METHOD AND APPARATUS FOR DRYING PRODUCTS

Abstract: A method for drying products (3) in a drying space (2), wherein in a direct or indirect manner, thermal energy is supplied to a flow of gas and the flow of gas is passed along the products. The flow rate PHI of the flow of gas through the drying space and/or the amount of supplied thermal energy Q is controlled, and in or djacent the drying space, measurements are carried out for determining the drying rate of the products in the drying space. According to the invention, the flow rate PHI of the flow of gas and/or the amount of supplied thermal energy Q is controlled depending on the drying rate determined. The invention also relates to an apparatus (1) for drying products.

Description: Title: Method and apparatus for drying products The invention relates to a method and apparatus for drying products in a drying space, wherein in a direct or indirect manner, thermal energy is supplied to a flow of gas and the flow of gas is passed along the products, while the flow rate Φ of the flow of gas through the drying space and/or the amount of supplied thermal energy Q is controlled. Such known method is, for instance, used for drying products to be subjected to a further processing operation, such as ceramic products which, in a next processing operation, are to be baked or wood which, in a further processing operation, is to be (further) sawed. Further, such drying method can be used as finishing operation, for instance the drying of vegetables or fruit. In particular in the case of products which shrink during extraction of moisture, it is of importance that the drying proceeds in a controlled manner, because unduly fast drying may cause stresses, deformations and cracks in the product. In the known method for drying products, the flow rate Φ of the flow of gas and/or the amount of thermal energy Q is controlled by operating an air supply valve or energy supply mechanism, depending on previous experiences with the products to be dried, for instance gained during simulation processes and/or measurement of the temperature of the flow of gas. A drawback hereof is that the chance of damage to the products to be dried is relatively substantial. This is in particular the case when the starting moistness content of the products to be dried is not always the same and/or when different types of products are to be dried in the drying space each time. In practice, this leads to the situation that the products are dried relatively slowly, to be on the safe side. This does not only cost much time, but also results in a waste of energy. The object of the invention is to provide a method of the type mentioned in the preamble, in which the above-mentioned drawbacks are avoided. To that end, the invention provides a method for drying products in a drying space, wherein in a direct or indirect manner, thermal energy is supplied to a flow of gas and the flow of gas is passed along the products, the flow rate Φ of the flow of gas through the drying space and/or the amount of supplied thermal energy Q being controlled, while, further, in or adjacent the drying space, measurements are carried out for determining the drying rate of the products in the drying space and the flow rate Φ of the flow of gas and/or the amount of supplied thermal energy Q is controlled depending on the drying rate determined. By controlling the drying process on the basis of the drying rate of the products in the drying space, the effect achieved is that a direct control of the process is possible and that the chance of the occurrence of damage through unduly fast drying can be reduced considerably. Further, the safety margin between the drying realized and the maximum drying rate of the products can be chosen to be smaller, allowing the products to be dried in a faster and more energy-saving manner. Also, during the drying process, a deviant starting moistness content of the products to be dried can readily be anticipated and the type of product to be dried can readily be changed. Further, by controlling the flow rate Φ of the flow of gas and/or the amount of supplied thermal energy Q depending on the difference between the drying rate determined and a predetermined, desired drying rate, the effect achieved is that for a specific type of product having a specific degree of moistness, the drying rate in the drying space can approach the optimal drying rate even closer. The ideal drying rate of a product can be predetermined in a theoretical manner on the basis of laboratory tests. In an embodiment of the method according to the invention, for determining the drying rate, a moisture balance of the flow of gas through the drying space is set up. Preferably, for determining the drying rate, the (change of the) degree of moistness of the flow of gas that is passed along the products is measured. In another embodiment of the method according to the invention, for determining the drying rate, the degree of moistness or the change of the degree of moistness of at least a part of the surface of one or more products in the drying space is measured. In another embodiment of the method according to the invention, for determining the drying rate, the weight or the change of the weight of one or more products in the drying space is measured. In yet another embodiment of the method according to the invention, for determining the drying rate, a thermal energy balance of the flow of gas passed along the products is set up. Further, in an advantageous manner, in a method according to the invention, the variation in temperature and/or in the moisture content of the flow of gas is recorded as function of time. Optionally, the flow rate Φ of the gas flow and/or the amount of supplied heat Q can also be recorded as function of time. The effect thus achieved is that it is possible to perform measurements for determining the drying rate of the products in the drying space only during a test period and that subsequently, control takes place on the basis of a prerecorded variation in temperature, humidity, gas or energy supply. In this manner, drying spaces can be regulated and controlled in a simple and inexpensive manner. The invention also relates to an apparatus for drying products. Further elaborations of advantageous embodiments of the method and the apparatus for drying products according to the invention are described in the subclaims. When in this context, the drying of products is mentioned, this should also be understood to mean the drying of only one product or a composition of products. Further, "passing a flow of gas along the products" should be understood to mean that at least a part of the flow of gas is passed along one or more products. Hereinafter, the invention will be specified with reference to exemplary embodiments shown in a drawing. In the drawing, Figs. 1-4 are each a schematic representation of an embodiment of a drying apparatus according to the invention, in which the drying rate is determined by setting up a moisture balance over the drying space, measuring the change of weight of the products to be dried, measuring the degree of moistness of the surface of the products to be dried, and setting up a thermal energy balance over the drying space respectively. It is observed that the Figures are only schematic representations of preferred embodiments. In the Figures, identical or corresponding parts are designated by corresponding reference numerals. Fig. 1 shows an apparatus 1 for drying products. The apparatus 1 comprises a drying space 2 in which products 3 to be dried can be disposed. The apparatus 1 further comprises supply means, designed as a supply line 4, and discharge means, designed as a discharge line 5, for supplying and discharging respectively a flow of gas to be passed through the drying space 2. The supply means 4 further comprise a burner 6 for directly supplying thermal energy to the drying space 2 via the supply line 4. By placing the burner 6 in the drying space 2, it can be effected that the thermal energy is supplied directly to the drying space 2. It is observed that instead of a burner 6, other types of means for supplying thermal energy may also be used, such as a glowing coil. The apparatus 1 further comprises control means for controlling the flow rate Φ of a flow of gas through the drying space and/or an amount of supplied thermal energy Q. In this exemplary embodiment, the control means are designed as a valve 7 incorporated into the supply line 4 and a control 8 coupled thereto, whereby the flow rate Φ of the flow of gas, supplied to the drying space 2 via the valve 7, can be controlled. The control means further comprise a regulator, not shown, coupled to the control 8, whereby the amount of thermal energy Q supplied by the burner 6 to the drying space can be controlled. The apparatus 1 further comprises measuring means for determining the drying rate of the products 3 in the drying space 2. In this exemplary embodiment, the measuring means comprise a flow meter for measuring the flow rate Φ of the flow of gas through the drying space 2. Preferably, a first flow meter 9A is incorporated into the supply line 4, while a second flow meter 9B is incorporated into the discharge line 5. The measuring means further comprise sensors 10 for measuring the degree of moistness of the gas flow through the drying space 2. Preferably, a first sensor 10A is incorporated into the supply line 4 and a second sensor 10B is incorporated into the discharge line 5. The control 8 is coupled to the flow meters 9A, 9B and to the sensors 10A, 10B, so that by means of the control 8 the drying rate of the products 3 in the drying space 2 can be determined and the flow rate Φ and/or the amount of supplied thermal energy Q can be controlled depending on the drying rate determined, as will be explained in more detail hereinbelow. The drying of products 3 in the drying space 2 by means of the apparatus 1 can be performed as follows. After the products 2 to be dried have been disposed in the drying space, a flow of gas, for instance ambient air, is supplied to the drying space 2 via the supply line 4. By means of the burner 6, the flow of gas is heated up to a temperature of, for instance, about 100-150°C and subsequently, by circulation means not shown, passed along the products 3 for drying them. Next, the flow of gas is discharged by means of the discharge line 5. By means of the flow meters 9A and 9B, a gas flow Φ(t) is determined as function of time through the drying space 2. Further, by means of sensors 10A and 10B, the moistness of the gas flow is determined as function of time, for instance Yλ(t) and Yε(t). Hence, in this embodiment, the measurement of the drying rate comprises a difference measurement of moisture in gas flows before and after the supply to products, for determining the amount of moisture added by the products to the gas flow. On the basis of these data, a moisture balance over the drying space 2 can be determined. From this moisture balance, the amount of moisture leaving the drying space per unit of time can be calculated, from which, through division by the number of products, the drying rate per product can be determined. Other units are also possible for expressing the drying rate, for instance the drying rate per kilo of product. By means of control 8, the drying rate W(t) can hence be calculated by means of the formula Wσ(t) = Φιs(t) * Yβ(t) - ΦA(t) * YA(t). Next, by means of the control 8, this measured drying rate WG(t) can be compared with the theoretical or experimentally determined ideal drying rate W,(t). By means of the control 8, the regulator and/or the valve can be operated for setting the supplied amount of heat Q and the flow rate Φ so that the measured drying rate Wβ(t) approaches the ideal drying rate Wι(t) as close as possible. It is observed that the determination of the degree of moistness of the supplied air YA( can in some cases be omitted, for instance when use is made of preheated air of a constant, known degree of moistness or when the degree of moistness of the ambient air is already known from climatological data. Further, when no leakage of the gas flow occurs, one single flow meter 9 may suffice or, at a constant flow rate, the flow rate may even be determined once by a temporary flow meter. It is observed that determining the moisture balance, measuring the flow rate and determining the moistness content of the flow of gas can of course also be performed in other places than those shown in the example. Fig. 2 shows a second embodiment of the apparatus 1 for drying products 3. The products 3 in the drying space 2 are disposed on a weighing apparatus 11 whereby the weight m of the products 3 can be measured and whereby the change of the weight can be determined as function of time dm(t)/dt. The weighing apparatus 11 is coupled to the control 8. On the basis of theoretical and/or experimental data, the drying rate W(t) of the product can be determined by the control 8 on the basis of the change of the weight of the products 3 during drying. Indeed, due to evaporation of moisture from the product to be dried, the mass of each product decreases. Thus, the drying rate WG(t) measured can be determined. Analogously with the example discussed hereinabove, the drying rate Wc(t) measured can be compared with the ideal drying rate Wi(t) by the control 8, so that the burner 6 and/or the valve 7 can be controlled to reduce the difference between the drying rate WG(t) measured and the ideal drying rate Wi(t). It is observed that the weighing apparatus 11 may also be used for measuring the change of weight of only a part of the products, for instance by measuring the change of weight of one or more reference products. Fig. 3 shows a third embodiment of an apparatus 1 for drying products 3, with the provision of sensors 12 for measuring the degree of moistness of the surface of products 3 placed in the drying space 2, for instance sensors whereby the electric resistance of the product surface can be measured, or so-called "heat flux" sensors which measure the flux of the heat through the surface. The sensors 12 are coupled to the control 8. By means of these sensors, the degree of moistness of the surface can be measured as function of time Vo(t). By means of theoretically and/or experimentally determined values, a measured drying rate WG(t) can then be derived from the change of the degree of moistness of the product surface in the time by means of the control 8. Next, the drying rate Wc(t) measured is compared with the ideal drying rate Wi(t) to control the flow rate Φ of the flow of gas and/or the amount of added thermal heat Q, as already explained hereinabove. Of course, several sensors 12 may be used on one product surface. Also, different surfaces of one product may be provided with sensors 12. In addition, several products 3 may be provided with sensors 12. It is observed that the sensors 12 can also be designed for measuring the evaporation rate at the product surface in order to determine the drying rate on the basis thereof, for instance via the determination of the loss of weight per unit of time. Fig. 4 shows a fourth embodiment of an apparatus 1 for drying products 3. In addition to a flow meter 9, the apparatus 1 further comprises temperature sensors TA and TB for measuring the temperature of the supplied and the discharged gas flow respectively, and a temperature sensor 15 for measuring the product temperature Tprod. The apparatus 1 further comprises an energy meter 14 whereby the amount of supplied thermal energy Q can be determined. By means of this apparatus, a thermal energy balance can be set up of the flow of gas through the drying space 2 for determining the drying rate WG(t) of the products 3. By means of the control 8, a thermal energy balance can be set up over the drying space with the following formula, with the temperature TA of the ingoing gas flow being measured before the heat Q is supplied thereto by the burner 6: P(t) = Φ(t) * C P .a, r * (TB - TA) + Q. After heating of the space and compensation for losses, this energy should be equal to the energy required for product heating and drying: P(t) = Wθ(t) * hvT(Tprod) + mprod * Cp, prod + dT P rod/dt. In these formulae: Cp = heat capacity hvT = evaporation heat as function of temperature mprod = product weight Q.1 prod = change of product temperature TA = temperature of ingoing air TB = temperature of outgoing air Q = heat supplied From the above formulae, a measured drying rate Wcι(t) can be determined by means of the control 8, which drying rate can then be used, analogously with the first example, for controlling the drying process. In this case, measurement of the drying rate hence comprises a difference measurement of energy in gas flows before and after supply to the products, to determine the amount of evaporation energy taken up by the products. It is observed that if TA is measured after the gas flow has been heated by means of the burner 6, measurement of the amount of supplied heat Q can be omitted. It is further observed that the temperature of the product may approximately be equated with the air temperature minus a correction factor, so that measurement of the product temperature can be omitted. It will be understood that the accuracy with which the energy and/or moisture balance is set up and the accuracy with which the terms of the energy and/or moisture balance is measured, determine the accuracy with which the drying rate can be determined. When thermal energy is supplied by means of a gas burner, for instance, during the setup of the moisture balance, moisture released upon gas combustion may or may not be taken into account. Further, loss factors may or may not be used for expressing energy losses, for instance a correction factor for expressing heat leakage via the walls of the drying space. It is further observed that the invention is not limited to the preferred embodiments discussed hereinabove. Thus, the gas flows may be circulated and, for instance, energy may be supplied by means of steam. Further, combinations of the different embodiments are possible and particular aspects may be designed differently. For instance, instead of a drying chamber, a drying tunnel may be used through which the products are conveyed during drying. A drying tunnel may then consist of a succession of drying spaces. Further, the measuring means and the control means may be of displaceable design and there may be provided means for recording the variation in temperature of the flow of gas as function of time, so that the drying apparatus can be regulated and controlled during a test period. Further, the flow rate Φ of the flow of gas and or the amount of supplied thermal energy Q may be controlled depending on the determined drying rate without being compared with a predetermined, desired drying rate. Such variations will readily occur to anyone skilled in the art and are understood to fall within the framework of the invention as set forth in the following claims.

Claims: Claims 1. A method and apparatus for drying products in a drying space, wherein in a direct or indirect manner, thermal energy is supplied to a flow of gas and the flow of gas is passed along the products, the flow rate Φ of the flow of gas through the drying space and/or the amount of supplied thermal energy Q being controlled, while, further, in or adjacent the drying space, measurements are carried out for determining the drying rate of the products in the drying space and the flow rate Φ of the flow of gas and/or the amount of supplied thermal energy Q is controlled depending on the drying rate determined. 2. A method according to claim 1, wherein the flow rate Φ of the flow of gas and/or the amount of supplied thermal energy Q is controlled depending on the difference between the drying rate determined and a predetermined, desired drying rate. 3. A method according to claim 1 or 2, wherein for determining the drying rate, a moisture balance of the flow of gas through the drying space is set up. 4. A method according to any one of the preceding claims, wherein for determining the drying rate, the degree of moistness or the change of the degree of moistness of the flow of gas is measured, preferably before and after the supply of the flow of gas to the products. 5. A method according to any one of the preceding claims, wherein for determining the drying rate, the degree of moistness or the change of the degree of moistness of at least a part of the surface of one or more products in the drying space is measured. 6. A method according to any one of the preceding claims, wherein for determining the drying rate, the weight or the change of the weight of one or more products in the drying space is measured. 7. A method according to any one of the preceding claims, wherein for determining the drying rate, a thermal energy balance of the flow of gas through the drying space is set up. 8. A method according to any one of the preceding claims, wherein for determining the drying rate, the flow rate Φ of the flow of gas is measured. 9. A method according to any one of the preceding claims, wherein for determining the drying rate, the amount of supplied heat Q is measured. 10. A method according to any one of the preceding claims, wherein for determining the drying rate, the temperature of the flow of gas is measured, preferably before and after the supply of the flow of gas to the products. 11. A method according to any one of the preceding claims, wherein the variation of temperature of the flow of gas, the variation of the moistness content, the variation of the gas flow Φ and/or the variation of amount of supplied thermal energy is recorded as function of time. 12. A method for drying products in a drying space wherein in a direct or indirect manner, thermal energy is supplied to a flow of gas and wherein the flow rate Φ of the flow of gas and/or the amount of supplied thermal energy Q is controlled on the basis of a variation recorded according to the method of claim 11. 13. An apparatus for drying products, comprising a drying space for disposing products to be dried therein, supply and discharge means for supplying and discharging respectively a flow of gas to be passed through the drying space, means for supplying thermal energy in a direct or indirect manner, and control means for controlling the flow rate Φ of a flow of gas through the drying space and/or an amount of supplied thermal energy Q, the apparatus further comprising measuring means for determining the drying rate of the products in the drying space and the control means being adapted to control the flow rate Φ and/or the amount of supplied thermal energy Q depending on the drying rate determined. 14. An apparatus according to claim 13, wherein means for measuring the degree of moistness of the flow of gas are provided. 15. An apparatus according to claim 13 or 14, wherein means are provided for measuring the degree of moistness or the change of the degree of moistness of at least a part of the surface of one or more products in the drying space. 16. An apparatus according to any one of claims 13-15, wherein means are provided for measuring the weight or the change of the weight of one or more products in the drying space. 17. An apparatus according to any one of claims 13-16, wherein means are provided for measuring the flow rate Φ of the gas flow. 18. An apparatus according to any one of claims 13-17, wherein means are provided for measuring the amount of supplied heat Q. AMENDED CLAIMS [received by the International Bureau on 25 August 2000 (25.08.00); original claims 1 and 8 amended ; original claims 3,5,7 and 9 cancelled; new claim 9 added; remaining claims unchanged (2 pages)] 1. A method and apparatus for drying products in a drying space, wherein in a direct or indirect manner, thermal energy is supplied to a flow of gas and the flow of gas is passed along the products, the flow rate Φ of the flow of gas through the drying space and/Or the amount of supplied thermal energy being controlled, while, further, in or adjacent the drying apace, measurements are carried out for determining the drying rate of the products in the drying space and the flow rate Φ of the flow of gas and/or the amount of supplied thermal energy Q is controlled depending on the drying rate determined, and wherein for determining the drying rate, a moiBture balance of the flow of gas through the drying space is set up. 2. A method according to claim 1, wherein the flow rate Φ of the flow of gas and/or the amount of supplied thermal energy Q is controlled depending on the difference between the drying rate determined and a predetermined, desired drying rate. 3. A method according to any one of the preceding claims, wherein for determining the drying rate, the degree of moistness or the change of the degree of moistness of the flow of gas is measured, preferably before and after the supply of the flow of gas to the products. 4. A method according to any one of the preceding claims, wherein for determining the drying rate, the flow rate Φ of the flow of gas is measured. 5. A method according to any one of the preceding claims, wherein for determining the drying rate, the temperature of the flow of gas is measured, preferably before and after the supply of the flow of gas to the products. 6. A method according to any one of the preceding claims, wherein the variation of temperature of the flow of gas, the variation of the moistneBs content, the variation of the gas flow Φ and or the variation of amount of supplied thermal energy Q is recorded as function of time. 7. A method for drying products in a drying space wherein in a direct or indirect manner, thermal energy is supplied to a flow of gas and wherein the flow rate Φ of the flow of gas and/or the amount of supplied thermal energy Q is controlled on the basis of a variation recorded according to the method of claim 6. 8. An apparatus for drying products, comprising a drying space for disposing products to be dried therein, supply and discharge means for supplying and discharging respectively a flow of gas to be passed through the drying space, means for supplying thermal energy in a direct or indirect manner, and control means for controlling the flow rate Φ of a flow of gas through the drying space and/or an amount of supplied thermal energy Q, the apparatus further comprising measuring means for determining the drying rate of the products in the drying space and the control means being adapted to control the flow rate Φ and/or the amount of supplied thermal energy Q depending on the drying rate determined and wherein the measuring means comprise means for measuring the degree of moistness of the flow of gas such that a difference of moisture in gas flows before and after the supply to products can be measured. 9. An apparatus according to claim 8 where the measuring means comprise a first sensor for measuring the degree of moistness incorporated in a supply line and a second sensor for measuring the degree of moistness incorporated into a discharge line. 10. An apparatus according to claim 7 or 8, wherein means are provided for measuring the flow rate Φ of the gas flow. 11. An apparatus according to any one of claims 7-9, wherein means are provided for measuring the amount of supplied heat .

Inventors: VELTHUIS JOHANNES FRANSISCUS M, VELTHUIS, JOHANNES, FRANSISCUS, MARIA
Applicants: TNO, VELTHUIS JOHANNES FRANSISCUS M, NEDERLANDSE ORGANISATIE VOOR TOEGEPAST-NATUURWETENSCHAPPELIJK ONDERZOEK TNO, VELTHUIS, JOHANNES, FRANSISCUS, MARIA
Application year: 2000
Main classification: F26B25
IPC codes: F26B21, F26B25