Centrifugal Blower Fundamentals - Siemens LME7 Series Technical Instructions

Technical Instructions
Document No. LME-1000

Centrifugal Blower Fundamentals

Since a centrifugal blower is the piece of machinery being controlled by the LME7, a brief mention of its
basic characteristics is warranted. Specifically, there are three fundamental "fan laws" that a person
working with such equipment should be aware of. These are:
1. Air flow varies linearly with the speed of the blower. In other words, the CFM of the blower is
directly proportional to the RPM of the blower.
2. The static output pressure of the blower (SP) varies by the square of the change in RPM:
3. The required brake horsepower of the blower (BHP) varies by the cube of the change in RPM:
Example: A blower spinning at 1750 RPM produces 10 in WC of static pressure, 4500 CFM of air flow,
and requires 20 BHP. What happens if the RPM is increased to 2750 RPM?
Assumptions: There is no air damper on the inlet to the blower, and system effects (such as the
restriction due to heat exchangers, the burner's diffuser, etc...) are not taken into account.
Flow: CFM (new) = (2750 / 1750) * 4500 = 7071 CFM
Pressure: SP (new) = (2750 / 1750)
Power: BHP (new) = (2750 / 1750)
Section 6
2
* 10 = 24.7 in WC
3
* 20 = 78 BHP
Page 2
LME Series
SCC Inc.