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Based on the energy conservation low, Venturi
tube is one of the easiest to use, not expensive and very
accurate instrument for flow rate measuring in pipe systems.
Determine pressure drop from the inlet to the throat and
calculate flow using calculator. |
(requires
Java Runtime Environment http://java.com/en/download/index.jsp
)
| Explanation
of used values |
D1
D2
p1
p2
p1 - p2
ni
mi
T
rho
R
|
diameter of tube in position 1 - inlet
diameter of tube in position 2 - throat
pressure in the position 1 - inlet
pressure in the position 2 - throat
difference in pressure between the position 1 and 2
kinematic viscosity
dynamic viscosity
temperature in the position 1
density in the position 1
gas constant
|
| Resulting
values |
Q
G
V1
V2
Re
C |
volumetric flow rate
mass flow rate
velocity in position 1 - inlet
velocity in position 2 - throat
Reynolds number
coefficient of discharge |
Theory
Calculation of flow rate using orifice plate
calculator is for incompressible flow, based on the Bernoulli
principle:
where is:
p - pressure
rho - density
V - velocity
g - gravitational constant (9.81 m/s2)
z - geodetic height
Assumption that pressure lost is negligible
(pressure drop is obvious and included with coefficient of
discharge which is introduced bellow):
and:
and if velocities substituted with flow
rate:
 
where is: Q - volumetric flow rate
D - diametar
Pressure drop through the orifice because
of velocity increase can be calculated as follows:
or:
Expressing flow rate from the previous equation
leads to:
Substituting:
flow rate can be determined as:
where C is coefficient of discharge. The
above equation is main one used for flow calculation in calculator.
Other values are calculated using following equations:
mass flow:
velocities:
If calculator is used for gas flow, then
gas is considered as incompressible and ideal. Equation for
ideal gas:
can be used for calculation of temperature
T:
as well as density rho:
where R is gas constant (R=287 J/kgK for
air).
Coefficient of discharge C
As fluid exits a reservoir through a small
hole and enters another one, or flows out to the open air,
stream lines tend to contract itself, mostly because of inertia.
Coefficient of discharge C is used to include this effect.
For the Venturi tubes with diameters in range of D = (200
- 1200 mm), D2/D1 = (0.4 - 0.7) and
ReD = (2 *105 - 2 *106) the coefficient
of discharge is C = 0.985.
In this calculator for coefficient off discharge C following
equation has been used:
where a, b, and c depend on the type of
Venturi tube.
For welded tube, these coefficients are:
a=0.70304970
b=0.00490015
c=-0.00024547
For casted tube are:
a=0.60892370
b=0.00659844
c=-0.00033123
And for machined are:
a=0.49670179
b=0.00873339
c=-0.00044367
Reynolds number on inlet ReD is calculated using well known
equation:
Appliance
Flow through Venturi tube calculator can
be used for both liquids and gases. Fluid is considered as
incompressible, so density (rho) and temperature (T) are constant
through tube. Also, gas is considered as ideal.
Units of measure are both in SI and English
system.
Flow through Venturi tube calculator can
be used for calculation of:
volumetric flow - Q
mass flow - G
velocity on inlet, bigger diameter - V1
velocity on smaller diameter - V2
For calculation of those values, necessary values for input
are:
inlet, bigger diameter - D1
smaller diameter - D2
inlet pressure - p1
either pressure on smaller diameter - p2, or pressure drop
p1-p2
kinematic viscosity - ni, or dynamic viscosity mi
for gases only:
either temperature - T, or density - rho
gas constant - R
for liquids:
density - rho
Beside four values (Q, G, V1, V2), which
calculation is main purpose of calculator, values that are
not defined by user are determined in process of calculation
(for example: if pressure p1 and temperature T are specified
for gas flow - value for rho is calculated together with four
main values (Q, G, V1, V2))
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