Chapter VI: Part 6

REPRODUCED BY PERMISSION FROM MARKS AND DAVIS "STEAM TABLES AND DIAGRAMS" (Copyright, 1909, by Longmans, Green & Co.) __________________________________________________________________ | | | | | | | Degrees of Superheat | |Pressure| |_______________________________________________| | Pounds |Saturated| | | | | | | |Absolute| Steam | 50 | 100 | 150 | 200 | 250 | 300 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 162.3 | 212.3 | 262.3 | 312.3 | 362.3 | 412.3 | 462.3 | | 5 v| 73.3 | 79.7 | 85.7 | 91.8 | 97.8 | 103.8 | 109.8 | | h| 1130.5 |1153.5 |1176.4 |1199.5 |1222.5 |1245.6 |1268.7 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 193.2 | 243.2 | 293.2 | 343.2 | 393.2 | 443.2 | 493.2 | | 10 v| 38.4 | 41.5 | 44.6 | 47.7 | 50.7 | 53.7 | 56.7 | | h| 1143.1 |1166.3 |1189.5 |1212.7 |1236.0 |1259.3 |1282.5 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 213.0 | 263.0 | 313.0 | 363.0 | 413.0 | 463.0 | 513.0 | | 15 v| 26.27 | 28.40| 30.46| 32.50| 34.53| 36.56| 38.58| | h| 1150.7 |1174.2 |1197.6 |1221.0 |1244.4 |1267.7 |1291.1 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 228.0 | 278.0 | 328.0 | 378.0 | 428.0 | 478.0 | 528.0 | | 20 v| 20.08 | 21.69| 23.25| 24.80| 26.33| 27.85| 29.37| | h| 1156.2 |1179.9 |1203.5 |1227.1 |1250.6 |1274.1 |1297.6 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 240.1 | 290.1 | 340.1 | 390.1 | 440.1 | 490.1 | 540.1 | | 25 v| 16.30 | 17.60| 18.86| 20.10| 21.32| 22.55| 23.77| | h| 1160.4 |1184.4 |1208.2 |1231.9 |1255.6 |1279.2 |1302.8 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 250.4 | 300.4 | 350.4 | 400.4 | 450.4 | 500.4 | 550.4 | | 30 v| 13.74 | 14.83| 15.89| 16.93| 17.97| 18.99| 20.00| | h| 1163.9 |1188.1 |1212.1 |1236.0 |1259.7 |1283.4 |1307.1 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 259.3 | 309.3 | 359.3 | 409.3 | 459.3 | 509.3 | 559.3 | | 35 v| 11.89 | 12.85| 13.75| 14.65| 15.54| 16.42| 17.30| | h| 1166.8 |1191.3 |1215.4 |1239.4 |1263.3 |1287.1 |1310.8 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 267.3 | 317.3 | 367.3 | 417.3 | 467.3 | 517.3 | 567.3 | | 40 v| 10.49 | 11.33| 12.13| 12.93| 13.70| 14.48| 15.25| | h| 1169.4 |1194.0 |1218.4 |1242.4 |1266.4 |1290.3 |1314.1 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 274.5 | 324.5 | 374.5 | 424.5 | 474.5 | 524.5 | 574.5 | | 45 v| 9.39 | 10.14| 10.86| 11.57| 12.27| 12.96| 13.65| | h| 1171.6 |1196.6 |1221.0 |1245.2 |1269.3 |1293.2 |1317.0 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 281.0 | 331.0 | 381.0 | 431.0 | 481.0 | 531.0 | 581.0 | | 50 v| 8.51 | 9.19| 9.84| 10.48| 11.11| 11.74| 12.36| | h| 1173.6 |1198.8 |1223.4 |1247.7 |1271.8 |1295.8 |1319.7 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 287.1 | 337.1 | 387.1 | 437.1 | 487.1 | 537.1 | 587.1 | | 55 v| 7.78 | 8.40| 9.00| 9.59| 10.16| 10.73| 11.30| | h| 1175.4 |1200.8 |1225.6 |1250.0 |1274.2 |1298.1 |1322.0 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 292.7 | 342.7 | 392.7 | 442.7 | 492.7 | 542.7 | 592.7 | | 60 v| 7.17 | 7.75| 8.30| 8.84| 9.36| 9.89| 10.41| | h| 1177.0 |1202.6 |1227.6 |1252.1 |1276.4 |1300.4 |1324.3 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 298.0 | 348.0 | 398.0 | 448.0 | 498.0 | 548.0 | 598.0 | | 65 v| 6.65 | 7.20| 7.70| 8.20| 8.69| 9.17| 9.65| | h| 1178.5 |1204.4 |1229.5 |1254.0 |1278.4 |1302.4 |1326.4 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 302.9 | 352.9 | 402.9 | 452.9 | 502.9 | 552.9 | 602.9 | | 70 v| 6.20 | 6.71| 7.18| 7.65| 8.11| 8.56| 9.01| | h| 1179.8 |1205.9 |1231.2 |1255.8 |1280.2 |1304.3 |1328.3 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 307.6 | 357.6 | 407.6 | 457.6 | 507.6 | 557.6 | 607.6 | | 75 v| 5.81 | 6.28| 6.73| 7.17| 7.60| 8.02| 8.44| | h| 1181.1 |1207.5 |1232.8 |1257.5 |1282.0 |1306.1 |1330.1 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 312.0 | 362.0 | 412.0 | 462.0 | 512.0 | 562.0 | 612.0 | | 80 v| 5.47 | 5.92| 6.34| 6.75| 7.17| 7.56| 7.95| | h| 1182.3 |1208.8 |1234.3 |1259.0 |1283.6 |1307.8 |1331.9 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 316.3 | 366.3 | 416.3 | 466.3 | 516.3 | 566.3 | 616.3 | | 85 v| 5.16 | 5.59| 6.99| 6.38| 6.76| 7.14| 7.51| | h| 1183.4 |1210.2 |1235.8 |1260.6 |1285.2 |1309.4 |1333.5 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 320.3 | 370.3 | 420.3 | 470.3 | 520.3 | 570.3 | 620.3 | | 90 v| 4.89 | 5.29| 5.67| 6.04| 6.40| 6.76| 7.11| | h| 1184.4 |1211.4 |1237.2 |1262.0 |1286.6 |1310.8 |1334.9 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 324.1 | 374.1 | 424.1 | 474.1 | 524.1 | 574.1 | 624.1 | | 95 v| 4.65 | 5.03| 5.39| 5.74| 6.09| 6.43| 6.76| | h| 1185.4 |1212.6 |1238.4 |1263.4 |1288.1 |1312.3 |1336.4 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 327.8 | 377.8 | 427.8 | 477.8 | 527.8 | 577.8 | 627.8 | | 100 v| 4.43 | 4.79| 5.14| 5.47| 5.80| 6.12| 6.44| | h| 1186.3 |1213.8 |1239.7 |1264.7 |1289.4 |1313.6 |1337.8 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 331.4 | 381.4 | 431.4 | 481.4 | 531.4 | 581.4 | 631.4 | | 105 v| 4.23 | 4.58| 4.91| 5.23| 5.54| 5.85| 6.15| | h| 1187.2 |1214.9 |1240.8 |1265.9 |1290.6 |1314.9 |1339.1 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 334.8 | 384.8 | 434.8 | 484.8 | 534.8 | 584.8 | 634.8 | | 110 v| 4.05 | 4.38| 4.70| 5.01| 5.31| 5.61| 5.90| | h| 1188.0 |1215.9 |1242.0 |1267.1 |1291.9 |1316.2 |1340.4 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 338.1 | 388.1 | 438.1 | 488.1 | 538.1 | 588.1 | 638.1 | | 115 v| 3.88 | 4.20| 4.51| 4.81| 5.09| 5.38| 5.66| | h| 1188.8 |1216.9 |1243.1 |1268.2 |1293.0 |1317.3 |1341.5 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 341.3 | 391.3 | 441.3 | 491.3 | 541.3 | 591.3 | 641.3 | | 120 v| 3.73 | 4.04| 4.33| 4.62| 4.89| 5.17| 5.44| | h| 1189.6 |1217.9 |1244.1 |1269.3 |1294.1 |1318.4 |1342.7 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 344.4 | 394.4 | 444.4 | 494.4 | 544.4 | 594.4 | 644.4 | | 125 v| 3.58 | 3.88| 4.17| 4.45| 4.71| 4.97| 5.23| | h| 1190.3 |1218.8 |1245.1 |1270.4 |1295.2 |1319.5 |1343.8 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 347.4 | 397.4 | 447.4 | 497.4 | 547.4 | 597.4 | 647.4 | | 130 v| 3.45 | 3.74| 4.02| 4.28| 4.54| 4.80| 5.05| | h| 1191.0 |1219.7 |1246.1 |1271.4 |1296.2 |1320.6 |1344.9 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 350.3 | 400.3 | 450.3 | 500.3 | 550.3 | 600.3 | 650.3 | | 135 v| 3.33 | 3.61| 3.88| 4.14| 4.38| 4.63| 4.87| | h| 1191.6 |1220.6 |1247.0 |1272.3 |1297.2 |1321.6 |1345.9 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 353.1 | 403.1 | 453.1 | 503.1 | 553.1 | 603.1 | 653.1 | | 140 v| 3.22 | 3.49| 3.75| 4.00| 4.24| 4.48| 4.71| | h| 1192.2 |1221.4 |1248.0 |1273.3 |1298.2 |1322.6 |1346.9 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 355.8 | 405.8 | 455.8 | 505.8 | 555.8 | 605.8 | 655.8 | | 145 v| 3.12 | 3.38| 3.63| 3.87| 4.10| 4.33| 4.56| | h| 1192.8 |1222.2 |1248.8 |1274.2 |1299.1 |1323.6 |1347.9 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 358.5 | 408.5 | 458.5 | 508.5 | 558.5 | 608.5 | 658.5 | | 150 v| 3.01 | 3.27| 3.50| 3.75| 3.97| 4.19| 4.41| | h| 1193.4 |1223.0 |1249.6 |1275.1 |1300.0 |1324.5 |1348.8 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 361.0 | 411.0 | 461.0 | 511.0 | 561.0 | 611.0 | 661.0 | | 155 v| 2.92 | 3.17| 3.41| 3.63| 3.85| 4.06| 4.28| | h| 1194.0 |1223.6 |1250.5 |1276.0 |1300.8 |1325.3 |1349.7 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 363.6 | 413.6 | 463.6 | 513.6 | 563.6 | 613.6 | 663.6 | | 160 v| 2.83 | 3.07| 3.30| 3.53| 3.74| 3.95| 4.15| | h| 1194.5 |1224.5 |1251.3 |1276.8 |1301.7 |1326.2 |1350.6 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 366.0 | 416.0 | 466.0 | 516.0 | 566.0 | 616.0 | 666.0 | | 165 v| 2.75 | 2.99| 3.21| 3.43| 3.64| 3.84| 4.04| | h| 1195.0 |1225.2 |1252.0 |1277.6 |1302.5 |1327.1 |1351.5 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 368.5 | 418.5 | 468.5 | 518.5 | 568.5 | 618.5 | 668.5 | | 170 v| 2.68 | 2.91| 3.12| 3.34| 3.54| 3.73| 3.92| | h| 1195.4 |1225.9 |1252.8 |1278.4 |1303.3 |1327.9 |1352.3 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 370.8 | 420.8 | 470.8 | 520.8 | 570.8 | 620.8 | 670.8 | | 175 v| 2.60 | 2.83| 3.04| 3.24| 3.44| 3.63| 3.82| | h| 1195.9 |1226.6 |1253.6 |1279.1 |1304.1 |1328.7 |1353.2 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 373.1 | 423.1 | 473.1 | 523.1 | 573.1 | 623.1 | 673.1 | | 180 v| 2.53 | 2.75| 2.96| 3.16| 3.35| 3.54| 3.72| | h| 1196.4 |1227.2 |1254.3 |1279.9 |1304.8 |1329.5 |1353.9 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 375.4 | 425.4 | 475.4 | 525.4 | 575.4 | 625.4 | 675.4 | | 185 v| 2.47 | 2.68| 2.89| 3.08| 3.27| 3.45| 3.63| | h| 1196.8 |1227.9 |1255.0 |1280.6 |1305.6 |1330.2 |1354.7 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 377.6 | 427.6 | 477.6 | 527.6 | 577.6 | 627.6 | 677.6 | | 190 v| 2.41 | 2.62| 2.81| 3.00| 3.19| 3.37| 3.55| | h| 1197.3 |1228.6 |1255.7 |1281.3 |1306.3 |1330.9 |1355.5 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 379.8 | 429.8 | 479.8 | 529.8 | 579.8 | 629.8 | 679.8 | | 195 v| 2.35 | 2.55| 2.75| 2.93| 3.11| 3.29| 3.46| | h| 1197.7 |1229.2 |1256.4 |1282.0 |1307.0 |1331.6 |1356.2 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 381.9 | 431.9 | 481.9 | 531.9 | 581.9 | 631.9 | 681.9 | | 200 v| 2.29 | 2.49| 2.68| 2.86| 3.04| 3.21| 3.38| | h| 1198.1 |1229.8 |1257.1 |1282.6 |1307.7 |1332.4 |1357.0 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 384.0 | 434.0 | 484.0 | 534.0 | 584.0 | 634.0 | 684.0 | | 205 v| 2.24 | 2.44| 2.62| 2.80| 2.97| 3.14| 3.30| | h| 1198.5 |1230.4 |1257.7 |1283.3 |1308.3 |1333.0 |1357.7 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 386.0 | 436.0 | 486.0 | 536.0 | 586.0 | 636.0 | 686.0 | | 210 v| 2.19 | 2.38| 2.56| 2.74| 2.91| 3.07| 3.23| | h| 1198.8 |1231.0 |1258.4 |1284.0 |1309.0 |1333.7 |1358.4 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 388.0 | 438.0 | 488.0 | 538.0 | 588.0 | 638.0 | 688.0 | | 215 v| 2.14 | 2.33| 2.51| 2.68| 2.84| 3.00| 3.16| | h| 1199.2 |1231.6 |1259.0 |1284.6 |1309.7 |1334.4 |1359.1 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 389.9 | 439.9 | 489.9 | 539.9 | 589.9 | 639.9 | 689.9 | | 220 v| 2.09 | 2.28| 2.45| 2.62| 2.78| 2.94| 3.10| | h| 1199.6 |1232.2 |1259.6 |1285.2 |1310.3 |1335.1 |1359.8 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 391.9 | 441.9 | 491.9 | 541.9 | 591.9 | 641.9 | 691.9 | | 225 v| 2.05 | 2.23| 2.40| 2.57| 2.72| 2.88| 3.03| | h| 1199.9 |1232.7 |1260.2 |1285.9 |1310.9 |1335.7 |1360.3 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 393.8 | 443.8 | 493.8 | 543.8 | 593.8 | 643.8 | 693.8 | | 230 v| 2.00 | 2.18| 2.35| 2.51| 2.67| 2.82| 2.97| | h| 1200.2 |1233.2 |1260.7 |1286.5 |1311.6 |1336.3 |1361.0 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 395.6 | 445.6 | 495.6 | 545.6 | 595.6 | 645.6 | 695.6 | | 235 v| 1.96 | 2.14| 2.30| 2.46| 2.62| 2.77| 2.91| | h| 1200.6 |1233.8 |1261.4 |1287.1 |1312.2 |1337.0 |1361.7 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 397.4 | 447.4 | 497.4 | 547.4 | 597.4 | 647.4 | 697.4 | | 240 v| 1.92 | 2.09| 2.26| 2.42| 2.57| 2.71| 2.85| | h| 1200.9 |1234.3 |1261.9 |1287.6 |1312.8 |1337.6 |1362.3 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 399.3 | 449.3 | 499.3 | 549.3 | 599.3 | 649.3 | 699.3 | | 245 v| 1.89 | 2.05| 2.22| 2.37| 2.52| 2.66| 2.80| | h| 1201.2 |1234.8 |1262.5 |1288.2 |1313.3 |1338.2 |1362.9 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 401.0 | 451.0 | 501.0 | 551.0 | 601.0 | 651.0 | 701.0 | | 250 v| 1.85 | 2.02| 2.17| 2.33| 2.47| 2.61| 2.75| | h| 1201.5 |1235.4 |1263.0 |1288.8 |1313.9 |1338.8 |1363.5 | |________|_________|_______|_______|_______|_______|_______|_______| | t| 402.8 | 452.8 | 502.8 | 552.8 | 602.8 | 652.8 | 702.8 | | 255 v| 1.81 | 1.98| 2.14| 2.28| 2.43| 2.56| 2.70| | h| 1201.8 |1235.9 |1263.6 |1289.3 |1314.5 |1339.3 |1364.1 | |________|_________|_______|_______|_______|_______|_______|_______|

t = Temperature, degrees Fahrenheit. v = Specific volume, in cubic feet, per pound. h = Total heat from water at 32 degrees, B. t. u.

[Graph: Temperature of Steam--Degrees Fahr. against Temperature in Calorimeter--Degrees Fahr.

Fig. 15. Graphic Method of Determining Moisture Contained in Steam from Calorimeter Readings]

MOISTURE IN STEAM

The presence of moisture in steam causes a loss, not only in the practical waste of the heat utilized to raise this moisture from the temperature of the feed water to the temperature of the steam, but also through the increased initial condensation in an engine cylinder and through friction and other actions in a steam turbine. The presence of such moisture also interferes with proper cylinder lubrication, causes a knocking in the engine and a water hammer in the steam pipes. In steam turbines it will cause erosion of the blades.

The percentage by weight of steam in a mixture of steam and water is called the _quality of the steam_.

The apparatus used to determine the moisture content of steam is called a calorimeter though since it may not measure the heat in the steam, the name is not descriptive of the function of the apparatus. The first form used was the "barrel calorimeter", but the liability of error was so great that its use was abandoned. Modern calorimeters are in general of either the throttling or separator type.

Throttling Calorimeter--Fig. 14 shows a typical form of throttling calorimeter. Steam is drawn from a vertical main through the sampling nipple, passes around the first thermometer cup, then through a one-eighth inch orifice in a disk between two flanges, and lastly around the second thermometer cup and to the atmosphere. Thermometers are inserted in the wells, which should be filled with mercury or heavy cylinder oil.

The instrument and all pipes and fittings leading to it should be thoroughly insulated to diminish radiation losses. Care must be taken to prevent the orifice from becoming choked with dirt and to see that no leaks occur. The exhaust pipe should be short to prevent back pressure below the disk.

When steam passes through an orifice from a higher to a lower pressure, as is the case with the throttling calorimeter, no external work has to be done in overcoming a resistance. Hence, if there is no loss from radiation, the quantity of heat in the steam will be exactly the same after passing the orifice as before passing. If the higher steam pressure is 160 pounds gauge and the lower pressure that of the atmosphere, the total heat in a pound of dry steam at the former pressure is 1195.9 B. t. u. and at the latter pressure 1150.4 B. t. u., a difference of 45.4 B. t. u. As this heat will still exist in the steam at the lower pressure, since there is no external work done, its effect must be to superheat the steam. Assuming the specific heat of superheated steam to be 0.47, each pound passing through will be superheated 45.4/0.47 = 96.6 degrees. If, however, the steam had contained one per cent of moisture, it would have contained less heat units per pound than if it were dry. Since the latent heat of steam at 160 pounds gauge pressure is 852.8 B. t. u., it follows that the one per cent of moisture would have required 8.5 B. t. u. to evaporate it, leaving only 45.4 - 8.5 = 36.9 B. t. u. available for superheating; hence, the superheat would be 36.9/0.47 = 78.5 degrees, as against 96.6 degrees for dry steam. In a similar manner, the degree of superheat for other percentages of moisture may be determined. The action of the throttling calorimeter is based upon the foregoing facts, as shown below.

Let H = total heat of one pound of steam at boiler pressure,
L = latent heat of steam at boiler pressure,
h = total heat of steam at reduced pressure after passing
orifice,
t_{1} = temperature of saturated steam at the reduced pressure,
t_{2} = temperature of steam after expanding through the orifice
in the disc,
0.47 = the specific heat of saturated steam at atmospheric pressure,
x = proportion by weight of moisture in steam.

The difference in B. t. u. in a pound of steam at the boiler pressure and after passing the orifice is the heat available for evaporating the moisture content and superheating the steam. Therefore,

H - h = xL + 0.47(t_{2} - t_{1})

H - h - 0.47(t_{2} - t_{1})
or x = --------------------------- (4)
L

Almost invariably the lower pressure is taken as that of the atmosphere. Under such conditions, h = 1150.4 and t_{1} = 212 degrees. The formula thus becomes:

H - 1150.4 - 0.47(t_{2} - 212)
x = ------------------------------ (5)
L

For practical work it is more convenient to dispense with the upper thermometer in the calorimeter and to measure the pressure in the steam main by an accurate steam pressure gauge.

A chart may be used for determining the value of x for approximate work without the necessity for computation. Such a chart is shown in Fig. 15 and its use is as follows: Assume a gauge pressure of 180 pounds and a thermometer reading of 295 degrees. The intersection of the vertical line from the scale of temperatures as shown by the calorimeter thermometer and the horizontal line from the scale of gauge pressures will indicate directly the per cent of moisture in the steam as read from the diagonal scale. In the present instance, this per cent is 1.0.

Sources of Error in the Apparatus--A slight error may arise from the value, 0.47, used as the specific heat of superheated steam at atmospheric pressure. This value, however is very nearly correct and any error resulting from its use will be negligible.

There is ordinarily a larger source of error due to the fact that the stem of the thermometer is not heated to its full length, to an initial error in the thermometer and to radiation losses.

With an ordinary thermometer immersed in the well to the 100 degrees mark, the error when registering 300 degrees would be about 3 degrees and the true temperature be 303 degrees.[19]

The steam is evidently losing heat through radiation from the moment it enters the sampling nipple. The heat available for evaporating moisture and superheating steam after it has passed through the orifice into the lower pressure will be diminished by just the amount lost through radiation and the value of t_{2}, as shown by the calorimeter thermometer, will, therefore, be lower than if there were no such loss. The method of correcting for the thermometer and radiation error recommended by the Power Test Committee of the American Society of Mechanical Engineers is by referring the readings as found on the boiler trial to a "normal" reading of the thermometer. This normal reading is the reading of the lower calorimeter thermometer for dry saturated steam, and should be determined by attaching the instrument to a horizontal steam pipe in such a way that the sampling nozzle projects upward to near the top of the pipe, there being no perforations in the nozzle and the steam taken only through its open upper end. The test should be made with the steam in a quiescent state and with the steam pressure maintained as nearly as possible at the pressure observed in the main trial, the calorimeter thermometer to be the same as was used on the trial or one exactly similar.

With a normal reading thus obtained for a pressure approximately the same as existed in the trial, the true percentage of moisture in the steam, that is, with the proper correction made for radiation, may be calculated as follows:

Let T denote the normal reading for the conditions existing in the trial. The effect of radiation from the instrument as pointed out will be to lower the temperature of the steam at the lower pressure. Let x_{1} represent the proportion of water in the steam which will lower its temperature an amount equal to the loss by radiation. Then,

H - h - 0.47(T - t_{1})
x_{1} = -----------------------
L

This amount of moisture, x_{1} was not in the steam originally but is the result of condensation in the instrument through radiation. Hence, the true amount of moisture in the steam represented by X is the difference between the amount as determined in the trial and that resulting from condensation, or,

X = x - x_{1}

H - h - 0.47(t_{2} - t_{1}) H - h - 0.47(T - t_{1})
= --------------------------- - -----------------------
L L

0.47(T - t_{2})
= --------------- (6)
L

As T and t_{2} are taken with the same thermometer under the same set of conditions, any error in the reading of the thermometers will be approximately the same for the temperatures T and t_{2} and the above method therefore corrects for both the radiation and thermometer errors. The theoretical readings for dry steam, where there are no losses due to radiation, are obtainable from formula (5) by letting x = 0 and solving for t_{2}. The difference between the theoretical reading and the normal reading for no moisture will be the thermometer and radiation correction to be applied in order that the correct reading of t_{2} may be obtained.

For any calorimeter within the range of its ordinary use, such a thermometer and radiation correction taken from one normal reading is approximately correct for any conditions with the same or a duplicate thermometer.

The percentage of moisture in the steam, corrected for thermometer error and radiation and the correction to be applied to the particular calorimeter used, would be determined as follows: Assume a gauge pressure in the trial to be 180 pounds and the thermometer reading to be 295 degrees. A normal reading, taken in the manner described, gives a value of T = 303 degrees; then, the percentage of moisture corrected for thermometer error and radiation is,

0.47(303 - 295)
x = ----------------
845.0

= 0.45 per cent.

The theoretical reading for dry steam will be,

1197.7 - 1150.4 - 0.47(t_{2} - 212)
0 = ------------------------------------
845.0

t_{2} = 313 degrees.

The thermometer and radiation correction to be applied to the instrument used, therefore over the ordinary range of pressure is

Correction = 313 - 303 = 10 degrees

The chart may be used in the determination of the correct reading of moisture percentage and the permanent radiation correction for the instrument used without computation as follows: Assume the same trial pressure, feed temperature and normal reading as above. If the normal reading is found to be 303 degrees, the correction for thermometer and radiation will be the theoretical reading for dry steam as found from the chart, less this normal reading, or 10 degrees correction. The correct temperature for the trial in question is, therefore, 305 degrees. The moisture corresponding to this temperature and 180 pounds gauge pressure will be found from the chart to be 0.45 per cent.

There are many forms of throttling calorimeter, all of which work upon the same principle. The simplest one is probably that shown in Fig. 14. An extremely convenient and compact design is shown in Fig. 16. This calorimeter consists of two concentric metal cylinders screwed to a cap containing a thermometer well. The steam pressure is measured by a gauge placed in the supply pipe or other convenient location. Steam passes through the orifice A and expands to atmospheric pressure, its temperature at this pressure being measured by a thermometer placed in the cup C. To prevent as far as possible radiation losses, the annular space between the two cylinders is used as a jacket, steam being supplied to this space through the hole B.

The limits of moisture within which the throttling calorimeter will work are, at sea level, from 2.88 per cent at 50 pounds gauge pressure and 7.17 per cent moisture at 250 pounds pressure.

Separating Calorimeter--The separating calorimeter mechanically separates the entrained water from the steam and collects it in a reservoir, where its amount is either indicated by a gauge glass or is drained off and weighed. Fig. 17 shows a calorimeter of this type. The steam passes out of the calorimeter through an orifice of known size so that its total amount can be calculated or it can be weighed. A gauge is ordinarily provided with this type of calorimeter, which shows the pressure in its inner chamber and the flow of steam for a given period, this latter scale being graduated by trial.

The instrument, like a throttling calorimeter, should be well insulated to prevent losses from radiation.

While theoretically the separating calorimeter is not limited in capacity, it is well in cases where the percentage of moisture present in the steam is known to be high, to attach a throttling calorimeter to its exhaust. This, in effect, is the using of the separating calorimeter as a small separator between the sampling nozzle and the throttling instrument, and is necessary to insure the determination of the full percentage of moisture in the steam. The sum of the percentages shown by the two instruments is the moisture content of the steam.

The steam passing through a separating calorimeter may be calculated by Napier's formula, the size of the orifice being known. There are objections to such a calculation, however, in that it is difficult to accurately determine the areas of such small orifices. Further, small orifices have a tendency to become partly closed by sediment that may be carried by the steam. The more accurate method of determining the amount of steam passing through the instrument is as follows:

A hose should be attached to the separator outlet leading to a vessel of water on a platform scale graduated to 1/100 of a pound. The steam outlet should be connected to another vessel of water resting on a second scale. In each case, the weight of each vessel and its contents should be noted. When ready for an observation, the instrument should be blown out thoroughly so that there will be no water within the separator. The separator drip should then be closed and the steam hose inserted into the vessel of water at the same instant. When the separator has accumulated a sufficient quantity of water, the valve of the instrument should be closed and the hose removed from the vessel of water. The separator should be emptied into the vessel on its scale. The final weight of each vessel and its contents are to be noted and the differences between the final and original weights will represent the weight of moisture collected by the separator and the weight of steam from which the moisture has been taken. The proportion of moisture can then be calculated from the following formula:

100 w
x = ----- (7)
W - w

Where x = per cent moisture in steam,
W = weight of steam condensed,
w = weight of moisture as taken out by the separating
calorimeter.

Sampling Nipple--The principle source of error in steam calorimeter determinations is the failure to obtain an average sample of the steam delivered by the boiler and it is extremely doubtful whether such a sample is ever obtained. The two governing features in the obtaining of such a sample are the type of sampling nozzle used and its location.

The American Society of Mechanical Engineers recommends a sampling nozzle made of one-half inch iron pipe closed at the inner end and the interior portion perforated with not less than twenty one-eighth inch holes equally distributed from end to end and preferably drilled in irregular or spiral rows, with the first hole not less than one-half inch from the wall of the pipe. Many engineers object to the use of a perforated sampling nipple because it ordinarily indicates a higher percentage of moisture than is actually present in the steam. This is due to the fact that if the perforations come close to the inner surface of the pipe, the moisture, which in many instances clings to this surface, will flow into the calorimeter and cause a large error. Where a perforated nipple is used, in general it may be said that the perforations should be at least one inch from the inner pipe surface.

A sampling nipple, open at the inner end and unperforated, undoubtedly gives as accurate a measure as can be obtained of the moisture in the steam passing that end. It would appear that a satisfactory method of obtaining an average sample of the steam would result from the use of an open end unperforated nipple passing through a stuffing box which would allow the end to be placed at any point across the diameter of the steam pipe.

Incidental to a test of a 15,000 K. W. steam engine turbine unit, Mr. H. G. Stott and Mr. R. G. S. Pigott, finding no experimental data bearing on the subject of low pressure steam quality determinations, made a investigation of the subject and the sampling nozzle illustrated in Fig. 18 was developed. In speaking of sampling nozzles in the determination of the moisture content of low pressure steam, Mr. Pigott says, "the ordinary standard perforated pipe sampler is absolutely worthless in giving a true sample and it is vital that the sample be abstracted from the main without changing its direction or velocity until it is safely within the sample pipe and entirely isolated from the rest of the steam."

It would appear that the nozzle illustrated is undoubtedly the best that has been developed for use in the determination of the moisture content of steam, not only in the case of low, but also in high pressure steam.

Location of Sampling Nozzle--The calorimeter should be located as near as possible to the point from which the steam is taken and the sampling nipple should be placed in a section of the main pipe near the boiler and where there is no chance of moisture pocketing in the pipe. The American Society of Mechanical Engineers recommends that a sampling nipple, of which a description has been given, should be located in a vertical main, rising from the boiler with its closed end extending nearly across the pipe. Where non-return valves are used, or where there are horizontal connections leading from the boiler to a vertical outlet, water may collect at the lower end of the uptake pipe and be blown upward in a spray which will not be carried away by the steam owing to a lack of velocity. A sample taken from the lower part of this pipe will show a greater amount of moisture than a true sample. With goose-neck connections a small amount of water may collect on the bottom of the pipe near the upper end where the inclination is such that the tendency to flow backward is ordinarily counterbalanced by the flow of steam forward over its surface; but when the velocity momentarily decreases the water flows back to the lower end of the goose-neck and increases the moisture at that point, making it an undesirable location for sampling. In any case, it should be borne in mind that with low velocities the tendency is for drops of entrained water to settle to the bottom of the pipe, and to be temporarily broken up into spray whenever an abrupt bend or other disturbance is met.

Case 1--Horizontal pipe. Water flows at bottom. If perforations
in nozzle are too near bottom of pipe, water piles against
nozzle, flows into calorimeter and gives false reading.
Case 2--If nozzle located too near junction of two horizontal
runs, as at a, condensation from vertical pipe which collects at
this point will be thrown against the nozzle by the velocity of
the steam, resulting in a false reading. Nozzle should be
located far enough above junction to be removed from water kept
in motion by the steam velocity, as at b. Case 3--Condensation
in bend will be held by velocity of the steam as shown. When
velocity is diminished during firing intervals and the like
moisture flows back against nozzle, a, and false reading is
obtained. A true reading will be obtained at b provided
condensation is not blown over on nozzle. Case 4--Where
non-return valve is placed before a bend, condensation will
collect on steam line side and water will be swept by steam
velocity against nozzle and false readings result.]

Fig. 19 indicates certain locations of sampling nozzles from which erroneous results will be obtained, the reasons being obvious from a study of the cuts.

Before taking any calorimeter reading, steam should be allowed to flow through the instrument freely until it is thoroughly heated. The method of using a throttling calorimeter is evident from the description of the instrument given and the principle upon which it works.

SUPERHEATED STEAM

Superheated steam, as already stated, is steam the temperature of which exceeds that of saturated steam at the same pressure. It is produced by the addition of heat to saturated steam which has been removed from contact with the water from which it was generated. The properties of superheated steam approximate those of a perfect gas rather than of a vapor. Saturated steam cannot be superheated when it is in contact with water which is also heated, neither can superheated steam condense without first being reduced to the temperature of saturated steam. Just so long as its temperature is above that of saturated steam at a corresponding pressure it is superheated, and before condensation can take place that superheat must first be lost through radiation or some other means. Table 24[20] gives such properties of superheated steam for varying pressures as are necessary for use in ordinary engineering practice.

Specific Heat of Superheated Steam--The specific heat of superheated steam at atmospheric pressure and near saturation point was determined by Regnault, in 1862, who gives it the value of 0.48. Regnault's value was based on four series of experiments, all at atmospheric pressure and with about the same temperature range, the maximum of which was 231.1 degrees centigrade. For fifty years after Regnault's determination, this value was accepted and applied to higher pressures and temperatures as well as to the range of his experiments. More recent investigations have shown that the specific heat is not a constant and varies with both pressure and the temperature. A number of experiments have been made by various investigators and, up to the present, the most reliable appear to be those of Knoblauch and Jacob. Messrs. Marks and Davis have used the values as determined by Knoblauch and Jacob with slight modifications. The first consists in a varying of the curves at low pressures close to saturation because of thermodynamic evidence and in view of Regnault's determination at atmospheric pressure. The second modification is at high degrees of superheat to follow Holborn's and Henning's curve, which is accepted as authentic.

For the sake of convenience, the mean specific heat of superheated steam at various pressures and temperatures is given in tabulated form in Table 25. These values have been calculated from Marks and Davis Steam Tables by deducting from the total heat of one pound of steam at any pressure for any degree of superheat the total heat of one pound of saturated steam at the same pressure and dividing the difference by the number of degrees of superheat and, therefore, represent the average specific heat starting from that at saturation to the value at the particular pressure and temperature.[21] Expressed as a formula this calculation is represented by

H_{sup} - H_{sat}
Sp. Ht. = ----------------- (8)
S_{sup} - S_{sat}

Where H_{sup} = total heat of one pound of superheated steam at any
pressure and temperature,
H_{sat} = total heat of one pound of saturated steam at same
pressure,
S_{sup} = temperature of superheated steam taken,
S_{sat} = temperature of saturated steam corresponding to the
pressure taken.

TABLE 25

MEAN SPECIFIC HEAT OF SUPERHEATED STEAM CALCULATED FROM MARKS AND DAVIS TABLES _______________________________________________________________ |Gauge | | |Pressure | Degree of Superheat | | |_____________________________________________________| | | 50 | 60 | 70 | 80 | 90 | 100 | 110 | 120 | 130 | |_________|_____|_____|_____|_____|_____|_____|_____|_____|_____| | 50 | .518| .517| .514| .513| .511| .510| .508| .507| .505| | 60 | .528| .525| .523| .521| .519| .517| .515| .513| .512| | 70 | .536| .534| .531| .529| .527| .524| .522| .520| .518| | 80 | .544| .542| .539| .535| .532| .530| .528| .526| .524| | 90 | .553| .550| .546| .543| .539| .536| .534| .532| .529| | 100 | .562| .557| .553| .549| .544| .542| .539| .536| .533| | 110 | .570| .565| .560| .556| .552| .548| .545| .542| .539| | 120 | .578| .573| .567| .561| .557| .554| .550| .546| .543| | 130 | .586| .580| .574| .569| .564| .560| .555| .552| .548| | 140 | .594| .588| .581| .575| .570| .565| .561| .557| .553| | 150 | .604| .595| .587| .581| .576| .570| .566| .561| .557| | 160 | .612| .603| .596| .589| .582| .576| .571| .566| .562| | 170 | .620| .612| .603| .595| .588| .582| .576| .571| .566| | 180 | .628| .618| .610| .601| .593| .587| .581| .575| .570| | 190 | .638| .627| .617| .608| .599| .592| .585| .579| .574| | 200 | .648| .635| .624| .614| .605| .597| .590| .584| .578| | 210 | .656| .643| .631| .620| .611| .602| .595| .588| .583| | 220 | .664| .650| .637| .626| .616| .607| .600| .592| .586| | 230 | .672| .658| .644| .633| .622| .613| .605| .597| .591| | 240 | .684| .668| .653| .640| .629| .619| .610| .602| .595| | 250 | .692| .675| .659| .645| .633| .623| .614| .606| .599| |_________|_____|_____|_____|_____|_____|_____|_____|_____|_____| |Gauge | | |Pressure | Degree of Superheat | | |-----------------------------------------------------| | | 140 | 150 | 160 | 170 | 180 | 190 | 200 | 225 | 250 | |---------+-----+-----+-----+-----+-----+-----+-----+-----+-----| | 50 | .504| .503| .502| .501| .500| .500| .499| .497| .496| | 60 | .511| .509| .508| .507| .506| .504| .504| .502| .500| | 70 | .516| .515| .513| .512| .511| .510| .509| .506| .504| | 80 | .522| .520| .518| .516| .515| .514| .513| .511| .508| | 90 | .527| .525| .523| .521| .519| .518| .517| .514| .510| | 100 | .531| .529| .527| .525| .523| .522| .521| .517| .513| | 110 | .536| .534| .532| .529| .528| .526| .525| .520| .517| | 120 | .540| .537| .535| .533| .531| .529| .528| .523| .519| | 130 | .545| .542| .539| .537| .535| .533| .531| .527| .523| | 140 | .550| .547| .544| .541| .539| .536| .534| .530| .526| | 150 | .554| .550| .547| .544| .542| .539| .537| .533| .529| | 160 | .558| .554| .551| .548| .545| .543| .541| .536| .531| | 170 | .562| .558| .555| .552| .549| .546| .544| .538| .533| | 180 | .566| .561| .558| .555| .552| .549| .546| .540| .536| | 190 | .569| .565| .562| .558| .555| .552| .549| .543| .538| | 200 | .574| .569| .566| .562| .558| .555| .552| .546| .541| | 210 | .578| .573| .569| .565| .561| .558| .555| .549| .543| | 220 | .581| .577| .572| .568| .564| .561| .558| .551| .545| | 230 | .585| .580| .575| .572| .567| .564| .561| .554| .548| | 240 | .589| .584| .579| .575| .571| .567| .564| .556| .550| | 250 | .593| .587| .582| .577| .574| .570| .567| .559| .553| |_________|_____|_____|_____|_____|_____|_____|_____|_____|_____|

Factor of Evaporation with Superheated Steam--When superheat is present in the steam during a boiler trial, where superheated steam tables are available, the formula for determining the factor of evaporation is that already given, (2),[22] namely,

H - h
Factor of evaporation = -----
L

Here H = total heat in one pound of superheated steam from the table, h and L having the same values as in (2).

Where no such tables are available but the specific heat of superheat is known, the formula becomes:

H - h + Sp. Ht.(T - t)
Factor of evaporation = ----------------------
L

Where H = total heat in one pound of saturated steam at pressure
existing in trial,
h = sensible heat above 32 degrees in one pound of water at the
temperature entering the boiler,
T = temperature of superheated steam as determined in the trial,
t = temperature of saturated steam corresponding to the boiler
pressure,
Sp. Ht. = mean specific heat of superheated steam at the pressure and
temperature as found in the trial,
L = latent heat of one pound of saturated steam at atmospheric
pressure.

Advantages of the Use of Superheated Steam--In considering the saving possible by the use of superheated steam, it is too often assumed that there is only a saving in the prime movers, a saving which is at least partially offset by an increase in the fuel consumption of the boilers generating steam. This misconception is due to the fact that the fuel consumption of the boiler is only considered in connection with a definite weight of steam. It is true that where such a definite weight is to be superheated, an added amount of fuel must be burned. With a properly designed superheater where the combined efficiency of the boiler and superheater will be at least as high as of a boiler alone, the approximate increase in coal consumption for producing a given weight of steam will be as follows:

_Superheat_ _Added Fuel_
_Degrees_ _Per Cent_
25 1.59
50 3.07
75 4.38
100 5.69
150 8.19
200 10.58

These figures represent the added fuel necessary for superheating a definite weight of steam to the number of degrees as given. The standard basis, however, of boiler evaporation is one of heat units and, considered from such a standpoint, again providing the efficiency of the boiler and superheater is as high, as of a boiler alone, there is no additional fuel required to generate steam containing a definite number of heat units whether such units be due to superheat or saturation. That is, if 6 per cent more fuel is required to generate and superheat to 100 degrees, a definite weight of steam, over what would be required to produce the same weight of saturated steam, that steam when superheated, will contain 6 per cent more heat units above the fuel water temperature than if saturated. This holds true if the efficiency of the boiler and superheater combined is the same as of the boiler alone. As a matter of fact, the efficiency of a boiler and superheater, where the latter is properly designed and located, will be slightly higher for the same set of furnace conditions than would the efficiency of a boiler in which no superheater were installed. A superheater, properly placed within the boiler setting in such way that products of combustion for generating saturated steam are utilized as well for superheating that steam, will not in any way alter furnace conditions. With a given set of such furnace conditions for a given amount of coal burned, the fact that additional surface, whether as boiler heating or superheating surface, is placed in such a manner that the gases must sweep over it, will tend to lower the temperature of the exit gases. It is such a lowering of exit gas temperatures that is the ultimate indication of added efficiency. Though the amount of this added efficiency is difficult to determine by test, that there is an increase is unquestionable.

Where a properly designed superheater is installed in a boiler the heating surface of the boiler proper, in the generation of a definite number of heat units, is relieved of a portion of the work which would be required were these heat units delivered in saturated steam. Such a superheater needs practically no attention, is not subject to a large upkeep cost or depreciation, and performs its function without in any way interfering with the operation of the boiler. Its use, therefore from the standpoint of the boiler room, results in a saving in wear and tear due to the lower ratings at which the boiler may be run, or its use will lead to the possibility of obtaining the same number of boiler horse power from a smaller number of boilers, with the boiler heating surface doing exactly the same amount of work as if the superheaters were not installed. The saving due to the added boiler efficiency that will be obtained is obvious.

Following the course of the steam in a plant, the next advantage of the use of superheated steam is the absence of water in the steam pipes. The thermal conductivity of superheated steam, that is, its power to give up its heat to surrounding bodies, is much lower than that of saturated steam and its heat, therefore, will not be transmitted so rapidly to the walls of the pipes as when saturated steam is flowing through the pipes. The loss of heat radiated from a steam pipe, assuming no loss in pressure, represents the equivalent condensation when the pipe is carrying saturated steam. In well-covered steam mains, the heat lost by radiation when carrying superheated steam is accompanied only by a reduction of the superheat which, if it be sufficiently high at the boiler, will enable a considerable amount of heat to be radiated and still deliver dry or superheated steam to the prime movers.

It is in the prime movers that the advantages of the use of superheated steam are most clearly seen.

In an engine, steam is admitted into a space that has been cooled by the steam exhausted during the previous stroke. The heat necessary to warm the cylinder walls from the temperature of the exhaust to that of the entering steam can be supplied only by the entering steam. If this steam be saturated, such an adding of heat to the walls at the expense of the heat of the entering steam results in the condensation of a portion. This initial condensation is seldom less than from 20 to 30 per cent of the total weight of steam entering the cylinder. It is obvious that if the steam entering be superheated, it must be reduced to the temperature of saturated steam at the corresponding pressure before any condensation can take place. If the steam be superheated sufficiently to allow a reduction in temperature equivalent to the quantity of heat that must be imparted to the cylinder walls and still remain superheated, it is clear that initial condensation is avoided. For example: assume one pound of saturated steam at 200 pounds gauge pressure to enter a cylinder which has been cooled by the exhaust. Assume the initial condensation to be 20 per cent. The latent heat of the steam is given up in condensation; hence, .20 × 838 = 167.6 B. t. u. are given up by the steam. If one pound of superheated steam enters the same cylinder, it would have to be superheated to a point where its total heat is 1199 + 168 = 1367 B. t. u. or, at 200 pounds gauge pressure, superheated approximately 325 degrees if the heat given up to the cylinder walls were the same as for the saturated steam. As superheated steam conducts heat less rapidly than saturated steam, the amount of heat imparted will be less than for the saturated steam and consequently the amount of superheat required to prevent condensation will be less than the above figure. This, of course, is the extreme case of a simple engine with the range of temperature change a maximum. As cylinders are added, the range in each is decreased and the condensation is proportionate.

The true economy of the use of superheated steam is best shown in a comparison of the "heat consumption" of an engine. This is the number of heat units required in developing one indicated horse power and the measure of the relative performance of two engines is based on a comparison of their heat consumption as the measure of a boiler is based on its evaporation from and at 212 degrees. The water consumption of an engine in pounds per indicated horse power is in no sense a true indication of its efficiency. The initial pressures and corresponding temperatures may differ widely and thus make a difference in the temperature of the exhaust and hence in the temperature of the condensed steam returned to the boiler. For example: suppose a certain weight of steam at 150 pounds absolute pressure and 358 degrees be expanded to atmospheric pressure, the temperature then being 212 degrees. If the same weight of steam be expanded from an initial pressure of 125 pounds absolute and 344 degrees, to enable it to do the same amount of work, that is, to give up the same amount of heat, expansion then must be carried to a point below atmospheric pressure to, say, 13 pounds absolute, the final temperature of the steam then being 206 degrees. In actual practice, it has been observed that the water consumption of a compound piston engine running on 26-inch vacuum and returning the condensed steam at 140 degrees was approximately the same as when running on 28-inch vacuum and returning water at 90 degrees. With an equal water consumption for the two sets of conditions, the economy in the former case would be greater than in the latter, since it would be necessary to add less heat to the water returned to the boiler to raise it to the steam temperature.

The lower the heat consumption of an engine per indicated horse power, the higher its economy and the less the number of heat units must be imparted to the steam generated. This in turn leads to the lowering of the amount of fuel that must be burned per indicated horse power.

With the saving in fuel by the reduction of heat consumption of an engine indicated, it remains to be shown the effect of the use of superheated steam on such heat consumption. As already explained, the use of superheated steam reduces condensation not only in the mains but especially in the steam cylinder, leaving a greater quantity of steam available to do the work. Furthermore, a portion of the saturated steam introduced into a cylinder will condense during adiabatic expansion, this condensation increasing as expansion progresses. Since superheated steam cannot condense until it becomes saturated, not only is initial condensation prevented by its use but also such condensation as would occur during expansion. When superheated sufficiently, steam delivered by the exhaust will still be dry. In the avoidance of such condensation, there is a direct saving in the heat consumption of an engine, the heat given up being utilized in the developing of power and not in changing the condition of the working fluid. That is, while the number of heat units lost in overcoming condensation effects would be the same in either case, when saturated steam is condensed the water of condensation has no power to do work while the superheated steam, even after it has lost a like number of heat units, still has the power of expansion. The saving through the use of superheated steam in the heat consumption of an engine decreases demands on the boiler and hence the fuel consumption per unit of power.

Superheated Steam for Steam Turbines--Experience in using superheated steam in connection with steam turbines has shown that it leads to economy and that it undoubtedly pays to use superheated steam in place of saturated steam. This is so well established that it is standard practice to use superheated steam in connection with steam turbines. Aside from the economy secured through using superheated steam, there is an important advantage arising through the fact that it materially reduces the erosion of the turbine blades by the action of water that would be carried by saturated steam. In using saturated steam in a steam turbine or piston engine, the work done on expanding the steam causes condensation of a portion of the steam, so that even were the steam dry on entering the turbine, it would contain water on leaving the turbine. By superheating the steam the water that exists in the low pressure stages of the turbine may be reduced to an amount that will not cause trouble.

Again, if saturated steam contains moisture, the effect of this moisture on the economy of a steam turbine is to reduce the economy to a greater extent than the proportion by weight of water, one per cent of water causing approximately a falling off of 2 per cent in the economy.

The water rate of a large economical steam turbine with superheated steam is reduced about one per cent, for every 12 degrees of superheat up to 200 degrees Fahrenheit of superheat. To superheat one pound of steam 12 degrees requires about 7 B. t. u. and if 1050 B. t. u. are required at the boiler to evaporate one pound of the saturated steam from the temperature of the feed water, the heat required for the superheated steam would be 1057 degrees. One per cent of saving, therefore, in the water consumption would correspond to a net saving of about one-third of one per cent in the coal consumption. On this basis 100 degrees of superheat with an economical steam turbine would result in somewhat over 3 per cent of saving in the coal for equal boiler efficiencies. As a boiler with a properly designed superheater placed within the setting is more economical for a given capacity than a boiler without a superheater, the minimum gain in the coal consumption would be, say, 4 or 5 per cent as compared to a plant with the same boilers without superheaters.