Induced Draft Fans
Induced Draft Fan Applications
Induced draft fans (ID fans) often process air in harsh conditions including high temperatures, acidic airstreams, and other extreme exhaust gases. They focus the volume and pressure at the inlet of the fan to pull air and other materials or elements through the system. Extreme conditions can require special attention due to imbalance, excessive vibration, temperature, maintenance, and operating costs. However, they can also operate at higher efficiencies relative to forced draft fans.
The induced draft fan is situated above the bundle, creating negative air pressure to draw the air through to the next step of the process, which could be another application such as a dryer or an air treatment system for pollution control. Induced draft fans can work in concert with forced draft fans to maintain efficient and steady air pressure. Your induced draft fan application environment and airflow will dictate fan selection and specifications.
Primary Industries for Induced Draft Fans
Induced draft fans are found across industries. However, because they are instrumental in processing air, gas, and materials under harsh conditions, you can be sure you’ll need an induced draft fan 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
- Power Plants
- Printing, Packaging & Labeling
- Pulp & Paper
- Rubber & Plastics
- Semiconductor & Electronics
- Wastewater Treatment
Selection & Specification of Induced Draft Fans
As mentioned above, induced draft fans typically operate at higher efficiencies relative to forced draft fans, though it depends on the nuances of your application. They also have better process control and stability with reduced exposure to elements like wind, rain, and hail. Select and design your fans based on function, flow, and pressure. Here’s a quick overview of the top fans for each main function:
Typical Types of Induced Draft Fans
- Backward Curved (BCHS)
- Backward Inclined (BIHS)
- Industrial Exhauster Air Handling (IEAH)
- Industrial Exhauster Paddle Wheel (IEPW / IEFB)
- Radial Tipped (RTHS)
Fan Arrangements indicate how the motors mount to and drive the fan. For induced draft fans, we typically use a direct drive arrangement. Usually, Arrangement 4 or Arrangement 8, unless the blower is very large, in which case we might specify an Arrangement 7:
Arrangement 4 – Direct Drive with an overhung impeller. A low-cost arrangement, the fan wheel mounts directly on the motor shaft.
Arrangement 4V / Top Mount – Direct Drive with an overhung impeller. The wheel also mounts directly on the motor shaft, but in a vertical position.
Arrangement 7 – Direct Drive with a center-hung impeller. The motor is directly coupled to the fan shaft with a bearing on either side of the fan wheel. Involves a bearing on either side of the fan wheel and is supported by the fan housing.
Arrangement 8 – Direct Drive with an overhung impeller. The fan shaft is directly coupled to the motor shaft with a flexible coupling.
Accessories & Design
The same harsh conditions (high temperatures, acidic airstreams, and other extreme exhaust gas) that make induced draft fans the right choice also make them critical components of operations and present risks to your fan’s health. If harsh environments present higher threats to your fan’s health, then it’s probably safe to assume your maintenance and operating costs will be higher.
Frequent vibration and temperature testing, fan cleaning, bearing lubrication, and higher power consumption are some of the factors you may want to consider when selecting your induced draft fan. For example, choosing a fan with high static efficiency may significantly lower your operating costs.
Here are just some of the accessories and features you may see in induced draft fan applications:
- 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