Forced Draft Fans

Forced draft fans, also referred to as pressure blowers or FD fans, focus the pressure and volume at the outlet of the fan to push air through a system, creating positive air pressure.

Applications calling for positive pressure include burners or combustion air, cooling, and air drying processes, fumes and gas elimination, tempered glass, and other conditioning processes. Accommodating higher process inlet temperatures, industrial applications use forced draft fans to push ambient air through burners to preheat air for boiler system efficiency.

Primary Industries for Forced Draft Fans

Forced draft fan applications are found across industries. However, because they are often used to feed combustion and high-temperature processes like dryers, burners, and boilers, you can be sure you’ll need forced draft fans in any or all of these industries:

Agricultural Processing
Automotive OEM
Building Materials
Chemical Processing
Food Processing
Glass Manufacturing
Metals & Mining
Oil & Gas

Paint & Surface Coatings
Pharmaceuticals
Power Plants
Printing, Packaging & Labeling
Pulp & Paper
Rubber & Plastics
Semiconductor & Electronics
Wastewater Treatment

Selection & Specification of Forced Draft Fans

Forced draft fans accommodate high process inlet temperatures, often working to push ambient air through burners to preheat air for boiler system efficiency. They can require less structural support and may offer a longer mechanical life than many induced draft fans because the cleaner ambient air is not as hard on the fan as induced draft applications.

Selection & Specification of Forced Draft Fans

Custom High-Pressure Blowers (HP#S)
High-Pressure Radial Shrouded (HPSM / HPSL)
Backward Curved (BCHS, BCLS, BCMS)

Backward Inclined (BIHS)
High-Pressure Radial Open (HPRM / HPRL)

Fan Arrangements

Fan Arrangements indicate how the motors mount to and drive the fans. Forced draft fans typically call for direct-drive, Arrangement 4 fans with variable frequency drives. Larger systems may require direct-coupled configurations while some users prefer belt-drive for speed flexibility. These are the arrangement options:

Arrangement 1 – Belt Drive with an overhung impeller. The fan and motor mount separately to a unitary base.

Arrangement 4 – Direct Drive with an overhung impeller. A low-cost arrangement, the fan wheel mounts directly on the motor shaft.

Arrangement 8 - Direct Drive with an overhung impeller. The fan shaft is directly coupled to the motor shaft with a flexible coupling.

Accessories & Design

“Pushing” ambient air involves less wear-and-tear on the fan than dirty and hot airstreams. The location of forced draft fans frequently allows for accessibility of upper bearings and the fan blades for maintenance and protects the fan and v-belt assembly from the hot-air stream. It also requires less structural support and can offer a longer mechanical life than many induced draft fans.

While ambient air may not be as hard on the fan compared to more extreme conditions, fan/system imbalance and excessive vibration can still be a problem. It’s critical that fan vibration levels are monitored and addressed if they exceed recommended levels.

Here are just some of the accessories and features you may see in forced draft fan applications:

Windbox Package that forms a large pressurized cavity that feeds combustion air into individual or multiple burners and balances the airflow through the burner registers
Flex Connectors to accommodate shifting during rapid temperature change
Spring Vibration Isolators to protect the fan from excessive vibration
Inlet Silencer for noise control on the fan inlet
Inlet Vane Damper, Variable Frequency Drive (VFD), Outlet Damper, or a combination for volume and pressure control
Inlet Filter or Screen to keep the airstream clean and avoid damaging the dampers
Inlet Box if space is tight for the ductwork
Purge Shaft Seal helps prevent hot air from getting to the bearings through the shaft hole opening
Vibration and temperature sensors to monitor bearing and fan health and prevent unplanned outages
Split Housing for easier maintenance access to the fan wheel and shaft
Proper installation and maintenance to ensure safe startup and operations

Case Studies

Talk with Our Application Engineers

If you feel overwhelmed by all this information, don’t despair! We have a team of application engineers who are happy to talk with you and select the best fan for your application. You can call 715-365-3267, email [email protected], or send us your information, and one of our engineers will connect with you.

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