Suction dredge (not a question)

Just wanted to share this with the uC guys here.

We were talking about airlifts on another forum. A suction dredge is used to remove silt from rives and harbors works on this principle.

The one wekipedia talked about works by releasing a bubble into a 4 inch pipe 10 or more feet long. Air is injected into the bottom of the tube for 1 to 3 seconds forming an air bubble. When the air bubble reaches the top of the pipe the next bubble is generated.

I was thinking what a fun uC project. Generating the timing for the bubble and controlling the duration of the bubble should be standard fare.

The fun part comes with detecting when the bubble reaches the top. This is not guaranteed to be a constant especially during start up. One could use probes at the top of the pipe to detect the lack of an electrical path. But it would be a lot more fun to play with the PIC CTMU to notice the capacitance change when the bubble passes by.

It would be a blast to set this up and play with bubble size and bubble release times based on where the previous bubble was.

Am I going to do it? I already have too many irons in the fire but it is tempting.

This sounds more like a mechanical engineering project rather than an electrical engineering project. As you said, controlling an air pump would be trivial.

I would think a pressure sensor would be the best for detecting the bubble hitting the top. It depends on the speed of the flow, but generally the water above the bubble will have a higher pressure than below as the bubble pushes the water in front of it. Then again it could just by timed, I don't believe you need to worry about timing the next bubble that closely. You could have multiple bubbles travelling up a pipe at the same time without issue.

3v0 said:

When the air bubble reaches the top of the pipe the next bubble is generated.

Click to expand...

Is it necessary to wait until the bubble reaches the top of the pipe before injecting the next bubble? Is it a problem if there is 2 or 3 bubbles in the pipe simultaneously? I would imagine it would be better to inject more than one bubble.. to a limit of course.

So far I only know what I read and I did not record the source...

Typically, the airlift is constructed from a 3 metre to 10 metre long, 10 cm diameter pipe. A controllable compressed air supply vents into the inside, lower end of the pipe (The input end always being the lower end). Compressed air is injected into the pipe in one to three second bursts with an interval long enough to let the resulting bubble to rise to the higher, output end of the pipe. The bubble moves water through the pipe sucking debris from the lower end and depositing it from the upper end of the pipe.

Click to expand...

The nice thing about using is a uC is that one could rapidly try variations,

I am thinking it might be a great way to supply lots of water to a water garden water fall.

A similar principle invented by Skeggs and commercialized by Whitehead revolutionized clinical chemistry in the early 1960's (https://en.wikipedia.org/wiki/AutoAnalyzer). It led to the term "euboxic" on clinical rounds meaning a patient's test results were all within the normal range. The term was derived from the way Technicon presented results on a strip chart recorder. That methodology is no longer used, and it is good to know it has found another home in the world.

If you are interested in the theory, there are loads of papers, both theoretical and practical, on it the effect of bubble size and frequency.

John

Compressed air is injected into the pipe in one to three second bursts with an interval long enough to let the resulting bubble to rise to the higher, output end of the pipe.

Click to expand...

That does not say that the bubble has to travel all the way through and out of the pipe. Only higher, (towards the) output end of the pipe. That is my interpretation and all the illustrations I've found support this:


Anyway, Interesting technique. Could be easy to experiment in small scale at home.. I don't know why, but could be fun.

misterT said:

That does not say that the bubble has to travel all the way through and out of the pipe. Only higher, (towards the) output end of the pipe. That is my interpretation and all the illustrations I've found support this:

Click to expand...

Yes, but why slow down the mass transfer with extra bubbles?

John

jpanhalt said:

Yes, but why slow down the mass transfer with extra bubbles?

John

Click to expand...

Are you sure more bubbles slows it down?

Generally speaking, the procedure is as it is described. You want to inject more bubbles. The onus is on you to show that would be valuable. I suggest that you read some of the AutoAnalyzer theoretical articles. The conclusions are not all that intuitive.

John

I looked at couple of papers and they all say that there is an optimal airflow for efficiency and optimal airflow for maximum capacity. The flow pattern also affects these optimal values. Not one paper said that best thing is to send one bubble at a time through the pipe. In fact it looks like that it is better to have lots of small bubbles traveling up the pipe.
http://digitalcommons.bucknell.edu/cgi/viewcontent.cgi?article=1027&context=honors_theses
**broken link removed**

Increasing the air flow rate results in increasing water flow rate up to a point, after which the water flow rate levels off or may even decrease with a further increase in air flow rate.

Click to expand...

jpanhalt said:

Generally speaking, the procedure is as it is described. You want to inject more bubbles. The onus is on you to show that would be valuable.

Click to expand...

I'm just questioning the method of sending one bubble at a time.. the onus is on you to show that that even works, or is better than multiple bubbles.

I read the first paper I linked and based on that I doubt that sending one bubble can lift any water. Maybe if the bubble is large enough, but then the pump is more like a geyser pump.

jpanhalt said:

A similar principle invented by Skeggs and commercialized by Whitehead revolutionized clinical chemistry in the early 1960's (https://en.wikipedia.org/wiki/AutoAnalyzer). It led to the term "euboxic" on clinical rounds meaning a patient's test results were all within the normal range. The term was derived from the way Technicon presented results on a strip chart recorder. That methodology is no longer used, and it is good to know it has found another home in the world.

If you are interested in the theory, there are loads of papers, both theoretical and practical, on it the effect of bubble size and frequency.

John

Click to expand...

jpanhalt, I just realized that we are talking about two different things here. AutoAnalyzer has nothing to do with Airlift pumps.. and yes, adding more bubbles to the AutoAnalyzer transfer tubing will slow the mass transfer, but I was not talking about AutoAnalyzer. I was talking about airlift pumps.

Over 10 years ago created non optimized airlifts using a continual supply of smaller bubbles.



I do think there are two different methods here. With the fine bubbles you are lowering the specific gravity of the the water in the column and the water around it pushed it up. Also used in foam fractioners. One problem is that as the foam moves up the column the bubbles combine into larger ones and lets the water fall while the air goes up.

The dredge is inserting a plug of air into the column and it rises lifting the water above and below it. I think

Compressed air is injected into the pipe in one to three second bursts with an interval long enough to let the resulting bubble to rise to the higher, output end of the pipe

Click to expand...

I may have misinterpreted this on reading it again I see the same thing. An interval between bubbles to let the first to near the top prior to creating another one.

This is similar to the technique used in fish tanks with gravel filters.

Air is bubbled into a vertical tube, as a result the hydrostatic pressure at the bottom of the tube is lower than when the tube is simply full of water.
The hydrostatic pressure at the bottom of the fish tank outside the tube remains the same.
This results in water flowing from the higher pressure (in the tank) to the lower pressure (inside the tube) and so circulating the water through the gravel filter.

On a much larger scale, a technique known as "gas lift" is used in the oil industry to improve the flow rate of produced oil.
Gas is injected at the bottom of the production tubing to lower the density of the oil in the tubing, and so reduce the hydrostatic pressure, resulting in an increased flow of oil from the reservoir.

JimB

found that reference

Also found this on Two-phase flow which maybe linked to what misterT was linking to. Have not had the time to read all the material. Very little.

@MisterT
You are quite right. I was impressed with the superficial similarity of the AutoAnalyzer. The mechanism by which the fluid is made to flow is quite different.
John

jpanhalt said:

@MisterT
You are quite right. I was impressed with the superficial similarity of the AutoAnalyzer. The mechanism by which the fluid is made to flow is quite different.
John

Click to expand...

Yeah, I read the AutoAnalyzer article in wikipedia. That was interesting too

I'll need to look at the Bucknell thesis closer. It seems there is an optimal bubble size/number. Flow around the bubble is a variable that may be hard to predict. 3v0's second link is interesting.

John