Ventilator Fan Question

It will have a small effect, and it is difficult to determine whether it will be an increase, by ducting the air more effectively, or a decrease, by adding some fluid friction at the duct walls without seeing the installation.

It can make the air flow seem to change in speed as it may slightly change the air flow pattern inside the shop.

Overall, I don't think so; but if an interior shroud is 'aimed' toward the part of the shop where you are standing you might get an effective increase around you if that's what you want. Depending on how far you are from the fan, you might need more that 10 or 12 inches of extra ducting; you'd probably need to be within a few feet of the intake.

OK.

So I am guessing that it serves no real purpose to extend the shroud around the fan on the inside or out.

If it ain't broke we don't need to fix it.

Thank you all.

Don't waste your time. It's very difficult to "pull" air, unless your room is very tight and the air inlets are placed strategically for flow. Turn the fan around so that it blows air into the room. You have an unlimited amount of air outside the building to draw from.

Cheers.

No. Extending the length of your duct will not increase air flow or velocity. Increasing its width or height will increase the cross-sectional area and will actually reduce air velocity unless the volumetric flow rate is also increased, and to do that you will need a higher capacity fan.

Thank you all again.

The all knowing Good Ol' Boy down the dock came into the shop and offered this as another solution to the heat problem.

* I am going ahead with my project to mount a huge sunscreen over the top the shop. The material should be in by next week. We all discussed that in another thread a week or so ago.

Thank you all for debunking this new myth..............

Have you considered using the "mesh" cloth greenhouses use over their units in order to reduce the amount of sun that is radiated onto the roofs? They reduce the heat by reducing the amount of sun radiated onto the units.

We have several exhaust fans in our facility. In the 24 to 48 inch range, belt driven and running about the same speed as yours. However none of them have a 1/2 Hp motor. The smallest is 1-1/2 on our 24" centrifugal which flows about 8500 cfm (advertised) The next smallest is a 30" axial also with a 1-1/2 hp motor and pushes approx 11,000 cfm (advertised). I'm not following how your getting that flow with a 1/2 hp motor on a 42" fan. Are you sure of these numbers? Could someone have changed out the motor and not used the correct replacement? our 48" (closest sized to yours) uses a 3 Hp motor and pushes (advertised) 32,000 cfm.

The specs on the fan say:

42" x 4 blade

1/2 hp. T-E (?)

60 mhrtz.

115 volts

7.6 amps

14,445 cfm

rpm hi/lo 500

Its stamped on the side . It comes from Triangle Engineering of Arkansas.

When I set up the sun shade over the top of the shop, I'm expecting things to cool down enough to my liking. Right now, the inside of the building feels warmer than outside the building.

It will have very little effect. To increase air moved, either speed up the fan, or increase the fan diameter.

<<< shroud around the entire fan >>>

Spacing between fan blades tip & shroud's inner wall hs a great effect in pushing the air. That is no air flow back through it.

I recommend Impeller type fans.

It is belt-coupled, but small direct-shaftcoupled are also in the market

Refer to: for wide range of exhaust fans.

Put a water sprinkler on the roof. That should cool things down. Or either install an attic fan. They pull air in from outside pretty quickly. I can remember having one when I was a little girl--but then again I think it was way cooler back then than it is now.

Thank you.

No attic in this shop. The fan is located very high up on the wall.

The sprinkler system was tried but made mold in here so I stopped.

Thank you for the idea.

Thank you.

I will look into this alos if the other remedies do not work as planned.

Your Good Old Boy had the right idea, but vastly underestimated the complexity. Shroud design can greatly affect the efficiency of the fan. A longer duct will reduce turbulence at the fan inlet, improving efficiency. You'll get more effect by minimizing flow disturbances. For example, any support struts should be rounded instead of using channel steel or similar. Removing any geometry transitions (from square duct to round for the fan, for example) also helps. The greatest impact is by minimizing the gap between the shroud and the blade tips.

Shroud design for improved efficiency is discussed beginning on the 4th page of this paper. Here's another paper which covers shroud effects on thrust in aircraft propeller design.

Thank you.

The blade tips are 1.25" from the shroud.

* I did mistake the fan blades, there are 6.

Are you saying that if I did lengthen the shroud inside the shop it would suck more air from the inside and pull it outside?

How long would you suggest a trial piece? 12" 14" , etc.

This is the exact fan we have , minus the wheels and it is set up in an almost round hole, elevated 7 feet up off the floor. It vents to the outside.

Photo Triangle Engineering

The minimum inlet distance for laminar flow is about 2 shroud diameters. If you have a 24" shroud, you need at least 48" of inlet duct. You will get some better efficiency, but I'm not certain the amount of improvement will be worth the cost in time and money.

You'll get a much greater effect by reducing the gap at the blade tips to the minimum you can reasonably achieve without rubbing. The curvature of the blade is such that the air tends to "slip" off the end of the blade tip. The low pressure on the suction side of the fan pulls that slip air back around to the inlet, causing recirculation in the fan itself. You want all the air (or as much as possible) to continue straight out the shroud & exit the shop. Eliminating the recirculation will give you the biggest bang for your buck.

You should also look at where the air is entering your shop. Ideally the inlet will be at floor level at one end of the long axis of the room, with the exhaust fan at the opposite end. You might also consider ducting the fan intake to the center of the room so it can grab hot air from the entire area.

If the issue is how hot the employees feel in the room, it may be easier to increase the air velocity without having to increase the flow rate. Greater wind speed will increase the mass flow rate across the body, improving the the evaporation rate of perspiration. It sounds counterproductive, but fewer points of air entry (to a point) will actually make the air move faster and feel cooler.

Extending the shroud maybe 6 inches on the discharge side and then another 24 inches expanding the diameter at say 15 to 20 degree slope would decrease the discharge turbulence and produce some increase in air flow but whether it would be enough to justify the cost and trouble is hard to predict. Generally speaking just a small increase in back pressure on a propeller type fan will cause a decrease in air volume so a small decrease in back pressure will result in increased flow. Lou Bindner

I have done about all the moving I can do with the fan and the windows. Its just HOT and after all it is South Louisiana.

The only employees are my wife and I.

With the economy and the BP thing , we had to stall the plans to remodel , insulate fully and drop a central AC unit outside. So I am just looking at inexpensive ways to cool things down until I see how we all fair with all the mess going on down here.

The shade cloth is on its way and I am hopefull that will cut down this internal heat by decreasing how hot the metal roof gets ,by a few degrees. Right now it is hotter inside than outside. It never really bothered me so much until after a surgery I had last year. Now I get vertigo every day when the temp hits around 90F.

Or.........it might just be I am turning into a Candy @$$ .......... ha ha!

Thank you all again.