Abstract:
Disclosed herein is a method and apparatus for improving the heating capacity of steam-heated hot plates, and in particular, to steam-heated hot plates used in the corrugating industry. One or more separators and thermocompressors are added to the output of the hot plates to separate blow through steam from condensate and pressurize and inject the steam back into the hot plates.

Description:
RELATED U.S. APPLICATION DATA 
     This application claims the benefit of U.S. Provisional Application No. 61/528,825, filed Aug. 30, 2011. 
    
    
     FIELD OF THE INVENTION 
     The invention pertains to a method and apparatus for improving the heating capacity of steam-heated hot plates, and in particular, to steam-heated hot plates used in the corrugating industry. 
     BACKGROUND 
     Corrugated containerboard is manufactured on machines that combine one or more “liners” in a stack with fluted webs (“medium”) in between with the peaks of the medium flutes glued to the surfaces of the liners. The adhesive between the fluted medium and the liners of the combined board (that is, the corrugated containerboard) is then dried by passing the board through a double face heating section. The double face heating section (“double-backer”) consists of a series of steam-heated “steam chests” or “hot plates”. Individual steam chests and hot plates are generally less than two feet in machine direction length and extend to the width of the corrugator, which is typically 100 ″ to 120″ in width. The containerboard is held against these steam chests and hot plates by belts and ballast rollers that serve to keep the board in good thermal contact with the top surfaces of the hot plates/steam chests. 
       FIG. 1  shows a steam chest  10  according to the prior art.  FIG. 2  shows a hot plate according to the prior art. Steam chests  10  and hot plates  40  are examples of steam heating devices designed to transfer heat from steam to a heating surface. Steam chests  10  can be constructed as large metal boxes  12  that are designed to hold the steam input at “A” the box interior  16  under pressure. The steam condenses on the top inside surface of the box  12  and the condensed steam (“condensate”) falls onto and collects on the bottom of the box  12 . From there, the condensate is drained by gravity to a steam trap  22  from which the condensate is returned to the boiler at “B.” The box upper surface  14  is in contact with a containerboard  15  to be dried, which is held down to the upper surface  14  by a belt  20 . Steam chests  10  are conventionally heated by steam that is supplied under pressure to each of the steam chests. The steam pressure to each group of steam chests  10  is typically controlled by a pressure control valve (not shown) working in conjunction with a pressure transmitter and a pressure indicating controller. 
       FIG. 2  shows the “hot plate”  40  (herein distinguished from the “steam chest”  10 ), which is similar in function to the steam chest  10 , except the hot plate  40  has drilled internal passages  44  adjacent to a hot plate surface  46 . The hot plate surface  46  and internal passages  44  are formed as part of a hot plate frame  42 . These passages  44  generally extend from one side of the hot plate frame  42  to the opposite side, and then back again, forming several loops before the passage leaves the plate. The steam flows into inlet  43  and through these internal passages  44  and condenses as it transfers its heat to a corrugated containerboard  48  on the outside of surface  46 . The condensate flows slowly by gravity toward a drain  45 . The drain line is conventionally connected to a steam trap. Steam traps open to drain the condensate from the hot plate and then close to prevent the passage of uncondensed steam. The condensate that leaves the steam trap is returned to the boiler. At high condensing rates, the condensate that forms inside the passages of the hot plates  40  tends to accumulate and result in a reduction in rate and uniformity of heat transfer. The corrugated containerboard  48  can be held down to the hot plate surface  46  by a belt  50 . 
     A typical corrugated containerboard making machine  300  with its associated double backer section  314  is shown in  FIG. 3 . The corrugated containerboard making machine  300  includes supply rollers  302  for the first liner, supply rollers  304  for the medium and supply rollers  306  for the second liner. The corrugated containerboard making machine  300  also includes a corrugator  308 , drive rollers  310  and adhesive applicator  312 . The corrugated containerboard making machine also includes a hot plates section  318  in a double backer section  314  for drying the adhesive applied at  312 . 
     In order to minimize the non-uniformity of heat transfer, a multitude of hot plates are used in each double backer section  314 . The pressure is adjusted on the belt  316  that holds the board to the hot plates  318  in an attempt to correct for these reductions in rate and uniformity of heat transfer. In conventional corrugators, the hot plate performance is controlled by the belt pressure, adding backing rolls, loading the backing rolls, increasing the steam pressure, venting some steam to atmosphere, adding more hot plates, or running the corrugating machine at a slower speed. 
     An example of prior art hot plates and their steam control system  400  are shown in  FIG. 4 .  FIG. 4  shows a steam line  402  inputting steam at “A.” The steam line  402  delivers steam either directly to a hot plate  408  via delivery lines  414  or to pressure control valves  404 ,  406 , which regulate the steam pressure and deliver steam to hot plates  410 ,  412  via delivery lines  416 ,  418 . The steam heats the hot plates  408 ,  410 ,  412  and condenses, forming a condensate that is collected by condensate trap lines  420 ,  422 ,  424  and carried to separators  426 ,  428  which separate condensate from steam and return separated steam to the delivery lines  416 ,  418  or directly to a pump  432 . Condensate is routed to pumps  430 ,  432  to be returned to the steam boiler (not shown) via a return line  434  at “B.” 
     The prior art hot plates and their steam systems are not suitable for high-speed corrugated boxboard production where uniformity, high heat transfer rates, and energy efficiency are important. 
     SUMMARY OF THE INVENTION 
     Aspects of disclosed embodiments include an improved method for transferring heat from a steam heating device including introducing steam into the steam heating device with a steam supply system; circulating the steam through the steam heating device creating steam condensate and collecting the steam and steam condensate with a separator tank which separates the steam from the steam condensate; returning the steam condensate to the boiler to be reheated; returning the steam to a thermocompressor which heats and pressurizes the steam and introduces it back into the steam supply system; and wherein the steam heating device includes a ratio of steam to steam condensate of at least 20:1 by volume. 
     Aspects of disclosed embodiments also include an apparatus for transferring heat from a steam heating device including a steam supply system for supplying steam; a steam heating device; a separator tank which separates the steam from the steam condensate; a thermocompressor which heats and pressurizes the steam and introduces it back into the steam supply system; and wherein the steam heating device includes a ratio of steam to steam condensate of at least 20:1 by volume. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a prior art steam box; 
         FIG. 2  is a diagram of a prior art hot plate; 
         FIG. 3  is a diagram of a prior art corrugating system; 
         FIG. 4  is a diagram of a prior art steam control system; 
         FIG. 5  is a diagram of a steam control system according to disclosed embodiments; and 
         FIG. 6  is a diagram of a steam control system according to disclosed embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The method and apparatus of the subject disclosure includes a steam-heated hot plate of the type typically used in the double-face heating section of machines that manufacture corrugated board, a steam pressure transmitter, a steam pressure indicator controller, a steam and condensate separator tank, a blow-down valve, a steam jet thermocompressor, and a pressure powered condensate pump. 
     The subject disclosure is applicable both to steam chests and to hot plates. Steam chests and hot plates can be referred to collectively as steam heating devices. If, when the steam chests or hot plates are first heated, the residual non-condensing gases (mostly air) are not purged, this can result in a further reduction in rate and uniformity of heat transfer. Steam heating devices can be equipped with a trap or separator which separates the live steam from condensed steam (water). In order to help purge air from the steam heating devices, a small line or passageway can be installed around the trap to by-pass the trap and allow “live” (uncondensed) steam to purge the air. The discharge of the live steam, however, gives rise to poor thermal efficiency and lack of process control. This escape of live steam with residual non-condensing gasses is called blow through. 
     Further, the collection of sub-cooled condensate in the bottom of the steam chest or on the bottom of the cross-machine flow passages of the hot plate gives rise to a thermal bowing of the heaters. This thermal bowing causes non-uniform thermal contact between the steam chest/hot plate surfaces and the corrugated container board which in turn results in non-uniform setting of the adhesive bonds. 
     In an embodiment of this disclosure, a steam pressure indicating controller maintains the desired steam pressure in the header that feeds one or more of the hot plates in the double-backer section. The drain line from the hot plate(s) discharges to the steam and condensate separator. The condensate is returned to the boiler through the pressure powered condensate pump. The blow through steam from the separator is piped to the suction port of the thermocompressor from where it is boosted in pressure by the thermocompressor and recirculated back to the supply header for the hot plate section. With this concept, the entire blow through steam is re-used. 
       FIG. 5  is a diagram showing a steam control system  500  for supplying steam to a number of steam heating devices, in this example hot plates  508 . The steam pressure indicating a controller  530  is used to maintain a hot plate header  506  pressure. This is accomplished by modulating the actuator on a thermocompressor  504  using the controller  530 . The controller  530  is connected to a transducer  532  which can measure steam pressure and temperature. Steam enters a high pressure steam input  502  at “A” and is routed to the thermocompressor  504  and a blow down valve  528  via a line  518 . Steam from the high pressure steam input  502  is combined with pressurized circulated steam at the thermocompressor  504  an routed to the hot plate header  506 , which distributes the steam to the hot plates  508 , under the direction of the controller  530 . 
     The steam circulates through the hot plates  508  and partially condenses. The circulated steam and condensate is output from the hot plates  508  through the return lines  510 . The return lines  510  route the circulated steam and condensate to a separator tank  512  where circulated steam is separated from condensate. The condensate is removed from the separator tank  512  via condensate line  514  to pump  516 , which pumps the condensate back to the steam boiler (not shown) via line  520  in direction “B”. 
     Circulated steam exits the separator tank  512  via re-circulation line  522  which can, in cooperation with valves  524  and  528 , permit the system to blow-down at start up to remove non-condensable gasses from the hot plates. Otherwise the circulated steam is returned to the thermocompressor  504  via the re-circulation line  522  to be pressurized and blended in with the new steam arriving from the high pressure steam header  502  to be returned to the hot plate header  506  and thereby to the hot plates  508 . 
     In  FIG. 5 , the amount of blow through flow and the differential steam pressure across the hot plates  508  depend on the operation of the thermocompressor  504  and are not primary control parameters. The thermocompressor  504  ensures the drainage of condensate from the hot plate(s)  508  and maintains high and uniform heat transfer from the hot plates  508  by a continuous and appropriate flow of blow through steam through the hot plate section. 
       FIG. 6  shows another disclosed embodiment of a steam supply system  600 . In  FIG. 6 , the amount of blow through flow and the differential steam pressure across hot plates  608  are alternatively selected as control parameters for a thermocompressor  604 . The steam pressure in a hot plate steam supply header  606  is controlled directly by a steam pressure control valve  638 . The thermocompressor  604  set point ensures the drainage of condensate from the hot plate(s)  608  and maintains high and uniform heat transfer from the hot plates  608  by a continuous and appropriate flow of blow through steam through the hot plate section. 
     The thermocompressor  604  is supplied with steam at a pressure that is equal to or suitably higher than the steam supply header  606  to the hot plates  608 . The high pressure (“motive”) steam that is supplied to the thermocompressor  604  is mixed with the low pressure steam from a separator tank  612  and discharges the mixture to the steam supply header  606  at a pressure that is at least as high as the steam supply header  606 . The thermocompressor  604  mixes high pressure steam from a high pressure steam inlet  602  with pressurized circulated steam from the separator tank  612  under the control of a differential pressure transmitter  630 , which gets information from a digital pressure transducer  632 . The output of the thermocompressor  604  is controlled by the control valve  638  that mixes high pressure steam from the high pressure steam inlet  602  with pressurized circulated steam under the control of a pressure indicating controller  636 , which gets information from a pressure transducer  636 . 
     Circulated steam and condensate exit the hot plates  608  via return lines  610  which route the circulated steam and condensate to the separator tank  612 , which separates the circulated steam from the condensate. The condensate is sent through condensate line  614  to a pump  616 , which pumps the condensate back to the steam boiler (not shown) via boiler return line  620 . Circulated steam is routed from the separator tank  612  via steam return line  622 . The returning steam can be routed through valve  624  to blow down line  626  to blow down the system upon start-up or be routed to thermocompressor  604 . 
     This method and apparatus maintains a flow of blow through steam that is by volume that can be 20-30 times higher than the condensate flow volume. This high volume of steam quickly purges the hot plate section  608  of all non-condensable gases, flushes the condensate through the passages in the hot plate  608  to decrease the amount of sub-cooled water that is in the passages, and prevents passages from flooding with condensate, thermally bowing, and losing heat transfer. 
     This concept allows the simultaneous achievement of high and uniform heat transfer and high operating efficiency, because the high volume of blow through steam is reused in the hot plate section  608 . Still further, this concept can quickly purge non-condensable gases from the heaters and reduce the amount of sub-cooled condensate in the heaters that would otherwise cause thermal bowing of the heaters and the corresponding loss of adhesive bond uniformly. 
     In an embodiment of this disclosure, the discharge from the thermocompressor  604  can be directed to the hot plate steam header of a down-stream hot plate section (not shown). This would be termed a “cascade thermocompressor system.” Embodiments of this disclosure include aspects in which the differential pressure transmitter  630  of  FIG. 6  is configured to measure the pressure drop across an appropriate orifice plate (not shown) in the uncondensed steam (blow through) line  622  so that the position of the control spindle in the thermocompressor  604  will be adjusted to maintain a fixed flow rate of uncondensed steam. 
     A further feature of the subject invention is the addition of a blow-down system to facilitate the start-up of the corrugator by purging air and other non-condensable gases from the corrugator system. This is accomplished by suitable control of the blow-down valves  524 ,  624  that discharge as shown in  FIGS. 5 and 6  to blow-down lines  526 ,  626 . A suitable thermostatic trap  540 ,  640  is used to clear non-condensable gases from the separator tank  512 ,  612  intermittently directing the discharge flow as needed to the blow-down lines  526 ,  626 . 
     The above-described implementations have been described in order to allow easy understanding of the present invention and do not limit the present invention. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structure as is permitted under the law.