# RE: NFC: A filter idea

```

> -----Original Message-----
> From: owner-nfc at actwin_com [mailto:owner-nfc at actwin_com]On Behalf Of D.
> Martin Moore
> Sent: Sunday, January 09, 2000 6:11 PM
> To: nfc at actwin_com
> Subject: Re: NFC: A filter idea

[snip]
> All I have in mind is replacing the existing impellor.  This is
> similar to
> the idea behind the non-cavitating propellor used by some ships.  By
> reducing cavitation you increase efficiency (and decrease noise).
> Compare the shape of the screw to they typical impellor used in
> today's OPF's - I think they must generate a LOT of friction.
>

Aquarium power filters do not generate pressures low enough to cause
cavitation.  If they did, the the plastic vanes would quickly be destroyed
by the extreme wear and tear that cavitation causes.  Cavitation will pit
and erode even the strongest steel propellers.

It is true that the swept vane design is more efficient than the straight
vanes found in most impellers.  However, the type of AC motor used in these
filters does not have a preferred direction of rotation, and it may start
rotating in either direction when the motor is plugged in.  For this kind of
motor, the straight vanes make sense.  It's not the most efficient design,
but when you're dealing with a motor that consumes less than 10 watts, other
factors can easily outweigh efficiency when designing a filter.

By the way, it's pretty easy to calculate the efficiency of a filter.  My
AquaClear 150 (rated at 6 watts) pumps water at 150 gallons/hour to a height
about 4 cm above the water level in the aquarium, before it cascades back
into the tank.  Fluid work equals pressure times flow rate, so

W = p*F
p = rho*g*h = 1000 kg/m^3 * 9.8 m/s^2 * 0.04 m = 392 Pa
F = 150 gal/hr * 3.785 L/gal * 0.001 m^3/L * 1 hr/3600 s = 0.000161 m^3/s

W = 392 Pa * 0.000161 m^3/s = 0.063 Pa m^3/s = 0.063 W.

eff. = 0.063 W/6 W = 0.011 = 1.1%

So there is some room for improvement.  Keep in mind that there are
unavoidable friction losses within the intake and filter media, so improving
the efficiency of the motor can do only so much.  And of course, you can
save only about 5+ watts.

Andrew Dalton

```

Follow-Ups:
References: