I am being asked to compare a fan which is going obsolete with its proposed replacement. From a mechanical and electrical point of view, they are almost completely identical so I have no concerns from that point of view. However, when I look at their respective performance curves, they are somewhat different, especially in the low air flow region. This is outside of my area of competence, and although I have done some reading about it, I am not sure what to make of it. Can somebody explain to me which one is best and why (I have purposefully not put any legend)?
The yellow curve corresponds to a fan that will provide mostly a set pressure differential, say, keeping fumes from escaping a tank through elsewhere than dedicated vents. If there is a circumstance that reduces the pressure differential, it will speed up and increase the flow, to compensate.
The blue curve is a fan that doesn't expect any significant pressure differential and will provide a constant air flow, e.g. for cooling a machine or providing air circulation through a room, maintaining near-constant flow regardless of factors that may restrict the pressure - likely higher RPM would result in extra noise, so it peaks at a certain threshold and doesn't attempt to pump more air than necessary.