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
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
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
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.