How do you size an industrial fan?
Finding high-quality, low-maintenance exhaust and air movement equipment can save money for agencies and business owners. Commercial and industrial ventilation can be expensive to install or update, and finding the right type and size of fan will keep companies and industries in compliance with codes and regulations and make working conditions safe for employees. Whether an industrial exhaust fan is used in a school, commercial kitchen, storage facility, gymnasium, factory, or outbuilding, getting the right equipment can make a difference.
There are different types of fans to consider for ventilation and exhaust. Local exhaust ventilation fans, for example, can help prevent indoor air contamination by capturing fumes and pollutants before they are spread. These type of hooded systems can be found in restrooms, commercial kitchens, chemical industrial environments, wood-working factories, and metal shops, just to name a few.
Size exhaust systems based on room or area dimensions, the type of environment, and the acceptable temperature and humidity range. ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) defines acceptable indoor air quality by: (1) the dilution of odors, (2) the concentration of airborne pollution, (3) heat, and (4) humidity. Exhaust fans and equipment can help provide ASHRAE-acceptable indoor air quality in terms of ACH (air changes per hour). According to ASHRAE standards, a rate of .35 or 3 CFM (cubic feet per minute) per each 100 square feet in a space is an adequate rate of airflow.
Along with sizing, ventilation exhaust should be designed and fitted with pollution control in mind. Exhaust ducts should be laid out in the most direct route possible to outdoor vents. Exhaust vents should never be located near supply air vents to avoid contaminating fresh outdoor air with polluted exhaust air. Improper design and installation can cause static pressure imbalances, indoor air contamination, odors, drafts, and inefficient environmental control.
Whether you’re installing ventilation fans for building safety or comfort, it’s important to estimate the fan calculation as closely as possible. Fan size is based on its cubic feet per minute (CFM). It’s important to match the CFMs to the cubic feet (CF) inside the building.
Industrial Fans Direct has a cubic feet per minute calculator that’s useful for fan sizing, along with a table that shows recommended air changes per minute based on the building’s use. The fan calculator makes calculating CFM’s fast and accurate.
However, if your building has varied ceiling heights, you may need to calculate the space separately to find total CF for the fan calculation.
Variable-Height Ceilings
Instead of measuring the total length and width, separate the building by ceiling plates. Measure wall length and width for each section separately and its ceiling height. Complete the cubic footage measurements for each section separately, and add the cubic footages together.
Pitched Ceilings
To measure the cubic footage of building that has a ceiling open to a pitched roof, use this formula for finding the area inside the unfinished attic area:
Multiply the length and width together, along with the tallest point in the roof (in feet). Divide the product of these three by two to find the CF inside the topmost area of the space. Add the CF of the lower portion and the upper portion to find total CF.
Selecting the Fan
Choose the fan or fans that most closely matches the number of air changes per minute you need to adequately ventilate the building. A 15,000 cubic foot area that needs an air change every 3 minutes would need a fan with a CFM of 5,000. The fan's product specifications will list its CFM on its packaging or data sheet.