TERMO

4
Pauta Tarea nº2 (Grupal 2 personas) OBS.I. Responda de forma clara y precisa y enuncie todos los postulados utilizados según corresponda. OBS.2. No se recibirán tareas atrasadas Pregunta nºI Una planta termoeléctrica de vapor de agua opera en el ciclo Rankine con recalentamiento. El vapor entra a la turbina de alta presión a 12.5 MPa y 550ºC, a razón de 7.7 kg/s y sale a 2 MPa. El vapor luego se recalienta a 450ºC antes de expandirse en la turbina de baja presión. Las eficiencias isentrópicas de la turbina y la bomba son 85% y 90%, respectivamente. El vapor sale del condensador como líquido saturado. Si el contenido de humedad del vapor a la salida de la turbina no debe exceder el 5%, determine: a) La presión del condensador, en kPa b)La producción neta de Potencia, en MW c) La eficiencia térmica Obs. Para enumerar las corrientes, especifique la corriente de salida del condensador, con el número (1), salida de la bomba (2) y así correlativamente, según el esquema visto en la clase. Solución ! " ! "! " ! " ! "! " HRSHC E " E%$T D " $UO& $ " EEP& &P " % $ " EEP& V UP " % V K HRSHC $&DP " T HRSHC $ " EEP& 4 OP% *W. $ HRSHC E " E%$T D " $UO& P " EOTQ &P " % P " EOTQ HRSHC D " $UO& *W. $ K HRSHC QEDT " Q HRSHC P " EOTQ 4 PP% *W. P " D$ Q P P Q Q P Q P Q P Q Q Q Q Q P P P P O E E O O E O E O E O O E E E E # $ $ # $ $ # %& % $ $ # # ( ) # # # ( ) # # * # # ( ) + # # # $ $ # $ $ # & $ $ # # ( ) # # * # # ( ) + # # s T s T s s T s T s s h h h h h h h h h s s P h x P s h T P h h h h h h h h h s s P s h T P , , , , D P $s Qs s T E Os D$"P *W. P \V Q O $ E Q D $ ’@-?28) ^523)- 458>)89)- W@67 P O Así, el resultado de la interpolación reiterada, nos permite determinar que: Prof. José Luis Salazar N.

description

termodinamica

Transcript of TERMO

Page 1: TERMO

Pauta Tarea nº2 (Grupal 2 personas)OBS.I. Responda de forma clara y precisa y enuncie todos los postulados utilizados según corresponda. OBS.2. No se recibirán tareas atrasadas

Pregunta nºI Una planta termoeléctrica de vapor de agua opera en el ciclo Rankine con recalentamiento. El vapor entra a la turbina de alta presión a 12.5 MPa y 550ºC, a razón de 7.7 kg/s y sale a 2 MPa. El vapor luego se recalienta a 450ºC antes de expandirse en la turbina de baja presión. Las eficiencias isentrópicas de la turbina y la bomba son 85% y 90%, respectivamente. El vapor sale del condensador como líquido saturado. Si el contenido de humedad del vapor a la salida de la turbina no debe exceder el 5%, determine:

a) La presión del condensador, en kPab) La producción neta de Potencia, en MWc) La eficiencia térmica

Obs. Para enumerar las corrientes, especifique la corriente de salida del condensador, con el número (1), salida de la bomba (2) y así correlativamente, según el esquema visto en la clase.

Solución

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Prof. José Luis Salazar N.

Page 2: TERMO

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Con ésto:

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PROPRIETARY MATERIAL"!#!$%%&!'()!*+,-./01233!4567.82)9:!;8+"!!<262=)>!>29=-2?@=258!7)-62==)>!583A!=5!=).+()-9!.8>!

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=()!8)=!75/)-!5@=7@=:!.8>!=()!=()-6.3!)BB2+2)8+A!.-)!=5!?)!>)=)-628)>"!

Assumptions!1!I=).>A!57)-.=28C!+58>2=2589!)J29="!2!K28)=2+!.8>!75=)8=2.3!)8)-CA!+(.8C)9!.-)!8)C32C2?3)"!

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PROPRIETARY MATERIAL"!#!$%%&!'()!*+,-./01233!4567.82)9:!;8+"!!<262=)>!>29=-2?@=258!7)-62==)>!583A!=5!=).+()-9!.8>!

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=()!8)=!75/)-!5@=7@=:!.8>!=()!=()-6.3!)BB2+2)8+A!.-)!=5!?)!>)=)-628)>"!

Assumptions!1!I=).>A!57)-.=28C!+58>2=2589!)J29="!2!K28)=2+!.8>!75=)8=2.3!)8)-CA!+(.8C)9!.-)!8)C32C2?3)"!

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Page 3: TERMO

Pregunta nº2 Una planta eléctrica de turbina de gas opera en in ciclo simple Brayton entre los límites de presión de 100 y 1.2 MPa. EL fluido de trabajo es aire, que entra al compresor a 30ºC y una razón de 150 m3/min y sale de la turbina a 500ºC. Usando calores específicos variables para el aire y suponiendo una eficiencia isentrópica de compresión de 82% y una eficiencia isentrópica de turbina de 88%, determine:

a) La producción de potencia, en kWb) La relación de Retrotrabajo, definida por la fracción de trabajo de la turbina que se emplea para

accionar el compresor.Solución

PROPRIETARY MATERIAL. © 2008 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and

educators for course preparation. If you are a student using this Manual, you are using it without permission.

9-75

9-105 A gas-turbine plant operates on the simple Brayton cycle. The net power output, the back work ratio,

and the thermal efficiency are to be determined.

Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are

negligible. 3 Air is an ideal gas with variable specific heats.

Properties The gas constant of air is R = 0.287 kJ/kg·K (Table A-1).

Analysis (a) For this problem, we use the properties

from EES software. Remember that for an ideal gas,

enthalpy is a function of temperature only whereas

entropy is functions of both temperature and pressure.

Process 1-2: Compression

KkJ/kg 7159.5kPa 100

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the isentropic efficiency relation, we find h3 = 1404.7 kJ/kg, T3 = 1034ºC, s3 = 6.5699 kJ/kg.K. The

solution by hand would require a trial-error approach.

h_3=enthalpy(Air, T=T_3)

s_3=entropy(Air, T=T_3, P=P_2)

h_4s=enthalpy(Air, P=P_1, s=s_3)

The mass flow rate is determined from

+ ,+ ,kg/s 875.2

K 27330K/kgmkPa 0.287

)s/m 0kPa)(150/6 100(3

3

1

11 "-!!

""RT

Pm

!!!

The net power output is

kW 1100)kJ/kg60.30324kg/s)(686. 875.2()( 12inC, ")")" hhmW !!

kW 1759)kJ/kg62.792.7kg/s)(1404 875.2()( 43outT, ")")" hhmW !!

kW 659")")" 11001759inC,outT,net WWW !!!

(b) The back work ratio is

0.625"""kW 1759

kW 1100

outT,

inC,

bwW

Wr

!

!

(c) The rate of heat input and the thermal efficiency are

kW 2065)kJ/kg24.686.7kg/s)(1404 875.2()( 23in ")")" hhmQ !!

0.319"""kW 2065

kW 659

in

net

Q

Wth !

!

*

1

Combustion

chamber

Turbine

23

4

Compress.

100 kPa

30°C

500°C

1.2 MPa

De aquí, podemos determinar que:

PROPRIETARY MATERIAL. © 2008 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and

educators for course preparation. If you are a student using this Manual, you are using it without permission.

9-75

9-105 A gas-turbine plant operates on the simple Brayton cycle. The net power output, the back work ratio,

and the thermal efficiency are to be determined.

Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are

negligible. 3 Air is an ideal gas with variable specific heats.

Properties The gas constant of air is R = 0.287 kJ/kg·K (Table A-1).

Analysis (a) For this problem, we use the properties

from EES software. Remember that for an ideal gas,

enthalpy is a function of temperature only whereas

entropy is functions of both temperature and pressure.

Process 1-2: Compression

KkJ/kg 7159.5kPa 100

C30

kJ/kg 60.303C30

11

1

11

!"#$%

"

&"

"'('&"

sP

T

hT

kJ/kg 37.617kJ/kg.K 7159.5

kPa 12002

12

2 "#$%

""

"sh

ss

P

kJ/kg 24.68660.303

60.30337.61782.0 2

212

12C "'('

))

"'(')

)" h

hhh

hh s*

Process 3-4: Expansion

kJ/kg 62.792C500 44 "'('&" hT

ss hh

h

hh

hh

43

3

43

43T

62.79288.0

)

)"'('

)

)"*

We cannot find the enthalpy at state 3 directly. However, using the following lines in EES together with

the isentropic efficiency relation, we find h3 = 1404.7 kJ/kg, T3 = 1034ºC, s3 = 6.5699 kJ/kg.K. The

solution by hand would require a trial-error approach.

h_3=enthalpy(Air, T=T_3)

s_3=entropy(Air, T=T_3, P=P_2)

h_4s=enthalpy(Air, P=P_1, s=s_3)

The mass flow rate is determined from

+ ,+ ,kg/s 875.2

K 27330K/kgmkPa 0.287

)s/m 0kPa)(150/6 100(3

3

1

11 "-!!

""RT

Pm

!!!

The net power output is

kW 1100)kJ/kg60.30324kg/s)(686. 875.2()( 12inC, ")")" hhmW !!

kW 1759)kJ/kg62.792.7kg/s)(1404 875.2()( 43outT, ")")" hhmW !!

kW 659")")" 11001759inC,outT,net WWW !!!

(b) The back work ratio is

0.625"""kW 1759

kW 1100

outT,

inC,

bwW

Wr

!

!

(c) The rate of heat input and the thermal efficiency are

kW 2065)kJ/kg24.686.7kg/s)(1404 875.2()( 23in ")")" hhmQ !!

0.319"""kW 2065

kW 659

in

net

Q

Wth !

!

*

1

Combustion

chamber

Turbine

23

4

Compress.

100 kPa

30°C

500°C

1.2 MPa

El flujo másico será:

PROPRIETARY MATERIAL. © 2008 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and

educators for course preparation. If you are a student using this Manual, you are using it without permission.

9-75

9-105 A gas-turbine plant operates on the simple Brayton cycle. The net power output, the back work ratio,

and the thermal efficiency are to be determined.

Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are

negligible. 3 Air is an ideal gas with variable specific heats.

Properties The gas constant of air is R = 0.287 kJ/kg·K (Table A-1).

Analysis (a) For this problem, we use the properties

from EES software. Remember that for an ideal gas,

enthalpy is a function of temperature only whereas

entropy is functions of both temperature and pressure.

Process 1-2: Compression

KkJ/kg 7159.5kPa 100

C30

kJ/kg 60.303C30

11

1

11

!"#$%

"

&"

"'('&"

sP

T

hT

kJ/kg 37.617kJ/kg.K 7159.5

kPa 12002

12

2 "#$%

""

"sh

ss

P

kJ/kg 24.68660.303

60.30337.61782.0 2

212

12C "'('

))

"'(')

)" h

hhh

hh s*

Process 3-4: Expansion

kJ/kg 62.792C500 44 "'('&" hT

ss hh

h

hh

hh

43

3

43

43T

62.79288.0

)

)"'('

)

)"*

We cannot find the enthalpy at state 3 directly. However, using the following lines in EES together with

the isentropic efficiency relation, we find h3 = 1404.7 kJ/kg, T3 = 1034ºC, s3 = 6.5699 kJ/kg.K. The

solution by hand would require a trial-error approach.

h_3=enthalpy(Air, T=T_3)

s_3=entropy(Air, T=T_3, P=P_2)

h_4s=enthalpy(Air, P=P_1, s=s_3)

The mass flow rate is determined from

+ ,+ ,kg/s 875.2

K 27330K/kgmkPa 0.287

)s/m 0kPa)(150/6 100(3

3

1

11 "-!!

""RT

Pm

!!!

The net power output is

kW 1100)kJ/kg60.30324kg/s)(686. 875.2()( 12inC, ")")" hhmW !!

kW 1759)kJ/kg62.792.7kg/s)(1404 875.2()( 43outT, ")")" hhmW !!

kW 659")")" 11001759inC,outT,net WWW !!!

(b) The back work ratio is

0.625"""kW 1759

kW 1100

outT,

inC,

bwW

Wr

!

!

(c) The rate of heat input and the thermal efficiency are

kW 2065)kJ/kg24.686.7kg/s)(1404 875.2()( 23in ")")" hhmQ !!

0.319"""kW 2065

kW 659

in

net

Q

Wth !

!

*

1

Combustion

chamber

Turbine

23

4

Compress.

100 kPa

30°C

500°C

1.2 MPa

Y la potencia neta:

PROPRIETARY MATERIAL. © 2008 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and

educators for course preparation. If you are a student using this Manual, you are using it without permission.

9-75

9-105 A gas-turbine plant operates on the simple Brayton cycle. The net power output, the back work ratio,

and the thermal efficiency are to be determined.

Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are

negligible. 3 Air is an ideal gas with variable specific heats.

Properties The gas constant of air is R = 0.287 kJ/kg·K (Table A-1).

Analysis (a) For this problem, we use the properties

from EES software. Remember that for an ideal gas,

enthalpy is a function of temperature only whereas

entropy is functions of both temperature and pressure.

Process 1-2: Compression

KkJ/kg 7159.5kPa 100

C30

kJ/kg 60.303C30

11

1

11

!"#$%

"

&"

"'('&"

sP

T

hT

kJ/kg 37.617kJ/kg.K 7159.5

kPa 12002

12

2 "#$%

""

"sh

ss

P

kJ/kg 24.68660.303

60.30337.61782.0 2

212

12C "'('

))

"'(')

)" h

hhh

hh s*

Process 3-4: Expansion

kJ/kg 62.792C500 44 "'('&" hT

ss hh

h

hh

hh

43

3

43

43T

62.79288.0

)

)"'('

)

)"*

We cannot find the enthalpy at state 3 directly. However, using the following lines in EES together with

the isentropic efficiency relation, we find h3 = 1404.7 kJ/kg, T3 = 1034ºC, s3 = 6.5699 kJ/kg.K. The

solution by hand would require a trial-error approach.

h_3=enthalpy(Air, T=T_3)

s_3=entropy(Air, T=T_3, P=P_2)

h_4s=enthalpy(Air, P=P_1, s=s_3)

The mass flow rate is determined from

+ ,+ ,kg/s 875.2

K 27330K/kgmkPa 0.287

)s/m 0kPa)(150/6 100(3

3

1

11 "-!!

""RT

Pm

!!!

The net power output is

kW 1100)kJ/kg60.30324kg/s)(686. 875.2()( 12inC, ")")" hhmW !!

kW 1759)kJ/kg62.792.7kg/s)(1404 875.2()( 43outT, ")")" hhmW !!

kW 659")")" 11001759inC,outT,net WWW !!!

(b) The back work ratio is

0.625"""kW 1759

kW 1100

outT,

inC,

bwW

Wr

!

!

(c) The rate of heat input and the thermal efficiency are

kW 2065)kJ/kg24.686.7kg/s)(1404 875.2()( 23in ")")" hhmQ !!

0.319"""kW 2065

kW 659

in

net

Q

Wth !

!

*

1

Combustion

chamber

Turbine

23

4

Compress.

100 kPa

30°C

500°C

1.2 MPa

La relación de retrotrabajo será:

PROPRIETARY MATERIAL. © 2008 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and

educators for course preparation. If you are a student using this Manual, you are using it without permission.

9-75

9-105 A gas-turbine plant operates on the simple Brayton cycle. The net power output, the back work ratio,

and the thermal efficiency are to be determined.

Assumptions 1 The air-standard assumptions are applicable. 2 Kinetic and potential energy changes are

negligible. 3 Air is an ideal gas with variable specific heats.

Properties The gas constant of air is R = 0.287 kJ/kg·K (Table A-1).

Analysis (a) For this problem, we use the properties

from EES software. Remember that for an ideal gas,

enthalpy is a function of temperature only whereas

entropy is functions of both temperature and pressure.

Process 1-2: Compression

KkJ/kg 7159.5kPa 100

C30

kJ/kg 60.303C30

11

1

11

!"#$%

"

&"

"'('&"

sP

T

hT

kJ/kg 37.617kJ/kg.K 7159.5

kPa 12002

12

2 "#$%

""

"sh

ss

P

kJ/kg 24.68660.303

60.30337.61782.0 2

212

12C "'('

))

"'(')

)" h

hhh

hh s*

Process 3-4: Expansion

kJ/kg 62.792C500 44 "'('&" hT

ss hh

h

hh

hh

43

3

43

43T

62.79288.0

)

)"'('

)

)"*

We cannot find the enthalpy at state 3 directly. However, using the following lines in EES together with

the isentropic efficiency relation, we find h3 = 1404.7 kJ/kg, T3 = 1034ºC, s3 = 6.5699 kJ/kg.K. The

solution by hand would require a trial-error approach.

h_3=enthalpy(Air, T=T_3)

s_3=entropy(Air, T=T_3, P=P_2)

h_4s=enthalpy(Air, P=P_1, s=s_3)

The mass flow rate is determined from

+ ,+ ,kg/s 875.2

K 27330K/kgmkPa 0.287

)s/m 0kPa)(150/6 100(3

3

1

11 "-!!

""RT

Pm

!!!

The net power output is

kW 1100)kJ/kg60.30324kg/s)(686. 875.2()( 12inC, ")")" hhmW !!

kW 1759)kJ/kg62.792.7kg/s)(1404 875.2()( 43outT, ")")" hhmW !!

kW 659")")" 11001759inC,outT,net WWW !!!

(b) The back work ratio is

0.625"""kW 1759

kW 1100

outT,

inC,

bwW

Wr

!

!

(c) The rate of heat input and the thermal efficiency are

kW 2065)kJ/kg24.686.7kg/s)(1404 875.2()( 23in ")")" hhmQ !!

0.319"""kW 2065

kW 659

in

net

Q

Wth !

!

*

1

Combustion

chamber

Turbine

23

4

Compress.

100 kPa

30°C

500°C

1.2 MPa

Pregunta nº3Un sistema de refrigeración en cascada (ver apuntes de clase) de dos etapas, opera entre los límites de 1.2 MPa y 200 kPa con refrigerante 134a como fluido de trabajo. El rechazo de calor del ciclo inferior al superior tiene lugar en un intercambiador adiabático a contracorriente en donde las presiones en los ciclos superior en inferior son 0.4 y 0.5 MPa, respectivamente. En ambos ciclos el refrigerante es un líquido saturado a la salida del condensador y un vapor saturado a la entrada del compresor. La eficiencia isentrópica del compresión es del 80%. Si el flujo másico del refrigerante en el ciclo inferior es 0.15 kg/s, determine:

a) El flujo másico del ciclo superior, en kg/sb) La tasa de remoción de calor del espacio refrigerado, en kW

Prof. José Luis Salazar N.

Page 4: TERMO

c) El coeficiente de operación (COP) de este refrigerador

Solución

PROPRIETARY MATERIAL. © 2008 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and

educators for course preparation. If you are a student using this Manual, you are using it without permission.

11-35

11-49 A two-stage cascade refrigeration cycle is considered. The mass flow rate of the refrigerant through

the upper cycle, the rate of heat removal from the refrigerated space, and the COP of the refrigerator are to

be determined.

Assumptions 1 Steady operating conditions exist. 2 Kinetic and potential energy changes are negligible.

Analysis (a) The properties are to be obtained from the refrigerant tables (Tables A-11 through A-13):

kJ/kg.K 9377.0

kJ/kg 46.244

kPa 200 @1

kPa 200 @1

!!

!!

g

g

ss

hh

kJ/kg 30.263kPa 500

212

2 !"#$

!

!sh

ss

P

kJ/kg 01.26846.244

46.24430.26380.0 2

2

12

12

!%&%''

!

'

'!

hh

hh

hh sC(

kJ/kg 33.73

kJ/kg 33.73

34

kPa 500 @3

!!

!!

hh

hh f

kJ/kg.K 9269.0

kJ/kg 55.255

kPa 400 @5

kPa 004 @5

!!

!!

g

g

ss

hh

kJ/kg 33.278kPa 1200

656

6 !"#$

!

!sh

ss

P

kJ/kg 02.28455.255

55.25533.27880.0 6

6

56

56

!%&%''

!

'

'!

hh

hh

hh sC(

kJ/kg 77.117

kJ/kg 77.117

78

kPa 1200 @7

!!

!!

hh

hh f

The mass flow rate of the refrigerant through the upper cycle is determined from an energy balance on the

heat exchanger

kg/s 0.212!%&%'!'

'!'

AA

BA

mm

hhmhhm

!!

!!

kJ/kg)33.7301kg/s)(268. 15.0(kJ/kg)77.117.55255(

)()( 3285

(b) The rate of heat removal from the refrigerated space is

kW 25.67!'!'! kJ/kg)33.7346kg/s)(244. 15.0()( 41 hhmQ BL!!

(c) The power input and the COP are

kW 566.9kJ/kg)46.24401kg/s)(268. 212.0(kJ/kg)55.25502kg/s)(284. 15.0(

)()( 1256in

!')'!

')'! hhmhhmW BA!!!

2.68!!!566.9

67.25COP

in

L

W

Q

!

!

.

5

6 7

8

QH

Condenser

Evaporator

Compressor

Expansion

valve Win

1

2 3

4

Condenser

Evaporator

Compressor

Expansion

valve

QL

Win

.

.

.

El flujo másico de refrigerante se determina con un balance de energía en el Intercambiador de calor (Evaporador-Condensador) según:

PROPRIETARY MATERIAL. © 2008 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and

educators for course preparation. If you are a student using this Manual, you are using it without permission.

11-35

11-49 A two-stage cascade refrigeration cycle is considered. The mass flow rate of the refrigerant through

the upper cycle, the rate of heat removal from the refrigerated space, and the COP of the refrigerator are to

be determined.

Assumptions 1 Steady operating conditions exist. 2 Kinetic and potential energy changes are negligible.

Analysis (a) The properties are to be obtained from the refrigerant tables (Tables A-11 through A-13):

kJ/kg.K 9377.0

kJ/kg 46.244

kPa 200 @1

kPa 200 @1

!!

!!

g

g

ss

hh

kJ/kg 30.263kPa 500

212

2 !"#$

!

!sh

ss

P

kJ/kg 01.26846.244

46.24430.26380.0 2

2

12

12

!%&%''

!

'

'!

hh

hh

hh sC(

kJ/kg 33.73

kJ/kg 33.73

34

kPa 500 @3

!!

!!

hh

hh f

kJ/kg.K 9269.0

kJ/kg 55.255

kPa 400 @5

kPa 004 @5

!!

!!

g

g

ss

hh

kJ/kg 33.278kPa 1200

656

6 !"#$

!

!sh

ss

P

kJ/kg 02.28455.255

55.25533.27880.0 6

6

56

56

!%&%''

!

'

'!

hh

hh

hh sC(

kJ/kg 77.117

kJ/kg 77.117

78

kPa 1200 @7

!!

!!

hh

hh f

The mass flow rate of the refrigerant through the upper cycle is determined from an energy balance on the

heat exchanger

kg/s 0.212!%&%'!'

'!'

AA

BA

mm

hhmhhm

!!

!!

kJ/kg)33.7301kg/s)(268. 15.0(kJ/kg)77.117.55255(

)()( 3285

(b) The rate of heat removal from the refrigerated space is

kW 25.67!'!'! kJ/kg)33.7346kg/s)(244. 15.0()( 41 hhmQ BL!!

(c) The power input and the COP are

kW 566.9kJ/kg)46.24401kg/s)(268. 212.0(kJ/kg)55.25502kg/s)(284. 15.0(

)()( 1256in

!')'!

')'! hhmhhmW BA!!!

2.68!!!566.9

67.25COP

in

L

W

Q

!

!

.

5

6 7

8

QH

Condenser

Evaporator

Compressor

Expansion

valve Win

1

2 3

4

Condenser

Evaporator

Compressor

Expansion

valve

QL

Win

.

.

.

La tasa de remoción de calor del espacio refrigerado será:

PROPRIETARY MATERIAL. © 2008 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and

educators for course preparation. If you are a student using this Manual, you are using it without permission.

11-35

11-49 A two-stage cascade refrigeration cycle is considered. The mass flow rate of the refrigerant through

the upper cycle, the rate of heat removal from the refrigerated space, and the COP of the refrigerator are to

be determined.

Assumptions 1 Steady operating conditions exist. 2 Kinetic and potential energy changes are negligible.

Analysis (a) The properties are to be obtained from the refrigerant tables (Tables A-11 through A-13):

kJ/kg.K 9377.0

kJ/kg 46.244

kPa 200 @1

kPa 200 @1

!!

!!

g

g

ss

hh

kJ/kg 30.263kPa 500

212

2 !"#$

!

!sh

ss

P

kJ/kg 01.26846.244

46.24430.26380.0 2

2

12

12

!%&%''

!

'

'!

hh

hh

hh sC(

kJ/kg 33.73

kJ/kg 33.73

34

kPa 500 @3

!!

!!

hh

hh f

kJ/kg.K 9269.0

kJ/kg 55.255

kPa 400 @5

kPa 004 @5

!!

!!

g

g

ss

hh

kJ/kg 33.278kPa 1200

656

6 !"#$

!

!sh

ss

P

kJ/kg 02.28455.255

55.25533.27880.0 6

6

56

56

!%&%''

!

'

'!

hh

hh

hh sC(

kJ/kg 77.117

kJ/kg 77.117

78

kPa 1200 @7

!!

!!

hh

hh f

The mass flow rate of the refrigerant through the upper cycle is determined from an energy balance on the

heat exchanger

kg/s 0.212!%&%'!'

'!'

AA

BA

mm

hhmhhm

!!

!!

kJ/kg)33.7301kg/s)(268. 15.0(kJ/kg)77.117.55255(

)()( 3285

(b) The rate of heat removal from the refrigerated space is

kW 25.67!'!'! kJ/kg)33.7346kg/s)(244. 15.0()( 41 hhmQ BL!!

(c) The power input and the COP are

kW 566.9kJ/kg)46.24401kg/s)(268. 212.0(kJ/kg)55.25502kg/s)(284. 15.0(

)()( 1256in

!')'!

')'! hhmhhmW BA!!!

2.68!!!566.9

67.25COP

in

L

W

Q

!

!

.

5

6 7

8

QH

Condenser

Evaporator

Compressor

Expansion

valve Win

1

2 3

4

Condenser

Evaporator

Compressor

Expansion

valve

QL

Win

.

.

.

Y el COP

PROPRIETARY MATERIAL. © 2008 The McGraw-Hill Companies, Inc. Limited distribution permitted only to teachers and

educators for course preparation. If you are a student using this Manual, you are using it without permission.

11-35

11-49 A two-stage cascade refrigeration cycle is considered. The mass flow rate of the refrigerant through

the upper cycle, the rate of heat removal from the refrigerated space, and the COP of the refrigerator are to

be determined.

Assumptions 1 Steady operating conditions exist. 2 Kinetic and potential energy changes are negligible.

Analysis (a) The properties are to be obtained from the refrigerant tables (Tables A-11 through A-13):

kJ/kg.K 9377.0

kJ/kg 46.244

kPa 200 @1

kPa 200 @1

!!

!!

g

g

ss

hh

kJ/kg 30.263kPa 500

212

2 !"#$

!

!sh

ss

P

kJ/kg 01.26846.244

46.24430.26380.0 2

2

12

12

!%&%''

!

'

'!

hh

hh

hh sC(

kJ/kg 33.73

kJ/kg 33.73

34

kPa 500 @3

!!

!!

hh

hh f

kJ/kg.K 9269.0

kJ/kg 55.255

kPa 400 @5

kPa 004 @5

!!

!!

g

g

ss

hh

kJ/kg 33.278kPa 1200

656

6 !"#$

!

!sh

ss

P

kJ/kg 02.28455.255

55.25533.27880.0 6

6

56

56

!%&%''

!

'

'!

hh

hh

hh sC(

kJ/kg 77.117

kJ/kg 77.117

78

kPa 1200 @7

!!

!!

hh

hh f

The mass flow rate of the refrigerant through the upper cycle is determined from an energy balance on the

heat exchanger

kg/s 0.212!%&%'!'

'!'

AA

BA

mm

hhmhhm

!!

!!

kJ/kg)33.7301kg/s)(268. 15.0(kJ/kg)77.117.55255(

)()( 3285

(b) The rate of heat removal from the refrigerated space is

kW 25.67!'!'! kJ/kg)33.7346kg/s)(244. 15.0()( 41 hhmQ BL!!

(c) The power input and the COP are

kW 566.9kJ/kg)46.24401kg/s)(268. 212.0(kJ/kg)55.25502kg/s)(284. 15.0(

)()( 1256in

!')'!

')'! hhmhhmW BA!!!

2.68!!!566.9

67.25COP

in

L

W

Q

!

!

.

5

6 7

8

QH

Condenser

Evaporator

Compressor

Expansion

valve Win

1

2 3

4

Condenser

Evaporator

Compressor

Expansion

valve

QL

Win

.

.

.

Prof. José Luis Salazar N.