Thermal measuring and testing – Heat flux measurement
Reexamination Certificate
1999-02-22
2001-07-24
Gutierrez, Diego (Department: 2859)
Thermal measuring and testing
Heat flux measurement
C374S121000, C374S186000, C374S149000
Reexamination Certificate
active
06264362
ABSTRACT:
The invention relates to the measurement of heat flux 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through which the material is introduced into the oven and, at the other end, an exit through which the material is withdrawn.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 surface sensor with measurements made using a reflective surface 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 reflective 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 reflective sensor behaves as a perfect reflector the better the apparatus will perform. Throughout the specification, references to a sensor being radiation-absorbing or radiation-reflecting are to be understood taking account of those facts.
An apparatus having two such sensors and arranged to operate in that way is described in U.K. 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.
Because of the 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. It has now been found, however, that placing the two sensors sufficiently close together to render negligible the effects of transverse variations in the heat flux results in an inadequate degree of thermal insulation between them.
This invention provides apparatus for measuring heat flux in a tunnel oven, which comprises first and second sensors and which can be conveyed through such an oven by the means used to convey through the oven the material to be heated with the first and second sensors in line astern, wherein each sensor comprises a layer of a thermally insulating material and means for providing a signal representing the temperature difference between the two surfaces of the layer, one surface of the sensor being in contact with a heat sink and the other surface of the sensor being exposed, and each sensor including means for giving a signal representing the temperature of the exposed surface of the sensor, the first sensor is radiation-absorbing and the second sensor is reflecting, the exposed surfaces of the two sensors are substantially coplanar and the two sensors are spaced apart from one another, and which apparatus includes means for giving a signal representing the gas temperature in the vicinity of the exposed surfaces of the sensors, and which apparatus also comprises means for periodically recording data derived from the signals from each of the sensors and from the means for measuring the gas temperature in the vicinity of the two exposed surfaces, and wherein the recording means is arranged to correlate the signals from one of the sensors at one instant, with the corresponding signals relating to the other sensor at a later instant, the difference in time between the two instants being, or being adjustable to be, equal to the time taken, when in use the apparatus is being conveyed through the tunnel oven, for one sensor to reach a position formerly occupied by the other sensor.
In the apparatus of the invention, because the two sensors pass through the oven, not side-by-side, but in line astern, any effect that variations in the heat flux across the width of the oven may have on the data gathered using the apparatus is not affected by the separation between the two sensors, with the result that a separation large enough to secure adequate thermal isolation of the sensors can be used. The minimum separation between the two sensors is advantageously at least 5 mm and preferably at least 7 mm.
The manner in which the recording means operates avoids the need for a side-by-side arrangement of the sensors because, instead of recording the data from the two sensors at the same instant, when the two sensors are at different positions along the length of the oven, the data from the two sensors are recorded at different instants so chosen that the data from one sensor, when it is in one position, is compared with data from the other sensor when it is in the same position.
Advantageously, the means for measuring the gas temperature in the vicinity of the exposed surfaces of the two sensors comprises means for measuring the gas temperature in the vicinity of the exposed surface of one sensor and means for measuring the gas temperature in the vicinity of the exposed surface of the other sensor, and the recording means is arranged to correlate the signal representing the gas temperature in the vicinity of the exposed surface of each sensor with the other signals from that sensor. Although the gas temperature will often not vary greatly over a distance equal to that separating the centres of the two sensors, the provision of separate gas temperature measuring means for each sensor does give improved accuracy.
Advantageously, in each sensor, a thermopile constitutes the layer of a thermally insulating material and the means for providing a signal representing the temperature difference across the layer. A thermopile comprises a piece of thin film of a plastic material in which are embedded thermocouples connected in series and so arranged that the cold junction of each thermocouple is located close to one surface of the film and the hot junction of each thermocouple is located close to the other surface of the film. The use of a plurality of thermocouples connected in series both gives a spread of readings over the area where the thermocouple junctions are situated and, more importantly, gives a larger signal for a given temperature difference. Further, because of the construction of a thermopile, it is possible to arrange that it has a
De Jesus Lydia M.
Gutierrez Diego
Millen White Zelano & Branigan P.C.
United Biscuits (UK) Limited
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