Dinc Ibrahim. Refrigeration systems and applications 2th edition

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Refrigeration System Components 141

1. Temperature senso

2. External equalizer

3. From condenser

4. To coil

Figure 3.25 An electronic expansion valve (Courtesy of Danfoss A/S ).

3.8.2 Constant-Pressure Expansion Valves

The constant-pressure valve is the forerunner of the thermostatic expansion valve. It is called an

automatic expansion valve because of the fact that it opens and closes automatically without the

aid of any external mechanical device. These expansion valves are basically pressure regulating

devices. These valves maintain a constant pressure at outlet. They sense and keep the evaporated

pressure at a constant value by controlling the liquid-refrigerant ﬂow into the evaporator, based

on the suction pressure. The refrigerant ﬂows at a rate that exactly matches compressor capacity.

Their applications are limited because of the constant cooling load.

3.8.3 Float Valves

These valves are divided into high-side ﬂoat valves and low-side ﬂoat valves. They are employed

to control the refrigerant ﬂow to a ﬂooded-type liquid cooler. A high-side ﬂoat valve is located

on the high-pressure side of the throttling device. It is used in a refrigeration system with a

single evaporator, compressor, and condenser. A low-side ﬂoat valve is particularly located on the

low-pressure side of the throttling device and may be used in refrigeration systems with multiple

evaporators. In some cases, a ﬂoat valve operates an electrical switch controlling a solenoid valve

which periodically admits the liquid refrigerant to the evaporator, allowing the liquid level to

ﬂuctuate within preset limits.

3.8.4 Capillary Tubes

The capillary tube is the simplest type of refrigerant-ﬂow-control device and may be used in place

of an expansion valve. The capillary tubes are small-diameter tubes through which the refrigerant

ﬂows into the evaporator. These devices, which are widely used in small hermetic-type refrigeration

systems (up to 30 kW capacity), reduce the condensing pressure to the evaporating pressure in

a copper tube of small internal diameter (0.4–3 mm diameter and 1.5–5 m long), maintaining a

constant evaporating pressure independently of the refrigeration load change. These tubes are used

to transmit pressure from the sensing bulb of some temperature control device to the operating

element. A capillary tube may also be constructed as a part of a heat exchanger, particularly in

household refrigerators.

142 Refrigeration Systems and Applications

Temperature

control

Head

pressure

control

Liquid line

Filter-Drier

Solenoid

valve

Moisture liquid

indicator

"A" Series

thermo

valve

Fan

cycling

control

Condenser

Receiver

Ball

valve

Solenoid

valve

Hot gas

bypass

regulator

Temperature

control

Evaporator

pressure

regulator

Suction

accumulator

Suction line

filter-drier

Low pressure

control

Oil pressure

safety control

Dual

pressure

control

Compressor

Oil

filter

Traxoil

Oil

separator

Liquid

indicator

Muffler

Figure 3.26 A practical vapor-compression refrigeration system with all control devices (Courtesy of ALCO

Controls).

With capillary tubes, the length of the tube is adjusted to match the compressor capacity. Other

considerations in determining capillary tube size include condenser efﬁciency and evaporator size.

Capillary tubes are most effective when used in small-capacity systems.

Figure 3.26 shows an excellent diagram of a practical vapor-compression refrigeration system

with all control devices.

3.8.5 Energy and Exergy Analyses of Throttling Devices

Throttling devices are used to decrease pressure of a ﬂuid. In a vapor-compression refrigeration

cycle, the refrigerant enters the throttling valve as a liquid and leaves as a saturated liquid–vapor

mixture (Figure 3.27). The conservation of mass principle requires that

˙m

=˙m

(3.33)

Figure 3.27 The schematic of a throttling valve considered for mass and energy analysis.

Refrigeration System Components 143

Referring to Figure 3.27, energy is entering and leaving by the refrigerant stream. Heat transfer

with the surroundings is negligible and there is no work interaction. Then the steady-ﬂow energy

balance can be written as (with negligible kinetic and potential energies)

˙mh

=˙mh

−→ h

= h

(3.34)

That is, a throttling valve is essentially an isenthalpic (i.e., constant enthalpy) device. Noting

that enthalpy is deﬁned as the sum of internal energy u and ﬂow energy Pv,wehave

= h

−→ u

+ P

= u

+ P

(3.35)

In a throttling valve, pressure decreases and speciﬁc volume increases. The internal energy must

decrease in order to achieve a drop in temperature. This requires that the increase in speciﬁc volume

is greater than the decrease in pressure. The large increase in speciﬁc volume is made possible by

turning some of the liquid into vapor.

Referring to Figure 3.27, an entropy balance on the throttling valve may be written as

−

out

gen

= 

sys

= 0

gen

out

−

(3.36)

=˙ms

−˙ms

=˙m(s

− s

)

Then the exergy destruction in the throttling valve becomes

dest

= T

gen

=˙mT

− s

) (3.37)

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

−

out

−

dest

= 0

dest

−

out

dest

−

=˙m

[

− h

− T

− s

)

]

(3.38)

The exergy efﬁciency of the throttling valve may be expressed as the ratio of the exergy recovered

to the exergy expended.

ex,ExpValve

= 1 −

dest

−

= 1 −

−

(3.39)

Note that there is no exergy recovered in an expansion valve, and thus the exergy efﬁciency is

zero.

Example 3.4

Refrigerant-134a enters the throttling valve of a heat pump system at 800 kPa as a saturated liquid

and leaves at 140 kPa. Determine (a) the temperature of R-134a at the outlet of the throttling valve

and (b) the entropy generation and the exergy destruction during this process. Take T

= 25

◦

144 Refrigeration Systems and Applications

Solution

(a) The properties of refrigerant at the inlet and exit states of the throttling valve are (from

Table B.4)

= 800 kPa

= 0



= 95.47 kJ/kg

= 0.35404 kJ/kg · K

= 140 kPa

= h

= 95.447 kJ/kg



=−18.8

◦

= 0.3797 kJ/kg · K

(b) Noting that the throttling valve is adiabatic, the entropy generation is determined from

gen

= s

− s

= (0.3797 − 0.35404) kJ/kg · K = 0.0257 kJ/kg · K

Then the irreversibility (i.e., exergy destruction) of the process becomes

dest

= T

gen

= (298 K)(0.0257 kJ/kg · K) = 7.65 kJ/kg

3.9 Auxiliary Devices

3.9.1 Accumulators

It is well known that compressors are designed to compress vapors, not liquids. Many refrigeration

systems are subject to the return of excessive quantities of liquid refrigerant to the compressor.

Liquid refrigerant returning to the compressor dilutes the oil, washes out the bearings, and in some

cases causes complete loss of oil in the compressor crankcase. This condition is known as oil

pumping or slugging and results in broken valve reeds, pistons, rods, crankshafts, and the like. The

purpose of the accumulator is to act as a reservoir to temporarily hold the excess oil-refrigerant

mixture and to return it at a rate that the compressor can safely handle. Some accumulators include

a heat-exchanger coil to aid in boiling of the liquid refrigerant while subcooling the refrigerant in

the liquid line (Figure 3.28), thus helping the system to operate more efﬁciently. Note that proper

installation of a suction accumulator in the suction line just after the reversing valve and before

the compressor helps eliminate the possible damage.

In large holdover plate refrigerator and freezer systems, refrigerant can accumulate in the plates

and suction line when the compressor is not running. On start-up, this liquid refrigerant can be

suddenly dumped into the compressor, creating a situation of liquid slugging of refrigerant and oil.

This can cause damage to the compressor. When installed in the suction line of the compressor, a

suction accumulator protects the compressor from this liquid slugging by gradually feeding liquid

refrigerant into the compressor.

Note that accumulators should be selected according to the tonnage, evaporator temperature, and

holding capacity.

3.9.2 Receivers

Some of the refrigeration units have enough space within the condenser to accommodate the

entire refrigerant charge of the system. If the condenser does not have sufﬁcient space, a receiver

tank should be provided. The amount of refrigerant required for proper operation of the system

determines whether or not a receiver is required. In practice, when proper unit operation requires

approximately 3.6 kg or more of refrigerant, the use of a receiver is essential (Langley, 1982).

Refrigeration System Components 145

Figure 3.28 An accumulator (Courtesy of Standard Refrigeration Company ).

(

)(

)

Figure 3.29 Receivers. (a) Horizontal design. (b) Vertical design (Courtesy of Standard Refrigeration

Company).

Receivers (Figure 3.29) are required on refrigeration systems that use an expansion valve for

refrigerant control. The receiver provides a place to store the excess refrigerant in the system when

the expansion valve restricts the ﬂow to the evaporator. Receivers are not required, however, when

using a capillary metering system. In addition to accommodating ﬂuctuations in the refrigerant

charge, the receiver aims to maintain the condenser drained of liquid, thereby preventing the liquid

level from building up in the condenser and reducing the amount of effective condenser surface area.

146 Refrigeration Systems and Applications

Figure 3.30 A coalescing oil separator (Courtesy of Standard Refrigeration Company).

3.9.3 Oil Separators

Oil separators (Figure 3.30) provide oil separation and limit oil carry-over to approximately

0.0003−0.001% of the total amount of refrigerant, depending on various system characteristics,

for example, operating conditions, refrigerant, start/stop, load/unload frequency, and so on. These

separators are normally used for a large variety of refrigerants, for example, ammonia, R-134a

and propane. Note that all the separators require the mounting of an external ﬂoat assembly to

control return from the separator to the compressor.

3.9.4 Strainers

Strainers remove foreign matter such as dirt and metal chips from the refrigerant lines. If left in

the system, unwanted matter could clog the small oriﬁces of the ﬂow-control devices and check

valves and also enter the compressor. Various types are available such as straight-through sealed

type, cleanable angle type, and the cleanable Y type.

3.9.5 Driers

In refrigeration systems, moisture is the single most detrimental factor in a refrigeration system. A

unit can stand only a very small amount of moisture. For this reason, the majority of both ﬁeld-

and factory-assembled refrigeration systems are equipped with driers. Some factors inﬂuence the

selection of the correct size of drier (Langley, 1983), for example,

• type and amount of refrigerant,

• refrigeration system tonnage,

• line size, and

• allowable pressure drop.

When the refrigerant type, line size, and equipment application are known, the drier is generally

selected on the basis of recommended capacities, which take into account both drying and refrigerant

ﬂow capacity.

3.9.6 Check Valves

Check valves are used for two essential goals: (i) to cause the refrigerant to ﬂow through the ﬂow-

control device and (ii) to allow the refrigerant to bypass the ﬂow-control device. These valves are

Refrigeration System Components 147

installed in a loop that bypasses the ﬂow-control device and only open when pressure is exerted

in the right direction; therefore, they should be installed with the arrow pointing in the proper

direction of refrigerant ﬂow at the point of installation. In operation, the refrigerant pushes either

against the valve seat to close it tighter or against its face to cause it to open and allow refrigerant

to pass through. These valves are usually spring loaded and will open when the pressure difference

on the seat reaches about 100 to 135 kPa.

3.9.7 Solenoid Valves

Solenoid valves are extensively used in all types of refrigeration applications. These valves are

employed as electrically operated line stop valves and perform in the same manner as hand shut-

off valves. These valves are convenient for remote applications because of the fact that these are

electrically operated and controlled easily.

3.9.8 Defrost Controllers

A defrost controller with timer (Figure 3.31) operates various control valves and fan relays to

quickly and efﬁciently remove frost and ice accumulation from evaporator surfaces. There are four

easy-to-set defrost steps:

• pump out,

• hot gas,

• equalize, and

• fan delay.

This controller uses reliable, solid-state electronics with a precision quartz time clock and time

interval adjusting slide knobs to sequentially operate through the four steps for smooth defrosting.

Figure 3.31 A defrost controller with timer (Courtesy of Hansen Technologies Corporation).

148 Refrigeration Systems and Applications

Each step is clearly indicated by a bright light emitting diode (LED) during operation. Terminals

for optional sensor defrost initiation and termination are provided. A 24-hour quartz time clock

facilitates simple setting in 15-minute increments of defrost start times. A 7-day quartz time clock

for weekly scheduling is also available. All time clocks have 72-hour battery backup in case of

short-term power failure. Because of its time-adjustable four-step defrost operation, this controller

is suitable for almost every defrost application including top and bottom feed unit coolers, blast

freezer evaporators, ice makers, etc.

3.10 Concluding Remarks

This chapter has dealt with a large number of theoretical and practical topics in refrigeration systems,

covering the history of refrigeration, refrigeration system components, and auxiliary equipment

and their technical and operational aspects, and their mass and energy analyses along with the

representative examples.

Nomenclature

A area, m

COP coefﬁcient of performance

constant-pressure speciﬁc heat, kJ/kgK

CR compression ratio

ex speciﬁc exergy, kJ/kg

E energy rate, kW

h enthalpy, kJ/kg

˙m mass ﬂow rate, kg/s

P pressure, kPa

Q rate of heat transfer, kW

R ratio of clearance volume to displacement volume

s entropy, kJ/kg

gen

entropy generation rate, kJ/kg · K

T temperature,

◦

CorK

u internal energy, kJ/kg

v speciﬁc volume, m

/kg

V volume, m

V volumetric ﬂow rate, m

W work input to compressor, kW

x quality

Greek Letters

η efﬁciency

ρ density, kg/m

Study Problems

Refrigeration System Components

3.1 What are the major components of a vapor-compression refrigeration system?

Refrigeration System Components 149

Compressors

3.2 What are the two main functions of a compressor in a refrigeration cycle?

3.3 What are the two main categories of refrigerant compressors?

3.4 What are the desirable characteristics of a compressor?

3.5 What criteria are considered in the selection of a proper refrigerant compressor?

3.6 What are the main characteristics of hermetic compressors?

3.7 What are the main applications of semihermetic compressors?

3.8 What is the difference between hermetic and semihermetic compressors?

3.9 What are the three types of positive displacement compressors?

3.10 What are the main parameters affecting the efﬁciencies of reciprocating compressors?

3.11 What are the general design conﬁgurations of rotary compressors?

3.12 Describe operating principle of rotary compressors.

3.13 What are the suitable applications of rotary compressors?

3.14 What are the basic advantages of vane compressors?

3.15 Is screw compressor a positive displacement compressor? What is the temperature range for

screw compressors?

3.16 Describe the operating principle of a screw compressor?

3.17 What are the basic advantages of screw compressors?

3.18 What is the basic operating principle of dynamic compressors? What are the main types?

3.19 What are the suitable applications of centrifugal compressors in place of positive displace-

ment compressors? What is the suitable load range for centrifugal compressors?

3.20 What is the basic operating principle of a centrifugal compressor?

3.21 How are compressors rated? Deﬁne compression ratio for a compressor.

3.22 What are the factors inﬂuencing the performance of a compressor?

3.23 Does lowering suction temperature decrease the power input to a refrigerant compressor?

Explain.

3.24 It is known that the higher the compression ratio of a compressor the lower the efﬁciency.

Explain how the higher compression ratios be avoided.

3.25 List the methods of compressor capacity control.

3.26 Refrigerant-134a enters the compressor of a refrigeration cycle at 120 kPa gage pressure.

The condenser is maintained at an absolute pressure of 800 kPa. If the atmospheric pressure

is 95 kPa, determine the compression ratio of the compressor.

3.27 Refrigerant-134a enters the compressor of a refrigeration cycle at 100 kPa and −20

◦

C with

a ﬂow rate of 1.8 m

/min and leaves at 700 kPa and 50

◦

C. Determine (a) the power input,

(b) the isentropic efﬁciency, and (c) the exergy destruction and the exergy efﬁciency of the

compressor. Take T

= 25

◦

150 Refrigeration Systems and Applications

3.28 Refrigerant-134a enters the compressor of a refrigeration cycle at 160 kPa as a saturated

vapor with a ﬂow rate of 6.5 m

/min and leaves at 900 kPa. The compressor isentropic

efﬁciency is 75%. Determine (a) the temperature of R-134a at the exit of the compressor

and (b) the exergy destruction and the exergy efﬁciency of the compressor. Take T

= 25

◦

Compressor

160 kPa

Sat. vap.

6.5 m

/min

900 kP

3.29 Refrigerant-134a enters the compressor of a refrigeration cycle at 100 kPa and −20

◦

C with a

ﬂow rate of 0.18 m

/min and leaves at 700 kPa and 50

◦

C. The ratio of the clearance volume

to the displacement volume is 0.05. Determine the volumetric efﬁciency of the compressor.

3.30 Refrigerant-134a enters the compressor of a refrigeration cycle at 160 kPa as a saturated

vapor and leaves at 900 kPa. The compressor volumetric efﬁciency is 85% and the ratio

of the clearance volume to the displacement volume is 0.04. Determine the temperature of

R-134a at the exit of the compressor.

3.31 Refrigerant-134a enters the evaporator of a refrigeration cycle at 200 kPa with a vapor mass

fraction of 0.15 and leaves at 1200 kPa as a saturated vapor with a ﬂow rate of 0.045 m

/min.

The volumetric efﬁciency of the compressor is 92%. Determine the refrigeration capacity

of the system.

Condensers

3.32 What criteria are used in the selection of condensers?

3.33 What are the main types of condensers?

3.34 What are the advantages and disadvantages of air-cooled condensers?

3.35 Describe the operating principle of a cooling tower.

3.36 What is the effect of climatic conditions on the effectiveness of evaporative condensers?

3.37 Refrigerant-134a enters the condenser of a refrigeration cycle at 1000 kPa and 80

◦

C with a

ﬂow rate of 0.038 kg/s and leaves at the same pressure subcooled by 4.4

◦

C. The refrigerant

is condensed by rejecting its heat to water, which experiences a temperature rise of 9

◦

Determine (a) the rate of heat rejected in the condenser, (b) the mass ﬂow rate of water,

and (c) the rate of cooling if the COP of this refrigeration cycle at these conditions is 1.4.

Refrigerant, m

Condenser

Water, m