[Pg 121]

The gain due to superheat cannot be predicted from the formula for the efficiency of a perfect steam engine given on page [119] . This formula is not applicable in cases where superheat is present since only a relatively small amount of the heat in the steam is imparted at the maximum or superheated temperature.

The advantage of the use of high pressure steam may be also indicated by considering the question from the aspect of volume. With an increase of pressure comes a decrease in volume, thus one pound of saturated steam at 100 pounds absolute pressure occupies 4.43 cubic feet, while at 200 pounds pressure it occupies 2.29 cubic feet. If then, in separate cylinders of the same dimensions, one pound of steam at 100 pounds absolute pressure and one pound at 200 pounds absolute pressure enter and are allowed to expand to the full volume of each cylinder, the high-pressure steam, having more room and a greater range for expansion than the low-pressure steam, will thus do more work. This increase in the amount of work, as was the increase in temperature, is large relative to the additional fuel required as indicated by the total heat. In general, it may be stated that the fuel required to impart a given amount of heat to a boiler is practically independent of the steam pressure, since the temperature of the fire is so high as compared with the steam temperature that a variation in the steam temperature does not produce an appreciable effect.

The formulae for the algebraic expression of the relation between saturated steam pressures, temperatures and steam volumes have been up to the present time empirical. These relations have, however, been determined by experiment and, from the experimental data, tables have been computed which render unnecessary the use of empirical formulae. Such formulae may be found in any standard work of thermo-dynamics. The following tables cover all practical cases.

[Table 22] gives the heat units contained in water above 32 degrees Fahrenheit at different temperatures.

[Table 23] gives the properties of saturated steam for various pressures.

[Table 24] gives the properties of superheated steam at various pressures and temperatures.

These tables are based on those computed by Lionel S. Marks and Harvey N. Davis, these being generally accepted as being the most correct.

[Pg 122]

[TABLE 23]
PROPERTIES OF SATURATED STEAM REPRODUCED BY PERMISSION FROM
MARKS AND DAVIS “STEAM TABLES AND DIAGRAMS”
(Copyright, 1909, by Longmans, Green & Co.)
Pressure,
Pounds Absolute 		Temperature
Degrees F. 		Specific Volume
Cu. Ft. per Pound 		Heat of the
Liquid, B. t. u. 		Latent Heat of
Evap., B. t. u. 		Total Heat of
Steam, B. t. u.
1101.83333.0 69.81034.61104.4
2126.15173.5 94.01021.01115.0
3141.52118.5 109.41012.31121.6
4153.01 90.5 120.91005.71126.5
5162.28 73.33 130.11000.31130.5
6170.06 61.89 137.9 995.81133.7
7176.85 53.56 144.7 991.81136.5
8182.86 47.27 150.8 988.21139.0
9188.27 42.36 156.2 985.01141.1
10193.22 38.38 161.1 982.01143.1
11197.75 35.10 165.7 979.21144.9
12201.96 32.36 169.9 976.61146.5
13205.87 30.03 173.8 974.21148.0
14209.55 28.02 177.5 971.91149.4
15213.0 26.27 181.0 969.71150.7
16216.3 24.79 184.4 967.61152.0
17219.4 23.38 187.5 965.61153.1
18222.4 22.16 190.5 963.71154.2
19225.2 21.07 193.4 961.81155.2
20228.0 20.08 196.1 960.01156.2
22233.1 18.37 201.3 956.71158.0
24237.8 16.93 206.1 953.51159.6
26242.2 15.72 210.6 950.61161.2
28246.4 14.67 214.8 947.81162.6
30250.3 13.74 218.8 945.11163.9
32254.1 12.93 222.6 942.51165.1
34257.6 12.22 226.2 940.11166.3
36261.0 11.58 229.6 937.71167.3
38264.2 11.01 232.9 935.51168.4
40267.3 10.49 236.1 933.31169.4
42270.2 10.02 239.1 931.21170.3
44273.1 9.59 242.0 929.21171.2
46275.8 9.20 244.8 927.21172.0
48278.5 8.84 247.5 925.31172.8
50281.0 8.51 250.1 923.51173.6
52283.5 8.20 252.6 921.71174.3
Pressure,
Pounds Absolute 		Temperature
Degrees F. 		Specific Volume
Cu. Ft. per Pound 		Heat of the
Liquid, B. t. u. 		Latent Heat of
Evap., B. t. u. 		Total Heat of
Steam, B. t. u.
54285.9 7.91 255.1 919.91175.0
56288.2 7.65 257.5 918.21175.7
58290.5 7.40 259.8 916.51176.4
60292.7 7.17 262.1 914.91177.0
62294.9 6.95 264.3 913.31177.6
64297.0 6.75 266.4 911.81178.2
66299.0 6.56 268.5 910.21178.8
68301.0 6.38 270.6 908.71179.3
70302.9 6.20 272.6 907.2 1179.8 [Pg 123]
72304.8 6.04 274.5 905.81180.4
74306.7 5.89 276.5 904.41180.9
76308.5 5.74 278.3 903.01181.4
78310.3 5.60 280.2 901.71181.8
80312.0 5.47 282.0 900.31182.3
82313.8 5.34 283.8 899.01182.8
84315.4 5.22 285.5 897.71183.2
86317.1 5.10 287.2 896.41183.6
88318.7 5.00 288.9 895.21184.0
90320.3 4.89 290.5 893.91184.4
92321.8 4.79 292.1 892.71184.8
94323.4 4.69 293.7 891.51185.2
96324.9 4.60 295.3 890.31185.6
98326.4 4.51 296.8 889.21186.0
100327.8 4.429298.3 888.01186.3
105331.4 4.230302.0 885.21187.2
110334.8 4.047305.5 882.51188.0
115338.1 3.880309.0 879.81188.8
120341.3 3.726312.3 877.21189.6
125344.4 3.583315.5 874.71190.3
130347.4 3.452318.6 872.31191.0
135350.3 3.331321.7 869.91191.6
140353.1 3.219324.6 867.61192.2
145355.8 3.112327.4 865.41192.8
150358.5 3.012330.2 863.21193.4
155361.0 2.920332.9 861.01194.0
160363.6 2.834335.6 858.81194.5
Pressure,
Pounds Absolute 		Temperature
Degrees F. 		Specific Volume
Cu. Ft. per Pound 		Heat of the
Liquid, B. t. u. 		Latent Heat of
Evap., B. t. u. 		Total Heat of
Steam, B. t. u.
165366.0 2.753338.2 856.81195.0
170368.5 2.675340.7 854.71195.4
175370.8 2.602343.2 852.71195.9
180373.1 2.533345.6 850.81196.4
185375.4 2.468348.0 848.81196.8
190377.6 2.406350.4 846.91197.3
195379.8 2.346352.7 845.01197.7
200381.9 2.290354.9 843.21198.1
205384.0 2.237357.1 841.41198.5
210386.0 2.187359.2 839.61198.8
215388.0 2.138361.4 837.91199.2
220389.9 2.091363.4 836.21199.6
225391.9 2.046365.5 834.41199.9
230393.8 2.004367.5 832.81200.2
235395.6 1.964369.4 831.11200.6
240397.4 1.924371.4 829.51200.9
245399.3 1.887373.3 827.91201.2
250401.1 1.850375.2 826.3 1201.5 [Pg 124]