ECONOMICAL METHODS AND STUDY OF ECONOMIES
In studying possible economies of construction and operation for power stations, one must consider three factors: The rental due to the first cost of building and equipment, the cost of fuel, and the cost of labor. A power plant which is used 10 or 12 hours a day for weekdays only will not always pay the interest on expensive equipment and must get along with moderately economical machinery. A plant which has a fluctuating load cannot afford compound or triple-expansion engines because of the extra expense of reserve units. A pumping station, on the other hand, with its 24-hour schedule and its reservoirs and storage tanks to steady the load, may well afford the best equipment which can be bought. The saving of fuel and labor in this case is continuous and will offset a much larger interest account than for the plants first mentioned. To illustrate, I will quote a few figures from ordinary power station practise, since the principle involved is the same. The cost of buildings and equipment for a first-class power house having 1,000-horsepower capacity will be about $80,000, or $80 per horsepower. Possibly $60 of this sum represents the cost of engines and boilers, and the remaining $20 the cost of building and accessories, Assuming In per cent, for interest, depreciation, etc., we have the following division of annual cost per horsepower in a plant of this size:
Various authorities give the annual cost of a horsepower in a plant of this size as from $35 to $4.1 for 24-hour service. It may be seen from these figures that a saving of 10 per cent, in the fuel and labor bills will pay the interest on more than one-third the cost of equipment. We may, then, expect to see in a pumping station, compound or triple-expansion condensing engines, water tube boilers with chain grate or underfeed stokers, and mechanical methods of unload ng, crushing and feeding the coal. The engine should be selected with a view to its economy and its durability. The test guarantee insures the former, and the weight per unit of capacity is a fair gauge of the latter. In choosing between an engine which guarantees 18 pounds of steam per horsepower and one which guarantees Hi pounds, under the same conditions, we must remember that this difference represents a saving of 11 per cent, in fuel alone, or approximately $1.75 per horsepower per year. One thousand seven hundred and fifty dollars will pay the interest and depreciation on quite a respectable sum of money, probably much more than the difference in cost of the two engines. Condensation adds 20 or 25 per cent, to the capacity and economy of the engine and in a water pumping station costs but little. After all, it is in the boiler room that the greater losses occur, and it is here rather than in the engine room that the up-to-date engineer will look for improvements. The boiler room is apt to be a rather dark, dirty sort of a place and its dull surfaces and dusty atmosphere to be m marked contrast to the polished brass and steel of the room where the pumps are located. A little attention paid to the proper lighting and ventilating of this usually gloomy spot will be a profitable investment. The efficiency of a boiler is the ratio of the beat units actually put into tin steam to those available in the fuel and is usually about 60 per cent, when it might well be 70. Four hundred steaming tests of western coals reported by Professor Brcckcnridge in 1907 show boiler efficiencies ranging from 55 to 70, with an average of about 65 per cent. An efficiency of 60 per cent, means that the coal is doing about one-thirteenth less work than can reasonably be expected at a loss of from 15 to 20 cents for each ton of coal burned. The losses which go to make up the 30 or 40 per cent, to be accounted for may be considered in the order of their importance as:
- Escape of heat in chimney gases.
- Loss of unburned gases.
- Loss of unburned fuel in ash.
- Radiation to ash pit and from outer brick work.
The first may be 15 or 20 per cent., the second 5 or 10 per cent., the third should not exceed 5 per cent., while the fourth is not to be measured directly, but represents whatever is otherwise unaccounted for. The first three losses above mentioned may be reduced to their lowest terms by simply insuring reasonably complete combustion of the fuel used. This involves the use of a mechanical stoker of a sufficient draft with good damper control and intelligent attention on the part of employes. In other words, efficient combustion requires a uniform supply of fuel and of air in the proper proportions and a thorough mixing of the two at a proper temperature. Doubtless, hand firing might be made to meet these conditions, but it rarely, it ever, does meet them. Irregular supplies of fuel cause the escape of unburned gases to the stack and unburned carbon to the ash pit. Irregular or poorly adjusted air supply causes variations in the stack temperature and, consequently, poor combustion. Too much air checks combustion and lowers stack temperatures; too little air causes incomplete combustion, smoke and dirty tubes. To properly regulate these two important factors, one must resort to mechanical aids, which are more or less automatic in their action. To make this need clear, let us consider the aids to boiler-room economy in their order. The coal should be delivered in hopper cars, weighed on track scales at the plant and occasionally sampled by a competent chemist to determine its constituents. This procedure has an excellent moral effect oil the coal company, even though it lead to no direct results. Properly. coal should be bought cm a sliding scale in proportion to its beat value, but this is rarely done. The coal should be dumped from the cars into a bin underneath the track; and from that point, until it reaches the hoppers of the automatic stokers, it should be handled entirely by machinery. The labor expense will be reduced to one-fourth or one-fifth of that required when the fuel is unloaded and transported by hand. In like manner the ashes should be carried by machinery to the car or to the dump, and should be weighed in transit. The weight of ash is one indication of had or good firing. If the chain grate stoker is use_____, experiments should be made with different thicknesses of fire and different rates of speed to determine which are best adapted to the kind of fuel burned and the particular load carried. In like manner, tests should be made with various degrees of damper regulation to decide how much and how little draft can be advantageously used. Automatic damper control can then he adjusted to care for this permanently. Every boiler plant has its own personal equation, which must be worked out by the engineer in charge. A CO2 recorder should be installed to show the composition of the stack gases and a good dial thermometer inserted to show the temperature. Some kind of automatic feed regulator should control the water supplv of the boilers; a uniform boiler feed at a uniform temperature tends to make other factors more uniform and to improve the economy all around. Recording gauges and a recording feed water meter are good investments if kept in order. So far we have considered the installation of suitable machinery and apparatus for getting economical consumption of fuel. When all this is done, there remains the equally important problem of labor; unless that is efficient and well organized, all our pains have been for nothing. While the introduction of improved machinery cuts down the number of men employed, it necessitates a better quality of labor. Engineers and firemen who are to care for triple-expansion engines, water tube boilers and expensive coal handling machinery must be the best in their several classes men who have the mechanical instinct, a good degree of commonsense, and who are absolutely dependable. No man who drinks should be allowed inside the building, and politics should be kept outside the gates. A daily log should be kept. which may include most, if not all, of the following items:
- Weight of coal burned.
- Weight of ash rejected.
- Weight of boiler feed.
- Pressure at steam gauge.
- Temperature of stack gases.
- Temperature of boiler feed.
- Per cent, of CO2.
- Volume of water pumped.
- Pressure at head gauge.
- Level of suction well.
- Pressure at condenser.
- Temperature of superheat.
The gauges and recorders should be of the recording dial type and the cards submitted with the log. A graphic log of the more important data should be kept by the chief engineer and hung on the wall, where it may be inspected at any time. It takes but little time each day to make these records, and they inspire everyone about the plant with an interest in its efficiency and the methods ot improving it. After a time the firemen will, of their own pleasure, watch the readings of the CO. recorder and the various gauges and appreciate their significance. The bonus system may be made effective by paying each employe a monthly bonus graded to his rate of pay and to the improvement in operating conditions. The pumping station I have described is an expensive one and might not be justified in all its details when the plant is a small one. but the principle is the same throughout. We must try to save fuel and labor, rather than to economize on overhead charges, if we are to have n efficient station and one that will justify itself for a term of years.
*Read at the recent convention of Indiana Sanitary and Water Supply Association, held in Indianapolis.