Re: CO2 reactor placement, post or pre?

["S. Hieber" <shieber at yahoo_com>]

Tom said:

"The post filter side has to deal with the vertical head
pressure in the
line. Does the prefilter line also have head pressure to
deal with?"

You have gravity working with you on one side and working
against you on the other.  With a canister the dynamic head
evens out to zero --  not counting the static head from
from friction, tubing size, bends, and any other things
that impede flow.  On the suction side the vapor pressure
is lower which theoretically means the CO2 doesn't absorb
as readily -- but the pressures on hobbyist pumps are so so
small, the efect is very light.  You'll still get al the
CO2 into the water.

"Why is there a limit of the height a canister filter will
be able to pump
water to (w/o reactor added)? Is this merely from the
frictional losses of
the water/sides of the hose?"

If you increase the relative height of the ut put (relative
to the input, that is) then the pressure declines to zero
at some point and you have no water coming out.  But in a
normal cansiter set up, that's not an issue. Well it's not
the dynamic head anyway in a normal set up since there
isn't any -- well, technically there is but gravity push on
suction side gives an opposite, contervailing effect.  As a
single system, from the intake screen to the output nozzle,
the dynamic head, the effect of gravity on the water, nets
out.

"But you can place a larger input diameter in the suction
side, generally
3/4" in, and the output side will be 1/2" out.  Many/most
pumps/canisters
work best with a larger suction side and small output for
good head pressure
lifting with minimal flow rate losses as the head
increases. Many filters
are also set up this way(Via Aqua that I have etc). As the
head pressure
increases, pump designs go to different smaller sized
outputs to make up for
for this.   "

Based on the tubing sizes on many canisters, smaller tubing
on the pressure suggests that impediments to flow have less
overall effect on the presure side and should be avoided on
the suction side, where the manufactures tend to use
larger, less restricting hoses.  Smaller hoses on the
output side don't increase the water flow via-a-vis larger
hoses.  

"This results in better flow with this placement rather
than post filter."

Or the opposite is true or there is essentially no net
improvement either way ;-) .  It probably matters, but only
very slightly, that we're talking about dynamic (in this
case centrifugal) pumps and not positive displacement
pumps.  Centrifugals are less accepting of restrictions on
the suction side, but restrictions aren't an issue *unless*
they are substantial ones that induce cavitation at the
impeller.  Caviation results when the vapor pressure gets
low enough for "bubbling" gas out of the water due to the
suction and reduced pressure due to restricted flow into
the impeller.  [Behavior would be diff with a postive
displacement pump.] The vapor presure is higher on the
pressure side, but the diffs are slight unless there is
significant restriction on one side or the other.

"The other thing is simpler vertical placement. With post
filter placement,
you would need a loop going in the top and exiting the
bottom since the CO2
reactor requires vertical placement to prevent bubbles from
escaping out the
bottom. This results in longer hose return lines/total
length. Longer pipe
length= more friction = more work= less flow."


Since you feed the water into the top of a reactor, you add
the length of the reactor when installed on the output
side. But if 12-15" of tubing is having a noticable effect
on your water flow, get a bigger filter, you've got bigger
problems than deciding where to put your reactor.  Otoh, yo
get that slight imporovement is absorbtion from the
increased vapor pressure.

But the amounts are so small -- mount wherever it's easier
-- it works fine both ways.  You should get 100% CO2
absortion eaither way unles you have a substantial
restrction on the input side.

"It's easier to put it at the pre filter side as far as
installation goes and
the potential for leaking is less on this side as well."

If you're relying on pump suction to avoid leaks, get
better fittings -- the pressures involved with hobbyist
pumps and filters are so low, you should have secure
connections whether the pump/filter is turned on/off.

Scott H.

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