One for you teccy guys or failing that Nigel!

[K.H]

I dont doubt this for one moment but i just cant get my head round it,why dont venturis slow/restrict the flow?
In my mind if you took say two large funnels and cut one spout to a diameter of 22 mm,the other to 8 mm then filled both with the same amount of liquid the 22 one would empty first-yes? so how does it work with a venturi or do i need to look at it another way?
Plain english please 

[Julian]

To my way of thinking, any constriction will reduce the flow (unless the pipe and constriction in it are ridiculously large).  I think with the size throats we use based on Jim's recommendations/calculations, the answer is that they do restrict the flow, but not very much.  But don't take my word for it ... I'm incapable of doing the sums to prove it, so it's just educated guess work on my part!

I have a positive displacement pump I can run in tandem with my Leo.  One of the things I want to try is a smaller throat dia in conjunction with the pressure the PD pump will create.  It should increase the velocity in the discharge cone with yet to be discovered results!  This would be more for distilling than mixing, I'm hoping an eductor will improve the mixing.

Anyway, why are you, a confirmed GLophobe, asking about venturies?  You're not weakening under the relentless procession of new GL's being built are you?

[K.H]

Sorry did,nt realise they were exclusive to those old dated non washing GL,s,always found them useful for introducing air when drying,only reason i hav,nt got one now is i cant work out how to build one in 2"
Notice i have resisted the urge to use smillies

[nigelb]

I think, as Julian says that there is a reduction in flow but not enough to make any difference. Surprising really considering the restriction, often 22mm down to 8mm. I wish I could explain it but I'm not suitably qualified. I'm more of a chemist than a mathmatician. What does concern me is the strain on the pump when an eductor is introduced. A further reduction to 6mm seems to really load a pump.

Are you saying that if you had a venturi in 2" then you would fit one? What size pump have you got?

[Julian]

Now you're a whiz at silver soldering, draw two developments based on Jim's formula (provided you know the flow rate of the pump), cut out of sheet steel, bend and join.  Silver solder a 2" fitting at each end, drill a suction port Silver solder on a bit of 22mm copper, or what ever, and you're done.

Or you could do what Jim does, make the cones, with three suction ports in the discharge and slip them into a bit of 2" pipe.  Then just drill the pipe and weld on the suction pipe.

Temped to say simples, but I won't.

[Julian]

Nige ...


Iit's quite a reduction in diameter, but only over a very short, (theoretically infinitely small) distance.  When I used to play with Inert Gas Systems on oil tankers we used orifice plates to measure flow, both of sea water and gas.  Again the reduction in diameter looked large but had little effect on the flow.

Look at it from another perspective ... if all the pipe work was 8mm diameter instead of 22mm then the resistance would be enormous.

[K.H]

I think, as Julian says that there is a reduction in flow but not enough to make any difference. Surprising really considering the restriction, often 22mm down to 8mm. I wish I could explain it but I'm not suitably qualified. I'm more of a chemist than a mathmatician. What does concern me is the strain on the pump when an eductor is introduced. A further reduction to 6mm seems to really load a pump.

Are you saying that if you had a venturi in 2" then you would fit one? What size pump have you got?

Probably but introducing something new to the steel pipework would be a headache,240lpm pump

[K.H]

BTW still dont really understand restriction not affecting flow,my last venturi was on a separate circuit and as Nigel comments,switching to the venturi made the pump seem to strain

[Tony]

Yes same here K.H.  When I had a TAM105 switching in the venturi made an obvious different to the pump whine (it sounded well loaded down).

Very different story with the multistage pump, bypassing it or running it through Nigel's venturi makes little difference to the loading noise, and from the pump output pressure checked on pump curves only a few percent reduction in flow.

I think a lot of it comes down to the pump and the curves, some of the very high flow ones with low input power (100s of litres per minute) have a steep curve and don't push much head - even a hint of resistance will dramatically reduce the flow.  Whereas the multistagers will take a lot of loading to only slightly reduce the flow.

I've just picked up another pump with quite incredible charateristics - it's rated for maximum flow (80lpm) with a back pressure of nearly 3 bar!

I've not seen the curves for the TAM105, it would be interesting to see what 1 bar of back pressure does to its flow.

To add to the variables, the back pressure to the pump is different sucking in Methoxide vs sucking in air (more back pressure with air, confusingly).

So I don't think there is one equation to rule them all due to different pump characteristics, but it would appear that what Jim has come up with is pretty good for the general case.