Myth or Fact: DO Depletion by Pumps and Use of Eductors

Sleepe

I agree that it should not be dismissed out of hand.
EB there are a number of instances in which you have 'free' head ie excess pump requirement or gb drain. While it may be better for aeration to thin film it down wide channels you very rarely see it happening. As for moving water the offset pipe head may not be as efficient. No I am not Frank

DrLuke

I'm not being dismissive either. I really want to know the answer to the question I asked. (not that I expect everyone to look it up for me)

Firstly, whether the motive nozzle in the diag Charlie posted for me (thanks for that

, I'm using phone ) is actually necessary for something? Secondly, if it is necessary, will the loss of flow from the first reducing nozzle (forget about the main one for now) be in the ball park of the quoted 4x increase introduced by the Venturi effect?

DrLuke

Hmm, the Wikipedia page seems to be a bit ambiguous to me as to which of the two nozzles is producing the main Venturi effect.

This page (http://www.pumpsofoklahoma.com/jet-educator-systems) explains the roles of the two nozzles a little better, but seems to contradict the Wikipedia article slightly.

Sorry, if I had a full screen and mouse I would post which part I think is ambiguous.

DrLuke

Okay. So I've re read the two pages I posted a number of times and I no longer believe that the Wikipedia page is "as" ambiguous.

It does mention the motive nozzle, but focuses on the converging diverging nozzle as the central component forming the low pressure/increased velocity, and implies it occurs at the troat of this nozzle. It mentions very little about the role of the motive nozzl explicitly, though I guess someone smarter might be able to infer.

The other page, on the other hand, implies that this occurs at the motive fluid inlet nozzle and largely neglects the role of the converging part of the converging diverging nozzle highlighted by Wikipedia.

PLJ

The diagram from Wikipedia posted by Charlie is of an ejector/injector. These differ from eductors sufficiently to perhaps negate the need for your question, DrLuke. As I understand it, there is no reduction in diameter, ie convergence, after the inlet point in an eductor. Looking at the diagram, there is a diffuser without the nozzle.
The following from Wikianswers may help:

The ejectors are used to maintain a system vacuum in the upstream (Example maintaining the vacuum column pressure) whereas eductor's main objective is to take out the volume of any fluid out of the system by maintaining a system pressure in the upstream.
example:
1. The eductors are used to remove the air 4 times the volume of the vessel per hour for the vessel entry jobs. We use compressed air as a motive fluid.
2. The water jet eductors are used to suck the muddy water, or a oily water from the storm water channel or a pit (an alternative for a de-watering pump). Here the motive fluid is high pressure water.
Hence the Eductors can be used to transfer a considerable volume of fluid from low pressure to high pressure with high compression ratio than ejectors. whereas the ejectors just suck the excess volume of the system and maintains the system pressure accurately.
The difference is with respect to their function and not with respect with their motive fluid. The diameter of the ejector's throat is lesser than the eductor.

RupertofOZ

They all seem dependent on a pressure differential though....

For the sake of moving water around in the fish tank.... are they really any better or efficient than some of the very low wattage "wavemaker" pumps available??

Ronmaggi

I really wish I had the equipment to measure those things, because it sounds like fun to figure that out. An ideal setup would be four simple IBC setups, all run constant flood. One the control, that is just a pump running up to the growbed, water draining back into the tank. The second, a wave maker pump. Third, an eductor at the pump. Fourth, an eductor on the drain back from the grow bed. All measuring wattage and d.o. My gut, is that an eductor on the drain line will consume no more energy than the control, but either increase d.o., or clog up easly.

SuperVeg

Actually if you are restricting the flow of your pump you will be reducing its power consumption. So potentially less energy

DrLuke

Thanks plj,

I guess the fact that the wiki article is trying to cover three technologies in one should have got my attention.

Charlie

If you restrict a pump SV it requires more power to push through the restriction = more load, more power consumption.

bythebrook

Charlie wrote:

If you restrict a pump SV it requires more power to push through the restriction = more load, more power consumption.

It's more likely that the power consumption will remain the same, but the flow rate will drop - just like the pump curves show, increasing head (restriction), lower flow.

chillidude

+1 bythebrook

RupertofOZ

I still don't get the "supposed" benefit... of moving water around within the tank...

And if it's for some perceived DO gain.... how it's more beneficial than just introducing a venturi into a water return.... :dontknow:

mantis

Thats all I have in my 2000L tank. 25mm pipe from sump pump going down to about half way under the water, with venturi and 90mm bend so it swirls the tank. With a sloping floored tank and centre drain this works a treat

SuperVeg

Charlie wrote:

If you restrict a pump SV it requires more power to push through the restriction = more load, more power consumption.

Luckily I have already had this debate and I have also done real world tests

Me wrote:

It seems counter intuitive but the principle is the same as when you put your hand over a vacuum cleaner. What happens is that when you restrict the flow the pump actually has LESS work to do and therefore uses less power. When you put your hand over the vacuum cleaner the moter revs up, or spins FASTER. Why? Because it has less load, and less load means it can spin faster.

I have evidence as well:.
My old 200W ebay pump:
-running with only a short length of hose (max water flow) 135W
-Pumping up to my GB through lots of little holes 100W
-My finger jammed in the pipe so ZERO flow 85W

New Jabao eco pump (awesome pump)
-running with just included fittings (full flow) 48.5W
-running with my finger jammed in the end (ZERO flow) 44W

of course for the SAME FLOW RATE at a higher head you will need more power, thats obvious. But that is not using the same pump, you just need a bigger more power hungry pump.
With the same pump at a higher head, your flow rate decreases (of course) and the power consumption also decreases (not so obvious)

Go on, test it and prove me wrong

It MIGHT be different with different pump architectures, but I doubt it.

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Myth or Fact: DO Depletion by Pumps and Use of Eductors

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