Venturi Theory

What is a venturi?

Many of ChemIndustrial's blending, batching and metering systems exploit the capabilities of process venturis. So exactly what is a process venturi?

Simply stated, a process venturi is an engineered restriction in a pipeline.

Fluid flowing in the pipeline (the "motive fluid") speeds up to pass through the restriction and in accordance with Bernoulli's equation creates a vacuum in the restriction. A side port at the restriction allows the vacuum to draw a second fluid (the "injectate") into the motive fluid through the port. Turbulence downstream of the port entrains and mixes the injectate into the motive fluid.

Now for a simple physics refresher: In mechanical systems, energy may change form but the total amount of energy remains constant. An example:

A sled at the top of a steep, icy hill has potential energy related to the height of the hill and the weight of the sled and occupant.

When the sled slides down the hill, it goes faster and faster, as the potential energy that was available at the top of the hill is converted into kinetic energy (the energy of movement) as it descends the hill. At the bottom of the hill, all of the sled energy is in the form of kinetic energy.

In undulating terrain, the sled continues some distance up the next rise, and its kinetic energy is converted back to potential energy as the sled climbs. Of course, the sled never reaches the height of the starting point before gravity overcomes its momentum. This is because some energy is lost to friction.

Friction losses actually show up as heat at the runners and as heat in the surrounding air resisting the forward motion of the sled. Heat losses in this "system" are usually not noticeable because they are small relative to the ability of the surroundings to dissipate the heat.

The basic energy relationship in a working process venturi is similar:

Motive fluid in the pipeline upstream of the restriction has a combination of potential energy (fluid pressure) and kinetic energy (fluid velocity). At the restriction, velocity increases which is to say: the kinetic energy of the motive fluid increases. The increase in kinetic energy must come from somewhere. The source of the additional kinetic energy is the reduction of the potential energy of the motive fluid, which is to say that the increase in motive fluid velocity requires that the pressure of the motive fluid in the restriction must drop.

Process venturi systems can be designed so that under normal flow conditions the motive fluid attains a constant, near-absolute vacuum in the restriction. The difference between the low pressure inside the venturi restriction and the higher atmospheric pressure in the ambient surroundings provides the energy that allows the venturi to draw in the injectate.

Downstream of the restriction, the pipeline is usually the same diameter as the upstream pipework. The fluid slows back down due to the increased pipe diameter. The kinetic energy that was present in the fluid at the restriction converts back to potential energy as the motive fluid slows down, raising pipeline pressure. Of course, the final energy condition downstream from the restriction must also account for the energy used to bring the injectate to the same velocity and pressure as the motive fluid.

Just as the sled is unable to regain its original elevation after sliding downhill, the motive fluid pressure downstream of the restriction never fully recovers to the inlet pressure, even if nothing is injected. This is because some of the energy in the system is lost to friction and dissipated as heat into the motive fluid.

ChemIndustrial builds systems that use process venturis for metering, blending and batching applications.