The Coanda Effect, Exhaust Fans and HVLS – What You Need to Know
26 MAR 2013

Posted by Eddie Boyd Categories: Blog, How HVLS Fans Work Comments: No Comment

HVLS Fans Coanda EffectHave you ever seen a ping pong ball suspended above a blowing blow drier? Or a stream of water attracting a spoon and then bending around it? These are common examples of what is known as the “Coanda Effect.” In the early 1900s, an aeronautical engineer, Henri Coanda, identified an application of the effect during experiments with an aircraft that mounted an unusual engine designed by Coanda. The motor-driven turbine pushed hot air rearward, and Coanda noticed that the airflow was attracted to nearby surfaces. The airflow actually bent around a curved surface. This phenomenon became known as the Coanda Effect. It is now also known as the “wall-attachment effect,” and is defined as the tendency of a moving fluid, either liquid or gas, to attach itself to a surface and flow along it.

Coanda Effect and Exhaust Fan Ventilation

What does the Coanda effect have to do with exhaust fan ventilation? Plenty. Exhaust fans, mounted either on a ceiling or wall, for lack of a better term, are supposed to suck air out of a room. Then, fresh air from the outside is supposed to enter the room to replace all the bad air that has been sucked out. The exhaust fan creates a pressure differential to “exchange” the air. Most exhaust fan diagrams show the clean outside air coming straight in through a warehouse door, or a window, or other venting source, traveling straight through the room, past the occupants in the room, and then out the exhaust fan again. The problem with that graphic is that is does not take into account the Coanda Effect on the incoming, fresh air. The incoming air, instead of coming straight into the room, will actually tend to travel from the inlet opening (door, window or vent) and then, due to the Coanda (or wall-attachment) Effect, go up the wall, across the ceiling and out again through the exhaust fan. For the air, that is the path of least resistance. That is unfortunate for the people in the room, who need the benefit of the clean, replacement air. What this means is that, in reality, most of the air that is exhausted from the building is the fresh air that has just been drawn in, instead of the old, stale air that was supposed to be exhausted.

Importance of Ventilation Standards

Let’s get technical for a moment to find out why all this is important. The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHREA) created ventilation standards most recently known as ASHRAE 62.1-2010. The stated purpose of that standard is to “specify minimum ventilation rates and indoor air quality that will be acceptable to human occupants and are intended to minimize the potential for adverse health effects.” Acceptable indoor air quality is defined in section 3 of that standard as “air in which there are no known contaminants at harmful concentrations as determined by cognizant authorities and with which a substantial majority (80% or more) of the people exposed do not express dissatisfaction.” When this standard is adopted by local authorities, it can become a required part of the building code. So, it becomes very important to be sure the bad air is truly exhausted and the good, replacement air is actually available to be enjoyed by the occupants. It has been stated: “The majority of HVAC systems do not meet the minimum ventilation rates prescribed during operation.” (“ASHRAE Standard 62, Analysis and Recommendations, Jan. 2003, EBTRON, Inc., as published in AutomatedBuildings.com)

How HVLS Fans Help Meet ASHRAE Standards for Indoor Air Quality

MacroAir understands why most HVAC systems, alone, have a hard time meeting the ASHRAE standards for indoor air quality. Exhaust fan systems do not account for the Coanda effect, and it is hard for the high speed fans move enough air to overcome the Coanda effect. Typical exhaust fans run at 900rpm and move between 5,400 and 64,000 cubic feet per minute (CFM) of air. But they need lots of horsepower and use lots of energy to do this. This is where High Volume, Low Speed (HVLS) fans can make all the difference. HVLS fans, with blade diameters up to 24 feet, move massive amounts of air at ranges of 80,000 to 375,000 CFM. The blades turn at only 50 to 160 rpm. MacroAir’s HVLS fans, moreover, typically run on only a 1HP motor and are widely known to be incredibly energy efficient. But the important part is that HVLS fans, when used to complement a traditional exhaust system, will effectively mix the incoming fresh, replacement air, and overcome the Coanda effect that limits the potential of the exhaust fan to truly refresh the air in the room. The HVLS fan will stop the incoming air from sticking to the walls and ceiling, and get it down to the people in the room. The HVLS fans will let the exhaust fan create the pressure change to enable air exchange, but the HVLS fan will then do what the exhaust fan alone cannot do, continually mix the air so that less incoming air is required to produce the correct number of air exchanges needed to satisfy the people in the room. This can reduce the number of exhaust fans needed (and thereby cutting energy costs) and reduce the loss of heated or cooled air since less fresh air must be drawn in from the outside, creating an overall savings for heating and air conditioning.










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