hour. The total radiation surface was 1057.5 square feet; hence the loss in B. T. U. per square foot per hour was 0.229 per degree difference of temperature.

This for a difference of temperature of 150° corresponds to 0.17 B. T. U. per degree difference per square foot per hour, an amount about 10 per cent of that which would have been given off from a naked pipe. (See page 66.)

The loss by condensation varied from 3 to 8 per cent, the loss of pressure and consequent ability to do work about 6 per cent. The total loss was not far from 10 per cent from both these causes; if this had been proportional to length, it would have been 13.5 per cent for a line 1000 feet in length.

The diagram shows variations in the observed quantities as they occurred from time to time. It is to be noted that as the demand for steam at the engine was large the moisture in the steam delivered was correspondingly reduced.

CHAPTER XII.

HEATING WITH HOT AIR.

139. General Principles.-The general laws which apply to hot-air heating have already been considered in the articles relating to Ventilation and to the Methods of Indirect Heating with Steam or Hot Water.* The method of heating with hot air, as usually practised, consists in first enclosing a suitable heater, termed a furnace, in a small chamber with brick or metallic walls, which is connected to the external air by a flue leading to its lower portion and to the various rooms to be heated by smaller flues leading from the upper part. In operation the cold air is drawn from the outside, is warmed by coming in contact with the heated surfaces of the furnace, and is discharged through the proper flues or pipes to the various rooms. The rapidity of circulation depends entirely upon the temperature to which the air is heated and the height of the flue through which it passes; the velocity will be in every case essentially as given in the table on page 45. In order that a system of circulation may be complete flues must be provided for the escape of the cooler air from the room to be heated, otherwise the circulation will be very uncertain and the heating quite unsatisfactory. Registers and flues for the escape of the air from the room are often neglected, although fully equal in importance to those leading to the furnace.

Regarding the relative merits of hot-air heating by furnace as described and of the various systems of steam or hot-water heating, little can be said in a general way, since so much depends on circumstances and local conditions. It is rarely that these systems come in direct competition. The force which

* See pages 52 and 211.

causes the circulation of the heated air is a comparatively feeble one and may be entirely overcome by a heavy wind; consequently it is generally found that the horizontal distance to which heated air will travel under all conditions is short; hence the system is in general not well adapted for large buildings. When properly erected and well proportioned, this system gives, in buildings of moderate size, very satisfactory results.

It may be said, however, that, in erecting a hot-air system of heating, competition has been in many cases so sharp as to induce cheap, rather than good, construction. Small furnaces have been used in which the temperature of the exterior shell had to be kept so high, in order to meet the demands for heat, that the heated air absorbed noxious gases from the furnace and entered the room in such condition as to impair, rather than to improve, the ventilation. Ventilationducts for removing the air from the rooms have often been neglected, and hence the results obtained have been far from satisfactory. Such faults are to be considered, however, as those of design and construction rather than as pertaining to the system itself.

In order that the hot-air system should be satisfactory in every respect, the furnace should be sufficiently large, and the ratio of heating surface to grate such that a large quantity of air may be heated a comparatively small amount rather than that a small quantity shall be heated a great amount. As air takes up heat very much more slowly than steam or water, it would seem that the relative ratio of heating surface to grate surface should be more than that commonly employed in steam-heating. By studying the proportions which have already been given for steam-heating boilers (page 125) it will be seen that the ratio of heating surface to grate surface for the steam-boiler varies between 20 and 45, averaging about 32. From a study of the results in catalogues of manufacturers of furnaces the ratio of air-heating surface to grate surface in hotair furnaces seems to vary from 20 to 50 as extremes. These proportions are essentially the same as used in steam-heating and are much too small for the best results in hot-air heating. It is quite evident that since air cannot be heated by radiation,

and is warmed only by the contact of its particles against the heated surface, that the exterior form of the furnace should be such as will induce a current of air to impinge in some portion of its course directly against the surface.

Regarding the economy of this or any other system of indirect heating, it is simply a question of perfect combustion and relative wastes of heat. If the fuel is perfectly burned and all the heat which is given off is usefully applied, the system is perfect. The waste of heat in any system of combustion is that due to loss in the ashes, to radiation, and to escape of hot gases into the chimney. If the furnace is properly encased and if the hot-air pipes are well covered, there is no reason why losses from imperfect combustion and from radiation should not be a minimum. The chimney loss depends largely upon the temperature of the surface of the heater: if this is high, the loss will be large. In general, it may be said that the larger the heating surface provided the lower may be its temperature, and the greater the economy. It should be noted, however, that this or any system of indirect heating requires the consumption of more fuel than when the heating surfaces are placed directly in the room, and for that reason the operating expense must be considerably greater than that of direct systems of hot-water and steam heating. (See page 202.)

Furnaces, or in fact heating-boilers of any kind, are uneconomical if operated with a deficient supply of air. In this case the product of combustion will contain carbon monoxide,* an extremely poisonous and inflammable gas, which is quite likely to take fire and burn, on coming in contact with air, at the base or top of the chimney.

140. General Form of a Furnace. The principles which apply in furnace construction are not essentially different from those already given in Chapter VII for steam and hot-water boilers. In the case of a hot-air furnace the fire and heated products of combustion are on one side of the shell and the air to be warmed on the other. In the case of steam or hotwater boilers the water and steam occupy the same relative

* See Article 24, page 26.

The

positions as the air in the case of the hot-air furnaces. types and forms of furnaces which are in use may be classified exactly the same as heating-boilers, Articles 77 and 82, as having plain or extended surface, and as being horizontal or vertical, tubular or sectional; it may be said that the forms which are in use are fully as numerous as those described for steam-heating and hot-water heating.

The material which is employed in construction is usually cast iron or steel, and there is a very great difference of opinion as to the relative merits of the two. It seems quite probable that cast iron, because of its rough surface, may be a better medium for giving off heat than wrought iron or steel, but it is quite certain that at a very high temperature, some carbon from the cast iron will unite with the oxygen from the air forming carbonic acid. When very hot it may be slightly permeable to the furnace gases. Such objections are, however, of little practical importance, since the temperature of a furnace never should, and never does if properly proportioned, exceed 300 or 400 degrees Fahr., and for this condition the difference in heating power of cast iron and steel is very slight. It is of great importance that the shell of the furnace be tight, so that smoke and the products of combustion cannot enter the air-passages.

Furnaces can be purchased with or without magazine feed, but the demand of late years is principally for those without the magazine, since it has not been proved to present any special advantages.

Furnaces are often set in a chamber surrounded with brick walls, as explained for steam-boilers, but they are more frequently set inside a metallic casing, this latter being termed at portable setting; this casing varies somewhat as constructed by different makers, but usually consists of two sheets of metal, the outer of galvanized iron, with intervening air-space empty or filled with asbestos. The casing is placed at such a distance from the furnace as to provide ample room for the passage of air.

Some form of dumping or shaking grate which can be readily and quickly cleaned is almost invariably employed. The draft-doors which admit air below the grate and checkdampers in the stovepipe are usually arranged so they can be