The term"ultrafiltration"as used in the present application is intended to encompass microfiltration, nanofiltration, ultrafiltration, reverse osmosis and gas separation. A typical ultrafiltration device comprises a plurality of spiral wound filtration modules through which a fluid to be filtered passes.

Such a spiral wound filtration module consists of leaves in which a layer of a permeate carrier material, usually a porous felt or fabric material sold under the tradename TRICOT, is sandwiched between two membrane sheets. The membrane sheets comprise a membrane material integrally joined to a backing material. The membrane sheets are oriented relative to the permeate carrier material so that the membrane material is facing outwardly. The membrane sheets are typically folded once along their length to present a leaf with two halves integrally joined. The outside edges of the leaves are then sealed on all but one side, allowing access to the permeate carrier only from a radial direction through the membrane. The membrane module is made by winding one or more membrane leaves around a permeate tube which has holes therein to collect the filtered product, or permeate. The membrane leaves are placed with the unsealed edge of each leaf adjacent to the permeate tube and oriented along the length of the tube, allowing the permeate to flow into the permeate tube.

Each membrane leaf is separated from adjacent leaves by feed spacer screens, which are of a relatively large mesh size to accommodate feed fluid flow. The membrane leaves and feed spacer screens are spirally wound around the permeate tube.

After the leaves are wound, some type of external restraining means such as a hard shell, straps or a bypass screen, or a combination thereof may be used to hold the spirally wound leaves in tight formation around the tube. The spiral module is then loaded into a pipe-like housing or pressure vessel which is operated at a slight pressure drop across the module as the fluid being filtered flows through.

In use, the product to be filtered is introduced under pressure at one end face of the module and is allowed to travel axially along the module through the feed spacer screens. Because the outside edges of the membrane leaves are sealed, the feed

fluid is prevented from entering into the permeate carrier material without first passing through a membrane sheet. As the feed fluid flows axially through the module along the feed spacer screen, the permeate is allowed to pass through the membrane sheets and will be directed to the permeate carrier tube by the permeate carrier sheet. Concentrate is removed from one end of the module and permeate is removed through the permeate tube. This type of filtering, with a spiral wound module, is advantageous for a number of different applications. However, the manufacturing of these modules presents certain difficulties.

When winding a spiral wound filtration module, the layers of the leaves and feed spacer screens must be able to slide relative to one another, because the outside layers will be required to travel a greater distance than the inside layers due to the increasing circumferential distance. Therefore, it is now the practice of most manufacturers to use a wet adhesive, usually a two part epoxy or urethane, to seal the outer edges of the membrane leaves against one another, with the permeate carrier material sandwiched therebetween. This adhesive is either applied to the permeate carrier material or to the back sides of the membrane sheets, or both, as they are positioned one on top of the other. The spiral is then wound while the adhesive is wet, and the adhesive is allowed to cure after the spiral is wound. In this prior art method, an additional bead of adhesive is applied across the bottom of each leaf axially along the module, from one side seam to the other to seal the outer axial seam of each leaf. This axial bead can be applied either before or after winding the module.

The location of the axial glue bead, among other things, determines the overall outside diameter for the spiral wound module. Because the leaves are wound about the permeate tube, as the axial bead is placed further down the leaf, the diameter of the resulting spiral wound module increases. The precise location of the axial glue bead is therefore important because these spiral wound modules are designed to fit inside a housing with a specific inside diameter. It is desirable for the wound module to fit snugly within the housing to prevent excess wasted flow through the annulus between the outside of the module and the inside wall of the housing. It is also desirable for the axial glue bead to be placed as far down the leaves as possible since the membrane area circumferentially beyond the glue seam is inactive this results in the maximum possible active membrane area per module. In manufacturing these modules, however, care must

be taken to avoid placing the axial glue bead too far down the leaves, so as to prevent the finished spiral from having an outside diameter that results in an interference with the inside diameter of the housing. Placing the glue bead too far down the leaves can also require that the glue bead be cut off to reduce the module diameter to fit within the housing. If this must be done, the result is an open passage between the feed spacer sheet and the permeate carrier sheet, which is unacceptable.

As stated above, application of the axial glue bead in prior art methods can be done both before and after winding. When the axial glue bead is placed before winding, the bead can be placed as each leaf is placed onto the stack of material to be wound about the permeate tube. Placing the axial bead after winding requires that each leaf be located and pealed apart to apply the bead. Typically, in the method of applying the bead before winding, the axial glue bead is placed at the same time as the side seam glue beads are placed. It has been found that application of the glue bead before winding in this manner is less labor intensive than applying the axial bead after winding.

Placing the axial bead prior to winding using a wet adhesive does, however, present a number of disadvantages. First, it is difficult to properly position the glue bead that will result in the needed module diameter. The variation in sheet material thickness of the components can lead to a wound spiral that is larger or smaller than is desired. This diameter is fixed, because the axial seam location has been set by the application of the glue bead prior to wind up. Second, placing the axial bead prior to winding increases the risk that the materials of the leaves will wrinkle. The increased risk of wrinkling is caused by the restricted relative movement of the membrane sheets during wind up caused by the adhesive. In other words, the wet adhesive placed along the axial seam area restricts the ability of the membrane sheets to slide relative to one another. This sliding occurs during wind up because the outer material is required to travel further due to the increase in circumferential distance. Another cause of the wrinkling may be air trapped in the end of the leaf. If the axial glue bead is already in place, this trapped air is not allowed to easily escape and can cause wrinkling.

Application of the axial glue bead after winding addresses some of the above problems. First, a more precise seam location can be achieved because the spiral can be pre-wound to the desired outer diameter, and can thus take into account the effect of sheet material thickness variation. Further, application of the axial glue bead after

winding reduces the risk of wrinkling and air entrapment. As stated above, however, application of the glue bead after winding is labor intensive because it requires that after the stack of leaves has been wound, each leaf must be located and peeled apart to allow application of the adhesive. Moreover, even when the bead is placed at the end of wind up, the seam area for all leaves must be accessible. This requires that up to two feet of material must still be rewound after application of the wet adhesive beads. As with the application before winding, the wet adhesive tends to prevent the smooth sliding of the sheets, which can again result in wrinkles. It is also common for air to be trapped in the leaf upstream from the glue bead. When the spiral is tightened, this air tends to be forced through the glue. This can result in trapped air bubbles and air channels within the glue bead. The channels may result in unacceptable leaks. The trapped air bubbles are undesirable in a sanitary application because fluid can become trapped in them.

Another disadvantage of the prior art methods occurs when the resulting spiral wound modules are used in a sanitary environment. One requirement of the axial seam in modules used in sanitary applications is that there be no exposed open pockets at the outer edge of the axial seam, and that all three material layers of the leaf end at the outer edge. When the axial seam is achieved using a bead of wet adhesive, the outer edge is likely to have exposed open pockets. The prior art method therefore normally requires a secondary trim step, in which the trim is executed through the glued area of the axial seam. By trimming in this manner, the risk of an open pocket along the outer edge is reduced.

Another disadvantage of the wet axial glue bead method is that extra membrane material must be used beyond the optimum because the glue tends to flow and ooze during final wind up. If the glue flows past the end of the membrane sheets, it can flow into the feed channel and stick to the neighboring active membrane surfaces. When flow is introduced to the spiral, any such glue can pull and tear the active membrane surface, resulting in a fatal leak. To prevent the glue from flowing beyond the end of the membrane sheets, a longer membrane sheet is used and the glue bead is applied spaced from the outermost edge. In a sanitary module, the extra membrane used to catch the excess of flowing glue must be cut off to ensure that there are no exposed open pockets at the outer edge of the axial seam. Thus, the extra membrane is not used in the final product, adding expense to the manufacture of the module. In non-sanitary modules, the

excess does not need to be cut off. However, the extra membrane is not active and therefore wastes potential filtering area by taking up circumferential space that could otherwise be used by active membrane.

Yet another disadvantage of the wet glue axial bead method relates to the tendency of the membrane above the axial glue seams to form undesirable blisters. When glue penetration is inadequate to seal all the way through the backing layer up to the outer separating surface of the membrane layer from the back, the spongy unsealed membrane under the outer separating surface of the membrane, but on top of the glued membrane backing, can sometimes blister. This is particularly undesirable in a sanitary module since fluid can become trapped in the blister and since membrane polymer from the blistered region can sometimes flake off and get into the feed or concentrate flow. The blister is caused by low molecular weight fluids passing through the separating membrane surface into the spongy layer underneath. When fresh water is later introduced for cleaning, the osmotic pressure difference between the trapped fluid and the water can cause a surge of water inflow to the spongy region, which can lift and blister the membrane surface area.

The membrane above the axial seam tends to be predisposed to blistering because the adhesives used tend to be highly viscous so as to resist running during and after application. The highly viscous adhesives are thus less likely to completely penetrate into the membrane layer up to the outer separating surface of the layer. As stated above, this lack of penetration can cause blistering. It is also difficult to get high and uniform compression of the axial seam after application of the adhesive, because the leaf end is sandwiched between layers of feed spacer and because excessive compressive force used after winding can damage the active membrane in the spiral. Moreover, because the adhesive typically applied to form the side seams acts as a spacer, most of the compressive force of the wind up is focused over the side seam area, which makes it difficult to adequately compress the axial seam all the way across. All of these factors tend to result in a reduction in the degree and consistency of the penetration of the axial seam adhesive through the membrane backing and substructure up to the outer separating layer of the membrane, thus increasing the likelihood of blistering.

Therefore, a device is needed which overcomes the drawbacks and deficiencies of the existing constructions and methods discussed above.

BRIEF SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an effective method of forming the axial seam of a spiral wound membrane module.

Another object of the present invention is to provide a method of forming the axial seam of a spiral wound membrane module that will resist blistering.

Yet another object of the present invention is to provide a method of forming the axial seam of a spiral wound membrane module that will reduce the risk of wrinkling during winding.

A further object of the present invention is to reduce the amount of membrane that must be removed in the trim step.

A still further object of the invention is to provide a method of forming a spiral wound membrane module that will reduce the risk of trapped air bubbles and air channels in the axial seam.

Yet another object of the invention is to provide a method of forming a spiral wound membrane module that allows a more precise location of the axial seam.

According to the present invention, a method for preparing a spiral wound filtration module utilizing a central permeate carrier tube includes winding at least one filtration leaf around the permeate carrier tube. The filtration leaf includes a first membrane sheet, a permeate carrier sheet, and a second membrane sheet with the backing side of each membrane sheet facing inward toward the permeate carrier sheet and the membrane separating surfaces facing outward. Each leaf is typically accompanied by a feed spacer sheet which is positioned adjacent to one of the outward facing surfaces of the membrane sheet such that the feed spacer sheet keeps neighboring membrane surfaces from contacting one another during and after winding. In the winding step each leaf is oriented to present an edge generally adjacent the tube, a pair of side edges and an axial edge distal from the tube and oriented to be in parallel with the axis of the tube. A strip of thermoplastic sealant is positioned between the membrane sheets in a location near the axial edge of the leaf that corresponds to a desired location for an axial seam of the leaf.

Heat is applied to the thermoplastic sealant strip to fuse the thermoplastic, the membrane sheets and the permeate carrier together to define a fused axial area of the membrane leaf.

The afore-described method achieves the objects heretofore set forth and others which are inherent from the description and claims which follow.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS In the accompanying drawings which form a part of the specification: Fig. 1 is a partially exploded, perspective view of a filtration module embodying the principles of the present invention; Fig. 2 is a side perspective view of the module of Fig. 1 in a partially assembled state and schematically showing a fusing bar and a vacuum tube; and Fig. 3 is a schematic drawing illustrating the method of making a spiral wound module according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION Referring initially to Fig. 1, a spiral wound filtration module manufactured according to the method of the present invention is designated generally by the numeral 10. The module 10 has at its center a permeate carrier tube 12 which has a plurality of openings 14 spaced about its perimeter. Openings 14 allow liquid communication between the exterior of tube 12 and the interior. Permeate tube 12 is constructed of a suitable rigid material, such as high strength inert plastic. Examples of suitable materials include polysulfone, polyvinylchloride and polyphenylene oxide. Other suitable materials which are rigid and which are compatible with the materials to be filtered are acceptable for use.

Surrounding permeate tube 12 and in liquid communication therewith is a permeate carrier material 16. Permeate carrier 16 acts to transport the filtered permeate in a direction perpendicular to the axial length of the tube, as indicated by the arrow B in Fig. 1. Suitable materials for permeate carrier 16 are well known in the art and include porous felts or fabric materials. For example, a material sold under the trade name TRICOT is a suitable material for permeate carrier 16.

Located on either side of permeate carrier 16 is a membrane sheet 18 which has a membrane surface 20 and a backing material 22. Membrane sheets 18 are oriented so that membrane surface 20 faces outwardly with respect to the permeate carrier 16. In other words, backing 22 faces permeate carrier 16. Membrane surface 20 and backing material 22 are integrally joined by techniques well known in the art to form membrane sheet 18. Acceptable materials for membrane 20 include a wide-range of

thermoplastic resins which can be fabricated into a sheet having a known pore structure and filtration capability. A preferred material is polyethersulfone.

Materials acceptable for use as backing material 22 include woven or nonwoven synthetic materials having the strength necessary to reinforce membrane 20 and the ability to be integrally bound to the membrane while not interfering with the passage of permeate through the membrane. Suitable backing materials include polyester, polypropylene, polyethylene, and the family of polyamide polymers generally referred to as"nylon".

Disposed adjacent surface 20 of membrane sheet 18 is a feed spacer screen 24 which has a relatively large mesh size to allow the fluid to be filtered to travel axially along membrane module 10. In most instances, feed spacer 24 will be utilized, but it is possible and known in the art to construct a module without this component. In general, feed spacer 24 is formed of any inert material which maintains a space between the facing membrane surfaces 20. Further, the feed spacer screen 24 must allow the fluid to be filtered to travel axially along the membrane module. Preferred materials are adequately open, channel forming grid materials, such as polymeric grid, or corrugated or mesh materials. Preferred among these are polypropylene or other polyolefin netting materials, such as those commercially available from Nalle Plastics under the tradename VEXAR.

As known in the art, the edges of adjacent membrane sheets 18 which lie along the axial length of permeate tube 12 are sealed so that fluid flowing through feed spacer screen 24 is prevented from access to permeate tube 12. Alternatively, membrane sheet 18 may be folded with the fold being adjacent to the permeate tube 12 and with feed spacer screen 24 located within the fold such that membrane surfaces 20 face one another. In this construction, access to permeate tube 12 is allowed only through the permeate carrier material 16.

Permeate carrier 16, membrane sheets 18, and feed spacer screens 24 are thus spirally wound around permeate carrier tube 12 with permeate carrier 16 located adjacent tube 12 and in liquid communication therewith. Referring to the series of layers of membrane sheet 18, permeate carrier 16 and a second membrane sheet 18 as a filtration leaf 26, typically a plurality of filtration leaves 26 are spirally wound about permeate tube 12 with a feed spacer screen 24 located between each leaf. As is known in the art, the module may optionally be formed without feed spacer screen 24.

After membrane module 10 has been wound, the assembly is held in a wound state through the use of restraining bands or outer wraps, or a combination thereof, as is known to those of skill in the art. The modules can then be loaded into a housing or pressure vessel which is operated at a slight pressure drop across the module as the fluid being filtered flows through. In operation, the fluid to be filtered is introduced at one end face of the membrane module 10, as indicated by the arrow A in Fig. 1. The fluid travels axially along membrane module 10 through feed spacer screen 24. As the feed fluid encounters surface 20 of sheet 18, permeate passes through membrane 18 in a direction perpendicular to the axis of tube 12. After the permeate passes through the membrane, it will travel along permeate carrier 16 in the direction of arrow B, eventually passing into permeate tube 12 through openings 14. The permeate exits the membrane module through tube 12 and the filtrate travels axially through the module along feed spacer screen 24.

As will be appreciated, it is necessary to seal all of the edges of membrane sheets 18 to the corresponding edges of permeate carrier 16, with the exception of the edge adjacent permeate tube 12, in order to prevent the feed fluid from entering the permeate carrier 16 without first passing through membrane sheet 18. This is necessary to prevent the feed fluid from entering permeate carrier 16 without first being filtered as desired. Prior art filtration modules have utilized a wet adhesive, which is typically a one-part or two-part epoxy or urethane to achieve this sealing. In the prior art, the wet adhesive is placed on the permeate carrier 16 and/or the backing material 22 along both the side edge 28 and the axial edge 30 prior to winding. The module is then wound while the adhesive is wet and held in place while the adhesive is allowed to cure. As discussed above, this prior art method presents a number of difficulties.

In the method of making the module according to the present invention, the side edges 28 are sealed as in prior art methods, by applying a wet glue bead 32 to permeate carrier 16, to backing 22 or to both the backing and the permeate carrier.

Alternatively, the side edge 28 can be sealed after winding, using a centrifugal potting method as described in a co-pending patent application filed January-, 1999 and titled Method for Sealing Spiral Wound Filtration Modules. If the centrifugal potting method is used, the axial seam will first be sealed as disclosed below, followed by sealing the side seams. The side seams may also be formed by any other suitable method that

achieves the same results. Using the prior art method, glue bead 32 is applied adjacent to the side edges 28 of permeate carrier 16 prior to winding. An additional bead of adhesive 34 may be placed along the axial edge 30, as best seen in Fig. 1. Bead 34 is optional and is used as needed to hold the leaves in proper registration with one another.

When axial bead 34 is used, only a small strip is needed as compared to the wet axial bead in prior art methods. For example, a strip of approximately one inch has been found to be sufficient.

A strip of thermoplastic adhesive 36 is placed between membrane sheets 18, and preferably between one membrane sheet 18 and permeate carrier 24. Strip 36 is placed in a position desired for the axial seam of the spiral wound module. This position is determined by the desired outside diameter of the module as well as the thickness of the leaves and feed spacers. Suitable thermoplastics for strip 36 include those that will fuse membrane sheets 18 and permeate carrier 24 together upon the application of heat or heat and pressure, such as polypropylene and polyethylene. A preferred material is an ionomer resin sold under the trademark SURLYN by E. I. DuPont de Nemours and Company of Wilmington, Delaware, U. S. A. The glass transition temperature of the preferred material is approximately-40° F, and the melting point of the material is 185° F. After strip 36 is in place, a second membrane sheet 18 is placed on top of the glued permeate carrier 24, forming a membrane leaf 26. Alternatively, a sealant may be used in place of strip 36 that is presented in the form of a pre-applied coating to permeate carrier 24, or to one or both membrane sheets 18. Further, more than one strip 36 may be used. For example, a strip 36 could be placed between the bottom membrane sheet 18 and permeate carrier 24 and another strip 36 could be placed between the top membrane sheet 18 and permeate carrier 24. Also, permeate carrier 24 or the backing 22 may be serve as the necessary sealant, provided the materials are appropriately selected so as to enable the necessary fusing by the application of heat to the axial seam, such as if the sealant were incorporated into carrier 24 or backing 22.

After all the membrane leaves have been assembled in this manner, the <BR> <BR> <BR> <BR> <BR> leaves are wound about permeate tube 12. In an alternative embodiment, strip 36 is placed between membrane sheets 18 after winding. In other words, strip 36 may be positioned between the membrane sheets 18 either before or after winding. The spiral is then held in a wound state while the side glue seams, if present, are allowed to solidify.

If desired, solidification of the side seam spiral adhesive may be accelerated by holding the spiral in an oven. An oven having a temperature of 50°C has been found sufficient to cure one of many suitable adhesives overnight. After the glue has cured, the circumferential length of all membrane leaves 26 is fixed and maintained by the adhesive.

In cases in which fusing is to be performed and side glue seams are not present, considerable care must be taken to avoid excessive relative motion among the material layers during handling. As shown in Fig. 2, each leaf 26 is then unwound to expose about the outer one foot of the leaf. The leaf is positioned under a fusing bar, shown schematically in Fig. 2 as 38, with the thermoplastic strip 36 directly under bar 38.

Prior to engaging bar 38, one end of permeate tube 12 is typically coupled to a vacuum source, as indicated by numeral 40 in Fig. 2. The other end of tube 12 is either connected to a vacuum gauge or is capped. The vacuum operates to remove steam via permeate carrier 16 that is generated during the fusing process. It has been observed that during the fusing process the membrane adjacent the fused area can become irregular and deformed. The cause of this deformation of the membrane sheet 18 is believed to be caused by the pressure and heat of the escaping vapors and gases from the materials and fluids (water and solvents) in the fusing area. It has been found that an applied vacuum reduces the localized pressure build up and reduces or eliminates the resulting deformation of the membrane. This step may not be required for all membrane and module types.

While a vacuum of approximately one to two pounds per square inch is applied, fusing bar 38 is placed in contact with membrane leaf 26 to fuse the axial seam of the leaf. It has been found that a temperature of 350°F and a pressure of about one hundred pounds per square inch, applied for approximately four minutes is sufficient to form an adequate seal. A thermal impulse sealer utilizing a nichrome wire shielded inside of a TEFLON (D (registered trademark of E. I. DuPont de Nemours and Company of Wilmington, Delaware) covered flat bar and operated by a skilled technician is an acceptable and efficient means for accomplishing this fusing. Preferably, fusing bar 38 will have a radius of curvature on each of its jaws, as shown in Fig. 2, that generally matches the radius of curvature of the module in the location of the axial seam.

It is, of course, understood that other methods of fusing the thermoplastic sealant strip could be used, such as ultrasonic welding and radiation, as well as other

known techniques combining heat or heat and pressure to the thermoplastic strip. While the fusing bar 38 is shown contacting both the top and bottom of leaf 26, it is to be understood that a fusing bar could be used that contacts only the top or only the bottom of the membrane leaf. It is also contemplated that heat without pressure, or only a nominal pressure may be used to fuse the axial seam.

In the fusing process, the membrane surface pores are sufficiently collapsed so as to render the membrane surface impermeable. This eliminates structural voids and reduces sanitation problems caused by such voids. A reliable indication of when the desired level of fusing has taken place is when the membrane appearance changes from milky white to highly translucent. After fusing, the portion of leaf 26 beyond the fused area is removed in a trimming operation. This trimming operation presents a smooth completely sealed axial edge for the membrane. After trimming, the sealed membrane leaf is rewound, and the above procedure is repeated for each membrane leaf.

The invention thus encompasses a method of preparing a spiral wound filtration module which comprises assembling the desired number of membrane leaves, as indicated at station 42 in Fig. 3. In this assembly step, a first membrane sheet is placed under a permeate carrier sheet, and a wet adhesive is applied to the side edges of the permeate carrier sheet at step 44. An additional, smaller bead of adhesive may be placed adjacent the axial edge of the permeate carrier sheet, although this is not required. Other methods of sealing the side edges are to be understood as within the scope of this invention, such as sealing the side edges via potting methods. A second membrane sheet is placed in overlaying relationship to the permeate carrier, the first and second membrane sheets and the permeate carrier forming a membrane leaf. A thermoplastic sealant strip is placed between the two membrane sheets, and more specifically between one of the membrane sheets and the permeate carrier, as indicated at step 46. The sealant strip is positioned to be in the area that is desired for the axial seam. Multiple leaves are similarly assembled and are typically separated by a feed spacer. After all of the leaves have been assembled, they are wound about a permeate tube and the adhesive is allowed to cure, at step 48. After curing, a portion of each membrane leaf is unwound at step 50 to allow access to the portion of the leaf containing the thermoplastic sealant. The leaf is then sealed along the area desired for the axial seam by applying heat, or heat and

pressure, to the area containing the thermoplastic sealant, as indicated at step 52. After the axial seam has been formed, the portion of the leaf extending circumferentially beyond the seam is trimmed at station 54. The portion of the leaf is then rewound, as indicated at 56, and the remaining leaves are axially sealed in a similar manner.

The above method provides an effective method of forming the axial seam of a spiral wound membrane module that resists blistering. The blistering is reduced because the problem of inadequate adhesive penetration along the axial seam is eliminated. Further, because the use of a wet adhesive along the axial edge is either reduced or eliminated, the risk of wrinkling during wind up is also reduced. The use of the thermoplastic sealant strip allows a more precise positioning of the axial seam and reduces the amount of membrane that must be removed in the trim step. Finally the above method reduces the risk of trapped air bubbles and air channels existing in the axial seam.

From the foregoing, it will be seen that this invention is one well adapted to attain all of the ends and objects hereinabove set forth, together with other advantages which are inherent to the structure. It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

Since many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.