Thermal measuring and testing – Heat flux measurement – By differential temperature measurement along undisturbed...
Reexamination Certificate
2001-10-01
2003-03-25
Gutierrez, Diego (Department: 2859)
Thermal measuring and testing
Heat flux measurement
By differential temperature measurement along undisturbed...
C374S166000, C374S179000, C136S224000, C702S136000
Reexamination Certificate
active
06536945
ABSTRACT:
This application claims priority under 35 U.S.C. §§ 119 and/or 365 to application Ser. No. 0024702.3 filed in the United Kingdom on Oct. 9, 2000; the entire content of which is hereby incorporated by reference.
The invention relates to the measurement of heat flux in heated chambers, for example, in ovens of the kind that are suitable for use in continuous processes in which material to be heated (which may be in the form of discrete articles) is transported through the oven and is heated progressively during its passage through the oven. Such ovens are known as tunnel ovens because they are elongate and have at one end an entrance through which the material is introduced into the oven and, at the other end, an exit through which the material is withdrawn. Tunnel ovens are used for a variety of purposes, for example, to dry material or to effect the baking of food products.
The heat flux to be measured is that incident on a surface of the material to be heated. In general, the heat flux will have radiative and convective components. The material will normally be supported from below on the upper run of an endless band conveyor and, where the band is imperforate, the only exposed surface of the material will be its upper surface. Where the band is a mesh, the lower surface of the material will be partly exposed to the heat flux, but it is the heat flux incident on the upper surface of the material that is here of prime concern.
It is often important to measure separately the radiative and convective components of the heat flux, and that can be done by comparing measurements made using a radiation-absorbing sensor with measurements made using a reflecting sensor. In each case, the heat flow can be determined by measuring, together with certain other quantities, the temperature difference across a thermally insulating layer located between the exposed surface of the sensor and a heat sink. Essentially, the radiation-absorbing sensor responds to the total heat flux whereas the reflecting sensor responds only to the convective component of the heat flux.
Of course, neither sensor will behave either as a black body or as a perfect reflector; each sensor will both absorb and reflect radiation incident on it. It is strictly necessary only that the two sensors should have different absorptivities, but the more nearly the radiation-absorbing sensor behaves as a black body and the more nearly the reflecting sensor behaves as a perfect reflector the better the apparatus will perform. Throughout the specification, references to a surface or a sensor being radiation-absorbing or radiation-reflecting are to be understood as taking account of those facts.
An apparatus have two such sensors and arranged to operate in that way is described in UK Patent Specification No 2 183 346B. Typically, the heat flux will vary significantly along the length of a tunnel oven, with considerable variations occurring over relatively small distances. That will be especially marked when, for example, the heat flux is primarily radiative and derives from burners or other heating elements extending across the width of the oven at intervals along its length. Thus, the axial profile of the heat flux will show pronounced peaks and troughs, and the apparatus of the invention is intended to enable the precise form of that profile to be ascertained.
Where there is high spatial frequency of the fluctuations of the heat flux along the length of the oven, it seems clear that the two sensors must pass through the oven side-by-side. In a well designed oven, variations in the heat flux across the width of the oven will be small, but they will not usually be entirely negligible. Therefore, in order to minimise the effect of those variations across the width of the oven, the two sensors must be situated close together.
In International Specification no. WO98/09143, the relatively large variations in the heat flux along the length of the oven and the relatively smaller variations across the width of the oven are taken account of by conveying through the oven, one behind the other, a radiation-absorbing sensor and a radiation-reflecting sensor, and time-adjusting the readings from the sensors according to their speed of travel such that readings corresponding to the same position along the length of the oven are compared.
It has now been found that, by adoption of certain configurations of sensors, it is possible to compensate for the variations in heat flux along and across a heated chamber, especially a tunnel oven, without the need to provide for time-adjustment of readings.
The invention provides a heat flux measuring device for transporting through a heated chamber, the device having an array of sensors, each sensor comprising first and second surfaces bounding a region, a thermally insulating layer substantially occupying said region, and means for providing a signal which is a measure of the temperature difference across the layer, said first surface of each sensor being in thermal contact with a heat sink and said second surface of each sensor being exposed, a plurality of the said exposed surfaces of the array being reflective to radiation and a further plurality of the said exposed surfaces of the array being absorbent to radiation, the sensors of the array being so arranged that the area of radiation-absorbing exposed sensor surface and the area of radiation-reflecting exposed sensor surface are each equally distributed about a mid-line of the array extending along the direction in which in use the device will be transported and about a mid-line of the array extending along the direction transverse to the direction in which in use the device will be transported.
The device of the invention can be of relatively simple construction, but nevertheless make allowance for variations in heat flux within the chamber and thus permit relatively accurate measurements of the heat flux to be made in a relatively straightforward manner.
Each reflecting exposed surface may be a surface of the insulating layer of the respective sensor. Each reflecting exposed surface may instead comprise a layer of heat-conductive material having a reflecting surface that is in thermal contact with the insulating layer of the respective sensor.
Each absorbing exposed sensor surface may be a surface of the insulating layer of the respective sensor. Each absorbing exposed sensor surface may instead comprise a layer of heat-conductive material having a radiation-absorbing surface that is in thermal contact with the insulating layer of the respective sensor. Advantageously, the array is so arranged that the reflecting and absorbing surfaces form portions of a substantially continuous surface. Thus, in a preferred form of the device, the exposed sensor surfaces may each be a portion of a continuous layer, for example, of a flexible sheet of material which is arranged to be reflecting in selected regions and arranged to be radiation-absorbing in other regions. For example, the sensor surfaces may each be a region of sheet of a metal foil, for example an aluminium foil, of which selected regions have been treated, for example, by blackening with a matt black paint, to render them absorbing to radiation.
It is preferred for the radiation-reflecting and radiation-absorbing exposed sensor surfaces to be spaced from one another. In that case, the separation between the radiation-reflecting surfaces and the radiation-absorbing surfaces is advantageously at least 5 mm.
The device may be arranged to generate a cumulative signal representative of the sum of the individual sensor signals of those sensors having a reflecting surface. In general, that will be achieved by connecting in series those sensors having a reflecting surface. Similarly, the device may be arranged to generate a cumulative signal representative of the sum of the individual sensor signals of those sensors having an absorbing surface, and that will in general be achieved by connecting in series those sensors having a radiation-absorbing surface. Such arrangements of the device are preferred, but i
Burns Doane Swecker & Mathis L.L.P.
Gutierrez Diego
Pruchnic Jr. Stanley J.
United Biscuits (UK) Limited
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