FUELS AND FIRING AND BOILER HORSEPOWER
FUELS AND FIRING
Air consists of two gases, oxygen and nitrogen, mixed in proportion to one part of the former to four of the latter. Air is necessary to make a fire burn, because combustion or burning consists of the union of oxygen with the carbon and hydrogen of the fuel. Air supplies the necessary oxygen for combustion. Nitrogen does not aid combustion in the least, but rather hinders it. If nitrogen were supplied to the fire, and no oxygen, the fire would go out as quickly as though water was poured upon it.
When oxygen unites with carbon, it forms a new chemical substance called carbon dioxide, which goes out with the smoke. Smoke, then, consists of the nitrogen of the air which passes through the furnace without being changed, carbon dioxide gas, some steam, and some finely divided particles of soot, which is carbon that has not been burned, and, which gives smoke its black color. If more air is admitted to the furnace than is necessary to supply the exact amount of oxygen for the fuel burned, then there will always be some free oxygen in the smoke, and a much larger amount of nitrogen.
Fuels — Different fuels have different heating values. For example, a pound of coal will heat more water than a pound of wood, and a pound of wood more than a pound of straw. The heat value of a fuel is based on the amount of water it will heat and is expressed in what is called heat units.
A heat unit is the amount of heat necessary to heat one pound of water through one degree. Measured on this basis, pure carbon would, if all its heat could be utilized, raise 14,650 pounds of water one degree, Fahrenheit. In other words, one pound of pure carbon contains 14,650 heat units. Ordinary soft coal contains from 12,000 to 13,500 heat units. The following table gives the heating volume in heat units of the different fuels used in traction engines.
TABLE II
| KIND OF FUEL | | HEAT UNITS |
| |
| Good soft coal | --------------------------------------------------------------------------------------- | 13,500 |
| Hard wood | --------------------------------------------------------------------------------------- | 8,400 |
| Soft wood | --------------------------------------------------------------------------------------- | 9,000 |
| Crude oil | --------------------------------------------------------------------------------------- | 20,000 |
| Flax straw | --------------------------------------------------------------------------------------- | 7,500 |
| Wheat straw | --------------------------------------------------------------------------------------- | 5,500 |
As inspection of Table II shows that one pound of crude oil is equal to about 1.8 pounds of coal, and one pound of flax straw equal to one-half a pound of coal. It also shows why flax straw is so much better for fuel than wheat straw.
Duty of the Fireman — The first duty of the fireman in the morning when he comes to the engine is to clean the flues. The tool used for this purpose should be some sort of a scraper that can be adjusted to the size of the flue by screwing the rod in or out the proper amount. The flues should be scraped clean, as a very thin coating of soot on the inside prevents heat from passing through the metal as rapidly as it should and, consequently, it takes a long time to get up steam. As an example of the effect of soot inside the tubes, the writer, in order to prove the matter to his students, allowed an engine to run three full afternoons without cleaning the flues. The next afternoon it took hard firing from one o’clock until almost five to get up steam. On the following day the flues were cleaned and a full head of steam was gotten up in about an hour. It is often advisable to clean the flues at noon also. Of course doing so when steam is up is apt to cool the front ends of the flues and perhaps cause them to leak, but if the fire is allowed to go down rather low at first and the work is done quickly, it ought not to do any particular damage. Some boilers are fitted with a device for cleaning the flues with a blast of steam. This arrangement is a good device to use once in a while, but should not take the place of a regular flue cleaner. All the blower does is to blow out the loose soot, and does not affect the dense scale-like portion which does the real damage, and which sticks closely to the metal.
After cleaning the flues, the next thing to do is to see if there is plenty of water in the boiler before building the fire. It is not enough to merely look to the glass, but the gauge cocks should be tried also. When the boiler is shut down for the night both the upper and lower glass connections should always be shut and the glass drained. Then in the morning it must be turned on, and when tested with the gauge cocks, there is no doubt about where the water level is in the boiler.
The next thing to do is to build the fire. If coal is used for fuel, first start a fire with kindling and wood. When it is going in good shape, throw in some coal, which has been broken in small pieces. It is not best to throw in very much coal at first, as it cools the fire and makes starting slower. When the first two or three shovelfuls get to going well, throw in some more, and keep on adding fuel in this way until the grates are covered all over to a depth of four or five inches. This is about the proper depth of fire to carry with soft coal in order to get the best results. It is very important that the fuel be spread evenly over the grates and that no dead or open space be allowed. Air always follows the line of least resistance and if there is a place in the grates where there is no fuel, most of the draft will go through this opening instead of going up through the fuel where the oxygen can get to the right place and aid combustion.
The result of open places in the grates is always a poor fire, and trouble in keeping up steam. For the same reason, the coal should be broken up fine — in pieces not larger than a man’s fist. Large lumps of coal do not lie closely together and so make open places in the grates. It is best for the beginner to follow a system in firing; that is, throw a shovelful first in one corner, then in another, and so on, around, and in that way keep an even thickness. The fire door should be opened immediately after the coal is thrown in to prevent cold air from passing into the flues. The strong draft, due to the exhaust, causes a heavy inrush of cold air every time the fire door is opened and is the principal cause of leaking flues. If the fireman is careful about opening and closing the doors, he can save the flues a great deal.
The fire should not be poked very much, but should be cleaned of ashes and clinkers occasionally, by running the slice bar down along the grates and in under the fire. What ever clinker is present should be raked out.
Care should be taken to keep ashes from piling up under the grates, because they not only obstruct the draft but cause the grate bars to burn out. If the grate bars burn it is always due to too much ashes in the ash pan, which prevent cold air from coming in below and keeping them cool.
If firing with coal there is not much danger of sparks being thrown from the stack, but with old dry wood and with straw there is danger unless the spark arrester is kept in place. In starting the fire, the spark arrester must be lifted out of the way, and may be kept lifted even after the blower is turned on, unless the fuel is very dry, but when the engine is started and the exhaust causes a heavy draft, it should be lowered in the chimney. Wire screen spark arresters should be cleaned at frequent intervals, because, like the flues, they get coated with soot, which clogs the meshes and prevents a good draft. In fact, one of the first places to look, if an engine does not steam well, is at the spark arrester to see if it is dirty. If it is, the draft will be poor and the fire can not burn properly.
Firing with straw appears to be a very simple process, but in reality it is harder than firing with coal and requires constant attention, whereas a coal fire is attended to only at intervals. A man may be an expert in firing with coal and still make a failure when he tries straw. Straw requires a strong draft and plenty of air, and so if coal grates are used all but two or three should be removed — only enough being left in place to keep whatever straw that falls into the fire box from dropping into the ash pan. The straw chute should be kept packed with straw at all times to prevent cold air from passing through into the flues. Only a small forkful should be fired at a time and this should not be crowded into the firebox. The object to be aimed at in straw firing is to keep the end of the column of straw that passes through the chute just far enough inside the fire box to burn well, but so that none of it falls onto the grates.
Straw forms a great amount of loose ash that must be raked out of the ash pit at frequent intervals or the draft will be poor. It also forms considerable hard clinker, which accumulates not only in the ash pit but also on the upper side of the brick arch. This clinker must be cleaned out at frequent intervals also. There is also trouble from the ends of the flues in the fire box end of direct flue boilers being capped with soot and a sort of clinker. This must be looked after every little while, and the caps removed with a poker through the little door in the side of the firebox that opens above the arch. When a fireman attends to all these little things and keeps steam up and at the same time attends to the pump or injector, he is kept pretty busy.
Many firemen make the mistake of firing too hard, thinking it a mark of ability to make the boiler “pop off” every few minutes. As a matter of fact, it rather shows lack of ability. The steam pressure ought to be kept as high as necessary, say five or ten pounds (per square inch) below the popping off point. A good fireman will exert himself to keep steady pressure, because by so doing he saves fuel, saves water, and saves the boiler. A change in steam pressure means a change in temperature also, and that means either expansion or contraction of the metal and consequent wear and tear. The steam pressure can be controlled fairly well by opening or closing the drafts, by using the right amount of fuel, and by pumping in cold water at the right time.
If the engine is fitted with a variable exhaust nozzle, this will also help to control the steam pressure. When it gets a little too high, the relief nozzle should be opened; this will cause less draft and at the same time make the engine a little stronger by reducing the amount of back pressure in the exhaust pipe. Taking away back-pressure is just the same as adding an equal amount of forward pressure.
When an engine is shut down for the evening, the fireman should see that plenty of water is left in the boiler. He should pump water up above the middle gauge cock. This will reduce the steam pressure and there will not be much loss of water through the pop valve’s working after the engine is shut down. The draft doors should be closed and all parts in which water accumulates during the day properly drained.
If the fireman or engineer has a system that he follows every day, there is not much danger of overlooking anything. The parts that need draining are the water glass, lubricator, pump, injector, feed pipe, cylinder, steam chest and throttle valve.
In all the Northern wheat-raising states frost is apt to occur almost any night in the fall during the threshing season, and it is better to start in by draining all parts that need it the first thing in the season, and keep it up every night. There will be no danger of forgetting some cold night to open the drip cocks and no need of having to say by way of an excuse that you did not think it was going to freeze.
HORSEPOWER OF ENGINES AND BOILERS
The term “horsepower” is used more frequently, perhaps, than any other term in connection with boilers and engines. Undoubtedly, most people think horsepower means the same thing whether applied to an engine or a boiler. As a matter of fact, however, boiler horsepower and engine horsepower are quite different things.
In order to explain the two terms clearly, it will first be necessary to define work and power.
Work may be defined as the overcoming of resistance through distance, or as a force acting through distance. The force or resistance is measured in pounds and the distance in feet. Work, then, is expressed as the product of the two factors resistance and distance, or, force and distance. The result is expressed in
foot pounds. For example, if a force of one pound moves through a distance of one foot, one foot pound of work is accomplished. Or, if it requires ten pounds of force to overcome a certain resistance, and this force acts through a distance of twenty feet, then 10×20 or 200 foot pounds of work are done. There must always be two factors, force and distance, involved when work is done. Looked at in this way, work is a purely mathematical quantity.
When only one factor is involved, no work is done that can be measured. For example, a man holding a weight at arm’s length is doing no work in the sense above described, because the force he exerts does not pass through any distance. The man may get very tired and feel as though he had done a great deal of work, but according to the accepted definition of work, he has done nothing. Just as the unit for measuring distance is the foot, so the unit for measuring work is the foot pound.
Power measures the
rate of doing work. Consequently, in all discussions of power, time must be taken into consideration, because the rate or speed at which work is accomplished is measured by the amount of time taken. Three factors enter into all considerations of power; namely, force, distance, and time. The more power an engine has, the faster it can do its work, or, the more work it can do in a given length of time, which amounts to the same thing. If two men each do the same amount of work, and the first gets through in one hour, while the second takes two hours, then the first exerts twice as much power as the second.
Horsepower — When James Watt, the inventor of the steam engine, first put his engines on the market he found it necessary to adopt some method of measuring their power that could be readily understood and appreciated. He accordingly conceived the idea of comparing the power of his engine with that of horses. In order to do this he measured the work done by the large London dray horses, and found that on an average, they were able to do 33,000 foot pounds of work in one minute. Since that time the term “horsepower” has meant the accomplishment of 33,000 foot pounds of work in one minute. Thus it will be seen that the term horsepower is a perfectly definite quantity and does not depend upon what a horse may or may not be able to do. As a matter of fact, few horses can actually perform a horsepower of work for a considerable length of time. Authorities on the subject state that a 1,200 pound horse, working eight hours per day, is able to accomplish only about twothirds of a horsepower of work. Larger horses, of course, can do proportionately more. For short periods of time, an ordinary horse may be able to work at a much faster rate, doing perhaps 3- or 4- horsepower. An ordinary man, working eight hours per day, can do from one-eighth to one-tenth of a horsepower of work.
In view of the above discussion, it will be readily seen that it is an error to say that a 25-horsepower engine is equal to twenty-five horses. If the engine is actually doing 25-horsepower of work, it is doing about as much as thirty-six ordinary horses. Moreover, most engines are capable of doing considerably more work than they are rated at. This is especially true of traction engines, which, by the way, are underrated a great deal. For example, the writer recently tested a 15-horsepower traction engine on the brake, and found that it easily developed 42-horsepower. Even then it was not working as hard as it could, and in an hour’s test no trouble was experienced in keeping up steam. .As a general rule, it may be stated that traction engines are capable of developing from two to three times as much power as their rating specifies. On account of the necessity for having a large amount of reserve power to enable them to handle sudden large increases in the load, it has become the habit to underrate them much more than stationary engines. Stationary gasoline engines, on the other hand, have not generally been rated any higher than their actual capacity. Consequently, men who have replaced steam engines with gasoline engines of the same rated horsepower have often been greatly disappointed in not having sufficient power. It is only fair to say, however, that at the present time gasoline traction engines are also largely underrated and compare quite favorably in actual power with steam engines of the same rating.
Boiler Horsepower — This term has an entirely different meaning from the term horsepower as applied to an engine. In fact, it is more misleading than descriptive. Originally, it meant that a boiler of a certain horsepower was capable of supplying steam for an engine of the same power. Later, when the engines were improved, it was found that what might be a 10-horsepower boiler for one engine might be a 20- or even a 30-horsepower for another. In order, therefore, to give the term a definite meaning, a committee of engineers met in Philadelphia, in 1876, and after a good deal of experimenting, decided to base the power of a boiler on the amount of water it is able to evaporate in one hour. The report of this committee stated that a boiler horsepower shall be equal to the evaporation of thirtyfour and one-half pounds of water in one hour into steam at atmospheric pressure, starting with the feed water at a temperature of 212 degrees Fahrenheit. It was further specified that the boiler should work under normal conditions as to firing and that a good, ordinary grade of soft coal should be used. It will thus be seen that the term boiler horsepower is quite different from engine horsepower. In fact, it is not correct, strictly speaking, to say the horsepower of a boiler, because a boiler does not work in the strict sense of the term. It does not overcome any resistance through a distance. It simply stores away energy in the form of steam.
The power of boilers is often based upon their heating surface, or upon the area of the grates. Ordinary multi-tubular stationary boilers are given twelve square feet of heating surface per boiler horsepower, and one-third of a square foot of grate surface. Locomotives have about four and one-half square feet of heating surface, and only seven one-hundredths of a square foot of grate area. According to their rated horsepower, direct flue traction boilers are given about eleven and a half square feet of heating surface, and forty-two hundredths of a square foot of grate area per horsepower. Some builders exceed these values a slight amount, while others fall below.
Measuring the Horsepower of an Engine — There are two common methods in general use for determining the horsepower of an engine. In the first method a Prony brake is used, and in the second, an indicator. Brake horsepower shows exactly how much work the engine is doing at the flywheel, while the indicator shows how much work is done in the cylinder. Indicated horsepower is always greater than brake horsepower, because the work done in the cylinder shows the total amount of work done by the piston, and includes not only the work done at the fly wheel, but the work necessary to run the engine itself as well.
An indicator is an expensive and delicate instrument, and the services of an experienced man are required to secure satisfactory results. A brake is something that any engineer can make and is easy to operate. Figures 29 and 30 show how it may be constructed, and the following directions explain how it should be used and how the horsepower is figured.
 Figure 29 |
 Figure 30 |
1. Place the brake on the fly wheel in the position shown, with the nuts A loosened so that the brake hangs free on the wheel. Now find out how much weight the brake exerts on the scales. This weight is constant and must be subtracted from all of the scale readings.
2. Oil the rim of the fly wheel, so that it will not stick to the brake, and start the engine. Now tighten nuts A as much as possible without slacking the speed of the engine, thus putting a load on the engine. In running it is often necessary to keep a stream of water on the wheel to keep the brake from burning.
3. Find the average speed of the engine per minute during a run of at least ten minutes, and the average load on the scales. Subtract the weight of the brake on the scale, as found in paragraph 1, and the result will be the load exerted by the engine.
4. Now measure the length of the brake arm in feet, and proceed as follows: Multiply the length of the brake arm by the average load on the scales, and by the average revolutions of the flywheel. Then multiply this product first by 2, and then by 3.1416. Divide the final product by 33,000, and the quotient will be the brake horsepower.
Example — Suppose in a certain test, that the brake arm was seven feet long, the load on the scales 150 pounds, and the average number of revolutions 240 per minute; what is the horsepower of the engine?
Solution — (7×150×240×2×3.14160)÷33,000=47.7 horsepower — answer.
Information Sources
- The Thresher’s Guide, Volume I, 1910 pages 44-52
Being a reprint from the Threshers’ School Of Modern Methods of the American Thresherman
Published By The American Thresherman, Madison, Wisconsin
Copyright 1910 By The American Thresherman
Prepared By Professor P.S. Rose, North Dakota Agricultural College
This book courtesy of Brian Szafranski