Conveyor oven having an energy management system for a modulated gas flow

Abstract

A conveyor oven has a modulating gas valve which raises and lowers the thermal output of a burner that heats the oven. A bypass line passes a minimum amount of gas to prevent the burner from completely shutting down during periods while the oven remains above a threshold demand for more heat. electrical isolation is provided between temperature controlling devices to prevent an electrical feed back which might otherwise prevent a complete shut down of the oven during hazardous conditions.

Description

This is a continuation-in-part of Ser. No. 09/760,194 filed Jan. 12, 2001 which and, blower fan compartment compartments 77 which together define an air supply plenum 78 between them. This and two air supply plenums 78. Each plenum is pressurized by hot air that is driven into the oven via upper and lower fingers 79, 80, respectively. A conveyor 81 passes between the lingers 79, 80, carrying food products which are baked by the heat of the hot air discharged from the fingers. After the hot air is emitted from the fingers, it passes between the fingers and into an air return plenum 82, 83 above and below the fingers.

The oven contains three thermal couples which are seen in line in FIG. 7A so that only a high limit thermocouple sensor 84 appears. The high limit sensor 84 causes the oven to shut down if a predetermined threshold temperature is reached. This shut down is well below a hazardous level. The other two of these thermocouples which feed controller 32 are the control sensors 85, 86 seen in the plan view of FIG. 7B.

The controls are in a compartment 87 at the front of the oven which is cooled by fans seen in FIG. 8.

FIG. 8 is a circuit diagram showing a control section 108 relating to the mechanical aspects and a control section 110 relating to the energy management system of the inventive oven.

The mechanical aspects 108 include blower motors 113, 114 which are connected to a suitable commercial power source 112 via switches (such as 115) and circuit breakers (such as 116). The blowers 76 77 are in the plenum 82 plenums 78 to drive a stream of hot air into the oven cavity which is recycled between the fingers and back into the plenum. The high limit thermostat 118 does not normally provide an active function unless the temperature in the oven exceeds some preset threshold safety limit, beyond which the oven might go into a runaway condition. If that limit is exceeded, the contacts 120 open to shut down the oven while contacts 122 close to light a reset pilot lamp 124.

Next, there is a series circuit 126 of door switches which will prevent the oven from Operating unless all doors are suitably closed. If there are any other mechanical parts which have to be in any particular condition for the burner to switch on, associated switches (not shown) may also be provided in the series circuit 126.

A power transformer 128 provides power to the conveyor motor 130 which is turned on/off at switch 132. The motor has a suitable sensing mechanism for maintaining a stable speed selected at and under the control of a known circuit in control box 134.

The energy management system 110 controls the heat in the oven by modulating the delivery of gas to the burners. More particularly, a set of switches 135 either enables or disables the blower. If the blower shuts down, the contacts 136A open and the oven cannot be placed in operation until the manual switch 136B is closed. A coil 146 is energized when the burner blower switch 135 is closed to provide an interlock when the blower is on. The two fans 142, 144 blow cool air over the electronic and other controls in the control compartment 87 in order to prevent a malfunction as a result of over heating.

The temperature controller 32 receives signals from two sensors 85, 86 located at plenum in plenums 78 in the front end of a hot air stream for supplying heated air in the oven. The controller 32 is a product of the Honeywell Company. The switches 152 detect the presence of the various air streams in the oven. Basically, these switches have air sails in the oven at a location where they are moved by the air stream in order to open or close electrical switches. These sail switches are simply on/off switches to indicate the presence or absence of the air stream. Switch 155 is closed in order to heat the oven.

First and second isolation transformers 156, 157 supply 24V power for the electronic equipment coupled to their secondary windings respectively. In particular, transformer 156 supplies an ignition module 158 while transformer 157 supplies signal conditioner 31, thereby isolating the two from each other. The controller 32 is supplied from the power line 112. Signal conditioner 31 responds to signals which it receives from the controller 32 and converts them into signals which control the modulating valve 30. A push button 159 must be pressed to reset the modulating valve 30 after it shuts down.

An ignition module 158 is adapted to ignite and maintain a pilot flame that initiates the gas of burner 50 (FIG. 4). An associated sensor 160 signals the presence of a pilot flame. The safety gas valve shown at 29 has the characteristics prescribed by an appropriate governmental regulation in order to shut down the oven under predetermined hazardous conditions.

Means are provided to enhance the integrity of the safety features. Accordingly, even though there are many safety features on the oven so that it automatically shuts down well before any catastrophic condition is reached, it is conceivable that there could be a feed back condition in a loop extending between the controller 32, the signal conditioner 31, and the ignition module 158, which might interfere with the response of the safety valve 29.

Therefore, it is desirable to provide a safety integrity enhancement by the electrical isolation between the controller 30, conditioner 31 and ignition module 158, which would prevent such feed back.

The power line voltage (here 240V) applied at 112 appears across line conductors 162 and 164. Controller 32 is connected directly across the line so that there will not be any feed back to it via the lint 162, 164 from either signal conditioner 31 or the ignition module 158. The two transformers 156 and 157 have primary windings 168 and 170 connected in parallel across the power line conductors 162, 164, so that the primaries are not affected by events after the secondaries. The secondary winding 172 of transformer 156 supplies the 24V power to ignition module 158 and to the governmental prescribed safety switch 29. The secondary winding 174 of transformer 174 supplies 24V to the signal conditioner 31 and modulating valve 30.

Hence, there is no single path forming a feed back loop for causing an interaction between the controller 32 and conditioner 31, and ignition module 158.

A layout of a front panel control panel is shown at the top of FIG. 9. There are a display panel and eight push buttons. A list of control panel functions are shown at the bottom of FIG. 9 to help explain the features of controller 32. The push button “1” is a blank which has no preselected purpose, but which is available to control any customized feature which the owner of the oven may request.

A four digit numeral display 200 displays either the process variables or the setpoint during normal operation. The right-most digit of this display shows whether the displayed value is in terms of degrees in Fahrenheit or in Centigrade. Alternate information is also displayed during the service depending upon the function being carried out.

A “heat on” indicator is illuminated when the controller is applying an output of 3% or more heat. When an on/off control is configured, the “heat-on” indicator is illuminated when an output heat is provided. Either the present temperature or the programmed setpoint temperature may be displayed. A simultaneous pressing of both the unlock and the display keys 208, 202 will display the value of the heat output with the heat on indicator on. With the output unlocked at key 208, the user is able to cycle through actual temperature display indicators to reveal the actual temperatures, setpoint temperature, and the heat on condition. When the setpoint displays “yes”, it is not possible to change the setpoint.

A locked setpoint condition can be temporarily unlocked at key 210 for making an adjustment of the programmed oven temperature. Sixty seconds later, the setpoint automatically returns to the locked state if no further control operations are carried out after the last press of the display key 202. When the 60 second time period expires, the controller 32 is locked and the display return to the default display.

The output display is automatically locked in a default display when a service man or operator places the controller in the service mode by pressing key 204 or when the hidden key 206 is pressed while the output display is shown. A failsafe condition occurs when any one of the various tests fail, at which time, a flashing signal display is alternately displayed with temperature.

If the oven has not reached 200° F. within fifteen (15) minutes after an initial power-up of the oven, a message is flashed on the display panel indicating that the controls need to be reset (power-cycled). If a thermocouple sensor fails to operate properly, the display will flash “open”. If the polarity of the thermocouple leads are reversed, the display will flash a signal indicating the thermocouple leads are incorrectly connected to the instrument.

The displays also provide prompts for servicing the oven responsive to a simultaneous pressing of the unlock key 208 and the service tool key 204. Each additional press of the service tool key 204 advances the prompts in the order shown under “service mode” in FIG. 9. The user can continually sequence through the service prompts by repeatedly pressing the service tool key 204. The service mode is exited by either pressing the display key 202 or pressing no key for sixty seconds. Either way, the system automatically returns to the normal mode.

Setpoint lock key 210 automatically flashes the temperature that has been selected for an operation of the oven. The setpoint can be changed up or down by pressing either the increment or decrement keys 212, 214. The degrees (° F. or ° C.) used for the prompts is changed by pressing either the increment or decrement keys. While at the degrees ° F. or ° C. prompt, a selection of “F” or “C” automatically changes the units of all the display to ° F. or ° C. While the default display prompt is being displayed, an indicator flashes to indicate which display is chosen as the default display, which can be changed by pressing either the increment or decrement keys.

Those who are skilled in the art will readily perceive various modifications that fall within the scope and spirit of the invention. Therefore, the appended claims are to be construed to cover all equivalents.

Claims

1. A conveyor oven control comprising an energy management system for controlling a flowing stream of hot air through a cavity of an oven for baking a food product; a conveyor extending through said cavity for conveying said food product through said oven; a fuel gas line for conveying fuel gas from a source to a burner in said oven; said energy management system being interposed in said fuel gas line and between said source and said burner for controlling a flow of fuel gas to said burner; said energy management system comprising a controller, a signal conditioner, a fuel gas valve, and an ignition module; a safety shut down valve associated connected with said ignition module to prevent said ignition module from re-igniting said burner during hazardous conditions; a pair of sensors in said oven for sensing instantaneous oven temperatures; said controller being responsive to said sensed temperatures for controlling said energy management system to regulate operation of said fuel gas valve and, thereby, the flow of fuel gas to the burner in order to maintain oven temperature within a predetermined range, said signal conditioner being responsive to said controller for providing signals that control said fuel gas valve; and a pair of transformers, one transformer being coupled to energize said signal conditioner and to electrically isolate said signal conditioner from said ignition module, and the other transformer being coupled to energize said signal ignition module and to electrically isolate said ignition module from said signal conditioner, the pair of transformers connected in parallel with each other and with said controller to prevent an electrical feed back which might otherwise defeat the safety shut down valve between said signal conditioner and said ignition module.

2. The conveyor oven of claim 1 wherein said fuel gas valve comprises a diaphragm closing a chamber having an internal pressure controlled by fluctuations of oven temperatures, a main valve in said fuel gas valve coupled to move with said diaphragm for regulating an amount of fuel gas flowing from said source through said fuel gas line to said burner in response to movement of said diaphragm, and a tap line for applying said pressure in said chamber acting on said diaphragm in response to said sensors whereby the flow of said fuel gas through said main valve in said fuel gas valve to said burner is regulated as a function of said instantaneous oven temperature.

3. The conveyor oven of claim 2 and a by-pass line for enabling a limited amount of fuel gas to flow around said main valve of said fuel gas valve whereby said burner continues to burn and does not shut down while said main valve is closed.

4. The conveyor oven of claim 1 wherein said burner heats air in a plenum at an input end of said oven from which hot air is driven through said cavity, said sensors being located at different places in a plenum.

5. A conveyor oven for automatically baking a food product over a timed period under the control of an energy management system, said oven comprising a cavity having a burner associated therewith for providing heated air in said cavity, a fuel gas line for delivering fuel gas to said burner via said energy management system, said burner heating air in a plenum, a system for delivering said stream of heated air from said plenum through said cavity and returning at least a portion of said stream to said plenum, a pair of sensors at different locations in said plenum for sensing an instantaneous temperature of said heated air in said plenum, a valve for modulating the fuel gas delivered to said burner responsive to said sensed instantaneous oven temperature, said delivered fuel gas comprising at least a minimum amount of fuel gas so that said burner means remains in continuous operation regardless of said modulation of fuel gas delivered to said burner, a controller responsive to said sensors for providing signals for regulating said fuel gas valve, a signal conditioner, and an ignition module, said signal conditioner converting said signals provided by said controller into control signals for operating said fuel gas modulating valve, and a pair of transformers, one transformer being coupled to energize said signal conditioner and to electrically isolate said signal conditioner from said ignition module and the other transformer being coupled to energize said ignition module and to electrically isolate said ignition module from said signal conditioner, wherein the pair of transformers are connected in parallel with each other and with said controller to prevent an electrical feedback between said signal conditioner and said ignition module.

6. The oven of claim 5 and a conveyor for delivering a food product through said cavity over a timed period during which said burner continuously delivers heat to said cavity, said heat baking said product as it is conveyed through said oven, and a line for bypassing said minimum amount of fuel gas around said fuel gas modulating valve in order to prevent said burner means from shutting down during periods while said controller is not calling for heat.

7. An energy management system for a conveyor oven that bakes a food product during a passage through said oven, said energy management system delivering a flowing stream of hot air from a burner and over said food product during said passage and comprising a fuel gas valve, a controller for sensing and regulating the temperature of said stream of hot air, a signal conditioner responsive to said controller for producing signals required to operate said fuel gas valve, an ignition module, and a pair of transformers, one of said transformers delivering power to and providing electrical isolation for said signal conditioner, and the other of said transformers delivering power to and providing electrical isolation for said ignition module, wherein the pair of transformers are coupled in parallel with each other and with said controller to prevent an electrical feed back signal between them the signal conditioner and the ignition module.

8. The system of claim 7 wherein said fuel gas valve has a pressure chamber closed by a diaphragm which expands and contracts in response to the pressure in said chamber, said pressure increasing and decreasing in said chamber jointly responsive to said controller and said signal conditioner as a function of the temperature of said stream of hot air, a fuel gas line, and a main valve in said fuel gas line, said main valve being connected to said diaphragm whereby said main valve opens and closes as said diaphragm expands and contracts in order to modulate a flow of fuel gas in said line.

9. The system of claim 8 and a by-pass line around said main valve, said bypass line delivering enough fuel gas to continuously maintain said burner in at least a minimum heat condition despite operation of said main valve responsive to said diaphragm.

10. A conveyor oven comprising a modulating fuel gas valve for supplying heat to said oven; a controller; a signal conditioner for converting signals from said controller into signals for operating said fuel gas modulating valve; an on/off safety valve in said fuel gas line to automatically shut down said oven during predetermined conditions; an ignition module for igniting fuel gas delivered by said safety valve; and an electrical control circuit for operating said fuel gas modulating valve, controller, conditioner, and ignition module, said circuit having a first section relating to mechanical parts of the oven and a second section relating to energy management of heat delivery in said oven, and said electrical control circuit having electrical isolation for preventing a feed back of electrical signals which might prevent shut down of said safety valve during said predetermined conditions between said signal conditioner and said ignition module;

wherein said second section includes first and second transformers coupled in parallel with each other and with said controller to provide said isolation between their secondary windings of the first and second transformers, one of said secondary windings supplying power to and electrically isolating said signal conditioner and not supplying power to said ignition module, and the other of said secondary windings supplying power to and electrically isolating said ignition module and not supplying power to said signal conditioner, said safety valve being coupled to supply fuel gas via said ignition module.

11. The oven of claim 10, and a pair of sensors for detecting heat in said oven, said sensors being coupled to drive said controller, said signal conditioner being coupled to operate said fuel gas modulating valve, and a coupling from said controller to said signal conditioner whereby heat detected by said sensors controls said fuel gas modulating valve.