Posted by Tom Stanton on November 25, 2001 at 13:42:22:
In Reply to: draft posted by afdmello on November 22, 2001 at 05:00:10:
Draft is the difference in pressure between the atmosphere and the inside of the boiler. Natural draft is what you get when you use a fireplace or wood stove. It is the difference between the pressure in the fireplace and the pressure at the top of the chimney. Most boilers today do not use natural draft, although I have seen some old gas fired saturated steam boilers that do. Forced draft is air above atmospheric pressure being forced into the furnace, usually measured in inches of water column. FD is needed in muti-pass boilers and in boilers with superheaters, reheaters, economizers, air heaters etc. Most boilers at least use this method today. Induced draft is is when the combustion gases are removed from the boiler setting with the assistance of a fan. An ID fan is usually located after the air heater and prior to the chimney. Most large boilers today use both FD and ID fans. The ID fan will creat a slight negative pressure in the boiler setting to prevent the combustion gases from leaking into the boiler room if there is a leak in the boiler casing. This condition is known as balanced draft.
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