If you have read my posts on the Hollisterian and Florentine effects on direct-drive fans, you may be wondering if it is really worth the complication to chose this type of fan-drive system for your project.
But when you think about it, the lesson from those two effects is to strive to use fan selections that are at a synchronous speed when at design. And since you know you can vary the width of the wheel to get the CFM you need, this should be a simple trick that you (or any decent AHU provider) can do when laying out your equipment. And if this isn't possible, you know how to compensate for an asynchronous fan selection in your motor size and system design. The only question left is what do you gain for this effort?
Plenty.
Efficiency for one. Belts are a source of inefficiency. Friction between the belt and the pulleys causes heat and erosion of the belt which is exhibited in the system by a reduction in efficiency of the system. How much friction are we talking about? A sample chart might help quantify this:
That's right--About 4% of your motor energy is lost on systems with brake HP's around 50, and more than seven percent on systems less than two BHP. That's a lot of energy to just throw away.
Another reason is maintainability.
Guess how much belt maintenance needs to be done on a direct-drive system? Guess how many belts need to be stocked to replace broken belts? Guess how much time needs to be spent adjusting belt tension to spec? Guess how many times someone needs to be called in to correct a squealing belt?
A rule of maintenance is that if something is hard to do, it won't be done. A good corollary to that might be that if something doesn't need to be done at all, there's a good chance it won't cause a problem downstream because someone didn't do it.
A related advantage is the longevity of the system. For starters, A belt drive system requires at least four bearings, two at the motor and two at the fan. Since a direct drive fan only has the motor bearings, simple mathematics would indicate that you would have at least half the bearing failures. But, as is often the case, the real situation is a bit more complicated than simple math. In each system there is a force on the motor bearing perpendicular to the shaft. In the belt drive system, this force is from the belt tension, in the direct drive system it is from the fan wheel weight.
This is where real life makes things more complicated--because the belt tension is usually several times the weight of the fan wheel. This means that the bearings in the direct-drive fan motor see much lower stress than in the belted case. This translates to many times the expected life for these critical components.
Another advantage? No belt dust. This means that projects with critical air quality concerns may be able to avoid final filters
And let's not forget belt noise--Not just the squeaking that is caused by the slipping of an incorrectly tensioned or worn belt, but the inherent noise that is added to the system from the normal operation of the belt drive itself. (Which is not, by the way, accounted for in the fan sound data you get from the manufacturer. Those were measured on direct-drive fan wheels....)
So let's review:
(Some people might argue that changing pulleys to adjust speed is less of an advantage than a disadvantage)
Now, of course, there will always be applications for belt-drive fans. Sometimes they just make a better fit for the project than direct-drive. Since the motor on a belt-drive fan is supposed to be operating at its synchronous speed (by design) you don't need to oversize motors to reach operating points where a full wheel width is truly the most efficient or quietest solution possible. But direct-drive sure makes sense when it makes sense.
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