Popular Science Monthly/Volume 3/June 1873/Domestic Economy of Fuel I
MY endeavor will be, to show that there may be obtained, from a much-diminished consumption of coal in fireplaces used for domestic purposes, all the advantages which have hitherto resulted from the wasteful expenditure which has prevailed.
I have no expectation of stating any thing that is actually new, because the functions and the attributes of heat and combustion have long been thoroughly discussed in their application to industrial objects. I hope, however, to draw attention to important considerations which govern the application of heat, and which are very generally neglected in fireplaces, in kitchen-ranges, and in most warming apparatus.
I think I may say, without hesitation, that the quantity of fuel now absolutely wasted in our houses amounts to at least five-sixths of the coal consumed. That is to say, if the greatest care and the best method of applying the heat were in all cases adopted, we could effect in heating and cooking all that we now effect, with one-sixth of the coal we now use; and, if, in the construction of our fireplaces and cooking apparatus, simple principles were recognized and ordinary care was used, we might without difficulty save from two-thirds to half of the coal consumed.
In my remarks on this question I intend to confine myself rather to the enunciation of the principles which should govern the application of heat for domestic purposes, than to give descriptions, except in a general way, of special appliances.
The inventors of apparatus for warming and cooking are so numerous, and the merits of a large number of inventions which have come into common use are of so negative a value, that it would not be fair to single out some individual instance for condemnation, and leave unnoticed other apparatus which possess equal defects and maybe in equally extensive use. Mr. Edwards's very interesting and instructive treatise on domestic fireplaces clearly shows with what persistent perverseness the inventions which possess real merit have been almost invariably passed by. This result, I fear, is due mainly to the fact that architects and builders have not been penetrated with sound principles on the warming of our dwellings, and have encouraged the adoption of showy grates, based on false principles, instead of taking the trouble to make new designs of pretty grates based on sound principles of warming.
The question of the consumption of coal for domestic purposes divides itself into two branches:
- The quantity required for warmth.
- The quantity required for cooking.
The former is required only for the winter months, the latter is a permanent quantity during the year.
The waste of coal in domestic fireplaces is, however, no new question. It is quite eighty years since the subject was most fully treated of by Count Rumford, and afterward by Mr. Sylvester. They showed conclusively what enormous savings in fuel, for heating, cooking, and drying, were possible. Count Rumford' s principles have never been generally applied, because the price of coals has ruled so low that householders have not much cared for economy. "We hear Count Rumford's axioms now and then quoted by rival manufacturers in support of their newly-devised grates or kitchen-ranges; but, in many cases, the manufacturer, in the article he supplies, seems to be endeavoring to violate, rather than to follow, every axiom which Count Rumford ever laid down.
I do not mean to say that improvements have not taken place since Count Rumford's time, but the progress in the direction of economy has been very small, when we consider the great ingenuity displayed in devising new forms of apparatus. In respect of our fireplaces, our chief talent has been expended in providing a means of warming the outside air, and of polluting it by the smoke and soot we project into it.
The methods which have been adopted for warming houses fall under the several heads of—
- Open fireplaces.
- Close stoves (the German plan).
- The Roman hypocaust, or floors warmed by direct action of fire.
- Hot-water pipes, without ventilation.
- Hot air warmed by a cockle, or by hot-water pipes.
The class of apparatus to be adopted in any country will vary with the climate. In England the climate is of so very changeable a nature, that the amount of heat required for comfort in a house varies from day to day. There are many days in the middle of winter when it is quite possible to sit in an unwarmed room ; or, sometimes a warm morning is followed by a cold afternoon, when the sudden application of heat is desirable. It is probably for this reason that in England the open fireplace has, as a rule, held its own against all the proposals for warming houses by means of one central fire.
The open fireplace in ordinary use warms only by means of the direct radiation of the flame into the air of the room. It is the most primitive mode of warming, derived from the days when our ancestors inhabited caves. But these ancestors, by placing the fire in the centre of the floor of the cave, derived from it a larger portion of heat than we generally do, who place it against the wall of the room, and carry off the greater part of the heat up a flue separated from the room. The earlier fireplaces consisted of a large square brick opening, with a chimney carried up for the escape of smoke. The large square fireplace was adverse to the direct radiation into the room of the heat generated, and the large chimney removed from the room a very considerable quantity of air, which had necessarily to be replaced by cold air flowing into the room through all available apertures, and this created strong draughts.
Franklin, Count Rumford, and Sylvester, are the most prominent names of those who at an early period contributed improvements to the warming of our houses. The main principle of fireplace construction advocated by Count Rumford, eighty years ago, was, that the heat radiated from the fire directly into the room should be developed to the utmost. He brought the back of the fireplace as prominently forward as possible; he sloped the sides so as to reflect heat into the room; he advocated the use of fire-brick backs and sides instead of iron; he reduced the size of the chimney opening, so as to prevent the chimney carrying off the large quantity of warmed air it used to remove in his time. Our manufacturers of fireplaces have continued in the same groove. They have, undoubtedly, in some cases, largely developed the use of radiant heat. There are fireplaces, eminently successful as radiators of heat, of a circular or concave form, with polished iron sides, the fire being placed against a fire-brick back forming the apex of the concavity. So long as the concave surfaces are bright, the heat thrown out by them when a clear flame is burning is very great, but the gases from the flame pass directly off into the chimney while they are still at a very high temperature. The heat of the flame at that part will often be between 1,200° and 1,300° Fahr., and a very large proportion of this heat, to the extent of at least nine-tenths of that generated by the combustion of the fuel, is carried directly up the chimney.
One pound of coal is capable, if all the heat of combustion is utilized, of raising the temperature of a room, twenty feet square and twelve feet high, to ten degrees above the temperature of the outer air. If the room were not ventilated at all, and the walls were composed of non-conducting materials, the consumption of fuel to maintain this temperature would be very small, but, in proportion as the air of the room was renewed, so would the consumption of fuel necessary to maintain that temperature increase. If the volume of air contained in the room were changed every hour, one pound of coal additional would be required per hour to heat the inflowing air, so that, to maintain the temperature at ten degrees above that of the outer air during twelve hours, would require twelve pounds of coal.
The principle of the ordinary open fireplace is that the coal shall be placed in a grate, by which air is admitted from the bottom and sides to aid in the combustion of coal; and an ordinary fireplace, for a room of twenty feet square and twelve feet high, will contain from about fifteen to twenty pounds at a time, and, if the fire be kept up for twelve hours, probably the consumption will be about one hundred pounds, or the consumption may be assumed at about eight pounds of coal an hour.
One pound of coal may be assumed to require, for its perfect combustion, 150 cubic feet of atmospheric air; 8 lbs. would require 1,200 cubic feet; but, at a very low computation of the velocity of the gases in an ordinary chimney-flue, the air which would pass up the chimney at a rate of from 4 to 6 feet per second, or from 14,000 to 20,000 cubic feet per hour, with the chimneys in ordinary use, and I have often found a velocity of from 10 to 12 feet per second giving an outflow of air of from 35,000 to 40,000 cubic feet per hour—this air comes into the room cold, and when it is beginning to be warmed it is drawn away up the chimney, and its place filled by fresh cold air. A room 20 feet square and 12 feet high contains 4,800 cubic feet of space. In such a room, with a good fire, the air would be removed four or five times an hour with a moderate draught in the chimney, and six or eight times with a blazing fire; the air so removed would be replaced by cold air. The atmosphere of the room is thus being cooled down rapidly by the continued influx of cold air to supply the place of the warmer air drawn up the chimney. The very means adopted to heat the room produces draughts, because the stronger the direct radiation, or rather the brighter the flame in open fireplaces, the stronger must be the draught of the fire and the abstraction of heat. The only way to prevent draughts is to adopt means for providing fresh warmed air to supply the place of that removed.
The most natural way of providing warmed air is to utilize the excess of heat which passes up the chimney, beyond what is required for creating an adequate draught, and to use this heat to warm fresh air; and the warmed air should be admitted into the room in such places as will enable it to flow most easily into the currents prevailing in the room. These considerations led to the construction of the ventilating fireplace, which has been so extensively used in barracks. This fireplace will keep a room at a given temperature with one-third of the quantity of fuel usually required in most ordinary fireplaces, and with less than one-half the quantity required in the very best-constructed radiating fireplaces.
The open ventilating fireplace, if properly constructed, is the simplest and most effectual means of warming and ventilating a single room, because it absorbs all spare heat from the chimney beyond what is necessary to create a draught ; and, while it admits warmed air into the upper part of the room in an imperceptible current, the action of the fire draws air from the lower part of the room, and thus provides for a circulation of the warmed air toward the floor of the room.
The ventilating fireplaces invented by me, and now called by my name, but which have never been the subject of a patent, were a consequence of the efforts made by the late Lord Herbert and Miss Nightingale to improve the health of the army. The death-rate of the soldiers, when this question was taken up, was found to be larger than that of many unhealthy civil populations. Soldiers are, however, a body of men picked out as the healthiest members of the nation; they should, therefore, have had an exceptionally low death-rate in peacetime. A main element in the improvement of their health lay in improving the ventilation of their barrack-rooms. But soldiers, whenever they became aware of the existence of any fresh-air currents, insisted on closing the inlets. It was also made a sine qua non by the Government that the barrack-rooms should be warmed by open fireplaces; and, moreover, the Government required that the increased amount of ventilation declared to be necessary on medical grounds should be provided without any increase in the amount of fuel allowed. By the adoption of these fireplaces, and by the introduction of simple and improved arrangements for cooking the soldiers' food, the Government were enabled to effect a saving on the fuel supplied, instead of being obliged to incur a large increased expenditure on account of the additional ventilation introduced into the barrack-rooms. The manufacturer of these fireplaces informs me that he has supplied between 9,000 and 10,000 to the military departments up to this time.
The principle of warming by means of an open fireplace, or by means of a German stove or a Gill stove, is applicable to single rooms, that is to say, each room must have its own appliance, and each room may be self-contained as far as regards its heating and ventilation.
The close stoves employed in Germany use less fuel in warming the room than any open fireplace, but they are economical because the heat generated is not removed by the frequent renewal of the air. This element of their efficiency in warming, however, makes them most unhealthy.
The most recent improvements in the use of the German stove for warming have been introduced by Dr. Bohm, in the Rudolf Hospital at Vienna. He there warms fresh air by means of passages con- structed in the fire-clay stoves, placed in the ward, and the fresh warmed air passes into the ward from the top of the stove. He provides flues of a large size, and proportioned to the size of the ward, from the level of the ward floor to above the roof, and the difference of temperature between the air in the ward and the outer air causes a sufficient current in these flues to ventilate adequately the ward. By this means the fresh warmed air, instead of passing off to the upper part of the ward and then away by flues there, is made to circulate toward the floor of the ward, thus bringing into action the principle by which the open fireplace is useful in ventilation. But this arrangement destroys one element of economy in the German stove, because the heat generated, instead of being left to pass slowly off into an unventilated room, is removed rapidly by the fresh air passed into the ward, and has, therefore, to be renewed at intervals, instead of, according to usual custom, the stove being left shut irp for twenty-four hours to give off its heat slowly. The larger the supply of warmed air, the larger must be the consumption of fuel; and, if the heat is to be supplied economically, it must be through a good conducting medium; but the material of the German stove is a bad conductor of heat.
The old Roman system of warming by means of a fire under the floor produced a most agreeable and equable temperature, but it did not assist the ventilation, and' it was not economical, in that the floor, being of tiles, was of a bad conducting material, and much of the heat was absorbed in the ground or surrounding flues. According to Pliny, the smoke was carried to the wood-house to be used in drying the wood for burning. I recently made an experiment to compare the effect of warming by means of a heated floor with the heating effect of a ventilating fireplace; the experiment lasted, with each mode of warming, for two days. It showed that, in the case of the warmed floors, the room was maintained at a temperature of about 18° above the temperature of the outer air with an expenditure of 56 lbs. of coal and 112 lbs. of coke, while with the ventilating fireplace the expenditure was only 75 lbs. of coal; the cost being 3s. 4d. for the warmed floor as compared with 1s. 4d. for the ventilating fireplace.
A more complete plan of warming a building is by means of a fire from which the heat is conveyed, either by hot-water pipes or hot air, to the various parts of the building.
Warming by means of air conveyed by flues to various parts of the building, will answer, as a rule, in ordinary existing houses, best in connection with open fireplaces, which draw in the warmed air to the various rooms, because there must be some means of forcing or drawing the warmed air into the house, and it would not be convenient to keep a steam engine in an ordinary house to pump in the warmed air. These open fireplaces would then, however, be wasting the spare heat which each fireplace sends up its own chimney; but, on the other hand, very much smaller fires would be needed, to keep the rooms warm, than when the rooms are not supplied with fresh warmed air. Theoretically, however, it can be shown that if we. are prepared to give up open fireplaces, and arrange our houses on the plan of having flues which would draw off the air from near the floors of our rooms, and which would also warm fresh air, heated from a central fire, to be constantly admitted near the ceilings, and if the climate were such as to make us desire to have the system in continuous operation, such a system would probably be by far more economical of fuel than open fireplaces, because the fuel used could then be made to do its full duty. The variations of our climate and the low price of fuel, which have hitherto prevailed, have prevented such systematic arrangements from being adopted in this country.
The plan of carrying the heat from the fire to the air to be warmed by means of hot-water pipes affords also a very economical method of warming air, because the best-constructed hot-water apparatus will enable the full heating value to be got out of the fuel. Fuel may be consumed to far greater advantage in a close furnace than in any open grate, because the admission of air for the combustion of the fuel can be regulated to any required extent. The heating surface of the boiler may also be so arranged as to absorb a very large proportion of the heat generated by the fire.
But in deciding on the amount of heat in hot-water pipes which is most favorable to economy, the following considerations occur: At least twice the quantity of air which is strictly necessary by theory passes through the fire in the best-constructed furnaces. In an ordinary grate this consumption is enormously increased. Each part of oxygen supplied by the air and necessary for combustion is accompanied by four parts of nitrogen, which is of no value for combustion. Consequently, if twice as much oxygen passes through the fire as is strictly necessary, we have one part which combines with carbon and produces combustion, and nine parts which, being inert, must act, in the first place, to lower the temperature of the fire, and, secondly, to carry a larger amount of unutilized heat up the chimney. Moreover, when water is heated sufficiently to generate steam, each particle of water converted into steam absorbs or makes latent 960° Fahr. of temperature. In experiments on the evaporation of water, the temperature of the gases passing off in the chimney was ascertained to vary from 430° to 530°, diminishing to 415° at the top of a flue 35 feet high, with the dampers open; and about 380° at the bottom of the flue with the dampers closed. With a boiler of which the temperature of the water is maintained at 200° without evaporation, the temperature of the flue need not exceed from 230° to 240°.
It is clear from these considerations, that, in order to insure the maximum effect from the fuel, the heating surface of the pipes should be sufficiently large to warm all the air required without its being necessary to raise the temperature of the water in the boiler to any great extent, and the proportion between the boiler-surface and the pipe-surface, that is to say, between the surface which absorbs heat, and the surface which gives out heat, should be such as to render it unnecessary for the fire to be forced, because, the lower the temperature at which the gases from the fire pass off up the chimney, the greater will be the economy.
In order to show the waste which results from forcing the boiler, i. e., from passing the gases into the flue at a high as compared with a low temperature, I will give an instance of one experiment. The proportion of heating surface in the boiler to the heating surface of the pipes is assumed by some manufacturers as 1 to 100, or, when great heat is required, 1 to 40. An experiment made on 4,000 feet of pipe, heating certain greenhouses by a wagon-shaped boiler with 40 square feet of heating surface, showed that a certain temperature was kept up for 8 hours with 8 bushels of coal; but when, by the addition of another boiler, the heating surface of the boiler was increased to 80 square feet, the temperature could be maintained for the same period with 4 bushels of coal. The outer temperature was the same on the two days.
On these grounds it is not so economical, so far as the consumption of fuel is concerned, to use steam instead of water, either water heated to a high temperature under pressure, or to heat air for warming purposes, because the gases from the fire employed to produce the higher degree of heat will pass off at a high temperature, and the heat they contain be wasted. On the other hand, the capital outlay required, where highly-heated pipes are used, is smaller than with hot-water pipes, because a smaller heating surface, and therefore smaller pipes, will suffice when the temperature is high; and, moreover, a very small pipe will convey steam to any required place, whereas with hot water, at a relatively low temperature, much larger pipes are required. It follows that where the price of fuel makes it necessary to reduce the permanent annual expenditure, the original capital outlay must be increased. There is a further consideration in regard to economy with hot-water pipes, steam-heating, and all appliances for warming buildings from a central fire, viz., that if the heat has to be conveyed for long distances before its useful application comes into force, very much heat is lost, and consequently fuel is wasted. On the other hand, against the saving which would result from a more immediate application of the heat to the place to be warmed, there is to be weighed the diminished expense of attendance consequent upon the use of one fire instead of several fires, each with its attendance and supply of fuel. There remains one source of economy to be applied to close grates used for heating water, which has not yet been adopted. I mean the application of some of the heat which is passing into the chimney to warm the air which feeds the tire. Theoretical considerations show that an advantage of from six to nine per cent, might be obtained from this source, and the experiments which I have made bear out this result.
But, after we have designed the most effective arrangements for economizing the fuel which warms our dwellings, if that object is to be fully secured, we must arrange to retain the heat in our houses. The architect should devote to these considerations the same care which he now is frequently satisfied with bestowing upon the beauty of the design for a building. The arrangements of the plan should be adapted to the retention of heat. All portions of houses exposed to the air should be formed of materials which are found to be the slowest conductors of heat. Whatever may have been the mistakes of the manufacturers of fire-grates or kitchen-ranges, the nation has latterly very much disregarded the means of retaining heat in the house. The uniform model house of the speculating builder is constructed with thin walls, thin glass windows, ill-fitting casements, and a roof of slates, with nothing under them. The old half-timbered house was warm, because it had an air space between the inner and outer skin; the brick-built, stone-faced house is warm because it has, so to say, a double wall. In modern houses it has long been shown that, without much increased expense, the use of walls built hollow will keep the rooms effectually warm and dry, and yet this mode of building is the exception rather than the rule, possibly because it gives the architect or the builder a little additional trouble. A slated roof, if ill-constructed, is a material agent in allowing of the escape of heat, because there is necessarily an inlet for air where the slates overlap. The old thatched roof, although most dangerous in cases of fire, was a great preserver of heat. In well-built modern houses the slates are laid on felt, which is laid on close boarding, and this arrangement keeps the house warm in winter and cool in summer. As regards the windows, glass ranks high as a non-conductor of heat, and the effect of using thick glass, instead of the very thin glass so often seen, is very largely to economize the heat. Evidence of the cooling effect on the air of a room of a window of thin glass is afforded by the cold draught which any one perceives when sitting on a cold day near a closed window of thin glass. Proposals have been often made to glaze a window with double panes, and no doubt such a plan is a good means of retaining heat in the room, but the inside of the glass between the panes will in time become dirty, and then it can only be cleansed by removing one of the panes. A more convenient, but more expensive, plan is to adopt the system, which prevails universally in the northern parts of Europe, of a double casement.
It is not, however, my object here to give a treatise on building. The conclusion which I would draw from these various considerations is, that, if we desire to economize to the utmost the daily expenditure of fuel, we must increase our outlay of capital. So long as coal was cheap, it may have been better worth the while of the individual consumer to employ coal wastefully rather than spend money upon the arrangements for economizing heat. On the other hand, when coal is dear, the daily expense from the waste of fuel will induce a capital outlay to secure economy of heat.—Journal of the Society of Arts.