12. Keep the Conditions Uniform.The boiler should be run continuously, without stopping for meal-times or for rise or fall of pressure of steam due to change of demand for steam. The draught, being adjusted to the rate of evaporation or combustion desired before the test is begun, should be retained constant during the test, by means of the damper.
If the boiler is not connected to the same steam pipe with other boilers, an extra outlet for steam with valve in same should be provided, so that in case the pressure should rise to that at which the safety-valve is set, it may be reduced to the desired point by opening the extra outlet, without checking the fires.
If the boiler is connected to a main steam pipe with other boilers, the safety-valve on the boiler being tested should be set a few pounds higher than those of the other boilers, so that in case of a rise in pressure the other boilers may blow off, and the pressure be reduced by closing their dampers, allowing the damper of the boiler being tested to remain open, and firing as usual.
All the conditions should be kept as nearly uniform as possible, such as force of draught, pressure of steam, and height of water. The time of cleaning the fires will depend upon the character of the fuel, the rapidity of combustion, and the kind of grates. When very good coal is used, and the combustion not too rapid, a ten-hour test may be run without any cleaning of the grates other than just before the beginning and just before the end of the test. But in case the grates have to be cleaned during the test, the intervals between one cleaning and another should be uniform.
13. Keeping the Records. The coal should be weighed and delivered to the fireman in equal portions, each sufficient for about one hour's run; and a fresh portion should not be delivered until the previous one has all been fired. The time required to consume each portion should be noted, the time being recorded at the instant of firing the first of each new portion. It is desirable that at the same time the amount of water fed into the boiler be accurately noted and recorded, including the height of the water in the boiler and the average pressure of steam and temperature of feed during the time. By thus recording the amount of water evaporated by successive portions of coal, the record of the test may be divided into several divisions, if desired, at the end of the test, to discover the degree of uniformity of combustion, evaporation, and economy at different stages of the test.
14. Priming Tests. In all tests in which accuracy of results is important, calorimeter tests should be made of the percentage of moisture in the steam, or of the degree of superheating. At least ten such tests should be made during the trial of the boiler, or so many as to reduce' the probable average error to less than one per cent; and the final records of the boiler test should be corrected according to the average results of the calorimeter tests.
On account of the difficulty of securing accuracy in these tests, the greatest care should be taken in the measurements of weights and temperatures. The thermometers should be accurate within a tenth
of a degree; and the scales on which the water is weighed, to within one-hundredth of a pound.
15. As each fresh portion of coal is taken from the coal-pocket, a representative shovelful should be selected from it and placed in a barrel or box, to be kept until the end of the trial, for analysis. The samples should then be thoroughly mixed and broken. This sample should be put in a pile, and carefully quartered. One quarter may then be put in another pile, and the process repeated until five or six pounds remain. One portion of this sample is to be used for the determination of the moisture and heating value; the other, for chemical analysis.
16. The ashes refuse should be weighed dry, and a sample frequently taken to show the amount of combustible material passing through the grate. To get a representative ash sample, the ash-pile should be quartered as required for the coal.
17. The quality of the fuel should be determined by heat test, by analysis, or by both.
18. The analysis of the flue gases is an especially valuable method of determining the relative value of different methods of firing or of different kinds of furnaces. Great care be taken to procure average samples, since the combustion of the gases may vary at different points in the flue; and as the combustion of flue gas is liable to vary from minute to minute, the sample of gas should be drawn through a considerable period of time.
19. It is desirable to have a uniform system of determining and recording the quantity of smoke produced. This is usually expressed in percentages, depending upon the judgment of the observer.
20. In tests for the purpose of scientific research in which the determination of all variables is desirable, certain observations should be made which in general are not necessary—such as the measurement of air-supply, the determination of its moisture, the determination of the heat loss by radiation, the infiltration of air through the setting, etc.—but as these determinations are rarely undertaken, no definite instructions are here given.
21. Two methods of defining and calculating the efficiency of the boiler are recommended. They are: (1) Efficiency of the boiler = (Heat absorbed per pound of combustible) / (Calorific value of one pound of combustible) (2) Efficiency of boiler and grate = (Heat absorbed per pound of coal) / (Calorific value of one pound of coal) The first of these is the one usually adopted.
22. An approximate statement of the distribution of the heating value of the coal among the several items of heat utilized, may be included in the report of a test when analyses of the fuel and chimney gases have been made.
23. Record of the Test. The data and results of the trial should be recorded in a systematic manner, according either to Table 1 (see Vol. XXI, Transactions of the American Society of Mechanical Engineers), or Table 2, taken from those "Transactions."