A fan is a critical and the most energy-consuming component of a ventilation system. In today’s scenario of sustainable development, energy efficiency has become the predominant criterion for selecting a fan for a given application. A system designer specifies certain criteria to be followed by the fan manufacturer to select the right fan, motor and drive for the specified duty point. A review of these criteria and specifications over the years has led the authors to believe there are a lot of misconceptions about an HVAC fan and its operating behaviour. These misconceptions may have arisen historically due to the unavailability of good design fans and the lack of certified fan performance data. Incorrect specifications based on these misconceptions may lead to less-than-optimal fan selection. This paper discusses many such misconceptions and their implications for fan selection. Many examples are included for illustration and ease of understanding. It is hoped that the paper will help in clearing up these misconceptions and lead to amended specifications resulting in more judicious fan selections. Most of the material in this paper has been taken from the HVAC Fan Guidebook, an ISHRAE Publication (1). This guidebook was written by ISHRAE’s Fan Technical Group (TG), which was then chaired by Mr Sushil Choudhury. Mr Sachin Maheshwari is the current Chair of the Fan TG.
It is commonly assumed that lower fan speeds will necessarily result in reduced fan noise. This assumption is incorrect and should not be used as a basis for fan speed specifications. Fan noise is influenced by both speed and size, and focusing solely on speed is misleading. Imposing arbitrary speed limits (e.g., not exceeding 1000 rpm or 1500 rpm) can severely restrict the selection of suitable fans for a given application. Generally, smaller propellers and circular inline fans selections come with 2800 rpm motor options; however, many specifications demand not more than 1440 rpm motor. The same applies to smaller backwards curved and tube axial fans in 250-450 mm size, which have to operate at more than 1440 rpm to achieve the design pressure levels in practical applications.
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