Venturi

Principle

Bernoulli equation states that the sum of all forms of energy in a fluid flowing along an enclosed path is the same at any two points in that path (or streamline). Its formulation in the simplistic hypothesis of incompressible flow (fluid motion with negligible changes in density) is:

where:

v        : fluid velocity along the streamline
g        : acceleration of gravity on Earth
h        : height
p        : pressure along the streamline
r          : fluid density

As a consequence of this law a fluid passing through smoothly varying constrictions is subjected to changes in velocity and pressure. A Venturi is a system for speeding the flow of the fluid, by constricting it in a cone shape tube. In the restriction the fluid must increase its velocity reducing its pressure and producing a partial vacuum. As the fluid leave the constriction, its pressure increase back to the ambient or pipe level.

Applications

Venturi are used to measure the speed of a fluid, by measuring the pressure changes from one point to another along the venture.

A venturi can also be used to inject a liquid or a gas into another liquid. A pump forces the liquid flow through a tube connected to:

• A venturi to increase the speed of the fluid (restriction of the pipe diameter)

• A short piece of tube connected to the gas source

• A second venturi that decrease the speed of the fluid (the pipe diameter increase again)

After the first venturi the pressure in the pipe is lower, so the gas is sucked in the pipe. Then the mixture enters the second venturi and slow down. At the end of the system a mixture of gas and liquid appears and the pressure rise again to its normal level in the pipe.

This technique is used for ozone injection in water.

The newest injectors design causes complete mixing of injected materials (air, ozone or chemicals), eliminating the need for other in-line mixers. Venturi injectors have no moving parts and are maintenance free. They operate effectively over a wide range of pressures (from 1 to 250 psi) and require only a minimum pressure difference to initiate the vacuum at the suction part. Venturis are built in thermoplastics (PVC, PE, PVDF), stainless steel or other metals.

The cavitation effect at the injection chamber provides an instantaneous mixing, creating thousand of very tiny bubbles of gas in the liquid. The small bubbles provide and increased gas exposure to the liquid surface area, increasing the effectiveness of the process (i.e. ozonation).

Lenntech can help you to design the venturi that best suits your application. Do not hesitate to contact us for a direct technical and commercial offer.