for example but the rolling unit 33 is not limited thereto as shown in fig7 each circulation tube 31 includes a straight portion 311 and two u like cornering portions 312 respectively extended from two opposite ends of the straight portion 311 the length of the straight portion 311 of each circulation tube 31 is greater than the length of accommodating channel 22 the ends of two cornering portions 312 of each circulation tube 31 are facing to each other and the distance of the ends of two cornering portions 312 of each circulation tube 31 is smaller than the length of accommodating channel 22 one of the cornering portions 312 of each circulation tube 31 includes a fill in opening 313 formed on a portion thereof adjacent to the straight portion 311 the separating sheet 32 is arced and installed on the circulation tube 31 to shield the fill in opening 313 such that the circulation tube 31 and the corresponding separating sheet 32 are jointly defines a rolling path in which the rolling path communicates with the outer space via the ends of cornering portions 312 of the corresponding circulation tube 31 as shown in fig8 the straight portion 311 of.