How do I determine the losses in an elbow?

As a rule-of-thumb, the pressure loss at an elbow in a piping system is generally reckoned to be equivalent to 90 diameters of straight pipe.

Any slow air stream movement is very sensitive to duct changes. If you reduce the cross sectional area, you increase the air speed and increase the pressure drop, thus increasing the pressure on the machine which is moving the air. If you increase the pressure on the apparatus (fan) you may well choke it to the point where it will stall completely and move no air at all. Check your minimum air quantity requirements, measure the existing air quantity and total static pressure. Then recalculate the new velocity and corresponding pressure increase, caused by your alteration to the duct, then compare with the fan performance curve. If you have trouble with these calculations refer to ASHRAE, where you will find the actual calculations.

Good luckand

Important considerations for doing any changes already were pointed out by Philiplyon. Before doing anything please make you familar with the flow patterns your arrangement probably will have: unfortunately the pressure drop (resistance) of an ellbow can vary a lot depending on ellbows geometry (ratio inner diameter to ellbow radius) AND the flows Reynolds number! Saying "flow pattern" not only means the investigation if the flow is laminar or turbulent, it also means the pattern of steaming lines when changing a flows direction. You may have a look to bionic systems: natural (=resistance minimised) flows never go in a shape of a circular ellbow! Changing the flow direction the flow pattern becomes asymmetrical across the pipe section, with sections of "dead water" before and in the trailing sections of the bow (or behind the bow). This I think is the reason for mentioned fall-out.

Your idea with a "false bottom" in the horizontal section in front of the bow probably goes into right direction. It would be perfect if the false bottom would eliminate all sections of "dead water" or local turbulences. This not only will require a false bottom before entering the ellbow, but also a similar modification to the inner race of the trailing ellbow region (down into the continuation pipe). With both modifications the arrangement changes to a kind of "soft ellbow venturi".

For the flow resistance of a venturi and an ellbow I must refer you to the literature but I guess, you won't find a calculation model for an "ellbow venturi". For the guys running the digital flow pattern simulations (CFD) this matter would be very easy to solve (just an etude for learning modeling). It might be valuable to go in contact with an university (a student can do it) or a company who are running those CFD software.

Here is a handy calculator to determine the friction loss http://www.freecalc.com/ductloss.htm

Good Luck

If you're using air flow to transport fine material, you need to know the recommended air velocity. (for example, paper scrap is approximately 5000 FPM.) Once you know the minimum transport velocity of the material, the duct system needs to be designed to maintain this velocity. The turn you are describing will generate high and low velocity air streams that could cause the fallout you're describing.

You could put an evase transition ahead and after a reduced size elbow to generate the higher velocity through the transition. I think this would minimize your overall pressure drop for the fan.

In my experience.....ground limestone dust.... your scheme will only change the point/location of the drop....not remove it...you might try turbulation by inserting a twisted ...one and a half twists... linear fin section in about 1.5 to two diameters of pipe immediately ahead of the elbow,,,will depend on material weight/velocity/particulate size factor.

This moves the air inside of the el flow to the outside... in rotation like a tornado...producing a sort of scrubbing action and equalizing the linear travel distance of the air flow through the el section.

MR. GUY

For a fine powder, I will guess your idea offers a lower pressure drop. I give you a "good answer" vote.

By the way, what is the maaterial in question from the original poster?

The material is wood fiber at about 15% moisture content. The fiber is introduced through two airlocks into the duct about 80 feet prior to this elbow. There was thought about adding turning vanes or something into the elbow to improve the flow. The problem is the probability that there will be buidlup on the leading edge of the vanes. If this buildup becomes too great and breaks off into the air stream we will have defects in our product.

Acccording to Table 24.1 in my eighth edition "Fan Engineering" handbook (1983), conveying air velocity for sawdust is 3000fpm, shavings is 3500fpm and paper is 5000fpm.

A 48" dia duct has about 12 sq. ft. area, so the fan should probably be moving around 40,000 to 45,000 CFM. Has anyone verified either the air velocity or CFM?

Another idea might be to use compressed air nozzles around the perimeter of the duct set at an angle to swirl the air flow. This might eliminate a sticking point that vanes would introduce.

Instead of 90 degree elbow install 2 45 degree bends, it will cut the static pressure in half ______________ | | _______ _______| / / ____________/