Ductwork System Effect

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:

• With a free inlet fan (no inlet duct), a minimum of one half the fan wheel diameter, D, clearance should be maintained from the inlet to the wall or structure.
• An elbow entrance to the fan should be at least two times the inlet diameter distance from the flex connector at the fan, C=0.15 (remember, the fan entrance is a high-velocity duct).
• A 2D cone to the inlet flex (vibration isolator) should be used to decrease friction losses (C=0.10).
• If this 2D inlet cone is attached to a rectangular, vertical duct, its dimensions should be two times the fan's inlet diameter by one half the fan's inlet diameter and extend one diameter past the center line of the fan's inlet (Figure 1).
• A duct expansion from the fan discharge should be at a 10° angle up on a top horizontal discharge fan (Figure 2). Down angles and increases in the width of the duct are less efficient.
• A straight duct for a distance of three to six duct diameters from the fan discharge should be used in order to develop a full dynamic head. Branching and turning sooner causes system effect losses.
• A fan discharge diffuser should be used before air is released to the atmosphere.

Figure 7.1

Figure 7.2