US3914953A

US3914953A - Cryogenic fragmentation freezer

Info

Publication number: US3914953A
Application number: US465799A
Authority: US
Inventors: Keith A Miller
Original assignee: Air Products and Chemicals Inc
Current assignee: Air Products and Chemicals Inc
Priority date: 1974-05-01
Filing date: 1974-05-01
Publication date: 1975-10-28
Anticipated expiration: 1992-10-28
Also published as: ATA1025174A; FR2269689B1; CA1007877A; DE2459946A1; GB1495024A; AT333320B; FR2269689A1; JPS50141753A; NL7501015A; BE828484A; JPS5312699B2

Abstract

A cryogenic freezer is disclosed which is particularly designed for embrittling large metallic articles such as motors, transformers and scrap automobile bodies prior to the fragmentation of the embrittled articles in an impact mill or other comminution device in order to separate and reclaim the scrap metal and non-metallic materials.

Description

United States Patent Miller Oct. 28, 1975 CRYOGENIC FRAGMENTATION FREEZER 3.455,:20 7/1969 Schlemmer 62/266 3,472,570 10/1969 Moran 62/374 X [75] Inventor Keih Allentown 33,245 10/1970 Korberec er a], 62/266 [73] Assignee: Air Products and Chemicals, lnc.,

Allentown, Primary E.raminerWilliam F. O'Dea 22 F d: M l, 1974 Assistant Examiner-Ronald C. Capossela 1 Attorney, Agent, or FirmRonald B. Sherer; H. Barry {21] App]. No.: 465,799 Moyerman [52] 1.8. CI. 62/266; 62/374; 62/380; [57] ABSTRACT [98 I40 51 lm. (3|. FZSD 2 3/02 A "Yogcmc freeze is disclosed which Pamcularly [58] Field of Search H 62/63 266 374 375 designed for embrittling large metallic articles such as 432/242. 198]]89 motors, transformers and scrap automobile bodies 1 prior to the fragmentation of the embrittled articles in [56] Reierences Cited an impact mill or other comminution device in order to separate and reclaim the scrap metal and non- UNITED STATES PATENTS metamc materiah 3,302,423 2/1967 Morrison 62/266 3,434,30l 3/1969 Hice 62/374 11 Claims, 8 Drawing Figures F ,&\

U.S. Patent Oct.28, 1975 Sheet 1 of3 3,914,953

On Q

- |'l El CRYOGENIC FRAGMENTATION FREEZER BACKGROUND OF THE INVENTION Numerous prior proposals have been made for embrittling both metal and non-metallic articles by cooling them with cryogenic refrigerants followed by subjecting the embrittled articles to impact or other high energy forces so as to fragment the articles and thereby permit the separation and recovery of useful metals and non-metals. Such proposals have been relatively successful where the articles to be embrittled and fragmented or ground are small. However, the application of cryogenic embrittlement has been largely unsuccessful from the commercial standpoint with respect to relatively large articles such as motors. transformers and scrap automobile bodies. This has been largely due to the inefficient cooling of the large articles to their embrittlement temperature such that a large amount of relatively costly refrigerant has been required. One of the primary factors in such poor thermal efficiency has been due to the relatively large size of the tunnel inlet and outlet openings which are required for entrance and exit of the large articles. Also, such embrittlement tunnels must have large cross-sections and long lengths which increase the heat loss through the tunnel walls since such heat loss is a direct function of the total square feet of the surface of the tunnel. The weight and size of the large articles has also presented serious problems in the structural design of the insulated tunnel.

In addition to these problems, substantial difficulties have been encountered in controlling the net flow of the vaporized refrigerant in such large tunnels so as to effectively utilize the sensible heat of this cold gaseous refrigerant before it is exhausted from the embrittlement tunnel. Furthermore, such tunnels require substantially solid conveyor belts having hinged joints such that excess cryogenic liquid may flow downwardly through the hinged joints of the belt and become trapped in the bottom of the tunnel where there is very little gas circulation because both the upper and lower reaches of the essentially solid conveyor belt separate the bottom of the tunnel from the recirculation fans which are located in the upper portion of the tunnel above the conveyor.

BRIEF DESCRIPTION OF THE INVENTION It is a principal object of the present invention to provide a cryogenic embrittlement tunnel which is structurally and thermodynamically superior to previously known tunnels of this type. More specifically, the present invention substantially reduces the loss of cold refrigerant gas from the ends of the tunnel by the use of particular types of gas curtains; establishes an effectively controlled net flow of the cold gas in contact with the articles to be embrittled using variable direction fans; provides one or more internal gas curtains which divide the tunnel into distinct temperature zones, as well as, vaporizing the cryogenic liquid which may become trapped in the bottom of the freezer; and provides a novel structural form of the tunnel capable of withstanding the size and weight of the large articles while also providing a highly effective insulation system which can be readily removed for access to the tunnel interior.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a top vie of the tunnel;

FIG. 2 is a side elevational view. partly in Crosssection, showing the essential components of the k m brittlement tunnel in simplified form:

FIG. 3 is an enlarged cross-sectional view of the Iun nel taken along view line 33 of FIG. 2;

FIG. 4 is a cross-sectional view taken along vie line 4-4 of FIG. 2 showing certain details of the inlet end of the tunnel;

FIG. 5 is an enlarged. fragmentary view taken along view line 5-5 of FIG. 4;

FIG. 6 is an enlarged. fragmentary view taken along view line 6-6 of FIG. 2;

FIG. 7 is a fragmentary, side elevational view taken along view line 77 of FIG. 6; and

FIG. 8 is a fragmentary perspective view showing the construction details of the conveyor belt.

DETAILED DESCRIPTION Referring to FIGS. I4 and 78, the embrittlement freezer generally indicated by numeral 10 is formed by a rigid framework of fiberglass I-beams ll which are connected to the base 12 and extend upwardly with their top portions secured by transverse angle members 13. The l-beams ll carry angle brackets 14 which support upper and lower pairs of L-shaped conveyor support rails 15 which may be provided with suitable low friction coatings or strips 15' to reduce the friction of the long and heavy reaches of conveyor belt 16. The conveyor belt 16 is preferably of the type known as an apron conveyor having a plurality of Vshaped plates 17 forming "pans" the sides of which are closed by apron plates 18. The V-shapcd pans are hinged together and the conveyor belt includes chain links 19 which ride on the low friction strips 15. The chain links engage the sprocket of a conventional motordrive assembly 20 located at the product outlet end of the freezer shown at the right-hand portion of FIGS. 1 and 2.

As most clearly shown in FIG. 3, the tunnel housing is preferably formed of a plurality of modular, tongueand-groove panels each of which comprises two metal walls separated by four inches of urethane foam insulation. The panels forming the bottom wall 21 are supported by cross-beams 22, while the panels forming the side walls 23 are removably secured to the bottom wall 21 and the top wall 24 by suitable locking mechanisms such as those more fully disclosed in U.S. Pat. No. 3,353,314. In this manner. the insulated tunnel housing does not carry any of the weight of the conveyor or the articles, and the side walls may be partially or completely removed in which event the top wall 24 rests on cross members 13.

A cryogenic liquid, such as liquid nitrogen, or liquid carbon dioxide, or other cryogenic refrigerant having a normal boiling point below minus l00F., is injected into the embrittlement tunnel through a spray header 26 into direct contact with the articles to be embrittled. The latent heat of the cryogenic liquid refrigerant is immediately utilized in cooling the articles such that the liquid is vaporized and becomes a cold gaseous refrigerant having a temperature equal to its normal boiling point or sublimation temperature, which is minus 320F. for liquid nitrogen and minus 109F. for liquid carbon dioxide, respectively. While the majority of the liquid refrigerant is vaporized by contact with the arti cles to be embrittled, there is usually a small amount of excess liquid refrigerant which is not immediately vaporized. Of this excess liquid refrigerant, a minor portion may flow through the hinge joints of the conveyor and accumulate in the bottom of the tunnel wherein it is forced to vaporize and effect efficient heat transfer with the articles by special means which will be subsequently described, However, the majority of the excess liquid is caught in the \/-shaped pans 17 of the upper reach of the conveyor and is vaporized in these pans between the spray zone and the outlet end of the tunnel thereby providing direct refrigeration in the post-spray zone of the tunnel.

In order to efficiently utilize the substantial sensible heat of the cold refrigerant gas generated by vaporization of the liquid in the spray zone, as well as that vaporized from the conveyor pans in the post-spray zone, a plurality of recirculation fan assemblies 30 are provided along the length of the tunnel in both the precooling zone, ahead of the spray zone, and in the postspray zone. While only three of such fan assemblies are illustrated in FIGS. 1 and 2, it will be understood that many more may be provided in actual practice and, since each recirculation fan assembly is identical, only one assembly will be described in detail.

The axial-flow fan blades 31 are driven by shafts 32 connected to motors 33 through speed reduction pulley-belt drives 34. Each of the fans is surrounded by an adjustable position shroud ring 36 supported by four rods 38 having threaded lower ends to which the shroud rings are connected by adjusting nuts. Thus, the angular position of the shroud rings may be varied from the horizontal plane to give the recirculating gas a forward component of velocity toward the inlet end of the freezer and thereby control the desired degree of net forward movement of the gas.

in order to prevent exfiltration of cold gas from the outlet end of the embrittlement tunnel, as well as, prevent the entrance of warm ambient air, the preferred embodiment of the invention employs an air curtain assembly 40 as most clearly shown in FIGS. 1 and 2. This assembly includes a pair of motor-driven centrifugal air blowers 42 which draw in ambient air and discharge the air through intermediate ducts 44 into a plenum chamber 46 which has a slot extending across the width of the bottom of the plenum chamber so as to direct a high velocity curtain of air downwardly across the exit opening of the tunnel as shown by flow arrow A.

An effective gas curtain is also provided at the front or loading end of the freezer as illustrated in FIGS. 1, 2, 4 and 5. While the air curtain previously described operates on the principle of discharging a high velocity stream from a plenum chamber, hereinafter described as a positive gas curtain, the gas curtain provided at the inlet end of the freezer does not discharge a high velocity gas stream across the opening, but rather, operates on a suction principle and will be subsequently described as a negative gas curtain This negative gas curtain is created by a suction plenum chamber generally indicated by numeral 50 which is generally in the form of an inverted U-shaped chamber having an upper chamber portion and a pair of side chamber portions 52. Plenum chamber 50 is formed of sheet metal and is entirely enclosed except for a pair of ports 53 and an inverted U-shaped suction slot generally indicated by numeral 54 which includes a horizontal portion 55 and a pair of side portions 56 with intervening webs of sheet metal to provide structural strength. Alternatively, the suction slot 54 may be in the form of a plurality of circular or elongated apertures which collectively function as a single, U-shaped slot. Thus, the inlet tunnel opening is surrounded on three sides by the suction slot 54 of the suction chamber 50.

Ports 53 of the suction chamber are connected by suitable ducts through heaters 57 to the inlets of a pair of motor-driven centrifugal blowers 58. Thus, blowers 58 create a strong negative pressure in plenum chamber surrounding the tunnel inlet which creates a high velocity curtain of ambient air, indicated by flow arrow B, and refrigerant gas, indicated by flow arrow C, which flows through suction slot 54 into the plenum chamber 50, through ports 53, heaters 57 and blowers 58 to atmospheric discharge. In addition to preventing infiltration of the warm, moist ambient air, the negative pressure created at the product inlet end of the freezer assists in drawing the cold refrigerant gas forwardly from the spray zone in which it is generated in countercurrent heat exchange with the articles on the conveyor belt.

As further shown in FIG. 5, the suction slot 54 may be varied in width so as to vary the net flow of the cold gas toward the product inlet zone, while still maintaining an effective seal against the entrance of warm ambient air into the inlet end of the tunnel. The adjustment of the suction slot 54 may be achieved in various ways, but is shown in FIG. 5 as comprising a slidable damper 59 which is slot-bolted, or otherwise secured, to the plenum chamber so as to be slidable and cover more or less of the width of the suction slot. As previously stated, the slot may be in the form of a plurality of apertures, and it will be apparent that separate dampers may be used to individually control the flow through horizontal slot portion and that through the side slot portions 56.

Since the warm, moist ambient air is mixed with the gaseous refrigerant which may still be at a temperature significantly lower than that of the ambient air, a serious frost problem can develop depending upon the temperature of the ambient air and its relative humidity. For this reason, heaters 57 are provided so that the mixture of air and refrigerant gas is maintained below the frost point such that frost does not clog blowers 58. The heaters 57 may take any one of a number of forms such as electrical resistance coils surrounding the duct between plenum chamber 50 and blowers 58. Alternatively, any other convenient source of heat may be utilized such as warm air, or warm water, circulated in indirect heat exchange with the mixture of air and refrigerant gas.

In severe conditions where the ambient air is extremely warm and moist, a frost problem may also occur within the plenum chamber 50 wherein frost may become deposited on the internal surfaces of the plenum chamber. ln most instances, the plenum chamber can be designed of a sufficient size so as to permit a degree of frost buildup without interfering with the flow of the gas mixture. However, in view of uncontrollable changes in relative humidity, as well as, adjustments with respect to the temperature of the exiting gas which may be required for different size articles, the present invention further includes the provision of auxiliary.

heaters 57' within the suction plenum 50 as shown in H0. 4. Again, the heaters may take any number of forms, but are schematically illustrated as comprising electrical resistance coils in the side portions 52 and the upper portion 51 of the plenum chamber.

in addition to the positive air curtain at the outlet end of the freezer, and the negative air curtain just described at the inlet end of the freezer, it has been discovered that the provision of internal zone gas curtains has a number of substantial advantages. FIG, shows the utilization of two such zone gas curtain assemblies generically indicated by numerals 60A and 608. Since the construction of both assemblies is substantially identical, the same numerals indicate corresponding components, and only one assembly will be described in detail. Referring to FIGS. 1, 2, 6 and 7, a centrifugal blower 61 is mounted adjacent the top of the tunnel and is driven through a speed reduction unit compris ing a pulley and belt drive 62 connected to a motor 63. The center intake of the blower is connected to a transversely extending duct 64 having pairs of

ports

65 and 66. Ports 66 communicate directly with the interior portion of the tunnel, while ports 65 communicate with ducts 67 which extend downwardly on opposite sides of the conveyor belt in the spaces between the l-beams 11.

Ports

65 and 66 are provided with closure plates 65' and 66', respectively, which may be manually operated by movement of rods 68 so as to open or close either of the ports of each pair. Thus, cold refrigerant gas may be withdrawn from the upper portion of the tunnel, as shown by flow arrows D, and/or from the lower portions of the tunnel between and below the reaches of the conveyor as shown by flow arrows E and F in FIG. 7.

The discharge of blower 61 is connected through a short coupling conduit 70, which may be a flexible hose, to a discharge duct 72 having a downwardly directed nozzle or slot 73 which extends across the full width of the freezer so as to direct a downward curtain of high velocity refrigerant gas as shown by flow arrows G. Duct 72 is preferably supported from the cover 24 by a pivoted or hinged connector 74 and the angle of zone gas curtain G can be varied from the vertical direction by adjusting rod 75 so as to have a small forward or rearward component of velocity.

It will therefore be apparent that the internal zone gas curtain assemblies not only provide gas curtains which establish distinct temperature zones within the freezer so as to increase the rate of heat transfer, but also, they promote the circulation of gas below the solid conveyor belt so as to vaporize any cryogenic liquid in the bottom portions of the tunnel and efficiently utilize the refrigeration value of such otherwise trapped refrigerant in the high velocity gas contact with the articles on the conveyor belt. in addition, the gas curtain G. of assembly 60A cooperates with the positive air curtain A so as to provide a short intermediate zone which acts as a dead space of substantially stationary gas having a temperature substantially colder than that of the ambient air, but warmer than that in the spray zone. Thus, the combination of the two gas curtains and the dead space provide a highly efficient gas seal at the outlet end of the tunnel which prevents the very cold and dense gas from the spray zone from exiting the outlet end of the tunnel with substantial loss of thermal efficiency. In addition, the zone gas curtains enable a slight positive, above atmospheric, pressure of cold refrigerant gas to be maintained in the tunnel so that air is excluded and the tunnel is completely filled with only the cold gaseous refrigerant. While the zone gas curtain of assembly 60A is preferably directed downwardly at a right angle to the conveyor belt so as to form a vertical gas curtain parallel with that of positive air curtain A, the zone gas curtain of assembly 608 is preferably adjusted by rod so as to have a forward component of velocity toward the inlet end ofthe freezer. This further promotes and provides additional control of the net forward flow of cold gas from the spray zone through the zones of high velocity recirculation created by recirculation fan assemblies 30 toward the inlet end of the freezer from which it is exhausted by passage through suction slot 54, plenum chamber 50 and blow ers 58. While only one internal zone gas curtain assembly 608 has been illustrated between the spray zone and the freezer inlet for the sake of simplicity, the present invention contemplates the provision of a plurality of such internal zone gas curtains whereby a plurality of zones of different temperature are provided between the ambient temperature and the temperature of the spray zone, and whereby the discharge angle of each zone gas curtain may be varied to control the forward net velocity of the refrigerant gas in countercurrent heat exchange with the articles to be embrittled. Accordingly, it will be apparent that all of the objects of the invention are achieved such that the present invention provides a freezer of increased thermodynamic efficiency, mechanical reliability and ease of operation.

Of course, it is to be understood that the foregoing description is intended to be purely illustrative rather than limiting, and that the scope of the invention is not to be limited other than as set forth in the following claims.

I claim:

1. A cryogenic embrittlement freezer for cooling articles to their embrittlement temperature comprising an elongated, open-ended tunnel having insulated top. bottom and side walls, conveyor means extending through said tunnel for conveying articles to be embrittled from the open inlet end to the open outlet end, means for contacting said articles with a cryogenic liquid refrigerant and vaporizing a majority of said liquid refrigerant into a cold refrigerant gas, a plurality of fans mounted above said conveyor for recirculating said refrigerant gas in contact with the articles to be embrittled in a plurality of high velocity recirculation zones, means forming an air curtain at the outlet end of the tunnel for preventing the entrance of ambient air and the exit of cold refrigerant gas, means forming a suction plenum chamber at the inlet end of the freezer having an inverted U shaped suction slot surrounding the upper and side portions of said inlet, and blower means having a suction inlet connected to said plenum chamber for producing a negative gas curtain at the inlet of the tunnel, said negative gas curtain comprising a mix ture of ambient air and refrigerant gas being exhausted from the tunnel.

2. The embrittlement freezer as claimed in claim 1 including heater means upstream of said blower means for preventing the formation of frost in said blower means.

3. The embrittlement freezer as claimed in claim 2 including additional heater means in said suction plenum for preventing frost therein.

4. The embrittlement freezer as claimed in claim 1 further including at least one internal gas curtain assembly for producing a gas curtain within said freezer,

said gas curtain assembly including a blower having an inlet in communication with the interior of the tunnel to draw refrigerant gas into said blower. and discharge plenum means connected to the outlet of said blower for discharging a curtain of gaseous refrigerant perpendicular to the direction of movement of said conveyor means.

5. The embrittlement freezer as claimed in claim 4 wherein said internal gas curtain assembly is positioned between said spray zone and said tunnel outlet for de fining a distinct zone of substantially stationary gas between said air curtain and said internal gas curtain.

6. The embrittlement freezer as claimed in claim 4 wherein said internal gas curtain assembly is positioned between said spray zone and the tunnel inlet for defining at least two distinct zones of gas recirculation of substantially different temperatures.

7. A cryogenic scrap freezer for cooling articles to their embrittlement temperature comprising an elongated, open-ended tunnel having insulated top, bottom and side walls, a substantially solid apron conveyor belt extending through said tunnel for conveying articles from the inlet end to the outlet end, spray header means for spraying said articles with a cryogenic liquid refrigerant and vaporizing the major portion of said liquid refrigerant into a cold refrigerant gas by contact with the articles to be embrittled, a plurality of fans mounted above said apron conveyor for recirculating said refrigerant gas in contact with said articles in a plurality of high velocity recirculation zones, said conveyor belt including a plurality of hinged V-shaped pan sections for collecting excess liquid refrigerant not vaporized in said spray zone and vaporizing it between the spray zone and the outlet of the tunnel thereby providing liquid refrigerant for post-spray cooling of the articles.

8. A cryogenic embrittlement freezer for cooling articles to their embrittlement temperature comprising: a framework of l-beams, a plurality of modular insulated panels removably supported by said framework forming an elongated insulated tunnel housing, a conveyor belt supported by said framework, spray header means for spraying said articles to be embrittled with a cryogenic liquid refrigerant and vaporizing the major portion of said liquid refrigerant into a cold refrigerant gas, a plurality of fans mounted above said conveyor for recirculating said refrigerant gas in contact with said articles in a plurality of high velocity recirculation zones, said fans having axial flow fan blades, shroud rings surrounding said axial flow fan blades, and adjustment means for varying the angular position of said shroud rings from the horizontal plane so as to produce a forward component of velocity of the recirculating gas toward the inlet end of the tunnel.

9. The embrittlement freezer as claimed in claim 8 further including at least one internal gas curtain assembly for producing a gas curtain within said freezer, said gas curtain assembly including a blower having an inlet in communication with the interior of the tunnel to draw refrigerant gas into said blower, and discharge plenum means connected to the outlet of said blower for discharging a curtain of gaseous refrigerant perpendicular to the direction of movement of said conveyor means.

10. A cryogenic embrittlement tunnel for cooling articles to their embrittlement temperature comprising an elongated, open-ended tunnel having insulated top, bottom and side walls, a substantially solid conveyor belt extending through said tunnel for conveying articles from the inlet end to the outlet end, spray header means for spraying said articles with a cryogenic liquid refrigerant and vaporizing the major portion of said liquid refrigerant into a cold refrigerant gas by contact with the articles to be embrittled, a plurality of fans mounted above said conveyor for recirculating said refrigerant gas in contact with said articles in a plurality of high velocity recirculation zones, said conveyor belt including a plurality of hinged pan sections for collecting excess liquid refrigerant not vaporized in said spray zone and vaporizing it between the spray zone and the outlet of the tunnel thereby providing liquid refrigerant for post-spray cooling of the articles, and at least one internal gas curtain assembly, said assembly including blower means having a discharge above said solid conveyor and duct means having one end connected to the inlet of said blower and the other end extending downwardly along the side of the conveyor for producing forced circulation of gas below said solid conveyor belt and vaporizing excess liquid refrigerant in the bottom portion of the tunnel.

H. The embrittlement tunnel as claimed in claim 10 wherein said internal gas curtain assembly further includes an adjustable position discharge duct, and means for adjusting the angle of the gas curtain discharged from said duct so as to have a forward component of velocity for promoting and controlling the net forward flow of refrigerant gas from the spray zone to the tunnel inlet.

Claims

1. A cryogenic embrittlement freezer for cooling articles to their embrittlement temperature comprising an elongated, openended tunnel having insulated top, bottom and side walls, conveyor means extending through said tunnel for conveying articles to be embrittled from the open inlet end to the open outlet end, means for contacting said articles with a cryogenic liquid refrigerant and vaporizing a majority of said liquid refrigerant into a cold refrigerant gas, a plurality of fans mounted above said conveyor for recirculating said refrigerant gas in contact with the articles to be embrittled in a plurality of high velocity recirculation zones, means forming an air curtain at the outlet end of the tunnel for preventing the entrance of ambient air and the exit of cold refrigerant gas, means forming a suction plenum chamber at the inlet end of the freezer having an inverted U-shaped suction slot surrounding the upper and side portions of said inlet, and blower means having a suction inlet connected to said plenum chamber for producing a negative gas curtain at the inlet of the tunnel, said negative gas curtain comprising a mixture of ambient air and refrigerant gas being exhausted from the tunnel.

4. The embrittlement freezer as claimed in claim 1 further including at least one internal gas curtain assembly for producing a gas curtain within said freezer, said gas curtain assembly including a blower having an inlet in communication with the interior of the tunnel to draw refrigerant gas into said blower, and discharge plenum means connected to the outlet of said blower for discharging a curtain of gaseous refrigerant perpendicular to the direction of movement of said conveyor means.

5. The embrittlement freezer as claimed in claim 4 wherein said internal gas curtain assembly is positioned between said spray zone and said tunnel outlet for defining a distinct zone of substantially stationary gas between said air curtain and said internal gas curtain.

8. A cryogenic embrittlement freezer for cooling articles to their embrittlement temperature comprising: a framework of I-beams, a plurality of modular insulated panels removably supported by said framework forming an elongated insulated tunnel housing, a conveyor belt supported by said framework, spray header means for spraying said articles to be embrittled with a cryogenic liquid refrigerant and vaporizing the major portion of said liquid refrigerant into a cold refrigerant gas, a plurality of fans mounted aBove said conveyor for recirculating said refrigerant gas in contact with said articles in a plurality of high velocity recirculation zones, said fans having axial flow fan blades, shroud rings surrounding said axial flow fan blades, and adjustment means for varying the angular position of said shroud rings from the horizontal plane so as to produce a forward component of velocity of the recirculating gas toward the inlet end of the tunnel.

11. The embrittlement tunnel as claimed in claim 10 wherein said internal gas curtain assembly further includes an adjustable position discharge duct, and means for adjusting the angle of the gas curtain discharged from said duct so as to have a forward component of velocity for promoting and controlling the net forward flow of refrigerant gas from the spray zone to the tunnel inlet.