Improving sand blast booth extractor fans

[Strostkovy]

About a year and a half ago I built a sand blasting booth on a budget out of two shipping containers. I has three 36" fans that spin at 500 rpm and do a great job. They are rated to 750 rpm at 1.5hp each.

Unfortunately, they are failing for a variety of reason. One motor arrived damaged and we never fixed it so we've only had two fans running. The v belts have been constant issues with fast wear, and the set screw pulleys do not stay secured to the shaft. We've been running one fan due to a seized flange bearing, and a bearing on the last remaining fan is sounding bad.

It's about that time to fix this up. I want to replace the cheap single phase TEFC motors with three phase motors on a VFD. The speed control will be lovely as we can slow them down for using the small blaster or during heavy rain or speed them up when blasting something that produces a lot of dust, or if we add hardware cloth to the grill if dust outside becomes an issue. As of now we use aluminum oxide and the dust going outside presents us no problems, and does not linger in the air long before it falls. The acceleration limit of a VFD will also help belt wear, as frequent starts and stops on the very heavy fans will not allow the belts to slip in sandy conditions nearly as much.

The cooling space is not ideal for the motors, as I thought the motors were shorter when I ordered them. I'm going to oversize the new motors a bit for the environment and VFD. At 750 RPM the 1.5HP cheap single phase motors were overheating. I'm going to use a 2hp motor with a pulley reduction to get it down to 650 rpm max.

Set screws will be replaced with taper bushings. I've learned from other repairs I've had to make on some ovens and spray booths that set screws just aren't great, even with a snug fit and keyway.

As far as the pulley and belt wear goes, I think a large issue is due to sand in the groove while the belt slips, causing a lot of wear. I also feel like 1.5hp is pushing it for 1/2" belt with such a small drive pulley and long drive distance. I'm considering using a timing belt to eliminate that slip, and also a much wider belt. Otherwise I can use a serpentine style belt.

Anybody have any input on fixing this up?

 

[deltap]

Fans designed for say kitchen exhaust do not have belts, bearings, or motor in the air stream, so they would not be damaged by the grit. Would it be possible to use guarding on a prop fan to force air away from these wear components? A TEFC motor would solve one problem.

 

[Strostkovy]

Fans designed for say kitchen exhaust do not have belts, bearings, or motor in the air stream, so they would not be damaged by the grit. Would it be possible to use guarding on a prop fan to force air away from these wear components? A TEFC motor would solve one problem.

For the size of the fan I think the shaft would have to hang out prohibitively far to keep bearings out of the airflow. The existing shroud is also not well setup for that unfortunately. Pillow block bearings with air purge would be awesome but those seem hard to find or incredibly expensive. The motors are actually surviving fine other than being extremely low quality.

EDIT: totally valid suggestions though. If I were to build this again (hopefully I don't have to) I would make the shroud 45 degrees from the fans to fit motors out of the airflow.

 

[DDoug]

I thought you had proper tubaxial fans, where the belt is enclosed.
tubeaxial fan at DuckDuckGo

Are these behind a simple baffle to keep most of the buckshot off them ?

 

[Strostkovy]

I thought you had proper tubaxial fans, where the belt is enclosed.
tubeaxial fan at DuckDuckGo

Are these behind a simple baffle to keep most of the buckshot off them ?

All three fans share a rectangular housing

 

[millwrong]

Well, if you bought me a beer,I would tell you to go to larger pulley diameters. If you look up what you've got; I'm figuring an A section drive, you're maxed. Larger arc of contact increases hp transmission capabilities substantially.And is the drive pulley one of those POS adjustable, set-screwed,crap?

 

[DDoug]

All three fans share a rectangular housing

I can clearly see that in the pix...

 

[crossthread]

I would at least make up some sheet metal or even wood shrouds for the belt and pulley. This is one nasty ass environment. In addition to a shroud for the belt and pulleys, I would add a brush wiper to the inside of the shroud to clean any grit from the belt before it contacts the pulley. Cut a slot in the shroud so you can slide the brush out when it wears out.

 

[gustafson]

honestly I don't know how you make anything last in that environment.

It is probable the you are 'supposed' to have an outgoing filter element[not judging just saying] and while that would add maintenance it would certainly help belt/bearing/motor life

 

[GregSY]

Honestly...you're using a half-assed layout which is why people who are serious about doing a job go to better laid out system which generally cost more money.

Running open belts in a heavy sand environment is never gonna work well for long. Just ask the Saudis.

A proper layout would use TEFC purpose-designed motors and probably direct drive (no belts).

You can make do with what you have, just accept there will be higher maintenance costs.

 

[Strostkovy]

Honestly...you're using a half-assed layout which is why people who are serious about doing a job go to better laid out system which generally cost more money.

Running open belts in a heavy sand environment is never gonna work well for long. Just ask the Saudis.

A proper layout would use TEFC purpose-designed motors and probably direct drive (no belts).

You can make do with what you have, just accept there will be higher maintenance costs.

After making this I had to deal with redesigning belt driven fans for both our oven and our spray booth, neither of which I made. I have learned belts kind of suck, and switched to direct drive with great results.

I was hoping to put the fans directly on a motor shaft this go around, but the low rpm made it impractical.

 

[Strostkovy]

honestly I don't know how you make anything last in that environment.

It is probable the you are 'supposed' to have an outgoing filter element[not judging just saying] and while that would add maintenance it would certainly help belt/bearing/motor life

Maintenance costs aren't terrible at around $1000 annually (most of the is the blast equipment itself. The sand destroys valves and pipes and hoses and nozzles and lights and switched and filters, etc)

 

[Strostkovy]

It looks like my best option is to replace the bad motors with decent ones (both are TEFC, the first were just garbage), and double both pulley sizes and use tapered bushings.

I'll make up some nice belt and bearing shrouds, and use some bristle strip to pack the openings of the shrouds to reduce dust.

Seems like the whole fix can be done for under $1000 and should cover most of our issues.

 

[Flail]

Curious, is there a reason you can’t blow clean air in and duct bad air out?

 

[Strostkovy]

Curious, is there a reason you can’t blow clean air in and duct bad air out?

I think it would kick up a lot more dust and be bad for visibility. We also blast with the access door open, which would be bad in a pressurized system

 

[DDoug]

How much compressed air doo you have available ?

 

[GregSY]

I actually don't think the sand will create belt failures on a grand scale. I think the sand does have a negative effect, for sure, but a good V-Belt will digest lots of sand and still give a decent service life. That's especially the case given how cheap belts can be. If you enclose the belts, make sure the sand that does get in has somewhere to go....won't be good to have the belts running buried in sand.

There are direct drive fans sold....of course, you'd have to buy the fan/motor from the same source as a packaged, 'matched' unit.

 

[B1 SCOTT]

Needs a big squirrel cage fan so the grit doesn't go near the belts or motor. I built a 40 x 100 x 16 tall blast booth for a waste processing facility 20 years ago. It had a very large squirrel cage.

 

[Strostkovy]

How much compressed air doo you have available ?

40hp rotary screw compressor. But we use as much of that as possible for blasting.

 

[Strostkovy]

I actually don't think the sand will create belt failures on a grand scale. I think the sand does have a negative effect, for sure, but a good V-Belt will digest lots of sand and still give a decent service life. That's especially the case given how cheap belts can be. If you enclose the belts, make sure the sand that does get in has somewhere to go....won't be good to have the belts running buried in sand.

There are direct drive fans sold....of course, you'd have to buy the fan/motor from the same source as a packaged, 'matched' unit.

I think the tiny pulleys and large center distance are the biggest killer with excessive slip, and the sand accelerating the slip induced wear. Definitely leaving the shroud open at the bottom