The fuel for a coal-fired furnace or boiler may be fed either automatically or by hand. Both methods have certain advantages and disadvantages. For example, automatic (stoker) firing is initially more expensive because it requires the purchase and installation of a suitable mechanical stoker to feed the coal to the furnace or boiler. As a result, stoker firing is a common practice in larger buildings (e.g., stores, hotels) where the initial high cost of the equipment can be more easily absorbed into the total cost of the structure. Despite the relative high cost of stokers, there are some designed for use in single-family residences. These will be considered in detail at a later point in this chapter.
Because there is no need to invest in special and expensive coal- handling equipment, hand-firing the coal has been the traditional method used for firing house heating furnaces and boilers. Although hand-firing coal is less expensive than stoker-firing for these smaller installations, the following objections to the hand- firing method should be noted:
• The frequent opening of the furnace or boiler doors allows a large excess of air to enter and chill the flame. The combustion efficiency of the flame therefore tends to fluctuate.
• The dumping of a lot of fuel at each firing results in a smoke period until normal combustion conditions are restored.
• Hand-firing coal is by its nature an intermittent firing method.
The flame often reaches a low and inefficient level or is extinguished before new fuel is added.
Coal-Firing Draft Requirements
The amount of draft required for proper combustion is an important consideration, and it depends on a number of different considerations, including the following:
• Grate area
• Fuel size and type
• Fuel bed thickness
• Boiler pass resistance
The degree of resistance offered by the boiler passes to the flow of the gases is an important consideration in determining the required amount of draft. These gases must exist at a speed sufficient to pre- vent them from backing up into the combustion chamber and rob- bing the fire of necessary oxygen.
The total area of the grate, the type of coal burned (e.g., bitu- minous, semibituminous), the size of the coal, and the thickness of the coal bed all affect the amount of draft required for proper combustion.
Insufficient draft usually results in the accumulation of excess ashes in the ashpit. Moreover, it necessitates additional attention to the fire including more frequent cleaning. These and other aspects of improper firing contribute to fuel waste and higher operating costs.