Fig. 67.—Babcock and Wilcox Water-tube Boiler.
Not less than two boiler units should be used in any power station, regardless of the demands for power, and if the feed water is bad, three or even four units should be provided, as two units may be down at any time. An appreciable factor of safety is provided by the ability of a boiler to be operated at 30 to 50 per cent overload, if sufficient draft is available, but with resulting reduction in the economy of operation. The number of units provided should be such that the maximum load on the pumping station can be carried with at least one in every 6 units or less, out of service for repairs or other cause.
| TABLE 28 | |||||||
|---|---|---|---|---|---|---|---|
| Efficiencies of Steam Boilers | |||||||
| From Marks’ Mechanical Engineer’s Handbook | |||||||
| Type | Horse-power | Furnace | Sq. Ft. Grate Area | Per Cent of Rated Capacity D’v’l’d | B.T.U. per Lb. Dry Coal | Evap. from and at 212° per Lb. Dry Coal | Combined Efficiency of Boiler and Furnace |
| Babcock & Wilcox | 300 | Hand-fired | 84 | 118.7 | 11,912 | 8.81 | 71.8 |
| Babcock & Wilcox | 640 | Hand-fired | 118 | 121.5 | 14,602 | 10.83 | 72.0 |
| Stirling | 1128 | B. & W. chain grate | 187 | 198.3 | 12,130 | 9.51 | 76.1 |
| Rust | 335 | Hand-fired | 68 | 210.5 | 13,202 | 9.42 | 68.9 |
| Heine | 400 | Green chain grate | 83.5 | 123.8 | 11,608 | 8.79 | 73.5 |
| Maximum efficiency recorded | 83 | ||||||
The steam delivered by a boiler is the basis of the measurement of its capacity or power. A boiler horse-power is the delivery of 33,320 B.T.U. per hour. It is approximately equal to the raising of 30 pounds of water per hour from a temperature of 100° Fahrenheit, to steam at a pressure of 70 pounds per square inch, or to 34 pounds of water per hour changed to steam from and at 212° Fahrenheit, at atmospheric pressure. The horse-power of a boiler is sometimes approximated by the area of its grate or heating surface. Such a method of measuring has a low degree of accuracy on account of the variations in the quality of the fuel, and the rate of combustion. For example, the rate of combustion under a locomotive boiler is high and there is less than ⅒th of a square foot of grate area and about 4.5 square feet of heating surface per boiler horse-power. The Scotch Marine type of boiler used on steam ships, has slightly more grate area and slightly less heating surface than the locomotive type of boiler, because the rate of combustion is lower. Stationary water-tube boilers may have 2 to 3 times as much grate area and heating surface per horse-power as is found in locomotive boilers. If a poor type of fuel is to be used the area of the grate should be increased about inversely as the heat content of the fuel. The approximate heat content of various types of fuels is shown in Table 29.
| TABLE 29 | ||
|---|---|---|
| Approximate Heat Value of Fuels | ||
| Fuel | B.T.U. per Pound | Pounds of Water Evaporated from and at 212° F. All heat utilized |
| Anthracite | 13,500 | 14.0 |
| Semi-bituminous, Pennsylvania | 15,000 | 15.5 |
| Semi-bituminous, best, West Virginia | 15,000 | 15.8 |
| Bituminous, best, Pennsylvania | 14,450 | 15.0 |
| Bituminous, poor, Illinois | 10,500 | 10.9 |
| Lignite, best, Utah | 11,000 | 11.4 |
| Lignite, poor, Oregon | 8,500 | 8.8 |
| Wood, best oak | 9,300 | 9.6 |
| Wood, poor ash | 8,500 | 8.8 |
83. Air Ejectors.—The Ansonia compressed-air sewage ejector is shown in Fig. 68. In its operation, sewage enters the reservoir through the inlet pipe at the right, the air displaced being expelled slowly through the air valve marked B. The rising sewage lifts the float which actuates the balanced piston valve in the pipe above the reservoir when the reservoir fills. The lifting of the valve admits compressed air to the reservoir. The air pressure closes valve A and the inlet valve at the right, and ejects the sewage through the discharge pipe at the left. As the float drops with the descending sewage it shuts off the air supply and opens the air exhaust through the small pipe at the top center. Sewage is prevented from flowing back into the reservoir by the check valve in the discharge pipe. Other ejectors operating on a similar principle are the Ellis, the Pacific, the Priestmann and the Shone.
84. Electric Motors.—The most common form of alternating current electric motor used for driving sewage pumps where continuous operation and steady loads are met is the squirrel-cage polyphase induction motor. These motors operate at a nearly constant speed which should be selected to develop the maximum efficiency of the pump and motor set. While Fig. 59 shows the best efficiency under varying heads to be obtained with variable speed, the advantages of cost, attention, and availability make the use of a constant speed motor common.[[47]] This type of motor is undesirable where stopping and starting are frequent because it has a relatively small starting torque and it requires a large starting current. Such motors can be constructed in small sizes for high starting torques by increasing the resistance of the rotor, but at the expense of the efficiency of operation.
Fig. 68.—Ansonia Compressed-Air Sewage Ejector.