An

that no air can pass into rooms without being warmed. automatic air-valve should be used to remove the air from the sections of the heater.

If the sections are of proper form, one connection will be sufficient for steam; but in nearly every case two connections, one for the supply and one for the discharge, will be required for water circulation.

70. Proportions of Parts of Radiators.-There is great difference regarding the relative volume of radiators of different make as compared with the surface; but the practice is quite uniform as regards the sizes of supply-pipes for either steam or hot water. Because of the high efficiency of a radiating surface formed of one-inch horizontal pipe, it has been argued that this should form a standard for relation of contents to surface. It is seen, however, by consulting the tests given in Chapter IV, that inch-pipe vertical radiators are not more efficient than cast-iron radiators with larger volume; so that it is doubtful if the relative ratio of volume to surface is of importance.

It is of importance that the steam or water should circulate through the radiators with the least possible friction, and that in the case of steam-radiators the base should be of such a form as to perfectly drain; otherwise the water which remains in will be certain to cause the disagreeable noise and pounding known as water-hammer.

The following table gives the standards which are almost universally adopted by the different makers for the size of inlet and outlet to the direct radiators; those for indirects are to be taken one size larger:

CHAPTER VII.

STEAM-HEATING BOILERS AND HOT-WATER HEATERS.

71. General Properties of Steam-Explanation of Steamtables. Steam has certain definite properties which always pertain to it and distinguish it from the vapor of other liquids than water.

Steam, at any given pressure above a vacuum, possesses a definite temperature. The atmospheric pressure is different at different localities and for different conditions of the weather, thus causing slight changes in temperature of the boiling-point. The pressure which is read by any steam-gauge is that in excess of the atmosphere; the pressure which is given in the steam-tables is that which is reckoned from a perfect vacuum, and is usually called absolute; hence, in order to use the steamtable which is given in the back of the book, the pressure as determined by a steam-gauge reading must be increased by the atmospheric pressure. The atmospheric pressure is given accurately by a barometer, but it will be sufficiently accurate, for most cases, to consider it as 14.7 pounds. To use the table add this quantity to the gauge-reading and the result will be the absolute pressure. For approximate purposes the atmospheric pressure may be considered as 15 pounds. The steam-tables referred to give, in the first column, the pressure above a vacuum; in the second column, the temperature Fahrenheit; in the third, the heat, expressed in heat-units, required to raise one pound of water from zero Fahrenheit to the required temperature. If the specific heat of water were unity at all temperatures, the heat contained in one pound of water would be numerically the same as the temperature. The difference is not great in any case.

The fourth column gives the value in heat-units of the latent heat of evaporation for each pound of steam. This quan

tity expresses the amount of heat which is stored, without change of temperature or pressure, during the physical change of condition from water to steam; and it has been termed latent because it cannot be measured by a thermometer (sce Art. 13, page 15). It will be noted that this quantity is relatively large as compared with the sensible heat. It is of importance, since it expresses the amount of heat which is contained in one pound of steam in excess of that in one pound of water at the same temperature.

The fifth column gives the total heat contained in one. pound of steam; this is the sum of the sensible and latent heat.

The sixth column gives the weight in pounds of one cubic foot of steam for various pressures. In many instances steamtables are arranged so as to give the heat in one pound of steam above 32° Fahr., the freezing-point of water, instead of above zero.

It should be noted that the temperature of steam corresponding to different pressures, as given in column (2), is also the boiling point of water corresponding to the same pressure.

As the temperature and absolute pressure of steam always bear definite relation to each other, it is quite evident that a steam-table could be arranged giving the properties of steam from measurements of temperature. This is generally not so convenient as the present arrangement. If temperatures are known, the corresponding pressure can be determined by inspection and interpolation in the present table.

72. General Requisites of Steam-boilers.-The steamboiler is a closed vessel, which must possess sufficient strength to withstand the pressure to which it may be subjected in use; but it may have almost any form, and may be constructed of various materials.

It is used in connection with a furnace, from which the heat required for evaporation is obtained by combustion of fuel. The heat is received on the surface of the boiler, and passes. by conduction through the metallic walls to the water or The surface which receives this heat is called heating surface, and is partly situated so as to receive the direct or radiant heat, and partly located so as to receive the convected. or indirect heat from the gases only. The heating surface in

steam.

most modern boilers is made relatively great, as compared with the cubic contents, by the use of tubes containing water or heated gases, or by subdividing the boiler so as to make the surface large with respect to the cubic contents and weight. The steam generated rises in the shape of bubbles through the water in the lower part of the boiler, and is liberated from the surface of the water at the water-line.

The power of the boiler depends upon the amount and orm of heating surface, upon its capacity for holding water and steam, and upon the extent of fire-grate surface. Its * economy depends upon the relative proportions of these, and the character and amount of fuel burned. Its ability to produce dry steam depends upon the circulation of its liquid contents, and also upon the extent of surface at the water-line.

For safety, the boiler must be provided with safety-valve, pressure and water gauges. For convenience automatic damper-regulators, water-feeding apparatus, etc., are desirable.

73. Boiler Horse-power.-As a boiler performs no actual work, but simply provides steam for such purposes, a boiler horse-power is entirely an arbitrary quantity, and may be transformed into a lesser or greater amount of work, as the character of the engine which uses the steam varies.

The standard established by the Committee of Judges at the Centennial Exhibition in 1876 as a boiler horse-power has been universally adopted, and would, no doubt, in absence of other stipulations, constitute a legal standard of capacity. This committee defined a boiler horse-power as the evaporation of 30 pounds of water from feed-water at 100° Fahr. into. steam at 70 pounds pressure; this is equivalent to the evaporation of 34.5 pounds of water from a temperature of 212° Fahr. into steam at atmospheric pressure.* Engines require from 12 to 40 pounds of steam per horse-power per hour, depending upon the grade or class to which they belong; hence the steam required to perform one horse-power of. work in an engine bears no definite relation to a boiler horsepower.

The condition of evaporating from water at 212° into steam at the same temperature will be referred to hereafter as evaporation, without other qualification.

Since the evaporation of one pound of water from and at 212° Fahr. requires 966 heat-units, one boiler horse-power is equivalent to 33,327 heat units.

For heating purposes a more convenient standard of power is the square foot of radiating surface. Each square foot of direct steam-radiating surface gives off 270 to 330 heat-units per hour when the difference of temperature is 150 degrees (see Art. 51), which is that usually existing in low-pressure steamheating. About two thirds as much is given off by one square foot of hot-water radiating surface. As the evaporation of one pound of water requires 966 heat-units, there is needed about one third of a pound of steam for each square foot of steamradiating surface per hour, hence one boiler horse-power will be sufficient to supply somewhat more than 100 square feet of direct radiating surface; that is, we can consider the boiler horse-power as equivalent to 100 square feet of direct steam radiation, with sufficient allowance to meet ordinary losses.

74. Relative Proportions of Heating to Grate Surface.— The relative amount of grate surface and heating surface required in a steam-boiler depends, to a large extent, upon the nature and amount of coal burned per unit of time. That part of the heating surface which is close to the fire and receives directly the radiant heat is much more effective than that which is heated by contact with hot gases only; but it will be found that considerable indirect heating surface will in every case be required, in order to prevent excessive waste of heat in the chimney. Power-boilers have been rated for a long time not on their actual capacity, but on the amount of heating surface; and this would seem to be a fair standard of rating for heatingboilers. It is the general practice to consider 11.5 square feet of heating surface in water-tube boilers or 15 square feet in plain tubular boilers as equivalent to one horse-power.

The actual power of the boiler depends more upon the method and management of the fires than upon the size; and either of the above classes of boilers can be made to develop under favorable circumstances from two to three times the capacity for which they are rated.

A rating of 15 sq. ft. of heating surface to one horse-power requires an evaporation of 2.3 lbs. of water per square foot of