<blockquote><cite>Posted By: SteamyTea</cite>Are fans limited in the same way as turbines? S have a max of a nats under 60%, put a restriction in the way and it drops rapidly.</blockquote>

Not really, the Betz limit doesn't apply. I used to play around with hovercraft years ago and we aimed to run lift fans at around 65%, more if we could get it. It's fairly easy to get a fan to operate at around 80% or more under fixed flow conditions, but efficiency falls off quite rapidly if the conditions change away from optimum.

Bigger is better, no question. A big fan turning slowly is more efficient than a small fan turning quickly. The same goes for propellers.

Right now I'm finishing a small light boat for the Cordless Canoe Challenge, a race for boats powered by cordless tools (see here for last years event: http://www.youtube.com/watch?v=t82OL0YObw4). Much testing, development and experimentation has shown that I can get a measured prop efficiency of about 88%, by using a large diameter, narrow blade, prop turning quite slowly. In comparison, a typical outboard prop will be around 60% efficient.

The same rules apply to props as to fans, but fans have the added advantage of running in a duct, so they have lower tip losses (much of the wasted power on a prop comes from vortices that spin off the tips).

For good efficiency a fan needs to have a low pressure ratio for the required flow. The pressure ratio is just the dynamic pressure difference between the inlet and outlet. If you increase the area swept by the fan, then for a given volume flow rate the velocity drops. As dynamic pressure is proportional to the square of velocity, this results in a significant pressure ratio change.

The other key factor is to get the flow resistance of the heat exchanger down to as low a figure as possible, as this then reduces the pressure ratio further.

The Betz limit only applies to extraction of power from an airflow, as it is based on the fundamental principle that a 100% efficient device would result in no air flow from the outlet (because all the energy in the incoming air had been extracted). Clearly this cannot ever happen, so Betz calculated what he thought was the most you could get (and it's arguable whether he got it right).

A fan can still operate with no inlet airflow, or no outlet airflow, although efficiency drops to zero for this case, so Betz doesn't apply. Just as well, really, as the fans that propel the majority of big aircraft run at around 80% or more efficiency, way over the Betz limit.

I cannot wait to retire and spend my time doing pointless things that are fun. Beats working for a living Eh.

Hedonist.

Posted By: Viking House0.432W/l/s,

So 0.43 J.l^-1

First DuckDuckGo find for “air filter pressure drop” is:

http://ateam.lbl.gov/Design-Guide/DGHtm/filterpressuredrop.htm

which indicates not more than 0.2" water gauge for a good (i.e., low pressure drop) HEPA filter at what sounds like sensible operating conditions.

32' water gauge is about 100 kPa so that's 100e3 / (32 x 12 x 5) = 52 Pa.

Work done in pushing some fluid through something is force x distance = pressure x area x distance = pressure x volume. So the extra SPF required for a good filter would be 52 J/m³ or 0.052 J/l if you insist or 0.052 W/(l/s) if you really insist.

If that calculation's right (beware: done while half way through a second can of beer) then, yes, you are being fed a line though that line might be a result of not-so-low-pressure-drop filtering either due to not-so-good filter material or restricted surface area - I guess.

The DS90 is quoted as having a G3 + F7 filter. No idea what that actually means!

On further searching I found this diagram which goes some way to explaining it. (http://www.aimfiltration.com/products_syn_air.asp). It does seem to me to be a limitation of the testing. I have seen somewhere that the F5 filter for the Vent-Axia is 'optional' so presumable not fitted for testing?

They are almost certainly on the supply side with G3 followed by F7. there is just a G3 on the exhaust.

The point of the discussion though, is the effect different filters will have on the power required to drive/pull the air. If you have better filtration, the fan will have to work harder - the question is by how much. This then distorts the SPF on the SAPQ results as it apparently does not take this into account making the data very suspect - unless someone can tell me otherwise.

On the flipside, the SPF is then a function of how good the filter is as that seems to vary widely.

How else can they do it?

I suppose my point is that a manufacturer might try to justify a higher fan power for their unit by claiming that this is due to better filtration, but if they can't produce clear evidence that that level of filtration has any tangible benefits with regard to pollen allergies then why take the financial hit?

Exactly my point. If for instance all units were tested with just a G3 filter then it would be a level playing field. At the very least there should be a statement of configuration in terms of filters and anything else that may affect the performance.