proper application of fan laws?

[jaywing]

I have an existing installation that I'm trying to apply the fan laws to. As shown in the attachment, we've had a testing and balancing firm perform tests to see what the system is currently doing. These tests were performed with the bypass dampers open. I am trying to calculate what the new airflow will be with the bypass dampers closed and with the air flowing through the heat exchanger.

Using the fan laws:

CFM1/CFM2=RPM1/RPM2
SP1/SP2=(RPM1/RPM2)^2

Substituting the two equations for RPM:

SP1/SP2=(CFM1/CFM2)^2

Knowing the pressure drop through the HX is 1":

2.00"/(2.00"-1.00")=(57,000 CFM/X CFM)^2

I get 40,305 CFM as the new airflow. Can the fan laws be used in this situation? I'm not sure if the two situations are considered 'similar conditions' for fan laws to apply, and I'm also concerned that I seem to be equating the flow at I with the flow at II.

 

[MintJulep]

You cannot use FAN laws to calculate SYSTEM induced effects.

You need the performance curve for your specific fan.

 

[chicopee]

The first three equations that you presented can only be used to develop additional fan characteristic curves from the original one provided by the fan manufacturer.

MintJulep is correct.

What I would do is assume that the initial system curve is quadratic in nature and develop that curve from the original values of 1" SP and 57000 cfm and 0"SP and 0 CFM. Then you can plot that curve from a series of SP and CFM values. Then plot you fan characteristic curve(s) depending which RPM you are using and find the intersection fo the two curves for your SP and CFM that you wanted.

 

[chicopee]

By the way, you fourth equation is faulty if you were to apply it to a new fan curve. You don't subtract the two SP values.