5.11 Steam and Air Jets

The energy derived from a jet of expanding air or steam may be utilized in producing a draft. Devices to accomplish this consist of a high-pressure jet of steam or air discharging into a larger pipe. The aspirating effect produced causes an additional amount of gases to be delivered.

Jets are seldom used as a primary means of producing draft in stationary practice. However, they have been used for many years to introduce over-fire air, create turbulence, and thereby promote more complete combustion.

The secret of smokeless combustion is in burning the hydrocarbons before they form soot particles. To this end, jets must supply the required over-fire air, distribute it advantageously, and at the same time create the required turbulence. As a result of research and operating experience, it is possible to design and successfully apply jets to specific furnace installations.

While over-fire air jets are not a cure-all for faulty design and operation, they are capable of performing a very important function in boiler operation. Early installations often were makeshift arrangements, and the results were disappointing. Consideration must be given both to the quantity of air to be supplied and to the distance it is to be carried into the furnace. The jets must be so located and spaced that their effects will overlap to prevent lanes of unburned gases from escaping. The air must be introduced close to the fuel bed to prevent the formation of smoke, because smoke is very difficult to...