Excessively large margins on design performance are bad practice because they impose restrictions on fan selection and prevent operation at best efficency. They should not be set. Nevertheless, there is a gap between design and commissioning: there may be design errors, late changes in the required performance, unforeseen installation difficulties, mistakes in the interpretation of the design intention by the erector of the duct system, and so on. There is also the possibility of air leakage. The HVCA (1998) deals with air leakage, giving acceptable leakage limits and specifying testing procedures. The CIBSE (1986b) deals with allowable leakage rates in detail.
Bearing in mind the above, it is desirable to augment the design duty by a small amount, for the purpose of fan selection. It is suggested that the volumetric airflow rate should be increased by 5 per cent and, since pressure drop in a system is proportional to the square of the airflow rate, the fan total pressure should be increased by 10 per cent.
Although the fan manufacturer usually selects the driving motor it is often useful to be able to make an estimate of the probable motor power at an early stage in the design. As is seen later, backward-curve impellers have a non-overloading characteristic curve for fan power versus volumetric airflow rate, whereas forward-curved impellers do not. It is suggested that, for the purpose of making a preliminary estimate of the power of the motor needed to drive a fan, margins of 25 per cent be added for backward-curved impellers and 35 per cent for forward-curved impellers. These allowances would be applied to the fan powers determined from equation (15.20), including the margins of 5 per cent and 10 per cent for the airflow rate and fan total pressure, respectively. The power would then be rounded up to the next commercial motor size.
It is to be noted that when the fan starts, enough torque has to be available to accelerate the impeller to the running speed in a reasonably short period of time, depending on the characteristics of the starter. About 18 seconds is a reasonable period. See Keith Blackman (1980).
When commissioning a system it is often necessary to increase the fan speed to achieve the design duty, in accordance with fan law Al. The success of this depends on the driving motor having enough power.
Posted in Engineering Fifth Edition