EVAPORATORS
AIR COILS
AND LIQUID
CHILLERS
6.1 WHERE THE REFRIGERATION
LOAD ENTERS THE SYSTEM
The evaporator is the component of the refrigeration system where a fluid stream
or a product is cooled. As Fig. 6.1 shows, the evaporator is the interface between
the process and the refrigeration system. With the exception of direct cooling a
product, such as in a plate freezer, most evaporators chill air or such liquids as
water, brine, or antifreeze.
6.2 TYPES OF EVAPORATORS
Two major categories of evaporators used in industrial refrigeration practice
are air coils and liquid chillers. A typical air coil is shown in Fig. 6.2 and several
types of liquid chillers are shown in Fig. 6.3. In the air coil, refrigerant flows
through the tubes and air passes over the outside of the tubes. For effective
heat transfer, fins are fastened to the outside of the tubes and the air flows
between the fins.
The liquid-chilling evaporators in Figs. 6.3a and 6.3b are of the shell-andtube
design, while Figs. 6.3c is a plate-type chiller. In Fig. 6.3a the refrigerant
boils in the shell while the liquid flows through the tubes. In Fig. 6.3b the roles
of the tubes and shell are reversed. The plate-type evaporator in Fig. 6.3c is
growing in market share and is an adaptation of the plate-type heat exchanger
used for many years in the food industry. Some of its popularity is attributable
to its compactness and also that the refrigerant charge is less than in a shelland-
tube evaporator. The reduced charge characteristic is attractive both when
ammonia and the new chlorine-free refrigerants are used. A low-charge ammonia
system is desirable for safety reasons, and a low-charge chlorine-free system
minimizes the غير مجاز مي باشدt of these expensive refrigerants.
This chapter is divided into three parts: Part I, Evaporator Performance,
addresses the principles applicable to all types of evaporators; Part II, Air Coils
PART I. EVAPORATOR PERFORMANCE
6.3 EVAPORATOR HEAT TRANSFER
This chapter is not intended for the engineers of evaporator manufacturers
who must decide on the refrigerant circuiting, tube arrangement, and other
important details. These designers consider the performance, غير مجاز مي باشدt of materials,
and ease of manufacturing in deciding the configuration of the evaporator.
Instead the audience of this chapter is intended to be the user of evaporators
who should understand the basic performance of the evaporator as a heat
exchanger, how to properly select the evaporator from manufacturer’s catalogs,
and how to install, operate, and maintain the evaporator properly.
As a heat exchanger, the evaporator follows the rules of heat transfer. In the
evaporator of Fig. 6.4 heat flows in series through three resistances—the fluid
side, the metal of the tube, and the refrigerant.
Many engineers visualize a heat-flow process as an analogy to the electric
flow process with the correspondence of terms shown in Table 6.1.
Ohm’s law for electricity states that
Application of the electrical analogy provides a simple means of deriving an
expression for the overall U-value, where U is a term which when multiplied by
the overall temperature difference, tf - tr, and the area yields the rate of heat
transfer q, W (Btu/hr
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6.4 EXTENDED SURFACE )FINS)
A comparison of the magnitudes of the various heat-transfer resistances in
Example 6.1 shows that the major resistance is on the fluid side, where the
fluid in this case is air. The heat-transfer resistance on the air side is 0.01305
m2·°C/W (0.0741 hr·ft2·°F/Btu) while on the refrigerant side the resistance is
0.000833 m2·°C/W (0.00473 hr·ft2·°F/Btu), so the air-side resistance is 20 times
that of the refrigerant side. This comparison suggests immediately where
attention should be directed if an increase in the U-value were desired. Certainly
not on the refrigerant side, because even if the coefficient could somehow be
doubled, the U-value would increase by only 3%. No, the resistance to attack is
that on the air side, and this resistance can be decreased only by increasing hf
or increasing the area ratio.
Several methods of increasing hf are to increase the air velocity and to increase
the turbulence by introducing irregularities in the heat-transfer surface.
Increasing the air velocity will increase hf but at the expense of additional fan
power. The coil manufacturer seeks the optimum air velocity which provides a
respectable hf but requires a reasonably sized fan and motor. A further
consideration affected by the fan power is that the power to the fan motor
ultimately appears as refrigeration load, and perhaps 10 to 20% of the heat
removed by the air coil was ultimately introduced by the fan and its motor.
The standard approach to reducing the air-side resistance is to increase the
area ratio Ao/Ai by the application of extended surface or fins. The air coil
shown in Fig. 6.2 is equipped with fins which are formed from flat metal plates
that are then punched and the tubes inserted in the holes. When the tubes are
in position, they are expanded either hydraulically or mechanically to provide
good thermal contact between the tube and fin. The section of fin associated
with one tube is shown in Fig. 6.6 in a situation where the tube temperature is
0°C (32°F) and the air temperature is 6°C (42.8°F). If the entire fin were at the
same temperature as the tube, namely 0°C (32°F), the resistance on the air
side would be as stated in Eq. 6.8, Ai/hfAo. Figure 6.6 indicates, however, that
the fin temperature increases at positions progressively further removed from
the tube.
So all of the air-side area Ao is not 100% effective. The effectiveness of the
fin is usually given the symbol and the equation for the U-value of the
finned coil is
(usually steel or aluminum), fin thickness, and distance from the tube. The
effects of several choices on fin effectiveness and on the overall heat-transfer
capacity of the coil are shown in Table. 6.2. The designer for the coil manufacturer
spends nights trying to juggle these various decisions to provide the maximum
heattransfer rate for a given غير مجاز مي باشدt of the coil.
Most of the foregoing discussion has implied that the evaporator is an
aircooling coil, and indeed fins are almost universal on air coils, but not always.
In some food plant applications the evaporator is composed of unfinned tubes.
The reason is that these coils are easier to clean for hygienic purposes, even
though the heat-transfer rate suffers. It should be pointed out that while the
bare-tube coil is easier to clean, the coils must often be deep with a large number
of rows of tubes in the air-flow direction which complicates cleaning.
A further comment to complete the discussion of extended surface is that
fins are sometimes used on the refrigerant side in liquid-chilling evaporators.
In water-chilling evaporators where refrigerant flows in the tubes, the tubes
are often equipped with inner fins that are sometimes rifled. This style is
especially adaptable to copper tubes which can be used in halocarbon
refrigeration systems, but not ammonia. For ammonia systems, aluminum tubes
can be provided with internal and/or external integral fins, and even for steel
tubes the possibility of external integral fins exists.
6.5 TEMPERATURE DIFFERENCE:
BETWEEN ENTERING FLUID AND
REFRIGERANT
Engineers who expect the appearance of the logarithmic-mean-temperature
difference or complex heat-exchanger effectiveness factors to be encountered in
selecting and analyzing the performance of evaporators will be pleased to know
of a simplification. The streamlined process is made possible by the fact that
6.6 REFRIGERANT BOILING INSIDE
TUBES
The mechanism of refrigerant boiling inside a tube of an evaporator is complex.
There are more than 4000 technical papers on the subject, so a person could
make a career of studying boiling heat-transfer. Even the prediction of heattransfer
coefficients for a given refrigerant in a certain size tube with a specified
flow rate is difficult. Fortunately, the designer and manufacturer of the
evaporator take over that task, and the engineer who selects or uses the
evaporator normally does not need to. However, the application engineer should
understand what occurs during the boiling process.
One evaporator concept is direct expansion, discussed further in Sec. 6.27.
The direct-expansion evaporator receives refrigerant from the expansion valve
with a small fraction of vapor as shown in Fig. 6.8.
As the warm tube adds heat to the refrigerant, progressively more refrigerant
evaporates, and the velocity increases until the refrigerant leaves the evaporaor
saturated or superheated. Figure 6.8 also shows typical boiling heat-transfer
coefficients corresponding to the position along.the evaporator tube. The changes
in the heat-transfer coefficient are associated with differing patterns of flow2 as
the fraction of vapor and the velocity change along the tube. At the entering
section of the evaporator, bubbles and plugs of vapor flow along with the liquid.
Further along the tube, the flow becomes annular with high-velocity vapor
rushing through the center and the liquid clinging to the inside surface of the
tube. Still later in the evaporator, the flow converts to a mist and eventually
there could be a nonequilibrium mixture of superheated vapor and liquid until
all the liquid finally evaporates.
The benefit of being aware of a distribution of heat-transfer coefficients as
in Fig. 6.8 is to understand why such concepts as liquid recirculation (Sec. 6.7)
have some heat-transfer advantages and to be able to diagnose operating
problems attributable to the refrigerant-side heat transfer.
6.7 METHODS OF SUPPLYING
REFRIGERANT TO
EVAPORATORS—DIRECT
EXPANSION, FLOODED
EVAPORATORS AND LIQUID
RECIRCULATION
The techniques for feeding the evaporators with refrigerant provide one form of
categorizing the types of evaporators.
Direct expansion. In what is referred to as direct expansion, liquid refrigerant
enters the expansion valve and only vapor leaves the evaporator, as shown in
Fig. 6.9. One of the most popular types of expansion valves that facilitates this
control is the superheat-controlled valve, which is also called a thermo-valve,
thermostatic valve, or a TXV. More on this type of control will be covered in
Chapter 11 on valves and refrigerant controls, but it is sufficient at this point to
explain that the valve controls the flow rate of refrigerant such that the vapor
leaves the evaporator superheated by from 4 to 7°C (7 to 12°F). Direct expansion
is limited to evaporators where the refrigerant evaporates in the tubes.
If the sensing bulb detects a higher-than-setpoint superheat at the evaporator
outlet, the valve opens further. The evaporator fed by a superheat-controlled
expansion valve is probably the lowest in first غير مجاز مي باشدt of the three methods described
here. It is used widely with halocarbon refrigerants at moderate refrigerating
temperatures, but its use is limited in low-temperature applications and for
ammonia. More on the possibilities of direct-expansion coils with ammonia will
be found in Sec. 6.27.
Flooded evaporator. The flooded evaporator, shown in Fig. 6.9, relies on
natural convection to circulate more refrigerant through the evaporator than
what evaporates. All inside surfaces of the evaporator are thus wetted with
liquid refrigerant. The vapor formed in the evaporator is separated in the surge
drum and flows to the suction line.
A level-control valve admits liquid refrigerant to replace the amount
vaporized. The difference in static pressure in the liquid leg is greater than
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