Pneumatic Conveying

Pneumatic Conveying

Have you ever heard of a fan being used to carry unwanted chicken parts (we’ll leave it at that without more explanation)? 

Many people only think that fans are good for moving air.  But there are many applications where fans are used to convey material.  Some of the more common applications are dust collection, wood working, grain transport, paper and cardboard trim handling.  The purpose of a conveying system is to move material from one place to another.  The beauty of using air is that it can turn corners and change elevations with relative ease.  Many materials that are granular, pulverized, or crushed can be successfully conveyed using air.

All of these applications have one thing in common – they handle a mixture of air and material.  As such, selection of material handling fans can get quite complicated.  There are four basic concepts to consider.  The first is the blade type required to successfully handle the material in question.  The second is the effect of the material on the system calculations, and the third is the effect of the material on the power requirements of the fan itself.  Finally, you need to consider if there is a spark potential created by moving the material.

There are many types and shapes of fan blades.  When looking at pneumatic conveying, you have to consider characteristics of the material being moved, as well as the ability of the fan to resist the effect of the material on the blades.  Some materials, like concrete dust, are very abrasive, and the fan blades and housing need to be made of a material that resists abrasion.  Many fans are made entirely of AR400, or have components made of some other extremely hard material.  Some of these materials do not lend themselves to forming or welding, so they are usually bolted in place.  There are even ceramic based materials that get used which are pieced together to line a fan housing (something like the Space Shuttle tiles). 

Other considerations are the stickiness of the material, the size of the particles, or the size of the strip of paper being picked up.  There are special wheel designs that minimize collection of material on the blades, and some designs attempt to use sharpened cutter bars bolted to the blades.  Our experience is that cutter bars are a maintenance headache, and it is better to design the wheel so that the material doesn’t clump up in the first place.  The Hartzell trim handling wheel is designed to keep the material moving through the wheel without hanging up on the leading edge of the blades.  In case a large amount of trim material hits the fan wheel at one time, we place axial bars around the inlet adapter, which tear the material off the wheel.  Of course, all material handling fans work better if the material has an even load per minute.

In any conveying system, energy is needed to pick up and suspend the material, accelerate it into the duct, overcome friction, and vertically lift it when necessary.  You have to consider the increased friction in the ductwork and elbows.  Designing these systems is more of an art than strict science, and some trial and error should be expected.  The scope of this article doesn’t lend itself to much discussion about how the material gets introduced into the duct system or fan, but there are several methods.  These can be a gate valve that periodically opens, a screw feed or rotary valve, or a suction venture.  We will limit this discussion to any system that provides a smooth introduction of material.

In order to calculate the effect of the material on the pressure drop calculations of the system, we need to first calculate the loading factor (R), which the weight of the material moved per minute divided by the weight of the air moved per minute.  As a rule of thumb, industrial type centrifugal fans can be used up to about R = 2.  Each material will have a transport velocity, which will keep the material moving and suspended, and minimize drop-out that will plug the system.  The conveying velocities are a function of the bulk weight of the material, the size of the particles, and the velocity and amount of air being used.  These factors can be looked up in several reference books.  For our example, we will consider a system conveying 600 pounds per minute of dried wheat, which has a transport velocity of 5800 fpm, and a friction factor of 0.4 when sliding on steel.

With a trial duct size and amount of material to be handled, we will calculate the pressure drop.  For this example, we’ll use 50 feet of horizontal duct, an 10 foot vertical rise, and two elbows.  The various formulas to calculate the effect of the material can be reduced to the following:

Accelerating the material (Am) = R * velocity pressure change

Horizontal runs (Hm) = friction factor (f) * R * length of duct in feet /69.2

Vertical lifts (Vm) = R * lift in feet / 69.2

Elbows (Em) = 3.14 * f * R * velocity pressure at the elbow.

                Note – the elbows should have a radius of at least 5 or 6 diameters for conveying.

Let’s try a couple system calculations.  First, we’ll try a 8” duct.  For a material loading factor of 2, the amount of air required would be 300 pounds per minute:

                600 lbs./min. of wheat / 2 (the material load factor) = 300 lbs./min. of air

At standard density of 0.075 lbs./cu. ft., the amount of air required is:

                300 lbs./min. of air / 0.075 = 4000 cfm

In an 8” pipe, that results in a duct velocity of 11,500 fpm, which is about twice the required transport velocity.  So, we need to select a larger duct size.  A 11” cut will provide 6040 fpm, and the velocity pressure will be 2.29”.

Once you calculate the duct resistance of the air alone (which we won’t do here), then you calculate the various factors resulting from the material, as noted in the equations above.

                Am = 2 * 2.29 = 4.58

                Hm = 0.4 * 2 * 50 / 69.2 = 0.58”

                Vm = 2 * 10 / 69.2 = 0.3”

                Em = 3.14 * 0.4 * 2 * 2.29 = 5.75”

The total additional pressure drop for the material is then 17” (don’t forget that there were two elbows).  From this example, you can see that moving material using air creates pretty high pressure drops, so care in designing the system is paramount. 

Thirdly, the material load increases the power required by the fan.  The effect is approximated by multiplying the power by the effective density divided by the air density.  The effective density is the total weight of the mixture divided by the volume.  Since dried wheat weighs 48 lbs./cu.ft., the volume of wheat is 12.5 cu.ft./min., so the total volume is 4012.5 cu.ft./min.  The total weight of the mixture is 900 lbs./min., so the effective density is 0.225.  If the selected fan horsepower is 10 bhp, then the actual power draw will be:

                Total Power = 10 * 0.225 / 0.075 = 30 bhp

The last item to address is whether the material will create an explosive atmosphere.  In this case, dried wheat can create explosive dust, so care should be taken to ground the system thoroughly.

Pneumatic conveying is very common.  Don’t let the numbers in this example scare you.  This is a high loading factor of a very heavy material, requiring a high transport velocity.  The next time you get asked about this application, you will know a bit about what is involved in designing a system.  Go find a new application for fans.  Aren’t you curious about what chicken parts were successfully conveyed?