Open fireplaces which were used at one time extensively are very wasteful, as little more than the direct radiant heat from the fire is absorbed in warming. They are also subject to all the wastes which pertain to stoves, and their probable efficiency cannot be considered as over 15 or 20 per cent. They are, however, valuable adjuncts of a system of ventilation, since large quantities of air are drawn from the room and discharged into the chimney. In the use, of a stove called a fireplace heater, the heated gases from an open fire pass through a drum or radiating surface in the room above, and the heat which otherwise would be discharged from the chimney and wasted is partly utilized in heating.

148. General Directions for Operating a Furnace.-The general directions for operating a furnace so far as regards the care of the fire are the same as those which have been previously given for the operation of steam-heating furnaces, page 169; there are, however, no steam-gauges or safety appliances needed. In regulating the temperature of the house the drafts of the furnace should be operated rather than the valves of registers leading to various rooms. In some instances if the circulation is strong in certain directions and weak in others so that certain rooms cannot be heated, it may be a good plan to shut all registers except the one to the room where heat is required until circulation is established, after which circulation will usually continue without further attention. In the opera

tion of a furnace great care should be taken that the metal never becomes red hot or even cherry-red. If it will not warm the building without being excessively hot, the furnace is too small, or else has too little radiating surface in proportion to the fire-pot. The water-pan should be kept filled with water. Thermostats arranged to open or close the drafts when desired are in use in many systems of furnace heating with success.

For protection of the furnace during summer months some makers recommend that the fire-pot be filled with lime. For burning soft coal, furnaces of special construction only should be employed.

NOTE.-Rules for Furnace Heating:

First. To find area of grate in square inches: Divide total window surface plus total exposed wall surface in square feet by 200.

Second. To find area of flue for any room in square inches: Divide window surface plus wall surface in square feet by 1.2 for first floor, by 1.5 for second floor, by 1.8 for third floor.

CHAPTER XIII.

FORCED-BLAST SYSTEMS OF HEATING AND VENTILATING.

149. General Remarks.-In the systems of hot-air heating which have been described the circulation of air is caused by expansion due to heating, which is a feeble force and is likely to be overcome by adverse wind currents, by badly proportioned pipes, or by friction; by employing a fan or blower of some character for moving the air the circulation will be rendered positive and so strong as to be unaffected by these

causes.

This system can be employed where power is available, and in many cases will be found to present an economical and satisfactory system of heating, comparing well with any that has been devised, especially when the amount of ventilation provided is considered. The cost of heating a large quantity of air is, however, in every case one of considerable amount, so that it is quite probable that in expense of operation no system of indirect heating, whether by furnace or steam-pipes, can compare with that of direct hot-water or steam radiation. The systems of forced-blast heating are in almost every case employed in connection with steam-heated surfaces, but in some instances the system has been applied successfully with furnace heated surfaces.*

150. Form of Steam-heated Surface. -The heating surface is generally built of inch pipe, set vertically into a square cast-iron base, connected at top with return-bends, although the box coil, Fig. 94, page 109, or any form of indirect radiating surface could be used. The fan or blower is placed either

* The Metal Worker, May 25, 1895, gives an interesting example showing the successful use of a blower and furnace for heating a church.

so as to draw the air by suction over the heated surface and then deliver by pressure into the rooms, or it is placed so as to force the air by pressure over the heating surface and thence into the conduits leading to the various rooms. The heating surface is usually surrounded with metallic walls forming a chamber through which the air is discharged. Fig. 213 shows the arrangement often adopted, in which a pressure fan is directly connected to an engine, and arranged to take air from

FIG. 213. BLOWER CONNECTED TO ENGINE.

the atmosphere and force it into the chamber in which the heating surface is placed.

151. Ducts or Flues-Registers.*--The dimensions of the ducts or flues leading from the heater should be such that the required amount of air may be delivered with a low pressure and velocity, so as to avoid excessive resistances due to friction. The velocity which will be produced by various pressures in

*General formulæ for the motion of air in long pipes is to be found in Weisbach's Mechanics, and in article Hydrodynamics in Encyclopædia Britannica, by Prof. W. C. Unwin. The formula given by Weisbach is elaborated in an article by Carl S. Fogh in Engineering Record, Feb. 16, 1895, and a graphical diagram given for practical application. The uncertainty which relates to the application of these elaborate formulæ is well shown by the fact that a factor of safety of 4 is used by Mr. Fogh, and serves in the writer's opinion to render such estimates as crude as those obtained by the approximate formulæ given here. The article is of great value, however, to those desiring to study the theory of motion.

excess of that of the atmosphere is given in table on page 42, from which it is seen that a pressure sufficient to balance inch of water (0.29 ounce per square inch) will produce a velocity of 30 feet per second in a pipe 100 feet long and I foot in diameter; this is generally considered to be the maximum velocity which should be pemitted in any of the pipes or passages. In proportioning apparatus in this system of heating it is generally required that sufficient air shall be brought in to change the cubic contents of the room four times per hour. By consulting the table on page 53, it will be seen that for this condition, and without allowance for friction, it will require a flue with 5.7 square inches of area for each 1000 cubic feet of space in the room. By adding two inches to the diameter obtained as above, a fair allowance for friction will be made.

The pipes are usually made of galvanized iron or bright tin and should have tight joints and be protected from loss of heat by some good covering (see page 197). Flues of brick or masonry cause more friction than those of galvanized iron, and if used should generally be about two inches larger in diameter than provided for by this table. As branch pipes for various apartments are taken off, the main pipe can be reduced in size; this should never be done abruptly, but only by the use of tapering tubes, the angle of whose sides measured from the line of the main pipe should rarely be greater than 15 degrees. The fan can be located in a chamber which is connected with the external air, as in Fig. 214, or it may be placed in a tube or passageway leading from the heating surface to the outside.

The area of the cold-air duct or passageway leading to the fan should be as great as possible in order to keep the velocity of entering air low; if the area of cross-section is equal to the sum of the areas of all the ducts leading from the heating surface, the velocity will probably be about three quarters of that in the hot-air pipes, and may draw in considerable dust and dirt from outside. The flues which convey air to the rooms should discharge near the upper part of the room substantially as described on page 49 and shown in Fig. 21. The friction in small pipes is greater than in large ones, being relatively proportional to the circumference or perimeter; hence the sum

of the areas of the branch pipes should be considerably greater than that of the main.*

The table on opposite page gives the number of small pipes which provide an area equivalent to that of one large pipe of similar cross-section; in case no table is at hand the same results may be obtained by dividing the larger diameter by the smaller one and taking the square root of the fifth power of the quotient.

The following table gives the actual amount discharged with constant resistance, and with pressure equal to one half inch of water column in round pipes, as computed from Unwin's formulæ, page 41:

VELOCITY AND QUANTITY OF AIR DELIVERED IN PIPES OF DIFFERENT DIAMETERS, EACH 100 FEET LONG, WITH AN AIR-PRESSURE EQUAL TO INCH OF WATER COLUMN.

*The velocity of flow of air is given in formulæ on page 41; the amount discharged is equal to the area of the pipe multiplied by the velocity, and will be equal in every case to the square root of the fifth power of a constant multiplied by the diameter of the pipe. If we denote diameter of larger pipe by D, of smaller pipe by d, and the number of smaller pipes required to make one of area equivalent to that of larger by n,

To find diameter of round pipe, d, which shall be equivalent in carrying capacity to a rectangular pipe with dimensions a and b, we would have