I wonder if someone can help me understand something i'm puzzled about.

I make industrial fans, which rotate heavy mass impellers.

Example: The power varies with the increase in volume of airflow, so on the left hand side of the fan performance curve the absorbed power will be lower than the right hand side,( btw this is constant rpm). So on the left hand curve the minimum absorbed power would on a 630mm fan size running at 1440 rpm would be 18.5 Kw.

A 630mm impeller would have Inertia of approx 1/2mr^2 of which would be approx 7 kg m^2.

Angular velocity = (1440/60) x 2pi = 150.79 radians per sec.

So RE = 1/2Iw^2 = 79581 J

I understand 1 Watt = 1 Joule/sec

So the bit i'm puzzled about

Is the rotational energy (RE) instantaneous energy? if so 79581 J /1000 would then equal 79.581 Kw if this is so then isn't this violation of energy conservation? considering the fan only needs 18.5 Kw? or should i turn 79581 J into Kw-hours? of which would be 79581/ (3.6 x 10^6) of which would equal 0.022 Kw???

Or am i totally misunderstanding something?

Just so that we're on the same page, what do you mean by "the right hand side"? Are you refferring to the right hand side of an energy balance equation?

There are two quantities to consider here
(1) The rotational inertia which determines how much energy a rotating body has for a given angular velocity.
(2) The efficiency of the fan, i.e. how much additional power must be supplied in order to give the air a given amount of power to the air. This will depend on the geomety of the impeller.

PMB
If you look at the top curve on left 1400, thats the rpm. read across to left see (Pa) at 4750 Pa, read down to volume M^3/hr = 5000, so on the left hand curve of 1400 rpm at 5000 m3/hr you will get 4750 Pa, thats the fan duty. The absorbed power lines are straight, so from 5000 m3/hr read up untill you meet the line, then read across left to Kw, it will be 18 Kw.

The impeller inertia is 7 Kg m^2

the efficiency will be 4750 x (5000/3600) / 18000 = 0.37 x 100 = 37%

Thanks.