Finding Fan Curve w/ Orifice Plates & Fixed RPM

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SUMMARY

The discussion focuses on determining the fan curve using orifice plates at a fixed RPM. The challenge arises from the need to calculate the flow rate, which requires knowing the Reynolds number, a value that depends on the flow itself. The suggested solution involves using the hydraulic diameter of the orifice to compute the Reynolds number, allowing for the calculation of the coefficient of discharge. Ultimately, the fan curve should be obtained from the manufacturer's specifications.

PREREQUISITES
  • Understanding of fluid dynamics principles, specifically orifice flow equations.
  • Familiarity with Reynolds number and its significance in flow measurement.
  • Knowledge of fan curves and their relevance in fan performance analysis.
  • Basic concepts of hydraulic diameter and its application in flow calculations.
NEXT STEPS
  • Research the calculation of Reynolds number using hydraulic diameter in fluid flow scenarios.
  • Study orifice flow equations and their application in determining flow rates.
  • Explore fan performance curves and how to interpret manufacturer data.
  • Investigate alternative methods for measuring flow in duct systems without relying solely on Reynolds number.
USEFUL FOR

Engineers, HVAC professionals, and anyone involved in fluid dynamics or fan performance analysis will benefit from this discussion.

gomerpyle
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If I had a fan sitting in a small duct, and used several different sized orifice plates to create different restrictions, how would I find the fan curve? I run the fan at the same RPM each test.

The problem I run into is using the equation for orifice flow that relates pressure drop, area ratio, density, and coefficient of discharge to find flow. The coefficient of discharge is a function of the Reynold's number, but if the flow is what I'm trying to measure, then I don't know the Reynold's number. As a consequence of that I cannot know the coefficient of discharge, and thus cannot calculate flow for each orifice plate. Sort of like a paradox it seems to me.

Am I missing something here? Is there an alternative method to generating a fan curve?
 
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Use the hydraulic diameter of the orifice at its exit to compute the reynolds number.
 
A fan curve goes with the fan, its manufacturer should have given it.
 

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