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Re: filter flow rates

> Date: Tue, 28 Dec 1999 12:12:37 -0500 (est)
> From: "Roger S. Miller" <rgrmill at rt66_com>
> Subject: Re: filter flow rates
> On Tue, 28 Dec 1999, Tom Bates wrote:
> > There are a number of factors to take into consideration[...snip]
> > Here is the formula to compute average turnover rate. Average turnover
> > is the amount of time, in hours, it will take for 99% of the water in a
> > given aquarium to pass through a given filter. This formula takes into
> > consideration the decreasing flow rates created by the filter media as
> > becomes clogged.
> >
> > T(ave) = 12.75 x G divided by F
> >
> > T(ave) is turnover average
> > 12.75 is a purity coefficient (a constant factor [given])
> > G is the net gallons of the aquarium
> > F is the manufacturers rated flow of the filter

On Dec. 28, 1999, Roger Miller wrote:

> Sorry, but I have to disagree.  The filter flow rate divided by tank
> volume is a good qualitative measure for filter sizing.
> The average turnover rate is (under condition of complete mixing in the
> tank) exactly the filter flow rate divided by the tank volume.

The filter flow rate as advertised by the manufacturer is a rating of
maximum flow with no media installed. As restriction is applied (clean, new
filter media) this flow rate is lowered. As the filter media becomes clogged
to the point that the filter is in need of servicing, this flow is now
severely compromised.

The filter flow rate divided by the tank volume will only be a viable
outcome if the filtered water was not returned to the unfiltered water. By
mixing this water together, even if completely mixed, the filter is
constantly taking in a combination of filtered and unfiltered water.

> This
> follows from rather intuitive logic and it can be shown with much more
> rigorous (and pointless) math.  Some parts of the tank volume will cycle
> through the filter more often than given by this ratio, and some parts
> will cycle through less often; the average is given by the flow
> rate/volume ratio.  I don't see how throwing a coefficient into the
> formula adds to the usefulness of the ratio.

The purity coefficient takes into effect the gradual decrease of the flow
due to the filters increasing restriction as the media becomes clogged. By
inserting this factor into the formula we can arrive at an average turnover
time (throughout the media's lifespan). Granted the times will be shorter
when the media is new and longer when the media needs replaced (or
serviced). This purity coefficient also leads to the implication that 99% of
the water is now in a filtered state.

> Roger Miller

Tom Bates

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