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Re: Pushing a thousand litres of water
- To: Aquatic-Plants at actwin_com
- Subject: Re: Pushing a thousand litres of water
- From: Jeff Ludwig <jeff at rockytop_net>
- Date: Mon, 21 Apr 2003 13:20:37 -0400
- In-reply-to: <200304211039.h3LAdVkY030721@otter.actwin.com>
From: "Greger Lindstrand" <greglind at algonet_se>
I use a Ehiem 2252 which (at least when it was new 18 years ago) is
able to push 1250 l/h of water.
With the CO2 system engaged on the Reaktor S, the Eheim manages to
keep 192 l/h output of CO2 enriched water.
Ok, I figured that it wasn't strong enough so I got myself an
AquaClear 5000 powerhead to do the job. It's good for 3550 l/h and can
lift 200 cm of H2O.
I was astonished when measuring the output of CO2 enriched water from
the Reaktor S. It was exactly the same 192 l/h!!!
The problem lies in the way that pump specs are reported to hobbyists
and the fact that you don't know the pressure drop over the reactor.
The LPM ratings on pumps are frequently reported at zero discharge head
or at best with a specified discharge head that varies from pump to
pump making practical comparison impossible.
I think a lot of mystery of practical fluid mechanics vanishes when you
think of an electrical system as an analogy... the pump is analogous to
the battery, the lines (size, shape of inlets, sudden constrictions,
valving and relative heights) and equipment (pressure drop over
reactor) supply the resistance, you are interesting in the current
(flow in this case). Saying a pump drives xxx LPM is like saying xyz
battery provides xx current without specifying the resistance. The
zero discharge head velocities are like reporting the amperage of a
battery when you short it; pretty worthless.
What you really are looking for is a pump curve (P vs. Q), which gives
discharge rate as a function of total discharge pressure drop. Expect
to loose 0.43psi per foot of vertical climb, pressure drop due to line
resistance in aquarium system is basically 0 (and would require the
friction factor of the tubing which is unknown anyway), tees and elbows
and frequently estimated in terms of equivalent pipe diameters. The
real problem you have is that pressure drop over the reactor is
generally a complicated function of flow... To really get this right,
you would need to measure this, the best you can do right now is a
simple proportional fit as follows.
If you have a pump curve for your current powerhead/pump, measure the
flow through the reactor, calculate the total pressure drop by backing
that off the pump curve. Then subtract the effect of gravity on the
system so you have DeltaP_reactor (pressure drop just due to the
reactor) and Q (flow), divide the two to get a proportionality
constant. Use this to estimate the pressure drop at the desired flow
rate, add back in gravity then look at pumps curves and figure out
which pump would give you Q at that pressure drop. Hope this makes
sense, I know a lot of this info isn't available for aquarium pumps but
it something a little better (and cheaper) than just guessing if the
info is available. Check with the hardcore marine guys over at
www.reefcentral.com, I think they might have some pump curves for
standard pumps. Let me know if you need more info.