This invention relates to apparatus and method for the quick freezing of food products and the like and in particular to a new and improved apparatus and method which provides for freezing in shorter time and with greater efficiency. The specific embodiment of the invention described herein is particularly suited for use with packaged vegetables, packaged meat and whole fowl, but is equally suited for quick freezing of other items.
Liquid nitrogen is widely used for quick freezing of various products and two basic systems presently are in use. One system is called the "freezing tunnel" which utilizes an insulated enclosure with a conveyor belt or the like for moving product therethrough. Usually about three quarters of the way along the conveyor path, spray nozzles are installed for spraying liquid nitrogen onto the product. A substantial portion of the liquid nitrogen is transformed into nitrogen vapor at the spray nozzle. The remaining liquid nitrogen is quickly vaporized on contact with the product, producing cooling of the product. The vapor is circulated through the enclosure by fans for increasing the efficiency of heat transfer between the vapor and the product. Normally a thermostat provides a control of the amount of liquid sprayed into the enclosure for maintaining the temperature within predetermined limits. The product is frozen during its passage through the tunnel and emerges ready for storage and/or shipment in refrigerated equipment.
The freezing tunnel method under favorable conditions transfer approximately 120 BTU of cooling to the product, from a theoretically possible 160 BTU, thus operating at approximately 75% efficiency. A typical tunnel is in the order of 60 feet long and requires several minutes for the product to move through the tunnel. This relatively slow freezing, the large size of the tunnel and the relatively high cost of the system are disadvantages.
The other system for quick freezing currently in use is the so-called "liquid immersion system" utilizing a liquid bath in an insulated tank with some form of conveyor for moving the product through the bath. Freezing is accomplished by moving the product on the conveyor belt through the liquid nitrogen with the fully frozen product emerging at the downstream end. One such liquid immersion system is shown in my U.S. Pat. No. 3,832,864, and other liquid immersion systems are shown in the references cited in said patent. The liquid immersion system has several advantages over the freezing tunnel system including size, typically being about 9 feet long, faster freezing, taking several seconds rather than several minutes, and lower cost, costing about 1/3 that of a freezing tunnel for the same capacity. However the efficiency of the liquid immersion system is about the same as that of the freezing tunnel system.
A combination liquid immersion and freezing tunnel system is shown in U.S. Pat. No. 3,376,710. In the system shown in this patent, product is introduced for freezing by sliding it over an entrance ramp until it drops into a liquid nitrogen bath. The product is suppose to drop to the bottom of the bath and is moved along the bottom by fingers attached to a drive belt positioned above the surface of the liquid nitrogen. The nitrogen bath provides the first stage of cooling. The fingers of the belt push the product up a slope out of the bath for discharge onto a conveyor belt for the second, vapor cooling stage.
The second stage uses nitrogen vapor from the liquid bath. Six recirculating fans are located in the second stage housing above the conveyor belt. Rounded directional top and bottom baffles are provided for each fan for controlling gas flow. Nozzles are provided for introducing nitrogen vapor adjacent each of the fans. The purposes of the fans and baffles is to provide relatively high vapor velocity, in the order of 1500 to 3000 feet per minute.
The aforesaid system suffers from a number of disadvantages. Because of the extremely low temperature at the entrance of the first stage, there tends to be substantial frost build-up so that the product sticks and does not freely fall into the liquid bath. The high gas velocity in the tunnel section tends to make products airborne. Also, the amount of energy required to produce this high velocity produces heat within the tunnel, reducing the efficiency of the system. The liquid bath and the subsequent cooling conveyor are positioned in a single insulated chamber, with additional nitrogen being sprayed into the conveyor system. This means that the overall system operates at substantially the same temperature, which must be the lower temperature at the liquid bath, necessarily at -320.degree. F. in order to prevent boiling of the nitrogen. Maintaining such a large piece of equipment at such a low temperature would result in low efficiency and also damage to a number of food products which have a tendency to crack when exposed to such a low temperature for such a long period of time. Also, each of the fans produces a doughnut shaped or annulus high velocity zone, with very low velocities in the areas between the fans. This non-uniform flow pattern tends to produce inequalities in freezing of product at different locations along the conveyor belt.
Another U.S. Pat. No. 3,485,055, discloses product precooling with cold liquid nitrogen vapor on one horizontal conveyor belt, immersion in liquid nitrogen on another vertical belt, additional cooling by liquid nitrogen spray, and final postcooling on another horizontal belt. An additional belt is used to wrap around the vertical belt for the purpose of preventing the product from falling off the vertical conveyor. Major portions of the belts are out of the liquid bath. A total of four conveyors are used. The entire system is in a single chamber.
U.S. Pat. No. 3,413,818 discloses a system similar to that of U.S. Pat. No. 3,485,055, but it is even more complicated. Product precooling is done on a horizontal regular conveyor. Immersion freezing is accomplished by another conveyor with baskets or buckets. For postcooling, another conveyor with buckets is utilized. Vapor circulation is accomplished by means of blower, duct, air locks and dampers. This patent proposes to push the vapor through several buckets full of product. The vapor pressure required to do so would have be to so high that the product would become airborne. Also the postcooling bucket conveyor could not run substantially slower and with heavier loading than the immersion conveyor. The cold vapor used for postcooling would pass around the buckets rather than contact all the product.
It is an object of the present invention to provide a new and improved quick freezing method and apparatus which can be operated with improved efficiency while at the same time holding down size and cost.
It is a further object of the present invention to provide a method and apparatus for preventing frost build-up at the inlet of the system, and in particular, to provide for withdrawal of a small quantity of nitrogen vapor at the inlet and mixing with outside air.
It is an additional object of the present invention to provide a new and improved method and apparatus which eliminates any need for recirculating fans and/or nitrogen spray in the second stage cooling tunnel. A particular object is to provide such a system incorporating an external exhaust for drawing nitrogen through the cooling tunnel and for providing control of the rate of flow of nitrogen vapor in the cooling tunnel for maintaining a desired temperature. An additional object is to provide a system wherein a relatively low uniform gas velocity is maintained in the cooling tunnel, and incorporating baffles for producing gas flow across the product as the product moves through the tunnel.
It is a particular object to provide a new and improved quick freezing method and apparatus with the liquid immersion system thermally insulated separate from the cooling tunnel so that the two components of the overall system can be maintained at different temperatures and also so that the conveyor speed of product through the liquid bath and through the cooling or freezing tunnel can be different. More particularly, it is an object to provide a system wherein there is a relatively low but substantially uniform vapor velocity through the freezing tunnel, with the product speed through the tunnel being reduced in order to extend the product exposure time, thereby increasing operating efficiency and improving product freezing.
An additional object of the invention is to provide such a method and apparatus wherein the exhaust magnitude is automatically varied to provide the desired temperature operation with variations in loading, such as the number, size and/or inlet temperature of product being frozen.
A further object of this invention is to provide method and apparatus wherein the exhaust nitrogen vapor may be utilized for reducing the temperature of the room in which the freezing system is operated and for creating a slight positive pressure in the room thereby reducing the likelihood of contamination by dust, insects and the like.
Other objects, advantages, features and results will more fully appear in the course of the following description.