Today
most reef keepers understand the importance of water
motion. Adequate water motion may be as important as
adequate lighting in maintaining a healthy reef tank.
Creating realistic water motion, however, is more challenging
than creating realistic lighting. Most of the options
available to hobbyists represent significant trade-offs.
Many
reef hobbyists use external pumps for circulation
and generating water motion in the tank. Until recently,
hobbyists who wanted to increase water motion in their
tanks had to step up to more powerful pumps. Unfortunately,
larger pumps generate more heat, consume more electricity,
and often make more noise. Now hobbyists have an alternative
that enables them to increase water motion in a closed
loop without using a larger pump. A device originally
developed for the chemical industry called an eductor
has now become available to hobbyists. It can increase
water flow generated by a pump without requiring a
larger pump.
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At
first glance an eductor just looks like a flared nozzle
(figure 1). Upon closer inspection, however, one discovers
that there are large slots surrounding a narrow nozzle
in the center of the eductor (figure 2). As water enters
the eductor, it flows into a narrow section that increases
the velocity of the water. As the water continues, it
exits the narrow nozzle into a section with a much larger
cross section. This expansion creates a pressure gradient
that draws water through the surrounding slots into
the stream. The process of drawing water from outside
the eductor effectively increases the apparent flow
from the pump over what a simple return would provide.
If the process sounds familiar, it is. Venturi driven
protein skimmers utilize the same principle to inject
air into a stream of water. The only difference is that
in the case of the eductor, the process draws water
into the stream rather than air.
The claimed output of an eductor varies by the manufacturer.
Some claim that it will generate six times the flow
of a pump. The more conservative claims suggest that
four times the flow is more likely. To reach these
kinds of efficiencies, the pump feeding the eductor
needs to be pressure rated. It needs to be able to
generate flow against the back pressure of the narrow
nozzle. Suitable pumps include the RLT versions of
Iwaki pumps and the MDQ versions of Little Giant pumps.
To
test the eductor, I mounted a ¾ inch eductor
from Aquatic Ecosystems on the output of a Blueline
40HD, a pump slightly more powerful than an Iwaki
40RLT. The pump is rated at 790 gallons per hour (gph)
at a height of four feet. Because of the construction
of my tank, I mounted the pump above the water and
rotated the impeller housing so that water from the
pump flowed directly down into the tank. Threaded
schedule 80 PVC fittings were used throughout.
I
first measured flow through the pump with standard
¾ inch fittings. Using a flow meter I measured
a velocity of 4.3 feet per second (fps) at the output
of a horizontal elbow mounted just below the water’s
surface. I then screwed the eductor onto the end
of the elbow and measured flow again. Water velocity
with the eductor measured 4.6 fps. Since the volume
of water exiting a pump is a product of the water
velocity times the cross-section of the flow, one
can estimate the flow generated by the pump. The
volume of water flowing through the ¾ inch
elbow at a speed of 4.3 fps is 0.55 gps. If we assume
that flow across the entire cross section of the
eductor is constant, flow works out to be 1.9 gps
for the eductor. This is about three and a half
times the flow of the pump alone. To achieve this
increase through the use of a larger pump, one would
have to use the equivalent of an Iwaki 55RLT or
larger.
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Increasing
flow is one benefit of an eductor, but a second
benefit is equally valuable. An eductor increases
the volume of water flowing into the tank, without
significantly increasing the velocity of the water.
In the case above, volume increased 245% while
velocity increased only 7%. The flare at the end
of eductor broadens the wave front making the
current less traumatic for corals. The impact
of a broader wave front is apparent if one adds
air bubbles to the water flowing from the pump.
In figure three water is flowing directly out
of the ¾ inch fitting and the bubbles flow
in a narrow stream. Figure four shows the effect
of the eductor. The bubbles are expanding into
a much broader stream as they exit the eductor.
The
eductors currently available are fairly large.
The smallest version, the ¾ inch model
is six inches long. Fortunately manufacturers
are working on smaller units that will work in
smaller tanks. My eductor has been running several
months now without a problem. The broad wave front
generated by the wide flare enables corals to
grow within a few inches of the eductor. My eductor
runs continuously and no animals have wandered
into the slots, but potential users should keep
in mind that a significant amount of water is
drawn through the radial slots. The velocity of
this water is fairly low because of the large
surface area, but one cannot rule out snails and
the like getting on the eductor; Captive animals
seem to have an odd habit of wandering into dangerous
situations. The safest course of action is to
do as I do and run the pump continuously. Centrifugal
pumps aren’t really designed for frequent
cycling and there are better means to randomize
water flow in the reef tank.
