Altitude Effect for a Centrifugal Fan/Blower

Quote: "Is the mass flow proportional to the density decrease?"

I would surely think so! The volume of air moved/second may remain the same, but the mass moved/second must be lower at lower densities. Your blower will not really run with the same load - think about the blower's 'load' in a vacuum...

You are correct in assuming that the mass flow falls of as the density reduces. This led to the development of turbochargers & superchargers for aircraft engines and, of course, for vehicle engines operating at altitude where black smoke is a problem for diesel engines.

From this early beginning the modern exhaust gas turbocharger with variable geometry turbines have developed giving huge benefits in economy and low NOX.

I think that at reduced inlet pressure (and constant temperature)

• inlet flow q₁ and ouput flow q₂ (measured at actual conditions) stay constant
• ΔP across the orifice is proportional to (absolute) inlet pressure P₁
• air mass flow is proportional to P₁
• power drawn is proportional to P₁ (varies as q₁*ΔP which is proportional to P₁ as q₁ is constant and ΔP is proportional to P₁)

It's common to control the output of a centrifugal fan/blower by means of a throttling valve on the suction (to reduce the inlet pressure at the impeller) which also reduces the power drawn. You would never throttle the inlet of a centrifugal pump handling liquid.

In such situations with Turbos on internal combustion motors (and with certain types of fan driven by electric motors), as the air atmospheric pressure reduces with altitude, the load on the impeller blades is also reduced. This allows the impeller to spin faster and compress more, but of a lower pressure, air. End result is that the motor gets about the same amount of air come what may.... This is why Turbo engines in cars do not suffer from significant power loss with altitude (upto a certain point at least!). Whereas a motor that has to 'suck' its air in has problems in this area as it gets less and less air to mix with its fuel...On older cars consumption increases. Talk to people in Colorado that have a standard east coast set up carburretor car (without Turbo) that try to drive up Pike's Peak! For those who have a Turbo, it flattens out the power curve over a great deal of air pressure differences (within reason of course). In Greece many years ago I was towing a caravan with a Turbo diesel 2.3L 80BHP motor up a mountain, a 3 liter (normally aspirated)Mercedes shot by at a low altitude, I overtook him about an hour later, he was down to about 20MPH max speed (the 5 passengers did not help him either.......). Now I shot by with no problems. (he overtook me again much later on the flat, low down on the other side of the same range of mountains!!!)