which is additional heat
transfer resistance) can easily be on the order of 0.00004 m2·°C/W (0.0002
hr·ft2·°F/Btu) or higher. In the condenser of Example 7.1 the water-side heat
transfer area is 3.78 m2 (40.7 ft2), so the U-value of the condenser based on the
water-side area is:
or 1533 Btu/(hr·ft2·°F). The resistance is the reciprocal of this U-value or 0.000114
m2·°C/W (0.000645 hr·ft2·°F/Btu). The resistance when the fouling factor is
included is 0.000154 m2·°C/W corresponding to a U-value of 6500 W/m2·°C
(1143.6 Btu/hr·ft2·°F). Tube foulingin this case reduces the condensing capacity
25% compared with the clean condition. The user has some protection because
condensers leave the factory with a higher U-value than indicated by catalog
data. They are derated using a fouling factor specified in the catalog. The user,
however, should be aware that if the cooling tower water is fouling the condenser,
frequent tube cleaning can improve system performance
7.6 COOLING TOWERS
A cooling tower cools water by spraying it through a stream of ambient air. A
schematic diagram of a cooling tower and the manner in which it serves the
refrigeration condenser are shown in Fig. 7.6. The air- and water-flow patterns
suggested in Fig. 7.6 are counterflow of air and water, a frequently used
configuration. Another popular geometry is crossflow, in which the air is blown
horizontally through the falling stream of water. Because some water evaporates
into the air, a supply of makeup water must be provided. Also, because the makeup
water contains some dissolved minerals, the concentration of these minerals in
the sump water would progressively increase were a blowdown not provided.
The explanation of the heat- and mass-transfer process in a cooling tower
starts with the recollection of the straight-line law first introduced in Sec. 6.13.
The straight-line law states that when air is in contact with water, the change
in air conditions is a straight line on the psychrometric chart directed toward
the saturation line at the water temperature. This information is used to
examine what happens to the enthalpy (heat content) of the air. If the enthalpy
of air increases in the process, the enthalpy and temperature of the water must
decrease. Consider first the special case shown in Fig. 7.7, where the wet-bulb
temperature of the air equals the water temperature.
The path of the air moves toward the saturation line at the water temperature,
which is along the wet-bulb temperature line. The wet-bulb temperature lines
and the enthalpy lines are essentially parallel, so there is no change in the
enthalpy of air, and the temperature of water does not change either. This is
the process that takes place in evaporative coolers that reduce the air
temperature in homes in arid regions.
If the temperature of the water is higher than the wet-bulb temperature of
the air, as in Fig. 7.8, the enthalpy of the air increases from point 1 to point 2,
so an energy balance requires that this heat must come from the water by
cooling it from point 1' to point 2
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