Using Forced and Induced Draft Fans for Efficient Furnace Operation
Furnace operation can be improved by understanding how to use forced and induced draft fans for maximum efficiency. Forced draft fans are used to supply combustion air and are critical for successful furnace operation. Induced draft fans are used to draw hot flue gas out of the furnace. Both types of fans must be properly selected, installed and maintained in order to ensure efficient furnace operation.
Understanding Forced Draft Fans
A forced draft fan is the key element in a forced-draft system. Fans are not usually 100% spared and failure of a fan can result in furnace shutdown in a single fan installation or in reduction of unit throughput if more than one fan is used to supply combustion air.
For natural draft burners, a single fan should be sufficient if drop-out doors are provided in the ductwork to allow for natural draft operation in case of fan failure. For forced draft burners, two 50% fans are normally provided; one fan only enables the furnace to operate at about 85% of its design capacity.
The fan flow rate must include burner requirements (hot air and cold air operation) plus 5% for potential ductwork leakage. Static head requirements include the pressure loss through the preheater, ducts, and burners. Typical fan heads are 6 to 8 inch water with natural draft burner and 15 to 20 inch water with forced draft burners. The recommended fan margins are 15% on air flow rate and 15% on fan head.
Utilizing Induced Draft Fans
Induced draft fans are used to draw hot flue gas out of the furnace. In such a case, a single fan is sufficient if the fan fails since the furnace stack damper opens and the hot flue gas does not pass through the air preheater, returning the furnace to cold air operation mode.
The flow rate is based on the flue gas rate and the air preheater outlet temperature. The static head requirements include the draft required at the preheater inlet; plus the pressure drop through the preheater, cold flue gas duct, and stack; minus the stack effect. Typically, the fan head is in the range of 5 to 7 inch water. The recommended fan margins are 25% on flue gas flow rate and 50% on fan head.
Important Considerations for Air and Flue Gas Ducts
The recommended arrangement of the air ducts includes a main supply duct, a distribution header and feed ducts to a limited number of burners. Isolation dampers should be provided in the feed ducts to allow for burner maintenance with the preheater in operation.
The designed air velocity in the supply duct is about 50 feet per second. The distribution header velocity should be limited so that the velocity head pressure is equal to less than about 5% of the pressure drop through the feed ducts and burners. In any case, the design shall provide a bypass duct around the air preheater to allow for control of the flue gas temperature out of the air preheater and also for operation with preheater out of service.
The design velocity for all flue gas ducts is also about 50 feet per second. The ducts should be insulated to minimize heat loss. Generally, for operation with an air preheater, the stack must be lined for corrosion protection.
Conclusion
Using forced and induced draft fans for efficient furnace operation is a critical part of successful heating systems. Fans must be properly selected, installed and maintained in order to ensure efficient furnace operation. Air and flue gas ducts must also be properly arranged and insulated to minimize heat loss and prevent corrosion. Understanding and implementing these elements will ensure optimal furnace performance.