the difference between direct drive fan and belt drive fan connection?

[hello2006]

Hi everyone,

would you please describe something about the difference between direct drive and belt drive fan. Compare their good and bad function please?

My fan is 18,000 cfm. But belt drive gave lots of trouble, therefore, want to change to direct drive. I am not sure what the benefits will be?

Thanks for any suggestion!

Good night!

 

[dvd]

Direct drive works well when you couple it with a variable frequency drive, so that you can soft-start the fan and have a wide speed range. Alignment and coupling choice is more critical with direct drive.

Belt driven fans have the capability of operating at different speeds by changing sheaves and belts. One advantage to belts is that they slip at start-up giving a cheaper version of a soft-start. You can sometimes fit a belt-drive fan in a more compact space.

 

[hello2006]

Thanks for your reply!

Since the coupling is so critical, what kind of coupling will you suggest?

my client want to use direct drive. Do they need to have a special technician on their site who will do maintenance work for alignment of direct drive?

The old one is belt drive, the sheave is a little bit more than 1. Or by the other word, the speed of the fan is a little bit faster than motor. So, do you think that direct drive can make the speed of fan faster than motor?

Glad to hear your more opinions!

thanks very much!

 

[dvd]

Obviously the fan will only rotate as fast as the motor is rotating because it is directly coupled.

I would recommend that you contact the fan manufacturer and request the maximum allowable speed of the fan. I would also ask for their recommendations on the coupling. They might be willing to sell you a conversion kit.

 

[hello2006]

How many kinds of direct drive and what are they? thanks!

 

[speco]

Hello2006,

Basically there is just one kind of direct drive. Direct drive usually means that the fan is mounted right on the output shaft of the motor. There is no "coupling" in the usual sense. There is just a bushing which is normally supplied with the fan.

Some fan manufacturers will provide the whole assembly including the fan ring, motor support, motor, fan and guard as an assembly. This may be a good way to go with this size of fan.

One word of caution though: As pointed out previuosly by DVD, the fan rpm is the same as the motor rpm. It may be advantageous to use a lower speed motor than an 1800 rpm motor to keep the fan tip speed within reason (say under 12,000 ft/minute). This is important if noise is a consideration. Fan noise is related more to the tip speed than anything else.

Regards,

speco (

http://www.stoneprocess.com)

 

[hello2006]

hi Speco,

Thanks very much for your reply! What is meaning "keep the fan tip speed within reason"? I am just not sure about the word "reason"?

I would like to use a bit smaller tip speed of the motor if I can. But my client want me to confirm the cooling ability of the fan equal or is higher than the original one. Therefore, I am worrying if I use direct drive coupling and smaller motor, Can I still get the enough cooling ability?

hope to hear more from you!

Thanks!

 

[djack77494]

hello,
I don't know what your client wants you to confirm. If you do not change the fan's speed, and you did not make any other significant mechanical changes, then the air flowrate will be unchanged. With constant air flow, I see no reason why the "cooling ability" of the fan would change. However, it sounds to me as if you may be trying to increase the fan's speed. That would tend to increase its cooling ability. It would also increase the tip speed, which is nothing but the speed at which the end of the fan (its tip) is traveling, which may lead to excessive noise, as speco attempted to describe to you. There are other issues that you should consider, such as the ability of the bearings, etc. to operate at different speeds. The best way to deal with these problems is to contact the fan's manufacturer.

 

[hello2006]

Hello Djack77494,

thank you very much for the message!

My client wants me to make sure that the cooling ability of the fan system can be the same as before or a little bit higher is no problem too, after My make some modification for this fan cooling system.

My client currently is using belt driven fan. Many maintenance work have been done for the belt, but it still fails quite often. So they want me to change it to direct drive coupling.

The original belt driven fan has a faster speed than motor and the sheave ratio is not 1:1. The fan is a little bit faster than motor. I am thinking that the direct drive coupling can only make the speed of fan the same as the speed of the motor and the speed of fan decides the cooling ability of it. The more fast, the more cooling ability I believe. That is the reason that I am thinking to choose a motor that has faster speed than the original one. I am not sure if my thinking is correct or not?

I will definitly try to contact the fan company and pursue some suggestion from them. But the company was too old, and it seems hard for me to find them now.

Personally, from what I learn from here, I do not want to increase the speed of the fan. But I just hope to confirm if I have another way to satisfy my client cooling requirement without increasing fan speed.

By the way, is there difference between the fan speed and the tip speed of the fan?

Many Many thanks!

 

[kenvlach]

hello2006,
Is your belt-driven fan similar to this one?

http://www.grainger.com/Grainger/wwg/itemDetailsRender.shtml?ItemId=1611631661

If you want direct-drive:

http://www.grainger.com/Grainger/wwg/itemDetailsRender.shtml?ItemId=1611768949

If you try to adapt a belt-driven to direct drive, you need to engineer a frame for the motor that can withstand all the vibration, the motor must have correct bearings for the full end thrust of the fan, and the motor should be TEFC if exposed to weather. Also, you shouldn't spin a fan faster than original design unless you have engineering details because bad things happen -- a little vibration becomes major, curvature of the blades tends to flatten out...
Fan speed is RPM, fan tip speed is the velocity at the blade outermost tip. If 48" diameter fan at 830 RPM, tip speed is 4 [π] x 830 = 10,430 feet/min. Pretty fast if there is a flaw in the old fan or aligned wrong!

Considering you have an old fan, probably best to buy an assembled, direct-drive fan. It will come with specifications, as shown for the Grainger fans, like CFM for different back pressures.

 

[speco]

Hello2006,

By "within reason" I meant a fan tip speed around 12,000 ft/min or less. Generally, smaller fans can run a bit higher without making too much noise. As suggested earlier, you need to rely on the fan manufacturer to provide a direct drive fan that will meet your needs. They can usually provide noise data for a a given application.

Here's a definition of tip speed, in case you need it:

Tip speed = Fan diameter X Pi X RPM

For example, a 3 ft. diameter fan at 1800 rpm has a tip speed of 16,965 ft/minute. This would be a pretty noisy fan, and I would not recommend using such a design.

Since your cooling rate is apparently related to the amount of air delivered by the fan, you will need to let the fan manufacturer know the required air volume and its associated static pressure to overcome. Normally the static pressure would be approximately proportional to the square of the air volume/minute.

Regards,

Speco

 

[hello2006]

centrifugal fan, clockwise downblast, double width, double inlet, airfoil type, class III, 24” diameter. The proformance is 18000 SCFM, 8”SP, OV = 2900 FPM, BHP=27.45, RPM = 1985


That is the specific of my fan. If you have any warning that you think I should be careful for this fan, please point those out.

I appreciate a lot for so many people's help and the organizer of this forum!

 

[speco]

Hello2006,

Everything I said early applies to axial flow fans. Your fan application is centrifugal, which is very different and operates at a much higher static pressure than an axial fan is capable of. Sorry for the confusion.

Regards,

Speco

 

[hello2006]

Hi Everybody,

would you please say something about bearing? What is the difference between bearings which are used for belt driven and bearings which are used for direct drive? Can they inter-changable or not?

Where is the bearing located in the fan? I do not the reason why the bearing used for belt driven can not be upgraded or used to direct drive for the fan?

thanks very much!

 

[kenvlach]

hello2006,
The shaft is mounted in the bearings. Start in the very center & move outward along the centerline. But, I don't really know much about blowers (high-pressure fans); have always taken them to a professional electric motor rebuildiing shop when needed bearings. It's perhaps not possible to alter the bearing type or geometry, and the manufacturer already furnished suitable quality bearings. Maybe a pressfit not easy to do yourself, anyway.
Your problems maybe due to poor fan installation & maintainance.

Re bearings difference between belt drive & direct drive. In direct drive, one end of shaft is supported by the motor & its bearings. It is even possible for small blowers to be fully supported by the motor & not have any bearings although unlikely in this case. There is also a huge difference between an axial fan (lots of end thrust!) and a blower (lots of torque). And, more heat generated by blowers. Advise that you locate some engineering drawings from the manufacturer or at least a matching style. Two places to look:

HRAI - The Heating, Refrigeration and Air Conditioning Institute of Canada. Links to manufacturers:

http://www.hrai.ca/site/manufacturers/member_companies.php

Air Movement and Control Association International, Inc.
Certified products search (by company name an/or product type):

http://www.amca.org/licenses/search.aspx

Good luck.