Steam:
Its Generation and Use

TESTING STEAM BOILERS* The object of testing a steam boiler is to de- termine the quantity and quality of steam it will supply continuously and regularly, under speci- fied conditions; the amount of fuel required to produce that amount of steam, and sometimes sun- dry other facts and values. In order to ascertain these things by observation it is necessary to exer- cise great care and skill, and employ the most per- fect apparatus, or errors will creep in sufficient to vitiate the test and render it of no value, if not actually misleading. This is most apparent in testing the quality of the steam by a “barrel cal- orimeter,” as at the Centennial Exposition, where an error of % lb. in either of two weighings of a mass of some 400 lbs. made a difference of 3 per cent, in the final result. 5. Pressures of the steam, of barometer, and of draft in chimney. 6. Weights of feed-water, of fuel, and of ashes. Water meters are not reliable as an accurate measure of feed water. 7. Time of starting and of stopping test, taking care that the observed conditions are the same at each as far as possible. 8. The quality of the steam, whether “wet,” “dry,” or “superheated.” From these data-all the results can be figured, giving the economy and capacity of the boiler, and the sufficiency or insufficiency of the condi- tions, for obtaining the best results. The amount of water evaporated per pound of coal is universally conceded to be the proper measure of the efficiency of a boiler, but in order The principal points to be ascertained and noted in a boiler test are : 1. The type and dimensions of the boiler, in- cluding the area of heating surface, steam and water space, area of water surface, and draft area through or between tubes or flues. 2. The kind and size of furnace ; area of grate with proportion of air spaces therein, height and size of chimney, length and area of flues. 3. Kind and quality of fuel and amount of ash and water therein. The latter is a more impor- tant item than is generally understood, as it not only adds to the weight without aciding to the value of the fuel, but the heat taken to evapo- rate, and send the steam up chimney in a highly superheated condition, adds to the unobserved waste. 4. Temperatures, of external air, of fire-room, of chimney gases, of fuel, water and of steam. * This subject will be found very fully treated in the re- port of a committee to the American Society of Mechanical Engineers, and the discussions on the same. Transactions A. S. M. E., Vol. VI, pp. 256-351. to compare one boiler with another, each should have equally good coal, be fed with water at the same temperature and furnish steam at the same pressure. As this is impractical in making tests, a standard has been accepted to which all tests should be brought for comparison. This is called the “equivalent evaporation from and at 212°” per pound of combustible; that is, what the evap- oration would have been if the coal had been without ash, the feed-water at boiling point and the steam delivered at atmospheric pressure. It may be determined by the following formulæ : Let W —■ the observed evaporation per lb. of combustible. “ t — the observed temperature of feed. “ T — the temperature of steam at observed pressure. “ H — the total heat of steam at the observed pressure. “ W = equivalent evaporation from and at 2120. W/ = w + °-3 (T —212) + (2T2 — £)\ 966 / or, . . . W' W — 32 “ Z 966 The value of T and H may be found by refer - ence to “steam table ” on another page, (49-) 80