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TN 65: Recommendations on Specific Fan Power and Fan System Efficiency

Schild P.G., Mysen M. , 2009
Bibliographic info: AIVC Technical Note 65, 2009, 42 pp
Languages: English Pages (count): 42

Energy use for fan operation can be significantly reduced by a 3-flanked approach: 

(1) The first step is prudent sizing of ventilation rates by minimizing the demand (e.g. low-emission building materials, passive cooling design), and by utilizing efficient air distribution. The latter reduces unnecessary over-ventilation by use of airtight ductwork, careful choice of room airflow principles (i.e. minimizing short-circuiting), and controls for demand-control of flow rate.

(2) Perhaps the most important measure is to minimise flow resistance, and hence fan pressure. This is achieved by aerodynamic design of fan inlets/outlets and ductwork layout (including optimal location of plant rooms and duct risers, to reduce duct length), liberal sizing of components in the duct system, and increasing AHU cabinet size, but without oversizing the fan system.

(3) Optimize efficiency of the fan system, including the fan, drive, motor, and variable speed drive (i.e. minimize total ‘wire-to-air’ losses). Oversizing must be avoided, since fan efficiency can decrease significantly if the combination of airflow and pressure rise is not near the combinations giving peak efficiency. Motor and drive efficiencies can also decrease rapidly at low loads. Thus, oversizing and load diversity are key factors affecting system efficiency. 

These three measures are far more important than any exploitation of natural driving forces, in climates where heating or cooling is needed. This guide focuses on points (2) & (3).

Besides reducing energy use, energy-efficient systems are generally less noisy than inefficient systems.

To achieve these potential savings, building developers/owners must dictate and verify fan power performance specifications. All countries should have building regulations set limits on fan power, and establish inspection/auditing schemes that include spot checks of fan power. Furthermore, there is a need for simpler calculation tools for calculating duct system pressure loss and specific fan power in buildings, and easy commissioning guidelines.

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