Shaw, James Thomas (96 Lady Margaret's Avenue, Deeping St James, Peterborough PE6 8TQ, GB)
Hodge, David Anthony (168 Stonald Road, Whittlesey, Peterborough PR7 1QR, GB)
Clair-thompson St., Stephen John (21 Crocket Lane, Empingham Oakham, Rutland LE15 8PW, GB)
Shaw, James Thomas (96 Lady Margaret's Avenue, Deeping St James, Peterborough PE6 8TQ, GB)
Hodge, David Anthony (168 Stonald Road, Whittlesey, Peterborough PR7 1QR, GB)
| 1. | A method of monitoring the pressure of a location (01, 02,03, 04,05) within the interior of a temperaturecontrolled enclosure (3) comprising providing a conduit (Cl, C2, C3, C4, C5) which is adapted to allow air to flow substantially freely therethrough, and positioning one end of the conduit at said location, the opposite end of the conduit being located at a reference position remote from said location at which the temperature is normally different from the temperature of said location whereby a difference in air pressure between said region and said reference position can give rise to a flow of air through said conduit, and providing a temperature sensor (T1, T2, T3, T4, T5) arranged to respond to the temperature of the air passing through the conduit. |
| 2. | A control system for a conveyor oven (3) comprising a plurality of conduits (C1, C2, C3, C4, C5) which are adapted to extend from positions exterior to the oven casing to spacedapart positions interior to the oven casing, the conduits being provided with a respective temperature sensor arranged to respond to an outflow of hot oven gases through the respective conduit, and a controller (10) which is responsive to the outputs of the plurality of temperature sensors to provide a control signal which is suitable for adjusting air flow through the oven. |
| 3. | An oven control system as claimed in claim 2 in which the inner ends (01, 02,03, 04 05) of the conduits (C1, C2, C3, C4, C5) are positioned in use at different heights in the oven (3). |
| 4. | An oven control system as claimed in claims 2 or claim 3 in which the conduits (C1, C2, C3, C4, C5) extend substantially horizontally. |
| 5. | An oven control system as claimed in claim in any one of claims 2 to 4 in which at least one of the conduit inner ends (01, 02, 03, 04,05) is positioned below the level at which products are conveyed in use through the oven (3). |
| 6. | An oven control system as claimed in any one of claims 2 to 5 in which at least one of the conduit inner ends (01, 02,03, 04,05) in positioned above the level at which products are conveyed in use through the oven (3). |
| 7. | An oven control system as claimed in any one of claims 2 to 6 in which at least one of the conduit inner ends (01, 02,03, 04,05) is positioned at substantially the level at which a product is conveyed in use through the oven (3). |
| 8. | An oven control system as claimed in any one of claims 2 to 7 in which the air flow into or out of the oven (3) is controlled primarily by use of a variable speed extraction fan (1), the speed of the fan being determined by the controller (10) in response to the measurements of the plurality of temperature sensors (T1, T2, T3, T4, T5). |
| 9. | A method of controlling the movement of air into and out of the ends of a conveyor oven comprising monitoring the temperatures of the air within a plurality of conduits (C1, C2, C3, C4, C5) extending through the oven casing (3) to positions (01, 02, 03, 04, 05) internal to the oven casing, in order to determine by temperature measurement which conduits are conveying hot air out of the oven, and which conduits are conveying cold air into the oven, and controlling air flow through the oven in response to the temperature measurements. |
| 10. | A multizone oven comprising a partition wall (Wl2) with a conduit (O12A, O12B, O12C) extending therethrough, opposite ends of the conduit communicating with the atmosphere of the zones (Zi, Zg) on opposite sides of the partition wall, the conduit containing a thermocouple (T12A, Tl2B, T12C), and the output of the thermocouple being connected to a controller (10) which is so arranged as in use to adjust the airflow passing through an exhaust outlet from one of those zones in response to the output of the thermocouple. |
| 11. | An oven as claimed in claim 10 in which control action is effected by use of variable speed exhaust fans (EF1, EF2, EF3) associated with respective zones (Zl, Z2, Z3). |
| 12. | An oven claimed in claim 10 in which a common exhaust is be employed, and the exhaust outlets from the respective zones are provided with respective adjustable dampers the settings of which are determined, at least in part, by the controller. |
| 13. | An oven as claimed in claim 11 or claim 12 in which one or more of the zones is provided with an individual zone air inlet (I1, I2, I3) to avoid a buildup of humidity within that zone (Zl, Z2, Z3). |
| 14. | An oven as claimed in claim 13 in which the size of the zone air inlet (It, I2, I3) is controlled by an adjustable damper (Dl, D2, D3) in response to the output of a humidity sensor (DD2) associated with that zone (Zl, Z2, Z3). |
| 15. | An oven as claimed in claim 13 in which the height of the baked products is measured and an adjustable air damper (D2) on a zone air inlet (I2) is adjusted by a suitable controller (20). |
| 16. | An oven as claimed in any one of claims 10 to 15 in which the conduit (C1, C2, C3, C4, C5) is provided with two thermocouples (TIA ; T2A; T2B; T3A; T3B; T4A; T4B; T5A; T5B), the temperature sensitive elements of the thermocouples being located respectively in portions of the conduit that are located on opposite sides of an oven wall (4) through which the conduit extends. |
| 17. | An oven as claimed in any one of claims 10 to 16 comprising reference thermocouples (A, Z), a first reference thermocouple being arranged to measure the temperature (TA) of the atmosphere in the region adjacent to one end of the conduit (24'), and another reference thermocouple being arranged to measure the temperature (Tz) of the atmosphere in the region adjacent to the other end of the conduit (24'). |
| 18. | A control system for a temperaturecontrolled enclosure, the control system comprising a first comparator (31) arranged to compare the output of the first thermocouple (X) with a first reference thermocouple (TA) to provide a control signal to a motor (33), which is either a variable speed motor driving a fan, or is a motor controlling a damper, and a second comparator (32) arranged to compare the output of a second thermocouple (Y) with the output of the second reference thermocouple (Z), the outputs of the first and second comparators being arranged to affect the motor in opposite senses. |
| 19. | A control system as claimed in claim 18 in which each of the first thermocouple (Tx), the first reference thermocouple (TA), the second thermocouple (Ty) and the second reference thermocouple (Tz) are provided with respective conduits (A, X, Y, Z), the first thermocouple measuring the temperature of a region in the respective conduit which is exterior to the temperaturecontrolled enclosure and the second thermocouple measuring the temperature of a region of the respective conduit which is internal of the temperaturecontrolled enclosure, the conduits of the first thermocouple, the second thermocouple and the second reference thermocouple extending through the oven outer casing (4). |
| 20. | An oven as claimed in any one of claims 10 to 17 and comprising an elongate housing for the thermocouple connections, said thermocouple housing (26', 26") extending substantially transverse to the length of the conduit (24'), the outer end of the thermocouple housing projecting from the oven outer casing (4). |
| 21. | An oven as claimed in claim 20 in which the conduit extends substantially horizontally through the outer casing (4) of an oven, and is provided with two thermocouples (X, Y), a first of which has a temperature sensitive element in a region of the conduit (24') that is external to the oven casing, and the second of which has a temperature sensitive element located in a region of the conduit that is within the oven, the conduit being arranged to extend through the oven casing at an acute angle thereto, and a thermocouple housing (26") for the second thermocouple projecting substantially transversely of the conduit and through the oven casing such that the second thermocouple is accessible from outside the oven. |
This invention relates particularly, but not exclusively, to oven control systems such as control systems for baking ovens.
Conveyor ovens are widely used in the baking industry for baking. The products are carried through the oven on a metal conveyor band. At the entrance and exit to the conveyor oven there will be a tendency for air to flow into the oven or out of the oven depending upon the pressure difference between the interior of the oven and the outside pressure.
For process and thermal efficiency reasons it is desirable to minimise the inflow or outflow of air at the open ends of a conveyor oven. One typical process reason is the desirability of maintaining oven atmosphere temperatures and humidities for the full extent of the oven between the entrance and exit.
The amount of this air movement through the ends of the oven is dependent on a plurality of variable factors, which are outside of the oven designer's control, for example the amount of product flow through the oven and the operator setting of extraction dampers and/or exhaust fan speed.
Attempts have been made to produce an oven control system that employs a highly sensitive pressure transducer to measure the difference in
pressure between a position inside the oven and the external atmospheric pressure. It is, however, difficult using conventional pressure transducers to achieve an accurate measurement of the pressure at a point in the oven and, moreover, it has been found difficult to set up and maintain effective control of the oven inflow/outflow of air utilising such pressure transducer outputs.
According to one aspect of the invention a method of controlling the movement of air into and out of the ends of a conveyor oven comprises monitoring the temperatures of the air within a plurality of conduits extending through the oven casing to positions internal to the oven casing, in order to determine by temperature measurement which conduits are conveying hot air out of the oven, and which conduits are conveying cold air into the oven, and controlling air flow through the oven in response to the temperature measurements.
The conduits preferably extend substantially horizontally.
The basis for the control action is that if the inner end of a particular conduit is located at a point in the oven where the pressure is greater than the ambient atmospheric pressure, there will be an outward flow of hot gases through the conduit which will give rise to an elevated temperature at the temperature sensor associated with the external portion of that conduit. If, on the other hand, another conduit inner end is positioned at a point where the local oven pressure is at atmospheric pressure, or below, then there will be no similar heating effect of the temperature sensor associated with that other conduit. Accordingly the control system is able to utilise the differences and rates of change of difference in temperature sensed by different temperature sensors associated with different conduits to monitor local pressure differences in
the oven, and such signals can be used by the control system to provide a more stable control of the oven atmosphere.
The air flow through the oven can then be automatically adjusted by the control system by means of adjusting one or more exhaust dampers, and/or by varying the speed of one or more exhaust fans.
According to a second aspect of the invention a control system for a conveyor oven comprises a plurality of conduits which are adapted to extend from positions exterior to the oven casing to spaced-apart positions interior to the oven casing, the conduits being provided with a respective temperature sensor arranged to respond to an outflow of hot oven gases through the respective conduit, and a controller which is responsive to the outputs of the plurality of temperature sensors to provide a control signal which is suitable for adjusting air flow through the oven.
Preferably the inner ends of the conduits are positioned in use at different heights in the oven.
Preferably the conduits extend substantially horizontally.
Preferably at least one of the conduit inner ends is positioned below the level at which products are conveyed in use through the oven.
At least one of the conduit inner ends in preferably positioned above the level at which products are conveyed in use through the oven.
Preferably at least one of the conduit inner ends is positioned at substantially the level at which a product is conveyed in use through the oven.
The air flow into or out of the oven is preferably controlled primarily by use of a variable speed extraction fan, the speed of the fan being determined by the controller in response to the measurements of the plurality of temperature sensors.
The control system may be responsive to another parameter, such as the humidity of the oven atmosphere, and that other parameter may be the main control parameter.
According to a third aspect of the invention a method of monitoring the pressure of a location within the interior of a temperature-controlled enclosure comprises providing a conduit which is adapted to allow air to flow substantially freely therethrough, and positioning one end of the conduit at said location, the opposite end of the conduit being located at a reference position remote from said location at which the temperature is normally different from the temperature of said location whereby a difference in air pressure between said region and said reference position can give rise to a flow of air through said conduit, and providing a temperature sensor arranged to respond to the temperature of the air passing through the conduit.
Preferably the conduits extend substantially horizontally.
The temperature of the air passing through the conduit will depend upon the direction of the air flow through the conduit since the air flow is between regions of different temperatures, and accordingly the temperature sensed will be a measure of the relative pressures between said location and said region.
The opposite ends of the conduit may be located at different locations that are both positioned within the enclosure. For example, in an
enclosure that is arranged to provide different temperature regimes at different regions within the enclosure, then the opposite ends of the conduit may be located in said different temperature regions.
One preferred arrangement is in a so-called'multi-zone oven', small tubes being arranged to extend through the wall between two zones of the oven, the tubes each being fitted with a respective thermocouple which will provide a measure of the differential pressure between the two zones.
In a multi-zone conveyor oven partition walls divide the interior of the oven into different zones, and each zone is provided with a respective exhaust outlet. The conveyor passes through an aperture provided in each partition wall. In order to help maintain the integrity of a zone, it is desirable to control or minimise the airflow going into a zone from its neighbours.
In accordance with a fourth aspect of the invention in a multi-zone oven we provide a partition wall with a conduit extending therethrough, opposite ends of the conduit communicating with the atmosphere of the zones on opposite sides of the partition wall, the conduit containing a thermocouple, and the output of the thermocouple being connected to a controller which is arranged to adjust the airflow passing through an exhaust outlet from one of those zones in response to the output of the thermocouple.
The control action is preferably effected on the exhaust of gases from the respective zones. This is preferably achieved by use of a variable speed exhaust fan associated with a respective zone. Alternatively, a common exhaust fan may be employed, and the exhaust outlets from the respective
zones are provided with respective adjustable dampers the settings of which are determined, at least in part, by the controller.
One or more of the zones may be provided with an individual zone air inlet to avoid a build-up of humidity within that zone. The zone air inlet can be open to atmosphere. The size of the zone air inlet may be controlled by an adjustable damper in response to the output of a humidity sensor associated with that zone.
Alternatively, since the humidity in a zone may affect the dimensions of a baked product, such as a biscuit, the height of the baked products may be measured and an adjustable air damper on a zone air inlet may be adjusted by a suitable controller.
Instead of providing a conduit with a single thermocouple, there can be advantage in providing a conduit with two thermocouples, the temperature sensitive elements of the thermocouples being located respectively in portions of the conduit that are located on opposite sides of an oven wall through which the conduit extends. The oven wall can be the exterior casing of the furnace, or an interior wall separating two zones of a multi-zone oven.
It will be appreciated that in the absence of any hot gas flowing through the conduit, due to the opposite ends of the conduit being exposed to the same pressure, the temperatures of the thermocouple elements is likely to be different due to differences in the temperature of the surroundings, even when the elements are insulated against conduction from the thermocouple supports.
Reference thermocouples are preferably provided, a first reference thermocouple being arranged to measure the temperature of the
atmosphere in the region adjacent to one end of the conduit, and the other reference thermocouple being arranged to measure the temperature of the atmosphere in the region adjacent to the other end of the conduit.
Thus, in the case where the conduit extends through the oven casing, one reference thermocouple may have the temperature sensitive element thereof exposed to the furnace atmosphere, and the other thermocouple may have the temperature sensitive element thereof exposed to the external atmosphere, whereas in the case where the conduit extends through an internal wall of an oven, the reference thermocouples may be exposed respectively to the oven atmospheres on opposite sides of said internal wall.
One preferred control system for use with two such reference thermocouples and two such thermocouples sensing gas flow through a single conduit, is one which comprises a first comparator arranged to compare the output of the first thermocouple with the first reference thermocouple to provide a control signal to a motor (which is either a variable speed motor driving a fan, or is a motor controlling a damper) and a second comparator arranged to compare the output of the second thermocouple with the output of the second reference thermocouple, the outputs of the first and second comparators being arranged to affect the motor in opposite senses.
We have appreciated the fact that it could be disadvantageous for the thermocouple mounts and connections to be accessible only from inside the oven. In order to overcome this difficulty, in cases where the temperature sensitive element of the thermocouple is positioned in a portion of a conduit that is located within the oven, we prefer to provide an elongate housing for the thermocouple connections, said thermocouple housing extending substantially transverse to the length of the conduit,
the outer end of the thermocouple housing projecting from the oven outer casing.
In one preferred arrangement where the conduit extends substantially horizontally through the outer casing of an oven, and is provided with two thermocouples, a first of which has a temperature sensitive element in a region of the conduit that is external to the oven casing, and the second of which has a temperature sensitive element located in a region of the conduit that is within the oven, the conduit is preferably arranged to extend through the oven casing at an acute angle thereto, and a thermocouple housing for the second thermocouple projects substantially transversely of the conduit and through the oven casing such that the second thermocouple is accessible from outside the oven.
The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 schematically illustrates a conveyor oven fitted with a control system in accordance with the invention, Figure 2 is a partial longitudinal schematic cross-sectional view of a multi-zone tunnel oven fitted with a control system in accordance with the invention, Figure 3 is a view similar to Figure 2 but showing a control system in accordance with the invention fitted to one zone of a multi-zone oven in which zones are provided with individual air inlets to provide zone integrity, Figure 4 shows an oven similar to that of Figure 3 but including a humidity control,
Figure 5 shows an oven similar to that of Figure 3 but including a product height detector controlling a zone air inlet, Figure 6 is an enlarged vertical cross-sectional view of a conduit extending through an oven wall and provided with a thermocouple in accordance with the invention, Figure 7 is an enlarged horizontal cross-sectional view of a conduit and thermocouple assembly in accordance with the invention, the conduit extending through a partition wall of an oven between two zones of the oven, Figure 8 shows a modification of the control system of Figure 1 in which each conduit is provided with two thermocouples, one of which senses the temperature of gas in the conduit at a position external to the oven casing and the other senses the temperature of gas in the conduit at a position internal to the oven casing, Figure 9 shows a modification of the control system of Figure 2 in which respective thermocouple pairs are provided for each conduit extending between one zone and the adjacent zone, Figure 10 is a horizontal cross-section of a modification of the conduit and thermocouple assembly of Figure 6 and showing two thermocouples associated with the conduit, Figure 11 is a control schematic showing how the outputs of the various thermocouples of Figure 10 can be used to control a motor driving a fan, or controlling a damper, and
Figure 12 is a schematic perspective view of an alternative control system to that shown in Figure 10.
In the illustrated system a variable speed extraction fan 1 is mounted in conventional manner on the roof 2 of the oven casing 3. It will be appreciated by the skilled person that the position of the fan is chosen to suit requirements. The fan is preferably located in the middle of the roof of a single zone oven, or leading from the middle of each zone of a multi-zone oven.
As shown, a plurality of conduits Cl to C5 are positioned at different heights and project through one side-wall 4 of the oven casing 3. Each conduit Cl to C5 projects inwardly of the oven casing 3 a short distance whereby the difference between local oven pressures at regions O1 to 05, at the inner ends of the conduits, and atmospheric pressure external to the oven determines the rate of flow of oven gases through the respective conduits.
Thermocouples T1 to T5 extend into the bore of the respective outwardly projecting portions of the conduits Cl to C5 to enable the temperature of the hot air passing through the respective conduits to be sensed by the thermocouples. It will be appreciated that the thermocouples need to be sufficiently spaced from the wall 4 so as not to be too greatly affected by the proximity of the oven itself. Figure 6 shows a suitable arrangement of conduit and thermocouple.
Typically the conduits have a length of 400 mm on either side of the wall through which they extend, and have an internal diameter of 10 mm.
Typically the conduits are made from stainless steel.
The outputs S1 to S5 of the thermocouples T1 to T5 are fed to a controller 10 which is configured to look for predetermined patterns of the outputs S1 to S5 and to make adjustments to the speed of fan 1 by a control signal CS depending upon the deviation of the signals S1 to S5 from a particular predetermined pattern that is appropriate to the required baking/treatment conditions for a particular product.
Figure 2 shows the invention applied to a multi-zone conveyor oven having three zones Z,, Z2, Z. The invention is shown applied to the second zone, zone Z2, of the oven.
As shown in Figure 2, at suitable heights relative to the inter-zonal product interface point small tubes O12A, O12B, O12C are positioned through the wall W, 2 between the zones Z,, Z2. A respective thermocouple T12A, T12B, T12C is fitted to each tube. Similarly tubes OzjA. 0238, 023c extend through the partition wall W2, between zones Z2 and Z,, and are fitted with respective thermocouples T23A, T23B, T23C respectively. If thermocouple T12A registers approximately, say, zone 1 temperature then zone 1 is pressurised relative to zone 2 at that point, but if it registers, say zone 2 temperature, then zone 1 is depressurised relative to zone 2 at that point.
A controller 10 system is connected to the thermocouples T12A, T12B, T12C, T23A, A, T23B, T23C. It can be arranged that the operator chooses at which thermocouple the balance point is to be established. Alternatively, the choice of thermocouple to use as the balance point may be determined according to predetermined parameters appropriate to a particular product. The control loop can be arranged to control the speed of the exhaust fan EF2 of zone 2 to maintain this balance point. In this way the airflow between zones Z1 and Z2 can be maintained constant, independently of operator settings and product throughput.
Figure 3 shows a modified oven in which each of zones Z1, Z2, Z3 is provided with a respective air inlet 1,, 1, and13. The purpose of providing an air inlet is to help avoid a build-up of moisture in a zone.
Figure 4 shows a further modification in which the air inlet I,, Is and I3 are provided with respective adjustable dampers D1, D2 and D, respectively. The damper D is shown in Figure 4 as being controlled by a humidity controller HC2 in response to a humidity detector DD2 located in zone Zz. Zones Z ; and Z3 may similarly be provided with respective humidity detectors and controllers.
Figure 5 shows a yet further modification in which, instead of employing a humidity detector, the height of a stack of baked products is monitored by a stack height detector 21, and a controller 20 responds to the output of the detector 21 to alter the setting of damper D2. This control action is based upon the fact the humidity in zone Z2 can alter the dimensions of the baked product.
Figures 6 and 7 show arrangements for mounting a thermocouple in relation to a conduit 24, the conduit being provided with a T- connection 25 for mounting a stainless steel tubular thermocouple well 26. As shown in Figure 6, the temperature sensing element 27 of the thermocouple 28 is positioned in the flow-path of any gases passing through the conduit 24, and will therefore respond to the temperature of any such gases.
Since the temperature sensing element 27 is insulated against conduction of heat from the stainless steel housing 26, the temperature sensed by the element 27 will largely depend upon the temperature of such gases flowing through conduit 24.
The arrangement of Figure 7 shows how the temperature of gases flowing though a conduit between two oven zones can be sensed by a thermocouple that is accessible from outside the oven casing, the thermocouple housing 26 projecting outwardly of the oven wall.
In Figure 8 each of the conduits Ci to Cs has been provided with two thermocouples, one of the thermocouples, such as TSA, having its temperature sensing element located in a portion of the conduit Cs external to the oven wall 4, and the other thermocouple, such as Tgs, having its temperature sensing element located in a portion of the conduit Cs that is internal of the oven wall 4. The respective thermocouple pair TSA and TSB are used to monitor gas flow through the conduit C, to determine whether the pressure at the inner end of conduit Cs is greater, less than, or equal to the pressure at the outer end of conduit Cs.
Similarly, Figure 9 indicates how thermocouple pairs can be used in a multi-zone oven to monitor the gas flow through conduits which extend between adjacent zones of the furnace.
Figure 10 shows an assembly of conduit and associated thermocouples similar to that of Figure 6 but which incorporates two thermocouples X and Y mounted in respective tubular thermocouple housings 26'and 26" which both extend transversely of a conduit 24', the conduit 24'being arranged to extend in a substantially horizontal plane at an acute angle with respect to the oven wall 4, such that the outer end of the housing 26"projects outwardly of the wall 4', to enable both thermocouples X and Y to be accessible from outside the oven.
For the purpose of providing references for control action, thermocouples A and Z are shown in Figure 10 to provide signals TA and Tz
representative of the ambient outside temperature, and of an oven temperature at the same level as the inner end of the conduit 24'.
Figure 11 shows how comparators 31 and 32 can be used in conjunction with the temperature readings TA and Tx, and Ty and Tz respectively, to control a motor 33. Motor 33 may be a variable speed motor which drives a fan, or may be a motor controlling a damper.
Figure 12 shows an alternative assembly which is essentially equivalent to that shown in Figure 10. The assembly comprises four horizontally- extending conduits A, X, Y and Z each provided with thermocouples TA, Tx, T. and Tz respectively. The thermocouple TA is located towards open end OA of the conduit A, and the conduit A is provided with two diametrically opposed elongate apertures SA (of which the uppermost slot only is shown in Figure 12). The conduits X and Y are substantially identical and each comprises upper and lower elongate slots Sx and Sy (of which the uppermost slot only of each conduit is shown in Figure 12).
The thermocouple Tx is located approximately one third the way along the conduit X from the slots Sx to the opening Ox, and the thermocouple Ty is located approximately two thirds the way along the conduit Y from the slots Sy to the opening Oy. The thermocouple Tz protrudes into the oven through the opening Oz of the conduit Z.
Since the purpose of the thermocouple TA is to measure the temperature of the air outside the oven, slots SA are provided to allow air to flow through the conduit so that the thermocouple is indeed responsive to the surrounding air. Slots Sx and Sy are provided to permit a flow of air through the conduits between the inside and outside of the oven and thus temperature is measured dependent on the direction and substantially the velocity of the air flow through the conduits. In other words, the
temperatures are dependent on the pressure difference between oven interior and exterior.
It will be appreciated that the various control systems described above could be installed in many types of temperature-controlled enclosures and not only multi-zone baking ovens.