Axial Fan efficiency vs Centrifugal fans efficiency

Can you show your calculations?

Do not have performance curves to easily see the efficiency points?

I have calculated efficiency with following formula: Efficiency = Pressure (Pa) x Flow (m3/s) / 102 x Power (kw)

Yes, there is a mistake with regard to the efficiency of the axial flow fan, unless the operating point is so far away from BEP that it would not work anyway. Axial fans are not so inefficient. I would guess that the mistake is more to do with the input figures than the calculation per se. The two duties you quote are substantially different.

When you say "I can achieve following parameters" does this mean within the same system, or that such fans are available or what?

Dear, I didnt understand your point. First, is there any other formula for calculation of efficiency of axial flow fan. Second: Input figures are 100% perfect, I have achieved this point from characteristic curves. Third: Also, I didnt got how two duties are substantially different. Request to give the solution. Regards, Keyur Shah

Substantially different; 10 Pa discharge to 100 Pa discharge. If in the same system how can the flow be the same?

Request you give the data.

keyur

Your calculations are OK but don't necessarily tell the whole story. You need to compare the best efficiency point for the 2 fans. Preferably check on several models of each type to give an overall picture. Also smaller machines tend to be less efficient as fixed losses are more significant etc.

Are you sure your measurements are accurate? 10Pa = 1mm w.g. and is not easy to read.

Cheers..........Codey

1 m.

Sorry mate, you were of course correct (1 mm) - sooooo glad I was not signed on.

Dear, I am 100% sure because I got this details from curves. And many of our clients are using these fans for condenser cooling. Regards, Keyur Shah

Unless I am not understanding this thing, your units are wrong. As Codey pointed out 10 Pa is just over 1 mm water column. This you will not be able to measure with any accuracy.

You have given the discharge pressure but you do not give the suction pressure. If this is atmospheric pressure then your efficiency has so many zeros in front of it as to be meaningless.

The power required to raise the pressure by 10 Pa is negligible and you probably would not notice the difference between this and running the blower under no load.

Also as stated before these blowers appear to run in two different systems as the system resistance is an order of magnitude different.

Your calculation appears to be very simplistic and would be just about OK for incompressible fluids.

For Polytropic efficiency when compressing gases think in terms of Eff pol = n/(n-1) * (k-1)/k

Your calculations are correct and everything is OK but the efficiency .

Axial flow fans are typically supposed to run at around 50-60% efficiency under rated condition. On either side there will be sharp drop in efficiency.

Can you check up the rated cfm of the fan vis a vis the cfm demanded by the system ?

And axial flow fans are supposed to run at very low Pa so this Pa is OK for it (Consider the Axial flow fan at the ceiling , or the propeller or the exhausters - they are not supposed to generate much Pa)

Just a quick link however very less details, but does compare various types

http://www.engineeringtoolbox.com/fan-types-d_142.html

sb,

With all due respect I believe that his calculations are not correct.

If you plug the numbers given into the calculator for pumps on the same site that you quote http://www.engineeringtoolbox.com/pumps-power-d_505.html you will see that it will not even give an answer to the power until you reduce the efficiency to some very low number.

I used 3000 m3/hr, assumed normal conditions so a density of 1.204 kg/m3 and a head of 1 mm.

If I have this totally wrong I would really appreciate learning where I am going wrong. Would also help the OP.

Dear, Can you give me your email ID. I will send curves for both Axial and CF Fan to you. Regard, Keyur Shah

Hello again keyur

Not quite sure what you're saying. Are your figures based on measurement or taken from curves?

I've just checked a couple of suppliers' data for axial fans on the web and they only give curves of flow vs pressure, not efficiency or power.

Your power calcs are OK, at these sorts of pressure rise the error caused by ignoring compressibility is negligible.

Also you need to be sure whether your pressures are static or total. 10 Pa dynamic pressure relates to an air velocity ~ 4 m/s, and fan discharge could be much higher than that. Some suppliers quote total discharge pressure, and if system requirement is calculated on static a well-designed transition piece (gentle taper) is needed to recover pressure.

Cheers........Codey

Dear, This figures are taken from curves. And pressure mentioned by me is static pressure only. Regards, Keyur Shah

I am trying to determine how to measure Axial Fan Efficiency and Centrifugal Fan Efficiency. I just posted a detail comment about centrifugal Fan Efficiency measurement and calculations. see

http://cr4.globalspec.com/thread/31413/Centrifugal-Air-Compressor-Efficiency-Calculation#comment727488

Once I determine how to measure enthalpy, will this method and formulas be applicable to measuring Axial Fan Efficiency as well as Centrifugal Fan Efficiency?