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I need assistance (or experience feedback) in estimating if a Venturi pump would work at low pressure. A boiler under vacuum (10% atmosphere) will produce steam (at a given speed depending on the boiler power) and the steam flow will be used to suck water at the same temperature (of boiling water under this vacuum, about 50 degrees C) and pressure in a Venturi style setup.

From the wiki articles I read about the Venturi tube, the usual equations giving the pressure difference between the constriction and the larger tube are for incompressible fluids. I do worry that 10% ATM steam would be very compressible though. So in short I need an equation for tying up the tube diameters, the steam feed speed, and the suction generated under such conditions

share|cite|improve this question contains formula for compressible flow, but not for steam. Can you make a rough sketch? How is the vaccuum in the boiler produced? – mart Jun 26 '13 at 7:10
The air is purged from the system with a vac pump initially and then the vacuum is maintained by the condenser, the diagram (not to scale :)) is here – Manu de Hanoi Jun 26 '13 at 9:54
I assume the T-piece at the right side of the U will be the Venturi-Nozzle? I honestly can't tell if this would work right now - but I like the question. – mart Jun 26 '13 at 11:24
yes it is mart. – Manu de Hanoi Jun 26 '13 at 11:57
At those conditions, the speed of sound in a saturated steam gas is 444 m/s. Compressibility effects kick in at roughly Mach 0.3, or 133 m/s. So figure out what the velocity would be using the incompressible equations and if it comes out to be bigger than, say, 100 m/s then compressibility might matter. Otherwise, run with the incompressible equations. – tpg2114 Jun 27 '13 at 5:25

1 Answer 1

It Possible to have it work. But it's not feasible to keep it working. The equation is simply Bernoulli's Principle/equation.

Very basics; -Venturi needs a rise in a velocity and a drop in a Pressure. -There is no negative pressure, so you end up having vacuum really easily with just 10% pressure left. Why is this a problem? The exit side of venturi is like a Drafttube in a hydroturbine. And once you lost the contact (boundary layer) because of some disturbance you will have a Turbulence and you must raise the pressure to stabilize the flow again (recover of the boundary layer).

There is a nice old video about these showing the expected problems of this setting;

At 2:30-> 4:00 is the most important.

enter image description here

Shortly, if you can easily accept that you pump often only 1/3 of the maximum, then it's feasible. But if you expect somehow stabile flow. -Forget it.

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