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re: Heating Coils revisited

From: Erdoz1 <erdoz1 at wt_net>
Date: Sun, 02 Jul 2000 22:18:02 -0500

Just when a post shows up that I want to answer, my home PC goes belly up! 
Go figure. Maybe my newly spun-off employer will give me a personal e-mail grace 
period while my ex-employer takes its sweet time with the warranty repairs on 
the PC it sold me.  

>Could a modified under gravel filtration plates serve the same function
>as the under gravel heating coil systems?
>
>Take an UGF plate with a slightly longer than normal return tube that
>rises about 1 cm above the normal level. Supply water into the return
>tube (from the filter outlow) so its serves as a constant overflow weir
>to impose a very slight static head.  This would result in a very small
>reverse flow through the substrate, duplicating (what I understand to be)
>the  primary benefit of heating coils - providing enrichment to the
>root system and avoiding totally anaerobic conditions by creating a very
>small small circulation pattern from the water column through the root
>system.

In theory, such a setup should provide almost all the supposed benefits of 
heating coils - minus the heat, of course. Nutrients added to the water column 
(naturally or otherwise) would be carried into the substrate.  If the substrate 
has something in it to bind the nutrients (clay-like additives, for example), 
the nutrients will be held in the proper state for plant roots to adsorb. A very 
slow flow will not bring too much O2 into the substrate, maintaining a reducing 
environment. 

However, heat IS one of the benefits of coils. Besides helping those few plants 
that like "warm roots", it also increases the rate of biological processes in 
the substrate. 

BUT (and that's a "big but" in case you didn't notice), in practice a very slow 
reverse flow may be difficult to achieve. Water, like electricity, takes the 
path of least resistance. In all likelyhood, all of the slow flow will find some 
channels in the substrate near the downlift tubes (or will make some over time) 
and simply escape via those routes, leaving most of the substrate devoid of 
circulation. In other words, it will be very difficult to generate a slow flow 
through the entire substrate. 

Futhermore, consider how much flow is needed in the downlift tubes to produce a 
slow flow in the substrate. Let's try a hypothetical example (hypothetical since 
I [nor anyone else?] don't have a clue as how much flow a proper heating coil 
setup creates). 

Let's take an 85 gallon aquarium with a footprint of 17.5" x 59.5" (such as one 
I had at one time), giving a substrate surface area of roughly 1040 square 
inches, or more conveniently, 6700 sq. cm. Let's also say we wanted a flow rate 
through the substrate of 1 cm per hour (I picked that because it doesn't sound 
like much). That is, a particular molecule of water would travel 1 cm in 1 hour 
and would travel from the bottom to the top of the substrate in around 7 hours.

I'm making a leap here so please correct me if I've run afoul of science and/or 
common sense, but that flow "speed" seems to correlate to 6700 cubic cm per hour 
or about 110 cu cm per minute.     

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 or about 
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. 

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.   

Food for thought. 

George Booth in Ft. Collins, Colorado (booth at frii_com)

Web site still active even though the old PC is not:
  http://www.frii.com/~booth/AquaticConcepts