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RULES FOR PROPORTIONING FURNACE HEATING
Rules for Grate Surface.-There are various empirical rules in common use for determining the amount of grate surface a furnace should have; some of the best ones are here given. Rule 1.-Divide the number of cubic feet representing the total volume of air to be heated per hour by 100. The quotient will be the required grate surface, in square inches.
Rule 1.-The sum of the glass surface and equivalent glass surface, in square feet, equals the required grate surface, in square inches.
Rule.III.-Divide the number of B. T. U. required per hour to heat the building to 70° during the coldest weather by 180 for furnaces in ordinary dwellings, or by 270 if a fireman or janitor is constantly employed tending the furnace. The quotient is the grate surface, in square inches.
These empirical rules should give good results in all ordinary-size furnaces that have a sufficient amount of heating surface properly arranged. In small furnaces, the grate surface may be a little in excess of that found by the above rules.
In calculating the number of B. T. U. lost per hour, it is good practice to figure on changing the air in the building four times per hour.
Furnace Proportions.-Some furnaces are fairly well proportioned, while others are very defective. In some of them the heating surface is altogether too small for the amount of grate surface. The following table has been compiled after a careful examination of a large number of furnaces; it shows the relative average proportions that should exist between the most important parts in order to obtain satisfactory results in heating.
The proportions shown in this table are based on a combustion of about 3 lb. of coal per sq. ft. of grate surface per hr., and a furnace efficiency of about 70 per cent. This rate of combustion may appear low, but it is an ordinary rate for an ordinary fire. When the rate of combustion is much higher
than 3 lb., the fire requires attention too often. The heating surface given in the table is larger than is found in most furnaces, but it is not too large for good results.
Furnace Efficiency.-The efficiency of a furnace is an indication of its ability to impart the heat of the burning coal to the air passing over the heated surfaces. The efficiency of a given furnace is a quantity varying largely with existing conditions, such as the kind of coal used, the chimney draft, and the quality of the attendance. It is found by dividing the number of B. T. U. per pound of coal usefully applied by the number of B. T. U. a pound of the coal used is capable of emitting. Thus, if a certain coal is capable of emitting 14,000 B. T. U. per lb., and only 7,000 B. T. U. are absorbed by the air as it passes over the heated surfaces of the furnace, the efficiency of the furnace is 7,000 ÷ 14,000 .5 50 per cent.
To measure the amount of heat transmitted from a furnace to the rooms it is necessary to measure the volume of air passing through the cold-air box and to record its temperature. Then, the mean temperature of the hot air and its volume per hour at each register is measured. Next is calculated the number of B. T. U. required to raise the different volumes of warm air delivered per hour at each register from the temperature of the cold air as it enters the
Glass surface and equivalent, sq. ft.
REQUIRED IN FURNACE
Volume of air to be heated per hr., cu. ft.
Space to be warmed, cu. ft.
Furnace heating surface, sq. ft.
cold-air box to the temperature found by actual measurement at each register face. The B. T. U. thus separately found are added and the sum represents the total amount of heat given off by the furnace to the air per hour.
SIZE OF HOT-AIR PIPES AND STACKS.
Size of Leaders.-The size of the leaders is influenced by their location, and their area should be made larger than that of the stacks they supply, on account of the extra friction encountered by the air in traveling in a horizontal direction.
In ordinary dwellings, the requirements of ventilation are not usually considered, it being assumed that the amount of air required for heating purposes is also sufficient for ventilation.
There are various empirical rules for determining the sizes of hot-air leader pipes, but no single rule gives satisfactory results in all cases. The following rule simplifies calculation, saves time, and gives satisfactory results for ordinary work. It is known as the I. C. S. rule for leader pipes, and may be remembered by the numerals 1-10-100.
Rule. Add together the area of the exposed glass surface, in square feet, the area of the exposed wall surface, in square feet, divided by 10, and the volume of the room, in cubic feet, divided by 100; the sum will be the area of pipe required, in square inches, for the first floor.
For the second-floor leaders, multiply the sum found by this rule by .8, and for the third floor, by .6. For north or west expos
ure increase the leader-pipe area 10 per cent.
A rule frequently used for rooms having an excess of exposed glass and wall surface bases the size of the leader pipe on the cooling surface only, and is as follows:
Rule. For rooms on the first floor, add together the total glass surface and one-fourth the area of the exposed wall, in square feet; the sum is the proper area of the pipe, in square inches.
For second-story rooms, multiply by .75 for a south and east exposure: for third-story rooms, multiply by .5 for a south and east exposure, and by .75 for a north and west exposure.
In a rule used by many furnacemen, the cooling surfaces are converted into an equivalent of the cubic contents of the room to be heated. 1 sq. ft. of cold floor surface is considered to equal 1 cu. ft.; 1 sq. ft. of outside wall surface is taken to equal 3 cu. ft.; 1 sq. ft. of outside door surface is considered as equal to 6 cu. ft.; and 1 sq. ft. of outside glass surface is considered equivalent to 12 cu. ft. The rule is as follows:
Rule. Find the cubic contents equivalent to the cooling surfaces and add them to the cubic contents of the room, in cubic feet; divide the sum by 40. The quotient will be the sectional area of the leader pipe, in square inches. Add 10 per cent. for a
north and west exposure.
Some furnacemen determine the area of the hot-air pipe, in square inches, by dividing the cubic contents of the room, in cubic feet, by a constant taken from the following table:
In the rule given below, the cubic contents and different cooling surfaces are reduced to an equivalent glass surface (abbreviated to E. G. S.), the following value being assigned the different quantities:
12 cu. ft. of contents = 1 sq. ft. of glass surface;
12 sq. ft. of cold floor 4 sq. ft. of exposed wall
= 1 sq. ft. of glass surface;
1 sq. ft. of glass surface;
2 sq. ft. of outside door surface 1 sq. ft. of glass surface.
Rule. To find the area of leader pipe, in square inches, multiply the sum of the actual and equivalent glass surface of the room by .3. For a north or west exposure add 10 per cent. to the area.
If the different rules for finding the size of hot-air leader pipes here given be applied to the same room, their results may agree very closely, or there may be an appreciable divergence, depending on the conditions of the case. case of a large divergence, it may be well to take an average. Size of Stacks.-For the first floor, the stack should have the same area as the leader pipe. A stack leading to the second floor may have .8 of the area of the leader pipe, and a stack leading to the third floor .6 to .7 the area of the leader pipe. Stacks should have their sectional area increased 10 per cent. for each offset.
Sizes of Registers.-In calculating the size of register necessary for a given stack or pipe, only two-thirds the area corresponding to the size given by manufacturers in their catalogue list should be regarded as effective, on account of the obstruction offered by the grille-work, and by the body and valves. AREA OF RECTANGULAR REGISTERS.
For instance, a 10-in. pipe, which has a sectional area of about 79 sq. in., requires at least a 10" X 12" register. A 10" X 14" size, however, is better, as it offers less resistance to the passage of the air. When the velocity of the air-current issuing from the register is liable to be unpleasantly high, it