Does Fan Efficiency Vary When The Number Of Turns Varies?

The manufacturer will be the best source of information on this topic. The name and telephone number will be derived from a label thereon, the installation instructions, the commissioning test records, etc.; none of those can be seen from here.

Base on my Simpson's instinct, no, it is yes.

As the fan is consuming only 4% of the power at the new speed compared to the old, why does efficiency at the new speed matter?

because, most of the time, the fan will work at the new condition: so it is important for me to know the power consumption in typical conditions. But I still want a fan which can work at high number of turns for some not-everyday conditions

The fan needs around 34kW on the shaft for the higher speed and around 1.4kW on the shaft for the lower speed.

So why does efficiency matter at the lower speed, if the motor selected is big enough to deliver 34kW on the shaft at the higher speed?

because, most of the time, the fan will work at the new condition: so it is important for me to know the power consumption in typical conditions. But I still want a fan which can work at high number of turns for some not-everyday conditions

You're assuming system resistance varies as Q². It most likely does, but if there is some static the speed and power at the lower flow will be higher.

My guess is the change of efficiency with speed is likely to be small, but will vary from one fan to another. The one with high efficiency at full speed is likely to be highest at turndown also. And (as PWSlack said a couple of times) power at turndown is low, and you're stuck with it anyway, so why worry.

Check the formulas for the efficiency.

Maybe this helps.

Here's an online calculator:

http://www.freecalc.com/fansfram.htm

Here's some more information:

http://www.slideshare.net/anurajthakkar/fans-and-blowers

There may be exceptions, but generally speaking, motors operating at well less than rated load or speed are not as efficient as at rated load/speed. At low loads/speeds, some nearly constant losses predominate more.

The question is about fan efficiency, but it's a good point. Change in motor efficiency is likely to affect wire power more than fan efficiency.

Thinking about that again - if the load is low, say 10% of rated, at normal speed, the efficiency is well down. But if the Hz and speed are also lower, maybe efficiency doesn't suffer as much. Don't know if anybody can confirm.

That's assuming speed change is via an inverter, not by changing the drive ratio, OP didn't say.

Fan output power is airflow x delta pressure. Efficiency is output power divided by input power. A fan with completely restricted airflow has no output power. A fan with completely unrestricted airflow also has no output power. Somewhere in between these two extremes is the maximum output power and maximum efficiency on the curve of pressure versus airflow.

Ref: http://www.nmbtc.com/pdf/engineering/Fan%20Efficiency%20-%20An%20Increasingly%20Important%20Selection%20Criteria.pdf

The system supplied by the fan likewise has a load curve, back pressure versus airflow. Typically, back pressure is proportional to the square of air flow. At the point where the curves cross, the fan produces the pressure needed to sustain that airflow. A properly designed system would arrange for this operating point to be close to the peak efficiency.

OK, this is the optimum operating point for a set RPM. What happens when we change RPMs? Airflow is proportional to RPM and pressure is proportional to the square of RPM.

I have got a fan curve only at a given number of turns, let's say for 1440RPM.
Now I would like to use the fan laws to see how my fan curve changes at different number of turns:

The fan curves would be compressed leftward by factor (500/1440) and downward by factor (500/1440)² .

can I assume that for the point 1 (n1=500RPM, q1=11.8m3/s, dp1=88Pa), the fan efficiency is still 0.74?

I think that the answer is yes. The formula for efficiency is:

Eff = (airflow)x(pressure)/(Input Power)

Input Power is motor torque x motor rpm

Motor torque is proportional to pressure, airflow is proportional to rpm so (airflow x pressure) is proportional to motor rpm x motor torque.

In overall system efficiency, you still have to consider motor efficiency at different rpm.

I still think a bit of care is needed, as the usual fan laws only apply when the system resistance is all friction, and varies as Q². That's likely to be the case here, but for blowers in eg sewage aeration the system curve might start at 0.4bar (4m deep tank) at zero flow, and go to say 0.5bar at maximum flow. It doesn't take much speed reduction for the closed-valve pressure to fall below the static, and flow stop altogether. Also if the flow is through a porous medium the pressure drop varies as Q, not Q².