Chapter 13
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.