The heart of the HVAC system is the fan. Improper fan design or installation can cause serious degradation of performance because of "system effects." System effects were first described by Farguhar (1973)ref178 and by Meyer (1973) ref192. System effect factors for connections between fan and duct system are presented in "Fans and Systems" (AMCA, 1973), the ASHRAE Fundamentals Handbook [ASHRAE, 1997], and the DFDB [ASHRAE, 1994]. System effects cause changes in a fan's aerodynamic characteristics that result in an entirely new fan performance curve. They appear when inlet and outlet connections cause non-uniform inlet and outlet flow and swirl at the fan inlet.
An engineer designing ductwork should provide uniform straight flow conditions at the fan inlet and outlet. An elbow at a fan's inlet causes turbulence and uneven flow into the fan opening. An elbow at a fan's discharge causes a non-uniform backpressure. An engineer can only estimate how much pressure will be lost as a result of system effects by reviewing the air distribution system design.
System effects are rarely measured during system start-up procedures. They become evident only when not enough air is delivered to the building. Start-up personnel must then speed up the fan by changing pulleys or even increasing the fan's motor size in order to get design air-flow rates.
System effects can be reduced or largely avoided by a few easy precautions for centrifugal fans, as follows: