Analytical Approach for the Performance Prediction of High Pressure Centrifugal Fans in Serial Arrangements

1.5 System & Preliminary Design

Centrifugal fans are commonly used as a component in industrial processes. The performance of the installed fan is often adapted to one defined operating point. By a change of the system resistances there is also a change in the operation point of the fan. Especially increased resistances, caused by an extension of the pipe system for example, can significantly shift the operating point of the fan to low efficiency areas up to the point of insufficient output. To meet these new conditions there are several options to increase the fan capacity. The first option to meet the higher demand is to accelerate the fan impeller which results in a higher operating speed, until the maximum permissible strength is achieved. If this option is not sufficient, the fan is often replaced by a larger fan. Another option to increase the available pressure is to combine two or more fans in a serial arrangement.
Despite the advantages of higher pressure, efficiency benefits at variable operating points as well as more variability in structural arrangement and electrical requirements, only theoretical descriptions to reliably design serially arranged fans are given in the relevant literature of fans. The most common method to calculate the performance of serially arranged fans is to sum up the pressure increases of each single stage at a constant volume flow rate. However, this method indicates deviations compared to measured results. In previous investigations it could be shown, that the first stage in a serial arrangement has a behavior comparable to the single fan. Despite the geometrically identical structure of the used fans, the second stage showed a different behavior. Hence, there is a need for the manufacturer to reliably predict the performance of serially arranged fans based on the data of the single fan. This paper introduces a new method to calculate the performance based on the specific energy. The method is evaluated with measurement results of different fans. The introduced method covers serial arrangements of identical fans as well as serially arranged combinations of different fans.