5.10 Mechanical Draft: Fans

There are essentially three methods of applying fans to boiler units:

1. Balanced draft. A forced-draft fan delivers air to the furnace and an induced-draft fan or a stack produces the draft to remove the gases from the unit. The furnace is maintained at 0.05 to 0.10 in of water (gauge) below atmospheric pressure.

2. Induced draft. A fan or stack is used to produce sufficient draft to cause the air to flow into the furnace and the products of combustion to be discharged to the atmosphere. The furnace is maintained at a pressure sufficiently below that of the atmosphere to induce the flow of combustion air. This type of system is not used on modern designs.

3. Pressurized furnaces. A forced-draft fan is used to deliver the air to the furnace and causes the products of combustion to flow through the unit and out the stack. The furnace is maintained at a pressure sufficiently above that of the atmosphere to discharge the products of combustion. The FD fan must be designed to handle both the air resistance and draft loss through the boiler.

Forced-draft (FD) fans handle cold (ambient temperature), clean air and provide the most economical source of energy to produce flow through boilers and their auxiliary systems. Induced-draft (ID) fans handle hot flue gas and thus a larger flue gas volume. Because of this higher temperature and greater volume of gases, they require higher electric power and are also subject to a...