Operating Point
The operating point of the fan and the system is the point where these two curves intersect. This intersection will determine the cfm and Ps delivered.
Adjusting Fan Performance
There is a direct relationship between cfm and rpm within a system. Doubling the fan rpm will double the cfm delivered.
Sample problem:
The figure on page 21 showed a fan curve at 700 rpm which had an operating point of 1000 cfm at 0.25 in. Ps. What rpm is required to move 2000 cfm through the same system?
Solution:
Within a system, cfm is directly related to rpm. Therefore, the new rpm (rpm2 ) can be determined from the following equation:
Varying Operating Points
rpm2 = rpm1 X ( cfmcfm12)
=700 rpm x (10002000 cfmcfm) = 1400 rpm
Referring to figure at right, this results in sliding up the system resistance curve from 700 rpm to 1400 rpm.
Notice that as we doubled our airflow from 1000 cfm to 2000 cfm, the Ps went up from 0.25 in. to 1.0 in. It must be kept in mind that we are not changing the system, only increasing fan speed. Therefore, we must remain on the system resistance curve. Within a system, Ps varies as the square of cfm. Since cfm and rpm are directly proportional, an equation relating Ps and rpm can be derived as follows:
Ps2 = Ps1 X ( rpmrpm12 )2
For our example,
2( 1400 rpm )2
Ps = 0.25 in. X 700 rpm = 1.0 in.
This verifies the operating point on the 1400 rpm curve (2000 cfm at 1.0 in. Ps). With this example, it should be clear how cfm, rpm and Ps tie together in a steady- state system.
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