Back to back Axial fans, angle of blade problem

[Ralph2]

Hello
Our company was asked to disassemble, clean, balance individual components, replace bearings and reassemble 2 axial fans. The fans, used as part of a hospital ventilation system, are about 24" OD with 10 blades. Mounted in a round duct with the fans about 14" apart. The inlet (stage 1) turns CCW and the second stage turns CW and both at 3540 RPM. The individual fan blades are clamped between a 2 part hub and the angle of the blade is set at the time of assembly.

My question, what angle should the blades should be set at.

The inlet had the blades set at 34~35 degrees and the second stage at 24~25. Intuitively, to me this is wrong. I wonder if at some point in the past the fans were mixed up and placed on the wrong motor. My feeling is that for the second fan to be able to impart any energy into the air flow the angle of the blades must be higher than that of the first fan. In trying to rationalize the current setup I wonder if the increased density of the air the second fan sees is enough to warrant the lower angle. Or......

Does anyone have any experience or thoughts on this issue. The heating and ventilating company that we are working for claims to have no information and suggest replacing as found. The ultimate solution would be to monitor the load on the individual motors and adjust the angle on both until the optimum draw was reached. But, this would likely change my balance as the clamping arrangement of the hub is not likely to be very consistent.

Thankd for any thoughts or advice

 

[btrueblood]

It makes sense to me that the 2nd stage fans run at a shallower angle - they are "seeing" a signficant amount of "swirl" flow from the first fan, thus the approach flow (relative to 2nd stage fan blades) is already at a higher angle of attack.

Now, another problem for the owner of this facility - is why the fans are the same blade number? The common blade number is going to lead to resonance and lots of noise, bearing wear, etc. The fans should have been spec'd with prime number blades (say 11 and 13) to avoid resonances.

My $.02 worth.

 

[friartuck]

The blade angles sound about right

As btrueblood states, the second set already have swirl from the first set. The general rule of thumb is that when you have two fans in series but rotating in the same direction, (and with a reasonable 'etoile' straightener, the net result is a doubling of the single fan pressure...BUT if you have contra-rotating fans as you have, the net result is a 2.4x increase in pressure assuming you have a decent fan manufacturers fan set.

Some manufacturers don't know what they are doing and don't do proper tests on their gear. For an accurate assessment, fo to either NuAire Ltd or Flakt Woods. Both have extensive fan testing facilities and do actual fan tests without extrapolating results or performing inaccurate mathematical modelling.



Friar Tuck of Sherwood

 

[btrueblood]

"...(they) do actual fan tests without extrapolating results or performing inaccurate mathematical modelling."

Friar Tuck of Sherwood

--- Friar Tuck, you mean there is such a thing as ACCURATE mathematical modelling??!!

 

[friartuck]

I used to work for a major fan manufacturer (worldwide sales and manufacturing facilities). You would be surprised at how many so called 'reputable' manufacturers massage their fan curves. I have known of units tested in garages before. It is also a common practice for manufacturers to out source their impellers. Truflo for instance make impellers for whoever wants to buy them and put them in a case.
You would think that taking an impeller and putting it in a cylindrical case would make little to the fan performance..but with Axials for instance, if you dont get the right blade clearance or have poor mounting positions for the motor (which interfere with the airflow), you can get quite variable results. Also the stall point can vary enormously.



Friar Tuck of Sherwood