- Steam Jet Draught Definition: Steam jet draught is defined as a system that uses pressurized steam to create draught in a boiler, eliminating the need for electric draught fans.
- Simple Construction: The steam jet draught system is simple to build and easy to maintain, which lowers maintenance costs.
- Forced Steam Jet Draught: This type feeds steam to the furnace entry, creating draught by dragging fresh air through the system.
- Natural Steam Jet Draught: In this type, a steam nozzle in the smoke box at the stack’s bottom creates draught by dragging flue gases.
- Advantages and Disadvantages: The system is economical and space-efficient but relies on steam production to create draught.
The boiler can use a small share of its own steam, blown through a nozzle, to move air and flue gas. The method is called steam jet draught. It needs no electric power for a fan. The steam still costs heat and water, so the plant pays in fuel rather than in fan power. Locomotives and some older boilers used this. Large modern plant uses fans.
A steam jet draught nozzle is a simple fitting with no rotating fan to service. High-speed steam entrains air or flue gas by momentum. Two layouts are used: forced steam-jet draught, with the nozzle on the air inlet, and natural (induced) steam-jet draught, with the nozzle in the smokebox or stack.
Forced Steam Jet Draught
In forced steam-jet draught, a share of boiler steam is fed through a pipe and nozzle at the furnace air inlet. The jet drags fresh air across the grate. The flue gas then follows the usual heating path through the economiser, air heater if fitted, and chimney.
Natural Steam Jet Draught
In the layout called natural steam-jet draught, a nozzle sits in the smokebox at the base of the stack. Steam leaving the nozzle entrains furnace flue gas and throws it up the chimney. Locomotive blastpipes work this way. The furnace then sits below atmosphere, so air is drawn through the grate.
The fittings are small and need little room. Draught from the exhaust steam starts only after the boiler is making steam. A separate blower jet is often used from cold until the exhaust blast can take over.
