Dinc Ibrahim. Refrigeration systems and applications 2th edition

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Refrigeration System Components 131

Figure 3.19 A typical air-cooled condenser (Courtesy of Trane Company).

On the other hand, they have some disadvantages as given below:

• high condensing temperatures,

• high refrigerant cost because of long piping runs,

• high power requirements per kW of cooling,

• high noise intensity, and

• multiple units required for large-capacity systems.

3.6.3 Evaporative Condensers

Evaporative condensers are apparently water-cooled designs and work on the principle of cooling

by evaporating water into a moving air stream. The effectiveness of this evaporative cooling process

depends upon the wet-bulb temperature of the air entering the unit, the volume of airﬂow, and the

efﬁciency of the air/water interface.

Evaporative condensers use water sprays and airﬂow to condense refrigerant vapors inside the

tubes. The condensed refrigerant drains into a tank called a liquid receiver. Refrigerant subcooling

can be accomplished by piping the liquid from the receiver back through the water sump where

additional cooling reduces the liquid temperature even further.

In an evaporative condenser (Figure 3.20a), the ﬂuid to be cooled is circulated inside the tubes

of the unit’s heat exchanger. Heat ﬂows from the process ﬂuid through the coil tubes to the

water outside, which is cascading downward over the tubes. Air is forced upward through the

coil evaporating a small percentage of the water, absorbing the latent heat of vaporization, and

discharging the heat to the atmosphere. The remaining water falls to the sump to be recircu-

lated by the pump, while water entrained in the air stream is reclaimed and returned to the sump

by the mist eliminators at the unit discharge. The only water consumed is the amount evapo-

rated plus a small amount which is intentionally bled off to limit the concentration of impurities

in the pan. With the optional extended surface coil, the recirculating water pump can be shut-

off and the unit operated dry during periods of below-design ambient temperatures. Air is still

132 Refrigeration Systems and Applications

(a) (b)

Figure 3.20 (a) An evaporative condenser. (b) A counter-ﬂow cooling tower (Courtesy of Baltimore Aircoil

International).

forced upward through the coil, but the heat is now dissipated to the atmosphere by sensible

cooling alone.

The following are some characteristics of these condensers:

• reduced circulating water for a given capacity,

• water treatment is necessary,

• reduced space requirement,

• small piping sizes and short overall lengths,

• small system pumps, and

• availability of large-capacity units and indoor conﬁgurations.

The volume of water used by evaporative condensers is signiﬁcant. Not only does water evaporate

just to reject the heat, but water must be added to avoid the buildup of dissolved solids in the basins

of the evaporative condensers. If these solids build up to the point that they foul the condenser

surfaces, the performance of the unit can be greatly reduced.

3.6.4 Cooling Towers

Cooling towers (Figure 3.20b) are like evaporative condensers, working on the principle of cooling

by evaporating water into a moving air stream. The effectiveness of this evaporative cooling process

depends upon the wet-bulb temperature of the air entering the unit, the volume of airﬂow, and the

efﬁciency of the air or water interface.

As mentioned above, cooling towers are essentially large evaporative coolers where the cooled

water is circulated to a remote shell and tube refrigerant condenser. Note that the cooling water

Refrigeration System Components 133

is circulating through the tubes while refrigerant vapor condenses and gathers in the lower region

of the heat exchanger. Notice also that this area subcools the refrigerant below the temperature

of condensation by bringing the coldest cooling tower water into this area of the condenser. The

warmed cooling water is sprayed over a ﬁll material in the tower. Some of it evaporates in the

moving air stream. The evaporative process cools the remaining water.

The volume of water used by cooling towers is signiﬁcant. Not only does water evaporate just

to reject the heat but it must also be added to avoid the buildup of dissolved solids in the basins of

the cooling towers. If these solids build up to the point that they foul the condenser surfaces, the

performance of the unit can be greatly reduced.

3.6.5 Energy and Exergy Analyses of Condensers

Condensers are used to reject heat from a refrigeration system. In a vapor-compression refrigeration

cycle, the refrigerant is cooled and condensed as it ﬂows in the condenser coils as shown in

Figure 3.21a. The conservation of mass principle requires that

˙m

=˙m

(3.13)

Referring to Figure 3.21a, energy is entering and leaving by the refrigerant stream and heat is

rejected from the condenser, (

out

). The steady-ﬂow energy balance can be written as (with

negligible kinetic and potential energies)

˙mh

=˙mh

out

(3.14)

out

=˙m(h

− h

)

In the condenser of a household refrigerator, heat is rejected from the refrigerant to the kitchen

air as it ﬂows in the condenser coils. Figure 3.21a represents operation of such a compressor. In

some refrigeration applications (especially large ones), heat is rejected from the refrigerant into

water (water-cooled condenser) in a refrigerant-to-water heat exchanger as shown in Figure 3.21b.

Assuming that heat exchanger is insulated, the energy balance in this case becomes

˙m

+˙m

=˙m

+˙m

(3.15)

˙m

− h

) =˙m

− h

)

where ˙m

and ˙m

are the mass ﬂow rates of the refrigerant and the water. The rate of heat rejected

to water is

out

=˙m

− h

) =˙m

− h

) (3.16)

Condenser

Refrigerant,

Water,

(

)(

)

Condenser

Figure 3.21 The schematic of condensers considered for mass and energy analysis. (a) Air-cooled condenser.

(b) Water-cooled condenser.

134 Refrigeration Systems and Applications

Referring to Figure 3.21a, an entropy balance on the condenser may be written as

−

out

gen

= 

sys

= 0

gen

out

−

(3.17)

gen

+˙ms

−˙ms

=˙m



− s



Then the exergy destruction in the condenser becomes

dest

= T

gen

=˙mT



− s



(3.18)

The exergy destruction can also be determined by writing an exergy balance on the condenser:

−

out

−

dest

= 0

dest

−

out

(3.19)

dest

= (

−

) −

=˙m

[

− h

− T

− s

)

]

−



1 −



The exergy efﬁciency of the condenser may be expressed as the ratio of the exergy of the heat

transferred to the high-temperature medium to the exergy decrease of the refrigerant across the

condenser:

ex,Cond

−



1 −



˙m

[

− h

− T

− s

)

]

= 1 −

dest

−

(3.20)

If we consider Figure 3.21b for the operation of an evaporator, an entropy balance may be

written as

gen

out

−

gen

= ( ˙m

+˙m

) − ( ˙m

+˙m

) (3.21)

=˙m

− s

) −˙m

− s

)

Then the exergy destruction in the evaporator becomes

dest

= T

gen

= T

[

˙m

− s

) −˙m

− s

)

]

(3.22)

Example 3.2

Refrigerant-134a enters the condenser of a refrigeration cycle at 800 kPa and 60

◦

C with a ﬂow rate

of 0.095 kg/s and leaves at the same pressure subcooled by 3.3

◦

C. The refrigerant is condensed by

rejecting its heat to water which experiences a temperature rise of 11

◦

C. Determine (a) the rate of

heat rejected in the condenser, (b) the mass ﬂow rate of water, (c) the COP of this refrigeration

cycle if the cooling load at these conditions is 12 kW, and (d) the rate of exergy destruction in the

condenser. Take T

= 25

◦

Refrigeration System Components 135

Solution

(a) We refer to Figure 3.21b for the schematic of the condenser. The properties of refrigerant at

the inlet and exit states of the condenser are (from Tables B.3, B.4, and B.5)

= 800 kPa

= 60

◦



= 296.81 kJ/kg

= 1.011 kJ/kg · K

= 800 kPa

= T

sat @ 800 kPa

− T

subcool

= 31.3 − 3.3 = 28

◦



∼

f @28

◦

= 90.69 kJ/kg

∼

f @28

◦

= 0.3383 kJ/kg · K

The rate of heat rejected in the condenser is

=˙m

− h

) = (0.095 kg/s)(296.81 − 90.69) kJ/kg = 19.58 kW

(b) The mass ﬂow rate of water can be determined from an energy balance on the condenser:

=˙m

− h

) =˙m

T

19.58 kW =˙m

(4.18 kJ/kg ·

◦

C)(11

◦

˙m

= 0.426 kg/s

The speciﬁc heat of water is taken to be 4.18 kJ/kg ·

◦

COP =

−

12 kW

(19.58 − 12) kW

= 1.58

(d) The entropy generation and the exergy destruction in the condenser are

gen

=˙m

− s

) +

= (0.095 kg/s)(0.3383 − 1.011) kJ/kg · K +

19.58 kW

298 K

= 0.001794 kW/K

dest

= T

gen

= (298 K)(0.001794 kJ/kg · K) = 0.5345 kW

3.7 Evaporators

Evaporator can be considered as the point of heat capture in a refrigeration system and provides

the cooling effect required for any particular application. There are almost as many different types

of evaporators as there are applications of heat exchangers. However, evaporators are divided into

two categories such as (i) direct cooler evaporators that cool air that, in turn, cools the product

and (ii) indirect cooler evaporators that cool a liquid such as brine solution that, in turn, cools

the product. Normally, the proper evaporator comes with the system. However, there may be an

occasion when designing a system, that one will need to determine the requirements and select the

proper evaporator from a manufacturer’s catalog or manual.

In practice, the following evaporators are commonly used for cooling, refrigerating, freezing,

and air-conditioning applications:

• liquid coolers,

• air coolers, and/or gas coolers.

136 Refrigeration Systems and Applications

Shell

Baffle and

support plate

Shell

fluid

Tube

fluid

Head

Tie rod and

spacer

Tube

Figure 3.22 A shell-tube type evaporator (Bejan, 2004).

3.7.1 Liquid Coolers

Shell and tube type heat exchangers (Figure 3.22) are the more common form of evaporation units

for water cooling and chilling applications. These are utilized to cool liquids, which can be used

as the secondary refrigerant or to cool the ﬁnal products directly. In practice, these types of heat

exchangers are known as liquid coolers or chillers.

Some example applications in food and refrigeration industry are

• chilling of drinkable water,

• chilling of water for air-conditioning coils,

• chilling of milk after pasteurization, and

• process cooling operations.

Chilled water systems can use either a ﬂooded evaporator or a direct expansion evaporator which

are typically shell and tube type heat exchangers. In a ﬂooded evaporator, refrigerant ﬂoods the

shell side of the heat exchanger and is controlled by a level valve. Water being chilled passes

through the tubes. Conversely, in a direct expansion evaporator, water is carried in the shell and

refrigerant is boiled inside the tubes. The rate of refrigerant ﬂow is throttled to insure that only

refrigerant gas exits the evaporator. Copper tubes mounted within a carbon steel shell is the most

common construction used for chilled water evaporators.

It is important to note that if the refrigerant vaporizes on the outside surface of the tubes the

evaporator is a ﬂooded cooler; if it vaporizes inside the tubes the evaporator is a dry cooler (note

that in this more common type, the mixture of liquid and vapor is evaporated completely), usu-

ally with some degree of superheating (Hewitt, Shires and Bott, 1994). In a ﬂooded cooler the

water or brine is circulated through the tubes, which are usually ﬁnned to provide an incre-

ment in the heat-transfer rate and a decrease in the evaporator size. In a dry cooler the liq-

uid refrigerant is contained within the tubes, and water or brine is circulated through the shell

of the cooler, which serves as an evaporator. Flooded coolers are often speciﬁed for applica-

tions where shell-side vaporization of refrigerant of other liquids is desirable. Due to rapid boil-

ing in the shell, in order to obtain high purity vapors, a vapor disengagement vessel is often

welded to the main shell. Flooded coolers are particularly employed in multiple compressor

systems.

Refrigeration System Components 137

(a)

(

)

Figure 3.23 Air coolers. (a) Room type. (b) Large-scale industrial type (Courtesy of Super Radiator Coils).

3.7.2 Air and Gas Coolers

These coolers are generally called direct expansion coils and consist of a series of tubes through

which refrigerant ﬂows (Figure 3.23). The tubes, which are ﬁnned to increase the heat-transfer rate

from the medium to be cooled (e.g., air) to the boiling point, are normally arranged into a number

of parallel circuits fed from a single throttling valve. The hot refrigerant vapor is accumulated in the

outlet (suction) gas header. These direct expansion coils are used only in the positive displacement

compressor systems, owing to quite low-pressure ratios. Like liquid coolers, these coolers are also

classiﬁed as ﬂooded and dry types. In a ﬂooded coil, a ﬂoat valve is used to maintain the preset

level in the coil, meaning that evaporator coil is kept close to full of the liquid refrigerant. This full

contact of the liquid with the tube walls provides a high heat-transfer rate. In practical applications,

ﬂooded-type evaporators are not preferable, because they require large amounts of refrigerant. A

dry coil requires only a small amount of refrigerant and this reduces the cost of the refrigerant

charge. Sometimes a metering device (thermal expansion valve) regulates the amount of the liquid

entering the coil to maintain a predetermined amount of superheat in the refrigerant at the coil

outlet. The dry expansion coil contains mostly liquid at the inlet and only superheated vapor at

the outlet, after absorbing heat from the medium to be cooled. In the air coolers, when the surface

temperatures fall below 0

◦

C, frosting occurs. Thick layers of frost act as insulation and reduce the

airﬂow rate (in the forced convection coils) and the available inner space.

Several methods are used for defrosting, for example, hot-gas defrost and water defrost. But

recently, frost-free refrigeration systems have become popular because of the problems men-

tioned above.

3.7.3 Energy and Exergy Analyses of Evaporators

Evaporators are used to absorb heat from the refrigerated space. In a vapor-compression refrigeration

cycle, the refrigerant is evaporated as it ﬂows in the evaporator coils as shown in Figure 3.24a.

The conservation of mass principle requires that

˙m

=˙m

(3.23)

Referring to Figure 3.24a, energy is entering and leaving by the refrigerant stream and heat is

absorbed from the cooled space, (

). The steady-ﬂow energy balance can be written as

(with negligible kinetic and potential energies)

˙mh

=˙mh

(3.24)

=˙m(h

− h

)

138 Refrigeration Systems and Applications

Evaporator

Refrigerant,

Evaporator

Water,

(a) (b)

Figure 3.24 The schematic of evaporators considered for mass and energy analysis. (a) Refrigerant absorbing

heat from a space. (b) Refrigerant absorbing heat from water.

In the evaporator of a household refrigerator, heat is absorbed from the freezer section as it ﬂows

in the evaporator coils. Figure 3.24a represents operation of such a compressor. Some refrigeration

systems are used to cool a ﬂuid stream in the evaporator. Figure 3.24b shows a heat exchanger

operating as an evaporator in which water is cooled as the refrigerant is evaporated. Assuming that

heat exchanger is insulated, the energy balance in this case becomes

˙m

+˙m

=˙m

+˙m

(3.25)

˙m

− h

) =˙m

− h

)

where ˙m

and ˙m

are the mass ﬂow rates of the refrigerant and the water. The rate of heat absorbed

by the refrigerant (and rejected from the water) is

=˙m

− h

) =˙m

− h

) (3.26)

Referring to Figure 3.24a, an entropy balance on the evaporator may be written as

−

out

gen

= 

sys

= 0

gen

out

−

(3.27)

gen

=˙ms

−˙ms

−

=˙m



− s

−



Then the exergy destruction in the evaporator becomes

dest

= T

gen

=˙mT



− s

−



(3.28)

The exergy destruction can also be determined by writing an exergy balance on the evaporator:

−

out

−

dest

= 0

dest

−

out

dest

=−

−

(3.29)

dest

= (

−

) −

=˙m

[

− h

− T

− s

)

]

−



−



1 −



Refrigeration System Components 139

The exergy efﬁciency of the evaporator may be expressed as the ratio of the exergy increase of

the cold space as a result of losing heat to the exergy decrease of the refrigerant due to receiving

heat from the cold reservoir.

ex,Evap

−



1 −



˙m

[

− h

− T

− s

)

]

= 1 −

dest

−

(3.30)

If we consider Figure 3.24b for the operation of an evaporator, an entropy balance may be

written as

gen

out

−

gen

= ( ˙m

+˙m

) − ( ˙m

+˙m

) (3.31)

=˙m

− s

) −˙m

− s

)

Then the exergy destruction in the evaporator becomes

dest

= T

gen

= T

[

˙m

− s

) −˙m

− s

)

]

(3.32)

Example 3.3

Heat is absorbed from a cooled space at 32

◦

F at a rate of 320 Btu/min by refrigerant-22

that enters the evaporator at −12

◦

F with a quality of 0.3 and leaves as saturated vapor at

the same pressure. Determine (a) the volume ﬂow rates of R-22 at the evaporator inlet and

outlet and (b) the rate of exergy destruction in the evaporator and the exergy efﬁciency of the

evaporator. Take T

= 77

◦

F. The properties of R-22 at the inlet and exit of the evaporator are

as follows:

= 102.67 Btu/lbm, s

= 0.2776 Btu/lbm · R, v

= 0.5332 ft

/lbm

= 169.82 Btu/lbm, s

= 0.4276 Btu/lbm · R, v

= 1.750 ft

/lbm

Solution

(a) The mass ﬂow rate of R-22 may be determined from an energy balance on the evaporator to

be (see Figure 3.24a)

=˙m(h

− h

) −→ 320/60 Btu/s =˙m(169.82 − 102.67) Btu/lbm −→ ˙m = 0.0794 lbm/s

The volume ﬂow rate at the evaporator inlet and outlet are

=˙mv

= (0.0794 lbm/s)(0.5332 ft

/lbm) = 0.04235 ft

/s = 2.54 ft

/min

=˙mv

= (0.0794 lbm/s)(1.750 ft

/lbm) = 0.139 ft

/s = 8.34 ft

/min

(b) The entropy generation and the exergy destruction are

gen

=˙m(s

− s

) −

= (0.0794 lbm/s)(0.4276 − 0.2776) Btu/lbm · R −

5.33 Btu/s

492 R

= 0.001073 Btu/s · R

dest

= T

gen

= (537 R)(0.001073 Btu/s · R) = 0.576 Btu/s

140 Refrigeration Systems and Applications

The exergy decrease of the refrigerant as it ﬂows in the evaporator is

−

=˙m(h

− h

) −˙mT

− s

)

= 5.33 − (0.0794 lbm/s)(537 R)(0.2776 − 0.4276) Btu/lbm · R

= 1.06 Btu/s

The exergy efﬁciency is then

ex,Evap

= 1 −

dest

−

= 1 −

0.576

1.06

= 0.458 = 45.8%

3.8 Throttling Devices

In practice, throttling devices, called either expansion valves or throttling valves, are used to reduce

the refrigerant condensing pressure (high pressure) to the evaporating pressure (low pressure) by a

throttling operation and regulate the liquid-refrigerant ﬂow to the evaporator to match the equipment

and load characteristics. These devices are designed to proportion the rate at which the refrigerant

enters the cooling coil to the rate of evaporation of the liquid refrigerant in the coil; the amount

depends, of course, on the amount of heat being removed from the refrigerated space. The most

common throttling devices are

• thermostatic expansion valves,

• constant-pressure expansion valves,

• ﬂoat valves, and

• capillary tubes.

Note that a practical refrigeration system may consist of a large range of mechanical and elec-

tronic expansion valves and other ﬂow-control devices for small- and large-scale refrigeration

systems, comprising thermostatic expansion valves, solenoid valves, thermostats and pressostats,

modulating pressure regulators, ﬁlter driers, liquid indicators, nonreturn valves and water valves,

and furthermore, decentralized electronic systems for full regulation and control.

3.8.1 Thermostatic Expansion Valves

The thermostatic expansion valves are essentially reducing valves between the high-pressure side

and the low-pressure side of the system. These valves, which are the most widely used devices,

automatically control the liquid-refrigerant ﬂow to the evaporator at a rate that matches the system

capacity to the actual load. They operate by sensing the temperature of the superheated refrigerant

vapor leaving the evaporator. For a given valve type and refrigerant, the associated oriﬁce assembly

is suitable for all versions of the valve body and in all evaporating temperature ranges.

When the thermostatic expansion valve is operating properly, the temperature at the outlet side

of the valve is much lower than that at the inlet side. If this temperature difference does not exist

when the system is in operation, the valve seat is probably dirty and clogged with foreign matter.

Once a valve is properly adjusted, further adjustment should not be necessary. The major problem

can usually be traced to moisture or dirt collecting at the valve seat and oriﬁce. Figure 3.25 shows

a common type of electrically driven expansion valve.