in fuel is the utilization of all the available exhaust steam for heating the feed water before it enters the boiler. Of course if the main engine is a condensing engine, the exhaust from that source is not directly available, except by interposing a closed heater between the cylinder and the condenser, or by using the water of condensation for feeding the boilers. This can be done with safety, provided a surface condenser is used, but with a jet condenser or an open heater in which the exhaust mingles with the water, it is advisable to have an oil separator to prevent the oil from getting into the boilers. Exhaust heaters are of two kinds, open and closed. In the open heater the exhaust steam mingles directly with the water and a portion of it is condensed. A well-designed open exhaust heater will raise the temperature of the water to very nearly the boiling point, 212° F. These heaters should be set so that the water will flow by gravity from them to the feed pump In the closed type of exhaust heaters, the exhaust steam and the water are kept separate. In some styles the steam passes through tubes, which are surrounded by water, while in others the water fills the tubes, which are in turn surrounded by the steam. either case the water in the closed heater is under the full boiler pressure while the feed pump is in operation, because the heater is between the pump and the boiler, while with the open heater the pump is between the heater and the boiler. In The saving effected by heating the feed water with exhaust steam can be easily ascertained by the use of a thermometer, BARAGWANATH STEAM JACKET FEED WATER HEATER. a steam table, and a simple arithmetical calculation. First, find by thermometer the temperature of the water before entering the heater; find its temperature as it leaves the heater. Next ascertain by the steam table the number of heat units above 32° F. in the water at each of the two temperatures. Subtract the less from the greater, and the remainder will be the number of heat units added to the water by the heater. Next find by the table the number of heat INTERIOR VIEW OF OPEN HEATER. units above 32° F. in the steam at the pressure ordinarily carried in the boiler, and subtract from this the number of heat units in the water before it enters the heater. The result will be the number of heat units that would be required to convert the water into steam of the required pressure, provided no heater were used. Then to find the percentage of saving effected by the heater, multiply the number of heat units added to the water by the heater by 100, and divide by the number of heat units required to convert the unheated water into steam, from the initial temperature at which it enters the heater. Example. Assume the boiler to be carrying 100 lbs. gauge pressure. Suppose the temper ature of the water before entering the heater is 60° F., and that after leaving the heater its temperature is 202° F., what is the percentage of saving due to the heater? The solution of the problem is as follows: Boiler pressure by gauge = 100 lbs. Initial temperature of feed water = 60° F. Heated temperature of feed water = 202° F. From the steam table (see Chapter IV., Table 5) it is found that = 170.7. Heat units in water at 202° F. = 170.7 28.01 = Heat units in steam at 100 lbs. gauge pressure 1185.0. = = Heat units to be added to water at 60° F. to make steam of 100 lbs. gauge pressure = 1185.0 - 28.01 1156.99. Percentage of saving effected by the use of the 142.69 × 100 heater = 1156.99 = 12.33 per cent. Suppose the coal consumed under this boiler amounts to two tons per day at a cost of $3.00 per ton, or a fuel cost of $6.00 per day. Then the saving in dollars and cents due to the heater in the foregoing example would be 12.33 per cent of $6.00, or $0.7398 (74 cents) per day. Heaters, especially those of the closed type, should have capacity sufficient to supply the boiler for fifteenor twenty minutes. There would then be a body of water continually in the heater in direct contact with the heating surface, and as it passes slowly through it will receive much more heat than if rushed through a heater that is too small. All heaters and feed pipes should be well protected by some good insulating covering to prevent loss of heat by radiation. In some cases the exhaust steam, or a portion of it at least, can be used to advantage in an exhaust injector. |