So the blasters cause a Venturi Effect?
Jeff
The venturi effect is how injectors work. That's a narrowing of a passage under pressure in order to increase the pressure\speed of the traveling fluid. By reducing the size of the pipe the fluid (in this case water) travels in, it increases the pressure to such a degree that it overcomes the resistance of any pressure at the end of it's travel. Think of like putting your thumb on a garden hose. What was originally a a bubbling flow is now a directed high pressure stream.
This is more that it's creating an area of low pressure in the smokebox by forcing the steam out through the stack. That action carries air with it, thus lowering the pressure in the smokebox. This is what's called an induced draft because it's downstream from the fire. Factories with steam boilers would get this draft from having giant smokestacks, the pressure difference from the top of the stack and the bottom would create a natural induced draft. Locomotives create a great deal more draft relative to the size of it's smokestack because of this exhaust.
Since the only source of air to replace that SHOULD be from under the fire (primary air), the induced draft pulls the air through first the fire, then the boiler. Assuming no dead spots or holes in the firebed, this makes the fire burn hotter from the additional oxygen being pulled through it.
Jason can better explain the fluid dynamics that take place between the exhaust nozzles and the stack.
