of the second fan 3 and turn on the power supply of the first fan 2 the processes are repeated under control when the rotation speed of either the first fan 2 or the second fan 3 decreases the rotation speed of other fan is increased to maintain the cooling effect for the elements which require heat dissipation and a warning signal is issued to increase the service life of the fans refer to fig3 for the invention in a use condition a server 4 a general personal computer has a casing 41 which has a rear panel or front panel 42 on the panel 42 there are the first fan 2 and the second fan 3 installed thereon when the first and the second fan 2 and 3 in operation they can discharge heat generated by the elements located in the casing 41 of the server 4 outside the casing 41 so that the elements in the casing 41 may function properly in the event that the rotation speed of either the first fan 2 or the second fan 3 decreases the rotation speed of other fan increases to maintain the cooling effect for the elements which require heat dissipation and a warning signal is issued to increase the service life of the fans refer to fig4 for the invention in another use condition a power supply 5 has a casing 51 which has a rear panel or front panel 52 on the panel 52 there are the first fan 2 and the second fan 3 installed thereon when the first and.