Finding Fan Curve w/ Orifice Plates & Fixed RPM

  • Thread starter gomerpyle
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In summary, the conversation discusses how to find the fan curve when using different sized orifice plates to create restrictions. The issue lies in not knowing the Reynold's number, which affects the coefficient of discharge needed to calculate flow. The speaker suggests using the hydraulic diameter to compute the Reynolds number and notes that the fan's manufacturer should have provided a fan curve.
  • #1
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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  • #2
Use the hydraulic diameter of the orifice at its exit to compute the reynolds number.
 
  • #3
A fan curve goes with the fan, its manufacturer should have given it.
 

1. What is a fan curve?

A fan curve is a graph that represents the performance of a fan in terms of its flow rate and pressure head at a fixed speed.

2. How do orifice plates affect the fan curve?

Orifice plates are used to measure the flow rate of a fluid in a pipeline. They create a pressure drop, which is reflected on the fan curve as a decrease in pressure head at a given flow rate.

3. Why is it important to find the fan curve with orifice plates?

Finding the fan curve with orifice plates allows us to accurately measure the performance of the fan in a real-world scenario, where there may be obstructions or flow restrictions in the pipeline.

4. Can the fan curve with orifice plates be used to determine the efficiency of the fan?

Yes, the fan curve with orifice plates can be used to calculate the efficiency of the fan by comparing the input power to the output power at a given flow rate.

5. How does a fixed RPM affect the fan curve?

A fixed RPM (revolutions per minute) means that the fan speed remains constant. This results in a linear relationship between flow rate and pressure head on the fan curve, with an increase in flow rate causing a decrease in pressure head and vice versa.

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