10 protective means in the form of angle irons and the like as shown at 400 401 are provided mounted above the hinges to prevent jamming of the hinge by material exiting from the unit the doors open into a chute 222 which has a second pair of similarly mounted doors 224 which function in a similar fashion as shown by the solid and phantom lines the chute 222 opens into the crusher 92 the ejector improves the operation of the system by recirculating a portion of the nitrogen which would otherwise be discharged this portion of the nitrogen is drawn into the ejector from the discharge header 218 as shown by the arrows in fig6 and is induced to flow from the discharge end of the drum toward the intake or upstream end of the drum the flow pattern provided by the recirculating system provides a pre cooling chamber in the upstream portion of the drum 10 between the port 86 and that nozzle which is positioned at the upstream end of the header 100 in this pre cooling chamber the nitrogen gas which has lost much of its cooling power by virtue of contact with the material in the drum in the downstream freezing portion thereof is exposed to the incoming material to reduce its temperature before it enters the freezing chamber thereby increasing the effectiveness of the system in the freezing portion of the drum since more nitrogen is constantly being introduced through the nozzles 102 there is a build up of nitrogen gas such that the excess must at times be withdrawn this.