difference in temperature, in degrees F., to obtain the total heat emission per square foot of radiating surface per hour. Then, the number of square feet of radiating surface required may be found by dividing the total amount of heat, in B. T. U., required per hour by the emission from 1 sq. ft. of radiating surface. The number of lineal feet of 1" pipe required in the coil may be found by multiplying the number of square feet of radiating surface by 2.9.

To find the number of B. T. U. required to evaporate the moisture in the wet goods in a dry room at a temperature of 120°, the following rule may be used:

Rule.-Multiply the number of square feet of wet surface in the dry room by the number of ounces of moisture in 1 sq. ft. of the material to be dried, divide by 16 and multiply the quotient by 1,020, which is the number of B. T. U. required to evaporate 1 lb. of water at a temperature of 120°.

The total emission of heat per square foot of radiating surface per hour for each degree difference in temperature is given in the following table:

The velocity of air when natural draft only is used rarely reaches 8 ft. per sec., and 3, 4, or 5 ft. are fair averages of velocities under conditions of natural circulation.

The efficiency of laundry drying apparatus depends not only on the thoroughness with which the wet materials are spread out and exposed to the air-currents, but also on the equal distribution of air in the drying chamber.

NON-CONDUCTING PIPE COVERINGS.

The efficiencies of various non-conducting pipe coverings now on the market vary much less than is usually stated in advertising literature. It has been found, by experiment, that for each material there is a certain thickness that gives the best result, and that a further increase in thickness adds but little to the efficiency. Therefore, data based on a uniform 1 in. thickness of the non-conducting material are liable to be misleading.

The appended table gives, in round numbers, the approximate percentage of heat transmitted through various pipe coverings, taking the heat loss from an uncovered pipe as 100 per cent. Pipe coverings on the market, it should be noticed, vary considerably in wearing quality and efficiency.

The various kinds of pipe coverings on the market are usually finished with a jacket of canvas sewed on; this is then given two coats of paint having the desired color. Either iron bands or, if an especially neat appearance is desired, polished brass bands are usually placed over the jacket every 2 or 3 ft. apart.

Per Cent.

HOT-WATER HEATING,

CIRCULATION.

The circulation in a hot-water heating system is a movement of hot water from the boiler to the radiators, where it parts with some heat, and a consequent movement of colder water from the radiators to the boiler to become reheated. Without circulation, heat cannot be conveyed from the boiler to the radiators. The velocity of circulation depends chiefly on: (a) the difference between the mean density of the ascending current and that of the returning current; (b) the vertical height of circuit above the boiler; and (c) the resistance to flow due to friction, change in direction, etc. The theoretical velocity could easily be computed, but as the actual velocity bears no definite ratio to the theoretical, the latter is of little value to the practical man.

With a given fall of temperature, the force of the circulation through radiators, etc., depends chiefly on the height of the return column, being, in many cases, practically independent of the height of the supply column. Thus, in the accompanying illustration, the force of circulation through the radiator a will be about 3 times as great as through b, notwithstanding the fact that the supply columns c and g are of equal height, because the return ƒ is about 3 times as high as the return e. The temperature in the pipes c and d is supposed to be nearly the same; consequently, the column d simply balances an equal height of column c, and fails to supply any force for circulation. The force for circulation in this circuit, therefore, depends on the preponderance of the water in e over that contained in the riser below the level of the radiator at the line h h.

A simple circuit is one in which the water flows directly to a radiator through a single pipe without branches, returning directly to the boiler. Each circuit of the system is then entirely independent of the others.

In a compound circuit, the water circulates in comparatively large flow mains and return mains connected by small

radiator branches that provide a number of direct circuits between the flow and return mains.

In some systems of hot-water circulation, the flow and return mains are connected only by the radiator branches,

and there is no way of maintaining a flow of water through them when the radiators are shut off. This arrangement of mains is called an open circuit. When the hot-water main

connects with the return main independent of the radiator branches, the arrangement is called a closed circuit. As long as a good fire is maintained in the boiler, there will be an active circulation in the mains, and the water will be always at the maximum temperature, so that any or all of the radiators may be promptly supplied with hot water as soon as the valves are opened.

There is quite a difference of opinion regarding the true meaning of the terms "open" and "closed" circuits. The terms and the meanings thereof, as adopted by us, may seem contrary to local usage, but they coincide with the meanings of the same words as applied in the electrical professions. and therefore will help to prevent confusion or misunderstanding.

PIPING SYSTEMS.

One-Pipe, or Single-Main, System.-Fig. 1 shows what is commonly called the one-pipe system, but the name is a misnomer. While it is practicable to operate a steam-heating system with

2nd. Story.

FIG. 1

a single main, and with single connections to the radiators, it is impracticable to do so with hot-water systems, because hot-water radiators must have two connections. The nearest