Dinc Ibrahim. Refrigeration systems and applications 2th edition

Подождите немного. Документ загружается.

Refrigeration Cycles and Systems 211

4.16 A refrigerator operates on the ideal vapor-compression refrigeration cycle with refrigerant-

134a as the working ﬂuid. The evaporator pressure is 160 kPa and the temperature at the exit

of the condenser is 35.5

◦

C. The ﬂow rate at the compressor inlet is 0.85 m

/min. Determine

(a) the rate of heat absorption in the evaporator, (b) the rate of heat rejection in the condenser,

4.17 An 18,000 Btu/h air conditioner operates on the ideal vapor-compression refrigeration cycle

with R-22 as the refrigerant. The evaporator pressure is 125 kPa and the condenser pressure is

1750 kPa. If the air conditioner operates at full cooling load, determine (a) the mass ﬂow rate

of R-22, (b) the power input, and (c) the COP. The enthalpies of R-22 at various states are

given as (R-22 tables are not available in the text): h

= 389.67 kJ/kg, h

= 458.68 kJ/kg,

= 257.20 kJ/kg, h

= 257.20 kJ/kg.

4.18 An ideal vapor-compression refrigeration cycle uses R-134a as the refrigerant. The refrigerant

enters the evaporator at 160 kPa with a quality of 25% and leaves the compressor at 65

◦

C. If

the compressor consumes 800 W of power, determine (a) the mass ﬂow rate of the refrigerant,

(b) the condenser pressure, and (c) the COP of the refrigerator.

4.19 A vapor-compression refrigeration cycle with refrigerant-134a as the working ﬂuid operates

between pressure limits of 240 and 1600 kPa. The isentropic efﬁciency of the compressor

is 78%. Determine (a) the heat absorption in the evaporator, (b) the heat rejection in the

condenser, (c) the work input, and (d) the COP.

4.20 A refrigerator operates on the vapor-compression refrigeration cycle with refrigerant-134a

as the working ﬂuid. The evaporator pressure is 100 kPa and the condenser pressure is

1400 kPa. The ﬂow rate at the compressor inlet is 0.22 m

/min. The isentropic efﬁciency of

the compressor is 84%. Determine (a) the rate of heat absorption in the evaporator, (b) the

rate of heat rejection in the condenser, (c) the power input, and (d) the COP.

4.21 A refrigerator operates on the vapor-compression refrigeration cycle with refrigerant-134a

as the working ﬂuid. The evaporator pressure is 20 psia and the condenser pressure is

180 psia. The ﬂow rate at the compressor inlet is 8.4 ft

/min. The isentropic efﬁciency of

the compressor is 86%. Determine (a) the rate of heat absorption in the evaporator, (b) the

rate of heat rejection in the condenser, (c) the power input, and (d) the COP.

4.22 An automotive air conditioner operates on the vapor-compression refrigeration cycle with

refrigerant-134a as the working ﬂuid. The refrigerant enters the compressor at 180 kPa super-

heated by 2.7

◦

C at a rate of 0.007 kg/s and leaves the compressor at 1200 kPa and 60

◦

R-134a is subcooled by 6.3

◦

C at the exit of the condenser. Determine (a) the isentropic

efﬁciency of the compressor, (b) the rate of cooling, and (c) the COP.

4.23 A vapor-compression refrigeration cycle with refrigerant-134a as the working ﬂuid operates

between pressure limits of 240 and 1600 kPa. The isentropic efﬁciency of the compressor

is 78%. The refrigerant is superheated by 5.4

◦

C at the compressor inlet and subcooled by

5.9

◦

C at the exit of the condenser. Determine (a) the heat absorption in the evaporator, (b) the

heat rejection in the condenser, (c) the work input, and (d) the COP. (e) Also determine all

parameters if the cycle operated on the ideal vapor-compression refrigeration cycle between

the same pressure limits.

4.24 A practical refrigerator operates on the vapor-compression refrigeration cycle with

refrigerant-22 as the working ﬂuid. The pressure of R-22 at the compressor exit is 2000

and 300 kPa at the inlet of the evaporator. The isentropic efﬁciency of the compressor

is 75%. The refrigerant is superheated by 5

◦

C at the compressor inlet and subcooled by

◦

C at the exit of the condenser. There is a pressure drop of 50 kPa in the condenser and

212 Refrigeration Systems and Applications

25 kPa in the evaporator. Determine (a) the heat absorption in the evaporator per unit mass

of R-22, (b) the work input, and the COP. (c) Determine the refrigeration load, the work

input, and the COP if the cycle operated on the ideal vapor-compression refrigeration cycle

between the pressure limits of 2000 and 300 kPa.

The properties of R-22 in the case of actual operation are obtained from R-22 tables to be

= 401.62 kJ/kg, h

= 487.29 kJ/kg, h

= 283.76 kJ/kg, h

= 283.76 kJ/kg

The properties of R-22 in the case of ideal operation are obtained from R-22 tables to be

= 399.18 kJ/kg, h

= 459.30 kJ/kg, h

= 293.30 kJ/kg, h

= 293.30 kJ/kg

4.25 An air conditioner with refrigerant-134a as the refrigerant is used to keep a large space

at 20

◦

C by rejecting the waste heat to the outside air at 37

◦

C. The room is gaining heat

through the walls and the windows at a rate of 125 kJ/min while the heat generated by the

computer, TV, and lights amounts to 0.7 kW. Unknown amount of heat is also generated

by the people in the room. The condenser and evaporator pressures are 1200 and 500 kPa,

respectively. The refrigerant is saturated liquid at the condenser exit and saturated vapor

at the compressor inlet. If the refrigerant enters the compressor at a rate of 65 L/min and

the isentropic efﬁciency of the compressor is 70%, determine (a) the temperature of the

refrigerant at the compressor exit, (b) the rate of heat generated by the people in the room,

at the compressor inlet for the same compressor inlet and exit conditions.

500 kPa

Condenser

Evaporator

Compressor

Expansion

valve

1200 kPa

20 °C

37 °C

4.26 A refrigerated room is kept at −27

◦

C by a vapor-compression cycle with R-134a as the

refrigerant. Heat is rejected to cooling water that enters the condenser at 16

◦

Catarateof

0.22 kg/s and leaves at 23

◦

C. The refrigerant enters the condenser at 1.2 MPa and 65

◦

Cand

leaves at 42

◦

C. The inlet state of the compressor is 60 kPa and −34

◦

C and the compressor

is estimated to gain a net heat of 150 W from the surroundings. Determine (a) the quality

of the refrigerant at the evaporator inlet, (b) the refrigeration load, (c) the COP of the

refrigerator, and (d) the theoretical maximum refrigeration load for the same power input to

the compressor.

Refrigeration Cycles and Systems 213

60 kPa

−34 °C

42 °C

Condenser

Evaporator

Compressor

Expansion

valve

1.2 MPa

65 °C

Water

16 °C

23 °C

Exergy Analysis of Refrigeration Cycles

4.27 A refrigeration cycle is used to keep a food department at −15

◦

C in an environment at

◦

C. The total heat gain to the food department is estimated to be 1750 kJ/h and the heat

rejection in the condenser is 3250 kJ/h. Determine (a) the power input to the compressor,

(b) the COP of the refrigerator, and (c) the second-law efﬁciency of the cycle.

4.28 A refrigeration cycle is used to keep a food department at −22

◦

F in an environment at

◦

F. The total heat gain by the food department is estimated to be 7900 Btu/h and the heat

rejection in the condenser is 4150 Btu/h. Determine (a) the power input to the compressor,

(b) the COP of the refrigerator, and (c) the second-law efﬁciency of the cycle.

4.29 A commercial refrigerator is to cool eggplants from 26 to 5

◦

C at a rate of 380 kg/h. The

power input to the refrigerator is 4.5 kW. Determine (a) the rate of cooling, (b) the COP,

law efﬁciency and the exergy destruction for the cycle. The speciﬁc heat of eggplant above

freezing is 3.92 kJ/kg·

◦

4.30 A refrigeration system absorbs heat from a space at 2

◦

C at a rate of 6.9 kW and rejects heat

to water in the condenser. Water enters the condenser at 16

◦

C at a rate of 0.27 kg/s. The COP

of the system is estimated to be 1.85. Determine (a) the power input to the system, (b) the

temperature of the water at the exit of the condenser, and (c) the second-law efﬁciency and

the exergy destruction for the refrigerator. Take the dead-state temperature to be the inlet

temperature of water in the condenser. The speciﬁc heat of water is 4.18 kJ/kg·

◦

4.31 A refrigerator using R-134a as the refrigerant is used to keep a space at −10

◦

C by rejecting

heat to ambient air at 22

◦

C. R-134a enters the compressor at 140 kPa as a saturated vapor

and leaves at 800 kPa and 70

◦

C. The refrigerant leaves the condenser as a saturated liquid.

The rate of cooling provided by the system is 2600 W. Determine (a) the mass ﬂow rate

of R-134a, (b) the COP, (c) the exergy destruction in each component of the cycle, (d) the

second-law efﬁciency of the cycle, and (e) the total exergy destruction in the cycle.

4.32 A refrigerator using R-134a as the refrigerant is used to keep a space at 15

◦

F by rejecting

heat to ambient air at 75

◦

F. R-134a enters the compressor at 25 psia as a saturated vapor

and leaves at 140 psia and 160

◦

F. The refrigerant leaves the condenser as a saturated liquid.

214 Refrigeration Systems and Applications

The rate of cooling provided by the system is 9000 Btu/h. Determine (a) the mass ﬂow rate

of R-134a, (b) the COP, (c) the exergy destruction in each component of the cycle, (d) the

second-law efﬁciency of the cycle, and (e) the total exergy destruction in the cycle.

4.33 A refrigerated room is kept at −18

◦

C by a vapor-compression cycle with R-134a as the

refrigerant. Heat is rejected to cooling water that enters the condenser at 14

◦

Catarateof

0.35 kg/s and leaves at 22

◦

C. The refrigerant enters the condenser at 1.2 MPa and 50

◦

and leaves at the same pressure subcooled by 5

◦

C. If the compressor consumes 5.5 kW of

power, determine (a) the mass ﬂow rate of the refrigerant, (b) the refrigeration load and the

COP, (c) the second-law efﬁciency of the refrigerator and the total exergy destruction in

the cycle, and (d) the exergy destruction in the condenser. Take speciﬁc heat of water to be

4.18 kJ/kg·

◦

1.2 MPa

5 °C subcool

Evaporator

Compressor

Expansion

valve

1.2 MPa

50 °C

Water

14 °C

22 °C

Condenser

4.34 An ideal vapor-compression refrigeration cycle with refrigerant-134a as the working ﬂuid

operates between pressure limits of 200 and 1600 kPa. The refrigerant absorbs heat from a

space at 3

◦

C and rejects heat to ambient air at 27

◦

C. Determine (a) the heat absorbed in the

evaporator and the work input, (b) the COP, (c) the exergy destruction in each component

of the cycle and the total exergy destruction in the cycle, (d) the second-law efﬁciency of

the cycle.

4.35 A 45,000 Btu/h refrigeration system operates on the ideal vapor-compression refrigeration

cycle with R-22 as the refrigerant. The evaporator pressure is 150 kPa and the condenser pres-

sure is 1500 kPa. The refrigerant exchanges heat with air at −5

◦

C in the evaporator and with

air at 19

◦

C in the condenser. If the air conditioner operates at full cooling load, determine (a)

the mass ﬂow rate of R-22, (b) the power input and the COP, and (c) the second-law efﬁciency

of the cycle and the total exergy destruction in the cycle. The enthalpies of R-22 at various

states are given as (R-22 tables are not available in the text): h

= 391.58 kJ/kg, s

= 1.8051

kJ/kg·K, h

= 450.98 kJ/kg, h

= 248.58 kJ/kg, s

= 1.1632 kJ/kg·K, h

= 248.58 kJ/kg,

= 1.2119 kJ/kg·K. Take T

= 19

◦

4.36 An automotive air conditioner operates on the vapor-compression refrigeration cycle with

refrigerant-134a as the working ﬂuid. The refrigerant absorbs heat from the air inside the car

at 23

◦

C and rejects heat to ambient air at 36

◦

C. The refrigerant enters the compressor at

Refrigeration Cycles and Systems 215

180 kPa superheated by 2.7

◦

C at a rate of 0.0095 kg/s and leaves the compressor at 1200 kPa

and 60

◦

C. R-134a is subcooled by 6.3

◦

C at the exit of the condenser. Determine (a) the

rate of cooling and the COP, (b) the isentropic efﬁciency and the exergetic efﬁciency of the

compressor, (c) the exergy destruction in each component of the cycle and the total exergy

destruction in the cycle, (d) the minimum power input and the second-law efﬁciency of the

cycle. Take T

= 36

◦

Air-Standard Refrigeration Systems

4.37 An ideal gas refrigeration cycle with a pressure ratio of four uses air as the working ﬂuid.

Air enters the compressor at 100 kPa and 0

◦

C and the turbine at 50

◦

C. Determine (a) the

temperature at the turbine exit, (b) the heat removed per unit mass of the air, and (c) the COP

of the cycle. Use constant speciﬁc heat for air at room temperature with c

= 1.005 kJ/kg·K

and k = 1.4.

4.38 A gas refrigeration cycle with a pressure ratio of four uses air as the working ﬂuid. Air enters

the compressor at 100 kPa and 0

◦

C and the turbine at 50

◦

C. The isentropic efﬁciencies of the

compressor and turbine are 84%. Determine (a) the temperature at the turbine exit, (b) the

heat removed per unit mass of the air, and (c) the COP of the cycle. Use constant speciﬁc

heat for air at room temperature with c

= 1.005 kJ/kg·Kandk = 1.4.

4.39 Argon gas enters the compressor of a gas refrigeration cycle at 40 kPa and −35

◦

Cataﬂow

rate of 7500 L/min and leaves at 130 kPa and 125

◦

C. The argon enters the turbine at 40

◦

The isentropic efﬁciency of the turbine is 88%. Determine (a) the minimum temperature

in the cycle, (b) the isentropic efﬁciency of the compressor, (c) the net power input to the

cycle, (d) the rate of refrigeration, and (e) the COP of the cycle. Use constant speciﬁc heat

for argon with c

= 0.5203 kJ/kg·K, R = 0.2081 kJ/kg·K, and k = 1.667.

4.40 Argon gas enters the compressor of a gas refrigeration cycle at 6 psia and −30

◦

Fataﬂow

rate of 265 ft

/min and leaves at 19 psia and 255

◦

F. The argon enters the turbine at 105

◦

The isentropic efﬁciency of the turbine is 83%. Determine (a) the minimum temperature in

the cycle, (b) the isentropic efﬁciency of the compressor, (c) the net power input to the cycle

in kW, (d) the rate of refrigeration in Btu/h, and (e) the COP of the cycle. Use constant

speciﬁc heat for argon with c

= 0.1253 Btu/lbm·R, R = 0.04971 Btu/lbm·R, and k = 1.667.

4.41 Air enters the compressor of an ideal gas refrigeration system with a regenerator at −20

◦

at a ﬂow rate of 0.12 kg/s. The cycle has a pressure ratio of 4.5. The temperature of the air

decreases from 15 to −28

◦

C in the regenerator. Both the turbine and compressor are assumed

to be isentropic. Determine (a) the rate of refrigeration, (b) the power input, and (c) the COP

of the cycle. Use constant speciﬁc heat for air at room temperature with c

= 1.005 kJ/kg·K

and k = 1.4.

4.42 Air enters the compressor of a gas refrigeration system with a regenerator at −20

◦

Cata

ﬂow rate of 0.12 kg/s. The cycle has a pressure ratio of 4.5. The temperature of the air

decreases from 15 to −28

◦

C in the regenerator. The isentropic efﬁciency of the compressor

is 85% and that of the turbine is 80%. Determine (a) the rate of refrigeration, (b) the power

input, and (c) the COP of the cycle. Use constant speciﬁc heat for air at room temperature

with c

= 1.005 kJ/kg·Kandk = 1.4.

4.43 Consider a gas refrigeration system with air as the working ﬂuid. The pressure ratio is 5.5.

Air enters the compressor at 0

◦

C. The high-pressure air is cooled to 35

◦

C by rejecting heat

to the surroundings. The refrigerant leaves the turbine at −95

◦

C and then it absorbs heat

from the refrigerated space before entering the regenerator. The mass ﬂow rate of air is

216 Refrigeration Systems and Applications

0.55 kg/s. Assuming isentropic efﬁciencies of 90% for both the compressor and the turbine,

determine (a) the effectiveness of the regenerator, (b) the rate of heat removal from the

refrigerated space, and (c) the COP of the cycle. Also, determine (d) the refrigeration load

and the COP if this system operated on the simple gas refrigeration cycle. In this cycle,

take the compressor and turbine inlet temperatures to be 0 and 35

◦

C, respectively, and use

the same compressor and turbine efﬁciencies. Use constant speciﬁc heat for air at room

temperature with c

= 1.005 kJ/kg·Kandk = 1.4.

Compressor

Heat

exchanger

Heat

exchanger

Regenerator

Turbine

4.44 A gas refrigeration cycle with a pressure ratio of six operates between a cooled space

temperature of 3

◦

C and an ambient temperature of 22

◦

C. Air enters the compressor at 90 kPa

and 10

◦

C and the turbine at 65

◦

C. Both the compressor and turbine are isentropic. Determine

(a) the heat removed per unit mass of the air, (b) the COP of the cycle, (c) the exergy

destruction in each component of the cycle and the total exergy destruction in the cycle,

and (d) the second-law efﬁciency of the cycle. Use constant speciﬁc heat for air at room

temperature with c

= 1.005 kJ/kg·Kandk = 1.4.

4.45 Helium gas enters the compressor of a gas refrigeration cycle at 65 kPa and −25

◦

Cata

ﬂow rate of 9000 L/min and leaves at 240 kPa and 160

◦

C. The helium enters the turbine at

◦

C. The isentropic efﬁciency of the turbine is 85%. Determine (a) the isentropic efﬁciency

and the exergetic efﬁciency for the compressor, (b) the rate of refrigeration and the COP of

the cycle, (c) the exergy destruction in each component of the cycle and the total exergy

destruction in the cycle, and (d) the exergy efﬁciency of the compressor, the minimum power

input, and the second-law efﬁciency of the cycle. The temperature of the cooled space is

−15

◦

C and heat is rejected to the ambient at −5

◦

C. Use constant speciﬁc heat for argon

with c

= 5.1926 kJ/kg·K, R = 2.0769 kJ/kg·K, and k = 1.667. Take T

=−5

◦

4.46 Air enters the compressor of a gas refrigeration system with a regenerator at −28

◦

Cata

ﬂow rate of 0.75 kg/s. The cycle has a pressure ratio of four. The temperature of the air

decreases from 12 to −35

◦

C in the regenerator. The isentropic efﬁciency of the compressor

is 85% and that of the turbine is 80%. Determine (a) the rate of refrigeration and the COP of

the cycle and (b) the minimum power input, the second-law efﬁciency of the cycle, and the

total exergy destruction in the cycle. The temperature of the cooled space is −40

◦

C and heat

Refrigeration Cycles and Systems 217

is rejected to the ambient at 7

◦

C. Use constant speciﬁc heat for air at room temperature with

= 1.005 kJ/kg·Kandk = 1.4. (c) Determine the minimum power input, the second-law

efﬁciency of the cycle, and the total exergy destruction in the cycle if the temperature of the

cooled space is −20

◦

Absorption-Refrigeration Systems (ARSs)

4.47 An ARS removes heat from a cooled space at 2

◦

C at a rate of 66 kW. The system operates

in an environment at 20

◦

C. If the heat is supplied to the cycle by condensing saturated steam

at 200

◦

C at a rate of 0.04 kg/s. Determine (a) the COP of the system and (b) the COP of a

reversible ARS operating between the same temperatures. The enthalpy of vaporization of

water at 200

◦

Cish

= 1940.34 kJ/kg.

4.48 Consider a reversible ARS that can be modeled to consist of a reversible heat engine and a

reversible refrigerator. The system removes heat from a cooled space at −7

◦

Catarateof

22 kW. The refrigerator operates in an environment at 25

◦

C. If the heat is supplied to the

cycle by condensing saturated steam at 175

◦

C, determine (a) the rate at which the steam

condenses and (b) the power input to the reversible refrigerator. The enthalpy of vaporization

of water at 175

◦

Cish

= 2031.7 kJ/kg.

4.49 Consider a basic ARS using ammonia–water solution as shown in the ﬁgure. Pure ammonia

enters the condenser at 400 psia and 150

◦

F at a rate of 0.013 lbm/s. Ammonia leaves the

condenser as a saturated liquid and is throttled to a pressure of 25 psia. Ammonia leaves the

evaporator as a saturated vapor. Heat is supplied to the generator by geothermal liquid water

that enters at 260

◦

F at a rate of 0.18 lbm/s and leaves at 220

◦

F. Determine (a) the rate of

cooling provided by the system in Btu/h and tons of refrigeration and (b) the COP of the sys-

tem. (c) Also, determine the second-law efﬁciency of the system if the ambient temperature

is 77

◦

F and the temperature of the refrigerated space is 32

◦

F. The enthalpies of ammonia at

various states of the system are given as follows: h

= 646.0 Btu/lbm, h

= 216.5 Btu/lbm,

= 616.8 Btu/lbm. Also, take the speciﬁc heat of water to be 1.0 Btu/lbm·

◦

4.50 Consider a basic ARS using ammonia–water solution as shown in the ﬁgure of the previous

problem. Pure ammonia enters the condenser at 3200 kPa and 70

◦

C. Ammonia leaves the

condenser as a saturated liquid and is throttled to a pressure of 220 kPa. Ammonia leaves

218 Refrigeration Systems and Applications

the evaporator as a saturated vapor. Heat is supplied to the solution in the generator by solar

energy, which is incident on the collector at a rate of 550 W/m

. The total surface area

of the collectors is 31.5 m

and the efﬁciency of the collectors is 75% (i.e., 75% of the

solar energy input is transferred to the solution). If the COP of the system is estimated to

be 0.8, determine the mass ﬂow rate of ammonia through the evaporator. The enthalpies of

ammonia at various states of the system are given as h

= 1491.9 kJ/kg, h

= 537.0 kJ/kg,

and h

= 1442.0 kJ/kg.

References

ASHRAE (1997) Handbook of Fundamentals, American Society of Heating, Refrigerating and AirConditioning

Engineers, Atlanta, GA.

Ataer, O.E. and Gogus, Y. (1991) Comparative study of irreversibilities in an aqua-ammonia absorption refrig-

eration system. International Journal of Refrigeration, 14, 86–92.

Dincer, I. (2003) Refrigeration Systems and Applications, 1st edn, John Wiley & Sons, Ltd., New York.

Dincer, I. and Dost, S. (1996) A simple model for heat and mass transfer in absorption cooling systems (ACSs).

International Journal of Energy Research, 20, 237–243.

Dincer, I., Edin, M. and Ture, I.E. (1996) Investigation of thermal performance of a solar powered absorption

refrigeration system. Energy Conversion and Management, 37, 51–58.

Dincer, I. and Ture, I.E. (1993) Design and Construction of a Solar Powered Absorption Cooling System ,

Proceedings of the International Symposium on Energy Saving and Energy Efﬁciency, 16–18 November,

Ankara, pp. 198–203.

Eames, I.W. and Wu, S. (2000) A theoretical study of an innovative ejector powered absorption-recompression

cycle refrigerator. International Journal of Refrigeration, 23, 475–484.

Gosney, W.B. (1982) Principles of Refrigeration, Cambridge University Press, Cambridge, UK.

Jelinek, M., Yaron, I. and Borde, I. (1980) Measurement of Vapour–Liquid Equilibria and Determination of

Enthalpy-Concentration Diagrams of Refrigerant-Absorbent Combinations, Proceedings of IIR, Commissions

B1, B2, E1, E2, Mons (Belgium), pp. 57–65.

Kaita, Y. (2001) Simulation results of triple-effect absorption cycles. International Journal of Refrigeration ,

25, 999–1007.

Kang, Y.T., Kunugi, Y. and Kashiwagi, T. (2000) Review of advanced absorption cycles: performance improve-

ment and temperature lift enhancement. International Journal of Refrigeration, 23, 388–401.

Keizer, C. (1982) Absorption refrigeration machines, Ph.D. Thesis, Delft University of Technology, Delft, The

Netherlands.

Khan, J.R. and Zubair, S.M. (2000) Design and rating of an integrated mechanical-subcooling vapor-

compression refrigeration system. Energy Conversion and Management, 41, 1201–1222.

Newell, T.A. (2000) Thermodynamic analysis of an electrochemical refrigeration cycle. International Journal

of Energy Research, 24, 443–453.

Norton, E. (2000) A look at hot gas defrost. ASHRAE Journal , 42, 88.

Patent Storm (2010) Triple Effect Refrigeration System, US Patent 5727397 issued on 17 March 1998,

http://www.patentstorm.us/patents/5727397/fulltext.html.

Rockwell, T.C. and Quake, T.D. (2001) Improve refrigeration system efﬁciency, process cooling equipment

(online magazine at: http://www.process-cooling.com ) October, p. 5.

Turpin, J. (2000) New refrigeration technology poised to heat up the market. Engineered Systems, October,4.

Van Den Bulck, E., Trommelmans, J. and Berghmans, J. (1982) Solar Absorption Cooling Installation,Pro-

ceedings of Solar Energy for Refrigeration and Air Conditioning, IIR Commission E1-E2, March 14–15,

Jeruzalem, Israel, pp. 83–87.

Advanced Refrigeration

Cycles and Systems

5.1 Introduction

Refrigeration cycles covered in Chapter 4 are simple and extensively used in most of the refrig-

eration needs encountered in practice. Household refrigerators, small coolers, and air-conditioning

systems are some examples. For other refrigeration applications, the simple vapor-compression cycle

may not be suitable and more advanced and innovative refrigeration systems may have to be used.

Other motivations include the search for improved performance and efﬁciency as well as require-

ments to achieve very low temperatures. In this chapter, we cover some advanced refrigeration

systems as well as special systems used in certain applications.

5.2 Multistage Refrigeration Cycles

Multistage refrigeration systems are widely used where ultralow temperatures are required, but

cannot be obtained economically through the use of a single-stage system. This is due to the fact that

the compression ratios are too large to attain the temperatures required to evaporate and condense the

vapor. There are two general types of such systems: cascade and multistage. The multistage system

uses two or more compressors connected in series in the same refrigeration system. The refrigerant

becomes a denser vapor while it passes through each compressor. Note that a two-stage system

(Figure 5.1) can attain a temperature of approximately −65

◦

C and a three-stage one about −100

◦

Single-stage vapor-compression refrigerators are used by cold storage facilities with a range of

+10 to −30

◦

C. In this system, the evaporator installed within the refrigeration system and the

ice-making unit, as the source of low temperature, absorbs heat. Heat is released by the condenser

at the high-pressure side.

In cases where large temperature and pressure differences exist between the evaporator and the

condenser, multistage vapor-compression systems are employed accordingly. For example, if

the desired temperature of a refrigerator (i.e., freezer) is below −30

◦

C, a several-stage compression

system is required in order to prevent the occurrence of high compression ratios. The following

are some of the disadvantages of a high compression ratio:

• decrease in the compression efﬁciency,

• increase in the temperature of the refrigerant vapor from the compressor, and

• increase in energy consumption per unit of refrigeration production.

Refrigeration Systems and Applications

Ibrahim Dinc¸er and Mehmet Kano

glu

 2010 John Wiley & Sons, Ltd

220 Refrigeration Systems and Applications

Condenser

Log P (kPa)

h (kJ/kg)

Expansion

valve

Flash

intercooler

Evaporator

High-pressure

compressor

Low-pressure

compressor

Expansion

valve

(

)(

)

Figure 5.1 (a) A two-stage vapor-compression refrigeration system, (b) its T −s diagram, and (c) its log P −h

diagram.

Figure 5.1 shows a schematic diagram of a two-stage vapor-compression refrigeration unit that

can provide temperatures below −30

◦

C (approximately to −50

◦

C), and its T −s diagram. This

system also uses an intercooler with air.

As an example, three-stage refrigeration systems can provide an evaporator temperature of

−100

◦

C. In the two-stage unit shown, the refrigerant is compressed in the ﬁrst stage and, after

being de-superheated by an intercooler, is further compressed in the second stage. An intercooler is

used in between the two compression stages for reducing the compression work. In other words, a

booster (ﬁrst-stage) compressor and a gas–liquid intercooler are attached to the single-stage cycle.

The intercooler subcools the refrigerant liquid supplied to the evaporator by vaporizing a portion

of the refrigerant after the ﬁrst throttling stage. The ﬂash gas returns at an intermediate point in

the compression process in order to improve the compression efﬁciency by cooling the superheated

gas. Not only a single compressor but a set of compressors is also required to be used in each stage,

depending on the capacity and temperature. In large systems with a number of evaporators and

large compression (temperature) ratios, the number of intercoolers and compression stages yields

increased system efﬁciency and hence increased coefﬁcient of performance (COP).

5.3 Cascade Refrigeration Systems

For some industrial applications that require moderately low temperatures (with a considerably

large temperature and pressure difference), single vapor-compression refrigeration cycles become

impractical. One of the solutions for such cases is to perform the refrigeration in two or more

stages (i.e., two or more cycles) which operate in series. These refrigeration cycles are called

cascade refrigeration cycles. Therefore, cascade systems are employed to obtain high-temperature

differentials between the heat source and heat sink and are applied for temperatures ranging from

−70 to 100

◦

C. Application of a three-stage compression system for evaporating temperatures below

−70

◦

C is limited, because of difﬁculties with refrigerants reaching their freezing temperatures.

Impropriety of multi-stage vapor-compression systems can be avoided by applying a cascade vapor-

compression refrigeration system.

Cascade refrigeration systems are commonly used in the liquefaction of natural gas and some

other gases. A large-capacity industrial cascade refrigeration system is shown in Figure 5.2.

The most important advantage of these cascade systems is that refrigerants can be chosen with

the appropriate properties, avoiding large dimensions for the system components. In these systems

multiple evaporators can be utilized in any one stage of compression. Refrigerants used in each

stage may be different and are selected for optimum performance at the given evaporator and

condenser temperatures.