Free-flow guards improve air volume over solid shrouds
Click here for more advantages shown in the above llustration |
It is important to know that in order to push a lot of air, a propeller has to have a lot of air reach it! Supplying more air TO the propeller simply results in more air being pushed BY the propeller.
When a VENTRY Fan runs, its propeller pulls air in from the back, the sides, and even the outer front edges of the guard -- then it shoots that air forward in a consistent, central stream of air. The majority of the propeller’s air supply reaches the propeller from the sides, through the open-flow guard. |
It is also helpful to understand that the outer quarter of a propeller does three quarters of the work. In other words, the tips of the propeller move most of the air. This makes sense if you consider the much longer distance traveled by a point on the tip in one revolution as compared to the distance traveled by a point near the center of the prop.
Solid shrouds block the air that reaches the propeller tips, and starves the prop of air, dramatically increasing noise. Fan users are surprised to find that most of the noise from conventional groundhugger fans is coming from the propeller, not the motor. VENTRY Fans, on the other hand, are known for being quiet. In the image at right, firefighters discuss training while the VENTRY Fan idles right next to them. |
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To demonstrate the noise problem for yourself, start a VENTRY Fan and hold your hand perpendicular to the propeller. As your hand approaches the guard, lay it flat against the side of the wire guard. You will hear a sort of “whump-whump-whump” sound as the propeller tips pass your hand. What happens is that every time the propeller tips pass your hand, they make a noise. It is prop starvation! If your hand makes that big of a difference, imagine how noisy a solid shroud clear around the propeller would be! For more on noise, click here.
The false claim is often made that solid shrouds improve the dynamics of a fan by entraining the air flow as it goes through the shroud. The tolerances required for this theory to apply to conventional PPV Fans are well beyond the capabilities of ppv fan manufacturers! Entrainment does happen during PPV, but it is through the entry, not the propeller.
To prove instead how solid shrouds DE-crease fan performance, wrap a long piece of cardboard around the guard of a VENTRY Fan to simulate a solid shroud. Stand in front of the fan when it is running and have someone remove the cardboard shroud. You will instantly hear a huge decrease in noise and feel a big increase in the air volume!
TRUE STORY: Years ago, a competitor of ours who also manufactured fans, experimented with using our VENTRY Safety Propellers on their fans. Shortly after sending it, we got a very nasty phone call. They were mad! They said our props didn’t work at all and asked how could we claim such high CFM?
James spent a minute troubleshooting, thinking maybe they had the prop on backwards, but eventually discovered that their fan had a solid shroud all the way around the guard. James asked them to try our propeller again, but this time to remove the shroud. A short time later, we received our Safety Propeller back, with a note saying essentially “Thanks, but no thanks.”
James followed up with a call to find out what happened...When they removed the shroud on their fan, thereby giving the VENTRY Safety Propeller some air to push, their fan fell over backwards! They did not want to re-engineer their whole fan to handle the thrust, so nixed the idea of using our VENTRY Safety Props. |
So why do other fans have solid shrouds? One reason solid shrouds may be used by other manufacturers is to prevent the propeller from being sand blasted by rocks, dirt and sand and to prevent the ejection of these materials into the air stream. Solid shrouds give the (false) impression of safety too; unfortunately, they do little to slow down shrapnel, released in the event of metal propeller blade failure.
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