226. A hot-air furnace may be described as a device for heating a continuous current of air by means of a fire contained within the apparatus and without mingling the fresh air with the products of combustion.

It combines the functions of a heat generator with those of a radiator, and operates without the aid of any intermediate heat-carrying agent. The transmission of heat is direct, through a single plate or sheet of metal, from the fire or hot gas on one side to the fresh air on the other.

It is evident, therefore, that, in order to operate effectively, the construction of a hot-air furnace must conform to the laws which govern the absorption of heat from hot gases, and also to those which control the emission of heat from hot surfaces. Many of the varieties of hot-air furnaces now upon the market are built with little regard to these laws, and consequently have a very low degree of efficiency.

Many forms of furnaces seem to have been designed upon the theory that the air can be heated sufficiently by bringing it within the range of the radiant heat, which is emitted from the fire-pot and other hot parts, and that the actual area of the heating surfaces presented to the air is a matter of minor importance. Nothing could be further from the truth. The radiated heat is useless unless it is intercepted and absorbed by some surface which will impart it to the air by contact and conduction. Hence, the efficiency of a furnace depends in a great measure upon the area of the hot surfaces over which the air is passed.

227. Extent Of Heating Surfaces

Extent Of Heating Surfaces. The actual rate of transmission of heat to air is much slower than the transmission from steam to metal, or from metal to water; consequently, the proportion of heating surface to the amount of fuel burned should be much greater in a hot-air furnace than in a steam boiler. The proportion of heating surface to grate area in steam-heating boilers ranges between 20 and 45 to 1, averaging about 35 to 1. The ratio in the hot-air furnaces now in general use is nearly the same, but should be much larger. In order to obtain the best results the ratio should average about 50 to 1.

The heating surfaces in a hot-air furnace should be sufficiently extensive to cool the hot gases of combustion, before they pass into the chimney, to a point not more than 100° above the temperature of the heated air which is discharged into the warm-air pipes. Any greater excess of temperature is unnecessary and wasteful.

228. Arrangement Of Heating Surfaces

Arrangement Of Heating Surfaces. In general features the construction of a hot-air furnace should approximate that of a tubular or water-tube boiler. The hot gases upon the inside, and also the fresh air on the outside, should be divided into thin sheets or small streams, and should be conducted through tubular channels or narrow passageways, having large areas of hot surface in proportion to the volume of the current. The hot gases should impinge upon the heat-absorbing surfaces as nearly as possible at right angles, and the movement of the gases should be in a direction opposite to that of the air.

On account of the slowness of the transmission of heat, to or from air and gases, it is necessary that the channels through which they pass should have considerable length, so that while the currents move with proper velocity, plenty of time will be afforded for transferring the heat. A momentary exposure of the air to a red-hot surface is far less effective than a prolonged exposure to surfaces having only a moderate temperature. Time is an important element in heating air.

229. Figs. 83 and 84 show a furnace which is designed for use in large buildings, such as shops, schools, churches, etc. In principle it resembles a tubular boiler, the hot gases being conveyed through the air chambers within tubes of comparatively small diameter. Fig. 83 is a vertical section parallel with the front, and Fig. 84 is a horizontal section on the line XX. The firebox is located in the middle of the structure and is surrounded by the air-heating chambers. The products of combustion pass to the rear, over the bridge wall d, and flow to the right and left into the combustion chambers /, /'. From these they pass through the upper sets of tubes a, a' into the front chambers n, n' and return through the lower tubes b, b' to flues at the rear, which conduct them to the chimney c. The air enters at the bottom of the casing, flows upwards between the tubes, and passes off at the top through the large central flue.

Fig. 83.

The tubes are secured to the tube-sheets in such a manner as to permit any tube to be readily removed by means of a common wrench when burned or clogged.

Fig. 84.

There are so many hot-air furnaces in the market that no attempt will be made to describe them in detail. Circulars and descriptions can easily be obtained upon application to manufacturers or their agents.