To facilitate the maintenance of the surface of a chamber plate having a number of small holes for ventilation by a method wherein a guide rail is provided on the main body of an assembly, a plate member is fitted to the guide rail, and the plate member is brought in the state wherein it is freely detachable to the assembly main body.

Guide rails 7a and 7b are provided on main body chassis 6s and 6b respectively. The plate members 11a and 11b, where the second chambers 12a and 12b are formed, are fitted with each other, and they are brought into a freely slidable state against the main body chassises. Normally, the main body chassisses 6a and 6b are fixed by screws 9a and 9b. When a maintenance operation is conducted, the screws 9 and 9b are removed, the plate members 11a and 11b are slidingly removed from the main body chassisses, and a cleaning and maintenance on the clogging and the like of ventilating small holes can be conducted easily.