Abstract:
An improved oven ( 1 ) comprising a conveyor configured and arranged to move a product ( 11 ) to be processed through an oven, a primary air delivery system ( 45 ) configured and arranged to provide a heated primary air flow ( 47 ), a secondary air delivery system configured and arranged to provide a heated secondary air flow ( 48 ), a processing enclosure ( 21 ) configured and arranged to receive and contain the product and the primary air flow, an insulated enclosure ( 2 ) configured and arranged to receive the heated secondary air flow, the processing enclosure configured and arranged to extend through the insulated enclosure and the heated secondary air flow and to separate the primary air flow from the secondary air flow.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to the field of ovens and dryers, and more particularly to an improved oven for processing fiber bundles or tows. 
       BACKGROUND 
       [0002]    Convection ovens and dryers that process continuous streams of product are in wide use. In many ovens the product moves horizontally at one or more levels, either carried on parallel moving conveyors or, in the case of textiles or webs, suspended under tension between external drives. A circulating hot air flow is brought in contact with the product for heating or drying. A technically important class of ovens treats polymeric or organic carbon fiber precursors in air to provide thermoplastic properties prior to carbonization. 
         [0003]    Ovens for providing oxidative heat treatment to carbon fiber precursor materials such as polyacrylonitrile (PAN) are known in the industry. U.S. Pat. No. 6,776,611 describes an oven in which the heating airflow is circulated around the PAN in tow format and contacts the fiber in a direction perpendicular to the direction of tow travel. U.S. Pat. No. 4,515,561 discloses an oven in which the heating airflow is circulated around the PAN in tow format and contacts the fiber in a direction parallel to the direction of tow travel. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    With parenthetical reference to corresponding parts, portions or surfaces of the disclosed embodiment, merely for the purposes of illustration and not by way of limitation, the present invention provides an improved oven ( 1 ) comprising a conveyor configured and arranged to move a product ( 11 ) to be processed through an oven, a primary air delivery system ( 45 ) configured and arranged to provide a heated primary air flow ( 47 ), a secondary air delivery system configured and arranged to provide a heated secondary air flow ( 48 ), a processing enclosure ( 21 ) configured and arranged to receive and contain the product and the primary air flow, an insulated enclosure ( 2 ) configured and arranged to receive the heated secondary air flow, the processing enclosure configured and arranged to extend through the insulated enclosure and the heated secondary air flow and to separate the primary air flow from the secondary air flow. 
         [0005]    The conveyor may be configured to move the product through the processing enclosure in a first direction ( 49 ), with individual passes moving either forward or backward, the processing enclosure may have a longitudinal enclosure axis ( 50 ) substantially parallel to the first direction, the primary air flow in the processing enclosure ( 47 ) may be substantially parallel to the first direction and the secondary air flow in the insulated enclosure ( 48 ) proximal to the processing enclosure may be substantially perpendicular to the first direction. 
         [0006]    The primary air delivery system may comprise an input chamber ( 10 ) configured and arranged to receive the primary air flow and the conveyed product and to output the primary air flow and the conveyed product to the processing enclosure. The conveyor may be configured and arranged to move the product through the processing enclosure in a first direction and the chamber may output the heated primary air flow and the conveyed product to the processing enclosure in the first direction. The input chamber may comprise an air input opening ( 38 ), a product input opening ( 39 ) different from the air input opening, an output opening ( 43 ) to the processing enclosure opposite the product input opening, and an airflow directional ( 37 ) configured and arranged to direct airflow from the air input opening to the output opening. The air input opening may be orientated substantially perpendicular to the output opening and the airflow directional may be configured and arranged to turn the airflow from a direction substantially perpendicular to the first direction to a direction substantially parallel to the first direction. The output opening may be larger in size than the product input opening. The chamber may further comprises a product input opening size adjustment mechanism, and the opening size adjustment mechanism may comprise a first plate ( 29 ) and a second plate ( 30 ), the first and second plates adjustable relative to each other so as to provide a variable gap ( 39 ) there between. A locking mechanism may be configured and arranged to adjustably lock the plates in a position relative to the chamber so as to vary the size of the product opening, and the locking mechanism may comprise locking screws ( 31 ). 
         [0007]    The oven may further comprise an output chamber ( 18 ) configured and arranged to receive the product and the primary air flow from the enclosure and to exhaust the primary air flow and discharge the product. The output chamber may comprise an input opening ( 44 ) from the processing enclosure, a product discharge opening ( 41 ) opposite the input opening and an air exhaust opening ( 42 ) different from the product discharge opening. The air exhaust opening may be orientated substantially perpendicular to the input opening. The output chamber may further comprise a product input opening size adjustment mechanism, and the opening size adjustment mechanism may comprise a first plate and a second plate, the first and second plates adjustable relative to each other so as to provide a variable gap ( 41 ) there between. A locking mechanism may be configured and arranged to adjustably lock the plates in a position relative to the chamber so as to vary the size of the product discharge opening, and the locking mechanism may comprise locking screws. 
         [0008]    The primary air delivery system may comprise one or more devices selected from a group consisting of a fan ( 3 ), a heater ( 4 ), a thermometer ( 6 ), a manifold ( 7 ), a valve ( 8 ), a flow meter ( 9 ) and a pipe ( 5 ). The primary air delivery system may comprise a single regenerative fan, a single in-line heater, a thermometer, a single manifold configured and arranged to split airflow into a plurality of downstream paths, each of the paths comprising a valve and a flow meter, wherein the primary air flow is generated and circulated through the heater, the manifold and the valve no more than once before being brought into contact with the product. The primary air delivery system may comprise a single regenerative fan, a manifold configured and arranged to split airflow into a plurality of downstream paths, each of the paths comprising a valve, a flow meter, an in-line heater and a thermometer, before being brought into contact with the product. The primary air delivery system may not re-circulate, in whole or in part, primary air flow exiting the processing enclosure. 
         [0009]    The secondary air delivery system may comprise a fan ( 12 ), a heater ( 13 ), a thermometer ( 35 ), a recirculating inlet ( 26 ) for receiving used air from the insulated enclosure, an air exhaust outlet ( 16 ) having a flow control valve ( 17 ) for exhausting air from the insulated enclosure, and a make-up air inlet ( 14 ) having a flow control valve ( 15 ) for receiving make-up air, wherein the secondary air flow may comprise a mix of the used air and the make-up air. The make-up air flow and the exhaust air flow may be controlled by the valves ( 15 ,  17 ) to vary the amount of the make-up air and the used air in the secondary air flow. The secondary air delivery system may comprise a plug fan ( 12 ) with an axis perpendicular to the processing enclosure axis ( 50 ), located on an insulation enclosure wall approximately midway along a product travel dimension of the oven, the fan having an upstream inlet cone ( 26 ) for receiving air and a discharge plenum ( 32 ) that directs flow downwards, a heater ( 13 ) positioned downstream and near the fan discharge port, a thermometer ( 35 ) positioned downstream and near the heater, a set of directing vanes ( 28 ) positioned near the heater and near a floor of the insulated enclosure that turn the flow 90 degrees to flow adjacent to the floor of the insulated enclosure, a second set of vanes ( 23 ) that split the flow approximately in half and turn a first half portion of the flow 90 degrees to be aligned with the first direction and turn the second half portion of the flow 90 degrees to be opposite the first direction, a third set of vanes ( 24   a ) that turn the first portion of the flow 90 degrees to flow upwards in a direction perpendicular to the enclosure axis, a fourth set of vanes ( 24   b ) that turn the second portion of the flow 90 degrees to flow upwards in a direction perpendicular to the enclosure axis, a flow conditioning device ( 22 ) that spans a length of the oven and is wider than a widest dimension of the processing enclosure and through which the upward air flow passes before contacting the processing enclosure, an upper perforated plate ( 27 ) above the processing enclosure, and an air collection plenum ( 36 ) separating air that flows through the upper perforated plate and into the fan inlet cone from air that is discharged from the fan and flows through the heater, turning vanes, flow conditioner and over the processing enclosure. The flow conditioning device may comprise two perforated plates with a cellular structures located there between, and the cellular structure may be a honeycomb structure. 
         [0010]    The primary air delivery system and the secondary air delivery system may be configured and arranged to deliver the primary air flow to the inside of the processing enclosure and to deliver the secondary air flow to the outside of the processing enclosure at a temperature range that is about the same. 
         [0011]    The processing enclosure may have a length and a cross-sectional characteristic dimension and the length may be at least about fifty times the cross-sectional characteristic dimension. The processing enclosure may have a cross-sectional shape that is circular, square, rectangular, oval or elliptical. 
         [0012]    The oven may comprise multiple processing enclosures configured and arranged to receive and contain the product and the primary air flow and extending through the insulated enclosure. The oven may further comprise multiple input chambers and output chambers communicating with the respective multiple processing enclosures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a perspective view of an oven in accordance with one embodiment of the present invention. 
           [0014]      FIG. 2  is an enlarged detailed view of the embodiment shown in  FIG. 1 , taken within the indicated area A of  FIG. 1 , with the top sheet metal of the end chamber removed for clarity. 
           [0015]      FIG. 3  is a rear perspective view of the embodiment shown in  FIG. 1 , with one wall of the insulating enclosure removed for clarity. 
           [0016]      FIG. 4  is a vertical transverse cross-sectional view of the embodiment shown in  FIG. 1 , taken generally on line B-B of  FIG. 1 . 
           [0017]      FIG. 5  is a cross-section view of a second embodiment of the oven shown in  FIG. 4 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    At the outset, it should be clearly understood that like reference numerals are intended to identify the same structural elements, portions or surfaces consistently throughout the several drawing figures, as such elements, portions or surfaces may be further described or explained by the entire written specification, of which this detailed description is an integral part. Unless otherwise indicated, the drawings are intended to be read (e.g., cross-hatching, arrangement of parts, proportion, degree, etc.) together with the specification, and are to be considered a portion of the entire written description of this invention. As used in the following description, the terms “horizontal”, “vertical”, “left”, “right”, “up” and “down”, as well as adjectival and adverbial derivatives thereof (e.g., “horizontally”, “rightwardly”, “upwardly”, etc.), simply refer to the orientation of the illustrated structure as the particular drawing figure faces the reader. Similarly, the terms “inwardly” and “outwardly” generally refer to the orientation of a surface relative to its axis of elongation, or axis of rotation, as appropriate. 
         [0019]    Referring to the drawings, and more particularly to  FIG. 1  thereof, this invention provides an improved oven for fiber heat treatment, of which a first embodiment is generally indicated at 1. While this invention has many applications for providing an efficient and high quality fiber heat treatment, it is described in this embodiment with regard to its application to an oxidative stabilization oven for carbon fiber precursor. 
         [0020]    As shown in  FIG. 1 , oven  1  includes rectangular insulation enclosure  2 , which is of conventional construction using structural and sheet steel and mineral or glass insulation. Product layers  11  are arranged and move in parallel horizontal planes through oven  1 . In the case of carbon fiber precursors in tow format, product layers  11  are tows arranged side-by-side in one horizontal layer and rollers or other pass-back devices are used to create one continuous serpentine path through the entire oven. 
         [0021]    The product contact air, or process air, is pressurized at fan  3  and passes through in-line heater  4 . Fan  3  may be any conventional fan capable of the required flow and pressure drop, and is preferably of a regenerative type. It is preferable that fan  3  draw air from a filtered source or that fresh air is drawn from outside the plant environment. In-line heater  4  may be either electric or fossil-fuel driven, and should be capable of raising the air to the desired process temperature in a single pass of air. Temperature ranges of the process air are preferably between about 100 and 600 degrees Celsius (C), more preferably between about 200 and 400 degrees C. The temperature of the air exiting heater  4  is controlled via a conventional electronic feedback loop using thermometer  6  to measure the temperature and a thyristor or gas flow control valve to modulate the power to heater  4 . 
         [0022]    The heated air enters manifold  7  and is split into a plurality of paths prior to entering oven  1 . Each such gas path through inlet piping  5  includes valve  8  and flow meter  9 , which measure and control the flow rate of heated air. Valves  8  may be any conventional control valve designed for the desired temperature range. While not shown on the figure, heater  4 , the downstream piping and manifold  7  are thermally insulated, preferably with fiberglass or mineral wool of about 50 mm or greater thickness. Alternative configurations for the process air inlet train may be used. For example, a separate heater could be installed in each gas inlet path  5  downstream of flow control valve  8 . 
         [0023]    Referring now to  FIG. 2 , in this embodiment the plurality of process gas inlets are directed via piping  5  through opening  38  in the side wall of end chamber  10 , where the gas is then directed by deflector  37  into tubular enclosures  21 , which are connected through opening  43  to the back wall of chamber  10  and pass through holes in insulation enclosure  2  and into oven  1 . Deflector  37  turns the flow 90 degrees from a lateral direction to a direction normal to the direction of travel of product  11 . Air is prevented from flowing out product entrance  39  of chamber  10  by having product entrance  39  of a reduced area. Product opening  39  is defined by an upper product slot plate  29  and a lower product slot plate  30 . The size of product slot or opening  39  may be adjusted by sliding the slot plates  29  and  30  vertically, with plates  29  and  30  locked in place or allowed to travel by means of locking screws  31 . In PAN oxidation ovens, thickness of product layer  11  varies but is generally about 3 mm or less. The gap  39  between plates  29  and  30  during operation is preferably between about 2 and 20 mm, and more preferably between about 6 and 10 mm. The maximum adjusted gap between plates  29  and  30 , for cleaning or other maintenance, is at minimum about equal to the height dimension of product enclosures  21 . Other means for fixing the position of plates  29  and  30  may be used. For example, spring loaded bolts may be employed. 
         [0024]    Process air enclosures  21  have a relatively small cross section compared to the oven dimensions, and are preferably tubes having a diameter between about 0.01 and 0.40 meters, and more preferable between 0.02 and 0.10 meters. The velocity of the product air flow within enclosures  21  is preferable between about 0.1 and 10 m/sec, and more preferably between about 1 and 6 m/sec. The ratio of the cross-sectional characteristic dimension (diameter in the case of a cylindrical tube) to the length of enclosures  21  is preferable greater than about 10 and more preferably greater than about 50. The high ratio of the cross-sectional characteristic dimension to the length ensures that the flow of air occurs along the direction of travel of product layers  11 . While enclosures  21  in the embodiment shown are round tubes, other cross-section tube shapes, such as square, rectangular, elliptical or oval, could be used as an alternative. It should be understood by those skilled in the art that, depending on the cross-sectional moment of inertia and length of enclosures  21 , they may require mechanical support along the length of the oven to prevent downward bowing or creeping. These supports can be positioned under enclosures  21  at regular intervals along the oven length and welded or bolted to the inside surface of insulation enclosure  2 . 
         [0025]    Referring now to  FIG. 3 , the plurality of process enclosures  21  and product layers  11  traverse the oven and pass through insulation enclosure  2  and into exit end chamber  18  through opening  44  in chamber  18 . Product  11  exits end chamber  18  through a slot  41  between a set of adjustable slot plates similar to plates  29  and  30  described with entrance end chamber  10 . The process air flows inside enclosures  21  as shown by arrows  47  and exits in the transverse direction through opening  42  in chamber  18  and a plurality of exhaust piping  40  that includes valve  19 . The exhausted air is then collected in exhaust header  20 , which is connected to an appropriate air discharge system. 
         [0026]    Referring again to  FIG. 1 , process air travels once through the oven system. It enters at fan  3 , and is heated and set to a control flow with heater  4 , valve  8  and flow meter  9 . Entrance end chamber  10  directs both product  11  and almost all the process air into process enclosures  21 , where the air transfers heat and mass with product layers  11 . The air and product  11  exit the oven through exit end chamber  18 , where the exhaust process air is directed through control valves  19  and into exhaust header  20 . The pressure inside of process enclosures  21  is preferably very close to ambient pressure, and most preferably within about 1 mbar and even more preferably within about 0.1 mbar. Valves  8  and  19  and the height of slot openings  39  and  41  in end chambers  10  and  18 , respectively, are the means for adjusting this pressure. The near ambient pressure ensures that very little air actually exits or enters process enclosures  21  through the product slots, which means that almost all of the process air, typically about 98% or more, contacts product layers  11 . The degree of control can be further increased if exhaust manifold  20  connects to an exhaust handling system with draw or negative pressure. In this case, the oven may be operated such that enclosures  21  have a slight negative pressure, virtually eliminating the escape of process gas at the product slots. 
         [0027]    The process air system described has the benefit that the gas contacting the product enters product enclosures  21  free from contaminants and picks up process contaminants only during a single air pass. For example, an oven such as shown in  FIG. 1 , heat treating  24000  filaments of 1.0 dTex PAN moving at 0.25 m/min, will generate about 1.1 gr/hr of hydrogen-cyanide (HCN) gas. With six oven enclosures  21 , each with a 50 mm diameter, and at an air velocity of 4.0 msec and temperature of 250 degrees C., the calculated maximum concentration of HCN in the air stream is about 8 ppm. This compares favorably to HCN concentrations seen inside typical industrial ovens that are between about 40 and 80 ppm. 
         [0028]    Referring again to  FIG. 1 , a secondary air flow is also provided to enclosures  21 . Secondary air flow is pressurized by fan  12  and heated by heater  13 . Fan  12  may be any conventional fan capable of the required flow, temperature and pressure drop, and is preferably of a plug type configuration. Heater  13  may be either electric or fossil-fuel powered, and should be capable of heating a circulating stream of air to the desired process temperature. The secondary air temperature is controlled via a conventional electronic feedback loop using thermometer  35  to measure the temperature and a thyristor or gas flow control valve to modulate the power of heater  13 . The purpose of the secondary air loop is to prevent heat loss or gain to the process air or product layers as they traverse the oven, so the temperature of the secondary air is set and controlled at a temperature substantially the same as the setting of the process air temperature. 
         [0029]    Referring to  FIGS. 2 ,  3  and  4 , secondary air flows vertically downwards from fan wheel  32  through heater  13 . It is turned 90 degrees to flow horizontally and transversally towards the back of oven  1  by a set of turning vanes  28 . The secondary airflow is then split in half and redirected horizontally and longitudinally, either toward the entrance or exit end of oven  1  by turning vanes  23 . The secondary airflow is then directed upwards vertically by turning vanes  24   a  and  24   b  and enters flow conditioner  25 . Flow conditioner  25  is designed to straighten the flow and make the air velocity uniform, and is preferably a device that contains a perforated steel plate and cellular honeycomb structures as described in U.S. patent application Ser. No. 13/180,215, entitled “Airflow Distribution System,” the entire disclosure of which is incorporated herein by reference. Flow conditioner  25  includes a second perforated plate  22  on top, through which the air flows at a uniform velocity and uniform vertical direction. The airflow just above plate  22  has velocity characteristics such that the ratio of the standard deviation to the mean is less than about 10%, and more preferably less than about 3%. The direction of flow just above plate  22  is preferably within about 10 degrees of vertical and more preferable within about 3 degrees of vertical. The mean velocity of the vertical flow is preferably between about 1 and 10 m/sec, and more preferably between about 3 and 6 m/sec. 
         [0030]    Referring again to  FIGS. 2 ,  3  and  4 , the secondary air flows upward over and around process air enclosures  21  and then continues upward through perforated plate  27 . The air then enters collection plenum volume  36 . Plenum  36  is separated from the airstream that flows upwards over process tubes  21  by vertical wall  33  and is separated from the flow that travels along the oven floor by horizontal wall  34 . The recirculating secondary airflow path is shown with arrows  48  in  FIGS. 3 ,  4  and  5 . The majority of the secondary airstream re-circulates through fan  12  by entering fan inlet cone  26 . A portion of the secondary air is exhausted at secondary oven air exhaust opening  16  and this flow is regulated by secondary air exhaust valve  17 . Make-up airflow for the secondary airstream enters the oven at secondary air inlet  14  and is regulated by make-up air valve  15 . Since the secondary airstream does not contact the product, it remains essentially clean, and therefore, at steady conditions, very little exhaust or make-up air is required. When it is desired to lower the oven temperature, however, the make-up air flow is useful for introducing cold room air into the oven. 
         [0031]    The secondary airstream keeps the temperature of the process air uniform as it flows along the interior length of the process air enclosures  21 . For example, if there were no secondary air flow, the temperature of the process air would, depending on velocity, drop by between about 20 to 50 degrees C. between the entrance and exit of the oven, with the largest temperature drops corresponding to the lowest air velocities. With a secondary airflow of about 3 m/sec or greater, the process air temperature change over the length of the oven is less than about 2 degrees C. 
         [0032]    The response time to a change in oven desired operation temperature, or setpoint, is determined in practice by the response time of the secondary airstream. This is because the process air consists of a once-through airflow that contacts only product layers  11  and the relatively small air enclosures  21 , and so has much lower thermal inertia than the secondary air system. The secondary air contacts the inside of the relatively large insulation enclosure  2  as well as the plug fan wheel  32  and all the other metal components inside the oven. For example, an oven similar to the embodiment shown in  FIGS. 1-4  with an insulation enclosure of dimensions of 5.0 m long×2.5 m high×1.0 m wide has a thermal inertial of about 800,000 Joules per degree C. If the oven is operating at a temperature of about 300 degrees C., there will be heat losses through the enclosure and ends of about 10 kW. In this example, heating element  13  with 30 kW of power capacity will thus have 20 kW power available to raise the temperature of the oven, which will result in a time of about 10 minutes to raise the oven temperature by about 15 degrees C. In this example it is assumed that valves  15  and  17  are closed to prevent makeup air from drawing power. Another example, using the same oven parameters just described, would be a lowering of the oven setpoint by about 15 degrees C. In this case, valves  15  and  17  are opened and heater  13  is shut off. In this example, a makeup airflow of about 170 Nm̂3/hr (100 scfm) results in about a 15 degree C. drop occurring in about 7 minutes. 
         [0033]    A calculation of the maximum temperature rise in product enclosure  21  during an exothermic runaway of PAN precursor will illustrate that the present invention does not require water quenching systems. The conditions assumed are 4×12,000 filament tows of 1.0 dTex at 1 m/min (mass rate of 0.288 kg/hr) in a single 51 mm diameter round enclosure  21 , and an air velocity of 1.0 m/sec at 250 degrees C. (mass rate of 6.2 kg/hr). Assuming PAN heat of reaction equals 2425 Joules per gram, and that all the reaction energy is absorbed by the flowing air, the calculated air temperature rise is about 110 degrees C. Thus, even with airflow near the bottom of the typical range, enclosure  21  should not experience a temperature above about 360 degrees C. 
         [0034]    While in principle enclosures  21  can be made of many different materials, the preferred materials are austenitic stainless steels such as  304  which maintain mechanical strength until above about 500 degrees C. and can therefore readily withstand this degree of exothermic runaway. The once-through airflow of the present invention promotes removal of the ash or other debris remaining after an exothermic runaway because the airflow itself tends to carry out lighter materials and is constantly being replaced by fresh air. Since the process air stream can be cooled rapidly, for example by about 100 degrees C. in less than about 5 minutes, the end chambers  10  and  18  can be opened within a short time after the exothermic event to facilitate inserting push rods or the like to remove any remaining debris. 
         [0035]      FIG. 5  shows a cross-section of another embodiment of the present invention. In this embodiment, the process air enclosure tubes  21  containing product layers  11  are arranged in multiple vertical rows and columns where the horizontal spacing is delineated by X and the vertical spacing delineated by Y. It is preferable that the ratio of vertical and horizontal spacing, Y/X, of enclosures  21  follows the principles used for conventional tube bundles in heat exchangers. In PAN fiber processing, the vertical spacing Y is established from tow transport considerations outside of the oven, with typical product layer spacing preferably between about 0.1 and 0.4 meters, and more preferably between about 0.15 and 0.20 meters. 
         [0036]    The described improvements provide a number of benefits. The oven provides uniform air velocity and consistent contact angle between the air and the fiber product throughout the heated length over a wide range of air velocities. Further, the temperature of the air is uniform for the entire heating length, independent of the velocity. Further, a uniform, stead-state temperature can be achieved rapidly, a benefit because delay in establishment of temperature wastes both time and process material. Further, the process contact air is introduced free of moisture, fiber fly, particulate, and process off-gas chemicals that can degrade the quality of the product. Also, the ability to control the process pressure prevents the escape of process off-gases. In particular, PAN based carbon fiber precursors are known to give off toxic hydrogen cyanide (HCN) which poses an inhalation hazard if allowed to concentrate outside the oven. 
         [0037]    Further, for carbon fiber precursors, the oven makes possible handling process upsets in an efficient manner. One type of process upset occurs when precursor tows break inside the oven. The broken tow ends can entangle with other tows, and other passes of tows at different elevations, either right after the break, or later when the broken tow is pulled out of the oven, until the entire process must be stopped and the oven cooled to ambient to allow internal access. With the design of oven  1  a tow break is contained within one minimum cross-sectional area enclosure  21 . The tow cannot fall far away from its normal path because of the enclosure, and is therefore unlikely to snag on oven parts or other tows. Oven  1  also facilitates pulling a broken tow out of the oven because the removal path is essentially a straight line and the tow removal point is from the ends outside of the oven and so does not require entering the oven or cooling the oven to ambient temperature. 
         [0038]    Another type of process upset occurs when carbon fiber precursor experiences an exothermic runaway reaction resulting in a fire. The oven limits fires from spreading throughout the entire oven volume. In the event of an exothermic process runaway, the heat generated is thus limited. The once-through process air stream carries products of combustion and generated heat out of the oven and there is no need to employ deluge water systems. After an exothermic event or fire, there is no need to stop the secondary air flow, no need to cool the oven to ambient temperature, and no need to enter the oven. Further, the oven limits fires from spreading without resorting to deluge water systems that are expensive to install and maintain, and which, when activated, require a time consuming cleanup inside an ambient temperature oven before the process can be restarted. This means the overall process upset due to an exothermic runaway or fire can be a matter of minutes, as compared to hours with conventional carbon fiber precursor ovens. 
         [0039]    The design of oven  1  provides uniform air velocity and consistent contact angle, temperature uniformity, short temperature response time, clean process gas, reduces or eliminates the need for post process treatment of the off gas, and makes possible efficient handling of process upsets. The fiber passes through the oven within enclosure  21  that are essentially the minimum possible cross-sectional area considering fiber catenary and natural vibrations. This small cross-section means that the ratio of the process enclosure length to its cross-sectional characteristic dimension is very large, creating boundary conditions that ensure the airflow is nearly exactly parallel to the fiber. The small cross-section area has the additional advantage that, for a given air velocity, the required amount of process air is kept to a minimum, thereby requiring minimum energy for pressurization and heating. 
         [0040]    The air passed through these product enclosures is filtered, pressurized, heated to the desired process temperature, and flow modulated upstream, flows parallel to the fiber through the enclosure, and exits to an exhaust system. Air only touches each element of the system one time. This means that the process air does not accumulate moisture, fiber fly, particulate, or other process off-gas chemicals that can degrade the quality of the product. Because there is no concentrating of process volatiles, the exhausted process air from PAN carbon fiber precursor does not necessarily require expensive incineration or other means of post treatment to destroy HCN. 
         [0041]    The once-through heating process is very fast thermally and thus the temperature of the process air can be changed rapidly, for example by 100 degrees C. in less than 5 minutes. This substantially reduces lost time and facilitates operator safety during tow removal. Tow removal can be done without changing the secondary air flow or temperature, so that once the broken tow is removed, the process air flow and temperature can be rapidly reestablished. This means the overall process upset due to a tow break can be a matter of minutes, as compared to hours with conventional carbon fiber precursor ovens. A benefit of the secondary air flow outside the process enclosures, and therefore not in contact with the fiber, is that it maintains a high degree of temperature uniformity within oven  1 . This re-circulated air flow is pressurized and heated to the desired process temperature with a dedicated fan and heater located integral to the oven casing. This air flows over and around the process air enclosures, keeping the outside surface at the desired process temperature, and thus preventing heat loss from the process air flowing parallel to the fiber. This affect provides temperature uniformity of the process contact air even at very low process air velocities, which is inherently difficult since in that case small heat loss or gain will tend to produce large temperature differences. The secondary air flow is provided with a modulated supply of cold fresh air. The secondary air temperature can be raised with increased heating power or lowered by increasing the intake of cold fresh air. This means that the secondary air temperature can be brought to equilibrium quickly whether the temperature change is an increase or a decrease. 
         [0042]    The present invention contemplates that many changes and modifications may be made. Therefore, while the presently-preferred form of the oven for fiber heat treatment has been shown and described, and several modifications and alternatives discussed, persons skilled in this art will readily appreciate that various additional changes and modifications may be made without departing from the spirit and scope of the invention, as defined and differentiated by the following claims.