BOILER ACCESSORIES: STEAM BOILER TRIALS The object of a boiler trial is to determine the quantity and quality of steam that the boiler will supply under given conditipns, the horse-power of the boiler, the amount of fuel it takes to make the required steam, and its efficiency.
The quantity of steam is taken as the amount of water evaporated, which, of course, is the total amount fed into the boiler during the test the water-level being the same at the beginning and the end.
The quality of the steam can be determined by some form of calorimeter already described; and the efficiency is the ratio of the heat units utilized in evaporating the water to the total heat supplied to the boiler. The heat utilized in evaporation can be found by multiplying the number of pounds of feed-water by the number of heat units required to change the water at the temperature of the feed into steam at gauge pressure. The heat units supplied can be determined by carefully weighing the fuel used during the test, and deducting the amount of ash and unburned fuel going through the grates, with proper allowance for moisture, multiplying the result by the total heat of combustion of the fuel. The heat of combustion can be obtained by calculation, or by means of a fuel calorimeter.
Under a short test the boiler must be in good working order and fired for some hours before the beginning of the test, so that the brickwork and chimney may be thoroughly heated. Shortly before the test is begun, the fire may be allowed to burn low; and by reducing the amount of steam taken from the boiler, the pressure can be kept constant. The fire may then be drawn, the grate cleaned, and a new fire quickly started, with wood and fresh coal. Toward the end of the test the fire may be allowed to burn low, and at the close may be drawn and quenched with water, the unburned fuel being allowed for. In along test of twenty-four hours or more, this is not necessary.
If the boiler is fed by a steam pump, the pump should be run b) steam taken from some other boiler, if convenient; if not, the amount of steam used by the pump must be determined and allowed for. If
the feed-water is supplied by an injector, it will take steam from the boiler itself. About 2 per cent of this steam is consumed in forcing the water into the boiler, the remainder going to heat the feed-water. During the boiler trial, observations of temperatures and pressures should be made at the same time, and at about 15-minute intervals. In order to obtain the result of the test, the following must be known: 1. Amount (in pounds) of coal burned, and number of pounds of ashes left; 2. Number of pounds of water pumped into boiler; 3. Temperature of feed-water when it enters boiler;.
4. Pressure of steam in boiler; 5. Quality of steam discharged from boiler—that is, the per cent of moisture in the steam.
The coal for the furnace can be conveniently weighed in barrels, and may be fired directly from these barrels or dumped on the fireroom floor. The barrels should be carefully weighed when full and empty, and the time recorded, so that there may be no possibility of counting one barrel twice or omitting any. The rate of combustion will be fairly uniform, and the calculations at the times of emptying the barrel will fairly indicate whether or not an error has been made. Any unburned coal should be weighed, and the amount subtracted.
The condition of the fire for a twenty-four-hour test should be the same at the beginning and the end. This condition is estimated by the eye; and unless great care is used, an appreciable error is likely to be made If the coal consumption is 15 to 20 lbs. per square foot of grate surface, an error of two inches in estimating the thickness of the fire may cause an error of as much as 2 per cent in the final results.
The wood used in starting the fire should be carefully weighed, and may be considered as equal to 4/10 of the same weight of coal. The clinker and ashes should be carefully collected and weighed, and a sample of the ashes examined to obtain the amount of unburned fuel.