When something is considered "at the heart of the opera […]
When something is considered "at the heart of the operation," people often think of it as an integral part of what's being discussed, and it may be in the middle of it. The human heart fits this description. This magical organ is located in the center of the chest cavity, moving blood, nutrients, oxygen and carbon dioxide into the body with amazing efficiency.
At the heart of the exhaust ventilation system is a fan, like a human heart. It first creates a vacuum in the intake hood, which is located strategically at the source of the pollution, drawing in contaminated air and guiding it through the ductwork. Sometimes the fan directs air to filters or other air cleaning devices, but eventually the dirty air is vented through the outdoor stack.
There are two main types of fans, axial and centrifugal. You may be most familiar with axial type because they appear very often in your home. These blades look like propellers, drawing air directly through the fan. Because they are useful in personal environments, axial fans are not typically used in local exhaust ventilation systems because the motor that drives the blades is in the path of the airflow. This setting may cause problems if the air flowing through the motor contains dust and flammable vapors. Dust can cause the motor to become dirty and overheated. Flammable vapors will ignite if motor wiring fails and an electric arc is generated.
The blades of a centrifugal fan are completely enclosed in an airtight enclosure. The motor that drives the fan can be safely placed outside the housing where there is no dust and no flammable vapors. If you look inside the case, you will find a moving part called an impeller similar to a squirrel cage.
To make all this work effectively, you have to choose the right centrifugal fan that provides enough suction to capture the air contaminated at the hood, and then overcome the plumbing, filters, and other air cleaners. Because these drag factors impede the ability of the blower to pass through the system, the blower must have sufficient strength to compensate for these factors. To determine the correct size of the centrifugal fan at work, engineers must calculate the expected airflow resistance and use the data provided by the component manufacturer as well as the tabular data sheet readily available in the project. Just as lawnmower engines do not provide enough energy to power a car, small fans will not be able to move air through systems that exceed their capacity limits.