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After combustion has taken place the heat of the coal appears in the form of sensible heat in the gases leaving the furnace or combustion chamber. The important problem is to cool the gases as much as possible with a minimum of boiler heating surface. In order to accomplish this the heating surface should be kept clean inside and out. Too much emphasis cannot be put on this point. Combustion is more complete with considerable excess air, but this excess air passing through the furnace reduces the temperature of the gases approaching the boiler and the temperature of the escaping gases remains about the same so that a larger percentage of the developed heat is lost up the stack. This condition might be compared with a steam engine running with low initial pressure and exhausting against a high back pressure. The amount of air excess is regulated by the intensity of draft and condition of the bed of fuel. Few firemen have ever had the opportunity of learning what was the best thickness of fire or intensity of draft under the conditions existing in their boiler plant when burning a certain kind of coal. Many people think the stronger the draft the better, but there is opportunity to save thousands of dollars every year in many plants by merely reducing the draft or better regulation of it. The installation of a damper regulator is not always the remedy for they often cause more loss than occurred when hand regulated dampers were used.
The analysis of the flue gases is the best criterion for regulating the conditions of a furnace so as to obtain nearly complete conibustion with a minimum of air excess. The perfecting of automatic gas indicators and recorders will do very much toward increasing the boiler room efficiency.
No one kind of boilers or heat absorbing apparatus will give equal satisfaction in all plants. This depends upon location of plant, kind of water, uniformity of load, kind of coal, etc., and must be determined in each individual case.
It may seem unnecessary to investigate so thoroughly what would be the most economical fuel, how it can best be burned, and how the largest percentage of the heat can be converted into useful work, but the money saved by doing so, even in the smaller plants, amounts to a surprising sum in the course of a year. The manufacturer who is too busy enlarging his mill and increasing his output to give corresponding attention to his boiler room usually regrets the mistake when all his labor is standing idle for lack of power or the coal bill becomes a disproportional percentage of his cost of operation.
The PRESIDENT. This is certainly a very interesting paper; has any one any questions to ask? I do not hear any comments.
Before calling for the report of the Committee on Ballots, I will ask for the last paper on the programme, Heating Systems for Mills, by Mr. A. G. HOSmer,
HEATING SYSTEMS FOR MILLS.
A. G. HOSMER, Clinton, Mass.
There are in general use in manufacturing buildings, three distinct types of heating systems: first, the direct, where the heat is supplied by means of pipes containing steam installed in the rooms; second, the indirect or blower system, by introducing air previously heated; and third, the hot water or forced circulation system, where the water heated in an outside heater is forced through pipes arranged much the same as those for steam, and returned to be re-heated again, thereby making a continuous circuit.
It is the writer's purpose to enumerate some of the merits and demerits of the several systems, and consider their adaptibility under the varying conditions met with in mills and factories.
Before taking up the subject however, it is well perhaps to mention the fact that while the theory is true that air will absorb heat from any source, provided the supply is warmer than itself, in order that a cold room may be warmed to a comfortable. degree and kept so in heating weather, it is necessary that the temperature of the heat supply shall be much higher than the desired temperature in the room; unless the area of heating surfaces or volume of heated air (in case of an indirect system) is much larger than it is advisable to provide in the average building for use only half the year.
THE DIRECT SYSTEM, We will first consider a plant using non-condensing engines and heating with exhaust steam wholly or partially during running hours, and direct steam for the remainder. It is a common custom to place a light back pressure on the engines to force the exhaust through the pipes at the proper velocity to absorb all the available heat contained therein and discharge the resultant condensation at the end of the lines at quite a low temperature.
If the areas of the pipes are ample and they are not too long or complicated this method works well, but as it is often not the condition, the steam is condensed before it reaches the end of its journey, rendering it almost useless as a source of heat for a considerable part of the way. In this case we must either raise the back pressure or turn in direct steam to accomplish the desired end.
It should be borne in mind that the adding of one pound of back pressure to an engine cannot be made up by adding one pound to the initial pressure, as back pressure is the same as mean effective pressure resisting through the whole stroke, while initial pressure extends its full force only to the point of cut off.
It is quite clear that adding back pressure to an engine, especially if it is large and well loaded, should be avoided if possible. From the foregoing statements it would seem that the entire heat from exhaust steam cannot usually be utilized without running more or less back pressure on the engines and therefore cannot be forced or hurried to any extent unless disagreeable conditions are brought about in the engine room. In some instances the circulation of the exhaust is assisted by establishing a vacuum at the discharge end of system by means of a pump or fan, thus doing away with the objectionable back pressure on the engines. This is a decided advantage if the engines are large, but it necessitates the complication of piping, with the cost and running expense of some more or less elaborate system for the maintainance of the vacuum. As a rule, the method of heating by exhaust steam leaves a great deal to be desired and is not advisable unless it is necessary to use noncondensing engines.
Taking low pressure steam from the receiver of a compound engine is a common practice in some mills. If the quantity of steam taken is fairly constant and the engine is designed with that object in view, the results are quite satisfactory, although this idea is usually more popular with the designing than the operating engineer.
It is well to remember when designing any supply of heat for the purpose of warming the buildings of a manufacturing plant that exhaust or receiver steam is available only during running hours, and that the heating system's highest duty is usually required when the engines are shut down, consequently it is more important to have an efficient service during non-running hours, than at times when machinery friction and other causes contribute to the warmth of the work rooms.
With a direct system installed and designed for the use of live or boiler steam alone, the engines are left out of the proposition, and direct steam usually reduced in pressure is sent to the heating pipes, remaining there until condensed and finally discharged through a trap to the atmosphere or to a tank to be fed back to the boilers.
With the trap working properly, nearly the same pressure is kept upon the entire length of pipes and all water automatically removed. This renders every foot of pipe equally efficient with the heat nearly uniform and due to the pressure carried.
In a building or series of buildings closely grouped and with it possible to have the drips carried to some point somewhat lower than any of the heating pipes, and not too far from the boilers, the condensation ought never to be cooler than 200 de. grees, and can be pumped or trapped to them without much loss of heat.
It piping is large and well installed with separate 'traps for each considerable unit of circulation this will be found to be one of the most economical and satisfactory methods of heating. It can be forced or moderated within reasonable limits by raising or lowering the pressure at reducing valve, and with everything right will give return for practically all the heat expended.
The benefit of returning the condensation to the boilers is