Industrial Ventilation - 4. Fans

This document is part of a series of documents on industrial ventilation, and includes general information about fans.

1. Introduction
2. Units and Measures
3. Ducts
4. Fans
5. Hoods
6. Air Cleaning Devices
7. Installation and Maintenance (general)
8. Troubleshooting
9. Glossary of Common Terms

Fans are the "workhorses" of the ventilation systems. To have an effective ventilation system, fans must be of the appropriate size and type. They must provide enough speed (air movement) to capture contaminants at the source, draw them through the hood, and carry them through the duct system, through the air cleaning devices and exhaust to the outdoors.

There are two main types of exhaust fans:

1. Axial Fans: These fans look like propellers and draw air straight through the fan.
2. Centrifugal fans: These fans look like "squirrel cages" that draw air into the centre of the fan and exhaust it at a 90-degree angle.

Figure 1
Types of fans

Axial or propeller fans

There are three basic types of axial fans: propeller, tubeaxial, and vaneaxial. Propeller fans are most commonly used for dilution ventilation or cooling. These fans have a propeller-shaped blade and a drive motor mounted on a flat frame. They are often mounted on a wall or ceiling. Common examples are your automobile radiator fan or a free-standing room fan. The basic characteristics of these fans include:

• that they can move large amounts of air if there is little resistance, and
• are not suited for local exhaust ventilation because they do not provide enough suction to draw air through the system.

Tubeaxial and vaneaxial fans are propeller fans made to fit in a duct. 

The tubeaxial fans have the propeller and the drive motors mounted in tubes. The clearance between the propeller and tube is very small to improve airflow efficiency.

The vaneaxial fans are tubeaxial fans with air straightening vane before and behind the propeller.

These fans usually limited to "clean air" applications such as exhaust ducts going through the roof.

Where the exhaust air is at a high temperature or contains contaminants (e.g., grease, corrosive, etc., such as in kitchens or paint booths) that could damage the drive motor, a bifurcated axial fan could be used. The design of this axial fan keeps out the drive motor of the air stream. 

Centrifugal fans

There are three types of centrifugal fans determined by the type of fan blades:

• forward inclined blades,
• backward inclined blades, and
• straight radial blades.

The fans in your home furnace, vacuum cleaner and hairdryer are examples of centrifugal fans. They can operate against a high resistance and are typically used in local exhaust ventilation systems. The rugged radial blade centrifugal fans are the best type for exhausting heavy amounts of dust because they are less likely to become clogged or abraded by the dust.

Selection of the proper fan can be complicated and should be done by a ventilation or fan expert. However, you can make the following observations to determine if the fan selected is appropriate:

Material handled through the fan

• If the exhaust air contains a small amount of smoke or dust, a backward inclined centrifugal or axial fan should be selected.
• If the exhaust air contains light dust, fume or moisture, a backward inclined or radial centrifugal fan would be preferred.
• If the particulate load in the exhaust air is high or when material is handled, the selection of a radial centrifugal fan is preferred.
• If the exhaust air contains explosive or flammable material, spark-resistant construction (explosion-proof motor if the motor is in the air stream) should be selected to conform to the standards of the National Fire Protection Association and jurisdictional government legislation. 
• If the exhaust air contains corrosive contaminants, a fan with a protective coating or made of special materials (stainless, fibreglass, etc.) may have to be used.

Airstream temperature

Fans that handle high-temperature exhaust air must be made from materials and parts resistant to high temperatures.

Capacity and physical limitations

You may or may not know how much air has to be moved by the fan. You may also not know the amount of resistance in the exhaust system that the fan has to overcome and what is the fan's efficiency. However, the following general information may be helpful:

• Fan size should be determined by performance requirements. Inlet size and location, fan weight and ease of maintenance also must be considered. The most efficient fan size may not fit the physical space available.
• On packaged fans, the motor is furnished and mounted by the manufacturer (direct-drive). On larger units, the motor is mounted separately and coupled directly to the fan or indirectly by a belt drive.
• Direct-drive fans offer a more compact assembly and assure constant fan speed. Fan speeds are limited to available motor speeds. Capacity is set during construction.
• Belt drive fans offer flexibility in the adjustment of the fan speed. The speed can be changed by altering the drive ratio. This flexibility may be important in some applications where there is the need to provide for changes in system capacity or pressure requirements determined by modifications of the process, hood design, equipment location or air cleaning equipment.
• It is normally a custom to select a fan that can supply the required airflow (volume and pressure) at no more than 80% of its full-rated speed. However, the motor selected should be able to handle the horsepower required to achieve that rated speed (i.e., a speed increase of 20%).

Safety

Safety guards are required for all danger points such as the inlet, outlet, shaft, drive, and cleanout doors. Construction should comply with applicable jurisdictional governmental safety requirements.

Except for low-speed fan units, fans usually are noisy. Noise can be distracting, irritating, or damaging to the ear. Fan noise can be a problem both in the plant and to neighbours outside. Most fan manufacturers publish sound ratings for their products and these levels should be considered when selecting a fan.

Common components of most fans that affect performance are listed below:

Bearings: Fan shaft bearings are often the single greatest source of trouble. Bearing life is reduced by overly tightened or excessively loose fan belts, fan vibration, uneven loading on the fan blades, high ambient operating temperatures, and improper (over or under) lubrication.

Belts (V belts): Improper belt tension can affect the fan performance. Audible belt squealing during start-up is a sign of insufficient belt tension. As a rule of thumb, belt tension should be tight enough so that the centre of the span will move 2.5 cm (1 inch)when moderate finger pressure is applied.

Blades and Housing: A buildup of solid material (dust, fumes, particulate matter) on the fan blades or the housing causes imbalance, vibration, and loss of capacity (reduced airflow).

Connectors and isolation foundations: Flexible connectors and isolation foundations are used to isolate fan vibrations from the building and the rest of the ventilation system. Flexible connectors attach the ventilation system duct to the fan while eliminating fan vibration that may travel through the ventilation system duct. If they are torn or corroded, the fan performance will be affected.

Louvers and dampers: Some fans have inlet or outlet louvers or dampers to adjust airflow. They may be manually, pneumatically or electronically controlled. If the louver linkage connections are not tight and secure, they may affect the fan performance.

Motors: Motor operating voltage must be maintained within 10% of the recommended voltage to ensure proper fan performance. Most motors are permanently lubricated for life and require no further maintenance.

Fans can go "out of balance" because material builds up on the fan blades, or because of wear. Imbalanced fans will vibrate and may cause damage to various parts of the fan (blades, housing, motor, etc.). It is important to keep fans clean and properly balanced, particularly if the air being removed contains abrasive, sticky or wet materials. Scheduled maintenance should check items including:

• bearings (lubrication, vibration),
• belt drives,
• coupling or belt alignment,
• fan blades (impellers) for proper alignment and rotation,
• bolts and screws for tightness,
• condition of impeller (wear or accumulation), and
• safety guards.

Air discharged from a fan should be kept away from inlets (intake) of the make-up air system. In this way, the make-up air system will draw only clean, outdoor air into the workplace.

The discharge exhaust stacks should be high enough from the roof so that contaminants do not re-enter the workplace. Generally, they should be located no closer than 15.24 m (50 feet) from the inlet to prevent recirculation of contaminants. Stacks work best when they are tall, usually at least 3 m (10 feet) above the roofline.