Patent Publication Number: US-6665492-B1

Title: High-velocity electrically heated air impingement apparatus with heater control responsive to two temperature sensors

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to heaters and, more particularly, to a high-velocity, accurately responsive impingement heater which is able to direct constant-temperature air to a process. 
     2. Description of Related Art 
     Heaters are used throughout industry to provide heat to a specific location to carry out a particular process. The location may be a joint between two aircraft parts, and the process may be a curing process of a chemical resin used at the joint. The heat provided by the heater, typically by blown air, is required to carry out or to facilitate the curing process. Many curing processes require very precise temperatures in order to be carried out accurately and with the highest degree of quality and reliability. 
     This is particularly true when the curing process cures composite materials used on high-performance aircraft. The builders of such aircraft absolutely need to maintain strict control over every aspect of the manufacturing process, including the curing to the composite material, to ensure that the aircraft perform as specified and to ensure the safety of pilots and crew. Other processes may include curing ultra-low-reflectance coatings used in stealthy applications. 
     One of the difficulties in employing heaters is maintaining control of the temperature or, more specifically, maintaining a constant temperature at the location of the process. Heaters use a heating element to heat air which is then blown through a manifold positioned at the location of the process. The manifold is attached to the heater by an air conduit which allows the manifold to be positioned at the location. 
     Accordingly, the air passing through the heating element travels some distance before reaching the manifold and being blown out to the location to carry out the process. Thus, the temperature of the heated air immediately downstream of the heating element may not be the same as the temperature of the air being blown out of the manifold. This temperature difference is known as offset temperature. Further, many of the processes may be located at a position which does not allow the heater to be positioned closely so that a relatively long air conduit needs to be employed to position the manifold close to the location at which the process is to be carried out. 
     In conventional heaters, the heating element is located inside a heat-exchanger enclosure and typically is a coiled stainless-steel air line. This poses a number of problems. For example, the length, the diameter, and the wall thickness of the tubing are critical variables that affect the overall performance of the heater. Accordingly, any variation from heater to heater in any of these dimensions eliminates identical performance between heaters. Therefore, highly strict tolerances need to be maintained, thereby increasing production costs. 
     Another drawback of conventional heaters is that the heat generated by the heating element radiates both outwardly and inwardly from the tubular heating element. The air rushing through the tube can only remove heat from the inside diameter of the tube. Accordingly, efficiency decreases as the heater cannot make use of the outwardly radiated heat, which heat is wasted and lost through the outer diameter. In addition, many conventional heaters use fans to blow air across open heating elements which is inefficient. 
     Furthermore, conventional heaters require a highly trained operator to manually control the amount of heat being applied to the cure area. This use of specialized operators is inefficient and results in higher production costs. For example, the temperature controller on a number of conventional heaters is programmable. An engineer can determine offset temperatures required internally to achieve the desired cure temperatures at the process. The engineer can then program this information into the controller, thereby allowing a cure to be accomplished generally with no further operator intervention once the curing process is under way. This process is known as profiling. A drawback of profiling a cure is that it is time consuming and needs to be done every time the process changes for another cure, resulting in lower efficiency and higher costs. In addition, controllers are not sufficiently responsive to temperature fluctuations in the air at the location of the process, resulting in temperature lags and overshoots. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing discussion, it is an object of the present invention to provide a high-velocity, accurately responsive impingement heater which mitigates and/or obviates the aforementioned drawbacks of conventional heaters. 
     It is another object of the invention to provide a high-velocity, accurately responsive impingement heater which provides heated air to a process location and maintains the heated air at a substantially constant process temperature. 
     These objects as well as other objects, features, and benefits of the present invention are achieved by providing a high-velocity, accurately responsive impingement heater which provides heated air to a process location for effecting a process. The heated air which is provided to the process location is maintained at a substantially constant temperature. 
     According to one aspect of the present invention, the heater includes an air line for conducting air from an inlet thereof to the process location. A heat exchanger, including a plurality of heating elements, is provided for heating air conducted through the air line. A driver, which is connected to the heat exchanger, receives power from a power supply and applies current to the heat exchanger. A controller is connected to the driver and receives a predetermined process temperature required for effecting the process. The controller also provides the driver with a control signal. 
     The heater further includes a pair of temperature sensors. A process temperature sensor is positioned at the process location and measures the temperature of the air provided to the process location. The process temperature sensor then provides a process temperature signal to the controller which is indicative of the air temperature at the process location. An internal temperature sensor is positioned immediately downstream from the heat exchanger and measures the temperature of the air at that location in the air line. The internal temperature sensor then provides an internal temperature signal to the controller which is indicative of the air temperature downstream from the heat exchanger. 
     Based upon the temperature signals from the temperature sensors, the controller provides the control signal to the driver for specifying the amount of heat required at the process location in order for the heated air to substantially equal the predetermined process temperature. The driver then applies a corresponding current to the heat exchanger in response to the control signal. 
     One of the advantages of the heater of the present invention is that profiling (as described above) is eliminated. Rather than an engineer determining the offset temperatures required for a particular process, the heater of the present invention simply requires an operator to enter the predetermined process temperature; offset temperatures do not need to be calculated, particularly from process to process. Accordingly, the heater according to the present invention is more efficient, economical, and easy to use than conventional heaters. 
     Another advantage of the heater of the present invention is that temperature lags and overshoots are substantially eliminated. As the internal temperature sensor is positioned immediately downstream from the heat exchanger, the air exiting the heat exchanger is immediately monitored. If any temperature fluctuation occurs, then the controller is able to quickly send a control signal indicative of such fluctuation to the drive to adjust the current applied to the heating elements. Accordingly, the heater is much more responsive to minor fluctuations in air temperature than conventional heaters, resulting in the substantial elimination of temperature lags and overshoots. 
     According to another aspect of the present invention, the heater may include a plurality of heater switches respectively connected to the plurality of heating elements of the heat exchanger. The heater switches enabling each of the heating elements to be manually energizable. This results in the current being applied by the driver to the energized heating elements of the heat exchanger to be inversely proportional to the number of heating elements manually energized. This manual switching on and off of heating elements allows an operator to select any number of heating elements to heat the air for effecting the process. 
     According to another aspect of the present, the heater may further include a three-phase transformer connected between the driver and the heat exchanger. The heating elements are preferably serially configured with the transformer so that a specific firing order of the heating elements is effected. In addition, a plurality of mercury-displaced switches may be respectively connected between the plurality of heating elements and the transformer. 
     The heater of the present invention employs a number of beneficial safety features. For example, according to a further aspect of the invention, the heater has a housing within which the heat exchanger, as well as the other components, is disposed. An emergency temperature sensor may be provided to monitor the temperature of ambient air within the housing. Accordingly, if the temperature within the housing reaches unsafe levels, the emergency temperature sensor may signal an alarm or high-temperature indicator to alert the operator. 
     The air line of the heater also has a number of advantages. For example, a manifold may be connected to the outlet of the air line for easy positioning near the process location and efficient distribution of heated air. Further, the air line is preferably configured to receive compressed air, rather than fan-driven air, for high-velocity, conduction to the process location, which aids in maintaining the temperature of the heated air while moving through the air line. 
     Other aspects, features, and advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view an exemplary embodiment of a high-velocity, accurately responsive impingement heater implemented in accordance with the present invention; 
     FIG. 2 is a block diagram illustrating the relationship between an electrical control system and an air system of the present invention; 
     FIG. 3 is a schematic diagram of a preferred embodiment of a power line of the present invention; 
     FIGS. 4A and 4B are schematic diagrams of control circuitry in accordance with a preferred embodiment of the present invention, with the circuitry of FIG. 4B connected to the circuitry of FIG. 4A at nodes A and B. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, particularly to FIG. 1, a high-velocity, accurately responsive impingement heater  10  is shown in a preferred embodiment. The heater  10  includes a housing  12  which encloses components of the heater  10 , which components may be accessed through a door  14 . A control panel  16  is formed on the housing  12  and includes a number of switches, gauges, and indicator lights, which will be discussed in more detail below. 
     With reference to FIG. 2, the heater  10  includes an electrical control system, shown generally by a single line, and an air system, shown generally by a double line. The electrical control system has a power line  20  connectable to a power supply  22 , for example, a 480-volt, 30-amp, three-phase power supply. The power line  20  may include a safety disconnect switch  24  and a plurality of fuses  26 , for example, 25-amp or 30-amp fuses, for safety and control purposes. 
     The power line  20  is connected to a transformer  28  and a driver unit  30 . The transformer  28  steps down the voltage of the power line  20  to a voltage which is useable by a controller  32 , for example, 115 volts. The output of the driver unit  30  is connected to a second transformer  34  which transforms the power of the power line  20  into a power supply which is useable by a heat exchanger  36 . An over-temperature alarm circuit  37  is preferably coupled to the heat exchanger  36 . An ammeter  38  may be connected to the input of transformer  34 , and a voltmeter  40  may be connected to the output of transformer  34  to monitor these respective parameters. The output of the controller  32  is connected to the driver unit  30 , indicated by reference numeral  41 , which will be described in more detail below. 
     The air system of the heater  10  has an air line  42  for conducting compressed air, having an inlet  44  and an outlet  46 . The air line  42  is provided downstream of the inlet  44  with a plurality of filters  48  and a regulator  50 . An air pressure gage  52  may be provided upstream of the outlet  46  of the air line  42 . A manifold  54  is removably attached to the outlet  46  of the air line  42  for directing and providing the heated air to a process  56 , for example, a curing process. A temperature-control master feed-back loop  58  is provided from a process temperature sensor  95  positioned at the process location  56  to the controller  32 , and a temperature-control slave feed-back loop  60  is provide from an internal temperature sensor  98 , downstream of the heat exchanger  36 , to the controller  32 . 
     With additional reference to FIG. 3, a preferred embodiment of the power line  20  is illustrated. The power supply  20  is preferably 480 volts, three phase, at 30 amps, although other power supplies may be used. The power line  20  includes three lines L 1 , L 2 , and L 3 , each tied in with the safety switch  24  and the fuses  26 . The driver unit  30  is preferably a phase-angle silicon-controlled rectifier (SCR) driver. An example of such a driver is model No. G 33 -480-24-AD1 produced by Watlow Controls of Winona, Minn. The inputs of the driver unit  30  are respectively coupled to the lines L 1 -L 3  of the power line  20  via capacitors, and the outputs of the driver unit  30  are respectively coupled to transformer  34  for stepping down the 480 volts of the power supply  20  to 240 volts, for example. Two of the lines L 2  and L 3  are coupled to transformer  28  for stepping down the 480 volts of the power supply  20  to 115 volts, for example. 
     Transformer  34  has three outputs, each corresponding to one of the three phases of the power supply  20 . The heat exchanger  36  includes a plurality of heating elements  62 , for example, four, to which each of the outputs of transformer  34  is coupled through a mercury-displaced switch  64 . The outputs of transformer  34  are coupled to the heating elements  62  in such a way that a preferable firing order of 1-2-3-4 is effected on heating elements  62   a,    62   b,    62   c,  and  62   d,  respectively. Specifically, the phase-A output is coupled to heating elements  62   a,    62   c,  and  62   d;  the phase-B output is coupled to heating elements  62   a  and  62   b;  and the phase-C output is coupled to heating elements  62   b,    62   c,  and  62   d.    
     The heat exchanger  36  is preferably model No. 007-10137 produced by Convectronics of Methuen,. Mass., with the mercury displace switches  64  preferably model No. KD20-1000-4400 produced by Watlow Controls. In the Convectronics heater exchanger, the heating elements are housed within a ceramic tube which is sleeved within a stainless-steel shroud which, in turn, is placed within a section of stainless-steel pipe for receiving compressed air. Accordingly, compressed air is channeled around the entire heating element for efficient and fast heating. Further, this heat exchanger is also extremely responsive to desired changes in the temperature of the air being heated. Each of the heating elements  62  is preferably rated at up to 6,000 watts, which is controlled by varying the amount of applied current. 
     The general operation of the heater  10  will be provided with additional reference to FIGS. 4A and 4B. The heater  10  is positioned near or adjacent to the process  56  such that the manifold  54  optimally directs heated air to the process  56 . The power line  20  is connected to the power supply  22  which, as mentioned above, is preferable  460  volts, three phase, at 30 amps. The fused safety disconnect, including switches  24  and fuses  26 , receives the power and, once energized, applies line voltage to contactor C and to transformer  28  (which is preferably a 0.5 KVA, 480V-115V step-down transformer). Transformer  28  supplies the control voltage for the heater  10 . Power is supplied to a cabinet fan  70  and a main power available indicator  72 . Power may also be supplied to an emergency-stop circuit  74  and a control power circuit  76 . At this point in the operation, the heater  10  may be considered to be in a “stand-by” mode. 
     The operator sets the desired temperature on the control panel  16  which is inputted into the controller  32 . Alternatively, the operator may program a desired cure or process parameters into the heater  10 , with which the controller  32  is able to carry out the process. In any case, the heater  10  is then started by turning on a control-power switch SW 1 , activating a control-power indicator  77 . Control power is supplied to the SCR driver  30 , a cascade temperature controller  78 , and a first and second alarm boards  80   a  and  80   b.  Upon receiving control power, the cascade temperature controller  78  initiates a start-up sequence, and the SCR driver  30  may activate one or more internal fan motors (not shown). 
     Control power is also supplied to the first mercury-displaced switch  64   a  (MDS 1 ), thus activating a first heating element  62   a  (HTR 1 ). The mercury-displaced switches  64   a-d  are wired in series so that the first heating element  62   a  (HTR 1 ) needs to be enabled before a second heating element  62   b  (HTR 2 ) is able to be energized, and so on, which will be discussed in more detail below. As shown, there are preferably four heating elements  62   a-d  (HTR 1 -HTR 4 ). Mercury-displaced switches are preferably used because of their good performance in volatile environments and because of their safe operation from the lack of contacts which eliminates arcing and sparks. 
     If a high-temperature condition within the heater  10  does not exist, as indicated by a high-temperature indicator  82 , relays  84  connected between the alarm boards  80  energize, closing the circuit therebetween and providing power to a heater-power contactor circuit  86 . With air pressure being applied to the heater  10 , causing compressed air to flow through the heater  10 , activating a start switch  88  will energize a heater power contactor  90 , thereby applying line voltage to the SCR driver  30  via node B. A heater power available indicator  92  will also be energized, indicating available power for the heaters  62 . In addition, a heater-element-hour meter  94  receives power via a switch tied to a first heater switch  96   a  (SW 2 ). At this point in the operation, the heater  10  is ready to generate heat. 
     Depending upon the temperature required to carry out the process  56 , an appropriate number of the heaters  62  are activated. The temperature is entered into the system, and the temperature controller  32  generates an electrical control signal, indicated by reference numeral  41 , which is input into and processed by the SCR driver  30 . The control signal  41  preferably ranges from about 4 mA to about 20 mA and is indicative of the level of power required to maintain the preferred process temperature. Based on the control signal  41 , the SCR driver  30  then applies a voltage, which preferably varies from zero to 450 volts, to stepdown transformer  34 . This applied voltage may be monitored on the voltmeter  40  (with current monitored on the ammeter  38 ). Transformer  34  then applies the stepped down voltage, which may vary from zero to 240 volts, to the heating elements  62 . 
     The controller  32  makes use of the dual-loop configuration of the master feedback loop  58  and the slave feedback loop  60 . Each of the feedback loops  58  and  60  includes a thermocouple  95  and  98 . The thermocouple  95  of the master feedback loop  58  monitors the temperature at process  56 , and the thermocouple  98  of the slave feedback loop  60  monitors the temperature at the output of the heat exchanger  36 . The controller  32  mediates the two signals from the thermocouples  98  and correspondingly adjusts and applies the control signal  41  to the SCR driver  30  to specify the amount of heat required at the process  56 . Upon receiving the control signal  41 , the SCR driver  30  then controls the amount of current applied to the heating elements  62  to produce the amount of required heat. This dual-monitoring process eliminates temperature lags and overshoots commonly associated with conventional devices. 
     The controller  32  and the feedback loops  58  and  60  are preferably configured such that the controller  32  only uses the signal from the slave feedback  58  in conjunction with the signal from the master feedback loop  60  in determining the control signal  41 . For example, if the signal provided by the slave feedback loop  60  indicates that the temperature is changing at the output of the heat exchanger  36  but not at the process  56 , the controller  32  will not generate a control signal. However, if the temperature at the output of the heat exchanger  36  is substantially constant but varying at the process  56 , then the controller  32  will generate a control signal  41  based on the both the master feedback loop  58  and the slave feedback loop  60 . 
     The heater  10  is configured such that each of the heating elements  62   a-d  may be manually turned on or off as needed while power is being applied by means of a respective heater switch  96   a-d.  Accordingly, the amount of current applied by the SCR driver  30  is proportional to and independent of the number of energized heating elements. For example, if two of the heating elements  62  are energized, the current applied to each of the energized heating elements will be proportionally more than if four of the heating elements  62  were energized. Such a situation results in the two energized heating elements operating more efficiently at a higher current than if four heating elements were energized and operating a low current. This situation is analogous to an automobile engine: a four-cylinder engine operating at high Rpm is more efficient than an eight-cylinder engine operating at low Rpm. Further, operating less heating elements  62  at higher currents substantially eliminates current surging and spikes often associated with operating more heating elements at less current per element. 
     Each of heating elements  62   a-d  preferably is connected to a respective indicator  97   a-d  which illuminates when the heating element is energized. The internal ambient temperature of the heater  10  may be monitored by a thermocouple  98 , for example, a type-J thermocouple, and displayed by a temperature indicator  100  provided on the control panel  16 . 
     As mentioned above, the heating elements  62   a-d  are preferably connected in cascaded series so that subsequent heating elements  62  may be enabled only if the preceding heating element is enabled. Each of the heating elements  62   a-d  may be monitored by a thermocouple  102   a-d  which feeds back into a respective circuit on the alarm boards  80   a  and  80   b.  Accordingly, if an unsafe high-temperature condition exists, the respective alarm board relay  84  will de-energize, thereby opening the heater-power contractor circuit  86 , causing line voltage to be removed from the SCR driver  30 , and shutting down all heat being generated by the heating elements  62 . The high-temperature indicator  82  illuminates to warn an operator of the high-temperature condition. In order for the heater  10  to resume normal operation, the high-temperature condition needs to be resolved, and the system needs to be reset, for example, by a reset switch  104 . 
     An unsafe high-temperature condition may result from a loss in air supplied to the air line  42  or a loss in air pressure within the heater  10 . In this case, either the heat exchanger thermocouple  98  or one of the heater thermocouples  102  would trigger a shut down of the system. The air pressure in the air line  42  may be regulated by a remote adjuster located on the control panel  16 , which takes pilot air pressure and feeds the in-line regulator  50  of the heater  10 . The pressure in the air line  42  at the output of the heat exchanger  36  is monitored by the air gauge  52 . A second air gauge may be provided to monitor the air pressure at the input of the heat exchanger  36 . 
     As an additional safety feature of the heater  10  of the present invention, an emergency stop switch or button  106  may be provided which, when activated, terminates all operations of the heater  10 , returning the heater to standby mode. 
     In view of the foregoing, the heater  10  may be configured for the following exemplary process. The manifold  54  may be connected to the outlet  46  of the airline  42  by about 20 feet of piping. The master thermocouple  95  may be positioned within about 2 inches from the orifice of the manifold and within about 4 inches of the location of the process  56 . A process temperature of 350° F., for example, may be entered into the controller  32  from the control panel  16 . Accordingly, the controller  32 , being preconfigured for heat loss from the piping and other variables, energizes the heating elements  62 . The temperature immediately downstream from the heat exchanger  36  may need to be maintained at about 150° F. higher than the process temperature, or at 500° F., in order to provide 350° F. air to the process  56 . As the temperature stabilizes at the process  56 , less power is required to maintain a relatively constant process temperature. The controller  32  generally has a ±5° F. bandwidth and, coupled with the thermocouples  98 , is preferably sensitive to at least about 2° F. 
     The heater  10  may be used to providing air heated up to about 600° F. but is most efficient at providing air heated in the range of about 250° F. to 350° F. The heater  10  may be configured with a higher output heat exchanger  36  and heating elements  62  to yield higher temperatures. 
     FIG. 5 illustrates an internal view of the housing  12  of the heater  10 . The housing  12  encompasses a substantial portion of the electronics used to operate the heater  10 , such as the power supply  22 , transformer  28 , controller  32  and drivers  30 . The housing  12  includes the transformer  34  and the elements relating to the generation and sensing of the hot impingement air flow. These elements include the air line filter  48 , regulator  50 , heat exchanger  36 , air pressure gauge  52 , internal temperature sensor  98  and emergency temperature sensor  102 . 
     The air line filter  48  is downstream of the air intake pipe  44  protruding out of the heater housing  12 , and the outlet pipe  46  is downstream of the air pressure gauge  52 . The outlet pipe  46  connects to the removably attached manifold  54 , whose end thereof includes the process temperature sensor  95  for sensing the temperature of the process  56 . The internal heat sensor  98  is positioned immediately downstream of the heat exchanger to effectively measure the temperature of the air flow thereat. The emergency temperature sensor  102  is positioned within the lower compartment of the housing  14  for monitoring the ambient temperature within the housing  14 . 
     Those skilled in the art will understand that the preceding exemplary embodiments of the present invention provide foundation for numerous alternatives and modifications. For example, the principles of the present invention may be employed with other types of heaters such as gas heaters. These other modifications are also within the scope of the appended claims of the present invention. Accordingly, the present invention is not limited to that precisely shown and described herein.