creating a make shift ventilation system...? cantrifugal fan

In the direction of the air flow, the diameter should increase from inlet to outlet, of any air carrying duct, to ballance the friction pressure drop created by the duct itself. However as the friction increases by the square of the velocity it is imperative to size your duct according to the air quantity in CFM (cubic feet per minute) at a velocity which will not cause a loss of air quantity below your design air quantity when measured in inches Water Guage. You need to look up the fan performance curve which will show the maximum pressure loss in inches WG at the design air quantity. You then keep increasing the duct size (regardless of the inlet size of the fan) until you reduce the air velocity and therefore the pressure drop (inches WG) that your fan can handle to give the design air quantity you want. Hope this hepls.

Phil

Is this because of the Boundary Layer thickness of the air flow..?

The American steam train had an expansion funnel to draw heat,

Your extractor mounted under such may draw better.

Hello Guest,

If your distance to the outside air for the intake cannot be reduced, there is a more efficient way to move more air that 60 feet and still using that 5 inch diameter pipe.

Place the fan at the outside end, in a round box, with the blades nearly touching the inside circumference of the box as they rotate.

The intake to that box will need to be at least 15 inches diameter, as short as possible, and the intake prevented from rain and other water or debris such as dust and/or leaves from entering that 15 inch dia. intake pipe.

So, your fan is spinning at some 1400+ RPM or thereabouts, and it will pressurise that 5 inch dia. pipe, causing a high-speed blast of fresh air at much higher pressure than atmospheric pressure (14.7 lbs/inch/squared).

Because the moving air in the delivery (5 inch dia.) pipe is at high pressure, many times more air will be moved than if the delivery pipe was just a suction pipe.

The incoming air is always at atmospheric pressure, so you would not have much pressure to begin with, if you used the 5" pipe as a suction pipe.

At the outlet end, connect the 5 inch dia. delivery pipe into a large expansion box, to lower the pressure, then take whatever outlet you require from that expansion box.

In your delivery (5") pipe, ensure you have pipe bends, not elbows, and if possible a straight run of pipe will minimise frictional losses.

Fan-blade shape is important, so that noise is minimised.

Motor mounts need to be rubber or spring, to also minimise noise.

The above is simplified, but I trust you understand enough to try the idea.

Kind Regards....

thanks for all the input i understand for the most part but i think i should clarify. i am venting to the outside and drawing in hot from the inside. more so just behind the fridgerators and not the whole room of air. basically the pipe will specify where the air will be drawn from. the run of coolers is about 60ft so i wanted to do somewhat of a reverse duct system. do you think that the pipe is necessary or would be more efficient to just have the centrifugal fane just be at one end and it naturally draw the length. the room has AC so i dont suck all the cool air out.

Do you have an idea how much hot air you want to remove? How many refrigerators? How many condensor fans on the refrigerators?

As others have posted, long small diameter fan suction ducts do not work very well. I would estimate at most the 6" duct would only remove a maximum of 400-500 cfm with a velocity of 2000-2500ft/min. This would be somewhat noisy, and as a W.A.G.(wile arse guess), would be about 1/4 of the amount of air you want to move.

Of course for a make-shift operation, you might be satisfied with this down and dirty solution.

The load on the air conditioner will not change much, if the make-up air comes from outside where the air is hot and possibly humid. You will be sucking out hot air near the refrigeration condensers, but you can't suck air out of the building without creating a slight vacuum, if there is no make-up air, and there will be a point where the air flow goes to nearly zero.

A way to alleviate such a quandry is to duct in outside air locally to each condenser, placing the supply and return air ducts in suitable positions so that a minimum of cool air is wasted.

Otherwise, it would be prudent to remove the internal (to the building) refrigeration condensers and place them outside. Of course, extra tubing would be needed to go through the wall, and the refrigeration units would need to be recharged.