GB1494104A - Freeze tunnel - Google Patents

Abstract

1494104 Tunnel systems CRYOGENICS CORP OF AMERICA 6 Dec 1974 [10 Dec 1973] 52943/74 Heading F4H [Also in Division H2] A tunnel refrigeration system comprises a number of modular units joined together by connectors 30 to form pre-cooling zones 12 and 14, and freezing zones 16, 18, 20, with load and discharge zones 22, 24 disposed at opposite ends of the tunnel. A conveyer 26 comprising an endless flexible bolt 40 on supports 50 passes through the zones, with an adjustable variable speed motor 42 driving the bolt. Each modular unit has side walls 32 and 34, a top 36 and bottom 38 formed of spaced sheets 38a, 38b of stainless steel with insulating foam 38c therebetween. A copper circular cooling coil is disposed in each zone; each coil having an inlet and outlet manifold joined together by a tubular run 74, 76, 78. An annular plenum chamber 85 is formed about each coil, an inner wall 88 of said chamber being adjacent the lower end of each coil. A fan 60 forces air downwardly from the coil through the spacing between 85 and 88 and upwardly through perforations in the belt back to the fan. Baffles (92, Fig. 4) extend across the tunnel in order to separate the zones. Zone 12 is preferably maintained at -20‹ F., zone 14 at -50‹ F., zone 16 at -100‹ F., zone 18 at -200‹ F. and zone 20 at -250‹ F. A container (100, Fig. 8) of liquid nitrogen, CO 2 &c. is connected to the inlet manifolds of the coils. The coils in zones 16 and 20 are connected in series, as are the coils in zones 14 and 18. The two sets of coils are themselves connected in parallel to a single exhaust line (114, Fig. 8). Under normal conditions (i.e. when defrosting is not required in zone 12), the cooling medium flowing through the exhaust line (114) pases through the cooling (62a) in zone 12 to a vent line (120, Fig. 8). When the coil in zone 12 becomes iced up, the cooling medium in exhaust line (114) passes directly to the vent line (120) by-passing the said coil. At the same time a bank of heating elements are energized for a predetermined period of time to defrost the coil. An electric control system is described in detail for operating the tunnel refrigeration system and the defrosting operation. The electric control circuit includes a defrost controller (128) in the form of a resistance bridge network including a first thermistor (136) mounted in the vapour exhaust line (120), through which the cooling medium vapour is exhausted to atmosphere, and a second thermistor (138) disposed inside the precooling zone 12. When frost on the surface of the coil in zone 12 creates a predetermined temperature differential, the resistance bridge becomes unbalanced, activating the defrost cycle.