US 3531946 A
Description (OCR text may contain errors)
J. D. HART 7 3,531,946
CRYOGENIC-MECHANICAL REFRIGERATION APPARATUS 3 Sheets-Sheet 2 Oct. 6, 1970 Filed July 9, 1968 Azzzz zzxywzzww% 4 W & 0 a mll 1 m a JAMES D. HART ,47-7-O4WVE/V Z/ZZZ/ Oct. 6, 1970 R 3,531,946
CRYOGENIG-MECHALIICAL REFRIGERATION APPARATUS Filed July 9, 1968 3 Sheets-Sheet 5 wFF-i=..- n n INVENTOR,
has JAMES D.HART' A m/V67 United States Patent 3,531,946 CRYOGENIC-MECHANICAL REFRIGERATION APPARATUS James D. Hart, Tucson, Ariz., assignor to Elmwood Liquid Products, Inc., New York, N.Y., a corporation of New York Filed July 9, 1968, Ser. No. 743,343 Int. Cl. F25d 3/10 US. Cl. 62-332 7 Claims ABSTRACT OF THE DISCLOSURE A hybrid cryogenic-mechanical refrigeration apparatus in which food having a high water content is conveyed on a foraminous belt through an open-ended, thermally insulated tunnel, and is cooled by cold air which is blown through mechanical refrigeration coils disposed below the belt, the cold gas passing upwardly through the belt to extract heat from the food conveyed thereby. Food, after entering the tunnel, is subjected to a spray of a cryogenic liquid which serves to superficially freeze the food to produce a thin ice glaze thereon serving to prevent the subsequent loss of moisture from the food, the expanding cold gas resulting from boiling of the liquid pressurizing the tunnel and acting to prevent ambient air and moisture from entering therein.
This invention relates generally to the freezing of comestibles, and more particularly to a hybrid food-freezing machine which coordinates cryogenic and mechanical freezing techniques to freeze food at a rapid rate without impairing the qualities thereof.
Three basic techniques exist for removing heat from food to effect freezing, namely: natural refrigeration, mechanical refrigeration, and cryogenic freezing. In natural refrigeration, use is made of ice, the heat to melt the ice being extracted from the food. The limit on this technique is the freezing point of water: hence natural freezing is of little practical value in modern mass-production freezing plants.
Mechanical refrigeration operates on the principle of a liquid-to-gas, gas-backto-1iquid cycle. The most widely used form of mechanical refrigerator utilizes a compressor, a condenser, an expansion valve, and an evaporator to achieve refrigeration. The refrigerant is a highboiling-point liquid such as ammonia or Freon.
In the compression refrigeration cycle, the refrigerant vapor leaving the evaporator is compressed to a high pressure, with subsequent elevation of the boiling point. The vapor, now at high temperature and pressure, enters the condenser and is condensed, releasing heat. The liquid refrigerant is partially vaporized by passing it through the expansion valve, where it undergoes a pressure drop.
This vaporization extracts heat from the remaining liquid,
and the temperature of the liquid is reduced. The mixture is then passed through the evaporator, where heat is absorbed and the liquid again vaporized. The absorption of the heat in the evaporation coils provides the refrigeration.
Food-s having high water content, such as fish, tomatoes, and citrus fruit usually cannot be satisfactorily frozen with mechanical refrigeration systems, for the relatively low freezing rates give rise to ice crystal growth which ruptures the delicate cell walls and tissues. Thus the frozen food 3,531,946 Patented Oct. 6, 1970 product, when later thawed, has a mushy consistency. Moreover, with slow freezing there is a loss of moisture and of volatile oils, which loss impairs the flavor of the product and also causes undesirable shrinkage thereof.
In an attempt to accelerate the freezing action of the mechanical refrigeration system and to overcome certain shortcomings thereof, arrangements have been proposed, such as that disclosed in the Overbye Pat. 3,115,756, to convey food through an open tunnel on a foraminous belt, below which are disposed the evaporation coils of a mechanical refrigeration system. Air is blown upwardly through the coils and the resultant cold air is then forced through the belt to effect rapid freezing of the food advancing thereon.
While this prior arrangement is more efiicient than most blast tunnel freezers, it has serious drawbacks, among which is the formation of snow and ice as a result of moisture extracted from the food being processed and from ambient air drawn into the tunnel. The formation of snow and ice on the refrigeration coils and on other components of the system markedly reduces the thermalefliciency thereof. Another important drawback of this arrangement is the shrinkage of food resulting from dehydration due to evaporative effects which are accelerated by the air blast.
Cryogenic liquids, such as liquid carbon dioxide, liquid nitrogen, or liquid air, having normal boiling points substantially below -l00 F., have been used to a limited degree to freeze foods which cannot be satisfactorily frozen by mechanical refrigeration systems. With cryogenic liquids, under carefully controlled conditions, freezing rates can be obtained which are so fast that high water-content products can be frozen in substantially amorphous form, whereby little or no collapse of the internal structure will occur upon thawing.
The rate of heat transfer between a solid product and a boiling liquid refrigerant such as liquid air is greater than that between the product and a gaseous refrigerant such as cold air. It is therefore preferable to have direct contact between the cryogenic liquid and the product to achieve maximum effectiveness of the latent heat of the refrigerant. Spray contact has been found superior to immersion, for the latter tends to produce a gaseous film about the product which functions as a contact barrier, whereas with a high-velocity spray the liquid droplets penetrate the film and produce a higher rate of heat transfer. One important advantage of cryogenic freezing is that it reduces food shrinkage, for the quick freezing of the water content limits the loss of moisture.
Thus in the patents to Macintosh 3,238,136 and 3,255,- 608 and in the patent to Harper 3,277,657, there are disclosed systems in which food conveyed on a belt through an open-ended tunnel, is sprayed with ultra-cold liquid nitrogen whose temperature is 320 F., to efiect quick freezing thereof. While such flash-freezing cryogenic arrangements are effective with certain foods, where the food has a high water content and is of somewhat delicate internal structure, the change in internal temperature and the resultant formation of ice which expands the structure may give rise to thermal shock damage, as a consequence of which the product is cracked and otherwise mutilated. Thus, in the case of breaded shrimp and other products having a thin or delicate crust, when the product is subjected to extreme cold which abruptly reduces the 3 internal temperature, the crust is caused to crack and the product rendered unacceptable.
Accordingly, it is the main object of this invention to provide a hybrid mechanical-cryogenic refrigeration machine which combines the best featues of these techniques without the drawbacks thereof.
More particularly, it is an object of this invention to provide a machine of the above-described type which functions to quick-freeze food having a high water content, without shrinkage or cracking thereof and at a high production rate.
Briefly stated, these objects are attained in a hybrid cryogenic-machinical refrigeration machine in which food is conveyed by a foraminous conveyor from an open inlet to an open outlet in a thermally-insulated box or tunnel, mechanical refrigeration coils being disposed below the belt. Fan means are provided to blow air upwardly through the refrigeration coils and through the belt in a recirculating loop, whereby the food advancing through the tunnel is subjected to cold air. At the same time, cryogenic spray means are provided to superficially freeze the food admitted into the tunnel to form an ice crust thereon which seals the food against the loss of moisture without, however, causing an abrupt change in the internal temperature thereof, the spray also serv ing in combination with the mechanical refrigeration system to maintain the temperature within the tunnel at a uniform temperature of about 40 F. The expanding cold gas resulting from volatilization of the cryogenic liquid pressurizes the interior of the box or tunnel and serves to prevent the intake of ambient moist air at the open inlet and outlet, thereby substantially reducing the formation of ice and snow.
For a better understanding of the invention, as well as other objects and further features thereof, reference is made to the following detailed description to be read in conjunction with the accompanying drawings, wherein:
FIG. 1 schematically shows, in side view, one embodiment of a hybrid cryogenic-mechanical refrigeration box in accordance with the invention;
FIG. 2 schematically illustrates the system shown in FIG. 1, in transverse section;
FIG. 3 is a right-side elevation of a hybrid refrigeration freezing tunnel in accordance with the invention;
FIG. 4 is a section taken in the plane indicated by line 4-4 in FIG. 3;
FIG. 5 is a section taken in the plane cutting through zones A and C in FIG. 4;
FIG. 6 is a section taken in the plane cutting through zones B and D in FIG. 4; and
FIG. 7 is a section taken in the plane indicated by line 77 in FIG. 6.
GENERAL DESCRIPTION OF FREEZING BOX Referring now to FIGS. 1 and 2, there is shown an existing form of mechanical refrigeration apparatus for freezing food having a high water content, such as IQF shrimp. The initials IQF stand for Individual Quick Frozen, which is shrimp peeled, deveined and rawfrozen. In the same figures, a cryogenic spray system has been added to the mechanical system to form a hybrid system in accordance with the invention.
The system comprises a thermally insulated box 10, which in practice is about twenty-four feet long, fifteen feet wide, and thirteen feet high. The box is constructed of thermally-insulated panels, the front panel 10A having an inlet opening 11 and an outlet opening 12, one above the other, to accommodate a foraminous feed-in or input belt 13, and a similar but shorter feed-out or output belt 14.
Input belt 13, which may be made of stainless steel mesh, is continuous and extends between a driven roll 15 outside the box and an idler roll 16 disposed adjacent the rear panel 10B of the box. The advancing (upper) and returning (lower) sections of belt 13 slide along T-bar runners bridging the parallel vertical walls of a frame 17 disposed within the box. Output belt 14, similarly supported between rolls 18 and 19, slides along T-runners mounted on the same frame. The advancing sections of the two belts run in opposite directions.
The advancing and returning sections of belts 13 and 14 are stacked in parallel horizontal planes within frame 17 and therebelow are evaporation coils 20 carrying ammonia or Freon and operating in conjunction with a conventional mechanical refrigeration system having a compressor and other elements (not shown).
Four fans 21, 22, '23 and 24 are mounted in short ducts disposed along one wall of frame 17 in the free space between the frame and the side panel 10C of the box, the fans being adjacent the bottom of the frame. The fans serve to blow air into the frame, the air recirculating at a high velocity in a clockwise direction through the evaporation coils 20 and the four sections of the conveyor belts, as indicated by the arrows in FIG. 2. The fans are of the highspeed propeller type.
The belts are driven by suitable variable-speed motors 25 and 26, operatively coupled by sprocket chains to sprocket wheels attached to rolls 15 and 19, respectively. That portion of input bet 13 which extends outside the inlet of box 10, serves as a feed table onto which shrimp or another food product is spread out for admission to the box. While the arrangement shown in FIGS. 1 and 2 also includes a cryogenic spray system, in order to explain the deficiencies of the mechanical refrigeration system in the absence of cryogenic means, the system will now be described as it operates without such cryogenic means.
OPERATION OF MACHINE LACKING CRYOGENIC FREEZING An unfrozen food product enters box 10 on input belt 13, where it is immediately exposed to an air-blast from a cold air stream blown by the fans upwardly through the evaporation coils 20. Since the cold air first passes through the returning and advancing sections of output belt 14 before reaching the returning and advancing sections of the input belt, the cold-air blast effect is, of course, weaker on the input belt.
The product is carried by input belt 13 to the end thereof, where a cutter bar 27, whose edge is closely adjacent the surface of the belt, acts to remove the product therefrom. The product falls down a chute 28 which serves to transfer it to output belt 14, whose direction of movement is the reverse of that of belt 13. The product is advanced on output belt 14 toward the outlet of the box, in the course of which movement it is subjected to a blast of colder air having a higher velocity, for the output belt is in closer proximity to the blowers than the input belt.
Belt 14 carries the product out of the box, where another blade 29 removes the product from the belt. The discharged product, which is frozen, is then packed and shipped. The amount of product fed into the box is controlled by the speed of input belt 13, whereas the core temperature of the product is controlled by the speed of output belt 14. Obviously, the slower the movement of belt 14, the longer the period of exposure and the lower the internal temperature of the product.
In practice, when used to freeze shrimp, the box is first pumped down to a temperature of about 25 F. before the product is run. As the box fills with shrimp, the interior temperature thereof rises from 25 F. to 5 F. or higher, depending on the amount of product fed therein. To maintain a box temperature of about 0 F., which is necessary for effective freezing, the production rate must be adjusted so that the temperature does not rise above this level.
Shrimp is a product having a very high water content water). After the machine has been in operation for several hours, moisture from the product and from ambient air sucked into the box by the fans, causes a considerable buildup of ice and snow on the fans and refrigeration coils as well as on the interior walls.
This ice and snow formation impairs the thermal conductivity between air and the refrigeration coils, causing the box to heat up and further reducing production. As the temperature of the box rises, the shrimp, instead of freezing solid, is still somewhat soft by the time it reaches the cutter 27 at the end of input belt 13. Hence the shrimp is not hard enough to be broken off the belt, but it is multilated by the cutting knife. It becomes necessary, therefore, to periodically shut down the machine to permit removal of the ice and snow.
OPERATION OF MACHINE WITH CRYOGENIC FREEZING In accordance with the invention, liquid air-spray manifolds are added to the system, one spray manifold 30 being disposed above input belt 13 adjacent inlet 11, and the other spray manifold 31 being mounted above the input belt adjacent roll 16. Each spray manifold consists of an array of spray nozzles deployed across the belt to shower the product carried thereby with liquid air. The amount of cryogenic liquid supplied to the box can be controlled by the size and number of the nozzles and the pressure in the liquid feed line. It is to be understood that while liquid air is the cryogenic liquid which is discussed herein, other cryogenic liquids, such as liquid nitrogen and Freon may also be used.
To allow a blow-down stream of liquid to be directed onto input belt 13, an externally-operated valve 32 is disposed above this belt near the first spray manifold. The blow-down effect makes it possible at the outset of operations to quickly cool down the machine. Also, in the course of operation, should the box temperature rise unduly, it may be quickly reduced by opening valve 32.
Liquid air is supplied through a line 33 under the control of a solenoid valve 34, the line conducting the liquid air to spray manifold 30, blow-down valve 32, and spray manifold 31. For products other than IQF shrimp, when less liquid is desired, a valve 35 interposed in the line makes it possible to shut off spray 31.
Liquid air sprayed on the product is volatilized, and the resultant cold gas sinks downwardly and is intercepted by the fans 21 to 24 and blown back over the refrigeration coils 20 and then through belts 14 and 13. Thus the interior temperature of the box depends on the combined effect of mechanical and cryogenic refrigeration.
The operating procedure is as follows:
First, fans 21 to 24 are turned on and the mechanical refrigeration system is put into operation to reduce the interior temperature of the box to a level of about -10 F. This usually requires about an hour. Then valve 34 is operated to open the cryogenic liquid line and the blow-down valve 32 is also opened. When liquid starts spraying into the box through the nozzles 30 and 31, the blow-down valve is closed.
The box temperature is measured by a thermocouple 36 disposed between the input and output belts 13 and 14. Liquid air is fed into the box until the interior temperature lies in the range of -20 F. to -40 F., and to attain this temperature quickly, the blow-down valve 32 is opened intermittently for short intervals until the desired level is reached. Thereafter, when the box is loaded with shrimp or other food products, the temperature will rise two to three degrees and remain steady,
In the hybrid cryogenic-mechanical refrigeration system, warm shrimp fed into the box by belt 13 is immediately sprayed by nozzles 30 with liquid air at a temperature of 312 F., as a consequence of which superficial freezing or glazing occurs and a thin ice crust forms on the surface of the shrimp, preventing further moisture dehydration.
The shrimp advancing on belt 13 is then subjected to a blast of cold air at a temperature in the range (20 F. to 40 F.) for a period determined by the speed of input belt 13, which in practice is about 4 feet per minute. Toward the end of travel on belt 13, the shrimp is again sprayed by nozzles 31, thereby assuring a solid freeze before the shrimp reach the transfer point where they are cut from belt 13 and transferred to output belt 14. Belt 14 serves to stabilize the temperature of the shrimp, sufficient time being allowed for the internal temperature of the shrimp to become fairly uniform throughout the body thereof.
It is to be noted that in the absence of the cryogenic system, the freezing-box arrangement is such that the advancing section of input belt 13 is effectively the hotspot of the system, whereas the returning section of output belt 14, which performs no freezing function, is the coldspot of the system. But with the addition of cryogem'c freezing, this relationship is reversed to afford a far more efficient operation.
Also to be noted is the fact that the box is pressurized by the cryogenic freezing action, for the expanding gas, produced by violatilization of the liquid cryogenic agent, creates a force preventing the admission of warm, moist ambient air, which otherwise would be sucked in by the action of the fans. Thus the moisture within the tunnel is primarily that derived from the product being processed therein. Since this moisture is sealed in the product at the very outset of freezing by the liquid spray which produces a thin ice crust thereon, relatively little moisture appears within the box and ice and snow formation is minimized, thereby significantly increasing the thermal efliciency of the system.
Because of the superficial sealing action, little moisture loss is experienced, and virtually no shrinkage occurs the product. This is important economically, for while using conventional mechanical freezing techniques a pound of shrimp may be frozen at relatively low cost, the shrimp when thawed weighs several ounces less than a pound. This decrease in value is so great as to make the savings effected by mechanical refrigeration immaterial. On the other hand, while the use of cryogenic freezing in combination with mechanical refrigeration adds somewhat to the cost of freezing per pound, this increase in cost is more than offset by the absence of shrinkage and the greater value of the frozen product.
While savings can be effected by a purely cryogenic system, the resultant thermal shock has deleterious effects. In the present invention, the cryogenic agent serves to supplement the action of the mechanical refrigeration system to produce an overall freezing temperature in the range of about 20 F. to -40 F., well above the temperatures produced by low-boiling-point cryogenic liquids, but sufficient to effect hard freezing of the product Without thermal shock, the cryogenic agent also serving to seal the product against dehydration.
Hence one does not need extreme levels of low temperature to obtain a low shrinkage rate and savings are effected in the amount of cryogenic liquid used. While the product is subjected to a cryogenic liquid spray, the exposure time is too brief to cause thermal shock damage, for the product during the main portion of the freezing cycle is subjected to an air blast in the 20 F. to 40 F. range.
The shrinkage of a food product is proportional to the water content; hence a product having a thirty percent water content will, under the same freezing conditions, shrink far less than one having an eighty percent water content. With conventional air-blast systems, shrinkage rates runing as high as twelve percent with shrimp, are encountered, whereas with the hybrid system according to the invention, the shrinkage factor is insignificant. Indeed, in some instances, the frozen product gains weight, for shrimp frozen with liquid air will not only glaze well, but will absorb some moisture during the freezing thereof.
While the freezing temperature which is in the range of 20 -F. to 40 F. is sufiicient to avoid the formation of destructive ice crystals, it is not so low (100 F. to 300 F.) as to cause so sharp a reduction in internal temperature as to produce thermal shock damage.
The invention is applicable to all foods having a relatively high water content, such as hamburgers, doughnuts, tomatoes, pies and the like. In the case of sliced tomatoes for example, the thin ice crust formed thereon by spraying followed by freezing in the 40 F. temperature region, prevents loss of moisture without, however, rupturing the delicate internal structure or damaging the skin.
By high water content is meant a content of at least 25% by weight. It is also to be noted that the cryogenic liquid spray may be directed upwardly through the foraminous belt as well as downwardly on the food to effect superficial freezing thereof.
FREEZING TUNNEL Referring now to FIGS. 3 to 7, there is shown an elongated freezing tunnel employing a hybrid system of cryogenic and mechanical refrigeration, in accordance with the invention.
The tunnel 40 is thermally insulated and is of generally cylindrical form, the tunnel being open-ended. Food is advanced through the tunnel by means of a single, continuous, foraminous belt 41, preferably fabricated of steel mesh and extending between a driven roll 42 and an idler roll 43. The belt is driven by a motor 44 coupled to roll 42, the portion of the belt between roll 42 and the inlet of the tunnel constituting a feed-in table.
As shown in FIG. 4, walkways 45 and 46 are provided on each side of the feed-in table, so that operating personnel can spread out the product to be frozen. It will be appreciated, however, that an automatic feeder system may be provided to supply the product to the feedin table.
As the product enters tunnel 40, it is subjected to a blast of cold air by a blower fan 47, which draws this air from the interior of the tunnel and blows it across the inlet thereto, to form an air curtain functioning as a thermal barrier between the cold interior and the warm exterior.
As will be evident in FIGS. and 6, the advancing and returning sections of conveyor belt 41 slide along T-bar runners adjacent the right side of the tunnel at about the center level therein, Whereas mechanical refrigeration coils 48 are disposed at the lower left quadrant of the tunnel. In order to blow cold air through the refrigeration coils and the belt sections, fans 49 are mounted above the refrigeration coils, the motors 50 therefore being mount d within an exterior compartment 5'1, in the upper left quadrant, such that, as indicated by the arrow, the circulating gas loop is through refrigeration coils 48, the advancing and returning sections of the belt.
Each fan and refrigeration coil region in the tunnel is provided with a temperature-control thermocouple 52. The arrangement is such that when the temperature in the tunnel reaches a pre-set level, say 40 F., this level will be maintained automatically by operating the mechanical refrigeration system intermittently to maintain the desired level.
It will be noted that, as distinguished from the arrangement in FIGS. 1 and 2, the fan motors are outside the freezing box or tunnel, hence the heat produced by the motors does not interfere with freezing operations. The refrigerant is supplied through a line 56 which leads to the compressor of the mechanical refrigeration system. In practice, however, in lieu of a mechanical refrigeration system, the liquid in refrigeration coils 48 may be the same cryogenic liquid which is used to spray the food, but in this instance, the cryogenic liquid flowing through the refrigeration coils is volatilized and exhausted into the tunnel. To maintain a constant temperature, the thermocouple 52 acts to control valves 53, 54 and 55 which vary the feed of the liquid cryogenic agent.
At two points in the tunnel (zones B and C), one near the inlet and the other a zone removed from the outlet, two spray manifolds are provided, one of which, 57, as shown in FIG. 6, is fed by a cryogenic liquid line 58 under the control of a solenoid 59. As shown separately in FIG. 7, below each nozzle in the spray manifold 57, between the advancing and returning sections of the belt, is an air-deflection pan 60, which serves to divert air blown upwardly toward these sections away from the spray to prevent interference therebetween.
Thus food entering the tunnel is subjected in zone A to a blast of cold circulating air which acts to pre-cool the food before it is subjected to a spray of the cryogenic liquid in zone B. The spray exposure time is controlled by the belt speed. At zone C, the product is again exposed to a direct spray of liquid air, and traveling through zone D to the exit, the product again comes under the circulating air blast. The last zone D is the temperature-stabilizing area.
At the outlet, a harvest blade 61 cuts the frozen product from the belt, the product being discharged through a chute 62 and transferred to suitable packing equipment. The returning belt is preferably washed by high water jets and blown dry by an air-blast so that it returns to the top of the feed-in table clean and dry, and in condition for loading.
To clean the tunnel without the need for dismantling, the interior thereof may be provided with a system of water lines and jets pointing in various directions and fed with a high-pressure water pump. When freezing is completed, and the freezing system shut down, the washing system is turned on, the water draining out of clean-out ports 63 located at the bottom of the machine, until the machine is fully defrosted and clean.
While there has been shown and described a preferred embodiment of hybrid cryogenic-mechanical refrigeration system and apparatus in accordance with the invention, it will be appreciated that many changes and modifications may be made therein without, however, departing from the essential spirit of the invention as defined in the annexed claims.
What I claim is:
1. A hybrid cryogenic-mechanical refrigeration apparatus for freezing food products having a high water content without subjecting said products to thermal shock damage and without substantial shrinkage thereof, said apparatus comprising:
(a) a thermally-insulated structure having an open inlet and open outlet,
(b) conveyor means for advancing said products from said inlet to said outlet, said means being foraminous to permit the circulation of gas therethrough,
(c) refrigeration means for said structure including coils disposed therein conducting a fluid refrigerant,
(d) blower means to force air through said coils to render the air cold, and then through said conveyor means to cool said products; and
(e) means to spray said advancing products with a cryogenic liquid at a zone adjacent said inlet to form a superficial ice glaze thereon sealing said products against dehydration in the course of freezing, the interior temperature of said structure resulting from the combined action of said spray and said refrigeration coils being above F. to prevent thermal shock damage.
2. Apparatus as set forth in claim 1, wherein said structure is a box having said inlet and said outlet on the front wall thereof, the conveyor means being constituted by a first continuous belt advancing the products toward the rear wall of the box, where they are transferred to a second continuous belt advancing the products toward the outlet.
3. Apparatus as set forth in claim 1, wherein said struc ture is an open-ended, elongated tunnel having a single continuous belt conveyor thereon advancing food from the inlet to the outlet.
4. Apparatus as set forth in claim 1, wherein said fluid refrigerant is ammonia and said cryogenic liquid is liquid air.
5. Apparatus as set forth in claim 1, wherein said products are shrimp.
6. Apparatus as set forth in claim 3, further including spray means adjacent said outlet.
7. Apparatus as set forth in claim 3, wherein said blower means are constituted by propellers disposed with- 3,125,867 3/1964 Rath 62-332X 3,294,553 12/1966 Benson 99-193 WILLIAM E. WAYNER, Primary Examiner US. Cl. X.R. 6265, 374