Ventilating a building simply replaces stale or foul air with clean, fresh air. Although the ventilation process is required for many different applications, the airflow fundamentals never change: undesired air out, fresh air in. The key variables that do change depending on applications are the fan model and the air volume flow rate (CFM). Other considerations include the resistance to airflow (static pressure or SP) and sound produced by the fan (sones).
Sometimes you need an exhaust fan to perform a particular function, but it’s not clear which model to use or even what CFM is needed. If this is the case, you’ll need to do some fan specification work. Fan specification isn't an exact science, but it can be done confidently when the fan application is understood.
Based on the application, four parameters need to be determined: the fan model, the cubic feet per minute (CFM) of airflow, the static pressure of the system and the loudness limit of the environment.
Fans all perform the basic function of moving air from one space to another. But the great diversity of fan applications creates the need for manufacturers to develop many different models. Each model has benefits for certain applications, providing the most economical means of performing the air movement function. The trick for most users is sorting through all of the models available to find one that is suitable for their needs. Here are some things to think about:
Once the fan type is known, the volume of air exchanged must be determined. Your local building codes should contain information pertaining to the suggested air changes for proper ventilation. The ranges specified will adequately ventilate the corresponding areas in most cases. However, extreme conditions may require airflow outside of the specified range. To determine the actual number of changes needed within a range, consider the geographic location and average duty level of the area. For hot climates and heavier-than-normal usage, select a lower number in the range to change the air more quickly. For moderate climates with lighter usage, select a higher number in the range.
Calculate the volume of a room by multiplying its length, width and height. Then use the following formula to calculate the CFM needed to adequately ventilate an area:
CFM = Room Volume ÷ Min/Change
Here’s a chart showing some suggested air changes for a range of room types.
|Area||Minutes per Change|
|Assembly Hall||3 to 10|
|Attic||2 to 4|
|Auditorium||3 to 10|
|Bakery||2 to 3|
|Bar||2 to 4|
|Barn||12 to 18|
|Beauty Parlor||2 to 5|
|Boiler Room||1 to 3|
|Bowling Alley||3 to 7|
|Cafeteria||3 to 5|
|Church||4 to 10|
|Classroom||4 to 6|
|Club Room||3 to 7|
|Corridors/Halls||6 to 20|
|Dairies||2 to 5|
|Dinning Hall||3 to 7|
|Dinning Room||4 to 8|
|Dormitories||5 to 8|
|Dry Cleaner||2 to 5|
|Engine room||1 to 3|
|Factory||2 to 7|
|Foundry||1 to 5|
|Garage||2 to 10|
|Generator Room||2 to 5|
|Gymnasium||3 to 8|
|Kitchen||1 to 5|
|Laboratory||2 to 5|
|Laundry||2 to 4|
|Machine Shop||3 to 6|
|Meeting Room||3 to 10|
|Mill||3 to 8|
|Office||2 to 8|
|Packing House||2 to 5|
|Plating Room||1 to 5|
|Printing Plant||3 to 8|
|Projection Rooom||1 to 2|
|Recreation Room||2 to 8|
|Residence||2 to 8|
|Restaurant||5 to 10|
|Restroom||5 to 7|
|Store||3 to 7|
|Transfer Room||1 to 5|
|Warehouse||3 to 10|
An accurate measurement of static pressure is critical to proper fan selection. Fan static pressure is measured in inches of water gauge. One pound per square inch is equivalent to 27.7 inches SP. Static pressure in fan systems is typically less than 2 inches SP, or 0.072 psi. This drawing illustrates how static pressures are measured in ductwork with a manometer:
A pressure differential between the duct and the atmosphere will cause the water level in the manometer legs to rest at different levels. This difference is the static pressure measured in inches of water gauge. In the case of the exhaust fan in the drawing, the air is being drawn upward through the ductwork because the fan is producing a low-pressure region at the top of the duct. This is the same principle that enables beverages to be sipped through a straw. The amount of static pressure that the fan must overcome depends on the air velocity in the ductwork, the number of duct turns (and other resistive elements), and the duct length. For properly designed systems with sufficient make-up air, the guidelines below can be used for estimating static pressure.
Static Pressure Guidelines
Important: Static pressure requirements are significantly affected by the amount of makeup air supplied to an area. Insufficient makeup air will increase static pressure and reduce the amount of air that will be exhausted. Remember, for each cubic foot of air exhausted, one cubic foot of air must be supplied.
The sound generated by a fan must be considered. For the fan industry, a common unit for expressing sound pressure level is the sone. In practical terms, the loudness of one sone is equivalent to the sound of a quiet refrigerator heard from 5 feet away in an acoustically average room. Sones are a linear measurement of sound pressure levels. For example, a sound level of 10 sones is twice as loud as 5 sones. Refer to the chart below to determine the acceptable sone range for the application. As a general guideline, choose a fan that has a sone rating within the range specified.
Suggested Limits for Room Loudness
|1.3 to 4||32 to 48||Private homes (rural and suburban)|
|1.7 to 5||36 to 51||Conference rooms|
|2 to 6||38 to 54||Hotel rooms, libraries, movie theaters, executive offices|
|2.5 to 8||41 to 58||Schools and classrooms, hospital wards, operating rooms|
|3 to 9||44 to 60||Court rooms, museums, apartments, private homes (urban)|
|4 to 12||48 to 64||Restaurants, lobbies, general open offices, banks|
|5 to 15||51 to 67||Corridors and halls, cocktail lounges, washrooms and toilets|
|7 to 21||56 to 72||Hotel kitchens and laundries, supermarkets|
|12 to 36||64 to 80||Light machinery, assembly lines|
|15 to 50||67 to 84||Machine shops|
|25 to 60||67 to 84||Heavy machinery|
Source: AMCA Publication 302 (R2008); see also Engineering Cookbook: Handbook for the Mechanical Designer, Third Edition.
Once the exhaust fan application is known, keep in mind these four parameters to choose the right one for the job.
The information contained in this article is intended for general information purposes only and is based on information available as of the initial date of publication. No representation is made that the information or references are complete or remain current. This article is not a substitute for review of current applicable government regulations, industry standards, or other standards specific to your business and/or activities and should not be construed as legal advice or opinion. Readers with specific questions should refer to the applicable standards or consult with an attorney.
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