LFP Super Venturi! Easy to make step-by-step guide!

[liveforphysics]

LFP Venturi


Venturi's are essential too the function of some important pieces of reefkeeping equipment. Protein skimmers and calcium reactors are common devices that can improve from the use of a good venturi.

It's sometimes thought that to get the ideal shapes of a venturi, one would need to use a lathe, or a CNC mill. These certainly can produce a perfect venturi, but most folks don't have those machines laying around.

The purpose of this thread is to show how you can make nearly ideal venturi's in any size or pressure ratio and flow requirement from just a few basic hand tools. They are also very clear, so you can watch the bubbles.

The tools used:

Drill and Drill bits (dulled edges for drilling acrylic)
Heat gun (you could use anything that will heat acrylic up enough to make a soft place)
Hacksaw
Pen
Glue


The materials:

Acrylic tubing in sizes that slip over each other. This means get something with an OD that equals another tube's ID. Often this just means you can grab a tube, and then grab the next size up. Most acrylic tubes are $4-15 depending on size.



Take your inside tube and roll it in your fingers in front of the heat gun. Check often to see when it become soft. If you get it too warm, the acrylic bubbles and becomes more brittle.




When the tube becomes soft, pull on the tube, and it slowly begins to stretch. Pull steady and even.



It stretches out nicely if you're careful. Keep holding pressure on the stretched tube or it will bunch up when it shrinks back. The greater the size ratio between the diameter and the contracted diameter will determine how hard the venturi is able to suck. If you have a lot of head pressure to fight, you will need to really shrink it down. If you have very little head, a less aggressive contraction will enable you to draw air and cause less water flow restriction. Since they are so quick and easy to make, I would recommend just trying a few designs to see what works best for your application.



When you see the shape you desire, it's helpful to blow to help cool the plastic faster. Not entirely needed, but your arms will get tired if you decide to stretch tubes about 50 times in a row while experimenting.



You will end up with a stretched tube. Unless you are crazy like me, you will just have a single tube in the size you need. I tried this with so many tubes so I could be sure that this method works well on both small and large tubing. It worked fine with all wall thicknesses and sizes of tubing I tried.




The next step is to mark a circle around the smallest diameter point. This is the point with the highest fluid speed. This gives it the lowest pressure. Look for whichever end has the most rapid increase in diameter, and make this the direction the air will move towards. Drill holes around this point, and then tip the spinning drill to the side so it cuts an angled hole in the venturi. Making a drill bit dull before trying this will help to avoid cracking.



I used a 1/16" bit for the smallest tubes, and up to 5/32" for the larger tubes. Remember, area increases at the square of radius, so a small increase in bit size has big effect on what a hole is able to flow.





Next, cut some of the acrylic tubing from the bigger tube, drill an air hole in it, and slide it over the smaller venturi tube. You could tap this hole and thread in a nipple, or glue in your airline or smaller tubing or whatever best suits your application. Run a bead of glue around the seams of the larger and smaller pipe. This reinforces it and makes it strong, along with sealing the housing.




To mount them, you could use quality chunk of silicone tubing/hose and clamps like red dragon skimmers, or epoxy a plumbing union to each end. I would avoid wanting to glue them in place, as that would limit your ability to make changes if you desired.

NOTE: For skimmer use, a venturi of this style will be able to draw more air than a pump can handle without cavitation. A valve to limit the incoming air will likely be needed. This type of venturi will cause less flow loss and be capable of injecting much more air than a flow-nozzle type of injector that is most often seen in protein skimmers. In other words, your venturi won't be limiting the amount of air you are able to draw anymore. You will be limited by how much air your pump can handle before it chokes.



Enjoy your LFP Venturi!

-Luke

[The0wn4g3]

Wow that's cool. Can you explain in more detail what a LFP venturi is? If you drill the holes at an angle, which direction would you position the tube, angles toward pump, or away?

[robertifly]

Great idea and DIY thanks!

[One Dumm Hikk]

LFP = Live For Physics ??

[The0wn4g3]

Originally posted by One Dumm Hikk
LFP = Live For Physics ??

that's what I'm assuming, too haha

[slandis3]

It looks like the holes are drilled in the direction of the water flow. So it would be toward the pump. I am going to try this with some pvc i have laying around tonight when i get home from work.

[liveforphysics]

LFP = liveforphysics of course


The holes are drilled so that the tip of the angled bit points towards the pump.


Slandis3 - You will need to have a lot of skills to make one out of PVC. PVC is flexible when warm, but tends to tear before it stretches into shapes with enough taper to make a good venturi. If you find you run into problems trying PVC, acrylic tubing is very stretchy when warm.

[slandis3]

I may give it a try. i could make some tapered molds at work pretty easily as well. There are not very many places in our area that carry acrylic tube in stock. I hate ordering things you never know if your going to get the right thing.

[coralfragger101]

I'm definately going to have to play around with this.

Pretty soon no one will be buying stock aquarium equipment. We will all be making our own

[herpboyben]

this is a good idea

[Young Frankenstein]

I Like this !!!!!!!!!

[iCon31]

Another DIY project for me.

[andrewfl]

where are you buying the tubes at,what wall thickness and are the tubes cast or extruded? good job guy!!

[andrewfl]

also do you have one set up and working that we can see in action,thanks...

[liveforphysics]

I'm going to see if I can make a video of this venturi sucking in a huge amount of air using the OUTLET side of a laguna 2400 while sitting on the bottom of an 8ft deep swimming pool. A venturi with the proper design can draw huge amounts of air. Even in situations with a large head pressure against it, like the bottom of a swimming pool.

[andrewfl]

is the pool salt water or at the right salinity. fresh water is less dence so the bubles will be very big!

[liveforphysics]

It's not the job of the venturi to make the bubbles small. That's the job of the pump's impeller.

The job of the venturi is to provide the ability for the pump to be fed as much air as it can handle. Which is exactly what this venturi design does very well. You will have the ability to adjust the amount of air that enters the pump to be the full amount your special needle or mesh-wheel fancy modded pump can handle.


Also, just as an FYI, the few percent density difference has nothing to do with why bubbles stay smaller in saltwater than freshwater. It's a very complex chemical effect with surface tension and some other factors I don't understand.

Best Wishes,
-Luke

[andrewfl]

what if you use it like a mazzei or beckett?

[BeanAnimal]

Originally posted by slandis3
I may give it a try. i could make some tapered molds at work pretty easily as well. There are not very many places in our area that carry acrylic tube in stock. I hate ordering things you never know if your going to get the right thing.

You can heat PVC pipe and do the same thing. The PVC will be a bit harder to work with though. You will also need to slide it into a larger diamter shell and fill it with epoxy or other material to support the, now thinner, walls.

I was not happy with the ability to keep consistent shapes, so had looked turning a small mold that could be forced into the softened pipes. I gave up on the project and decided to just use an Alita air pump.

[EnderG60]

HA this should be titled "When engineers get bored"

Ill have to try this with some PVC. Acrylic is expensive.

Now if you were a real engineer you would have supplied us with some formula's on diameter ratio's vs inlet sizes.

[miwoodar]

Thanks for the tips Luke. Nice pics and descriptions.

[BeanAnimal]

Originally posted by EnderG60
HA this should be titled "When engineers get bored"

Ill have to try this with some PVC. Acrylic is expensive.

Now if you were a real engineer you would have supplied us with some formula's on diameter ratio's vs inlet sizes.

I worked up just such a document with ratios and input/output angle recomendations a year or so ago. Part of the work was posted here in one of the "DIY Venturi" heated debates... I will have to look for my calculations when I get home this evening. It is tough to come up with a single "best fit" design because each pump has a different pressure profile/flow profile. In any case, these will work much better than the crude coupling and air hose method that most people use. Eddy currents and turbulance in those designs are parasitic, the smoother the transition, the more energy is preserved for the pressure drop that drives the air intake.

[slandis3]

the pvc is not very forgiving. I would have to machine up some supports and a bunch of other stuff to get nice shapes. Not worth it. I will try to find some small acrylic tubes somewhere and try it again.

[BeanAnimal]

Told ya it was kinda hard to work with

I used the schedule 20 stuff, that helped a lot.

[liveforphysics]

Most people would want 1" and 1.25" acrylic tubes. 1" was $8 for 6ft and 1.25" was $10.50 for 6ft. Not really something I would consider to be a cost prohibitive material to work with for most people. I bought mine from a plastic supply store, but I've seen them for sale in hardware stores, craft supply stores, hobby shops, and other places.


I told myself not to make this thread into useless engineer techno-babble. We could go that path, but nobody is going to have flow vs head charts for effects on the inlet side of pumps, and even if we did, the flow rates are going to all be very different with mesh and needle modified pumps. Without knowing water flow rates, it still puts you back at the guessing and trying step, which I figure with something so easy and cheap to make, isn't a bad place to be.

From my testing and playing around, it amazes me how much air these will draw. From my observations, you will need to choke things back quite a bit to keep from adding way more air than the pump can handle. I drilled 8 to 12 holes in most of the venturis I made. The more I play with them, I think drilling maybe 4-6 holes would be more than adequate to max the air volume into most people's modified pumps.


Best Wishes!

-Luke