US3454382A

US3454382A - Two-stage type gas producer

Info

Publication number: US3454382A
Application number: US509876A
Authority: US
Inventors: George M Hamilton
Original assignee: Mcdowell Wellman Engineering Co
Current assignee: Mcdowell Wellman Engineering Co
Priority date: 1965-11-26
Filing date: 1965-11-26
Publication date: 1969-07-08
Anticipated expiration: 1986-07-08

Description

July 8, 1969 I Filed Nov. .25, 1965.

TWO-STAGE G. M. HAMILTON TYPE GAS PRODUCER Sheet Y INVENTOR GEORGE M HAMILTON vATTORNEYS y 1969 G. M. HAMILTON 3,454,382 7 TWO-STAGE TYPE GAS PRODUCER Filed Nov. 26, 1965 Sheet 2 of3 W inn LA Q 95 65 S 57 L Q A -as 5-55 Q46 W5 w 2 s4 63-\ 4 5o M 66 X a. Jr W W v W "m e9 e4 as INVENTOR GEORGE M. HAMILTON QRNEY G. M. HAMILTON TWO-STAGE TYPE GAS PRODUCER July 8 1969 Sheet Filed Nov. 26, 1965 46 lump 46 INVENTOR GEORGE M HAMILTON ATTORNEYS United States Patent 3,454,382 TWO-STAGE TYPE GAS PRODUCER George M. Hamilton, Shaker Heights, Ohio, assignor to McDowell-Wellman Engineering Co., a corporation of Ohio Filed Nov. 26, 1965, Ser. No. 509,876 Int. Cl. Cj 3/20, 3/06, 3/62 US. CI. 4876 13 Claims ABSTRACT OF THE DISCLOSURE In an apparatus for making producer gas including a carbonization chamber mounted above a reduction chamber and through which a caking coal moves by gravity, stirring arms mounted for rotation on a shaft in the carbonization chamber, the stirring arms rotating at two different levels, and having drag bars extending between the spaced stirring arms at different levels for rotation with the arms. Separate radially extending stirring arms are provided in the reduction chamber.

This invention relates to an apparatus for producing gas, and more particularly to improvements in an apparatus employing two stages for making commonly called producer gas from caking coals, e.g., typical American bituminous coal.

A conventionally designed two-stage producer for making producer gas used for firing open hearth furnaces, glass tanks, tunnel kilns, etc. essentially comprises a vertically disposed vessel having a carbonization chamber or zone in surmounted relation to a reduction or gasification chamber or zone, and separate gas olftakes from each chamber. The volatile matter in the coal is distilled, and the coal is carbonized and reduced to coke in the carbonization chamber. The carbon in the coke is gasified in the gasification chamber.

The coal is charged into the carbonization chamber and freely moves through it and the reduction chamber by gravity. A substantially tar-free, coke producer gas is made in the reduction chamber. A portion of this gas continues upwards through the descending coal in the carbonization chamber, supplying the amount of heat necessary to distil or free the volatile matter from the coal, and carbonize it, reducing it to coke. The temperature of the gas leaving the gas offtake of the carbonization chamber is maintained at about 250 F. The remainder of the hot coke producer gas not required for the carbonization process, is taken off the lower gas offtake of the reduction chamber at about 1000 F.

The success of the two-stage technique is largely dependent upon the ability of the gases made in the reduction or gasification chamber to freely circulate or pass through the coal in the carbonization chamber. Heretofore, the two-stage process has been limited to using non-caking type coals, i.e. coals which do not agglomerate upon exposure to heat and which have a free swelling index of between one, and two and one half. The caking coals, e.g. typical bituminous coal found in the United States, agglomerate in the carbonization zone to cause channeling of the gases through the coal, which disrupts the process by causing loss of control of the gas outlet temperatures from the carbonization zone. This invention is directed to solving this problem.

Briefly stated, this invention is in a two-stage, or predistillation gas producer for making producer gas from caking coals such as typical bituminous coal found in the United States, and suitable for gasification. The producer includes a vertically disposed carbonization zone or chamber in surmounted relation to and communicating with a separate reduction or gasification zone or "ice chamber, the coal being fed into the carbonization chamber and freely moving by gravity through both chambers. In accordance herewith, means are provided in the carbonization chamber for agitating the coal to prevent agglomeration of the coal and keep it moving uniformly through the carbonization chamber. Such agitating means includes a shaft mounted in the carbonization chamber for rotation about, and axial movement along the longitudinal axis of the carbonization chamber. At least one pair of spaced arms are adjustably mounted on the shaft for rotation with the shaft. The arms radiate from the shaft and rotate in planes normal to the shaft axis. A plurality of drag bars are removably secured to, and extend between the spaced pair of agitator or stirring arms for rotation with the arms and shaft. The drag bars are radially spaced from the shaft axis along the agitator arms, the longitudinal axis of the drag bars, preferably, being parallel to the longitudinal axis of the shaft. The spaced arms and drag bars form a unit which is rotated in a predetermined zone within the carbonization chamber, to prevent agglomeration of the coal.

The following description will be better understood by having reference to the annexed drawings, wherein:

FIG. 1 is a diagrammatic illustration of a gas producing apparatus utilizing embodiments of this invention;

FIG. 2 is a sectional view of the agitators taken in the plane indicated by the line 22 of FIG. 3;

FIG. 3 is an enlarged top View of an embodiment of the agitators used with the gas producing apparatus as seen from the plane indicated by the line 3-3 of FIG. 2; and

FIG. 4 is an enlarged side view of an embodiment of the coupling mechanism for securing the agitators to the agitator shaft.

Referring more particularly to FIG. 1 of the annexed drawings, there is shown a gas producing apparatus generally indicated at 10. The gas producer 10 is mounted within a building 11 in spaced relation to the ground 12. The gas producer 10 comprises in successively surmounted axially aligned relation, an ash hopper 13, -a grate chamber 14, an elongated water-cooled cylindrical jacketed chamber or section 15, and an elongated cylindrical bricklined chamber or section 16. Section 15 is the gasification or reduction chamber 30, and section 16 is the carbonization chamber 45.

An eccentrically stepped grate 17 extending into the water-cooled section 15, eccentrically rotates in the grate chamber 14. Any suitable grate, however, may be mounted in the grate chamber 14. The grate 17 is rotated by any suitable means, e.g. a variable speed grate drive 18. The grate 17 may be provided with an automatic grate positioner 19 for controlling the grate position and its speed of rotation in response to temperatures received from the top grate 20 by any suitable means, e.g. wires 21.

The hopper 13 may be resiliently mounted on the grate chamber 14 and further provided with a vibrator 22 for shaking the ash from the hopper 13. In areas where air pollution must be controlled, the ash in the hopper 13 may be sluiced out with water.

The hopper 13 is provided with an ash lock chamber 23 in cases where the process is carried on under high pressure within the producer 10. The ash lock chamber 23 is provided with

power valves

24 and 25 at its

ends

26 and 27, respectively. The power valve 24 is opened to permit dry ash to drop into the ash chamber 23. The power valve 24 is then closed. The ash chamber 23 is opened to the atmosphere by a depressurizing valve 28. The powervalve 25 is then opened and the ash in the chamber 23 dumped into a truck (not shown) or any other suitable means for hauling the ash from the gas producer 10.

A shaft 36 extends through the brick-lined chamber 16 into the water-cooled jacket 15. The shaft 36 rotates about, and is axially movable along, the axes of the chamber 16 and jacket 15. The shaft 36 is mounted in the chamber 16 and jacket 15 by any suitable means,

e.g. bearings

37 and 38 in

chamber ends

33 and 39, respectively.

An agitator 41 is removably secured to the shaft end 42 extending into the water-cooled section 15 and rotates with the shaft 36. The agitator 41 lies in a plane normal to the axis of the shaft 36. The provision of the agitator 41 is not essential to the making of producer gas in the reduction zone or chamber 30, as the coal is carbonized to coke before it enters this zone, and the agglomeration of coke, if any, is not critical. In some instances then, it may be desirable to operate the gas producer without the agitator 41. However, the agitator 41 does increase the potential capacity of the gas producer 10, by diminishing the chenneling effect of the gases; that is, the tendency of the gases to form channels or passageways in the coal rather than dispersing freely upwardly through the coal, as may be caused by excessive resistance of the products of combustion through the fuel bed or by excessive fuel fines.

The shaft 36 in the carbonization zone or chamber 45 is designed to have at least one pair of spaced agitators or stirrers, e.g. spaced

stirrers

43 and 43a, removably secured to it. The stirrers may comprise a single arm, or a plurality of arcuately spaced arms radiating from, and mounted for rotating with the shaft 36. The

stirrers

43 and 43a rotate in planes normal to the shaft axis.

Drag bars 46 are detachably secured to, and extend between the pair of spaced

stirrers

43 and 43a mounted on the shaft 36. As seen in FIG. 3, the drag bars are radially spaced from the shaft, along the arms, e.g. arms 73-76 of stirrer 43, of the stirrers in a manner determined by actually operating the gas producer, and noting the areas in which the coal tends to agglomerate. For example, it may be found that the gas producer operates more efficiently by equally spacing the same number of drag bars on each stirring arm, or to alternately stagger the drag bars on the stirring arms. The location of the drag bars varies with the design, etc., of each individual gas producer. The spaced

stirrers

43 and 43a with attached drag bars 46 form a unit generally indicated at 47, for preventing agglomeration in the carbonization zone 45 and maintaining free and open passageways for the products of combustion. The agglomerate-preventing unit 47 is adjustable along the shaft 36 for greater flexibility.

The drag bars 46 can be chains, or solid bars having a round, flat or triangular configuration. The drag bars are, preferably, as thin as possible, as heavier bars meet greater resistance as they rotate through the bituminous coal, and have a tendency to produce a greater number of fines. A triangular cross-sectioned solid drag bar is preferable, because the leading edge, e.g. edge 94, or that edge first confronting the bituminous coal is pointed and meets less resistance as it moves through the material.

The shaft 36 is rotated by any suitable means, e.g. a conventionally designed agitator drive 44 as described in Patent No. 2,502,141. The agitator drive 44 is designed to axially move the shaft 36, whereby the agitator 41 and unit 47 are spirally oscillated, or moved to and from the grate 17. In other words, the reduction zone agitator 41 and carbonization zone agglomerate-preventing unit 47 may be adjusted, or moved to rotate in a plurality of predetermined fixed planes normal to the shaft axis, or spirally rotated through a plurality of planes normal to the shaft axis and defining a predetermined zone in the reduction and distillation chambers, respectively, Where the coal agglomerates. The speed at which the shaft is rotated varies with the number of arms provided in each stirrer and the number and location of the drag bars, as well as the rate at which the bituminous coal moves 4 through the carbonization and reduction zones, the rate of movement being dependent on the rate of fuel feed.

Referring more particularly to FIGS. 2-4, an elongated collar 48 is secured to, and surrounds the shaft 36. The collar 48 may completely surround the shaft 36 and extend through the chamber end 33, or it may be terminated in spaced relation from the brick-lined chamber end 36. In either case, the shaft 36 is protected by the collar 48 as it rotates in the hot bituminous coal.

The collar ends 49 and 50 (FIG. 3) are sealed on the adjacent shaft 36 by any suitable means, e.g. annular plate 51 and welds 52 and 53, and an agitator block 63 and weld 54, respectively. An annular recess 55 is formed between the shaft 36 and the collar 48. The recess 55 is designed to have cooling fluid, e.g. water, circulated in it.

The shaft 36 has a centrally disposed passageway or supply pipe 56 extending longitudinally through it. An annular recess 57 is formed in the shaft 36 in spaced relation from the pipe 56. Coolant, e.g. water, from a source of supply (not shown), is pumped into a water connection 58 (FIG. 1) disposed on the agitator drive 44 and into the pipe 56. The coolant is pumped through the pipe 56 and circulated to the agitator 41 and spaced

stirrers

43 and 43a. As the gases are relatively cool near the inlet 34, the stirrer 43a adjacent to or closest to the fuel inlet 34 may or may not be water cooled. The coolant is also circulated through the recess 55 for cooling the shaft 36 as it rotates in the hot fuel. The coolant is recirculated from the agitator 41, the spaced

stirrers

43 and 43a, respectively, and the recess 55, into the annular recess 57 (FIG. 2) from which it is pumped through the water outlet 59 (FIG. 1) disposed on the agitator drive 44. The recirculated coolant from the water outlet or connection 59 may be discarded or recirculated to the source of supply. In this manner, the shaft 36, the agitator 41 and the spaced

stirrers

43 and 43a, respectively, are cooled as they rotate within the hot bituminous coal.

The agitator 41 comprises a pair of oppositely disposed relatively fiat hollow

tubular arms

61 and 62 secured to and radiating from an agitator block 63 secured to the shaft end 42. A second pair of oppositely disposed

agitator arms

66 and 67 may be removably secured to the agitator block 63 by any suitable means, e.g., bolts 68. The

arms

66 and 67 are similar to the

agitator arms

61 and 62. A hollow recess formed in each of the tubular arms communicates with the annular shaft recesses 55 and 57, e.g. recess 64 of arm 61. A conduit or feed pipe is disposed in each of the hollow recesses, e.g. feed pipe 65 in recess 64 of arm 66, and communicates with the coolant supply pipe 56 through pipe taps, e.g. tap 60, disposed in the agitator block 63 and radiating from and communicating with the supply pipe 56. The free end 65a (FIG. 2) of each feed pipe 65 is in spaced relation from the end of each feed pipe receiving recess 64, thereby permitting water to be circulated fromthe feed pipe 65 into the recess 64 and recirculated into the annular recess 57 in the shaft 36.

An annular asbestos seal 69 surrounds each feed pipe and sealing coacts with the agitator block 63 to keep the coolant or water from seeping between the block 63 and agitator arms. The four

agitator arms

61, 62, 66 and 67 radiate from the agitator block 63, preferably, at right angles to each other. The agitator block 63 may be designed to accommodate more agitating arms. This is accomplished by providing more pipe connections or taps and more arm abutting surfaces in the agitator block 63.

The shaft 36 and attached collar 48 in the distillation zone 45, are provided with a plurailty of oppositely disposed arcuately spaced pipe taps or connect-ions, e.g. pipe connection 70, which radiate from and communicate with the supply pipe 56. The pipe connections are spaced along the collar 48, for example, about 2." apart. When the pipe connections are not in use, they are sealed by a cap or plug, e.g. plug 71, threadably secured in the collar 48 and. sealing the open end 72 of the pipe connection 70.

The

stirrers

43 and 43a are, preferably, similar and, as indicated, of any suitable design, i.e. a single arm, a pair of arms, or a plurality of arms. In the embodiment shown, the

stirrers

43 and 43a comprise a plurality of similar stirring arms,

e.g. arms

73, 74, 75 and 76. The stirring arms 73-76 are, preferably, arcuately spaced 90 apart, i.e. their longitudinal axes are angularly disposed 90 apart. Each arm has a flat hollow tubular configuration similar to the

agitator arms

61 and 62. Each arm is similarly provided with a recess and pipe for circulating coolant through the arm.

The stirring arms 73-76 are secured to and radiate from a hub 77 which is detachably mounted on the shaft 36 by any suitable means. For example, the hub is split into two

halves

78 and 79 which are hinged together by any suitable means, e.g. hinge 80. The hub 77 is configured to surround the shaft 36. The hub 77 is provided with a transversely extending recess 81 which is designed to receive a key or spline 82. The spline 82 is secured in a longitudinally extending recess 83 disposed in the collar 48 and is coextensive with the recess 83. The spline 82 when it is received in the hub slot 81, positions the stirrers,

e.g. stirrers

43 and 43a, on the collar 48 of the shaft 36.

The free ends 84 and 85 of the hub halves 78 and 79, respectively, have key-receiving

slots

86 and 87, respectively, disposed therein. Thus, when the stirrers are positioned on the shaft 36, i.e. the spline 82 is received in the hub slot 81 and the hub halves 78 and 79 are swung into overlapped relation, the key-receiving

slots

86 and 87 are in partially aligned relation.

A tapered key 90 is driven into the partially aligned

slots

86 and 87, wedging the hub halves 78 and 79 together, and clamping the stirrers in position on the collar 48 of the shaft 36. In this manner the stirrers are secured to the shaft 36 for rotating with it. In this clamped position, the feed pipes 65 disposed in the stirring arms each communicate with corresponding pipe connections disposed in the collar 48 and shaft 36, and communicating with, and radiating from the coolant supply pipe 56.

Similar annular asbestos seals 69 are disposed around the feed pipes 65 of the stirring arms adjacent the collar 48 and coact with the collar 48 to keep coolant from leaking between the stirrers and the collar 48.

The leading edge of the agitator and stirring arms, i.e. the edge of the arms first confronting bituminous coal as the arms rotate, are each provided with a protective wearing plate, e.g. wear plate 91. The wear plates 91 are detachably mounted on the arms by any suitable means, e.g. bolts (not shown), and wrapped around the free ends of the arms for protecting them. Thus, the arms of the agitator 41 and the

stirrers

43 and 43a are protected as they rotate in the hot bituminous coal in the water-cooled section and brick-lined chamber 16, respectively. The free ends of the agitator arms, and the free ends of the stirring arms, are angularly disposed to the longitudinal axis of the arms; that is, for example, the leading edge 92 (FIG. 3) of arm, 61 is closer to the inner wall surfaces of the section 15 than its trailing edge 93. This is done to keep the fuel from being crushed into fines between wall and end of arms as the arms rotate. Excessive fines create additional resistance through the fuel bed of the products of combustion, causing possible channeling, blow holes, lower calorific value gas, and lower gasification capacity.

In operation, bituminous coal is fed through the inlet 34' (FIG. 1) from a suitably controlled fuel feeding system into the brick-lined chamber end 33, and freely moves by gravity successively through the carbonization zone 45 and the reduction zone 30 in the brick-lined chamber 16 and the combination brick-lined and water-cooled jacketed chamber 15, respectively. The temperature of the bituminous coal rises gently and constantly as the coal passes through the carbonization zone 45, and by the time it reaches the reduction zone 30 the volatile matter has been distilled from the coal and partially carried olf through an offtake port 96 of the carbonization zone 45.

The stirrers,

e.g. stirrers

43 and 43a with attached drag bars 46, are rotating and preventing agglomeration of the bituminous coal. The bituminous coal is carbonized and reduced to coke in the carbonization zone 45. The reactive coke is gasified in the reduction zone 30 of the gas producer 10. The coke is reduced to ash in the firing zone 29 located in the chamber 16 adjacent the grate 17. The gas from the reduction zone 30 can leave either by way of the carbonization zone olftake 96, or a reduction zone ofitake 95. The flow of gas is so proportioned by control valve, e.g. valve 97, that just ls-uflicient heat is available in the carbonization zone 45 to effect the proper carbonization of the coal. The criteria of the proportioning is the temperature of the gas in the carbonization zone gas olftake 96 which is maintained at about 250 F. It is this low gas oiftake temperature of the gas which largely accounts for the remarkable performance of the gas producer 10.

The reduction zone gas offtake is, preferably, located in a system 103 of flues embedded in the refractory walls 104 of the carbonization zone 45. A portion of the gases from the reduction zone 30 pass upwardly through the flue system 103 to heat the refractory walls 104 to aid in heating the coal in the carbonization zone 45. The circulating gases from the flue system 103 then pass into the reduction zone offtake 95. The hot gases taken off in the reduction zone olftake 95, are at temperatures of about 1000 F. The agitator 41 is constantly rotating to retard channeling, and possible agglomeration. The water-cooled section 15 in conjunction with a steam drum, provides sufficient steam to mix with the air blast from an air-inducing apparatus 98, and is connected to steam inlet 99. The steam from inlet 99 and the air from apparatus 98 are pre-mixed, and fed through blast pipe 100 into the grate chamber 14 for circulating upwardly through the downwardly descending coal. A separate steam supply is necessary when the gas producer 10 is operated under high pressure conditions or if oxygen blast is used in place of air. CO may be substituted for steam depending upon the desired characteristics of the gas required.

The tar-laden gas from the carbonization zone offtake 96 is passed through a conventionally designed cyclone separator 101 in which the larger tar droplets are removed, the bulk of the tar remaining in the gas as a fine mist. The clear tar-free gas from the reduction zone offtake 95 is passed through a conventionally designed brick lined cyclone separator 102, where dust particles are removed. The two gas streams, i.e. the relatively cool and tar laden bas from the carbonization zone at temperatures of about 250 F., releases the larger tar droplets in the cyclone 101, and the tar-free gas from the reduction zone at temperatures of about 1000 F., are then mixed to give the final hot raw producer gas. As they mix, the combination of the high temperature gas from the reduction zone 30, causes the tar in mist form from the carbonization zone to revaporize, and the efficient removal of the dust from the reduction zone gas stream accounts for the subsequent freedom from deposition in the gas' mains.

Thus, there has been provided a two-stage gas producer in which caking type coals may successfully be used as a fuel burden. The adjustable and comprehensive automatic agitation system provided in the carbonization chamber, prevents agglomeration of the coal which would otherwise disrupt the process by causing loss of temperature control at the outlet of the carbonization zone. Producer gas may now be produced from typical bituminous coal found in the United States on a long term continuous basis, and the need for gas transmission line cleanouts, or providing refractory line gas mains required with single-stage bituminous gas producers, is substantially eliminated.

Other modes of applying the principle of this invention may be employed instead of those specifically set forth above, changes being made as regards the details herein disclosed, provided the elements set forth in any of the following claims, or the equivalent of such, be employed.

It is, therefore, particularly pointed out and distinctly claimed as the invention:

1. In an apparatus for making producer gas from caking coals, including a carbonization chamber in surmounted relation to a reduction chamber, the coal moving by gravity through both chambers; the improvement which comprises:

(a) at least one pair of spaced stirring arms mounted in the carbonization chamber for rotating in a plane normal to the direction in which the coal moves by gravity; and

(b) at least one drag bar mounted on, and extending between the spaced arms for rotating with the arms in a predetermined zone to prevent the coal from agglomerating as it moves through the carbonization chamber; and

(c) separate, radially extending stirring arms mounted on said shaft for rotation therewith in said reduction chamber.

2. In an apparatus for making producer gas from caking coals including a carbonization chamber in surmounted relation to a reduction chamber, the coal moving by gravity through both chambers; the improvement which comprises:

(a) a shaft mounted in the carbonization chamber for rotation on the longitudinal axis of said chamber;

(b) means coacting with the shaft for rotating it;

(c) at least one pair of spaced stirrers adustable along the shaft and removably secured to it for rotating it in said carbonization zone, the stirrers rotating in a plane normal to the shaft axis;

(d) a plurality of drag bars mounted on, and extending between the stirrers for rotating with the stirrers in said carbonization zone, said stirrers with attached drag bars being rotated through a predetermined zone to prevent the coal from agglomerating as it moves through the carbonization chamber; and

(e) separate, radially extending stirring arms mounted on said shaft for rotation therewith in said reduction chamber.

3. The improvement of claim 2, wherein the rotating means includes means for axially moving the shaft, whereby the stirrers with attached drag bars are spirally rotated through a predetermined zone.

4. The improvement of claim 2 wherein the drag bars are radially spaced from the shaft on the stirrers, the 1ongitudinal axes of the drag bars being parallel to the shaft IQXIS.

5. The improvement of claim 2, which includes means for cooling the shaft and stirrers.

6. The improvement of claim 2 wherein the drag bars have a triangular cross section taken in a plane normal to their longitudinal axis.

7. A two-stage apparatus for making producer gas from bituminous coal comprising:

(a) a water-cooled cylindrical jacketed chamber forming a reduction zone;

(b) a brick-lined cylindrical chamber forming a carbonization zone in surmounting relation to the jacketed chamber and communicating with it, the longitudinal axes of the zones coinciding, the coal moving by gravity through both zones;

(c) a shaft extending through the carbonization zone into the reduction zone and mounted for rotation about their common axis;

(d) means coacting with the shaft for rotating it;

(e) at least one pair of spaced stirrers rernovably secured to and adjustable along the shaft in the carbonization zone, the stirrers rotating in a plane normal to the shaft axis; and

(f) a plurality of drag bars mounted on, and extending between the stirrers for rotating with them;

(g) separate, radially extending stirring arms mounted on said shaft for rotation therewith in said reduction chamber.

8. The apparatus of claim 7 wherein the shaft rotating means includes means for axially moving the shaft.

9. The apparatus of claim 7 wherein the drag bars are radially spaced in a predetermined manner from the shaft axis and their longitudinal axes are parallel with the shaft axis.

10. The apparatus of claim 7 which includes:

(h) means coacting with the shaft for cooling it as it rotates in the hot bituminous coal.

11. The apparatus of claim 7 which includes:

(i) means coacting with the shaft cooling means for cooling at least one stirrer.

12. The .appaartus of claim 11 wherein the shaft cooling means includes a plurality of water taps spaced along the shaft in the carbonization zone.

13. The apparatus of claim 7 wherein the drag bars have a triangular cross section taken in a plane normal to their longitudinal axes.

References Cited UNITED STATES PATENTS 897,205 8/1908 Hellwig 259-134 XR 1,279,515 9/ 1918 Coleman 259-l34 2,440,940 5/ 1948- Galusha 48-76 3,233,875 2/1966 Vaughan v.. 259-134 XR MOR RTS O. WOLK, Primary Examiner.

R. E. SERWIN, Assistant Examiner.

U .8. C1. X.R.