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The losses shown in the diagram will be increased by circumstances approximately as follows:
Where the exposure is northerly, and the winds are strong. 35 per cent.
When the building is heated during the day and is allowed to cool off somewhat during the night, the exposure being moderate. 10 per cent.
Same, northerly exposure with high winds, 40 per cent. When the building is heated occasionally, for a day only, and is allowed to freeze for intervals of several days, such as churches and audience rooms. 50 per cent.
The temperature of cellars that are not warmed may be assumed for purposes of calculation to be 32°. Vestibules and corridors, frequently opened to the outer air and not heated, may be assumed to have a temperature of 20°.
In many instances the loss of heat from the room will be partly compensated for by the heat emitted from gas lights, electric lamps, etc., and by the heat given off from the persons occupying the room. The amount of heat from these sources is about as follows:
In lecture halls and large audience rooms, the amount of heat given off by the audience and the lights may equal or exceed that which is lost through the walls and windows. When this occurs, it becomes necessary to lower the temperature of the fresh air supply below the desired temperature of the room during the presence of the audience. Thus, the actual amount of heat required may vary from hour to hour, although the atmospheric temperature is stationary.
VOLUME AND TEMPERATURE OF HOT-AIR SUPPLY. The loss of heat per hour by conduction through windows, walls, etc. being given, and also the temperature of the hot-air supply and the desired temperature of the room, the required volume per hour may be computed by the following:
Rule. Multiply by 55 the amount of heat lost per hour by con duction and divide the product by the difference between the temperatures of the room and of the hot-air current. The quotient will be the volume of hot air, in cubic feet, required per hour.
The desired volume of the fresh-air supply, in cubic feet per hour, the amount of heat lost by conduction per hour, and the desired temperature of the rooms being given, the required temperature of the hot air entering the rooms may be computed by the following rule:
Rule.-Multiply by 55 the amount of heat lost per hour by conduction and divide the product by the volume of the air current. Add the quotient to the desired temperature of the room; the sum will be the required temperature of the hot-air supply.
KINDS OF FUELS.
Coal is divided into four classes, which are:
1. Anthracite, which contains from 92.31 to 100 per cent. of fixed carbon and from 0 to 7.69 per cent. of volatile hydrocarbons.
2. Semianthracite, which contains from 87.5 to 92.31 per cent. of fixed carbon and from 7.69 to 12.5 per cent. of volatile hydrocarbons.
3. Semibituminous coal, which contains from 75 to 87.5 per cent. of fixed carbon and from 12.5 to 25 per cent. of volatile hydrocarbons.
4. Bituminous coal, which contains from 0 to 75 per cent. of fixed carbon and from 25 to 100 per cent. of volatile hydrocarbons.
Anthracite is known to the trade by different names, according to the size into which the lumps are broken.
These names, and the generally accepted dimensions of the screens over and through which the lumps of coal will pass, are as follows:
Culm passes through-in. round mesh.
Rice passes over-in. mesh and through -in. square mesh.
Buckwheat passes over -in. mesh and through 1-in. square
Pea passes over 1-in. mesh and through 3-in. square mesh. Chestnut passes over 4-in. mesh and through 1-in. square
Stove passes over 18-in. mesh and through 2-in. square mesh.
Egg passes over 2-in. mesh and through 23-in. square mesh. Broken passes over 24-in. mesh and through 31-in. square mesh.
Steamboat passes over 31-in. mesh and out of screen.
Lump passes over bars set from 3 to 5 in. apart.
Semianthracite coal is broken into the same sizes as anthracite, the sizes having the same name.
Bituminous coal may be broadly divided into three general classes:
1. Caking Coal.-This name is given to coals that, when burned in the furnace, swell and fuse together, forming a spongy mass that may cover the whole surface of the grate. These coals are difficult to burn, since the fusing prevents the air passing freely through the bed of burning fuel; when caking coals are burned, the spongy mass must be frequently broken up with the slice bar, in order to admit the air needed for its combustion.
2. Free-Burning Coal.--This is often called non-caking coal from the fact that it has no tendency to fuse together when burned in a furnace.
Cannel Coal. This is a grade of bituminous coal that is very rich in hydrocarbons. The large percentage of volatile matter makes it valuable for gas making, but it is little used for the generation of steam, except near the places where it is mined.
Bituminous and semibituminous coals are known to the trade by the following names:
Lump coal, which includes all coal passing over screen bars 14 in. apart.
Nut coal, which passes over bars in. apart and through bars 14 in. apart.
Pea coal, which passes over bars in. apart and through bars in. apart.
Slack, which includes all coal passing through bars in. apart.
Lignite, according to the classification, comes under the general head of bituminous coal. Properly speaking, it occupies a position between peat and bituminous coal, being probably of a later origin than the latter. Exposure to the weather causes it to absorb moisture rapidly; it will then crumble quite readily. It is non-caking and yields but a moderate heat, and is, in this respect, inferior to even the poorer grades of bituminous coal.
Petroleum is occasionally used as a fuel, and, as such, possesses some advantages, among which are the ease of lighting and controlling the fire, the uniformity of combustion, and the economy in labor. Its disadvantages are: Danger of explosion, loss of fuel by evaporation, and high price in comparison with coal. The Standard Oil Company estimates that 173 gal. of petroleum is equal to 1 long ton (2,240 lb.) of coal, allowing for all savings incidental to its use.
Natural gas is abundant in parts of Ohio and Pennsylvania, and is there often used as a fuel for steam boilers, hotair furnaces, etc. On an average, 30,000 cu. ft. of natural gas is the equivalent of 1 ton of coal.
Waste gases from the furnaces of rolling mills and from blast furnaces are extensively used. Naturally, their use is limited to the places where they are produced.
The Babcock & Wilcox Co. states that on the average 1 lb. of good bituminous coal may be considered as the equivalent of 2 lb. of dry peat, 2 lb. of dry wood, 2 to 3 lb. of dry tan bark or sun-dried bagasse, 3 lb. of cotton stalks, 33 lb. of straw, 6 lb. of wet bagasse, and from 6 to 8 lb. of wet tan bark,
HEATING VALUE OF FUELS.
The heating value of a fuel is usually measured by the number of B. T. U. given out by the complete combustion of 1 lb. of the fuel. The average heating values, per pound of commonly used fuels, are:
The full heating value of a given fuel is not realized in practice. For house-heating work, between 8,000 and 9,000 B. T. U. per lb. of coal will be absorbed by the water at the ordinary combustion rate of from 4 to 8 lb. of coal per sq. ft. of grate surface per hr.
In power boilers, an average of 11,000 B. T. U. per lb. of coal is absorbed by the water. The reason for this difference in heat absorption is found in the different conditions of service. Heating boilers are operated under a dampened fire, in consequence of which there is an incomplete combustion of the coal during a large part of the time; power boilers, on the other hand, generate steam with a bright fire and very little incomplete combustion.
Assuming that in a heating boiler 8,000 heat units per lb. of coal, as ordinarily burned, will be absorbed by the water, and burning from 4 to 6 lb. of coal per sq. ft. of grate surface, from 32,000 to 48,000 B. T. U. per hr. will be absorbed per sq. ft. of grate surface. Since 1 sq. ft. of direct steam radiating surface requires about 300 B. T. U. per hr., 1 sq. ft. of grate is sufficient to insure a supply of steam for from 106 to 160 sq. ft. of radiating surface.
By increasing the combustion rate the same heating boiler will supply steam for an increased amount of radiation; but, to do so without making a proper increase in the amount of heating surface, and thus forcing the heater beyond its intended capacity, introduces a serious heat loss, due to the gases passing over the heating surfaces so fast that they give up only part of their available heat.