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Exhaust Fans

Regulating the movement air from one area to another has been a problem since the invention of the electric motor. The problems that are present when exhausting air are the energy costs (not only the cost of hydro to operate the fan), the noise the fan makes, the debris (dust, pollution odors) the fan spreads around and where the air to replace that that has been exhausted is coming from. Fans are manufactured in all sizes and styles from the propeller type made of stamped metal blades to wheels with curved blades . The volume of air that a fan moves is measured in Cubic Feet of Air, Cubic Meters or in Liters. The time that a volume of air is moved is measured in seconds, minutes or hours. Typical examples of this would be "CFM" for cubic feet per minute or "L/S" for liters per second. Another factor that affects the movement or flow of air is the resistance to the flow that is called "Static Pressure" and is measured typically in "inches or millimeters of water column". Greater restriction to the flow, generates greater static pressure. Things that affect the resistance to flow are the speed of the fan, length of a duct, size of a duct, the style of the duct and the number of turns the air has to make before exiting at a termination point. Static pressure can become so high that air flow is stopped. I am using the word "Air" as a generic substance but the fan could move any form of gas, mist, liquid vapor or particulate.

If you were to plot the efficiency of a fan on a graph it would resemble a hill (see the chart at the end of the section). The maximum amount of air able to be moved by a fan is at the top of the hill and the least amount of air is at the bottom of the hill. This is only worthy of note to explain that a fan can be turned at a rate so fast that it actually loses the ability to move air. In other words there are limits to what a fan can do and it is up to the manufacturers of fans to test and record the performance of each fan type that is produced so that engineers and customers can select the correct fan for their application.

Fans are made of all types of materials for specific applications from plastics, aluminum and steel. Some requirements may need a material that will not generate a spark (such as aluminum) where any failure of the fan could create a source of ignition for vapours or dust. Another consideration would be the cost of the material or the weight of the material. Plastic fans are light weight and inexpensive, but will melt under a wide range of heat. Aluminum fans are light weight, stronger than plastic and won't create a spark. Steel fans are heavy and strong and more costly than plastic or aluminum.

The problem with exhaust fans is that all the air that is exhausted has to be replaced by air from another area or source. Whether in your home or your office, when a fan removes air to the outdoors, the space air pressure is lowered in comparison to the air outside of the space. Air is always trying to reach equal pressure. When air is exhausted, air from outside of the space rushes in to equalize the pressure. This air eventually comes from outdoors through every crack, door or window in the structure. The problem with this is that the air is not filtered, heated, cooled, humidified or dehumidified and your HVAC system has to condition this air to provide your comfort. This is a costly fact of physics that can be greatly reduced with conservation and planning.

Typical appliances in your home that exhaust air are washroom fans, range hoods for your stove, clothes dryers, gas fired water heaters, some furnaces, HRVs' and ERVs'. The smallest of these is the average bathroom fan that is rated at 100 CFM (100 cubic feet of air per minute) This tiny fan if left on for 60 minutes will exhaust 100CFM x 60 minutes = 6,000 cubic feet of air. The same amount of air has to come back into the home from outside and be heated or cooled. An average home may be around 1,600 square feet with 8 foot ceiling for a total cubic volume of 1,600 x 8 = 12,800 cubic feet. In this example, the tiny bathroom fan that will exhaust 6,000 cubic feet in one hour will remove almost every ounce of air in your home in just over 2 hours. If this was a (-10) degree day in the winter you may wish to turn the fan off or install a 10 minute timer on it. In a commercial office space the men's and women's washrooms are commonly separate, but using the same roof exhaust fan. Washroom fans are specified at about 100 CFM per toilet fixture (including urinals). A typical office washroom might have 3 toilets in the women's and 2 toilets and a urinal in the men's for a total of 6 fixtures. Six fixtures X 100 CFM would require a roof fan of 600 CFM. Typically an office washroom fan is on the entire day for 9 to 10 hours. A 600 CFM roof fan would therefore exhaust 600CFM x 60 minutes per hour x 10 hours for a total of 360,000 cubic feet of air. In an office tower, this could easily be the same volume for each floor of the building. Not only does this use a ton of hydro, but the cost to condition the fresh air coming in is expensive. The solution to this problem is quite simple... Clean the air rather than exhaust. This is more practical in the commercial washroom facilities where a "Sanuvox" air purifier can be easily installed in the ceiling. Commercial washrooms always smell, 24 hours a day because of urinals with the big white mint in the men's washroom to napkins in the garbage in the women's along with perfumes and colognes that are liberally used. It is always a good idea to have a very slight negative pressure in a commercial washroom, but with the Sanuvox Air Purifier the amount of air exhausted can be reduced to a fraction of the designed amount See www.Sanuvox.com or the section Air Filtration and Purification in our web site under "Learn About".

Enough about washroom fans for now. Factory exhaust fans are primarily used to exhaust heat or fumes for employee comfort. These fans can be roof or wall mounted and automatically controlled by thermostats or manually controlled by switches. The size of these fans is typically large enough to put the factory into a severe negative pressure. I have witnessed cases so severe that it would take all of ones strength to open a normal man size door to the factory. Exhausting excess heat is normally only an issue in the summer months so a shipping or receiving door can be left open to neutralize the pressure difference.

Spray booths are another source of extreme exhaust in factory settings. These fans are described in the commercial / industrial section of "Learn About" under the heading of Make Up Air Units.

Signature Air Systems Ltd


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