The "rated horsepower" of a boiler, or the "builders' rating," is the number of square feet of heating surface in the boiler divided by a number. In the case of stationary boilers this number is 10 or 12, but 10 is very commonly taken as the amount of heating surface per horsepower. Assuming this value and assuming further that the boiler tested had 1,500 square feet of heating surface, its rated horsepower would be 1,500 10 = 150 boiler horsepower.
It is often desirable to know what per cent of the rated capacity is developed in a test. This is found by dividing the horsepower developed during the test by the builders' rating. In the case of the boiler tested, 109 horsepower was developed. The percentage of rated capacity developed, therefore, was 109 150 = 0.73, or 73 per cent.
HEATING SURFACE.
The heating surface of a boiler is the surface of metal exposed to the fire or hot gases on one side and to water on the other side.
Thus, the internal surface of the tubes of a fire-tube boiler is the heating surface of the tubes, but the outside surface of the tubes of a water-tube boiler is the heating surface of those tubes.
In addition to the tubes, all other surfaces which have hot gases on one side and water on the other must be taken into account. For instance, in a fire-tube boiler from one-half to two-thirds of the shell (depending on how the boiler is set) acts as heating surface.
In addition to this, the surface presented by both heads, below the water level, has to be computed. The heating surface of each head is equal to two-thirds its area minus the total area of the holes cut away to receive the tubes.
COST OF EVAPORATION.
The cost of evaporation is usually stated as the cost of fuel required to evaporate 1,000 pounds of water from and at 212 F. To find it, multiply the price of coal per ton by 1,000 and divide the result by the product of the equivalent evaporation per pound of coal and the number of pounds in a ton.
Suppose that the cost of the coal used in the foregoing test was $3.60 per ton of 2,000 pounds. The equivalent evaporation per pound of coal was 7.5 pounds. Therefore the cost of evaporating 1,000 pounds of water from 180 F. to steam at 100-pound gage, is--
$3.60 1,000 ------------- = $0.24, or 24 cents.
7.5 2,000
TABLE OF TEST RESULTS.
After the test has been made and properly worked up, as heretofore described, collect all the results of the test on one sheet, so that they can be kept in convenient form for reference and for comparison with later tests. A brief form of arranging the results is as follows:
1. Date of test May 20, 1918 2. Duration of test hours 10 3. Weight of coal used pounds 5,000 4. Weight of water fed and evaporated do. 35,000 5. Average steam pressure, gauge do. 100 6. Average feed-water temperature F. 180 7. Factor of evaporation 1.0727 8. Equivalent evaporation from and at 212 F. pounds 37,545
EFFICIENCY.
9. Efficiency of boiler and furnace per cent 54
CAPACITY.
10. Boiler horsepower developed 109 11. Builders' rated horsepower 150 12. Percentage of rated horsepower developed per cent 73
ECONOMIC RESULTS.
13. Actual evaporation per pound of coal pounds 7 14. Equivalent evaporation from and at 212 F.
per pound of coal as fired, pounds 7.5 15. Cost of coal per ton (2,000 pounds) $3.60 16. Cost of coal to evaporate 1,000 pounds from and at 212 F. $0.24
HOW TO USE THE TEST RESULTS.
The object of working up a test is to obtain a clear idea as to the efficiency of operation of the boiler or its operating cost.
Consequently, after the calculations have been made, they should be used as a basis for study with the idea of improving the boiler performance.
Take the matter of boiler efficiency, for example, as found from the test mentioned. Its value was 54 per cent. This is altogether too low and indicates wasteful operation. The efficiency of a hand-fired boiler ought not to be less than 65 per cent, and it can be increased to 70 per cent by careful management under good conditions.
The chart in figure 3 can be used to indicate the evaporation that should be obtained in order to reach a desired efficiency. Suppose, for example, that it is desired to know how much water per pound of coal must be evaporated to produce a boiler efficiency of 65 per cent with coal having a heating value of 13,500 B. t. u. per pound.
Locate 13,500 at the bottom of the chart, follow the vertical line until it meets the diagonal marked 65 per cent, and then from this point follow the horizontal line to the left-hand edge, where the figure 9 is found. This means that the equivalent evaporation from and by 212 F. per pound of coal must be 9 pounds of water. If the steam pressure is 100 pounds gauge, and the feed-water temperature is 180 F. the factor of evaporation is 1.0727, then the actual evaporation must be 9 1.0727 = 8.36 pounds per pound of coal. In other words, to increase the efficiency from 54 per cent to 65 per cent under the same conditions of pressure and feed-water temperature, it would be necessary to increase the actual evaporation from 7 pounds to 8.36 pounds. This would mean practically 20 per cent more steam from the same weight of coal used.
[Illustration: _Heating Value of Coal, in B. t. u. Per Pound_
FIG. 3.]
How to do this will require some study and experimenting on the part of the fireman or engineer. The three most common reasons for low-boiler efficiency are (1) excess air, (2) dirty heating surfaces, and (3) loss of coal through the grates. _The first of these items is the most important of the three._ In most cases the greatest preventable waste of coal in a boiler plant is directly due to excess air. Excess air simply means the amount of air which gets into the furnace and boiler which is not needed for completing the combustion of the coal. Very often twice as much air is admitted to the boiler setting as is required. This extra or excess air is heated and carries heat out through the chimney instead of heating the water in the boiler to make steam. There are two ways in which this excess air gets into the furnace and boiler setting.
First, by a combination of bad regulation of drafts and firing. The chances are your uptake damper is too wide open. Try closing it a little. Then, there may be holes in the fire. Keep these covered.
The second way excess air occurs is by leakage through the boiler setting, through cracks in the brickwork, leaks around the frames and edges of cleaning doors, and holes around the blow-off pipes.
There are also other places where such air can leak in.
Take a torch or candle and go over the entire surface of your boiler setting--front, back, sides, and top. Where the flame of the torch is drawn inward there is an air leak. Plaster up all air leaks and repair the brickwork around door frames where necessary.
You should go over your boiler for air leaks once a month.
In regard to best methods of firing soft coal, see Technical Paper No. 80 of the Bureau of Mines, which may be obtained from your State Fuel Administrator.
Dirty heating surfaces cause low efficiency because they prevent the heat in the hot gases from getting through into the water.
Therefore, keep the shell and tubes free of soot on one side and scale on the other. Soot may be removed by the daily use of blowers, scrapers, and cleaners. The problem of scale and pure feed water is a big one and should be taken up with proper authorities on the subject.
There are many things that may be done to increase the efficiency of the boiler and to save coal. For convenience a number of these points are grouped in the following list:
WHAT TO DO. WHY.
1. Close up all leaks in the boiler To prevent waste of heat due to setting. excess air admitted.
2. Keep shell and tubes free from To allow the heat to pass easily soot and scale. into the water.
3. Use grates suited to the fuel To prevent loss of unburnt coal to be burned. through air spaces.
4. Fire often, and little at a To obtain uniform conditions and time. better combustion.
5. Cover all thin spots and keep To prevent burning holes in bed fire bed level. and admitting excess air.
6. Do not allow clinkers to form Because they reduce the effective on side or bridge walls. area of the grate.
7. Keep the ash pit free from ashes To prevent warping and burning out and hot clinkers. of the grates.
8. Do not stir the fire except when Because stirring causes clinker necessary. and is likely to waste coal.
9. Use damper and not ash-pit doors Because less excess air is to control draft. admitted by so doing.
10. See that steam pipes and valves Because steam leaks waste heat are tight. and therefore coal.
11. Keep blow-off valves tight. Because leaks of hot water waste coal.
12. Cover steam pipes and the tops To prevent radiation and loss of of boilers. heat.
Make a boiler test under the conditions of operation as they now exist in your plant. Then make all possible improvements as suggested in this bulletin, make another test afterwards and note the increase in the equivalent evaporation per pound of coal used.
Remember that the _firing line_ in the boiler room can be just as patriotic and helpful as the _firing line_ at the front.