A firetube boiler has tubes that carry hot gases. The tubes are surrounded by water that is heated. The hot gases flowing through the tubes carry the by-products of combustion, primarily soot and scale, which are deposited on the inside surfaces of the tubes. The greater the amount of deposit, the lower the heat transfer to the water will be and the harder the boiler must work. Because soot has five times the insulating value of asbestos, the heat transfer loss in a dirty boiler rises dramatically as the layer of soot builds up.
Scale builds up over a longer period of time and also contributes to boiler inefficiency.
In a watertube boiler, water runs through tubes that are surrounded by the heating source. Because the water is heated to very high temperatures in this type of boiler, the solids in the water tend to accumulate quickly on the tubes inside surfaces, causing the same heat transfer problem and related cost increases as occur in firetube boilers. Both operate 260 10-hour days per year. Plant A operates at 65 percent efficiency. Plant B operates at 80 percent efficiency.