US2735275A

US2735275A - Inyentor

Info

Publication number: US2735275A
Authority: US
Publication date: 1956-02-21
Anticipated expiration: 1973-02-21

Description

Feb. 21, i956 l.. E. lammcHFLowER l 2,735,275

ICE MAKING MACHINE AND THE ART THEREOF l Filed Feb. 8. 1951 I 5'Sh9etS-Sh99't l A v Y 7/ 60 g l zr I 60 'Il i 43 1 .Ly/e E. Eremo/flo wer 77'aR/vfY Feb. 2l, 1956 Filed Feb. 8.

L. E. BRANcHFLowER 2,735,275

ICE MAKING MACHINE AND THE ART THEREOF 5 Sheets-Sheet 2 ATTO 1PA/Y Feb. 2l. 1956 L. E. BRANcHFLowER 2,735,275

ICE MAKING MACHINE AND THE ART THEREOF' Filed Feb. 8, 1951 5 Sheets-Sheet 3 vIN V EN TOR.

gy/e E. rancf/ower fn/MW ATTORNEY Feb. 21. 1956 L. E.` BRANcHFLowl-:R 2,735,275

ICE MAKINGMACHINE AND THE ART THEREOF Filed Feb. 8, 1951 5 Sheets-Sheet 4 1N V EN TOR. L y/e E. Brdncf/o wer ATTORNEY Feb. 21, 1956 L. E. BRANcHr-'LowER 2,735,275

A ICE MAKING MACHINE AND THE ART THEREOF Filed Feb. 8. 1951 5 sheets-sheet 5 )'NVENTOR. Lyle E. Brancf/owef ATTORNY United States Patent Oliice 2,735,275 Patented Feb. 2l, 1956 ICE MAKING MACHINE AND THE ART THEREOF Lyle E. Branchflower, Seattle, Wash. i

Application February 8, 1951, Serial No. 210,030

7 Claims. (Cl. 62--107) The present invention relates to ice making machines land the art thereof. In more particular the present invention is concerned with the making of sub-cooled ice in ake form by freezing a film of water on a rigid refrigerated surface and removing and breaking up from such surface the resulting thin sheet of dry ice.

Many of the prior art devices for making ice in small pieces or particles have the disadvantage that they produce ice that is wet or even in the form of slush. Such ice cannot be colder than 32 F. Such ice is difficult to handle and store as it freezes into a solid cake when stored in refrigerated bunkers. It will not handle easily by screw or belt conveyors. Naturally, such wet ice has a short storage life without refrigeration as compared with ice which is dry, sub-cooled, that is ice which is below 32 F. In many industries, a satisfactory ice must be below 10 F.

Many of the prior art devices feed water to the freezing surface from a tank by the partial or total immersion of the surface in such tank. This means that there is a large volume of water which does not progress thru the machine. Fungi and bacteria may accumulate. This may lead to unsanitary conditions, and the added cost of the tank is an important item of cost in the construction of the machine.

Also, many of the prior art devices freeze the water on an external surface. This increases the vapor in the surrounding atmosphere which is often undesirable. Further, such machines are difficult to insulate.

Some of the prior art machines for making sub-cooled ice use a flexible belt on which the ice is formed. While such machines are successful, the care required in the manufacture and operation of such belts is an important item of cost and maintenance.

A further item of disadvantage in many machines is the need for conveyors and/ or chutes for the transfer of the ice from the refrigerated surface to a storage bin.

In the removal of ice from a rigid refrigerated surface there have been developed several methods and associated devices which are based upon the concept of either scraping the ice from the surface as in:

Taylor, 2,080,639, May 18, 1937 Taylor, 2,280,320, April 2l, 1942 or the wedging of the ice from the surface as in:

Holden, 1,020,759, March 19, 1912 Short, 2,310,468, February 9, 1943 Raver, 2,308,541, January 19, 1943 Raver, 2,344,922, March 21, 1944 Raver, 2,431,278, November 18, 1947 Short and Raver wedge small pieces away from the main sheet of ice. Taylor scrapes against the main ice sheet. Holden wedges as in Short and Raver, or scores into the ice sheet with parallel closely spaced score lines. The Short and Raver devices require a rather close ternperature control of the ice and special material upon which to freeze the ice for satisfactory operation. Ice which does not come free stays on the surface and builds up until a cutter blade contacts it. InHolden, the scoring edges are so close together in opposition that theforce from one is directed against an adjacent one. This operation cannot be efficient as the scoring edges are working against each other and the ice is crushed to fine particles.

Raver has the disadvantage of using a tank in which his drum rotates for supplying water to the refrigerated drum surface. Taylor and Holden use vspray heads directed at various parts of the refrigerated surface but they do not secure uniform and efficient coverage. .Short has a series of holes in the bottom tank for running water onto his refrigerated surface. These holes will cause streams of water to channel down across the refrigerated surface. These channels may coalesce far down the surface but will not` form a continuous sheet of water near the top of his-refrigerated surface. Holden and Taylor clearly disclose that the ice cornes off their machinein a wet condi.

tion. From the construction shown by Short, it is doubtful that he can obtain a dry ice as excess water and ice would seem inevitably to be mixed in the outlet.

Raver in the later filed patents, Short, and Taylor have recognized the problem of obtaining velocity to the `refrigerant over the heat transfer surface to increase the heat transfer, to prevent the formation of gas pockets, and to reduce the accumulation of oil on the heat transfer surfaces. However, all of the prior art devices have the defect that velocity is attained by a loss of ,head and by a singlepass of the refrigerant across the heat transfer surface. In these devices the velocity lmust be controlled by the rate of evaporation of the refrigerant or much wet refrigerant must be returned to the compression cycle. Further, such devices do not provide for efficient separation of the gas from the liquid. Separation must occur in the machine which means that some of the heat transfer surface must be contacted by gas only and not liquid refrigerant. This means a low rate of heat transfer for these gas contacted areas.

Further, for eticient heat transfer, the presence of oil on heat transfer surfaces must be avoided.

Having in mind the above and other defects of the prior art, it is an object of the present invention to construct a machine that, will make sub-cooled, dry, ice in ake form.

A further object of the present invention is the provision of a construction that insures that the dry ice will not have water added thereto.

Another object of the present invention is the construction of a machine that can deliver ice ,to a storage bin without the use of chutes and/or conveyors.

A further object of the invention is the construction of a machine that does not use a tank for the supply of water for freezing.

A further object is the construction of a machine which has its freezing surface totally enclosed from the atmosphere and in which there cannot be any direct conduction or radiation from ambient conditions to the ice being formed.

A further object of the invention is the attainment of the objects herein by the freezing of the ice on a rigid surface.

Another object is the removal of the ice from the refrigerated surface by the isolation of small sections of the ice and the removal of such sections by a sweeping action in a manner that one section does not interfere with another during its removal.

A further object of the present invention is the provision of an ice removing sweep having a particular form.

Another object is the placing of the water on the refrigerated surface so that the entire surface is covered.

A further object is the construction of the heat transfer device so that there is a high velocity of refrigerant across the heat transfer surface.

Another obiect is the provision of a gas separator and liquid return for the refrigerant.

Another object is -the provision of means for cycling liquid refrigerant across the heat transfer surfaces without the passing of such refrigerant to the compression unit.

Another object is the provision of refrigerant cycling means to obtain a high refrigerant velocity across the heat transfer surfaces, and which means operates as a com` bined thermosiphon and gas pump.

A further object is the provision of a gas separation space outside of the refrigerant evaporator.

Another object is the provision of means for preventing the admission of oil to the evaporator.

Another object of the invention is the provision of a refrigerant precooler that also operates as an oil eliminator.

'Ihe above mentioned defects of the prior art are remedied and these objects achieved by the construction of a machine in which there is formed a sheet of thin ice on the inner surface of a refrigerated annular rigid sleeve. The axis of this sleeve is upright and the lower end of the sleeve is open and in communication with an ice storage bin so that ice when removed from the surface will fall directly into such bin. Water is supplied to the upper edge of the sleeve surface by a series of circumferentially spaced apart streams each of which has a velocity component tangential to the circumference of such surface. Ice is removed from the ice formation surface by a series of sweeps parallel to each other. all arranged parallel to the axis of the sleeve, and spaced a few thousandths of an inch from the surface. The sweeps travel in circular paths. Each sweep has a helically curved edge or an approximation thereof, the radius of which is close to that of the radius of the cylindrical ice formation surface. The forward end of this sweep edge enters the ice sheet in the direction of travel of the sweep or almost so. A collection trough for excess water is arranged below the lower edge of the ice surface. Moving with the sweeps are shields that prevent any of the falling ice from entering the collecting trough and prevent any excess water from entering the ice bin.

Exterior of and concentric of the sleeve on which ice is formed is an annular refrigerant chamber, or -evaporator chamber, which is flooded with a liquid refrigerant. The chamber is divided by an annular bale into two annular spaces that are in communication at the top and bottom of the spaces. The liquid level is maintained at or near the top of the bathe. Liquid and gas will rise in the inner, ascending, space and liquid alone ows downward in the outer, descending, space.V Gas and entrained liquid are carried out of the top of the annular space to a gas separator from which' the liquid returns to the outer annular space and the gas goes to a compressor.

The incoming refrigerant is expanded before reaching the evaporator and placed in heat exchange relationship with theincoming high pressure refrigerant to precool the refrigerant, and to condense and to facilitate removal of any oil that may be in the refrigerant.

Having thus briefly described a machine embodying the present invention, such is described in detail hereinafter and shown in the accompanying drawings in which:

Figure l is a perspective view of an assembled machine with parts thereof broken away.

Figure 2 is a sectional elevation view on the diameter of the machine.

Figure 3 is a plan view of the device shown in Figure l.

Figure 4 is a transverse sectional plan view on the line 4--4 of Figure 2.

Figure 5 is a transverse sectional plan view on the line 5-5 of Figure 2.

Figure 6 is a plan view of the water feed ring.

Figure 7 is an enlarged view of a portion of the ring shown in Figure 6.

Figure 8 is a section on the line 8 8 of Figure 7.

Figure 9 is a detail view in perspective of two of the ice sweeps.

Figure 10 is a detail view in of the doctor blade and guard.

Figure 1l is a detail view in perspective of the top edge scraper.

Figure l2 is a detail view in perspective of the bottom edge scraper and the water trough guard.

In the accompanying drawings, there is shown in Figure l a perspective view of an assembled flake ice machine. Parts of the machine are shown broken away for clarity of understanding. vIn this view there is shown the ice making machine mounted on the top of an ice storage bin 2 (shown in part only). The machine has as its principal parts a refrigerant precooler and oil eliminator 3, a gas separation chamber 4, an annular evaporator 5, a base ring assembly 6, a lop ring assembly 7, and a rotor assembly 8.

perspective of a portion Evaporator The evaporator iscomposed of an upright inner cylindrical shell 10 upon the inner, or ice making surface ll, on which is formed the thin sheet of ice to be removed therefrom in the form'o'f small flakes; an outer shell l2 concentric of the inner shell and spaced outwardly therefrom; a top end closure annulus 13; a bottom end closure annulus 14; and between said shells an annular circulation baille l5 that extends circumferentially and longitudinally of the annular evaporation chamber formed by said shells and end annuli, or rings, but is spaced from the ends and shells. The present evaporator is to be operated in a flooded condition with the refrigerant 16, preferably ammonia, flooding over the top of the circulation baille 15. Refrigerant is delivered from the precooler 3 to the interior of the evaporator by way -of the separator 4 thru a refrigerant inlet opening 17 formed in the lower part of the separator 4 and refrigerant is removed from the evaporator to the separator 4 thru a refrigerant outlet opening 18 formed in the outer shell 12 adjacent the top edge thereof. An inlet opening 20 is provided in the shell 12 for the return of liquid refrigerant from the separator 4. In actual use the outside of the outer shell 12 is covered with insulation which has not beenshown in the drawings as such would only confuse the showing thereof. Also, in actualuse, the pre-cooler and the separator would be covered with in- 'sulation.

Top and bottom rings The evaporator is set on the base ring 6 that carries on its interior and circumferentially thereof a water trough that opens upwardly and i'sformed by the outer trough and base ring 21, the inner trough ring 22 spaced inwardly from the outer ring, and the trough bottom annulus 23. The inner ring 22 has a slightly smaller diameter than the inner shell 11) of the evaporator. Water collects in the trough from the excess water running off the lower edge of the ice making surface 11 and from overflow from the water feed overflow chamber. Water in the trough is drained therefrom thru trough drain opening 24 formed in the outer ring 21. Spider arms 25, 26 are secured radially of the rings 21, 22 to support at the axis of the evaporator a radialthrust bearing 27.

Set on the evaporator is a top ring assembly 7 that is composed of the cylindrical ring 28 that has

spider arms

29, 30, 31 secured radially thereof to support at the axis of the evaporator a radial-thrust bearing 32. The annular opening thru the top ring may be closed by fixed annular closure plate 33 and removable ones similar to the xed ones but not shown in the drawings.

Rotor assembly ring assembly 7. This motor drives a pinion 41 that meshes with a gear 42 secured to and coaxial of a composite shaft having a top section 43 to which the gear 42 is secured, and a bottom section 44. In the present disclosure, in plan view, the shafthas counterclock rotation. Secured to the bottom section of the shaft are an upper and a

lower shaft arm

45, 46. These arms carry a sweep rail 47 that is rectangular in cross-section, that.

extends from the top edge to the lower edge of the evaporator and that is placed close to the ice making surface 11 of the evaporator with its side opposed to such surface.

Secured to the side of the rail in opposition to the ice freezing surface are a series of sweeps 48. yEach sweep (Figures 4 and 9) is somewhat in the form of a at plate having one edge secured to a sweep base 48A fastened to the side of the rail 47 opposed to the ,ice surface 11. Each sweep extends away from its base and the rail, andpast the trailing edge of the rail. The trailing portion 49 of the sweep may be said to have rake with respect to the rail 47. This trailing, or rake, portion has a sharp edge S0 in opposition to the freezing surface. This edge is obtained by beveling the top of the rake to leave or form the edge in the plane of the lower face of the rake. The sweep and this rake edge are at a slight angle, about 4 to 5, to a plane normal to the axis of the freezing surface. In a machine in vwhich the radius of the freezing surface is about two feet or more, the rake edge 50 may lie in a plane, but in smaller machines the edge would approach the form of a portion of a cylindrical helix whose diameter is that of the diameter of the freezing surface.

As a specific example, if the radius of the freezing surface is twenty inches, the rake edge 50 is two and onefourth inches long, lies in a plane, and is curved to a radius of twenty inches. The trailing end of the rake edge is seven thirty-seconds of an inch below the leading end of the edge. This slope of the rake edge sweeps the ice from the ice surface 11. The sweeps are spaced about one and one-half inches apart. All or most of the rake portion of the sweep trails the sweep rail. The sweeps may be secured to the rail by welding, bolting, or keying.

The angle that the rake edge makes with a plane normal to the axis of the evaporator may be considered to be the angle of lead, or the lead, as in a screw thread. This angle, or lead, is critical as too great a lead will cause the ice to powder and too small a lead will not effect satisfactory ice removal as to quantity. Under proper shaping and location of the rake edge, and with dry ice about one-eighth to one-sixteenth inch thick, the ice removal at each passage of the sweeps is in the order of ninty-eight percent complete. Also, with this shaping of the rake and its edge, all the forces exerted on the ice by the sweep are parallel, or tangential, to the surface of the ice, except for such forces as may result at the forward end of the rake edge where it enters the ice. There is no force component where sweep and ice contact that is normal to the ice freezing surface. Such a normal force causes the ice to powder along the rake edge with the result that forces are not transmitted thru the ice for any distance sufficient to loosen the ice between adjacent sweeps. This results in much or most of the ice being left on the freezing surface until a very thick layer is built up, as after several passages Aof the sweeps.

Frost builds up on the

end annuli

13, 14 and will extend inwardly beyond the ice making surface 11. This frost is scraped back flush with the surface 11 by upper and

lower Scrapers

51 and 52 which are secured by arms to the upper and lower end, respectively, of the sweep rail, and which Scrapers have their scraping edges adjacent and overhanging the ends of the freezing surface.

Also, carried at the lower end of the sweep rail is a trough guard S3 that prevents ice falling from the sweeps from entering the trough formed in the base ring assembly 6. Any ice hitting the guard will be deflected inside of the inner trough ring 22 and will fall into the Water system Water is delivered to the freezing surface 11 by a water recycle pipe 60 that empties into the uppervend of the hollow top section 43 of the composite shaft. The lower end of this top section is closed by a plug 61 which has extending therethru a short length of overow pipe 62 whose upper end is somewhat above the plug. Water collecting above the plug is carried away thru

nozzle ring pipes

63, 64 to a nozzle ring 65 that is concentric of the evaporator, adjacent the upper edge of the freezing surface, and carried on nozzle ring arms 66 secured to and radially of the upper end of the bottom section 44 of the shaft. The nozzle ring 65 is hollow, square in cross section and has around its lower outer edge` a series of orifices 67, or nozzles, that are shaped to direct streams of water outwardly, downwardly, and circumferentially in the direction of rotation and against the ice forming surface 11. This directing of the water against and circumferentially of the ice forming surface spreads the water over the surface and prevents its channeling. It, also, gives immediate coverage with water at the top of the freezing surface. The nozzle ring is segmental for about 270 from immediately behind the doctor blade 54 and its guard 55. In this relationship, the doctor blade and guard prevent water from reaching the falling ice. The ninety degrees of the nozzle ring that is open allows the ice time in which to dry and to be removed. The water delivered thru the ring is maintained constant by holding a fixed pressure, or head, on the ring. This is accomplished by the use of the overflow pipe 62 and by maintaining during operation of the machine a flow thru the overflow-'to cornpensate for variations in the amount of water delivered to the top section 43. The overflow water passes thru the overflow pipe 62 and into the top of the bottom section 44 where it is stopped by a plug 68. A drain pipe 69 leads from above the plug 68 t0 the water collecting trough in the bottom ring 6. This drain pipe 69 is secured to the rotor assembly 8 and moves around with it.

Water from the collecting trough ows from the outlet 24 to a water sump 70. Water from the sump is returned to the spray ring 65 by the feed pipe 60 in which is connected a water pump 71, driven by any suitable means. Make-up water is supplied from a water main 72 to the sump 70, and its flow into the sump is controlled by a water float valve 73.

Precooler-oil eliminator receiver of a compression system is delivered thru a l supply pipe 81 to a heat exchanger where it is cooled between an outer jacket 82 and an inner jacket 83 thereof. These jackets may be finned in any suitable manner. The exchanger is located inside of the vessel 80. Refrigerant, cooled to near zero degrees Fahrenheit between the iackets, passes thru the lowerI end of the heat exchanger and into the bottom of the vessel where the viscous oil settles out. This oil may be withdrawn thru the drain tube and valve 84 in the bottom of the vessel. Refrigerant is drawn olf the top of the vessel thru a pipe 85 leading to an expansion valve 86 thru which the refrigerant is expanded to a lower pressure and returned by the pipe 85 to the inside of the inner jacket 83 of the heat exchanger. Passage thru the expansion valve reduces the temperature of the refrigerant so that in passing thru the exchanger it will cool the high pressure incoming gas. Liquid refrigerant and gas from the exchanger is conducted to the separator thru a pipe 87 connected between the exchanger and the inlet opening 17 of the separator.

Separator i Gas and entrained liquid from the evaporator pass from the top of the evaporator thru the outlet opening 18 to the gas separation chamber 4. The chamber is in the form of a closed upright separation tube 90 having an inlet 91 in the side near the bottom connecting with the evaporator outlet 18, a liquid return opening 92 in the bottom of the separator connecting with the evaporator liquid return opening 20, and a gas outlet 93 in its side near the top which connects with the compression system which has not been shown but may be of any standard and suitable type. The cross sectional free area of the separator tube 90 is such that the rate of fall of the liquid particles in the gas stream rising in and thru the separation chamber will be greater than the upward velocity of such gas stream. A baffle 94 in front of the inlet 91 prevents short circuiting of the wet vapor from the evaporator to the gas outlet 93 and gives the incoming vapor a helical movement which aids in the gas-liquid separation. Y

Operation In the operation of the present device, the motor 40 for operation of the rotor assembly is energized to rotate the

shaft

43, 44; water is supplied to the sump l0 from the water main 72, its level in the sump is controlled by the iloat 73, and this water is circulated over the ice making surface 11 by the circulating water pump 71 and its associated piping including the nozzle ring 65; refrigerant such as liquid ammonia is supplied from the high side of a compression system to the precooler-eliminator 3, thence, thru the separator 4 to the evaporator 5, and the vapor from the evaporator has the entrained liquid separated out in the separator 4 and returned to the compression system.

When the temperature of the freezing surface l1 falls to and below the freezing point of water, ice will form on the surface. The open gap in the water nozzle ring 65 provides a period during which water is not applied to a portion of the ice surface which allows the ice to dry, harden, and sub-cool. This dry ice is swept from the freezing surface by the sweeps 48 and cascades down along the doctor blade 54 and its guard 55 to fall into the bin 2. Ice does not tend to lodge and pack between the sweeps because the rake portion of the sweeps trails the rail upon which the sweeps are mounted. Ice is prevented from falling in the water trough by the trough guard 53.

Oil is eliminated from the incoming refrigerant by being cooled and allowed to settlel out of the refrigerant in the precooler-eliminator 3.

A high rate of heat transfer is promoted by the rapid circulation of the refrigerant across the surface of the shell 10. This is accomplished by the circulation baille 15 forming an ascending passage and a descending passage between the inner shell and the outer shell 12 of the evaporator 5. Heat delivered to the refrigerant in the ascending passage and the gas formed in this passage induces an upward circulation of the refrigerant in the ascending passage and a downward'current in the descending passage. Gas and entrained liquid arey drawn ol the top of the refrigerant adjacent the top end closure annulus 13 and the liquid removed from the gas and l returned to the evaporator in the separator 4.

The above construction gives a high rate of heat transfer and a large output of ice per square foot of ice freezing surface. The ice is dry when removed from the freezsaid inner surface so that water deposited on said surface will form into ice; watersupply means depositing water on said inner surface; a driven member coaxially mounted of and in said cylinder; and a plurality of axially spaced apart ice removal blades carried by said driven member, each having an outer ice engaging edge portion disposedl generally in a horizontal plane, in close proximity to said inner surface, having the leading portion thereof disposed at a higher elevation than the trailing portion and having a substantially llat and horizontal lower surface portion and an inclined upper surface portion which surfaces join to form an edge, whereby ice forming on said inner surface is removed at a plurality of locations and moves downwardly toward said open bottom of said cylindrical shell.

2. A ake ice maker and removing device comprising a vertical cylindrical shell member open at the bottom and having an inner surface; refrigerating means refrigeratinglsaid surface sothat water deposited on said surface will form into ice; a driven member coaxially mounted of and in said cylinder; water supply means depositing water on said surface, said water supply means comprising a segmental circular reservoir carried by said driven member; and a plurality of axially spaced apart ice removing blades carried by said driven member and disposed in alinement with the space between the end portions of the segmental circular reservoir, each having an outer ice removing edge portion disposed generally in a horizontal plane, and in close proximity to said surface, whereby ice forming on said surface is removed therefrom at a plurality of locations. i

3. A flake ice maker and removing device comprising a vertical cylindrical shell member open at the bottom and having an inner surface; refrigerating means refrigerating the said inner surface so that water deposited thereon will form into ice; water supply means depositing water on said inner surface; a driven member coaxially mounted of and in said cylinder; a plurality of axially spaced apart ice removing blades carried by said driven member, each having an outer arcuate ice engaging edge portion disposed generally in a horizontal plane, and in close proximity to said'inner surface, whereby ice forming on said inner wall is removed at a plurality of locations; and a water collecting trough at the lower'edge of said inner surface.

4. A machine for the manufacture of liake ice, comprising: a cylindrical shell having an inner surface adapted to be refrigerated; means for supplying water to said surface so that ice in sheet form may be formed thereon; rotatable means arranged coaxially of said shell; base means extending axially of and secured to said rotatable means; and axially spaced apart ice removing blades secured to said base means and each extendingl toward and adjacent to said inner surface, each of said blades being constructed and secured to said base means so that the same trails said base means.

5. The combination set forth in claim 4 in which each of said blades is at an angle to its plane of rotation.

6. The combination set forth in claim 4 in which each of said blades has a face normal to said surface, in which each of said blades has this face at an angle to the plane of rotation of said blade, and in which said face at said angle is forward of said blade during rotation.

7. A machine for the manufacture of ake ice, comprising: a cylindrical shell having an inner surface adapted to be refrigerated; means for supplying water to said surface so that ice in sheet form may be formed thereon; rotatable means arranged coaxially of said shell; base means extending axially of and secured to said rotatable means; axially spaced apart ice removing blades secured to said base means adjacent to said inner surface, each of said blades having a face normal to said surface, and said face being at an angle to the plane of its rotation, and said face at said angle being forward of said blade during rotation.

References Cited in the file of this patent UNITED STATES PATENTS 10 Taylor Dec. 8, Taylor Ian. 5, Taylor May 18, Spiegl Oct. 21, Phelan Nov. 1l. McClure July 7, Raver -s Jan. 19, Short Feb. 9, Phelan Aug. 7, Zwickl Mar. 15, Ecabert July 5, Swanson May 8, Walsh Nov. 20, Lessard Feb. l2, Lessard Feb. 12, Nitsch May 20, Lees Nov. 17,