Holes required to vent

The day this stuff was taught in school I was either fishing or day dreaming. Hopful some of you were paying attention.
If a half inch sch 40 pvc pipe terminates into a closed tank,assuming the pipe is .622 inch I.D. ;
Aprox how many one eighth(.125) drill bit holes in tank are required to flow equal amount of air as an open ended pipe? Same question for a one sixteenth(.0625) drill bit. Do you think friction lose would be more than 30% ?

I didn't take any fluid theory, so I couldn't answer your question even if you'd given enough information, which you haven't. It matters what the length of the pipe is and the flow rate through the system. And I'm not sure what you mean by "friction loss". Measured where and how?

But let's step back a bit. I think, for starters, you want to know how many smaller holes it takes to equal the same cross-sectional area of a larger hole. The area of a circle increases as the the square of its radius, so when you double the diameter of a pipe you quadruple its cross-sectional area. A .125 diameter hole is .20 times the diameter of a .622 hole, so its area is .20² = .04 the size of the larger hole. So you need 25 0.125 inch holes to equal the area of a 0.622 inch hole. Quadruple that (100 holes) for 0.0625 holes.

That gives you an idea, but doesn't answer the question. If the flow rate through your system is zero, then there is zero pressure drop anywhere in the system and the hole size doesn't matter. As you increase the flow rate, flow through your holes becomes more turbulent and pressure drop increases. Even though the total area of the holes is the same for the eighth-inch versus sixteenth-inch holes, pressure drop will be greater through the smaller holes.

The other thing to consider is pressure drop through your half-inch pipe. If the pipe is very short, pressure drop is negligible until flow rates are high. But if the pipe is long, pressure drop through the pipe becomes significant; the pressure drop doubles each time you double the length of the pipe.

My guess is you'll need less total hole area than pipe area, due to pressure drop in the pipe. But the only way to know is to put a gauge on your tank and start drilling.

How about sharing what it is you're actually trying to do? You'll probably get a lot better answers than mine if provide more information.

One.
But it might flow a bit slow...

My first question is what are you trying to make or achieve. More info would be great. But sounds like Mark is on top of it. LOL

I agree with Mark and dabees, You give no information on flow rate and distance. Friction loss is just as dependent on length and flow as diam.

Several, and for a 1/16 drill, a bunch more.

Company hired a fermer engineer from IH, who displayed a plaque that stated: "For excellance in fluid dynamics". Turns out the plaque was awarded to him because he tipped over a whole pitcher of beer at a company function.

I doubt that he could answer your question. He didn't last long with the company.

"How about sharing what it is you're actually trying to do? You'll probably get a lot better answers than mine if provide more information."

Fabricating a means to airate a 3 tank conventional septic system. 4cfm,5psi pump/7' of 0.622 id line terminating into 2' of 1.049 id tube (tank with 0.125 holes). Tank submerged 30" below normal water level of center septic tank.
Thanks Mark,that is great. Those numbers allowed sizing air distribution tank to accomindate ample quinity of holes spaced as I invisioned. They will also alow me to drill 15 holes before starting to watch back pressure as #of holes are increased. I will work this out in a barrel of water before installing. I have plenty of small clear vinyl tubeing so I plan jerri rigging a water colum u tube for final tweeking. I will first check water colum I have with open ended pipe 30" deep in water barrel. I'm thinking with a 0.125 hole in pipe above water,drill 2 holes in distribution tank and retest until my u tube drops will indicate max cfm pump can maintaine.
Will that be enough info for those that asked before making calculations?

(quoted from post at 13:32:11 06/13/14) "How about sharing what it is you're actually trying to do? You'll probably get a lot better answers than mine if provide more information."

Fabricating a means to airate a 3 tank conventional septic system. 4cfm,5psi pump/7' of 0.622 id line terminating into 2' of 1.049 id tube (tank with 0.125 holes). Tank submerged 30" below normal water level of center septic tank.
Thanks Mark,that is great. Those numbers allowed sizing air distribution tank to accomindate ample quinity of holes spaced as I invisioned. They will also alow me to drill 15 holes before starting to watch back pressure as #of holes are increased. I will work this out in a barrel of water before installing. I have plenty of small clear vinyl tubeing so I plan jerri rigging a water colum u tube for final tweeking. I will first check water colum I have with open ended pipe 30" deep in water barrel. I'm thinking with a 0.125 hole in pipe above water,drill 2 holes in distribution tank and retest until my u tube drops will indicate max cfm pump can maintaine.
Will that be enough info for those that asked before making calculations?

Click to expand...

Is there a reason to not just buy an off the shelf part instead of making it yourself?

http://www.lagunakoi.com/Air
http://www.hakkoairpumps.com/subcat62.html

About $50, throw it in the water and call it a day. If the premade ones wont work for whatever reason, you can get more info from the aquaponics guys, they do alot of water circulation using airlift pumps. You can also get charts for building an airlift pump but they are more geared to volume of water recieved from X amount of air put in because they use airlift pumps for getting drinking water in 3rd world countries. Things should translate pretty well for your use though. Good luck, let us know how things work.

"Is there a reason to not just buy an off the shelf part instead of making it yourself?"

No reason that makes sense to all but in a nutshell,because I want to and can is the answer. I get similar questions about,tractors,tools and vehicles. I'm surprised you could be around this discussion group more than an hour without realizing that is just what we do. "$50 off the shelf,throw it in the water and call it a day"affords little satisfaction at times.

as for the resistance -

There are MANY variables at play here - and I'm no fluid mechanic myself - but if I HAD to guess - I'd go about it this way:

Assuming 1/8" holes.

Assuming uniform thickness - call it 1 to keep everything simple and take that out of the equation.

the surface area of friction causing contact would then be the circumference of the holes.

.622 in has a circumference of 1.95"

a .125 has a circumference of .393" times 25 holes = 9.83"

So you've matched the total area of the small holes to the large, but you've got quite a bit more friction causing surface area.

Let's just guess that it's .005" of air that is affected by the surface in question. (the air in the middle flows relatively friction free)

that's 9.83 x .005 = .049" total contact area.

Since .125" holes have an area of .012 - I'd drill 4 extra holes to make up for the loss in friction.

I could be WAY off - don't mean to pretend I know what I'm talking about - but again, if I HAD to guess - that'd be my way of thinking.

I'd also guess that was probably a little low on the .005 number since the friction must gradually taper off with distance from the surface, but in a small hole - it's probably more significant. I might go with 8 more holes just to be sure.

(quoted from post at 01:37:28 06/14/14) "Is there a reason to not just buy an off the shelf part instead of making it yourself?"

No reason that makes sense to all but in a nutshell,because I want to and can is the answer. I get similar questions about,tractors,tools and vehicles. I'm surprised you could be around this discussion group more than an hour without realizing that is just what we do. "$50 off the shelf,throw it in the water and call it a day"affords little satisfaction at times.

Click to expand...

I fully understand "just because" and thats why I asked, I didnt know if you knew there was a product that is available that does just what you want. Im not sure why you are trying to be so secretive yet asking questions but doing that is counter productive. In any case, for an effective aerobic septic tank installation, you need to use much smaller than 1/8" holes. No sense in doing all that work to not do much good, unless you are looking for the "thrill of failure" in addition to satisfaction of "just because".

Have fun with your project and good luck!

Thanks JR. Resistance has been a wild card from the first time I turned this over in my head. I do understand resistance to air flow is influanced by amoung other things,size of holes. Fortunatly,precision is not essential for satisfactory performance. I feel my pump is capiable of supplying more cfm and psi than is needed. My thinking is that my homemade manometer will alow me to see the resistance applied by 30" of water,which remains constant. Backpressure caused by all other elements,mainly resistance of collective circumferance walls of all those tiny holes will show up in addition to the 30" of water. You metioned that the effect of resistance through holes will deminish as depth of water increases. Let me say again that I will be working the kinks out in a barrel of clean water so experimentation and or backtracking and starting over will be simple. Here's a question. Shouldn't a 1/8th or perhaps 1/16th hole in air supply line ABOVE WATER alow putting a finger over it to see reaction of manometer,and thereby roughly where I'm at in adding holes in defuser? I almost forgot to tie this to tractors. If anyone has dual carbs or porting the intake manifold on a tractor,the manometer is inexpensive and very accurate for balancing vacuum.