CA1086059A - Apparatus for controlling flooding in the drawing of glass fibers - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE An orifice plate for use in a glass fiber drawing assembly of the type wherein the plate has a flat undersurface devoid of nozzles, and bulk gas is directed toward the under-surface to cool fibers being drawn through the plate. The plate is characterized in that the orifices therein are arranged in paired sets with the orifices within each set being of the same diameter and spaced from one another by a center-to-center distance equal to from lo 2 to 1.45 said diameter, the respective sets being spaced from one another by a distance greater than the distance between the orifices within the sets.

Description

6~9 BACKGROUND OF THE INVENTION
The present invention relates to an improved apparatus for the drawing of glass fibers and is particularly concerned with such an apparatus wherein the orifice plate of the drawing bushing is of the type having a generally planar undersurface toward which bulk flow gas is directed to achieve fiber cooling and attenuation. The invention is especially directed to an improved orifice pattern which provides for "self-healing"
in the event of the breakage of a fiber being drawn from the plate.
In its more specific aspects, the inven-tion is con-cerned with an improvement in the apparatus disclosed in United S-tates Application Serial No. 500,303, filed August 26, 1974, by Edward T. Strickland, now Patent No. 3,905,790. That appli-cation discloses a method and apparatus for forming glass fibers wherein the orifice plate has a generally planar undersurface and bulk flow gas is directed upwardly toward the undersurface to effect fiber cooling and attenuation. It also suggests that self-correction of localized flooding can be achieved by close ~0 orifice spacing and discloses a technique of such self-correc--tion wherein capillary grooves are provided between the orifices -to provide a pa-th for controlled glass flow from one orifice to another in the event of breakage of the fiber emanating from one of the orifices.
When glass defects (e.g., stones, crystalline parti cles, cords and seeds) pass through conventional tipped bushings, they generally cause fiber breakage. Then, the loose tail of the broken fiber either snaps out the rest of the fibers being drawn from the bushing, or else the drop that drains from the tip grows until it falls and breaks the other fibers. Either result causes an interruption of the fiber forming processO

L~ ~I

~86(~S9~

When similar defects pass through a non-tip bushing using a column of rapidly moving cooliny gas to maintain fiber separation, the fiber is similarly broken, but does not cause a snap-out of the other fibers. The drop left behind grows until it meets a cone of glass supplying a fiber being drawn from an adjacent orifice. It then, at times, causes a break of the fiber being drawn from the cone, which in turn floods to the next adjacent fiber, creating a "domino" effect that re-quires the operator's immediate attention.
In the preferred form of the non-tip bushing dis-closed in aforementioned Uni~ed States Patent Application Serial No. 500,303, the inventor contemplates the provision of capil-lary grooves between the orifices in order to provide for con-trolled flooding in the event that a fiber breaks. The capil-lary grooves are designed to cause the plate to act as though :
it had controlled, but perfect, wetability. Since only a small volume of glass from the oozing orifice will first contact the neighbor fiber, the increase of acceleration load on the neighbor fiber will be gradual and, as the whole fiber pulls more glass out of the groove, the fiber cross-section enlarges and the fiber becomes stronger until a single larger fiber is ~ed by two orifices. Although the capillary grooves are e~ective in that they encourage more rapid flooding to selec ;
-tive adjacent orifices, they have some disadvantages. For e~ample: they reduce the strength of the orifice plate; they aE.Eect the flow of electrical current, thus producing hot and cold spots; and, they increase plate fabrication costs.
SUMMARY OF THE INVENTION
The present invention contemplates a non-tip orifice plate wherein the orifices are arranged in paired sets, with the orifices within the respective sets being of substantially the -`

same diameter and closely spaced to provide for controlled .. . . , , , .:

~ ~o~s~

flooding therebetween, and the respective sets being spaced from one another by a distance greater than the distance be-tween the orifices within the sets. The paired orifices within the sets are spaced from one another by a center-to-center distance equal to from 1.2 to 1.45 times the orifice diameter.
With this spacing, in the event of the breakage of a fiber being drawn from one of the orifices, the glass from said one orifice floods to and joins the glass fiber being drawn from the orifice paired therewith before it has cooled to the extent where it would break out the fiber. The result is the forma-tion of a single enlarged fiber fed by a "double cone" being drawn from the paired orifices. This enlarged fiber may be readily separated to provide a pair of fibers wherein each fiber again is fed by a single orificeO Separation may occur naturally or, if necessary, be achieved through the application of localized cooling gas.
A principal object of the present invention is to provide an orifice plate for a non-tip bushing wherein flooding may be readily controlled both during start-up of the bushing and in the event of fiber break out.
Another, and more specific, object of the invention is to provide such an orifice plate wherein the orifices are so arranged as to be self-corrective in the event of fiber break out.
The foregoing and other objects will become more apparent ~hen viewed in light of the following detailed desc-ription and accompanying drawings.

~IL0~1D5g BRIEF DESCRIPTI~N OF T~IE DRAWINGS
Fig. 1 is an elevational view, with parts thereof broken away, diagrammatically illustrating a drawing assembly incorporating the orifice plate of the present invention;
Fig. 2 is an enlarged cross-sec-tional view of a portion of an orifice plate constructed according to the present invention, sequentially illustrating the manner in which the paired orifices within the plate cooperate -to achieve "self healing" in the event of flooding of one of the orifices;
Fig. 3 is an enlarged cross-sectional elevational view, taken on plane 3-3 of Fig. l;
Fig. 4 is a diagrammatic plan view of the underside of the orifice plate of the invention, taken on the plate designated by line 4-4 in Figure 3;
Fig. 5 is an enlarged plan view of the underside of a segment of a first embodiment of the inventive orifice plate;
Fig. 6 is an enlarged plan view of -the underside of that portion of the first embodiment circumscribed within line 6-6 of Fig. 5;

5~3 Fig. 7 is an enlarged plan view of a portion of the underside of a second embodiment of the inventive orifice plate;
Fig. 8 is an enlarged plan view of a portion of the underside of a third embodiment of the inventive orifice plate;
Fig. 9 is a bottom pian view of a portion of an orifice plate embodying the third embodiment of the invention, schemat-ically illustrating the arrangemen-t of a plurality of groups of orifices corresponding to the group circumscribed by the phantom line in Fig. 8; and, Fig. 10 is a curve plotting glass equilibrium contact angle versus temperature for glass typical of that with which the present invention is used.

DETAILFD DESCRIPTION OF THE INVENTION
, .. _ . _ _ . . . . _ Referring now to Fig. 1, the assembly there shown is of the same general type disclosed in my co-pending Canadian Patent Application Serial No. 254,353, filed June 8, 1976. This type of assembly may be used with any of the embodiments of the pre-sent invention. It incorporates, as a principal component, a direct melt forehearth 10 beneath which a bushing assembly 12 is removably secured. The orifice plate to which the present invention is primarily directed is incorporated into the bush-ing assembly and designated by the numeral 14.
In the Fig. 1 assembly, the molten glass contained with-in the forehearth is designated by the numeral 16 and is shown in~ drawn through the orifice plate 14 into a plurality of ~n Eine monofilament fibers 16a. The fibers are drawn over a ~inder applicator 18 and gathering shoe 20, from whence they are dixected to a collector and winding mechanism 22. A traverse 24 guides the fibers back and forth across the mechanism 22.

The glass within the bushing 12 is maintained at an elevated temperature by resistance heating the plate 14. The means for resistance heating the plate comprises a palr of term-inals 26 - ~086~S9 .~
and 28 secured to ears 30 integrally joined to opposite extremities of the plate. The ears and terminals are disposed to direct current lengthwise across the plate, as may be seen from Figs. 3 and 4.
The glass fibers being drawn from the orifice plate 14 are cooled by bulk gas directed toward the undersurface of the plate through means of a nozzle 32. The gas, typically air, is directed across the width of the plate in a direction generally normal to the current flow direction (See Fig. 4). The nozzle 32 is mounted beneath and to one side of the orifice plate 14 through means of a bracket 34 provided with means to adjust the angle of the nozzle relative to the undersurface of the plate.
The orifice plate 14 is similar to that disclosed in my co-pending Canadian Application Serial No. 254,353, in that it is reinforced through means of an l'egg crate" type of structure integrally joined to its inner surface. This structure comp-rises apertured ribs 36 extending transversely across the plate ; (as viewed in Fig. 4) and a perforated reinforcing plate, or screen, 38 of an area coextensive with the drawing area of the orifice plate, extending over the ribs in spaced parallel re-la-tionship to the upper surface of the orifice plate. The orif ice pla-te, ribs and reinforcing screen are all fabricated of the same material (e.g., an alloy of 90% platinum and 10% rhodium) and are integrally joined.
The bushing assembly 12 also includes a lining 40 inte-~r~lly joined to an extending upwardly from the orifice plate and a cle~lec-tor plate 42 joined to the lining and extending gener-ally across the supply flow passage, designated 44, leading to the bushing. The deflector plate 42 is of a peaked configuration 30 and tends to deflect glass entering the bushing to the sides of the orifice plate. Perforations are provided in the deflector plate and these perforations, together with the screening per-forations ~: ' .:

.
provided in the reinforcing plate 38, screen par-ticles, such as refractory stones or crystals, from entry into the orifices of the orifice plate.
The flow block of the forehearth illustrated in Figs. 1 and 3 corresponds to that disclosed in my copending Canadian Patent Application Serial No.254~353 and comprises an interior layer 46 fabricated of a highly heat and glass resistant material, such as zircon, and an exterior layer 48 fabricated o~ a material having high thermal shock resistant properties, such as MULLITE.
The supply flow passage ~4 extends through the interior and exterior layers and is lined with a platinum foil lining 50. The lining completely covers the flow passage and e~tends over the exterior peripheral surfaces surrounding the passage, as ma~ be seen from Fig~ 3.
The orifice plate of the present invention is characterized in -that the orifices are arranged in paired sets wherein the orifices within the respective sets are close enough to one another that, in the event of the breakage of a glass fiber being drawn from one orifice of a set, the glass from said orifice will flow to and join the glass fiber being drawn from another orifice o~ the set prior to reaching any other orifices within the orifice plate or cooling to the extent that it no longer has sufficient wetability to join and merge with the fiber being drawn from the other orifice. It is also characterized in that the distance ba-~w~en the paired ori~ices with the sets, hereinafter referred to as dimension "a"~ is sufficiently large that the fibers being drawn Erom the paired orifices within the sets will not coalesce under normal operating conditions (i.e., normal levels of bulk gas supply).
The dimension "a" is shown in Fig. 2 and in the three embodiments exemplified in the drawings (i.e., the embodiment of Figs. 5 and 6, the embodiment of Fig. 7, and the embodiment of ~ ~36~5~

Figs. 8 and 9), and is measured between the cen.ers of the paired orifices within the sets. The drawings also sho: the following dimensions:
Dimension Descri~tion :
"b" The center-to-center dis_ance between the sets of orifices within rows extending in the direction of current -low~ as measured : - , . -. . between adjacent orifices o~ the respective sets within the ror-s.

"c" The center to-center disLance between the orifices of adjacen~_ row, extending in the ~-direction of current flo-.Y-, as measured ,~ .
between adjacent orifices .herein (i.e., normal to the direction of current flow3.

"d" The orifice diameter.

"e" The center-to-center distance between adjacent groups of orifices, as measured between adjacent ou Lermoâ L orifices in the respective groups. (This dimension will not be present where the orifices are not arranged in groups--as, for example, with an orifice plate wherein -the orifices are uniformly arranged as exemplified in the embodiment of Fig. 7.) The dimension "b" is maintained larger than the dimension "a" to assure that, in the event of the breakage of a fiber being drawn Erom an ori~ice, ~he flood resulting at that orifice will ~low to and join the glass being drawn from the orifice paired thereto before it has the opportunity to reach an orifice of an ~30 adjacent set of orifices in the row within which the flooded orifice is located. This dimension is maintained as small as possible in order to maximize orifice density, but .~ill always be , . :................ . :
; ~

~ 5t6(~9 greater than the dimension "a".
The function of the dimensions "a" and "b" may best be appreciated by re~erence to the sequential illustration of Fig. 2.
This figure is a cross-sectional view taken through a row o~
orifices extending in the direction of current flow and illustrates a paired set of orifices, designated 0-1 and 0-2, and one orifice designated 0-3 of an adjacent set of orifices. In Fig. 2A, the orifice plate is shown in a condition wherein the fiber beiny drawn from the orifice 0-1 has broken and the glass from the orifice is , 10 in the process of flooding radially therearound, but has not yet reached an adjacent orifice. Fig. 2B illustrates the condition wherein the glass from the orifice 0-1 first reaches and joins the glass fiber being drawn from the orifice 0-2. It will be noted that, due to the relative dimensions "a" and "b'', the latter condition is achieved before the glass flooding from the orifice 0-1 can reach the orifice 0-3. Fig. 2C illustrates the next step in the progression after the condition illustrated in Fig. 2B and shows the glass from the orifice 0-1 fully joined with that from the orifice 0-2 to form a common enlarged fiber which i5 supplied wi~h glass from both of the orifices. It will be noted that, in the latter condition, the glass from the orifice 0-1 has been drawn away from the orifice 0-3, as compared to the condition illustrated in Fig. 2B.
Fi~s. 2D and 2E illustrate the manner in which the single anlaxg~d fiber being drawn from the orifices 0-1 and 0-2, as d~picted in Fig. 2C, bifurcates to "self heal" and return the oxiEices 0-1 and 0-2 to a condition wherein each orifice supplies a single fiber. Fig. 2F shows the final self-healed condition wherein the orifices 0-1 and 0-2 each supply but a single fiber.
It should be noted that, under ideal conditions, the sequential "self-healing" process depicted in Figs. 2A to 2F occurs automatically without operator assistance. Where operating 6~

conditions are difEicult, or the operator wishes to speed the natural process, he might manually assist the separating process illustrated in Figs. 2D, 2E and 2F by use of an air lance. It is also possible that an automatic air supply might be employed to facilitate the separation process.
The dimension "c" is maintained larger than the dimen-sion "b" because the plate area between the orifices in the "c"
direction tends to be hotter and, thus, more prone to flooding, than the plate areas between the orifices in the "b" direction.
This results because there is increased current flow and decreased gas flow in the areas measured in the "c" direction, as compared to those measured in the "b" direction. It should be noted that , current flows normal to the direction in which the "c" dimension is measured and that bulk gas is directed generally normal to the direction in which the "b" dimension is measured.
The relatively large expanses provided by the "e" dimens-ion between the groups or families of orifices permit clearing of separate areas of the orifice plate as the result of a non-flooded condition in these expanses. A "non-flooded" condition, as used herein, means a condition wherein the surface of the plate is not covered with glass. This clearing provision is very advantageous bo-th during start-up operation and in the course of breaking up large floods which will no-t self correct.
The paired sets of orifices also facilitate clearing by perm.itting the formation of enlarged fibers supplied with glass f~om the two or three orifices within a set, as exemplified in Fig. 2C. Such enlarged fibers are known as "doublets" where they are provided with glass from two orifices within a paired set and "triplets" where they are supplied with glass from three orifices within a paired set. Examples of paired sets wherein each set comprises two orifices may be seen in Fig. 5 and 6 embodiment and the Fig. 7 embodiment. An example of a paired set ... . .

~3605 ...
- wherein the set comprises -three orifices may be seen in the Fiy.
8 embodiment. The enlarged doublet or triplet fibers are advantageous durin~ clearing and start-up in that these fibers are s~ronger than would be a fiber supplied from a single ori~ice (~nown as a "singlet") and, thus, more resistant to breakage by the high gas flow which is typically employed during clearing and start-up operations.
It should also be appreciated that the doublet or triplet fibers provided by the closely spaced orifices of the paired sets are ideally suited for separation into singlet fibers because of the relatively close spacing of the orifices within the sets.
Where the spacing between orifices is relatively large, as, for example, measured in the "c" or "e" dimensions, and a doublet or triplet fiber is created between such orifices, it becomes difficult,~
i~ not impossible, to cleanly separate the fiber into singlet fibers.
The advantages of the present invention are evident throughout all of the operating conditions encountered during the dra~ing of glass fibers.
At start-up, the invention enables the opera-tor to clear ~0 the plate in an orderly and systema-tic manner. The sequence of op~rations during s-tart-up is generally as follows:
1. The plate is initially in a completely flooded condition with molten glass covering its undersurface.

2. The operator breaks the flood into family groups ~i.e., small floods) wherein the groups are separated by the spacing provided by the group spacing dimension "e".

3. The operator breaks the family groups down, generally in on~-at-a--time fashion, into double-ts and singlets.

4. The operator breaks the doublets into singlets~

5. Steps 3 and 4 are carried out on eac~ family group until the entire orifice plate is cleared and one fiber emanates from each orifice within the plate.

1~86~)5~

The same general steps are used to correct -the partial flood, with the number of steps required being dependent upon the extent of the flood.
During normal operation, in the event of fiber breakage, the breakage ideally self-corrects through the sequence depicted in Fig. 2. Where, for some reason, complete self-correction does not occur, partial self-correction to the extent shown in Fig. 2C
will generally take place. The latter condition results in the formation of doublets and triplets and effectively halts cont-inued flooding.
The or fice size and spacing employed in the present invention depends upon the throughput desired. The following tables give examples for three different output ranges:

I. Orifice Throughput of 0.2--0.3 Grams/Orifice~Minute "d" "a" "b" "c" "e"
1.20d--1.30d 1.40d--1.50d1.55d--1.65d 1.65d--1.75d .037 .044 .048 .052 .056 .057 .06l .061 .065 .040 .048 .052 .056 .060 .062 .066 .066 .070 .042 .050 .055 .059 .063 .065 .069 .069 .074 -II. Orifice Throughput of 0.3--0.5 Grams/Orifice/Minute "d""a" "b" "c" "e"
1.25d--1.35d 1.4Od--1.5Od1.55d--1.65d 1.65d--1.75d .045 .056 .061 .063 .068 .070 .074 .074 .079 .047 .059 .063 .066 .071 .073 .078 .078 .082 .050 .062 .067 .070 .075 .078 .083 .0~3 .088 .052 .065 .070 .073 .078 .081 .086 .086 .091 III. Orifice mroughput of 0.5--0O7 Grams/Orifice/Minute "d""a" "b" "c" "e"
1.30d--1.45d1.45d--1.55d1.60d--1.70d 1.70d--1.80d ~ .054 .070 .078 .078 .084 .086 .092 .092 .097 .056 .073 .081 .081 .087 .090 .095 .095 .097 .058 .075 .084 .0~4 .090 .093 .099 .099 .104 .060 .078 .087 .087 .093 .096 .102 .102 .108 The particular orifice pattern emplo~ved in the present :
~L~8 invention may vary considerably, as exemplified by the differen-ces between the three embodiments illustrated.
In the first embodiment, i]lustrated in Figs. 5 and 6, the orifices are arranged in generally diamond shaped groups, with each group comprising a plurality of rows of orifices, each row of which comprises at least one paired set. The sets in the Figs. 5 and 6 embodiment each comprise two orifices and, thus, in the event of flooding of an orifice, self-correction takes place as the resul-t of the formation of a double-t. Those areas oE Fig. 5 embraced within the phantom lines depict segments of the orifice plate corresponding to the segments shown in the composite plate illustrated in Fig. 4. The plate comprises a plurality of such segments and the respective segments are spaced ~rom one another by a distance greater than the dimension "e".
In the preferred arrangement, the reinforcing ribs (36) for the plate are disposed so as to be between the segments.
The Fig. 7 embodiment also employs an orifice pattern wherein two orifices are provided in each paired set. The sets are arranged in rows spaced by the dimension "c" and the sets within the rows are spaced by the dimension "b". As illustrated, however, the orifices of the Fig. 7 embodiment are not arranged ;~
in groups spaced by the dimension "e". If grouping is desired, such an arrangement might be provided with the Fig. 7 embodiment by simply arranging the orifices in rectangular groups and spac-in~ -the groups by the dimension "e".
In the embodiment of the invention illustrated in Fig. 9, the paired sets each comprise three orifices and the sets are arranged in rows wherein the sets within the rows are spaced by the dimension "b" and the rows are spaced from one another by dimension "c". The orifices within the sets are spaced by the dimension "a" and, thus, provide for the formation of "triplets".

The portion embraced within the phantom line in Fig. 8

6~S~
.
com?rises one group and -the manner in whic~. a plurali.y of such grou~s ~ould be arranged in a composite orifice plate is shown in Fig. 9. As there shown, six groups are included in the area which would be disposed between each pair of rein~orcing ribs (36) oS the orifice plate.
The foregoing examples assume a wet~bili ybet~Jeen the plate and molten glass wherein the equilibrium contact angle is betreen 30 and 40 degrees. This is the included angle between the undersurface of the orifice plate and a tan~ent to the liquid drop of glass which forms on the orifice plate when an orifice rloods. Complete wetting occurs when the contact angle is zero.
~o wetting occurs when the contact angle is higher than 90 degrees.
Fig. 10 is an equilibrium contact angle curve for type "E"
glass on an orifice plate of 90 percent platinum, 10 percent rhodium, alloy. The two cross-sections shown in the ~Sigure depict wetting angles of 30 and 60 degrees, respectively. The curve shows that maximum wetability occurs between about 1,050 degrees and 1,150 degrees centigrade. Temperatures in this range and of up to around 1,300 degrees centigrade are typical of those used in ~0 glass drawing processes.
Although the foregoing description and the examples therein have been concerned with glass, it should be understood that the invention is not necessarily limited to use with glass. The process and apparatus disclosed herein can also be used in the ;
m~nuEacture oE ceramic fibers which have processing properties similar to glass. These may include fibers containing various m~tal oxides, Eor example alumina borosilicate, alumina silica, irconia-silica, and the like. The bushing and the ori~ice plate, of course, should be made of an alloy or other material capable ~30 of withstanding the elevated temperatures of the various types of .~ ceramic material which can be formed into fiber.
` The invention is not intended to be limited to the specifics ~ .

.

.. . . .

~ ~986(~5~

of the afore-described embodiments, but rather is deEined by the following claims.

-15- ~:

Claims (12)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An orifice plate for the drawing of glass fibers, said plate having orifices extending therethrough for the draw-ing of fibers and a flat undersurface through which said orifices open and wherein the orifices are arranged in paired sets with the orifices within each set being of substantially the same diameter and spaced from one another by a center-to-center distance equal to from 1.2 to 1.45 times said diameter, and the respective sets are spaced from one another by a dis-tance greater than the distance between the orifices within the sets.

2. An orifice plate according to claim 1, wherein the sets are arranged in rows and the respective sets within the rows are spaced from one another, as measured between adjacent orifices therein, by a distance equal to 1.40 to 1.55 times the diameter of the orifices therein.

3. An orifice plate according to claim 2, wherein adjacent rows are spaced from one another by a distance greater than the distance between sets of orifices within the rows.

4. An orifice plate according to claim 3, wherein the orifices in adjacent rows are of substantially the same diameter and adjacent rows are spaced from one another, as measured between adjacent orifices therein, by a center-to-center distance equal to from 1.55 to 1.70 times said diameter.

5. An orifice plate according to claim 1, wherein the orifices are arranged in groups with a plurality of sets of orifices within each group and the respective groups are spaced from one another by a distance greater than the distance between the orifices within the groups.

6. An orifice plate according to claim 5, wherein the orifices are all of substantially the same diameter and the groups are spaced by a distance, as measured between adjacent outermost orifices in the respective groups, at least equal to 1.65 times said diameter.

7. An orifice plate for the drawing of glass fibers, said plate having orifices extending therethrough for the draw-ing of glass fibers and a flat undersurface through which said orifices open and wherein said orifices are arranged in triangu-lar sets of three with the orifices within each set being of substantially the same diameter and spaced equally from one another by center-to-center distance equal to from 1.2 to 1.45 times said diameter and the respective sets are spaced from one another by a distance greater than the distance between the orifices within the sets.

8. An orifice plate according to claim 7, wherein the sets are arranged in rows and the respective sets within the rows are spaced from one another, as measured between ad-jacent orifices therein, by a distance equal to 1.40 to 1.55 times the diameter of the orifices therein.

9. An orifice plate according to claim 8, wherein adjacent rows are spaced from one another by a distance greater than the distance between sets of orifices within the rows.

10. An orifice plate according to claim 9, wherein the orifices in adjacent rows are of substantially the same diameter and adjacent rows are spaced from one another, as measured between adjacent orifices therein, by a center-to-center distance equal to from 1.55 to 1.70 times said diameter.

11. An orifice plate according to claim 7, wherein the orifices are arranged in groups with a plurality of sets of orifices within each group and the respective groups are spaced from one another by a distance greater than the distance between the orifices within the groups.

12. An orifice plate according to claim 11, wherein the orifices are all of substantially the same diameter and the groups are spaced by a distance, as measured between adjacent outermost orifices in the respective groups, at least equal to 1.65 times said diameter.