Re: [compost_tea] Re: Re: Flow rate for air pumps

From: Robert Norsen <bnbrew_at_yahoo.com>
Date: Fri, 18 Feb 2005 12:38:27 -0800 (PST)

Dave some us know about water depth and back pressure, pump[ ratings, real tests. When you are advising based on "just a guess" as you say, maybe your advice is less than useful. Bob

David Anderson <squtch_at_gmail.com> wrote:
On Fri, 18 Feb 2005 08:36:52 -0800, Kirk Leonard wrote:
> .17 cfm/gal in 10 gallons would be a 1.7cfm air pump. .034 a/w in 50 gals -
> same pump, 1.7. (Math whizzes probably got this immediately.) "Linear" or
> diaphragm pumps are the best to use here - Alita, Sweetwater, Gast, Thomas,
> there are many... aquaticeco.com has a good selection. Aquarium pumps are
> (good) linear pumps but they're usually not big enough for making tea unless
> you gang several up.

Well, being a math wiz, I can tell you that you forgot a step in your
calculation. You are talking about "same pump" on 10 gallon and 50
gallon tanks. A pump rated at 1.7 cfm is not putting 1.7 cfm into the
10G tank. Just running an open line to the bottom of the tank, it
might only be putting in somewhere around half it's listed rate (just
a guess). Add a diffuser and it will drop more. That 50G tank is even
deeper, you are going to lose even more of your flow.

BTW, two 2.8 l/m aquarium pumps ganged together can do a very good job
of keeping a 5 gallon bucket rolling, as long as you are not using
stones. I use three, and don't think I could push it up to 4 without
reducing the water down to 3 gallons from the current 4.

> > 3.- Effectivity of circulation is critical since "dead volumes of ACT can
> pollute just as can anaerobic left over crud in dirty brewer.
>
> You want good air movement throughout the tank, so aerator design is
> important. Bubble size is important too. Smaller bubbles provide more air
> for critters, bigger bubbles move the water and support extraction better,
> generally.

Actually, the same volume (actual, not pump rating) of air will move
significantly more water with smaller bubbles. The large bubble move
through the water faster, but they don't move much of the water.

That is why the big bubbles are better for extraction. They do a much
better job of knocking the compost around.

Small bubbles provide no more air to the critters. There is the same
volume of air. There is more surface area, and they rise slower, so
there is certainly more surface area time, but it isn't going to take
much time at all for the air in the bubble to reach equilibrium with
the gasses in the water.

The most important thing (oxygen wise) is to move the stale water to
the surface by creating a current, and to create turbulance for higher
gas exchange.

> > 5 - The kind and quantity of the nutrients included in the "load".
>
> To me this is the most important variable - high nutrient loads will create
> greater oxygen demand, compost held constant. If you use a lot of nutrients
> you want a lot of air, but you can do with less air if you use less
> nutrients.

That is what I depend on with my homemade system. I'm not trying to
push the limits. I intentionally underload the nutrients.

That might change if I find myself battling any sort of diseases in my
garden, but for now, I am going for general soil improvement.

> KIS machines and many mentioned by OZ folks use much higher air/water
> ratios. I measured a little KIS fresh out of the box at a/w = 4! That's
> Four, not point four. Another person here measured it at 2 after some use.
> It's simply an air blower and wears out relatively fast. OZ folks seem to
> run above a/w = 1, even 2-3. You can get to the point where you're blowing
> tea out of the tank... How much roil is really useful?

A regular squirrel cage blower (which is what I assume you are
referring to for the KIS) can achieve huge volumes of air rather
effieciently, but only without backpressure. The open air rate will
have very little to do with how well it performs at depth. It would be
interesting to see what that blower produces when you drop it to the
bottom of a barrel instead of a bucket.

I'm not saying that they don't pump a hell of a lot of air into their
system, I'm just saying that unpressurized tests are pretty much
useless when comparing pumps or talking about their air volumes.

A 100 cfm roots pump will put a lot more air at the bottom of your 500
gallon tank than a 100 cfm diaphram pump will. A 100 cfm blower is not
likely to be able to put any are at the bottom of a 500 gallon tank.

Dave




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Received on Fri Feb 18 2005 - 16:31:51 EST

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