The boiler produces steam. If we utilize, a small portion of the pressurized steam for producing draught in a boiler system then we call the draught as steam jet draught. As the steam, causes the draught, generated in the boiler itself, there will be no need for extra electric power to run draught fans. Steam jet draught is a straight forward form of draught system in a boiler. No demand for additional electricity for running the draught fans hence reducing the cost of the system.
Construction of the system is simple and easy to maintain. Thus, maintenance cost is also low. In steam jet draught system a small portion of generated steam exhausts through a nozzle and the kinetic energy of the jet of high-velocity steam drags the air or flue gases in the boiler system. We can categorize the stream jet draught in two types. One is natural stream jet draught, and another is force steam jet draught.
Forced Stream Jet Draught
In forced stream jet draught, we feed a portion of the stream produced in the boiler to the entry point of the furnace through diffusion pipe. Due to the kinetic energy of steam, there will be a draught at the entry point because of which fresh air gets dragged to the grate and then to the economiser then to preheater then ultimately to the chimney.
Natural Steam Jet Draught
In natural steam Jet draught, we fit the steam nozzle in a smoke box attached to the bottom portion of the stack. The steam forcefully enters in the smoke box due to which flue gases created in the furnace get dragged in the smoke box because of the resultant draught created the kinetic energy of the steam jet. This process of creating draught in this way is called natural jet steam draught.
Steam jet draught is simple, economical and occupies very little space or occupies no space. But draught is only possible after steam gets produced which is the main disadvantage of steam Jet draught.