SYSTEM FOR DELIVERING COOLANT AIR TO A GLASS FIBER ATTENUATION ZONE

Abstract: A method and apparatus (10) for forming continuous glass fibers. The method includes the steps of supplying a plurality of streams of molten glass (16) from a bushing (14), drawing the streams (16) into continuous glass filaments (20), providing a stream of coolant air (12) parallel to the direction of draw of the streams of continuous glass filaments (20) at the front and back of the bushing (14) to entrain the coolant air (12) wherein the entrainment of the coolant air (12) is determined by the speed at which the glass filaments (20) are drawn; and then collecting the continuous filaments (20). The apparatus for delivering non-intrusive coolant air (12) to an attenuation zone of a glass drawing process of a bushing (14) including a bushing tip plate (14) having a plurality of bushing tips (18) includes at least two plenum chambers (26) having inlets (28) into which coolant air (12) is fed under pressure at a selected flow rate to discharge outlets (30), the discharge outlets (30) extend a longitudinal length of the bushing tip plate (14) to provide coolant air (12) to a front and back of the tip plate (14); wherein the entrainment of the coolant air (12) is a function of the speed at which the glass filaments (20) are drawn.

Description: SYSTEM FOR DELIVERING COOLANT AIR TO A GLASS FIBER ATTENUATION ZONE TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION This invention relates to a system for delivering coolant air to a glass fiber attenuation zone of a glass fiber mechanical drawing process. More particularly, this invention relates to the method and apparatus for delivering non-intrusive coolant air to the attenuation zone of a glass drawing process where the flow of the coolant air is determined by the speed at which the glass filaments are drawn. A localized vacuum is created by the fiber motion induces airflow at nominal velocities and allows the glass fibers to entrain coolant air as required without variation in fiber diameter. The apparatus provides the required coolant air supply to at least the front and rear of the bushing used to draw the glass, and preferably around the entire periphery of the tip plate. BACKGROUND OF THE INVENTION The present invention relates to glass filament forming and more particularly to the apparatus and method for providing a uniform thermal environment at the attenuation zone below a plurality of orificed filament forming tips on a heated glass fiber forming bushing. It is well known in the art to produce filaments from glass by flowing a stream of molten material from a plurality of orificed tips provided on the bottom of a heated bushing. The streams are attenuated, usually by mechanical means, into filaments. The filaments are gathered into strands and may, subsequently, be processed into a variety of commercial products. More particularly, in the production of glass fiber strands, molten glass flows from a suitable source into the heated bushing assembly. This bushing is generally an elongated channel having side and end walls and a generally planar bottom which carries a large number of nozzles or tips through which the molten glass passes. In the zone immediately below these tips, the molten glass is formed into filaments. This zone is the attenuation zone, in which the glass fibers are cooled, may have a sizing applied to them, and are gathered into a strand. Finally, the strands are wound on a spool into a glass package. The environment in the zone directly below these tips is crucial in the formation of the filaments because it is in this area that then molten glass cools. As the strand filaments cool, their mechanical properties and physical dimensions are established. For a more detailed description of a method and/or apparatus for making filaments reference is made to U.S. Patents Nos.4,118,210; 3,040,377; 2,300,736; 4,018,586; 4,636,234; 4,622,054; 4,325,722; and 4,866,536, incorporated herein by reference. There are four principal factors in the forming region which effect fiber formation. These are air, fibers, fins and the tip plate. The tip plate temperature distribution governs the glass throughput in each of the tips. Glass flows by gravity through the tips and is attenuated to the final diameters with the winder sustaining the tension. As the glass jets attenuate from their initial diameter to their final diameter, they lose heat by radiation to the fins and convection to the air around it. Also, air drag pulls air from the surroundings into the fiber fan. The air penetrates the fibers starting at the edge of the tip plate and works its way to the middle. During this process, there is heat exchange between the air and the fins, air and fibers as well as air and tip plate. The air gets progressively hotter towards the middle of the tip plate. Further, the velocity component parallel to the tip plate gets smaller (it is highest at the edge of the tip plate) as the entrained air is pulled downward by the fibers and is eventually "squeezed out" of the fiber fan. It is the attenuating history (local fiber diameter, velocity and temperature) of each of the fibers that dictates the air entrainment. The entrained air cools the tip plate. As a result of the changing air temperatures and velocities, the heat transfer coefficient below the tip plate can be a function of local position. This implies that the air can contribute to tip plate temperature gradients, which in turn means variations in glass flow from tip to tip. In addition, air influences fiber attenuation history (fiber diameter, velocity and temperature as functions of the attenuation direction). Since the entrained air is cooler at the tip plate periphery and hotter in the middle of the tip plate with changing velocities, air may be cooling part of the fins while heating the remainder of the fin. As has been observed, fiber attenuation contributes to the fin heat load, affects the air temperature and entrainment. Fiber attenuation is strongly influenced by glass physical properties. These physical properties include viscosity, surface tension, density, specific heat, emissivity (hemispherical total), and thermal conductivity. It will be appreciated that since glass is an absorbing-emitting medium, the hemispherical total emissivity (which determines the radiative heat loss from the fiber) and thermal conductivity (which determines conduction in the fiber) are dependent on absorption coefficient versus wave length data. These data are temperature dependent. Furthermore, both hemispherical total emissivity and thermal conductivity are governed by the optical thickness (absorption coefficient times distance) and temperature. Fins exchange radiative heat with the tip plate and attenuating fibers. They also participate in convection and conduction heat exchange with air. The fin location/orientation below the tip plate can be very important since radiation exchange view factors as well as air flows can be impacted. The heat transfer coefficient in the fin manifold influences fin heat removal and impacts the forming process. The tip plate, which determines the tip exit glass conditions, exchanges radiative heat with fins and convective and conductive heat transfer with air. It will be appreciated that air flow and temperature fields can lead to varying (from position to position) tip plate temperatures. It will be appreciated that due to the entrained air flow and temperature fields, each fiber experiences a different thermal environment and its attenuating history is therefore different. This means that the initial cone angle for each fiber can be different which, in turn translates to different glass throughputs since the throughput, among other quantities, also depends on the initial cone angle. Temperature fluctuations in this zone will result in diameter variations in the strands. Furthermore, if the environment in the zone immediately under the bushing tip is overcooled, the filaments formed by the bushing will have larger diameters and may not withstand the gathering and winding forces applied to them causing breakage of the filament. Conversely, filaments which are undercooled may break due to instability. Additionally, stray air currents can carry unwanted materials into the attenuation zone thereby breaking the filaments and decreasing production efficiency. Production efficiency is measured by the break rate or short term yardage. The production efficiency may also be measured in terms of reducing the required inputs, i.e. material, energy, time, and equipment to achieve the same break rate. It is also well known that forcing air into the attenuation zone, perpendicular to the fiber flow, may spread the fibers into random streams as compared to orderly filaments. The random streams are then collected on a rotating drum for use as a staple textile fiber. In a properly controlled environment, without a forced air stream, the ordered filaments may be combined into a high quality strand that may be wound onto a spool as a glass package. One typical use for such strand is in the formation of glass fabrics. In order that a satisfactory woven fabric be produced, it is imperative that the diameters of each glass strand be consistent. Variations in the diameters of glass strands along the length thereof results in a fabric that will not lie fiat but rather becomes "puckered." Such a fabric is unacceptable. Problems associated with prior art cooling apparatus include, for example, low production rate due to a high breakage, inequitable distribution of molten material to each tip, poor quality fabric due to glass strand diameter variation, inefficiency due to high process costs, and inefficiency due to high capital equipment investment. Compared to the heretofore known practice of blowing coolant air at the fibers, it will be appreciated that an air curtain in accordance with the present invention is non-intrusive and has the potential of aerodynamically isolating one position from the other. In view of the foregoing, it is an object of the present invention to provide an apparatus to produce high quality glass fiber. It is another object of the present invention to provide an apparatus to produce a glass fiber with reduced breakage rate. Another object is to provide an apparatus for producing glass fiber of simplified design and generally lower cost than prior art apparatus of the same type. Yet another object of the present invention is to provide an apparatus for producing glass fiber that efficiently utilizes coolant air. Still another object of the present invention is to provide a method for producing glass fiber using the apparatus as disclosed. SUMMARY OF THE INVENTION Briefly, there is provided a method and apparatus for forming continuous glass fibers. The method includes the steps of supplying a plurality of streams of molten glass from a bushing, drawing the streams into continuous glass filaments, providing a stream of coolant air parallel to the direction of draw of the streams of continuous glass filaments in at least the front and back of the bushing to entrain the coolant air wherein the entrainment of the coolant air is determined by the speed at which the glass filaments are drawn; and then collecting the continuous filaments. The apparatus for delivering non-intrusive coolant air to an attenuation zone of a glass drawing process of a bushing including a bushing tip plate having a plurality of bushing tips includes at least two plenum chambers having inlets into which coolant air is fed under pressure at a selected flow rate to discharge outlets. The discharge outlets extend a longitudinal length of the bushing tip plate to provide coolant air to a front and back of the tip plate. The entrainment of the coolant air is a function of the speed at which the glass filaments are drawn. BRIEF DESCRIPTION OF THE DRAWINGS Further features and other objects and advantages will become clear from the following detailed description made with reference to the drawings in which: FIG. 1 is a bottom view of a bushing including a coolant air delivery apparatus in accordance with the present invention; FIG. 2 is a cross sectional view of the bushing of FIG. 1 taken along line 2-2; FIG. 3 is a cross sectional view of the bushing of Fig. 1 taken along line 2-2 utilizing another coolant air delivery apparatus in accordance with the present invention; FIG. 4 is a cross sectional view of the bushing of Fig. 1 taken along line 2-2 utilizing another air delivery apparatus in accordance with the present invention; and FIG. 5 is a cross sectional view of the bushing of Fig. 1 taken along line 2-2 utilizing another air delivery apparatus in accordance with the present invention. DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS OF THE INVENTION Referring to the figures where like reference characters represent like elements, there is shown a system and apparatus 10 for delivering convective coolant air 12 to a glass fiber attenuation zone of a bushing 14. As is well known in the art, molten glass 16 is drawn through bushing tips 18 of the bushing 14. A plurality of bushing tips 18 are positioned in an array on the bushing tip plate. As the molten glass 16 is attenuated through the bushing tips 18, cones of glass are formed. Upon further attenuation these cones are formed into filaments 20 which are later gathered into composite strands. It will be appreciated that although the present invention is shown in the figures in cooperation with a single bottom bushing the invention may also be used with equal facility with a double bottom bushing, i.e. two tip plates separated by a gap, of a type well known in the art. The bushing tips 18 are typically cooled by means of a plurality of cooling fins 22 operatively attached to a liquid cooled manifold 24. The cooling fins 22 are operatively attached to the manifold 24 so that heat may be removed from the area surrounding the bushing tips 18. As shown in Fig. 1, the cooling fins 22 are arranged between rows of bushing tips 18. FIG. 1, also illustrates the connection of the cooling fins 22 to the manifold 24 and the direction of travel of the coolant within the manifold 24. The heat is removed through the cooling fins 22 and ultimately removed by the flowing liquid in the manifold 24. As shown in the figures, the manifold 24 may be a hollow pipe or the like and the cooling fins 22 may be in the form of solid fin members. In an alternate embodiment, a second manifold 24a may be operatively connected to an opposing side of the cooling fins 22 as shown in FIG. 5. Notwithstanding, it will be appreciated that the exact means employed for such cooling is not important to the operation of the instant invention and are well known in the art. Referring to the figures, the system and apparatus 10 includes at least two plenum chambers 26 into which a coolant gas such as air is fed under pressure at a suitable flow rate. In a preferred embodiment, the coolant air is at a temperature no greater than ambient temperature for efficient operation. However, the coolant air may be chilled as desired. The coolant air enters the plenum chambers 26 through inlets 28 and exits the system through discharge outlets 30. The discharge outlets 30 extend the longitudinal length of the tip plate 14 on both sides of the tip plate to provide satisfactory coolant air coverage. The discharge outlets 30 are designed to provide between 150 - 300 cfm (cubic feet/minute) of coolant air for yarn and reinforcement type bushings having a throughput of about 50 - 300 lbs/hr. However, it will be appreciated that the amount of coolant air may be adjusted as desired for the type of bushing design employed. The openings in the discharge outlets 30 comprise less than 1 % of the total surface area of the outlet. Referring to FIGS. 2-5, there are shown alternate embodiments of the apparatus for providing coolant air to the attenuation zone at both at least the front and rear of the tip plate 14, and preferably at least the entire periphery of the tip plate. The plenum chambers 26 may be rectangular shape (FIGS. 4 and 5) or the plenum chambers may be "boot shape" (FIGS. 2 and 3). The plenum chambers 26 may include vanes 32 for directing the coolant air flow perpendicular to the tip plate 14. The outlets 30 may be positioned above a plane formed by the cooling fins 22 (FIG. 4), below the plane formed by the cooling fins (FIGS. 2, 3 and 5) or parallel to the plane formed by the cooling fins (FIG. 5, shown in phantom line). In a preferred embodiment, the discharge outlets 30 of the plenum chamber 26 are positioned about 0-4 inches from the horizontal edge of the tip plate 14 and no more than about 3 inches from the bottom of the tip plate. The coolant air exits the outlets 30 substantially vertically downward into the attenuated zone directly adjacent the tips 18 to form an air curtain on at least the front and rear of the bushing, preferably the entire periphery of the bushing. A localized vacuum is created by the fiber motion and induces coolant airflow at nominal velocities and allows the glass fibers to entrain coolant air as required without variation in fiber diameter. It will be appreciated that a perforated screen (not shown) may be used to reduce turbulence in the coolant gas and also act as a filter to prevent any particulate matter from coming into contact with the glass fiber filaments being formed. It will be further appreciated that the air curtain delivers a majority of the coolant air substantially vertically downward on at least the front and rear of the tip plate 14, preferably the entire periphery of the tip plate, to allow the attenuating fibers to entrain the necessary quantity of coolant air as dictated by the movement of the fibers. A minor component of the coolant air may also be angled at the glass fibers so as to not disturb the attenuating zone. Compared to the heretofore known practice of blowing coolant air at the fibers, it will be appreciated that the air curtain is non-intrusive and has the potential of aerodynamically isolating one position from the other. Furthermore, it has been found that only providing coolant air parallel to the direction of flow of the glass fibers and on at least both the front and rear of the tip plate 14 a reduced amount of coolant air is required over heretofore known systems to achieve the same coolant effect and, in addition, as a further benefit short term yardage is improved. In operation, the bushing is supplied with molten glass which passes through the tips 18. The fin plates 22 are properly positioned below the tip plate 14 and a liquid coolant is passed through the manifold at a desired flow rate to extract heat from the fin plates. The coolant air is introduced into plenum chambers 26 passes through diffuse screens and flows in a non-turbulent manner parallel to the direction of pull of the glass fibers on both sides of the tip plate. The coolant air is drawn into the attenuated zone so that the filaments are attenuated in a uniform environment. The Patents and documents described herein are hereby incorporated by reference. Having described presently preferred embodiments of the invention it will be appreciated that the invention may be otherwise embodied within the scope of the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A method of forming continuous glass fibers comprising the steps of: supplying a plurality of streams of molten glass from a bushing; drawing the streams into continuous glass filaments; providing a stream of coolant air parallel to the direction of draw of the streams of continuous glass filaments on at least the front and back of the bushing to entrain the coolant air wherein the entrainment of the coolant air is determined by the speed at which the glass filaments are drawn; and collecting the continuous filaments. 2. The method of Claim 1 wherein between 150 - 300 cfm (cubic feet/minute) of coolant air is provided. 3. The method of Claim 1 wherein the coolant air stream is substantially vertically downward. 4. The method of Claim 1 wherein the coolant air stream is at ambient temperature. 5. The method of Claim 1 wherein the stream of coolant air is provided parallel to the direction of draw of the streams of continuous glass filaments on at the entire periphery of the bushing on at least the front and back of the bushing 6. An apparatus for delivering non-intrusive coolant air to an attenuation zone of a glass drawing process of a bushing including a bushing tip plate having a plurality of bushing tips, the apparatus comprising: at least two plenum chambers including inlets into which coolant air is fed under pressure at a selected flow rate to discharge outlets, the discharge outlets extending a longitudinal length of the bushing tip plate to provide coolant air to a front and back of the tip plate; wherein the entrainment of the coolant air is a function of the speed at which the glass filaments are drawn. 7. The apparatus of Claim 6 wherein the discharge outlets are positioned above a plane formed by cooling fins secured beneath the bushing tip plate. 8. The apparatus of Claim 6 wherein the discharge outlets are positioned below a plane formed by cooling fins secured beneath the bushing tip plate. 9. The apparatus of Claim 6 wherein the discharge outlets are positioned parallel to a plane formed by cooling fins secured beneath the bushing tip plate. 10. The apparatus of Claim 6 wherein the coolant air exits the discharge outlets substantially vertically downward directly adjacent the tips to form an air curtain at the front and rear of the bushing.

Inventors: BAKER JOHN, DOWLATI RAMIN, SNEDDEN ANDREW, SRINIVASAN SESHADRI, BAKER, JOHN, DOWLATI, RAMIN, SNEDDEN, ANDREW, SRINIVASAN, SESHADRI
Applicants: BAKER JOHN, DOWLATI RAMIN, OWENS CORNING FIBERGLASS CORP, SNEDDEN ANDREW, SRINIVASAN SESHADRI, BAKER, JOHN, DOWLATI, RAMIN, OWENS CORNING, SNEDDEN, ANDREW, SRINIVASAN, SESHADRI
Application year: 1999
Main classification: C03B37
IPC codes: C03B37