The Evaporative Efficiencies of Boilers

“Many tests have been undertaken to ascertain the evaporative power of different classes of boilers in actual work; but few of these are of any value, owing to the unreliable means usually employed to measure the quantity of water evaporated. The easiest method, and consequently the one most frequently adopted, is to measure the quantity by the difference of its height in the water-gauge glass at the beginning and end of the trial, and also at intermediate stages. This method is very rude and uncertain, since there can be little doubt that in many boilers at work the surface of the water is not level, but is usually higher over the furnace, or where the greatest ebullition occurs. The difference in height at any moment will greatly depend upon the intensity of the ebullition, which is ever varying during the intervals between firing. With mechanical firing the difference of height is probably reduced to a minimum.

“The meters employed for measuring the water are sometimes not trustworthy. The only sure method of ascertaining the quantity of water evaporated is by actual measurement with a cistern or vessel whose cubic contents are accurately known. The quantity of water in the boiler before and after the trial should be measured at the same temperature, which should not exceed 212° to insure accuracy. But even when the amount of water introduced and the quantity passed off from the boiler are accurately ascertained, there yet remains a doubt as to how much has been actually evaporated, and how much may have passed off in priming, unless the trial has been conducted with the boiler open to the atmosphere, which appears to be the only condition under which accuracy can be insured, unless a suitable apparatus can be provided for accurately measuring the weight and temperature of all the steam and water given off, when the boiler is working above atmospheric pressure.

“There are very few boilers that do not prime more or less, and the quantity of water passed off in this manner is sometimes very considerable, and has led to the impossible results of 16 and 17 lbs. of water evaporated per lb. of ordinary coal in locomotive and water-tube boilers being seriously recorded. Externally fired boilers, that have given the moderate result of 5 lbs. of water per lb. of coal at atmospheric pressure, have shown the unexpected result of 10 and 12 lbs. of water evaporated at 40 lbs. pressure. In fact, unless the amount of water passed over with the steam by priming or foaming, when working under pressure, can be accurately ascertained, the evaporative results are not to be relied upon, however careful in other respects the trial may have been conducted. It is customary to give the quantity of water evaporated from and at a temperature of 212°, or the boiling point at atmospheric pressure, to which the results of evaporation are reduced.”

The quantity corresponding to any temperature of feed water and working pressure can readily be found with the aid of the annexed table, taken from The Encyclopædia Britannica, wherein are presented the relations of the properties of steam, as now accepted by the best authorities.

Table Giving the Pressure, Temperature, and Volume of Steam.

Total
pres-
sure
per
square
inch
mea-
sured
from a
vacuum.
lbs. Gauge
pres-
sure or
pres-
sure
above
atmo-
sphere.
lbs. Sensi-
ble
tempe-
rature
in
Fahren-
heit
degrees. Total
heat in
degrees
from
zero of
Fahren-
heit. Weight
of one
cubic
foot of
steam.
lbs. Rela-
tive
volume
of steam
compared
with the
water
from which
it was
evapo-
rated.

1 — 102.1 1144.5 .0030 20582

2 — 126.3 1151.7 .0058 10721

3 — 141.6 1156.6 .0085 7322

4 — 153.1 1160.1 .0112 5583

5 — 162.3 1162.9 .0138 4527

6 — 170.2 1165.3 .0163 3813

7 — 176.9 1167.3 .0189 3298

8 — 182.9 1169.2 .0214 2909

9 — 188.3 1170.8 .0239 2604

10 — 193.3 1172.3 .0264 2358

11 — 197.8 1173.7 .0289 2157

12 — 202.0 1175.0 .0314 1986

13 — 205.9 1176.2 .0338 1842

14 — 209.6 1177.3 .0362 1720

14 .7 0 212.0 1178.1 .0380 1642

15 .3 213.1 1178.4 .0387 1610

16 1.3 216.3 1179.4 .0411 1515

17 2.3 219.6 1180.3 .0435 1431

18 3.3 222.4 1181.2 .0459 1357

19 4.3 225.3 1182.1 .0483 1290

20 5.3 228.0 1182.9 .0507 1229

21 6.3 230.6 1183.7 .0531 1174

22 7.3 233.1 1184.5 .0555 1123

23 8.3 235.3 1185.2 .0580 1075

24 9.3 237.8 1185.9 .0601 1036

25 10.3 240.1 1186.6 .0625 996

26 11.3 242.3 1187.3 .0650 958

27 12.3 244.4 1187.8 .0673 926

28 13.3 246.4 1188.4 .0696 895

29 14.3 248.4 1189.1 .0719 866

30 15.3 250.4 1189.8 .0743 838

31 16.3 252.2 1190.4 .0766 813

32 17.3 254.1 1190.9 .0779 789

33 18.3 255.9 1191.5 .0812 767

34 19.3 257.6 1192.0 .0835 746

35 20.3 259.3 1192.5 .0858 726

36 21.3 260.9 1193.0 .0881 707

37 22.3 262.6 1193.5 .0905 688

38 23.3 264.2 1194.0 .0929 671

39 24.3 265.8 1194.5 .0952 655

40 25.3 267.3 1194.9 .0974 640

41 26.3 268.7 1195.4 .0996 625

42 27.3 270.2 1195.8 .1020 611

43 28.3 271.6 1196.2 .1042 598

44 29.3 273.0 1196.6 .1065 595

45 30.3 274.4 1197.1 .1089 572

46 31.3 275.8 1197.5 .1111 561

47 32.3 277.1 1197.9 .1133 550

48 33.3 278.4 1198.3 .1156 539

49 34.3 279.7 1198.7 .1179 529

50 35.3 281.0 1199.1 .1202 518

51 36.3 282.3 1199.5 .1224 509

52 37.3 283.5 1199.9 .1246 500

53 38.3 284.7 1200.3 .1269 491

54 39.3 285.9 1200.6 .1291 482

55 40.3 287.1 1201.0 .1314 474

56 41.3 288.2 1201.3 .1336 466

57 42.3 289.3 1201.7 .1364 458

58 43.3 290.4 1202.0 .1380 451

59 44.3 291.6 1202.4 .1403 444

60 45.3 292.7 1202.7 .1425 437

61 46.3 293.8 1203.1 .1447 403

62 47.3 294.8 1203.4 .1469 424

63 48.3 295.9 1203.7 .1493 417

64 49.3 296.9 1204.0 .1516 411

65 50.3 298.0 1204.3 .1538 405

66 51.3 299.0 1204.6 .1560 399

67 52.3 300.0 1204.9 .1583 393

68 53.3 300.9 1205.2 .1605 388

69 54.3 301.9 1205.5 .1627 383

70 55.3 302.9 1205.8 .1648 378

71 56.3 303.9 1206.1 .1670 373

72 57.3 304.8 1206.3 .1692 368

73 58.3 305.7 1206.6 .1714 363

74 59.3 306.6 1206.9 .1736 359

75 60.3 307.5 1207.2 .1759 353

76 61.3 308.4 1207.4 .1782 349

77 62.3 309.3 1207.7 .1804 345

78 63.3 310.2 1208.0 .1826 341

79 64.3 311.1 1208.3 .1848 337

80 65.3 312.0 1208.5 .1869 333

81 66.3 312.8 1208.8 .1891 329

82 67.3 313.6 1209.1 .1913 325

83 68.3 314.5 1209.4 .1935 321

84 69.3 315.3 1209.6 .1957 318

85 70.3 316.1 1209.9 .1980 314

86 71.3 316.9 1210.1 .2002 311

87 72.3 317.8 1210.4 .2024 308

88 73.3 318.6 1210.6 .2044 305

89 74.3 319.4 1210.9 .2067 301

90 75.3 320.2 1211.1 .2089 298

91 76.3 321.0 1211.3 .2111 295

92 77.3 321.7 1211.5 .2133 292

93 78.3 322.5 1211.8 .2155 289

94 79.3 323.3 1212.0 .2176 286

95 80.3 324.1 1212.3 .2198 283

96 81.3 324.8 1212.5 .2219 281

97 82.3 325.6 1212.8 .2241 278

98 83.3 326.3 1213.0 .2263 275

99 84.3 327.1 1213.2 .2285 272

100 85.3 327.9 1213.4 .2307 270

101 86.3 328.5 1213.6 .2329 267

102 87.3 329.1 1213.8 .2351 265

103 88.3 329.9 1214.0 .2373 262

104 89.3 330.6 1214.2 .2393 260

105 90.3 331.3 1214.4 .2414 257

106 91.3 331.9 1214.6 .2435 255

107 92.3 332.6 1214.8 .2456 253

108 93.3 333.3 1215.0 .2477 251

109 94.3 334.0 1215.3 .2499 249

110 95.3 334.6 1215.5 .2521 247

111 96.3 335.3 1215.7 .2543 245

112 97.3 336.0 1215.9 .2564 243

113 98.3 336.7 1216.1 .2586 241

114 99.3 337.4 1216.3 .2607 239

115 100.3 338.0 1216.5 .2628 237

116 101.3 338.6 1216.7 .2649 235

117 102.3 339.3 1216.9 .2674 233

118 103.3 339.9 1217.1 .2696 231

119 104.3 340.5 1217.3 .2738 229

120 105.3 341.1 1217.4 .2759 227

121 106.3 341.8 1217.6 .2780 225

122 107.3 342.4 1217.8 .2801 224

123 108.3 343.0 1218.0 .2822 222

124 109.3 343.6 1218.2 .2845 221

125 110.3 344.2 1218.4 .2867 219

126 111.3 344.8 1218.6 .2889 217

127 112.3 345.4 1218.8 .2911 215

128 113.3 346.0 1218.9 .2933 214

129 114.3 346.6 1219.1 .2955 212

130 115.3 347.2 1219.3 .2977 211

131 116.3 347.8 1219.5 .2999 209

132 117.3 348.3 1219.6 .3020 208

133 118.3 348.9 1219.8 .3040 206

134 119.3 349.5 1220.0 .3060 205

135 120.3 350.1 1220.2 .3080 203

136 121.3 350.6 1220.3 .3101 202

137 122.3 351.2 1220.5 .3121 200

138 123.3 351.8 1220.7 .3142 199

139 124.3 352.4 1220.9 .3162 198

140 125.3 352.9 1221.0 .3184 197

141 126.3 353.5 1221.2 .3206 195

142 127.3 354.0 1221.4 .3228 194

143 128.3 354.5 1221.6 .3250 193

144 129.3 355.0 1221.7 .3273 192

145 130.3 355.6 1221.9 .3294 190

146 131.3 356.1 1222.0 .3315 189

147 132.3 356.7 1222.2 .3336 188

148 133.3 357.2 1222.3 .3357 187

149 134.3 357.8 1222.5 .3377 186

150 135.3 358.3 1222.7 .3397 184

155 140.3 361.0 1223.5 .3500 179

160 145.3 363.4 1224.2 .3607 174

165 150.3 366.0 1224.9 .3714 169

170 155.3 368.2 1225.7 .3821 164

175 160.3 370.8 1226.4 .3928 159

180 165.3 372.9 1227.1 .4035 155

185 170.3 375.3 1227.8 .4142 151

190 175.3 377.5 1228.5 .4250 148

195 180.3 379.7 1229.2 .4357 144

200 185.3 381.7 1229.8 .4464 141

210 195.3 386.0 1231.1 .4668 135

220 205.3 389.9 1232.3 .4872 129

230 215.3 393.8 1233.5 .5072 123

240 225.3 397.5 1234.6 .5270 119

250 235.3 401.1 1235.7 .5471 114

260 245.3 404.5 1236.8 .5670 110

270 255.3 407.9 1237.8 .5871 106

280 265.3 411.2 1238.8 .6070 102

290 275.3 414.4 1239.8 .6268 99

300 285.3 417.5 1240.7 .6469 96

Here we see that at 212° the total quantity of heat in the steam is 1178.1°, which gives a difference of 966.1°. This heat, usually termed latent, is absorbed in performing the work of expanding the particles of water from the liquid to the gaseous state. Now, suppose the water is evaporated at 60 lbs. pressure, the steam will have a temperature of 307°, and a total heat of 1207°. If the feed has been introduced at 60°, it is evident that 1147° of heat have been imparted. As the amount evaporated is inversely proportional to the quantity of heat required, we have 1147 ÷ 966 = 1.2. Multiplying by this factor, the quantity evaporated at 60 lbs. pressure from 60°, we obtain the amount that would be evaporated at 212° by the same quantity of fuel.

By the same table can be ascertained the comparatively small increase of heat required to evaporate water at higher pressures. Suppose we take water evaporated at 45 lbs. pressure from a feed temperature of 60°, then each lb. of water will require 1202.7 - 60 = 1142.7 for its conversion into steam. If we take the pressure at 100 lbs., we shall have 1216.9 - 60 = 1156.9° as the quantity required. The difference between these two total quantities is only 14.2°, and is so small as to be scarcely worth considering. Leaving out of account the loss due to the slight reduction of the conducting power of the material, the increased amount of heat required for the higher pressure will be only ⁄36 of the total heat required at 60 lbs. With an evaporation of 7 lbs. of water from 1 lb. of coal, it will be obtained by using ⁄563 more fuel, or about 1 lb. in about 556 lbs., a quantity not appreciable to the ordinary modes of weighing coal. The economy is then manifest of using steam of high pressures when at the same time advantage is taken of the facilities it offers for working the steam more expansively to the engine cylinders.

The saving that may be effected by heating the feed water may be shown as follows:

If we take the normal temperature of the feed water at 60°, the temperature of the heated water at 212°, and the boiler pressure at 20 lbs., the total heat imparted to the steam in one case is

1192.5° - 60° = 1132.5°

and in the other case 1192.5° - 212° = 980.5°

the difference being 152°, or a saving of 152°

1132.5°

which is 13.4 per cent. If the pressure be taken at 120 lbs., instead of 20 lbs., the saving will be 13.1 per cent, showing a slight diminution in the economy effected by heating the feed water when a high boiler pressure is employed.