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Pulse Jet Pointers
Vol. 8, No. 1

About Fans for Dust Collectors

We design dust collectors to any pressure or vacuum desired by the customer or as dictated by the job conditions. The fans are placed upstream or downstream as desired.

If the fan must see the dust (upstream), then it is usually a paddle wheel design, more expensive and less efficient than a centrifugal fan with a backwardly inclined wheel. The backwardly inclined fan however must have relatively clean air, such as the filtered air downstream of the dust collector.

The usual design for a rectangular dust collector is plus or minus 17" water gauge (0.6 psig). For a cylindrical dust collector the usual design is plus or minus 231" water gauge (8.25 psig). Rectangular dust collectors are less expensive and so are used unless higher pressures or vacuums are required, such as for central vacuum cleaning or pneumatic conveying.

If the dust is explosive, the design of relief devices often requires that the dust collector housing be capable of higher pressures. Thus, explosive applications often require cylindrical housings.

Most dust collectors have pressure losses of 4" to 10" static pressure water gauge, abbreviated SPWG. External system losses also range from 4" to 10", so most dust collector fans are required to be in the range of 8" to 20". This usually means a standard class paddle wheel fan (class 22) or a class three backwardly inclined centrifugal. So, 90% of the jobs will use one or the other of these fans, and again, based on whether the fan is located upstream (paddle wheel) or downstream (backwardly inclined) .

The other 10% of the jobs (central vacuum cleaning or pneumatic conveying) will use turbine type fans or positive displacement blowers. These air movers are very sensitive to dust in the air stream and so are almost always on the clean (downstream) side of the dust collector.


About now, you are wondering, how do these fans work?

Air is a fluid, and just as water, seeks its own pressure level. But air, unlike water, is a compressible fluid. So it stretches and shrinks like a rubber band. If you have ever tried to start a siphon by sucking on a tube, you have learned this first hand.

Putting air to work usually means putting it in a tube of some sort, like a sheet metal duct or even in a whole building where it is introduced into one window or door and goes out another.

Natural draft, or the wind if you will, occurs when vast areas are invaded by low or high pressure areas such as you see on the evening TV weather pictures. But to work our own will on the wind, we must confine the air in our own duct system and subject it to an air mover - a fan of some sort.

When energy is applied to the shaft of a centrifugal fan wheel, causing the paddles to rotate, air "particles" are thrown with centrifugal force into the fan housing scroll and on to the outlet which we have conveniently located somewhere in the periphery of the fan scroll. This air leaving has been "shrunk" (compressed), going down the outlet duct at a higher than atmospheric pressure. But by leaving the fan housing it has created a void, that is, it has stretched the air near the eye of the wheel, thus sucking new air into the inlet of the fan. This new air coming into the fan is at lower than atmospheric pressure.

So now the fan and its drive motor have done work for us, turning low pressure air into high pressure air. If we are smart, we pick a fan and motor with the highest possible efficiency for the job to be done. And because there are many different types of jobs for fans to do, there are many different types of fan wheels.

Your home furnace fan wants to be quiet so you can sleep at night, but still be very efficient so your power bill is low. Seeing as home heating equipment has very little pressure drop (the ducts are relatively large for the air flow required), the best fit is a forward curved "squirrel cage" fan. But woe unto you who don't keep a clean filter in place, because the forward curved fan wheel with its many little blades will load up with dust and fail to operate efficiently.

Dust collectors have significant pressure drop, so we seldom see forwardly curved fans applied. The backwardly inclined bladed centrifugal fan, placed down stream in relatively clean air, is the usual choice. Drawing A shows a typical performance curve for such a fan.

Drawing A. Typical backwardly inclined centrifugal fan performance curve.

The CFM volume (CFM means cubic feet of air per minute) delivered by the fan is plotted on the horizontal (abscissa for you math majors). The SPWG is on the vertical (ordinate for the math majors). SPWG means static pressure water gauge. Your tire gauge, when registering your car's tires at 35 psig, is actually registering 908" SPWG! That is because 28" SPWG = 1.0 psig, so 35 psig x 28" SPWG per psig = 908" SPWG.

The SE (static efficiency), and the BHP (brake horsepower) are also plotted on the vertical/ordinate.

If the fan inlet is blocked off, the pressure reaches its highest, but notice that the BHP and SE retreat to almost zero. When the fan inlet is left wide open, the CFM reaches its highest, but again see that the SE has retreated, and the BHP has also dropped off, but just a little.

So you see, if we want to get real work out of this fan, we have to select it properly. That value is known as the "normal selection range." In most fan tables, the points of peak optimum performance are shown underlined or in darker print. Normal selection range is below these underlined points. If you select above those underlined points, the fan will likely "fall off the curve" meaning that it will stall, just as an airplane wing stalls.

But let us open another can of worms. This stretchy, shrinky, unpredictable air must flow through more than our dust collector. It is captured in tight little hoods placed on dust producing operations, made to flow through long runs of duct, with elbows and sometimes other obstructions such as dampers, grilles and sound attenuators. It is made to carry dust, moisture and other substances which often dramatically change its flow characteristics. Nevertheless, we try to predict what it will do with a "system curve," an example of which is shown in Drawing B.

Drawing B. Typical System Resistance Curve

 

With the CFM is plotted on the horizontal/abscissa and the SPWG is plotted on the vertical/ordinate, the resulting characteristic curve is most always parabolic. Try to push the volume from 1000 CFM at 1" SPWG to 2000 CFM and you have
to allow for 4" of SPWG.


Now bringing our two curves together, we get something like Drawing C below.

Drawing C. Typical Fan & System Curve.

Note that the system curve, rising to the right, crosses the fan curve lowering to the right. This is the fan's operating point. If that intercept point is not to your liking and you want to change it, you must either change the system or change the fan RPM. The system can be changed by using larger or smaller ducts or hoods. The fan RPM can be changed if you buy a few v-belt drive components. Whoops! You bought the fan that is directly driven and the ductwork is all installed. Well, that is why we plan ahead, making long and laborious calculations before buying anything.

Suppose that you had made those calculations, and are fairly certain of them, but the fan is still not performing. Check the rotation of the wheel . A centrifugal fan will still deliver air - in the inlet and out the outlet - even though rotating in the wrong direction. It will do so at about fifty percent of its normal rated volume.

Finally, if you do have a belt drive fan, or if you can change the system in some way, you can "get legal" with this situation. You see, fans have laws they live by, and these laws cannot be broken. They are called "fan laws" (you knew that). Here they are:

Changes in the volume are directly proportional to changes in Speed, so CFMnew/CFMold = RPMnew/RPMold.

Changes in pressure vary as the square of the speed, so SPWGnew/SPWGold = {RPMnew/RPMold}2.

Changes in horsepower vary as the cube of the speed, so BHPnew/BHPold = {RPMnew/RPMold}3.

There is a lot more to know about air moving, but the above should get you by. Just be sure to tell your dust collector and fan providers everything about your proposed system, before they quote, and surely before you give them the order.

Bruce A. Beckert     
Beckert & Hiester, Inc.
Dust Collectors      

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