Fan Design

Ask the designer.

What designer? It's a homework problem.

If ever there were one!

Sorry did not get pls reply if u got the answer Iam from HVAC

How's the weather in HVAC?

Check any reputed fan manufactures technical literature. You can also refer any good academic books on fan & blowers or refer ASHRAE handbook

Get a copy of the fan affinity laws and review them carefully. These will guide you in the adjustments needed for changes in temperature and air density (altitude).

Once you understand the laws, the affects of changes will be straightforward to calculate.

Homework or not, this is an interesting question. Fan data I've seen is flow vs pressure rise, with inlet at atmospheric pressure. Do the fan affinity laws give more information?

Take a fan delivering a certain flow to a system which has ΔP as Q² (likely to be simplest case) and ΔP say 45 mbar to take OP's figure (low enough for compression to be negligible), the whole lot being at sea level.

If it's then taken up to where the pressure is 0.5 atm, what happens? My guess is the actual flow stays the same, ΔP falls to 1/2 (same velocity, density halved) and power drawn falls to 1/2 (same flow, ΔP halved).

What do you (and others) think?

Its been 40 years since I was in school. But I think you are getting close to understanding this. If in Codemaster's case pressure is halved, what happens to the mass flow of the fan while the volume delivered remains the same?

Is the pressure drop a result of volume flow, or mass flow, or velocity?

Thinking about this a bit more (prompted by your comments), halving the inlet pressure (and hence air density) is equivalent to reducing the liquid SG in a centrifugal pump. In that case, as is well known, if it's pumping into an all-friction, Q² system, the generated head in m liquid stays the same, so the pressure rise halves, and the system pressure drop also halves. That seems to support my guess.

To answer your question 1 - the mass flow halves (same flow, 1/2 density)

Is the pressure drop a result of volume flow, or mass flow, or velocity? Either volume flow or mass flow, velocity, and also density come into it. Whether you use actual volume flow, or mass flow (which is equivalent to standard or normal volume flow) density is still in the formula.