This document addresses the optimum efficiency only. It is, however, possible to select fans for a given duty well away from this point. Such selections may give a smaller fan size with reduced first cost at the expense of increased running costs. It is recommended that the fan supplier and user should select over the range of air flows where the efficiency is not less than 85 % of the optimum efficiency. The typical curves below, Figure D.1 to D.3, illustrate the range of acceptable air flow rates for a fan.