 Published on 21 October 2013
 Views: 45006
Venturi Calculator
The Venturi tube is one of the easiest to use inexpensive and accurate instruments for flow rate measurement in pipe systems.

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 *10^{5}  2 *10^{6}) 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 p1p2
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))