1. In an apparatus for flash freezing of articles comprising a hollow housing structure defining a substantially thermally isolated process chamber, means including a conveyor operating in said chamber for transporting articles in succession along a process path leading into, through and out of said chamber, means for contacting a cryogenic liquid with each article at a certain region along said process path to flash said liquid at least partially to gas for freshly generating refrigerant gas within said chamber, means defining a plenum chamber adjacent to said process path upstream of cryogenic liquid contact region, pumping means having a suction inlet communicating with a first portion of said process path upstream of said cryogenic liquid contact region for receiving said freshly generated refrigerant gas, said pumping means having a discharge outlet upstream of said first portion and communicating with a second portion of said process path through said plenum chamber to develop a high-velocity stream traveling along a recirculation flow path through said chamber from said discharge outlet, through said plenum chamber to said suction inlet, the improvement which comprises passage means in said plenum chamber between said first and second portions of said process path and communicating with said suction inlet, a control element adjacent said passage means, and means for adjusting the position of said control element relative to said passage means, whereby when said control element completely closes said passage means, the circulating gas travels along said entire recirculation path, and when said control element is spaced from said passage means at least some of said circulating gas travels along a shortened path through said passage means, said suction inlet and said discharge outlet.
2. The apparatus of claim 1 wherein said means for adjusting the position of said control element relative to said passage means is operatively responsive to the temperature at the exit end of said chamber.
3. The apparatus of claim 1 wherein said means for contacting a cryogenic liquid with each article comprises a plurality of nozzles.
4. The apparatus of claim 1 wherein baffle means are provided below and parallel to said conveyor to provide for higher velocity of said refrigerant gas stream.
5. In a method for flash freezing of articles in a substantially thermally isolated chamber wherein each article is transported along a process path leading into, through an out of the chamber, wherein a cryogenic liquid is brought into contact with each article within said chamber at a region along said path to flash at least partially to gas for generating fresh refrigerant gas within said chamber, wherein circulating gas is brought into contact with each article in a plenum chamber adjacent said process path upstream of said liquid contact region, wherein refrigerant gas is withdrawn from a first portion of said chamber upstream of said liquid contact region by a pumping means having a suction inlet communicating with said first portion and a discharge outlet upstream of said first portion and communicating with a second portion of said process path through said said plenum chamber, said pumping means causing said gas to flow along a recirculation path through said chamber from said discharge outlet, through said plenum chamber to said suction inlet, the improvement which comprises balancing the gas flow in said method by providing a passage means in said plenum chamber, passing a portion of said recirculating refrigerant gas from said plenum chamber by adjusting a control element relative to said passage means, thereby providing a shortened recirculation path for a portion of said recirculating refrigerant gas through said passage means, said suction inlet and said discharge outlet.
6. The method of claim 5 wherein said cryogenic liquid is liquid nitrogen.
7. The method of claim 5 wherein said cryogenic liquid is sprayed onto each article.
8. The method of claim 5 wherein the amount of said portion of recirculating refrigerant gas passed from said plenum to said chamber is controlled by the temperature at the exit end of said chamber.
9. The method of claim 5 wherein said recirculation path is of restricted cross section bounded on at least one side by a baffle from another portion of the cross section of said chamber.
SUMMARY OF THE INVENTION
The flash freezing of food products by the use of cryogenic liquids and the refrigerant gases produced by their vaporization is a field of growing economic significance. In such flash-freezing systems a hollow housing structure, or tunnel, is provided which is insulated over its entire length and open at both ends, and a conveyor is provided for passing food articles into, through and out of the tunnel. At one region in the path of the food articles they come into contact with the cryogenic liquid, vaporizing at least a portion of the liquid and utilizing the latent heat of such vaporization to be chilled thereby. In the preferred flash-freezing systems, the cryogenic liquid is sprayed onto the food articles on the conveyor.
At another region in the path of the food articles, they come into contact with a high velocity recirculating stream of refrigerant gas which stream is composed partly of freshly generated refrigerant gas from the aforementioned vaporization and partly of previously recirculated refrigerant gas. The recirculation path includes passage through a pump, or fan, outside of the tunnel, or chamber; and the passage through the chamber, or portion thereof, may be countercurrent, concurrent or crosscurrent to the passage of the food articles along the conveyor. This invention is applicable to systems in which a part of the gas recirculation path is through a plenum chamber which is adjacent to the flash-freezing process path.
Suitable systems in which this invention is applicable are disclosed in U.S. Pat. No. 3,277,657, issued Oct. 11, 1966 to John D. Harper et al., and in U.S. Pat. No. 3,345,828, issued Oct. 10, 1967 to David J. Klee et al.
In the flash-freezing systems of the prior art, means are provided for withdrawing a portion of the refrigerant gas from the recirculation path (or from each recirculation path when there is a plurality of them) in order to accommodate the freshly generated gas being added to the system by vaporization. And since the amount of gas being vaporized depends on the load of food articles passing through the tunnel and is thus variable, the amount of refrigerant gas being withdrawn must be adjusted to meet the changing load.
It has been found to be difficult to regulate and balance the flash freezing operation by the sole expedient of adjusting the amount of withdrawal of recirculating gas because at minimum loads, even the minimum withdrawal of gas from the recirculation path is greater than the rate of gas generation and this results in air infiltration at the exit end of the chamber.
Flow inward of atmospheric air dilutes and warms the refrigerant gas, and conversely, excessive flow of gas outward from the exit end of the tunnel results in inefficient utilization of the refrigerant gas. It is desired to maintain a slight outward flow of refrigerant at the exit end of the tunnel or chamber, but this is difficult to maintain when the load on the system is subject to substantial changes.
In accordance with this invention, a portion of the recirculating gas is short circuited by passage of said portion from the plenum chamber to the process path so that it does not travel the full length of either and a change in the load in the system produces a change in the portion of gas being short circuited. As said portion of diverted gas enters the process path and meets the onflowing main stream of circulating gas, it creates turbulence and eddy currents and slows down the net velocity of the main stream toward the fan inlet. Further, since a portion of the circulating gas travels a shorter path, the same fan capacity can be satisfied by a smaller amount of circulating gas.
The amount of gas diverted from the main stream is controlled by an adjustable opening between the plenum chamber and the process path; and this in turn is controlled by the temperature at the exit end of the tunnel.
When liquid nitrogen is the cryogenic liquid, there is only a short distance at the exit end of the tunnel separating freshly generated nitrogen gas at a temperature of about -320° F. from atmospheric air at a temperature of about 70° F. to 80° F. In a typical operation, a temperature-sensing device at the exit end of the tunnel is maintained at a temperature of from about -10° F. to about +10° F., indicating a slight purge of nitrogen gas. When the temperature drops below this range, the temperature-sensing device activates a control mechanism which in turn activates a motor operating to close the adjustable passage and thereby reduce or cut off the diverted gas flow. Conversely, a rise in temperature serves to open the adjustable passage and increase the gas flow therethrough.
DETAILED DESCRIPTION OF THE INVENTION
In the accompanying FIGURE the apparatus of the invention is shown schematically in vertical cross section.
In the FIGURE, the flash-freezing chamber 11 consists of a precooling section 12, a freezing section 13 and a postcooling, or equalization section 14. The precooling and postcooling sections have double walls 16 and 17, respectively, filled with a lightweight insulating material such as foamed polystyrene or polyurethane. The freezing section has double walls 18 having a high vacuum between the inner and outer walls.
A conveyor system having three reaches 19, 21 and 22 is provided to provide passage through the chamber for food articles 24 from inlet end 26 to outlet end 27. The terms "upstream" and "downstream" as used herein are related to the direction of the food articles on the conveyor.
Liquid nitrogen is supplied to the chamber by a system described in detail in copending application Ser. No. 487,359, filed Sept. 15, 1965, by John D. Harper, Frederick Breyer and Richard C. Wagner. As shown in the FIGURE herein, the liquid nitrogen system comprises an insulated liquid nitrogen storage vessel 28 containing liquid nitrogen 29, a cryogenic pump 31 suspended in submerged relation in the liquid nitrogen in the vessel, a drive motor 32 mounted externally of the vessel and having a drivebelt 33 connected to the exposed upper end of the pump drive shaft 34. The pump 31 is illustrated as being of the centrifugal type and is fully described in U.S. Pat. No. 3,379,132, issued Apr. 23, 1968 to Richard C. Wagner.
The discharge end of pump 31 leads through line 36 and valve 37 to manifold 38 to which a plurality of nozzles 39 are mounted so that liquid nitrogen sprayed through the nozzles will contact the food articles 24 passing thereunder, resulting in the vaporization of at least a portion of the nitrogen. Excess liquid nitrogen collects in pan 41 and is drawn off by filtered drain 42 for return to vessel 28 and recycle. Fresh liquid nitrogen to replace the liquid vaporized in the chamber is supplied by lines 43 and 66 and passes through solenoid valve 67 and regulating valve 48 into tray 41. The solenoid valve is controlled by level sensing device 68 in the nitrogen return vessel and is actuated to on or off position by the level of liquid nitrogen in the vessel. Regulating valve 48 controls the flow through line 66 when the solenoid valve is open.
The solenoid valve 67 may be bypassed in the event of malfunction by passing the liquid nitrogen through line 46 and valve 47. And valve 44 is provided to enable liquid nitrogen to be passed directly to the nozzles, as required in the startup operation.
The liquid nitrogen vaporized in the zone under the nozzles 39 passes into the gas contact zone moving countercurrently to the food articles as shown by the arrows. The temperature of the freshly generated nitrogen is lower than the temperature of the circulating nitrogen so that some of it will move along the bottom of the chamber, below baffle 50. Also, some of the freshly generated nitrogen will blend with the recirculating nitrogen deflected by baffle 51 after passage through plenum chamber 52, the combined stream moving toward the chamber inlet above the baffle 50, impelled by the suction of fan 53. The baffle 50 serves to restrict the cross-sectional area of the recirculating nitrogen and thus increase its velocity.
Fan inlet 54 draws gaseous nitrogen into the fan from both the recirculating stream above baffle 50 and the freshly generated stream below the baffle. The gas drawn into the fan inlet passes to the fan through a conduit (not shown) behind the chamber.
L-shaped gate 56, which is movable by means of rod 57 projecting through the insulated wall, regulates the amount of recirculating gas and the amount of freshly generated gas going into the fan inlet. Movement of the rod and gate toward the right as shown in the FIGURE reduces the amount of recirculation gas moving into the fan inlet, sending more gas to the precooling section, and also increases the amount of freshly generated gas going into the fan inlet from the bottom of the chamber.
After the circulating gas enters the fan 53 it is propelled thereby through the plenum chamber 52 to the baffle 51 for another cycle.
In accordance with this invention, passage 58 is provided through the wall separating plenum chamber 52 from the remainder of the chamber, and the width of passage 58 is controlled by adjustable baffle 59 which is attached to rod 61 actuated by motor 62. Motor 62 is controlled by a control mechanism, shown schematically at 63, in response to a temperature sensing device 64 at the exit end of the chamber.
In operation after startup of the apparatus with an excess of nitrogen discharging at the tunnel ends, gate 56 is adjusted to provide a balanced gas flow while baffle 59 is closed and the apparatus is under full load. At a balanced gas flow, there is no ingress of ambient air into the exit end of the chamber and there is a slight purge of nitrogen outward, as indicated by a desired temperature of about -10° F. to about +10° F. at the temperature-sensitive element 64.
Thereafter, gate 56 remains untouched in the same position and all adjustments necessary to accommodate for lesser loads are carried out by the raising or lowering of baffle 59 in response to temperature fluctuations at element 64. At minimum load, where the supply of food articles is temporarily suspended by mechanical failure of the food preparation mechanism or by time off for the loading operators, baffle 59 is in its most elevated position.
Under these circumstances, the amount of gas travelling the full recirculation path to baffle 51 and back is reduced; and most of the gas follows the short path through passage 58. Thus, a shorter average gas path is provided and the time interval for recirculation of an average aliquot portion of the gas is reduced. The full capacity of the recirculating fan is thereby satisfied by a greater number of cycles of a smaller amount of gas; and the smaller amount of gas required in the recirculation system compensates for the smaller amount of gas being vaporized by the lesser load.
While the invention has been described with respect to preferred constructional and operating features, it is to be understood that changes and variations may be made by those skilled in the art without departing from the spirit and scope of the appended claims.