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[December 2007]
Ceramic Bearings Solve Hospital Fan Failure Problems
When fans in the ventilation systems for operating theatres and other important zones in a five-year old hospital began to break down there was real concern. Not only was there risk of having critical zones out of service, there was also some doubt about the true cause of the fan failure. Meanwhile maintenance costs were becoming excessive. Specialist SKF knowledge determined the root cause of the problem and provided the solution with innovative hybrid bearings that have ceramic balls instead of steel balls.
Sunderby Hospital at Lulea, Sweden was built in 1999 to serve as a central hospital for a community of 250,000 inhabitants. Several complex ventilation systems with a total of 150 variable speed fans are installed on the 78,100 square metre site.
The systems serve different zones in the hospital and use variable speed fans because as well as saving energy they make each system more responsive to a zone’s varying air demands. To give precise control of airflow to a specific zone the fan speed is controlled by a frequency converter. Naturally, the hospital places a high demand on fan reliability for general air conditioning and also filtering and expelling air from operating theatres. So there was some distress when instances of fan failure began to occur.
Knowing that 50 percent of all electrical motor failures are attributed to bearings, the hospital maintenance manager, Kenneth Simu, arranged for examination of the failed motor bearings. “I could see that the bearings showed damage that would lead to a failure but could see no reason for the damage. After all, our fan motors are well maintained and were only five years old”.
Finding the cause behind the damage to the fan motor bearings became a top priority. The search was made even more important by the high cost of changing a bearing at the hospital. It takes two men two hours (working during the night shift) to replace a bearing at an estimated cost of 4000 SEK – excluding the cost of the bearing. So, because Kenneth Simu was unfamiliar with the type of damage seen on the failed bearings he asked SKF to investigate. The SKF specialists immediately identified the bearing damage as that caused by high frequency electrical discharges in the raceway-rolling element contact. This type of damage is revealed by micro-pitting of raceways, greying of rolling elements and raceways, washboard patterns, blackening of the grease and a smell of ozone. It occurs most often in smaller motors with standard bearings that are being speed controlled by a frequency converter.
To overcome the problem SKF recommended that hybrid bearings be fitted to the non-drive end of the fan motors to effectively prevent bearing damage from the electrical discharges. The balls of a hybrid bearing are made from a ceramic material that acts as an effective electrical insulator. In addition, the grease life of ceramic bearings is longer so that bearings last for years instead of months.
“I was pleased to have an effective solution to the bearing breakdown problem,” said Mr Simu.
Seven fans at the Sunderby Hospital were upgraded to hybrid bearings. It is estimated that the savings for these fans over five years will amount to 75,000 SEK.
ENDS