|
Pneumatics
101
FLOW
COEFFICIENT (Cv)
WHAT
IS Cv?
Cv,the
flow coefficient, is the relative measure of a device's ability to flow
air. In a constant, established air flow, the flow coefficient is determined
by the volume of air flow through the valve and the inlet and outlet pressures.
When holding the inlet and outlet pressures constant, the Cv of a given
valve is also constant. In other words, a pneumatic device has only one
Cv value.
WHY
DO WE USE Cv?
Cv
gives us a method to compare flow capabilities of different valves. The
flow coefficient allows us to determine what size valve is required for
a given application.
WHAT
AFFECTS Cv?
Several
factors have an impact on the flow coefficient. Some of them are listed
below:
- Orifice
size (diameter of the piping or opening through the valve)
- Length
of piping or opening through the valve
- Turbulence
caused by bends or turns in the piping
- Restrictions,
or anything that reduces the orifice size or the flow path
- Shape
of the orifice
TYPES
OF Cv
When
components of a pneumatic circuit are arranged in series with each other
,
air flow goes through all the components in the circuit, and there is
one flow path. Conversely,
when the components are connected in parallel, flow is divided among all
the components in the circuit, and there is more than one flow path.
THINGS
TO REMEMBER WHEN CONSIDERING Cv
When
comparing valves that have the same pipe size, line-mounted valves generally
have a higher Cv than base-mounted valves. This difference results because
line-mounted valves have straighter flow paths with fewer restrictions.
In addition, when calculating Cv, consider typical shop air to be approximately
100 psig with a 10 psig drop. The concept of a "10 psig drop" means that
if air pressure goes into a valve at 100 psig, then it comes out at 90
psig.
Back
to Pneumatics 101
|
