Fan System Effects: How Fan Ductwork Design Impacts Overall System Efficiency and What the Approach Should be for Optimization

Abstract

"In order to establish the aerodynamic performance characteristics of a custom fan or even a line of fans, the accepted practice of the industry is to carry out testing on a scale model in a laboratory to develop its fan performance curve. Then by applying affinity laws in conjunction with very specific rules that address scaling issues, the performance characteristics of geometrically similar fans can be predicted. These affinity laws (frequently referred to as fan laws) also allow the performance of fans operating at different speeds or handling gases at different density values to be accurately predicted.

However, it is often found that even fans with well defined and pedigreed performance curves are unable to meet their performance expectations once they are installed on the systems for which they have been sold. This is primarily due to the adverse aerodynamic impact of the inlet or outlet connections on the performance capacity of the fan and thus on the overall system efficiency. The general term for design conditions at inlets or outlets of fans that cause deficient aerodynamic performance is ‘system effects’. The characteristics of fan system effects are that they reduce fan capacity and are both velocity and geometry dependent. On the inlet side of a fan, this generally characterizes itself by a flow pattern that is highly non‐uniform. On the discharge side, the high and low velocity flow streams leaving the fan may simply be prevented from redeveloping a uniform flow profile and normal static pressure conversion before encountering a disturbance. The term can also apply to system elements such as silencers, elbows and transitions. For these components, the actual pressure drop across them may be significantly higher than their calculated or rated values if the velocity profile of the entering flow is skewed or non‐uniform. For either case (fan connection or system component), the result is that additional power will be required to address the flow rate required by the system. In many instances, system designers are simply unfamiliar with the importance of understanding system effects as it pertains to new fan selections and the attendant power requirements. At a minimum, a reasonable approach for new fan projects should be to establish the theoretical system effect of connection designs by using a recognized document such as Air Movement and Control Association Publication 201. The objective should be to first minimize their impact through appropriate connection design modifications in conjunction with potential fan inlet and outlet re‐orientation. Once the system effects are minimized, the residual value should be applied to the fan performance specification in order to ensure that the fan is selected for the required aerodynamic capacity. For system components, an approach that has proved to be of significant value is to predict the flow pattern using computation fluid dynamics (CFD) modelling tools and in this process, the design can be tweaked until the designer finds the overall pressure drops of a system are minimized to the greatest practical extent. Similarly, CFD can be used to predict the flow profile at a fan inlet to ensure that it is as uniform as practically possible.

This paper reviews the concept of system effects from the perspective of fan power requirements and provides a methodology for approaching system design from the perspective of optimizing fan energy use while achieving the required system capacity."