In the UK, Howden has a long history of developing and innovating solutions on industrial fans. One such innovation has been in the area of centrifugal fans running at variable speed. From our base in Renfrew, United Kingdom we have researched and engineered solutions that ensure the health of centrifugal fans operating with variable speed drives. Howden have devised a stringent set of rules for the assessment of fans. Each fan is designed to ensure it complies with our acceptance criteria for:-
Finite Element Analysis is a widely used design tool in this process.
Howden are experts in the design of rotating machinery and have the technical knowledge and experience to assess whether a switch to variable speed operation is structurally acceptable for a customer's operating requirements.
Howden would recommend the following before implementing a variable speed drive retrofit:
Summary – After VSD Installation
Variable speed drives were fitted to Waste Gas Sinter Fans in a steelworks and the operating regime changed. Instead of running at constant speed these fans were now running with several speed changes per day.
Soon after the change to variable speed, a routine inspection of the fan flagged numerous cracks at weld toe positions, some up to 50mm in length.
In addition full thickness cracks were noted at the centreplate cutaways, as shown in the photograph and image in figure 1.
The depth and extent of the cracking observed resulted in the fan being deemed unsafe to use and immediately taken out of service.
Howden were asked to carry out a Finite Element Analysis of the original fan and to design a replacement which would be suitable for variable speed operation.
The analysis of the original impeller showed it had acceptable structural integrity for constant speed operation. This had been demonstarted by its actual service life to date (over 20 years of start/stops).
However, an area of relatively high stress at the root of the centreplate cutaway was noted, as shown in the following plots. This was consistent with the location of the cracks found during inspection.
The estimated fatigue life for the fan, following convertion to variable speed operation was calculated to be only a few months. This hence confirmed that this fan was not suitable for variable speed operation.
The analysis was repeated for a new impeller design, This incorporated several modifications, including changes to weld specifications and to the cutaway dimensions and location.
Changes were also introduced to minimise the impeller inertia, thus allowing it to run on the existing supports and with the previously used motor.
The results confimed the following for the new design:-
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