FIRE BOAT AND ICE BREAKER "W. S. GRATTAN" OWNERS: CITY OF BUFFALO. BABCOCK & WILCOX BOILERS, 900 INDICATED HORSE-POWER the other hand it is more than likely to be very far from correct, and, to eliminate any errors of this kind, Mr. Barrus recommends a so-called "calibration" for dry steam. This, again, involves the assumption (which is open to some doubt) that steam, when in a quiescent state, drops all its moisture and becomes dry. No other practical method, however, has been proposed, and this is, therefore, the only method used at the present time. Some engineers, however, refuse to make any calibration, but, instead, make an assumed allowance for error. To make the calibration, close the boiler stop valve, which must be on the steam pipe beyond the calorimeter connection. Keep the steam pressure exactly the same as the average pressure during the test, for at least fifteen minutes, taking readings from the two thermometers during the last five minutes. The upper thermometer should read precisely the same as during the test, and the lower thermometer should show a higher temperature; this reading of the lower thermometer is the calibration reading for dry steam, which we will call T. Calculation of results, allowing for radiation by calibration method: -47 (Tt) in which x = percentage of moisture; T-calibration reading of lower thermometer; t-test reading of lower thermometer; L= latent heat of steam at boiler pressure. The method of taking a sample of steam from the main is of the greatest importance, and more erroneous results are due to improper connections than to any other cause. Use only a plain, open-ended nipple projecting far enough into the steam pipe to avoid collecting any condensation that may be on the sides of the pipe. Take care that no pockets exist in the steam main near the calorimeter, in which condensation can collect and run down into sampling nipple. Remember you are ascertaining the amount of moisture in the steam and not measuring the condensation on the walls of the steam piping. Make connections as short as possible. As mentioned above, there is a limit in the range of the throttling calorimeter which varies from 2.88 per cent. at 50 pounds pressure to 7.17 per cent. at 250 pounds. When this limit is reached a small separator may be interposed between the steam main and the calorimeter, which will take out the excess of moisture. By weighing the drip from the separator and ascertaining its percentage of the steam flowing through, and adding this to the percentage of moisture then shown by the throttling calorimeter, the total moisture in the steam may be ascertained. It is seldom, however, in a well-designed boiler, that any but a throttling calorimeter becomes necessary. ECONOMY DUE TO THE HEATING OF FEED WATER T HE importance of heating feed water before delivering it to a boiler can best be realized by considering exactly what takes place during the generation of steam. As explained on page 80, the total heat in steam consists partly of sensible heat, which marks the boiling point of the water, and partly of latent heat, which converts the water into steam. Therefore, in generating steam in a boiler, the water must first be heated to the boiling point and then enough heat added to evaporate it at the required pressure. PERCENTAGE OF FUEL SAVED BY HEATING FEED WATER The rate at which water absorbs heat varies slightly as its density decreases, but for rough calculations it can be assumed that the number of degrees Fahrenheit which a pound of water is heated, represents the number of British thermal units it has absorbed. Suppose, therefore, that a boiler is making steam at 180 pounds gauge pressure and is being fed with water at 60 degrees Fahrenheit. By reference to the steam tables, we find that the boiling point at 180 pounds gauge pressure is about 380 degrees Fahrenheit, and the latent heat equals about 845 heat units. When the water goes into the boiler, therefore, it has first to be heated from 60 degrees to the boiling point, which requires approximately (380–60) 320 heat units. This, with the latent heat afterwards added to convert it |