Fan types for wet scrubbers 1

[cokeguy]

(I originally posted this on the HVAC forum, but I believe this is a more appropriate forum, since it is a scrubber-specific problem).

Up to now, we have used radial type centrifugal fans in our scrubbers (blades extend from the hub straight out). We want to evaluate the use of backward-curved blades for our scrubber ID fans, could somebody point me to some web pages on the subject? I understand that radial fans are less prone to solids build-up, but I recently visited a plant whose scrubber uses backward-curved fans, and they are doing OK, with smaller fan motors for comparable applications.

Also, is there a rule of thumb for approximately how much less power you need on a particular application when using a backward-curved fan instead of radial fan? What about fan speeds, in general a backward would work faster or slower than a radial, for comparable sizes?

Last, any references of fan suppliers for scrubbers or other types of corrosive/abrasive environment applications?

Thanks a lot for your help.

 

[Compositepro]

You should probably consult a blower manufacturer but the basics are:

Forward curve blades will maximize the velocity of fluid leaving the fan. This allows for greater air speed, lower fan diameter, and greater presure output if velocity head is converted to pressure head. Due to turbulence it can be noisy. They collect dirt. Most common type for air.

Radial blades are simple, inexpensive and can be made very robust. They are easy to clean and shed dirt well. They are not the most aerodynamically efficient.

Backward curved blades do not generate much velocity or pressure but are quiet because they don't generate much turbulence. They require more diameter to move the same amount of air. They would be less prone to erosion due to particulates or liquid drops. Probably more energy efficient for moving air with no pressure output, like axial fans. Liquid pump impellers have backward curved blades.

 

[cokeguy]

Thanks for your reply, do you know what is the most common fan type used in wet scrubbers?

 

[rmw]

Radial tip fans are the most common fans used in wet scrubber applications. The inlet side of the blades are curved but the outlet side of the blade is radial. Sirocco type fans (also called squirrel cage) have been used but I don't like them.

Backward inclined straight blades are preferred over backward inclined curved blades. The curved blades have a concave side where solids will build up causing vibration and changing the fan characteristics. The build up isn't the problem, the sloughing off of large chunks of the build up on a random basis is.

Also, if airfoil blade shapes are used, they are made hollow and the noses erode and then solids build up (unevenly of course) inside the hollow spaces. This can be mitigated by making the nose of the airfoil shape out of round bar of a hard (very hard) material.

Airfoil shapes with flat (rather than curved) inside surfaces have been successful. Not quite as efficient as the pure 'curved both sided' airfoil shape, but better than a flat blade design.

Boy, this question cleared out a lot of cobwebs. I hope this helps.

rmw

 

[cokeguy]

Sure it helps, thanks a lot. What would you say are the main advantages/disadvantages of radial tip fans v.s. flat radial-blade extending-from-hub-type fans? What about the maintenance aspect?

Thanks for your help....

 

[rmw]

It combines the best of both worlds. The leading edge of the blade starts out like a backward inclined blade would but the trailing edge of the blade is the same as a radial bladed fan's trailing edge. You get the efficiency gains at the leading edge with the self cleaning characteristic of the radial bladed fan. It is not as good as the backwardly inclined blade, but not as bad as a purely radial bladed fan. It is a trade off design.

A backwardly inclined fan blade is a non overloading design but a radial bladed fan's power curve is wicked. It balances off the two effects.

rmw