Abstract:
A hybrid heater that includes a structural mass into which passages are provided to create a labyrinth for chemical flow through the structural mass, the passages being sized and disposed to receive a plurality of heater rods such that the chemical is traversed through the passages in direct contact with the heater rods. A coiled spring may be disposed or other spiral arrangement provided in the space between and against the walls of the passages and the heater rod to facilitate flow uniformity around the rods. A temperature sensor may be provided in direct contact with the heating element and may be fitted with a mass sleeve to draw off any excess heat on the sensor during transitions.

Description:
CROSS-REFERENCE TO RELATED APPLICATION FIELD 
     This application is a continuation of U.S. application Ser. No. 10/588,202 which is a national stage application of PCT Application PCT/US05/02892 filed Feb. 1, 2005, which claims priority to U.S. Provisional Application 60/542,062 filed Feb. 5, 2004. 
    
    
     FIELD OF THE INVENTION 
     This invention pertains to dedicated heaters for preheating chemical in mixing heads or spray guns for use in chemical processing, and more particularly to a heating unit that combines the beneficial features of both mass and direct contact style heaters. 
     BACKGROUND OF THE INVENTION 
     In chemical processing, such as plural component polyurethane processing, the proper mixing of the chemical components is essential to developing the final physical properties specified by the system supplier. In impingement designed mixing heads or spray guns, lowering the viscosities with heat helps to facilitate proper mixing. The two types of preheaters are typically utilized in impingement designed mixing heads/spray guns. 
     The first style, mass style, heats by conduction. Mass style heating utilizes a structural block, which is typically aluminum, into which holes are bored or small grooves cut and hydraulically connected to form a labyrinth through which the chemical passes. Heater rods are attached to or embedded in the block to raise the temperature of the surrounding structural mass, which in turn raises the temperature of the chemical within the holes/grooves. In this type of heating, the heater rods are isolated from the grooves or holes through which the chemical flows. Thus, heat is transferred from the heated mass to the chemical, which is either in a static or dynamic state within the chemical grooves, by means of conduction. The temperature of the mass, and, indirectly, the chemical, is maintained at the process temperature by means of a temperature controller and a sensor located within the mass. Typical mass style heating arrangements are disclosed, for example, in U.S. Pat. Nos. 2,866,885 to McIlrath, and 4,343,988 to Roller et al. 
     Mass style heaters have numerous advantages and disadvantages. Mass style heaters exhibit high thermal inertia in that, once at temperature, they tend to resist small temperature changes. As a result, mass style heaters generally provide stable temperature control if the chemical is maintained in a constant dynamic state or a constant static state. During the transition from the dynamic mode to the static mode, however, the mass ends to retain its temperature and pass it off to the static chemical causing an undesirable temperature spike. Conversely, as the chemical transitions from the static mode to the dynamic, the inefficiency of the mass heater causes a temperature drop at the outlet of the heater. Thus, mass style heaters are typically slow in responding to flow changes. Moreover, inasmuch as the labyrinth of drilled holes typically comprises relatively small grooves, it can develop backpressure during dynamic conditions. 
     The second style is the direct contact style heater. Direct contact style heaters utilize direct heating by placing heater rods into direct contact with the chemical. A heater rod is paced into a hydraulic tube of a given diameter. One or more such hydraulic tubes are typically connected to a manifold interconnecting other similarly configured tubes with an inlet and an outlet. The chemical traverses through the tubes in direct contact with the heater rods. Examples of direct contact style heaters are shown, for example, in U.S. Pat. No. 4,465,922 to Kolibas. 
     As with the mass style heater, direct contact style heating has both its advantages and disadvantages. Because there is little thermal inertia, direct contact style heating responds well to flow changes. Additionally, such heaters come to temperature quickly, providing a very fast warm up cycle. Direct style heaters provide more efficient heat transfer than mass style heaters. Direct style heaters provide a much greater difference in temperature between the set point temperature and the fire rod surface temperature such that the temperature control is less stable in steady conditions than mass style heaters. Further, direct contact heaters have historically been more costly to manufacture and assemble than mass style heaters. Moreover, the physical dimensions of direct style heaters constrain the number of tubes, thus shortening the contact surface area available for heat transfer. 
     Accordingly, there exists a need for a heating arrangement that provides the advantages of the currently available heaters, while minimizing or eliminating the disadvantages of the same. The invention provides such an arrangement. The advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention comprises a hybrid heater that combines aspects of both the mass style and direct contact style heaters. The hybrid heater includes a structural mass, similar to the mass style heater, into which passages are provided of a diameter similar to the inside diameter of the tubes of the direct contact style heater. A heater rod is placed in the passage, and the chemical is traversed through the passages such that it comes into direct contact with the heater rod within the passage, the passage being surrounded by the structural mass. 
     Thus, hybrid heater combines the advantages of both types of heaters while minimizing or eliminating the associated disadvantages of each. Among other things, the hybrid heater design provides very stable temperature control. As opposed to direct style heaters, the structural mass of the hybrid heater acts as a heat sink to draw off the excess temperature. The mass provides stability, and the controlled direct contact provides superior heat transfer. In the currently preferred embodiment, 30% greater heating surface area is provided within the same envelope as current mass style designs. The hybrid heater also provides more rapid warm up cycle and temperature control of the direct contact style heaters. The efficient heat transfer results in a delta T to flow rate not previously achieved in the prior art. Additionally, it is of a lower cost to manufacture than direct contact style heaters. 
     As another aspect of the design, a coiled spring may be disposed or other spiral arrangement provided in the space between and against the walls of the passages and the heater rod. This provides flow uniformity around the rod, defeating the random flow of chemical along the heating element, resulting in very efficient heat transfer and very low backpressure development during use. 
     Alternately or additionally, a temperature sensor may be provided in direct contact with the heating element, thus maintaining a relatively small delta T between the surface of the element and the process temperature. The temperature sensor may also be fitted with a mass sleeve, which draws off any excess heat on the sensor during transitions, resulting in very stable temperature control. 
     These and other advantages of the invention will be appreciated upon reading the brief description of the drawings and the detailed description of the invention, and upon review of the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partially exploded perspective view of a hybrid heater assembly constructed in accordance with teaching of the invention. 
         FIG. 2  is an exploded perspective view of the hybrid heater of  FIG. 1 . 
         FIG. 3  is a cross-sectional view of the structural mass taken along line  3 - 3  in  FIG. 2 . 
         FIG. 4  is a cross-sectional view of the structural mass taken along line  4 - 4  in  FIG. 2 . 
         FIG. 5  is a schematic view of the material flow path through the structural mass of  FIG. 2 . 
         FIG. 6  is a bottom view of the structural mass of the hybrid heater of  FIG. 2 . 
         FIG. 7  is a side view of the structural mass of the hybrid heater of  FIG. 2 . 
         FIG. 8  is a plan view of the structural mass of the hybrid heater of  FIG. 2 . 
         FIG. 9  is an opposite side view of the structural mass of the hybrid heater of  FIG. 2 . 
         FIG. 10  is an end view of the structural mass of the hybrid heater of  FIG. 2 . 
         FIG. 11  is a view of the opposite end of the structural mass of the hybrid heater of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings, there is shown in  FIG. 1 , a preheater assembly  20  constructed in accordance with teachings of the invention. The preheater assembly  20  includes a preheater  22 , which is covered by a preheater cover  24 . In the embodiment shown, the preheater cover  24  is spaced apart from the preheater  22  by spacers or standoffs  26  and secured by acorn nuts  28 , although any appropriate arrangement may be utilized. The preheater  22  comprises a structural mass or block  30  that is preferably formed of aluminum or the like. The structural mass  30  may be formed by any appropriate method, but is preferably machined from a block of aluminum. 
     In order to provide a flow of material to be heated, the preheater  22  is provided with an inlet  35  in the form of an inlet fitting  36  disposed in an inlet bore  38  in the mass  30 , and an outlet  31  in the form of an outlet fitting  32  disposed in an outlet bore  34  in the mass  30 . Internally, the mass  30  is provided with a series of parallel and perpendicular bores that provide an elongated path for the flow of material through the mass  30 . As may be seen in the cross-sectional drawing of  FIG. 3  and the schematic rendition of  FIG. 5 , material entering the structural mass  30  through the inlet bore  38  enters elongated bore  62 . The material flows down elongated bore  62  to its opposite end where it flows perpendicularly through vertical bore  60  to cross over to elongated bore  58 . After flowing down elongated bore  58 , the material again flows perpendicularly, vertically through bore  56  into elongated bore  54 . The material flows through elongated bore  54 , and, at the opposite end, flows perpendicularly through cross bore  52  and into elongated bore  50  (as may be seen in  FIG. 4 ). In a similar manner, the material flows through elongated bore  50 , then perpendicularly vertically through bore  46  into and then through elongated bore  44 , then perpendicularly vertically through bore  42  into and then through elongated bore  40 , and then outward through the outlet fitting in outlet bore  34 . 
     It will be appreciated by those of skill in the art, that the elongated bores or passages  40 ,  44 ,  50 ,  54 ,  58 ,  62  may be drilled into a solid block of a structural material such as aluminum. In the currently preferred embodiment, 6061 T6 Aluminum is utilized. The vertical bores  42 ,  46 ,  56 ,  60 , the cross bore  52 , the inlet bore  38  and outlet bore  34  may then be drilled to the appropriate depth in the block to properly construct the flow labyrinth. It will further be appreciated that the labyrinth may be of any appropriate arrangement so long as the design provides the required heating properties. In the currently preferred embodiment, on the order of 15%-30% of the mass  30  is open chemical flow paths, more preferably, approximately 22% is open flow paths. Following the construction of the labyrinth arrangement, the apertures opening into the bores  42 ,  46 ,  56 ,  60  may be sealed with appropriately sized plugs  42   a ,  46   a ,  56   a ,  60   a , and the inlet fitting  36  and outlet fitting  32  sealed to the inlet and outlet bores  38 ,  34  to complete the labyrinth. It will be appreciated that any appropriate method of sealing the same may be utilized. For example, threads may be provided as shown and an appropriate gasket, o-ring or other seal provided. 
     In order to increase the versatility of the mass  30 , alternate inlet and outlet openings  68 ,  66  may be provided that open into the adjacent elongated bores  62 ,  40  from an alternate surface. In the illustrated embodiment, the alternate inlet and outlet bores  68 ,  66  are provided in what is shown as the top surface of the mass  30  as opposed to the side surfaces to provide versatility in the design of the inlet and outlet configurations. When not in use, one of each of the inlet and outlet bores  38 ,  68 ,  34 ,  66  may be sealed using an appropriate plug  72 ,  70  by any appropriate arrangement, as explained above. 
     In accordance with the invention, the preheater  22  is further provided with a plurality of elongated heater rods  74 ,  76 ,  78 ,  80 ,  82 ,  84  that are disposed directly in the elongated bores  40 ,  44 ,  50 ,  54 ,  58 ,  62 , respectively, of the structural mass  30 . A pair of wires  85  is provided to a coupling  87  for each rod to provide power to heat the rods, as will be understood by those of skill in the art. In this way, the material flowing through the labyrinth of bores flows along and around the heating elements. 
     In order to further enhance the uniformity of the heating, a spiral flow path may be provided along the heater rods  74 ,  76 ,  78 ,  80 ,  82 ,  84 . This spiral flow path may be provided by any appropriate structure. In the preferred embodiment, however, the spiral flow path is provided by a coil  86 ,  88 ,  90 ,  92 ,  94 ,  96  that is sized such that it tightly contacts both the outer surfaces of the heater rods  74 ,  76 ,  78 ,  80 ,  82 ,  84  and the inner surfaces of the elongated bores  40 ,  44 ,  50 ,  54 ,  58 ,  62 . For purposes of explanation, a single such heater rod  80  and coil  92  is shown in  FIG. 4 , although the remaining heater rod and coil combinations will be essentially the same. Plugs  86   a ,  88   a ,  90   a ,  92   a ,  94   a ,  96   a  are provided to seal the coils  86 ,  88 ,  90 ,  92 ,  94 ,  96  within the bores  40 ,  44 ,  50 ,  54 ,  58 ,  62 . In this way, the coil  86 ,  88 ,  90 ,  92 ,  94 ,  96  forces the chemical material to uniformly flow between the heater rods  74 ,  76 ,  78 ,  80 ,  82 ,  84  and the bore  40 ,  44 ,  50 ,  54 ,  58 ,  62 , eliminating random flow that may result in inefficient heating. As a result, the preheater  22  provides every efficient heat transfer and very low backpressure development. 
     The preheater may additionally include a temperature sensor  100  to assist in temperature control. As shown in  FIG. 2 , the temperature sensor  100  is disposed in direct contact with the heater rod  74 , i.e. the heater rod adjacent the outlet bore  34 ,  66 . As a result, a relatively small delta T is maintained between the surface of the element and the process temperature of the chemical material flowing through the preheater. Additionally, the temperature sensor maybe fitted with a mass sleeve, which draws off any excess heat on the sensor during transitions and results in very stable temperature control. It will be appreciated by those of skill in the art that an over-temperature disk  102  may be provided along an outside surface of the mass  30  to cut power to the heater rods should an excessive external surface temperature be reached, i.e., over 210° F. 
     All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. For example, while the invention has been described with regard to the use of six elongated bores or passages and six heater rods, an alternate number may be provided. For example, two, three, four, five, seven, eight or more such passages and/or heating rods may be provided. Additionally, an alternate labyrinth arrangement may be provided. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.