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gree of purity essential to health, and which persons under certain pulmonary affections can so nicely appreciate.
It is evident that the former of these objects can never be attained by radiant heat; and yet, an open fire, which scarcely affords any other than radiant heat, is so connected with our domestic habits that it will be very long before the open grate will be entirely set aside. Under these circumstances, it has been found most expedient to use the combined effect of radiant heat with a constant supply of fresh air, raised to an agreeable temperature in the winter; and which, in certain cases may be cooled during the excessive heat of summer.
Great difficulties have been experienced in most of the means hitherto employed for warming air. In the first place, from what has been previously observed concerning the action of the solar rays on the earth, the air cannot be warmed by radiant heat passing through it; therefore we can only give heat to a transparent fluid by bringing its particles in contact with a heated surface, and, in proportion as elastic fluids are more expansible, they are heated with more difficulty.
There are a number of properties which a body should possess, to afford a surface proper for heating air intended to warın and ventilate rooms. For the sake of economy it should be a good conductor of heat, in order that the radiant heat which it receives on one surface may be safely transmitted to the other. The surface to be heated should be clean, that is, free from any foreign matter, but not polished; and when the temperature can be limited, it should never, under any circumstances be allowed to exceed 300°, Metals appear to be the best substances for heating air. The temperature is limited to 300° because the animal and vegetable matter, which is found mechanically mixed with the air at all times, will be decomposed if the temperature be raised a little higher. When this decomposition takes place, as is very observable when the heated surface is red hot, certain elastic fluids and vapours are produced, which give to the air a peculiar odour, and a deleterious quality which never fails to affect the health of those who inhale it for a length of time. This oppressive sensation has been mostly felt in churches and other places where large iron stoves are used and are sometimes heated to redness. The peculiar odour accompanying it has been erroneously attributed to the iron; and on this account, earthen ware or stone has been employed to form the exterior surface of the stove. It will, however, be found that whatever be the material, if the temperature at all approaches a red heat, the same smell will be perceived; as it arises entirely from the decomposition of the matter which is in the air, and not from the heating body. This matter is very visible to the naked eye, in a sunbeam let into a dark room.
When earthen ware or stone has been employed for stoves, its inferior conducting power has seldom allowed the exterior surface to get sufficiently hot, to produce the effect on the air above alluded to. And hence it has been less objectionable as affecting the purity of the air.
It must however be admitted, that if the body used for heating the air, does not undergo any change, a metal from its being a good conductor must be preferred to any other substance. Silver or platina, if it were not for the expense, would set aside every prejudice. But
long experience has shown that iron possesses every essential property. The slightly oxydated surface which is common to all iron coming from the forge or the mould in casting, is well fitted for receiving radiant heat. And if its temperature be kept below a red heat, there does not appear to be any limit to its durability. The latter point, therefore, is put out of all doubt, since it is essential, that the iron shall not be heated to a degree capable of decomposing animal and vegetable matter, in order to preserve the purity of the air which is warmed in contact with its surface.
With a view to insure the above objects, it will be necessary to dispose of the heat as it is produced from the combustion of the fuel, in such a way,
that an extensive surface of iron shall be heated uniformly without the risk of attaining a much higher temperature than 300°. This can be accomplished by making the fire of a size proportionate to the interior surface of an iron vessel, and it is found that radiant heat is much more efficacious than the heat produced by flame and conducting flues. Having heated the interior surface of an iron vessel it may be conceived that the exterior surface will quickly attain the same degree, and that whatever heat may be carried off from the exterior will be as quickly given from the interior, and instantly replaced by the radiant fire.
The next material object is the means of disposing of the heat from the exterior surface. If it be surrounded by an open space, and that be connected with a flue or tunnel of a certain height, supposing there to be no inlet at the bottom, or outlet at the top, the air will commence a circulation ; that on the heated surface would ascend, and its place be as constantly supplied by the surrounding air. In this way two currents will be established; one ascending from the heated surface, and the other descending on the outside of the tunnel; and these currents will go on, as long as any difference of density exists in the air of the different parts of the surrounding space. If now an opening be made in the bottom of this tunnel and another at the top, an ascending current will be kept up; which will be as the difference of density between external air and that of the heated column, and as the square root of the height of the tunnel.
Let D be the density of the external air ; d, that in the tunnel, which will be inversely as the heat supplied.
V = the velocity which a heavy body would acquire by ling through the height of the tunnel ; and v= the velocity of the ascending air.
D-d Then v=VX
D the density of the smoke.
The mere exposure of the heated surface in an open space, such as a small room, is not sufficient to produce the greatest effect. This is, however, the method at present used by sugar-bakers for heating the rooms in which they expose their sugars. The vessel so employed is of cast iron, and is called a cockle.
Various modifications of this method of heating air have been employed. The wall surrounding the heated vessel has been placed at various distances, in order to find the maximum of effect of a given fire. It was considered a great improvement, to place the wall at a
distance, to admit of a sufficient quantity of air, and make a number of apertures in the wall, about two and a half inches square, with a view to compel the air to blow upon the heated surface. This method was employed more than thirty years ago, by William Strutt, Esq. of Derby, in his cotton-works. He afterwards made a great improvement on this plan, by inserting tubes in the apertures in the wall reaching near to the heated surface. By these means, the air is prevented from ascending before it comes in contact with the heated surface. A further improvement was made in this apparatus, by inserting similar tubes over the surface of the cockle, the shape of which was a square prism with a groined top. The cold air was made to pass through one half of the tubes; and the air so heated, became still more heated by being compelled to pass in a contrary order through the other half,' into a chamber above, called the air-chamber. The stove, thus improved, has been employed by Messrs. Strutts in their works ever since, with complete success, and is similar to that by which the Derbyshire General Infirmary is warmed. This stove has been fixed in different parts of the country and in London, sometimes with success; but so many circumstances besides the stove itself interfere, in arrangements of this kind, that the plan has failed in many instances. And such will ever be the case with the best inventions, in the hands of men who are unacquainted with the principles on which they are founded.
Nothing can be more obvious, than the decided advantage which this stove possesses over all others, and nothing remained for its improvement but to give its different parts their proper proportions, and to vary its construction, so as to admit of its easy management in domestic use. By the former improvement, a larger quantity of air is admitted in proportion to the fuel consumed, and of course at a lower temperature. The advantages which result from this improvement will be obvious. The ventilation of the rooms warmed by it, is much more complete from a greater quantity of air being admitted; the temperature is more uniform, from the air being more dispersed; and, lastly, from the air being heated by a greater surface at a lower temperature, the apparatus is not in the least degree injured by the fire, and hence there does not appear to be any limit to its durability.
Nothing can be inore vague and uncertain, than the opinions which have been formed of the different apparatus used for warming rooms by heated air. It has in consequence appeared to me a desideratum in inquiries of this nature, to be able to ascertain the power and merits of a stove, as we do those of an engine. For this purpose, my first object was to get an instrument capable of measuring the velocity of currents. After trying a variety of methods, I have found one with which I am perfectly satisfied. It consists of a very light brass wheel, in the form of that for the first motion of a smoke-jack. An endless screw upon the same axis gives motion to a wheel of fifty teeth, on the axis of which is an index, which is watched by the eye, when the instrument is exposed to the current. The wheel acted on by the current, is about two and a half inches in diameter, and the vanes or sails are eight in number, and fill up the whole circle, when their faces are parallel to the plane of their motion, and they are adjusted to an angle of 45°. Under these circumstances, I have found that fifty re
volutions of the first motion take place, while the current causing those revolutions moves through forty-six feet.
In order to ascertain the power and merits of a stove, I generally take a period of twelve hours, beginning with a good fire, and leaving off with the same. During this time, the velocity and temperature in the main warm air-flue should be taken every half hour, and then the average
of each taken, keeping an account of the coal consumed in the same time. The temperature of the outer air being also known, the excess of the average temperature above the atmosphere is the datum required.
From the average velocity, the number of cubic feet of air passing through the flue in the twelve hours may be known. Put A = The number of pounds of air heated in twelve hours, allow
· ing 14 cubic feet of air to one pound.
same construction, should be generally a constant quantity:
advantages to be produced by W the weight of coal. E, the effect, will be directly as A and T, and inversely as W.
AT Therefore, E
W: To give an example in practice :-A stove which is capable of warming 100,000 cubic feet of space to 60° in the coldest season, when placed at the depth of nine feet below the level at which the warm air is discharged, will furnish about 45 cubic feet every second, raised 60 degrees above the temperature of the atmosphere. To keep up this current and excess of temperature for twelve hours, it will consume not more than three bushels of coals, or 252 pounds. In this case, 49 cubic feet of air in each second will be 1,944,000 in twelve
158,857 X 60 hours, equal to 138,857 pounds. Hence E
252 This number may be taken as a constant quantity, expressive of the power of any stove; but it also expresses the weight of air in pounds, which one pound of Newcastle coal heats one degree of Fahrenheit's thermometer.
This number will not be strictly a constant quantity, as small stoves will not act quite to the same advantage as larger ones; and local and other circumstances will in some degree alter the result of experiments made in the manner above stated.
This is more especially the case, when the admission of cold air and the discharge of foul air, are in any degree influenced by the wind.
The cold air is generally brought directly from the atmosphere; and, therefore, as its progress along this channel is affected by the wind, a greater or less quantity will pass through the stove. If the air be deficient, less heat is carried off from the heating surface, and a greater proportion goes up the chimney; on the contrary, when the wind blows into the cold air-flue, the two forces conspire, more air is admitted, more heat is carried off with the air, and of course less is wasted up the smoke-flue.
In all situations where it is practicable, I use an effectual means of regulating the admission of cold and the escape of foul air, by placing at the commencement and termination of these apertures a turn-cap or cowl, in which the vanes are so fixed as to let the wind blow into the one, and assist the escape of air from the other. Although this contrivance will always prevent a counter current, which without its use is sometimes the case, it does not prevent unequal quantities of air from entering, according to the strength of the wind. This is not found in practice to be a great inconvenience; for during the most perfect calm, the air admitted by the power of the stove alone, is sufficient for every purpose of warmth and ventilation : whilst with a tolerable fire in the stove when the wind is considerable, the air comes into the rooms at a higher temperature than the rooms require which is at least erring on the desirable side. If the quantity of air admitted under all states of the wind were required to be uniform, the aperture in the turn-cap for cold air might contain a self-adjustment, by the action of which its area would always be in the inverse ratio of the velocity of the wind; by which means equal quantities of air would always be admitted in equal times.
l'he turn-cap for the escape of foal air is placed at the top of the building, and is made common to the roof. Under this arrangement all the rooms into which the warm air is admitted have each a foul air Aue terminating in the cavity of the roof.
The contents of all the foul air-flues are therefore ultimately discharged at the turn-cap. This arrangement is adopted at the Derby. shire General Infirmary, and at the Wakefield Lunatic Asylum. In the summer season, when the stove is not in action, the ventilation will depend on the wind, which at some periods may not be adequate to that change of air required in hospitals. In such cases I have adopted an additional means of ventilation. Instead of making the foul air turn-cap common to the roof, I have placed it at the top of a cylindrical cavity built in the roof. Into this cavity I bring all the foul air-flues, which also in this case may be smoke-flues, if constructed with brick. I also connect with the same cavity, the stove chimney, and, if possible all the other smoke-flues in the building. By this means, it may be expected, that some degree of rarefaction in the cylindrical cavity in the roof will be constantly going on, and that hence a perpetual current will be established from every room towards the general outlet. It would be difficult to adapt such an arrangement to old buildings, without great alteration in the roof. But it would be easily introduced into new houses. The advantages derived from it in ordinary dwellings would be very great. In the first place, there could not be an instance of a smoky chimney; in the next, a down current in an unoccupied chimney could not occur, and therefore the passage of the smoke of one chimney down another would always be prevented; and lastly, by having only one outlet for smoke in every bouse, and that an object which may be made ornamental, we should ultimately get rid of the great deformity which arises from the present appearance
of chimneys in buildings. In all situations where it is practicable to make a cold air-flue, of considerable length under ground, the advantage is well worth securing. I have found by experience that a cold air-flue of fifty yards in length is capable of cooling the air in summer to about an arithmeti