# Re: Heating coils revisited

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On Thu, 6 Jul 2000, Aquatic Plants Digest wrote:

> Lift tubes are, what, 1" in diameter?  Assuming that is correct, each
> lift tube
> has an area of about 5 sq. cm. Assume 4 tubes in an 85 gallon aquarium
> 20 sq. cm. total area. To get 110 cu cm per minute (leaping again), you
> need a
> flow speed in the tubes of about 5 cm per minute. That would seem pretty
> fast to
> be generated by a 1cm head, as mentioned above.

Back when I was into quantifying every detail of one of my tanks, I
measured the discharge from it's air lift tubes and the head difference
between the open water and the base of the substrate which was plain,
uniform aquarium gravel.  The air lift generated a head difference of
about 1 cm and the flow rate through two tubes was (by my swiss cheese
memory) about 250 gallons per hour.  I was even able to build a digital
simulation of those conditions and match the simulation to my
measurements.  Geeky.

Conditions will vary, but 1 cm head difference seems like way more than
enough to push water down the lift tubes at 5 cm/min.

> Whether or not my example makes sense, the flow down the tubes is faster than
> the flow in the substrate by the ratio of the total tube area to the total
> substrate area.

This is true,but there is one change you need to make here and maybe a
conversion that needs patching up.  Not all of the substrate area actually
allows water to pass.  In groundwater literature it is usually assumed
that the ratio between the area that permits flow and the total area is
somewhat smaller than the porosity of the substrate, which is the ratio
between the volume of water in the substrate to the total volume of the
substrate.  Probably the typical substrate porosity is about 35%; from
that it would be reasonable to estimate that about 25% of the surface of
the substrate would transmit water.  These are rough estimates, but they
are fairly typical for natural sand/gravel deposits.

A substrate with a total area of 6700 square cm would provide about 1675
square cm that water could flow through and a movement of 1 cm/hour would
translate the discharge of 1675 cc/hour.  If the lift tubes have a total
area of 20 square cm then the downward flow would be at a rate of 1.4
cm/min.

Good to hear from you again, George.

Roger Miller

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