« ΠροηγούμενηΣυνέχεια »
BOILER HEATING SURFACE.
The actual heating surface of a boiler includes all parts that have water on one side of the metal and gaseous products of combustion having a higher temperature than the water, on the other. Parts that are covered by steam instead of water on one side have little value in increasing the steam-generating capacity of the boiler.
The portion of the heating surface close to the fire, under the direct influence of the radiant heat, is much more effective in generating steam than that portion called the indirect heating surface, which is heated only by contact with the hot gases. The relatively great amount of indirect heating surface found in the majority of house-heating boilers is provided to utilize as much of the available heat as possible.
It is customary for manufacturers to rate heating boilers on the basis that 1 sq. ft. of heating surface is approximately equivalent to 10 sq. ft. of direct radiation, so that a boiler having 200 sq. ft. of heating surface would be rated to supply 2,000 sq. ft. of direct radiation.
The heating surface of return-tubular boilers may be calculated quite closely by the following:
Rule. Multiply two-thirds of the circumference of the shell, in inches, by its length, in inches; multiply the number of tubes by the length of the tube, in inches, and by its circumference; add to the sum of these products two-thirds of the area, in square inches, of the two tube sheets; from this sum subtract twice the area of all the tubes and divide the remainder by 144; the quotient represents the number of square feet of heating surface of the boiler.
RATIO OF HEATING SURFACE TO GRATE AREA. In practice, the ratio between the heating surface and grate area varies with the type of boiler and the rate of combustion. The following are average values:
RATIO OF HEATING SURFACE TO RADIATION.
In ordinary forms of house-heating boilers, from 1,800 to 2,400 B. T. U. are absorbed per sq. ft. of heating surface per hr., and since 1 sq. ft. of direct steam radiating surface requires from 250 to 330 B. T. U., say an approximate average of 300 B. T. U. per hr., it is evident that 1 sq. ft. of boilerheating surface will generate enough steam to supply from 6 to 10 ft. of radiating surface. In other words, a vertical sectional boiler having 180 sq. ft. of heating surface will supply sufficient steam for 1,080 to 1,800 sq. ft. of direct radiation, including all losses due to condensation in the transmission of the steam through the supply piping.
The area of grate surface required in any given case depends on the type of boiler employed, the amount of water to be evaporated, the nature and amount of coal to be burned, and the rate of combustion, which varies from 3 to 20 or more lb. of coal per sq. ft. of grate per hr. The required grate surface may be approximately determined by the following:
Rule. To find the grate surface, in square feet, divide the weight of steam, in pounds required per hour, by the product
obtained by multiplying the number of pounds of coal burned, per square foot of grate per hour, by the number of pounds of water evaporated per pound of coal.
In this rule no account has been taken of the difference in the number of B. T. U. required to evaporate water from different feedwater temperatures into steam at different pressures. The rule is therefore only approximate, but close enough for practical work. The average evaporation per pound of coal for different types of boilers is given in the following table:
AVERAGE EVAPORATION PER POUND OF COAL.
RATIO OF GRATE SURFACE TO RADIATION.
The requisite area of grate necessary to supply a given amount of direct steam radiation may be found. approximately, by dividing the total amount of radiation surface by a factor varying between 100 and 160, selecting a factor in accordance with the character and probable management of the boiler. Ordinarily, heating boilers receive unskilful handling, a factor of 100 most nearly meeting the requirements, whereas with a skilled fireman in attendance a factor
of 160 may be used with satisfactory results. For example, a heating boiler rated to supply 1,600 sq. ft. of direct radiation, including ordinary losses, should have a grate area of 1600 16 sq. ft. if operated under ordinary conditions, or
10 sq. ft. if the boiler is to be carefully handled.
BOILER HORSEPOWER RATINGS.
Strictly speaking, there is no such thing as the horsepower of a boiler. The phrase was originally intended to mean that a boiler having a certain stated horsepower would furnish all the steam that was required to develop that amount of power in a given engine. This meant that if a certain boiler furnished steam for a 30-H. P. engine, it would be called a 30-H. P. boiler, and if the same boiler furnished steam for a 50-H. P. engine, it would be called a 50-H. P. boiler. It is therefore apparent that this rating had no particular significance.
Boilers are often rated by their ability to evaporate water from and at a temperature of 2120 into dry steam, 34.5 lb. per hr. being reckoned as 1 H. P. This method, however, is merely an indirect way of arriving at the number of B. T. U. transmitted through the boiler. The rating of boilers by the number of B. T. U. actually transmitted per hour is one that permits of a direct comparison of the relative heating capacities of all kinds of steam and hot-water boilers.
In order to have a definite standard of comparison, the American Society of Mechanical Engineers has accepted the report of a committee recommending that 33,330 B. T. U. per hr. transmitted from the fuel and absorbed by the water shall constitute 1 boiler H. P. The horsepower of a boiler thus expressed is the measure of its performance under certain given conditions, and does not indicate its capabilities when employed under other conditions. For instance, a boiler employed in heating water in an open system, with a slow fire, would develop a much smaller horsepower than
it would if employed in making high-pressure steam, with forced blast and intense fire. The standard boiler horsepower may be determined by the following:
Rule.-Subtract the temperature of the feedwater, in degrees Fahrenheit, from the total heat of 1 lb. of steam above 32° at the pressure of the actual evaporation. Add 32 to the remainder
and multiply this sum by the weight of water evaporated per hour into dry steam. Divide the product by 33,350.
In rating boilers according to their heating surface, the following factors are commonly used:
No hard and fast rules can be employed for determining the heating capacity of boilers, or for ascertaining the sizes best adapted for different jobs. The following table is deduced from practical tests and observations by Professor Carpenter, of Cornell University, and is considered to be within the limits of safety. Two cases are taken: (a) when the rate of coal consumption is 10 lb., and (b) when it is 8 lb. per sq. ft. of grate surface per hr. The latter is preferable for hot-water heating.
The usual dimensions of ordinary horizontal returntubular power boilers, which often are used for steam-heating purposes, are given in the table headed "Specifications for Horizontal Tubular Boilers."