The products of combustion being of a higher temperature transmit their heat more readily, and being smaller in quantity carry off less heat by way of the chimney: By suitable arrangements, these products may• be inter cepted and forced to impart a large amount of their heat to the air supply going to the com bustion chamber, thus returning a certain amount of heat to the system, which represents a saving in fuel equivalent to the amount of heat so returned.
Therefore, in considering any fuel the pri mary question does-not relate to the calorific value of the fuel so much as to what percentage of the heat energy represented by its calorific value is actually available for conversion into mechanical energy according to the manner of its utilization.
The latest experience shows, that in the case of a modern steam plant, burning coal by direct firing, the economy lies behind the stop valve, rather than in reducing the consumption of steam.
According to the best authorities, the mechanical arrangements and conditions most favorable for the most economical utilization of solid fuel are as follows: the height of the chimney should not be less than 120 feet, nor more than 180 feet, giving a natural draught ranging from 0.50 to 0.83 inches of water, with an average temperature of 300° F. For Lan cashire boilers (full size) the chimney area should equal six square feet per boiler up to a total of four boilers and five square feet per boiler for every additional boiler. The area
of the chimney entrance should be 10 per cent in excess of this amount and the flue should enter the chimney with a rising slope of not less than 45 degrees. The area through the economizer should be 20 per cent greater than the flue area and the economizer walls should be at least 14 inches thick and exceptionally well built and the combined area of the boiler side flues should not be less than twice that of the main flue.
As the price of fuel is influenced to a great extent by the convenience with which it can be burned, the general tendency is to use a higher priced fuel even though it may have a lower heating value than a cheaper grade. This tendency can be readily overcome and a large reduction made in the fuel bill by the use of high-class mechanical stokers which will burn coal almost impossible to fire by hand.
General Descriptions of For vari ous reasons coal is the most important of all fuels and in many localities constitutes the only one available; but in some places wood, oil and gas are abundant and take the place of coal. In certain industrial and manufacturing plants, their waste products, such as straw, tanbark, sawdust and bagasse are used as fuel more eco nomically than coal, owing to their availability. For general purposes, however, coal is the chief fuel and the available heating value of all other kinds of fuel are invariably expressed in terms of that of coal.