Inlet and outlet of Meridional Velocity on radial impeller design 1

[kflee0]

I have two pump books telling two different story about Cm1 and Cm2. First version said that Cm2 is equal or slightly less than Cm1. Is this statement correct? I highly suspect that this statement is true.

Second book said Cm2=Km(2gh)^0.5, if I use this formula, my Cm2 is much much less than Cm1, which is contradict to the statement of the first book,So which one is correct?

How does one calculate Cm1? Formula Cm1= Volume Flow rate/ Area of eye impeller. Is the area of impeller, which is D1 has to be predetermined before finding Cm1 assumed that shaft diameter is known?

Once you find your inlet angle, let say 20 degree, outlet angle= inlet angle + 9 according to the book. How does one know whether one should add nine, or ten or twelve in order to find the outlet angle. Does this make any different to the performance? Is there any books or graph can be referred saying certain Ns, the inlet and outlet angle should fall between certain value?

These question might sound stupid to some of you, but I really apprecaite someone can explain and answer my question above. Thanks

 

[vanstoja]

The two books you cite are probably referring to different methods of design which may be individually consistent but not comparable for specific design parameters. The method employing suction and discharge capacity constants Km1 and Km2 is the specific speed impeller design approach originally developed by Stepanoff. These constants are generally plotted versus specific speed for an inlet blade angle Beta 1 of 22.5 degrees which is ideal for best efficiency according to Stepanoff. The inlet blade angle can be varied about plus/minus 5 degrees, ie, between 17.5 and 27.5 degrees for better cavitation performance or better flow range. An example of Km amd cm values for a 22.5 degreee inlet angle at a specific speed of 2240 (RPM-GPM-Ft.) is Km1=0.19, Km2=0.14, cm1=24.09 fps, cm2=17.75 fps. In this case cm2 is substantially less than cm1. The impeller discharge angle Beta 2 depends on number of blades Z and blade channel discharge width, b2, and must be
calculated. An example for the 2240 specific speed impeller gives for Z=7, b2=2.0 to 2.4 in., Beta 2=18 to 24 degrees.

 

[kflee0]

Vanstoja, thank you so much for your explanation. So,
beta 2 does not have to be necessarily larger than beta1?

Would you mind explain how beta 2 depends on number of blazes and b2 ?? it interest me coz I remember reading this somewhere," the BEP and Q can be maintained by increasing b2 where number of vanes and beta2 are reduced." it seems like this 3 features are interrelated like you said.

However, according to the formula, b2=144*Q/(Cm2*PI*D2), here I don't see how b2 can have an effect on Z and beta2 or vice versa. Thanks in advance

 

[vanstoja]

Beta 2 and b2 are actually independent variables in the iterative design process employing Stepanoff's method. The example I cited calculated design parameters for 7 impeller blades with b2 in 0.2 inch increments from 1.8 to 2.4 inches and Beta 2 in 1 degree increments from 18 to 23 degrees. Number of blades was the nearest integer to Z=Beta 2/3.