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# Re: Under-tank heating coils.

```On Monday, 5 June 1995, George Booth wrote:

> I was reading through a thermodynamics book a few months ago trying to
> find some way to calculate how much flow would result from how much
> heat (this is a very difficult problem - way beyond my capability) but
> I did find that below a certain heat density, heat will travel by
> CONDUCTION in a liquid and will not produce movement.  Above that
> threshold and below another, CONVECTION will occur and produce a
> laminar flow.  Above the second threshold, turbulent flow will begin,
> something like boiling.

Awesome!  My thermodynamics text says exactly the same thing.  Unfortunately,
due to some oversight, the author neglects to mention what exactly will
happen in the substrate of a planted aquarium for a given amount of heat
applied in a specific manner.

> IMHO, coils can produce enough localized heating to generate a
> CONVECTION flow around them.  To heat the entire bottom surface enough
> to create the same amount of heat density would take a lot of heat.
>
> So it's not a matter of linear vs. whatever, it's how much heat
> density can be generated.

Strictly conjecture on your part, right?

> Assuming a Dupla 100w coil is 1/4" dia. by 22 ft long, it has about
> 200 sq in of surface area, or 0.5 watts per square in.  A 75 gallon
> tank (48"x18") has about 860 sq in of bottom area which would require
> 430 watts to produce the same heat density as the coil.  This may tend
> to overheat the tank.

This is true if in fact "heat density" is the appropriate metric,
something that hasn't been established yet.
--
Kevin Conlin   kcconlin at cae_ca   "We're Canadians.  We HAVE to be polite"

```