Scheduled maintenance - Thursday, July 12 at 5:00 PM EDT
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Fans are the "workhorses" of the ventilation systems. In order to have an effective ventilation system, fans must be 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:
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 are often mounted in 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:
Tubeaxial and vaneaxial fans are essentially propeller fans made to fit in a duct. They are usually limited to "clean air" applications such as exhaust ducts going through the roof.
There are three types of centrifugal fans determined by the type of fan blades:
The fans in your home furnace, vacuum cleaner and hair dryer 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:
Fans that handle high temperature exhaust air must be made from materials and parts resistant to high temperatures.
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 efficiency. However, the following general information may be helpful:
Safety guards are required for all danger points such as inlet, outlet, shaft, drive and cleanout doors. Construction should comply with applicable provincial governmental safety requirements.
Except for low speed fan units, fans usually are noisy. Noise can be distracting, irritating, and/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 one inch (1" or 2.5 cm) when moderate finger pressure is applied.
Blades & Housing: A buildup of solid material (dusts, 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:
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 50 feet from the inlet to prevent recalculation of contaminants. Stacks work best when they are tall, usually at least 10 feet above the roofline.