Simple Fan Engineering Question

I don't have your answer, but I feel it's necessary to mention, to make darned sure you are using an explosion proof motor to run that fan.

Dust can do some pretty impressive/destructive stuff.

Yes, we are aware of that. However, sawmills don't normally use explosion proof gear and I have not heard of a dust explosion in the type of sawmills that I work at in my 30-odd years in the game. With the proliferation of fuelwood pellet mills coming on line, I have heard of several explosions and fires in their facet of the business.

Sawmills currently use lots of dust and materials handling fans and blowers.

All I can attribute the difference is that pellet mills dry their material far more than sawmills and the wood is ground quite fine. I don't think we reach the LEL of wood dust concentrations in sawmills that pellet mills may encounter.

I'm hoping this question doesn't get hijacked by the safety aspect. All I can say is that we are well aware of it.

Jon.

If it's simple, why are you asking CR4?

As a rough estimate, I make fan pressure rise ~ 20 mbar, but you need to do detailed calcs using full layout details. Eg what do you mean by x "degrees of bends"? Flow ~ 4.8 m³/s, and input power 24 kW, assuming efficiency 40% (fans are not very efficient).

But it sounds like it's an existing fan. Does it have a motor? You still need to find what rpm to run it at to get the required performance. If possible, as a test you could fit a generously-sized motor fed from an inverter to find the right speed and power, and hence the drive ratio for a fixed-speed motor.

There's plenty of data on the internet, with Q/P curves at a range of rpm, but I doubt whether it gives details like no. of blades and their dimensions.

I strongly do doubt, that anybody has a useful curve for simply an old fan. But you can proceed two different ways:

You look up a motor / fan package, accept its diameter and splice it in with sheetmetal funnels. The maker may recommend funnel angles. With funnels, larger diameter fans move markedly larger volume, that translates to higher airspeed for the same volume moved in the smaller, existing pipe.

You stay with the present unknown paddles. You beg, borrow or buy a used motor of a (few) kilowatt. If three phase, their speed may differ depending on the wiring, so does their power consumption. Single phase is fixed. Mount it on a readiustable plate, and try out two or three sets of pulleys with belt, if needed. In your environment I would consider cogbelt (the one customarily is used as drive belt in sawmills). It costs more, comes sized for your need with cog pulleys. When the motor is loaded 60-75% of its plate value, you are allright. Observe the result, and modify as needed right. You need a clamp-on ammeter to measure the incoming current for every phase, to know when you are in the ballpark. Every good hardware , electrical or plumbing store carries them.

For a loose comparison, a force air heated larger home uses 1 -1,5 kWatts motor for circulation. When choked, it produces respectable suction.

Ideally you would use a fan curve from the manufacturer of the fan. If there is a name on it, look them up on-line. Otherwise use a reputable manufacturer that puts their fan curves on the internet. Look up a fan with the same basic dimensions and find your spot on the curve for it. This should be pretty close to the right information - at least close enough for road work. Kice is one manufacturer of many of fans, and cyclones that also has a very good engineering manual that they make available to customers. They would also be able to provide you with accessories for the system.

My thoughts are that your desired air velocity is a little high. This might be enough speed that you will sand-blast out the pipes over time with your sawdust. However, I don't think we need to get overly concerned with that at this point - this is just a design point, anyway.

Assuming the 65oo LFM, you will want about 10,600 or 10,700 cfm. An 8' cyclone will work out just fine, it appears that you just need about an 84"er, and bigger will only be more efficient (requiring less vacuum to do the job).

I am purely guessing here without pipe flow loss charts, but your cyclone will need about 6 or 8 inches of vacuum, and your system should probably survive with 10 to 14" wc. Your fan will therefore need to produce about 20", which a material moving fan should. With the 10,700 at 20", you should be able to determine the speed and horsepower for the fan, and you will be in good shape for a used equipment system and a first whack at the problem. Slide gates at the pick-ups and at the fan discharge should allow you to tune the system to work adequately to do what you want.

Good luck and be sure you either discharge the mess outside, or put a filter on it. You will fill the shop and everyone's lungs with dust otherwise.

Hi, Frank Designs,

I think you have got the gist of my layout and questions. I have a good used fan that has a 50 hp motor on it, and your suggestion of 10,000+CFM is the same as I estimated. I found a couple of companies that provided fan data for wheels pretty close to the OD that my fan has...it seems like there are some pretty standard radial blade fan sizes out there that just about everyone uses. My fan wheel has an OD of about 27" so it fits just between two standard fanwheel sizes. To get the high air velocity I'm looking for suggests at the 20" of WC is right at the top end of what the fan can do. It also looks like it will pull pretty much all of 50 hp--maybe a tad more.

Can you suggest where I can find tables of piping loss numbers and things like sizing a cyclone to the flow rate? You seem to have that at your fingertips--you suggested that my cyclone would be a certain pressure drop and might even be a little large. Once I learn to calculate the system pressure loss I should be able to get a lot closer to self-sufficiency in handling these designs.

My proposed system has the fan in the dirty side of the airflow--situated between the pickup points and the cyclone. This is common practise in the mills here in Western Canada. Probably less piping. The cyclone vents to atmosphere outside of the building and the dust, hopefully, will exit the bottom of the cyclone through an airlock feeder. I don't expect or hope to pick up much actual sawdust--I want to get the water vapor/oil mist and fine dust out of the sawbox, leaving the heavier sawdust to fall out the bottom of the machine to be conveyed away via a chain conveyor.

Another question--does it matter much just where the fan is placed into the system? It seems that quite a few systems tend to put the fan inlet suction onto the cyclone discharge to allow the use of more efficient fan wheel types. I always thought that one should put the fan as close as possible to the pickup points, to minimize the vacuum in the pipes and try to keep most of the system under a positive pressure.

Thanks for your suggestions!

Jon.

Try the chart shown here:

http://www.captiveaire.com/MANUALS/AIRSYSTEMDESIGN/DESIGNAIRSYSTEMS.HTM?catId=306&Model=GREASE+DUCT#Types_of_Pressure_Losses

There is also a ton on information on the design of systems. It is a little hard to find duct information online for a system designed to move stuff, and mine is all at the office. I mentioned Kice in the last note, and I like their engineering manual very much. If you are going to do this a lot, you may wish to talk your way into getting one of these for yourself. If you order from them, all you have to do is ask, and they will also help you educate yourself.

Figuring the pressure loss in the duct will likely buy you the reduction in suction you need, or just run with it and see what happens. The cyclone, if you already have it, is a bit too large, but it will just need less suction to work as you wish - probably down to 4" or possibly less... To know for sure you will need to look at manufacturer information - I was going off of the Kice information for the CK series in 8' dia.

Since you already have the 50 hp motor, go ahead and us it to start with, and set up the speed accordingly. If you don't have a VFD for it, buy one for a 75 HP motor, so you don't have to step up if the motor you have indeed ends up being small. A new/used motor is cheaper than a new fan.

The fan you described is OK to go anywhere in the product stream, however, the sawdust will slowly "sand blast" the blades away over time, so it is always best to put the fan after the cyclone. Many do it the other way (as you describe), so it is not necessarily wrong, but it is better after the cyclone (and filter) if possible. The other advantage is that if your duct inside the building is under vacuum pressure, a leak is not catestrophic. Leaks may hurt your suction ability, but it will remain clean and will still work. If the duct is under pressure and it leaks, you will have sawdust raining inside your building. Duct tape over a leak will pull the seal tighter, rather than blowing out away from the duct and creating a channel for the leak to continue.

If your budget is tight, I think you have everything you need, and can run with it in confidence. Tweaks to overcome obstacles will not be big or difficult, and may never rear their ugly heads.

Jon, go to the ACGIH site and order this manual, " Industrial Ventilation a Manual of Recommended Practice" This is the bible on ventilation design, it will help you in your quest.

This question is interesting and seems to have intelligent, competent input. The only suggestion I might have is that the fan blades are a possible area of modification to fine tune the air flow; though this should be done only if you have someone on site competent to do this. It sounds like fun. Running a test with a tiny or slow fan and collecting data that could be scaled up is another thought.