Any fan designers out there.?

[howardm]

a bit of a long shot, I know. I have been pricing backward inclined centrifugal fans for dust extraction. Boy, are they expensive. I was wondering if anybody had built these and knew about the dynamics and basic parameters for setting out the vanes. I have traced some pictures off the internet, but it's just not very scientific.

I am informed I am looking for 1000cfm at 5" static pressure max.

Thx
Howard

 

[Gareth J]

Ah, I've forgotten it all!

My knowledge was based around datacentre air handling units but I don't have any helpful info I'm afraid.

What I would say is that liebert were pretty good with publishing .PDF data and try googling for "EC fans" I know it refers to the motors but in these air handling units they are nearly always connected to one of those fans.

If you can find a fan that has a fan curve that suits your application, you maybe, just maybe might be able to then find a technical drawing of that fan and copy it.

Please post some build pics if you're going to make one, the fabrication involved in making a one off will be spectacular, especially keeping it balanced

 

[brightspark]

balancing a fan is quite easy .a small weight atached to one blade then run it if its still unballanced just move it around till it runs smoothly .u may have to add more weight or remove some till its correct .i get large sweep ceiling fans to ballance and just use a clip on weight

 

[chunkolini]

This is an amazing forum.
Somebody asks an impossible question, gets an answer from a specialist who has forgotten it all inside half an hour.
Then gets a knowledgeable bodge 30 minutes later.

 

[Rudi McAnichal]

Lots of info about fan types, ductwork design, etc in “Woods practical guide to fan engineering”, a standard textbook that you should be able to find in any good technical library. Woods of Colchester, and their associate company Keith Blackman, were major manufacturers of fans.

There was also a Keith Blackman text with details about all types of centrifugal fans - I think I still have a copy somewhere, and will have a look tomorrow.

Backward-inclined fans are quite commonly used in grain driers, especially the Aberdeenshire drive-on batch drier design, so you might find a secondhand one at a farm sale. Clark & Sutherland (Kingswells) have decades of experience in that type of installation.

 

[Rudi McAnichal]

I should have added that where there are heavy dust concentrations, paddle blade fans are often used in preference to the backward-inclined type (but paddle blades need more power).

 

[Maker]

But paddle blades need more power.

Aren't they louder than backwards curved too?

 

[bricol]

Be careful - a ceiling fan is a whole different kettle of wobbly fish to a centrifugal fan. A competitor company in the same business I was decided a proper fans were too expensive and how difficult could it be . . . after they'd rescued one impellor from the other side of a new hole in a brick wall, they replaced all four fans on their machine with a known manufacturer.

It's not only static and dynamic balance - get the housing wrong, or infeed/discharge, and you'll get problems with odd blades stalling and throwing the impellor out of balance. In my current work, a ducting "expert" contracted by the customer managed to get it wrong - two split fan casings later . .

I've used plug fans in housings of my design before, but only due to space rather than cost.

Bri

 

[howardm]

Ok thanks for the info guys, especially the lead to a text book. Yes a paddle fan is more robust, but needs 5hp to drive it. On domestic mains I am restricted to 3hp .! It's just an experiment/prototype I am building just now and it will be outside, in a sturdy box.! If you go on u tube there are videos of folk making fans from mdf which are used for dust collection, downstream of the cyclone.
I will have a look see at these folk in kings wells also, but everything up here is at gold plated price.
Thx
Howard

 

[Gareth J]

Prototype.... Air cooled beetle fan? Cheeeeeeap

 

[Rudi McAnichal]

A more recent book is “Fans and Ventilation, A Practical Guide” by W T W (Bill) Cory, published by Elsevier, 2005. It will probably tell you more than you ever wanted to know about fans, and seems to include everything that was in the earlier "Woods practical guide to fan engineering" and much more. The author was boss at Woods for many years.

[Edited - link to download website deleted, possible copyright infringement.]

Keith Blackman Centrifugal Fan Guide (2nd edition, 1981) contains much of the same information. Bill Cory sent me a copy about 30 years ago, in response to an enquiry about an old KB fan. My copy says price £10, but there is no ISBN number, so it may never have been widely available. From a quick look at the rather complicated charts, I think that 1000cfm and 5”swg (0.47m3/s and 0.625kPa) could be obtained using a KB286 backward inclined fan, 400mm diameter and 200mm wide, running at 1500rpm, 75% efficiency. Many more options are possible, but I just looked at 1500rpm because a direct drive fan with 4-pole motor is often a sensible choice (3000rpm fans can be more compact, but very noisy).

 

[Robotstar5]

A fan tale: we used big centrifugal fans to pull the air out of the foundry to wash the dust out of it, 8 foot diameter and about 10" deep, ran on 6" diameter shaft driven by 250hp motor. One evening a balance weight came off and it sounded like the end of the world as the fan wobbled enough to bend the drive shaft which then locked in it's bearings and stalled the motor, before the motor circuit breaker could react it had burnt out the first 18" of supply cable

Downside was we had to repair it while the adjacent fan was still running throwing black dust laden water everywhere (pre-HSE days) we ended up looking like black & white minstrels

 

[tomsutton529]

One of the most sketchy fans I have had to balance was at an incinerator plant, the fan NDE bearing had failed taking most of the fan with it and the housing, after being rebuilt and welded up we ran the fan and found 55mm/s of unbalance in the fan. Using some experience and large dose of good luck I got put the trial weight (adding the trial weight you want to see a 30% change in either force or angle, sometimes 30% of the force in the wrong direction is bad news) used for the calculating the correction weight in the right place and reduced the unbalance on the first run. Think i got that one down to 0.6mm/s of unbalance and it was asking for a correction weight of 2g..

If the OP does make a fan then get it in a workshop to be balanced, there are multiple types of unbalance and simply moving the weight round will not solve it. and dont just give it a spin and think that will be fine.. any idea how big this fan is going to be?

 

[Rudi McAnichal]

any idea how big this fan is going to be?

Just in case anybody failed to spot the deliberate error in my last post, 5” water gauge static pressure is of course 1.25kPa. The fan I had mentioned would be too small, and you would probably need one 560mm diameter and 224mm wide, if running at 1500rpm (76% efficiency). That is assuming the backward-inclined fan can handle the dust load.

On domestic mains I am restricted to 3hp

Starting currents for centrifugal fans can be quite high, and they may take several seconds to run up to full speed. It might be difficult to start a 3hp fan from a domestic mains supply, but probably some “soft start” experts on the forum can advise.

Have the air flow and static pressure been calculated fairly accurately, or is there a chance to reduce these?

It is usually best to fit hoods very close to where the dust is being produced, which allows you to have high air velocity to capture the dust, without high airflow. Smaller airflow means cheaper fans, ducting, cyclones and filters, combined with lower power requirement and less noise.

If in doubt, it may be wise to specify a belt-driven fan, which allows the shaft speed to be changed fairly easily and cheaply. Air flow is proportional to speed, and static pressure is proportional to the square of the speed.

 

[daleyd]

I'm certainly no design expert when it comes to fans but in my previous job we used to use a huge variety of fans and having seen the destruction caused when one throws a blade I would suggest thinking carefully about manufacturing your own.

I'm not advising against it, just throwing my tuppence into the ring as something to think about! I'll dig out some pics of some damage.


Good luck!

 

[tomsutton529]

a fan of those dimensions, if it is sensitive (usually a flimsy backplate) will spend most of its life out of balance. Sod all you can do it apart from keep it clean. More than likely it will require a two plane balance to correct any imperfections.

 

[howardm]

Anyone interested in dust collection should see the "oracle" just search for bill pentz.
I have found a helpful fan designer/manufacturer, so with a bit of luck the impeller won't damage the wallet too much.
More later,
Thanks guys