Flow Nozzle

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Flow Nozzle
  • The flow nozzle as shown in Fig.15.4 is essentially a venturi meter with the divergent part omitted. Therefore the basic equations for calculation of flow rate are the same as those for a venturimeter.
  • The dissipation of energy downstream of the throat due to flow separation is greater than that for a venturimeter. But this disadvantage is often offset by the lower cost of the nozzle.
  • The downstream connection of the manometer may not necessarily be at the throat of the nozzle or at a point sufficiently far from the nozzle.
  • The deviations are taken care of in the values of CdThe coefficient Cd depends on the shape of the nozzle, the ratio of pipe to nozzle diameter and the Reynolds number of flow.
Fig 15.4 A Flow nozzle
  • A comparative picture of the typical values of Cd, accuracy, and the cost of three flow meters (venturimeter, orificemeter and flow nozzle) is given below:
Type of Flowmeter
Accuracy
Cost
Loss of
Total Head
Typical Values
of Cd
Venturimeter
    High   High       Low
     0.95 to 0.98
Orificemeter
    Low   Low       High
     0.60 to 0.65
Flow Nozzle
Intermediate between a venturimeter and an orificemeter 
     0.70 to 0.80

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