BI and BC Impellers

Please check the performance characteristics of both fans supplied by the manufacturer. Generally backward curved fans of sheet metal construction have an efficiency up to 80% and with aerofoil section it can be even up to 90%. For BC fans volume delivered remains fairly constant. Change in system resistance doe not significantly affect volume and hence the efficiency. BI fans are energy efficient. B.C fans to run at higher speed to develop more pressure. It is good practice to refer the manufacturer's performance curve for more details and decide accordingly

BI and BC:

Which ever, if you want to keep the same extraction volume but with an increased back pressure, you will need an increase in motor power.

If you keep the same BC fan, the increase in back pressure required to overcome(from 650 to 775) will require a motor of approx 1.3 times (30% increase) in power if you want to keep the same volume extraction.

I used the affinity laws therefore, it is an approximation but gives some fair idea of the power required.

If you keep the same BC fan, the increase in back pressure required to overcome(from 650 to 775) will require a motor of approx 1.3 times (30% increase) in power if you want to keep the same volume extraction."

Is this correct? I thought that for the same fan and system that the Power varies as the pressure ratio, which in this case would be 775/650 = 1.19, not 1.3 or is this old memory mistaken?

from the afinity laws, powe is proportional to pressure to the power 1.5

i want to move from 650mmH2O to 775mmH2O by moving from BI to BC centrifugal fan of similar dimensions and the same motor power. Is it possible?

From the affinity laws, Pressure change is proportional to the square of speed change, Then Power Change is proportional to the cube of speed change.

- Obtain the speed at which you might achieve the pressure required, then use it to get the power required to achieve it. By resolving the equations, the speeds will be eliminated leaving a simple ratio of 1.3 times the original power.

Keep in mind that the conditions are for the same flow. If the flow is reduced because of the increased pressure, the final power will be less. After all these laws must be applied with caution. They give you a rough idea as a guide.

On the question of whether you can keep the same motor by changing from BI to BC with the same dimensions for the fan and have the pressure increase by the amount you wanted: I do not think that this will be possible because you need more power to keep the same volume and increase the pressure. The difference in efficiency will not cover the difference of 30%.

All the above is not taking in consideration whether the existing motor is over sized enough to still output the necessary power. You need to run and check how much the motor is loaded at present... and see how you can increase the speed even with a BC bladed fan. ( you can have a slightly bigger diameter fan with the same speed and use the affinity laws for same speed but different diametres... this will give you an idea of how much bigger the diametre... Still the proof will be at the experimental stage.)

From the affinity laws, Pressure change is proportional to the square of speed change, Then Power Change is proportional to the cube of speed change.

Agree, but no-one has mentioned changing the speed. If only the pressure changes then power varies as the pressure ratio. Is this not correct? The OP then wants to achieve this pressure increase by a design change in the shape of the blading.

Lama Sinto - I suggest that you look at this very good answer from AussiBob in another thread. It will depend very much on your system curve and what happens in the volute of your specific machine whether curving the blades backward or possibly even forward would be the best solution.

AussieBob Power-User Join Date: Oct 2008 Location: Auckland, New Zealand Posts: 299 Good Answers: 24	# 17 Re: Impeller Blades 11/03/2008 10:17 AM
	Hi babumechanical, Backward and forward curved blades of centrifugal fan impellers have different performance characterisitics. If you look at the velocity vector diagrams. For the same tip velocity(Vb), they produce different air velocity (Va). Vab is air velocity relative to the blade. The effect on fan performance is shown in the different performace curves. The forward curved type, often referred to as squirrel cage blowers basically run slower than the other types of centrifugal fans and are the quietest in operation.The forward curved (and radial) impellers use higher horsepower at low resistance than other types of impellers, and the least amount of horsepower at the higher pressures and low flows. The backward curved type of centrifugal fan performance characteristic curve shows that for increasing delivery volume, they startout at a lower horsepower, rise to a peak on the horsepower curve near the point of highest efficiency on the fan performance curve and then drop off again. Hoping this may help, AussieBob
	Good Answer (Score 4)

You are correct about the intentions of the OP.

But can he achieve this? From what I have been able to gather, Backward Inclined is less efficient than Backward Curved centrifugal fan. But how much can he hope to have increased efficiency by changing the blade design for the same fan dimensions (casing, diametre etc ?

His restriction is that he wants to keep the same motor which means the same speed if direct drive (assumed from the input of the OP), and not exceed the motor power.

Therefore, he needs to know how much the efficiency was with the BI fan blade and what he can hope to have improved upon using the BC fan. All this with an increase of pressure of ~20%.

if the objective is a research project by the OP then he does not need this forum except to get to talk to an expert in fluid dynamics! Also, he needs to be versant with the subject!

The only think that I can say is that it is likely that he needs an increase of the efficiency by 20% to keep the same motor power, the same speed and change the blades to have 20% increase in pressure. If the motor and BI combination at the moment has an efficiency of 60% then with BC he needs to have 80% eff. Can this be done? Is this difference in eff exists between BI and BC fans of the same standars? I doubt it can be there.

Thank you for replying.