The refrigerating effect

In the basic vapour compression cycle of refrigeration shown in Figure 9.2 a mixture of relatively cold, saturated liquid and saturated vapour enters the evaporator. The liquid part of the mixture is boiled to a saturated vapour and leaves the evaporator at state 1 (in Figure 9.2). The refrigerating effect (q{) is the enthalpy change across the evaporator. Using the notation of Figure 9.2, this is expressed by

Qx = (hx — h4) (9.4)

EXAMPLE 9.2

(a) Calculate the refrigerating effect for the plant used in example 9.1.

(b) If the duty is 352 kW of refrigeration, determine the mass flow rate of refrigerant.

(c) What is the volumetric flow rate under suction conditions?

Table 9.4 Superheated properties of R717, ammonia (NH3)

T-t,

II

1

O

O

U

O

<N

1

II

Ts = -1°C

K

P = 383.4 kPa

P = 398.3 kPa

P = 413.6 kPa

V

H

S

V

H

S

V

H

S

0

0.3224

1439.8

5.3753

0.3109

1440.9

5.3620

0.3000

1442.0

5.3487

5

0.3302

1453.0

5.4237

0.3185

1454.2

5.4105

0.3073

1455.4

5.3973

10

0.3379

1465.9

5.4700

0.3259

1467.1

5.4567

0.3145

1468.3

5.4436

T-t,

U = o°c

T, = i°c

U = 2°C

K

P = 429.4 kPa

P = 445.7 kPa

P = 462.5 kPa

V

H

S

V

H

S

V

H

S

0

0.2895

1443.1

5.3356

0.2795

1444.2

5.3225

0.2698

1445.2

5.3096

5

0.2966

1456.5

5.3842

0.2863

1457.6

5.3712

0.2765

1458.8

5.3584

10

0.3035

1469.5

5.4306

0.2930

1470.7

5.4177

0.2830

1471.9

5.4049

T-u

L = 3°C

Ts = 4°C

K = 5°c

K

P = 479.7 kPa

P = 497.5 kPa

P = 515.7 kPa

V

H

S

V

H

S

V

H

S

0

0.2606

1446.3

5.2967

0.2517

1447.3

5.2839

0.2433

1448.3

5.2712

5

0.2670

1459.9

5.3456

0.2580

1461.0

5.3329

0.2493

1462.0

5.3203

10

0.2733

1473.1

5.3922

0.2641

1474.2

5.3796

0.2552

1475.4

5.3671

T — /s

Ts = 6°C

H = 7°C

/, = 8°C

K

P = 534.5 kPa

P = 553.8 kPa

P = 573.6 kPa

V

H

S

V

H

S

V

H

S

0

0.2351

1449.2

5.2587

0.2273

1450.2

5.2461

0.2198

1451.1

5.2337

5

0.2410

1463.1

5.3078

0.2330

1464.1

5.2954

0.2253

1465.1

5.2831

10

0.2467

1475.5

5.3546

0.2385

1477.6

5.3423

0.2307

1478.7

5.3301

T-h

H = 9°C

U

O

O

II

F, = ll°C

K

P = 594.0 kPa

P = 614.9 kPa

P = 636.4 kPa

V

H

S

V

H

S

V

H

S

0

0.2126

1452.1

5.2214

0.2056

1453.0

5.2091

0.1990

1453.9

5.1969

5

0.2179

1466.2

5.2708

0.2108

1467.1

5.2587

0.2040

1468.1

5.2466

10

0.2231

1479.8

5.3179

0.2159

1480.8

5.3509

0.2089

1481.9

5.2939

T-h

T, = 26°C

H = 27 °C

U

0

00

CN

II

K

P = 1034.0 kPa

P = 1066.0 kPa

P = 1098.7 kPa

V

H

S

V

H

S

V

H

S

0

0.1245

1465.2

5.0232

0.1208

1465.9

5.0121

0.1173

1466.4

5.0011

5

0.1279

1480.8

5.0749

0.1241

1481.5

5.0640

0.1205

1482.2

5.0531

10

0.1311

1495.8

5.1238

0.1273

1496.6

5.1130

0.1236

1497.4

5.1023

15

0.1343

1510.3

5.1703

0.1304

1511.2

5.1597

0.1266

1512.1

5.1491

20

0.1374

1524.4

5.2149

0.1334

1525.4

5.2044

0.1296

1526.3

5.1939

25

0.1404

1538.2

5.2578

0.1364

1539.2

5.2473

0.1325

1540.3

5.2369

30

0.1434

1551.7

5.2992

0.1393

1552.8

5.2887

0.1353

1553.9

5.2784

35

0.1463

1565.0

5.3392

0.1421

1566.2

5.3288

0.1380

1567.3

5.3185

T-ts

Fs = 29 °C

II

O

O

N

Ts = 31°C

K

P= 1132.2 kPa

P= 1166.5 kPa

P= 1201.6 kPa

V

H

S

V

H

S

V

H

S

0

0.1139

1467.0

4.9902

0.1106

1467.6

4.9793

0.1075

1468.1

4.9685

5

0.1170

1482.9

5.0423

0.1137

1483.6

5.0316

0.1104

1484.2

5.0210

10

0.1201

1498.2

5.0916

0.1166

1498.9

5.0811

0.1133

1499.6

5.0706

15

0.1230

1512.9

5.1386

0.1195

1513.8

5.1281

0.1161

1514.6

5.1177

20

0.1259

1527.3

5.1834

0.1223

1528.2

5.1731

0.1189

1529.1

5.1628

25

0.1287

1541.2

5.2265

0.1250

1542.2

5.2163

0.1215

1543.2

5.2060

30

0.1314

1554.9

5.2681

0.1277

1556.0

5.2579

0.1241

1557.0

5.2477

35

0.1341

1568.4

5.3083

0.1303

1569.5

5.2982

0.1267

1570.6

5.2881

T~h

Ts = 32°C

Fs = 33°C

II

O

N

K

P = 1237.4 kPa

P = 1274.1 kPa

P= 1311.6 kPa

V

H

S

V

H

S

V

H

S

0

0.1044

1468.6

4.9577

0.1014

1469.1

4.9469

0.0986

1469.6

4.9362

5

0.1073

1484.8

5.0103

0.1043

1485.4

4.9998

0.1014

1486.0

4.9893

10

0.1101

1500.4

5.0601

0.1070

1501.1

5.0497

0.1041

1501.7

5.0393

15

0.1129

1515.4

5.1074

0.1097

1516.2

5.0971

0.1067

1516.9

5.0869

20

0.1155

1529.9

5.1525

0.1123

1530.8

5.1424

0.1092

1531.7

5.1323

25

0.1181

1544.1

5.1959

0.1148

1545.1

5.1858

0.1117

1546.0

5.1758

30

0.1207

1558.0

5.2377

0.1173

1559.0

5.2277

0.1141

1560.0

5.2177

35

0.1232

1571.7

5.2781

0.1198

1572.7

5.2681

0.1165

1573.8

5.2582

T-ts

Fs = 35 °C

Ts = 36°C

Fs = 37°C

K

P = 1349.9 kPa

P = 1389.0 kPa

P = 1429.0 kPa

V

H

S

V

H

S

V

H

S

0

0.0958

1470.0

4.9256

0.0931

1470.4

4.9149

0.0905

1470.8

4.9044

5

0.0985

1486.5

4.9788

0.0958

1487.1

0.9684

0.0931

1487.6

4.9580

10

0.1012

1502.4

5.0290

0.0984

1503.0

5.0188

0.0957

1503.7

5.0086

15

0.1037

1517.7

5.0767

0.1009

1518.4

5.0666

0.0981

1519.1

5.0565

20

0.1062

1532.5

5.1222

0.1033

1533.3

5.1122

0.1005

1534.1

5.0923

25

0.1086

1546.9

5.1658

0.1056

1547.8

5.1559

0.1028

1548.7

5.1461

30

0.1110

1561.0

5.2078

0.1079

1562.0

5.1980

0.1050

1562.9

5.1883

35

0.1133

1574.8

5.2484

0.1102

1575.9

5.2387

0.1072

1576.9

5.2290

40

0.1155

1588.4

5.2878

0.1124

1589.5

5.2781

0.1094

1590.6

5.2685

45

0.1178

1601.8

5.3260

0.1146

1603.0

5.3163

0.1115

1604.1

5.3068

50

0.1200

1615.1

5.3632

0.1167

1616.3

5.3536

0.1136

1617.4

5.3441

55

0.1222

1628.2

5.3995

0.1189

1629.4

5.3900

0.1157

1630.6

5.3805

60

0.1243

1641.1

5.4350

0.1210

1642.4

5.4255

0.1177

1643.7

5.4160

T-h

Fs = 38°C

Fs = 39°C

Rs = 40°C

K

P = 1469.9 kPa

P= 1511.7 kPa

P = 1554.3 kPa

V

H

S

V

H

S

V

H

S

0

0.0880

1471.2

4.8938

0.0856

1471.5

4.8833

0.0833

1471.9

4.8728

5

0.0906

1488.1

4.9477

0.0881

1488.6

4.9374

0.0857

1489.0

4.9371

10

0.0931

1504.3

4.9984

0.0905

1504.8

4.9883

0.0881

1505.4

4.9783

15

0.0954

1519.8

5.0465

0.0929

1520.5

5.0366

0.0904

1521.2

5.0267

20

0.0977

1534.9

5.0924

0.0951

1535.7

5.0826

0.0926

1536.4

5.0728

Table 9.4 (Contd)

T-k

Ts = 38°C

Ts = 39°C

WT*

II

O

O

N

K

P = 1469.9 kPa

P= 1511.7 kPa

P = 1554.3 kPa

V

H

S

V

H

S

V

H

S

25

0.1000

1549.6

5.1363

0.0973

1550.4

5.1266

0.0947

1551.2

5.1169

30

0.1022

1563.9

5.1786

0.0995

1564.8

5.1690

0.0968

1565.7

5.1594

35

0.1043

1577.9

5.2194

0.1016

1578.9

5.2098

0.0989

1579.8

5.2003

40

0.1065

1591.6

5.2589

0.1036

1592.7

5.2494

0.1009

1593.7

5.2400

45

0.1085

1605.2

5.2973

0.1057

1606.3

5.2878

0.1029

1607.4

5.2784

50

0.1106

1618.6

5.3346

0.1076

1619.8

5.3252

0.1048

1620.9

5.3159

60

0.1126

1631.8

5.3710

0.1096

1633.1

5.3617

0.1067

1634.3

5.3524

T-ts

/s = 41 °C

U

O

CM

II

Rs = 43°C

K

P = 1597.9 kPa

P = 1642.4 kPa

P = 1687.8 kPa

V

H

S

V

H

S

V

H

S

0

0.0810

1472.2

4.8623

0.0788

1472.4

4.8519

0.0767

1472.7

4.8414

5

0.0834

1489.4

4.9169

0.0812

1489.8

0.9067

0.0790

1490.2

4.8965

10

0.0857

1505.9

4.9682

0.0834

1506.5

4.9583

0.0812

1507.0

4.9483

15

0.0879

1521.8

5.0168

0.0856

1522.4

5.0070

0.0833

1523.0

4.9972

20

0.0901

1537.1

5.0631

0.0877

1537.9

5.0534

0.0854

1538.6

5.0438

25

0.0922

1552.0

5.1073

0.0898

1552.8

5.0978

0.0874

1553.6

5.0883

30

0.0943

1566.6

5.1499

0.0918

1567.4

5.1404

0.0894

1568.3

5.1310

35

0.0963

1580.8

5.1909

0.0937

1581.7

5.1815

0.0913

1582.7

5.1722

40

0.0982

1594.7

5.2306

0.0957

1595.8

5.2213

0.0932

1596.8

5.2120

45

0.1002

1608.5

5.2691

0.0976

1609.6

5.2599

0.0950

1610.6

5.2507

50

0.1021

1622.0

5.3066

0.0994

1623.2

5.2974

0.0968

1624.3

5.2882

60

0.1058

1648.7

5.3788

0.1031

1649.9

5.3697

0.1004

1651.1

5.3606

Reproduced from Thermodynamic Properties of Ammonia by W. B. Gosney and O. Fabris, with the kind permission of the authors.

Answer

(a) From Table 9.1, hu leaving the evaporator, is 398.68 kJ kg-1. Since the value of h4, entering the evaporator, is the same as that of hj, leaving the condenser as a saturated liquid at a temperature of 35°C, the value of h4 is found from Table 9.1 to be 248.94 kJ kg-1. Hence by equation (9.4) the refrigerating effect is

Qt = (398.68 — 248.94) = 149.74 kJ kg’1

(b) The mass flow rate of refrigerant, m, is given by

M = QMt (9-5)

Where QT is the refrigeration duty in kW.

Hence the mass flow rate of refrigerant handled is

M = 352/149.74 = 2.351kg s“1

(c) Assuming that there is no pressure drop in the suction line and that there are no heat exchanges between the suction line and its surroundings, the state entering the compressor is the same as that leaving the evaporator, namely, 0°C saturated. From Table 9.1 the

Specific volume at this state is 0.06935 m3 kg Hence the volumetric flow rate at the suction state is

Vj = 2.351 X 0.06935 = 0.16304 m3 s’1

Posted in Air Conditioning Engineering


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