The present invention relates to a closure for an opening of large size in a pressure vessel, more particularly to a closure for such an opening in a canning retort, for example, the upper or lower opening in a canning retort.
Canning is widely used in the food processing industry, and consists of a process in which food is sealed into a metal container. The advantages of canning are many and well known. However, an essential step in the process is sterilisation at high temperature and pressure. This operation is carried out in retorts, which are large pressure vessels into which the cans are placed. Generally the retorts have openings at the top and bottom to facilitate loading and unloading, and these openings must be sealed during the sterilisation process. The doors or lids provided to seal these openings are large and heavy and may typically exceed a square meter in area. Various means have been developed for closing and sealing these doors and a particular known closure is described in U.S. Pat. No. 3,386,206. The door is movable across the opening and mounted on rollers. The door is first moved from a position in which it exposes the opening into a position in which it is spaced from the opening. The rollers are themselves mounted eccentrically on shafts rotatably mounted on the door by means of bearings. These shafts are then rotated and the relative eccentricity of the shafts and rollers causes the door to to move perpendicular to its plane and to seal the opening by pressing against a sealing ring in a recess in the sealing edge of the opening. In order to avoid damage to the sealing ring it is important for the perpendicular movement to take place without the door moving in its own plane. Such movement would scuff the sealing ring or pull it out of its recess. Separate means are provided to move the door from the position in which it exposes the opening and to rotate the shafts to bring about the sealing or locking of the door. For the first-mentioned movement, hydraulic or pneumatic cylinders may be provided or a rack and pinion or a wire drive. For the sealing operation, a further pneumatic or hydraulic cylinder is used.
The above-mentioned U.S. patent is directed in particular to a top closure. U.S. Pat. No. 3,417,896 discloses a similar bottom closure in which the door sealing or locking movement is again brought about by rotation of eccentric rollers. In this instance the detailed construction differs, in order to suit the upward sealing movement necessary in the case of a bottom closure, and to provide secure sealing on all sides of the door. In particular, the eccentric rollers are arranged to act as cam members bearing upwardly on the lower surface of the door.
The closures described in the above Patent Specifications operate satisfactorily in respect of keeping the door away from the sealing edge during movement of the door across the mouth of the opening, and sealing and unsealing displacement of the door only takes place in a direction perpendicular to the plane of the door. Thus these constructions avoid causing damage to the sealing ring in the sealing edge. However they incorporate a considerable number of operating parts. Consequently the number of components susceptible to failure is large. In addition, many of them are exposed to the difficult environmental conditions in the region of the openings in the particular case of canning retorts or pressure vessels. It is accordingly an object of the invention to provide a closure for an opening in a pressure vessel having a small number of operating parts and to provide an improved, effective and simple means for closing a large sized opening in a pressure vessel.