Rangehoods

nofam:

Bung: I'm not doubting that removing the motor from the hood to the end of the flue gets a lot of the noise out of the kitchen but some of the Schweigen claims don't obey physical laws.

"The highly-efficient German IsoDrive Motor uses all it’s energy for extraction rather than pushing air up against gravity. This lets the rangehood concentrate all of its power on its primary purpose of removing large volumes of cooking steam, smoke, grease and odours from your home silently!"

As if pulling the air up the pipe is easier than pushing it.

I found that a lot of the noise in a conventional rangehood was air movement in the thin wall ducting and insulating the duct inside the outer cover made a big improvement.

Yeah, that claim reeks of marketing hyperbole, but given that cooking vapour is heavier than the air it's normally displacing, I'd think pulling that vapour up is nominally more efficient. The main thing is that you're not constrained by the size of the rangehood in terms of the motor's physical dimensions or noise, so you can go a bit larger that you otherwise could.

Schweigen clearly design for one main benefit: reducing the noise by placing the fan at the exterior outlet rather than at or near the interior inlet. The issue of vapour density is nowhere near as important as other factors:

The most efficient fans will usually be those with the largest diameter. This is the easiest spec to check because we can eyeball it.
Cheap fans tend to be of comparatively small diameter operating at higher speeds so they produce a lot of noise.
The quietest fans will usually be those with the slowest tip speed. Go see a wind turbine to see the effect of high tip speeds. But this is difficult to check this in a rangehood.
Operation in ducting ie. if your fan is meant to be ducted then don't place it at an inlet because performance will be observably reduced. Typically, installation instructions say that a fan be placed at least 4-5 duct lengths from the inlet or any bend in the inflow duct. And sometimes instructions say the same for distances from outlets.

Axial fans are usually rated as pushers (motor upstream so pushing on the impeller shaft) or tractors (motor downstream so pulling on the impeller shaft) so it isn't as simple as moving a fan from the inlet to the outlet or vice versa. There's also the motor position to consider. In general, fans perform more quietly and have a longer duty life in the tractor position than the pusher position.

The tractor configuration is more efficient mainly because of unimpeded inflows. It is kind of obvious if we think about non-turbulent flows allowing a fan to work evenly around the entire circumference of the fan. We can also see the preference for unimpeded inflows in aircraft design where tractor propellers are vastly more common than pusher propellers. The minimum efficiency difference in aviation is said to be a few percent but practical impacts can be 10-20%.

Turbulent flows create localised pressure changes which increase vibration and therefore noise and ultimately reduce the duty life of the fan. Ultimately the extremes of pressure result in two main failures: fans will stall (stop operating) at high enough pressure; and fans surge at lower air pressures, e.g. at higher altitude fans often have to be derated (power reduced below their maximum capability) to preserve their duty life.

nofam

Hammerer

MadEngineer