A Catechism of the Steam Engine Part 8

138. _Q._--In what way does the specific heat of a body enable the quant.i.ty of heat in it to be determined?

_A._--If any body has only half the specific heat of water, then a pound of that body will, at any given temperature, have only half the heat in it that is in a pound of water at the same temperature. The specific heat of air is .2669, that of water being 1; or it is 3.75 times less than that of water. An amount of heat, therefore, which would raise a pound of water 1 degree would raise a pound of air 3.75 degrees.

COMBUSTION.

139. _Q._--What is the nature of combustion?

_A._--Combustion is nothing more than an energetic chemical combination, or, in other words, it is the mutual neutralization of opposing electricities. When coal is brought to a high temperature it acquires a strong affinity for oxygen, and combination with oxygen will produce more than sufficient heat to maintain the original temperature; so that part of the heat is rendered applicable to other purposes.

140. _Q._--Does air consist of oxygen?

_A._--Air consists of oxygen and nitrogen mixed together in the proportion of 3.29 lbs. of nitrogen to 1 lb. of oxygen. Every pound of coal requires about 2.66 lbs. of oxygen for its saturation, and therefore for every pound of coal burned, 8.75 pounds of nitrogen must pa.s.s through the fire, supposing all the oxygen to enter into combination. In practice, however, this perfection of combination does not exist; from one-third to one-half of the oxygen will pa.s.s through the fire without entering into combination at all; so that from 16 to 18 lbs. of air are required for every pound of coal burned. 18 lbs. of air are about 240 cubic feet, which may be taken as the quant.i.ty of air required for the combustion of a pound of coal in practice.

141. _Q._--What are the const.i.tuents of coal?

_A._--The chief const.i.tuent of coal is carbon or pure charcoal, which is a.s.sociated in various proportions with volatile and earthy matters. English coal contains 80 to 90 per cent. of carbon, and from 8 to 18 per cent. of volatile and earthy matters, but sometimes more than this. The volatile matters are hydrogen, nitrogen, oxygen, and sulphur.

142. _Q._--What is the difference between anthracite and bituminous coal?

_A._--Anthracite consists almost entirely of carbon, having 91 per cent. of carbon, with about 7 per cent. of volatile matter and 2 per cent. of ashes.

Newcastle coal contains about 83 per cent. of carbon, 14 per cent. of volatile matter, and 3 per cent. of ashes.

143. _Q._--Will you recapitulate the steps by which you determine the quant.i.ty of air required for the combustion of coal?

_A._--Looking to the quant.i.ty of oxygen required to unite chemically with the various const.i.tuents of the coal, we find for example that in 100 lbs.

of anthracite coal, consisting of 91.44 lbs. of carbon, and 3.46 lbs. of hydrogen, we shall for the 91.44 lbs. of carbon require 243.84 lbs. of oxygen--since to saturate a pound of carbon by the formation of carbonic acid, requires 2-2/3 lbs. of oxygen. To saturate a pound of hydrogen in the formation of water, requires 8 lbs. of oxygen; hence 3.46 Fibs. of hydrogen will take 27.68 lbs. of oxygen for its saturation. If then we add 243.84 lbs. to 27.68 lbs. we have 271.52 lbs. of oxygen required for the combustion of 100 lbs. of coal. A given weight of air contains nearly 23.32 per cent of oxygen; hence to obtain 271.52 lbs. of oxygen, we must have about four times that quant.i.ty of atmospheric air, or more accurately, 1164 lbs. of air for the combustion of 100 lbs. of coal. A cubic foot of air at ordinary temperature weighs about .075 lbs.; so that 100 lbs. of coal require 15,524 cubic feet of air, or 1 lb. of coal requires about 155 cubic feet of air, supposing every atom of the oxygen to enter into combination.

If, then, from one-third to one-half of the air pa.s.ses unconsumed through the fire, an allowance of 240 cubic feet of air for each pound of coal will be a small enough allowance to answer the requirements of practice, and in some cases as much as 300 cubic feet will be required,--the difference depending mainly on the peculiar configuration of the furnace.

144. _Q._--Can you state the evaporative efficacy of a pound of coal?

_A._--The evaporative efficacy of a pound of carbon has been found experimentally to be equivalent to that necessary to raise 14,000 lbs. of water through 1 degree, or 14 lbs. of water through 1000 degrees, supposing the whole heat generated to be absorbed by the water. Now, if the water be raised into steam from a temperature of 60, then 1118.9 of heat will have to be imparted to it to convert it into steam of 15 lbs. pressure per square inch. 14,000 / 1118.9 = 12.512 Lbs. will be the number of pounds of water, therefore, which a pound of carbon can raise into steam of 15 lbs.

pressure from a temperature of 60. This, however, is a considerably larger result than can be expected in practice.

145. _Q._--Then what is the result that may be expected in practice?

_A._--The evaporative powers of different coals appear to be nearly proportional to the quant.i.ty of carbon in them; and bituminous coal is, therefore, less efficacious than coal consisting chiefly of pure carbon. A pound of the best Welsh or anthracite coal is capable of raising from 9-1/2 to 10 lbs. of water from 212 into steam, whereas a pound of the best Newcastle is not capable of raising more than about 8-1/2 lbs. of water from 212 into steam; and inferior coals will not raise more than 6-1/2 lbs. of water into steam. In America it has been found that 1 lb. of the best coal is equal to 2-1/2 lbs. of pine wood, or, in some cases to 3 lbs.; and a pound of pine wood will not usually evaporate more than about 2 1/2 lbs. of water, though, by careful management, it may be made to evaporate 4 1/2 lbs. Turf will generate rather more steam than wood. c.o.ke is equal or somewhat superior to the best coal in evaporative effect.

146. _Q._--How much water will a pound of coal raise into steam in ordinary boilers?

_A._--From 6 to 8 lbs. of water in the generality of land boilers of medium quality, the difference depending on the kind of boiler, the kind of coal, and other circ.u.mstances. Mr. Watt reckoned his boilers as capable of evaporating 10.08 cubic feet of water with a bushel or 84 lbs. of Newcastle coal, which is equivalent to 7 1/2 lbs. of water evaporated by 1 lb. of coal, and this may be taken as the performance of common land boilers at the present time. In some of the Cornish boilers, however, a pound of coal raises 11.8 lbs. of boiling water into steam, or a cwt. of coal evaporates about 21 cubic feet of water from 212.

147. _Q._--What method of firing ordinary furnaces is the best?

_A._--The coals should be broken up into small pieces, and sprinkled thinly and evenly over the fire a little at a time. The thickness of the stratum of coal upon the grate should depend upon the intensity of the draught: in ordinary land or marine boilers it should be thin, whereas in locomotive boilers it requires to be much thicker. If the stratum of coal be thick while the draught is sluggish, the carbonic acid resulting from combustion combines with an additional atom of carbon in pa.s.sing through the fire, and is converted into carbonic oxide, which may be defined to be invisible smoke, as it carries off a portion of the fuel: if, on the contrary, the stratum of coal be thin while the draught is very rapid, an injurious refrigeration is occasioned by the excess of air pa.s.sing through the furnace. The fire should always be spread of uniform thickness over the bars of the grate, and should be without any holes or uncovered places, which greatly diminish the effect of the fuel by the refrigeratory action of the stream of cold air which enters thereby. A wood fire requires to be about 6 inches thicker than a coal one, and a turf fire requires to be 3 or 4 inches thicker than a wood one, so that the furnace bars must be placed lower where wood or turf is burned, to enable the surface of the fire to be at the same distance from the bottom of the boiler.

148. _Q._--Is a slow or a rapid combustion the most beneficial?

_A._--A slow combustion is found by experiment to give the best results as regards economy of fuel, and theory tells us that the largest advantage will necessarily be obtained where adequate time has been afforded for a complete combination of the const.i.tuent atoms of the combustible, and the supporter of combustion. In many of the cases, however, which occur in practice, a slow combustion is not attainable; but the tendencies of slow combustion are both to save the fuel, and to burn the smoke.

149. _Q._--Is not the combustion in the furnaces of the Cornish boilers very slow?

A.--Yes, very slow; and there is in consequence very little smoke evolved.

The coal used in Cornwall is Welsh coal, which evolves but little smoke, and is therefore more favorable for the success of a smokeless furnace; but in the manufacturing districts, where the coal is more bituminous, it is found that smoke may be almost wholly prevented by careful firing and by the use of a large capacity of furnace.

150. _Q._--Do you consider slow combustion to be an advisable thing to practise in steam vessels?

_A._--No, I do not. When the combustion is slow, the heat in the furnaces and flues is less intense, and a larger amount of heating surface consequently becomes necessary to absorb the heat. In locomotives, where the heat of the furnace is very intense, there will be the same economy of fuel with an allowance of 5 or 6 square feet of surface to evaporate a cubic foot of water as in common marine boilers with 10 or 12.

151. _Q._--What is the method of consuming smoke pursued in the manufacturing districts?

_A._--In Manchester, where some stringent regulations for the prevention of smoke have for some time been in force, it is found that the readiest way of burning the smoke is to have a very large proportion of furnace room, whereby slow combustion may be carried on. In some cases, too, a favourable result is arrived at by raising a ridge of coal across the furnace lying against the bridge, and of the same height: this ridge speedily becomes a ma.s.s of incandescent c.o.ke, which promotes the combustion of the smoke pa.s.sing over it.

152. _Q._--Is the method of admitting a stream of air into the flues to burn the smoke regarded favorably?

_A._--No; it is found to be productive of injury to the boiler by the violent alternations of temperature it occasions, as at some times cold air impinges on the iron of the boiler, and at other times flame,--just as there happens to be smoke or no smoke emitted by the furnace. Boilers, therefore, operating upon this principle, speedily become leaky, and are much worn by oxidation, so that, if the pressure is considerable, they are liable to explode. It is very difficult to apportion the quant.i.ty of air admitted, to the varying wants of the fire; and as air may at some times be rushing in when there is no smoke to consume, a loss of heat, and an increased consumption of fuel may be the result of the arrangement; and, indeed, such is the result in practice, though a carefully performed experiment usually demonstrates a saving in fuel of 10 or 12 per cent.

153. _Q._--What other plans have been contrived for obviating the nuisance of smoke?

_A._--They are too various for enumeration, but most of them either operate upon the principle of admitting air into the flues to accomplish the combustion of the uninflammable parts of the smoke, or seek to attain the same object by pa.s.sing the smoke over or through the fire or other incandescent material. Some of the plans, indeed, profess to burn the inflammable gases as they are evolved from the coal, without permitting the admixture of any of the uninflammable products of combustion which enter into the composition of smoke; but this object has been very imperfectly fulfilled in any of the contrivances yet brought under the notice of the public, and in some cases these contrivances have been found to create weightier evils than they professed to relieve.

154. _Q._--You refer, I suppose, to Mr. Charles Wye Williams' Argand furnace?

_A._--I chiefly refer to it, though I also comprehend all other schemes in which there is a continuous admission of air into the flues, with an intermittent generation of smoke.

155. _Q._--This is not so in Prideaux's furnace?

_A._--No; in that furnace the air is admitted only during a certain interval, or for so long, in fact, as there is smoke to be consumed.

156. _Q._--Will you explain the chief peculiarities of that furnace?

_A._--The whole peculiarity is in the furnace door. The front of the door consists of metal Venetians, which are opened when the top lever is lifted up, and shut when that lever descends to its lowest position. When the furnace door is opened to replenish the fire with coals, the top lever is raised up, and with it the piston of the small cylinder attached to the side of the furnace. The Venetians are thereby opened, and a stream of air enters the furnace, which, being heated in its pa.s.sage among the numerous heated plates attached to the back of the furnace door, is in a favorable condition for effecting the combustion of the inflammable parts of the smoke. The piston in the small cylinder gradually subsides and closes the Venetians; and the rate of the subsidence of the piston may obviously be regulated by a c.o.c.k, or, as in this case, a small screw valve, so that the Venetians shall just close when there is no more smoke to be consumed;--the air or other fluid within the cylinder being forced out by the piston in its descent.

157. _Q._--Had Mr. Watt any method of consuming smoke?

_A._--He tried various methods, but eventually fixed upon the method of c.o.king the coal on a dead plate at the furnace door, before pushing it into the fire. That method is perfectly effectual where the combustion is so slow that the requisite time for c.o.king is allowed, and it is much preferable to any of the methods of admitting air at the bridge or elsewhere, to accomplish the combustion of the inflammable parts of the smoke.

158. _Q._--What are the details of Mr. Watt's arrangement as now employed?

_A._--The fire bars and the dead plate are both set at a considerable inclination, to facilitate the advance of the fuel into the furnace. In Boulton and Watt's 30 horse power land boiler, the dead plate and the furnace bars are both about 4 feet long, and they are set at the angle of 30 degrees with the horizon.

159. _Q._--Is the use of the dead plate universally adopted in Boulton and Watt's land boilers?

_A._--It is generally adopted, but in some cases Boulton and Watt have subst.i.tuted the plan of a revolving grate for consuming the smoke, and the dead plate then becomes both superfluous and inapplicable. In this contrivance the fire is replenished with coals by a self-acting mechanism.

160. _Q._--Will you explain the arrangement of the revolving grate?