Drying system

Abstract

A drying system (1) includes a drying chamber (2), a heater (3) for heating process gas and a fan (4) for circulating process gas through the drying chamber and the heater, and a dehumidifier (5) for dehumidifying process gas from the drying chamber and sending it back to the drying chamber. The dehumidifier includes a cooler (6) for cooling process gas to a temperature at or below its dew point and a gas-to-gas heat exchanger (7) for cooling process gas before entering the cooler and for heating process gas after the cooler. A heat pump system (8) has a thermal energy source arranged as the cooler of the dehumidifier and a thermal energy sink arranged as the heater. The drying system is adapted to pass a volume rate of process gas through the dehumidifier which volume rate is smaller than a volume rate of process gas circulated through the heater.

Description

[0001] DRYING SYSTEM

[0002] The present invention relates to a drying system for removing moisture from a product, the drying system including a drying chamber in which moisture may be evaporated from the product, a heater for heating process gas and a fan for circulating process gas through the drying chamber and the heater, the drying system including a dehumidifier for dehumidifying process gas, the dehumidifier being arranged for dehumidifying process gas from the drying chamber and sending dehumidified process gas back to the drying chamber, the dehumidifier including a cooler for cooling process gas to a temperature at or below its dew point, the dehumidifier including a gas-to-gas heat exchanger arranged to cool process gas before entering the cooler and to heat process gas after exiting the cooler, and the drying system including a heat pump system having a thermal energy source arranged as the cooler of the dehumidifier and a thermal energy sink arranged as the heater.

[0003] Traditionally, drying systems, such as convection ovens or laundry dryers, generate a convection flow by moving heated air through a drying chamber. Fresh air is taken from the surroundings and is heated, often by a gas burner, before entering the drying chamber. Thereafter, a mayor part of the hot airflow leaves the drying chamber to the surroundings through an exhaust pipe. However, in this way, a lot of energy is wasted.

[0004] WO 2024 / 061471 Al discloses a convection oven with a thermal energy system improving the energy efficiency of the system over conventional convection ovens. The convection oven comprises a baking chamber, a heating system, and a blower. A flow channel is provided, along which the convection flow circulates through the convection oven, and, along the flow channel, after the baking chamber, but before the heating system, a condensation heat exchanger is provided which extracts baking moisture from the convection flow through condensation and which by means of a heat pump system is connected to the heating system to supply the thermal energy, in particular the latent thermal energy, of the condensed baking moisture to the heating system. After the heating system, the flow returns to the baking chamber. However, when the flow meets the condensation heat exchanger, the temperature of the flow must be as close as possible to the dew point temperature in order to avoid that the heat pump system has to transfer too much sensible heat which would make the heat pump system inefficient and which would require a relatively large heat pump system. Therefore, flow leaving the baking chamber is first cooled by a dual internal gas-to-gas heat exchanger, before reaching the condensation heat exchanger. Subsequently, the flow leaving the condensation heat exchanger is again heated up by the dual internal gas-to-gas heat exchanger before finally reaching the heater. Unfortunately, in order to reach the required cooling effect of the dual internal gas-to-gas heat exchanger, a high efficiency of the dual internal gas-to-gas heat exchanger, such as more than 90 per cent, is needed which in practice results in a massive amount of internal gas-to-gas heat exchanger area. This in turn means that the system takes up excessive space and is even difficult to fit on, for instance, a 2 x 8 metres footprint with a height of 2 metres. As a consequence, it will be nearly impossible to retrofit the system in for instance existing traditional baking ovens or in existing traditional industrial laundry dryers. Furthermore, the required excessive amount of internal gas-to-gas heat exchanger area inevitably results in very high construction costs due to the amount of material necessary.

[0005] The object of the present invention is to provide a drying system occupying less space than existing systems without compromising energy efficiency.

[0006] In view of this object, the drying system is adapted to pass a volume rate of process gas through the dehumidifier which volume rate is smaller than a volume rate of process gas circulated through the heater.

[0007] In this way, compared to known solutions, a relatively lower volume rate of process gas has to pass through the dehumidifier which means that relatively less sensible heat needs to be transferred in the gas-to-gas heat exchanger before reaching the dew point temperature for water condensing. Therefore, less space may be required for the gas-to- gas heat exchanger. Furthermore, a higher fraction of the heat extracted by the cooler may be from latent heat, that is, not sensible heat, and this improves the overall efficiency of the system since the heat pump may transport less energy as compared to known systems. In fact, in known systems, the heat pump has to transport a lot of sensible energy, and because the heat pump has a certain coefficient of performance (COP), this means that a lot of energy is wasted in transporting such sensible energy, something which may be avoided according to the present invention. Consequently, according to the present invention, less space may be required for the system although the same energy efficiency may be obtained as compared to known systems. In particular, according to the present invention, the overall smaller space requirements of the dehumidifier may provide excellent possibilities of retrofitting existing traditional baking ovens or existing traditional industrial laundry dryers with a dehumidifier and heat pump system according to the present invention.

[0008] In an embodiment, the drying system is adapted to pass a volume rate of process gas through the dehumidifier which volume rate is between 1 and 80 per cent, preferably between 2 and 70 per cent, more preferred between 3 and 60 per cent, even more preferred between 4 and 40 per cent, and most preferred between 5 and 35 per cent of a volume rate of process gas circulated through the heater. Thereby, depending on process parameters of specific drying systems, space requirements of the system may be optimised although the same energy efficiency may be obtained as compared to known systems.

[0009] In an embodiment, the cooler is adapted to reduce the dew point of the process gas entering the dehumidifier by at least 6 degrees Celsius, preferably by at least 7 degrees Celsius, more preferred by at least 8 degrees Celsius, and most preferred by at least 9 degrees. Whereas in known systems in which the full flow passing through the heater is also passed through the dehumidifier, the cooler is adapted to reduce the dew point of the process gas entering the dehumidifier by only 2-3 degrees Celsius in order to optimise the COP of the heat pump system, according to the present invention, on the contrary, a higher reduction of the dew point, although slightly decreasing the COP of the heat pump system, may allow for even less space requirements for the system and better overall energy efficiency of the system as compared to known systems. In an embodiment, a heating flow conduit connects a process gas outlet of the drying chamber with a process gas inlet of the drying chamber, the fan and the heater are arranged in series along the heating flow conduit, and a process gas inlet conduit for the dehumidifier is branched off the heating flow conduit between the fan and the process gas inlet of the drying chamber. Thereby, a pressure rise caused by the fan in the heating flow conduit in order to urge process gas through the heater may also urge process gas through the dehumidifier. Consequently, a separate fan for the dehumidifier may not be necessary.

[0010] In an embodiment, a process gas outlet conduit for the dehumidifier is connected to the heating flow conduit between the fan and the process gas outlet of the drying chamber. Alternatively, the process gas outlet conduit for the dehumidifier may be connected directly to the drying chamber.

[0011] In an embodiment, the heater is arranged after the fan along the heating flow conduit, and the process gas inlet conduit for the dehumidifier is branched off the heating flow conduit between the fan and the heater. Thereby, process gas entering the dehumidifier has not yet passed the heater on its way from the fan, and therefore, it may be avoided cooling process gas in the dehumidifier which has just been heated in the heater. Consequently, better overall efficiency of the system may be obtained. Furthermore, according to this embodiment, pipe connections of the dehumidifier may fit directly into known traditional convection ovens not being provided with a thermal energy system. Thereby, a dehumidifier according to the present invention may easily be retrofitted into such known convection ovens.

[0012] In an embodiment, a damper is arranged in series with or is integrated in the dehumidifier. Thereby, the flow rate of the process gas passing through the dehumidifier relative to the flow rate of the process gas passing through the heater may easily be regulated and / or controlled in a cost effective manner. In an embodiment, a separate fan is arranged in series with or is integrated in the dehumidifier. Thereby, the flow rate of the process gas passing through the dehumidifier relative to the flow rate of the process gas passing through the heater may be regulated and / or controlled even more precisely and more independently of the flow of process gas through the heating flow conduit.

[0013] In an embodiment, a filtering system is arranged in the dehumidifier downstream the cooler and upstream the gas-to-gas heat exchanger. Thereby, a relatively high filtration efficiency, in particular of fats, may be obtained, because the filtration system according to this embodiment is arranged where the lowest temperature in the system is obtained. According to this embodiment, in particular, fouling of the secondary side of the gas-to-gas heat exchanger may be avoided.

[0014] In an embodiment, a filtering system is arranged upstream or at the entrance of the dehumidifier. Thereby, in particular, large debris, such as a ball pin from laundry, may be catched before the process gas enters the gas-to-gas heat exchanger of the dehumidifier. This embodiment may therefore be particularly advantageous for laundry dryers.

[0015] In an embodiment, the dehumidifier is arranged in the drying system as a separate exchangeable unit having an inlet connection for process gas to be dehumidified, an outlet connection for dehumidified process gas, and pipe connections for the heat pump system. Thereby, the dehumidifier may be retrofitted into existing traditional baking ovens or existing traditional industrial laundry dryers and a heat pump system with heater according to the present invention may be connected to the dehumidifier. The heater of the heat pump system may be also be retrofitted into the existing system in that the existing heater, such as a gas heater or an electrical heater, may be replaced by the heater of the heat pump system which may typically fit into the confinements of the existing system.

[0016] The present invention further relates to a method of drying a product, whereby moisture is evaporated from the product in a drying chamber, whereby process gas is heated by circulating the process gas through the drying chamber and a heater by means of a fan, whereby process gas from the drying chamber is passed through a dehumidifier and back to the drying chamber, whereby the process gas is dehumidified in the dehumidifier by means of a cooler by cooling the process gas to a temperature at or below its dew point, whereby thermal energy is transferred from the cooler to the heater by means of a heat pump system, whereby the process gas passing through the dehumidifier is cooled in a gas-to-gas heat exchanger before being cooled by the cooler, and whereby the process gas passing through the dehumidifier is heated in the gas-to-gas heat exchanger after being cooled by the cooler.

[0017] The method is characterised in that a volume rate of process gas passing through the dehumidifier is smaller than a volume rate of process gas circulating through the heater. Thereby, the above-mentioned features may be achieved.

[0018] In an embodiment, the dew point of the process gas leaving the dehumidifier is at least 6 degrees Celsius less than, preferably at least 7 degrees Celsius less than, more preferred at least 8 degrees Celsius less than, and most preferred at least 9 degrees Celsius less than the dew point of the process gas entering the dehumidifier. Thereby, the above-mentioned features may be achieved.

[0019] In an embodiment, process gas is passed through the dehumidifier by means of the fan. Thereby, the above-mentioned features may be achieved.

[0020] In an embodiment, the volume rate of process gas passing through the dehumidifier is controlled or regulated by means of a damper. Thereby, the above-mentioned features may be achieved.

[0021] The invention will now be explained in more detail below by means of examples of embodiments with reference to the very schematic drawing, in which

[0022] Fig. 1 illustrates a prior art drying system in the form of a convection oven;

[0023] Fig. 2 illustrates an embodiment of a drying system according to the present invention; Fig. 2A illustrates a further embodiment of the drying system of Fig. 2;

[0024] Fig. 3 illustrates another embodiment of a drying system according to the present invention;

[0025] Fig. 4 illustrates a heat pump system of the drying system according to the present invention;

[0026] Fig. 5 illustrates an embodiment of a drying system according to the present invention in the form of a convection oven;

[0027] Fig. 6 illustrates an embodiment of a drying system according to the present invention in the form of a convection oven including several sections each provided with a dehumidifier and wherein the system is provided with a central heat pump system;

[0028] Fig. 7 illustrates an embodiment of a drying system according to the present invention, wherein the system is provided with a Cleaning in place (CIP) system;

[0029] Fig. 8 illustrates an embodiment of a drying system according to the present invention, wherein the system is provided with a number of possible filtering systems;

[0030] Fig. 9 illustrates a simple embodiment of a drying system according to the present invention;

[0031] Fig. 10 illustrates an easy to control embodiment of a drying system according to the present invention;

[0032] Fig. 11 illustrates an embodiment of a drying system according to the present invention whereby the dehumidifier may be arranged independently of the heating flow conduit; Fig. 12 illustrates an embodiment of a drying system according to the present invention in the form of a baking oven;

[0033] Fig. 13 illustrates an embodiment of a drying system according to the present invention in the form of a laundry dryer;

[0034] Fig. 14 illustrates a prior art system as that illustrated in Fig. 1, wherein various process points have been provided with reference signs for use in a comparative example below;

[0035] Fig. 15 illustrates an embodiment of a drying system according to the present invention, wherein various process points have been provided with reference signs for use in the comparative example below;

[0036] Fig. 16 is a graph illustrating the humidity ratio in kilograms of water per kilograms of air in dependence of the dew point in degrees Celsius; and

[0037] Fig. 17 is a graph illustrating temperature in dependence of energy added to process gas during evaporation or condensation of water.

[0038] In the following, generally, similar elements of different embodiments have been designated by the same reference numerals.

[0039] Fig. 1 illustrates a prior art drying system 1 for removing moisture from a product. The drying system has the form of a convection oven and includes a drying chamber 2 in the form of a baking chamber in which moisture may be evaporated from the product, which in this case is bread, cookies or the like. In the convection oven, while passing the baking chamber, heated air bakes dough and absorbs moisture, which evaporates from the dough.

[0040] The prior art drying system 1 further includes a heater 3 for heating process gas and a fan 4 for circulating process gas through the drying chamber 2 and the heater 3. A heat- ing flow conduit 9 connects a process gas outlet 10 of the drying chamber 2 with a process gas inlet 11 of the drying chamber 1, and the fan 4 and the heater 3 are arranged in series along the heating flow conduit 9.

[0041] The prior art drying system 1 in addition includes a dehumidifier 5 for dehumidifying process gas circulating through the flow conduit 9.

[0042] The dehumidifier 5 includes a cooler 6 for cooling process gas to a temperature at or below its dew point and a gas-to-gas heat exchanger 7 arranged to cool process gas before entering the cooler 6 and to heat process gas after exiting the cooler 6. The prior art drying system 1 further includes a heat pump system 8 having a thermal energy source arranged as the cooler 6 of the dehumidifier 5 and a thermal energy sink arranged as the heater 3. As seen in Fig. 1, in the prior art drying system 1, the dehumidifier 5 is arranged in series along the flow conduit 9 so that the full flow through the heater 3 is dehumidified before entering the heater.

[0043] As mentioned above, in the prior art system illustrated in Fig. 1, when the flow meets the cooler 6, the temperature of the flow must be as close as possible to the dew point temperature in order to avoid that the heat pump system 8 has to transfer too much sensible heat which would make the heat pump system inefficient and which would require a relatively large heat pump system. Unfortunately, in order to reach the required cooling effect of the gas-to-gas heat exchanger 7, an efficiency of the gas-to-gas heat exchanger of more than 90 per cent is needed which in practice results in a massive amount of internal gas-to-gas heat exchanger area. Therefore, this prior art system takes up excessive space and will be impossible to retrofit in existing traditional baking ovens or in existing traditional industrial laundry dryers. Furthermore, the required excessive amount of internal gas-to-gas heat exchanger area inevitably results in very high construction costs due to the amount of material necessary.

[0044] Fig. 2 illustrates an embodiment of a drying system 1 according to the present invention for removing moisture from a product. In the illustrated embodiment, the drying system 1 has the form of a convection oven and includes a drying chamber 2 in the form of a baking chamber in which moisture may be evaporated from the bread, cookies or the like, and heated process gas in the form of air may bake dough and absorbs moisture, which evaporates from the dough.

[0045] The drying system 1 of Fig. 2 further includes a heater 3 for heating process gas and a fan 4 for circulating process gas through the drying chamber 2 and the heater 3. The drying system 1 additionally includes a dehumidifier 5 arranged for dehumidifying process gas from the drying chamber 2 and sending dehumidified process gas back to the drying chamber 2. The dehumidifier 5 includes a cooler 6 for cooling process gas to a temperature at or below its dew point, and the dehumidifier 5 includes a gas-to-gas heat exchanger 7 arranged to cool process gas before entering the cooler 6 and to heat process gas after exiting the cooler 6. The drying system 1 further includes a heat pump system 8 having a thermal energy source arranged as the cooler 6 of the dehumidifier 5 and a thermal energy sink arranged as the heater 3. Although in schematic diagrams illustrating the embodiments of the present invention, the heat pump system 8 is illustrated as a separate box, the heat pump system 8 also includes the cooler 6 of the dehumidifier 5 and the heater 3 of the drying system 1.

[0046] The cooler 6 of the dehumidifier 5 may have any suitable configuration and may be a closed or open cooler. In a closed cooler, such as for example in a finned-tube exchanger or similar, the process gas is cooled indirectly via a heat exchanger, typically in the form of a plate heat exchanger in which a working fluid forming a cooling medium, such as water, is separated from the process gas by thin metal plates. Heat is transferred through the plates without direct contact between process gas and cooling medium connected to the heat pump source system. As the hot, humid process gas cools below its dew point, moisture condenses on the plates of the heat exchanger and can be collected. This method is compact and simple but may be prone to fouling or clogging in heavily contaminated (dusty) process air.

[0047] In an open cooler, the cooling medium, typically water, comes into direct contact with the process air. A typical setup involves a cooling tower, wet scrubber or the like, in which cooling fluid is sprayed directly into the process gas stream. As the gas cools, moisture condenses and is collected along with the now-heated cooling fluid. This fluid mixture contains both the recovered thermal energy and possible contaminants from the process gas, such as dust, dissolved gases, or organic compounds.

[0048] Fig. 2A illustrates an embodiment of the drying system 1 of Fig. 2 in which the cooler 6 has the form of an open cooler 31. As seen, process gas from the gas-to-gas heat exchanger 7 flows via a process gas inlet conduit 30 to the open cooler 31. In the cooling tower or the like forming the open cooler 31, the process gas flows vertically upwards and out through a process gas outlet conduit 35 back to the gas-to-gas heat exchanger 7. As the process gas flows vertically upwards in the open cooler 31, it is by means of a spray head 38 sprayed with cooling fluid cooled by the heat pump system 8. However, in order to transfer the recovered heat to the heat pump system 8 without risking fouling or damage of heat pump system, the cooling fluid circuit of the open cooler 31 is in the illustrated embodiment connected to the heat pump system 8 indirectly via a fluid-to- fluid heat exchanger 34. As seen, cooling medium from the heat pump system 8 flows via a heat pump system flow conduit 36 to the fluid-to-fluid heat exchanger 34 and back to the heat pump system 8 via a heat pump system flow conduit 37 from the fluid-to- fluid heat exchanger 34. The fluid-to-fluid heat exchanger 34 cools the cooling medium of the open cooler 31 before it enters the spray head 38 thereof. The fluid mixture 39 collected at the bottom of the cooling tower or the like forming the open cooler 31 is by means of a fluid circulation pump 32 and a filtering system 33 circulated back to the fluid-to-fluid heat exchanger 34.

[0049] Depending on the type of heat pump system 8, however, and by taking appropriate steps in order to filter and / or clean the cooling medium exiting the open cooler 31, the fluid-to-fluid heat exchanger 34 could be omitted so that the cooling medium of the open cooler 31 would be directly circulated through the heat pump system 8.

[0050] Although the open cooler 31 enables higher heat transfer efficiency, especially for latent heat recovery, it may require additional components such as a pump for fluid circula- tion, filters to remove contaminants from the cooling fluid before it is reused, and a secondary fluid-to-fluid heat exchanger to transfer energy to a heat pump system 8. This makes the system more complex, larger, and costly.

[0051] Both closed and open coolers may remove water-soluble pollutants from the process gas with the condensate, but their suitability depends on process gas quality and maintenance requirements. In some cases, a hybrid solution combining open and closed cooling may offer the best balance of performance and reliability.

[0052] Fig. 4 specifically illustrates the heat pump system 8 including the cooler 6 of the dehumidifier 5, the heater 3 of the drying system 1, a working fluid circuit 24, a compressor 22 and a valve 23. The heat pump system 8 according to the present invention is a high temperature heat pump (HTHP) system typically operating with supply temperatures at the heater 3 (heat sink) above approximately 100 °C. The working principle of the heat pump system 8 could be of any suitable type, such as mechanical, absorption, solid state or chemical. In an embodiment, the heat pump system 8 is a double heat pump system. All, however, does the same thing, that is upgrading low temperature heat to high temperature heat. The COP (coefficient of performance) of the heat pump system 8 is the heat pump efficiency. Higher COP's equate to higher efficiency, lower energy (power) consumption and thus lower operating costs. COP is highly dependent on the temperature difference between the source and the sink (the temperature lift). Less temperature lift will improve / increase COP.

[0053] Fig. 17 illustrates how sensible heat changes the temperature of air, whereas latent heat involves a change of state without a change in temperature. Overall efficiency of the drying system 1 according to the present invention is depending on reducing the amount of sensible heat used as source to the heat pump system 8. Optimal would be to keep all sensible heat in the system to increase overall efficiency of solution.

[0054] As opposed to known systems, the drying system 1 according to the present invention is adapted to pass a volume rate of process gas through the dehumidifier 5 which volume rate is smaller than a volume rate of process gas circulated through the heater 3. Of course, such volume rates have to be measured at equal densities of the relevant process gas flows in the system. For instance, in the embodiment of Fig. 2, the volume rate of process gas circulated through the heater 3 may be measured just before the inlet to the heater 3 and the volume rate of process gas through the dehumidifier 5 may be measured just before the inlet of the damper 14, because at these points, the densities of the process gas flows may be assumed to be practically equal. If it is not possible to measure the relevant process gas flows at equal densities, a compensation factor must be used, so that in reality, the mass flow rates may be compared.

[0055] In the embodiment of the drying system 1 illustrated in Fig. 2, this is done by arranging the dehumidifier 5 to dehumidify a bypass volume of the volume of process gas circulating through a heating flow conduit 9 connecting a process gas outlet 10 of the drying chamber 2 with a process gas inlet 11 of the drying chamber 2 wherein the fan 4 and the heater 3 are arranged in series along the heating flow conduit 9. However, according to the present invention, the dehumidifier 5 may be arranged in different ways as discussed in further detail below. In general, the drying system 1 according to the present invention is also referred to as a partial flow dehumidifying system or bypass flow dehumidifying system.

[0056] Compared to known solutions, according to the present invention, a relatively lower volume rate of process gas has to pass through the dehumidifier 5 which means that relatively less sensible heat needs to be transferred in the gas-to-gas heat exchanger 7 before reaching the dew point temperature for water condensing. Therefore, less space may be required for the gas-to-gas heat exchanger 7. Furthermore, a higher fraction of the heat extracted by the cooler 6 may be from latent heat, that is, not sensible heat, and this improves the overall efficiency of the drying system 1 since the heat pump system 8 may transport less energy as compared to known systems. As already explained above, in known systems, the heat pump has to transport a lot of sensible energy, and because the heat pump has a certain coefficient of performance (COP), this means that a lot of energy is wasted in transporting such sensible energy, something which may be avoided according to the present invention. Consequently, according to the present invention, less space may be required for the drying system 1 although the same energy efficiency may be obtained as compared to known systems. In particular, according to the present invention, the overall smaller space requirements of the dehumidifier 5 may provide excellent possibilities of retrofitting existing traditional baking ovens or existing traditional industrial laundry dryers with a dehumidifier 5 and heat pump system 8 according to the present invention.

[0057] In the illustrated embodiments, the process gas is air, and the gas-to-gas heat exchanger 7 is an air-to-air heat exchanger.

[0058] In an embodiment, the cooler 6 is adapted to reduce the dew point of the process gas entering the dehumidifier 5 by at least 6 degrees Celsius, preferably by at least 7 degrees Celsius, more preferred by at least 8 degrees Celsius, and most preferred by at least 9 degrees. Whereas in known systems in which the full flow passing through the heater is also passed through the dehumidifier, the cooler is adapted to reduce the dew point of the process gas entering the dehumidifier by only 2-3 degrees Celsius in order to optimise the COP of the heat pump system, according to the present invention, on the contrary, a higher reduction of the dew point, although slightly decreasing the COP of the heat pump system 8, may allow for even less space requirements for the drying system 1 and better overall energy efficiency of the system as compared to known systems.

[0059] In an embodiment, the drying system 1 is adapted to pass a volume rate of process gas through the dehumidifier 5 which volume rate is between 1 and 80 per cent of a volume rate of process gas circulated through the heater 3. Preferably, according to this embodiment, the cooler 6 is adapted to reduce the dew point of the process gas entering the dehumidifier 5 by at least 6 degrees Celsius, more preferred by at least 7 degrees Celsius, even more preferred by at least 8 degrees Celsius, and most preferred by at least 9 degrees. Thereby, depending on process parameters of specific drying systems, space requirements of the drying system 1 may be optimised although the same energy efficiency may be obtained as compared to known systems. In an embodiment, the drying system 1 is adapted to pass a volume rate of process gas through the dehumidifier 5 which volume rate is between 2 and 70 per cent of a volume rate of process gas circulated through the heater 3. Preferably, according to this embodiment, the cooler 6 is adapted to reduce the dew point of the process gas entering the dehumidifier 5 by at least 6 degrees Celsius, more preferred by at least 7 degrees Celsius, even more preferred by at least 8 degrees Celsius, and most preferred by at least 9 degrees. Thereby, depending on process parameters of specific drying systems, space requirements of the drying system 1 may be optimised although the same energy efficiency may be obtained as compared to known systems.

[0060] In an embodiment, the drying system 1 is adapted to pass a volume rate of process gas through the dehumidifier 5 which volume rate is between 3 and 60 per cent of a volume rate of process gas circulated through the heater 3. Preferably, according to this embodiment, the cooler 6 is adapted to reduce the dew point of the process gas entering the dehumidifier 5 by at least 6 degrees Celsius, more preferred by at least 7 degrees Celsius, even more preferred by at least 8 degrees Celsius, and most preferred by at least 9 degrees. Thereby, depending on process parameters of specific drying systems, space requirements of the drying system 1 may be optimised although the same energy efficiency may be obtained as compared to known systems.

[0061] In an embodiment, the drying system 1 is adapted to pass a volume rate of process gas through the dehumidifier 5 which volume rate is between 4 and 40 per cent of a volume rate of process gas circulated through the heater 3. Preferably, according to this embodiment, the cooler 6 is adapted to reduce the dew point of the process gas entering the dehumidifier 5 by at least 6 degrees Celsius, more preferred by at least 7 degrees Celsius, even more preferred by at least 8 degrees Celsius, and most preferred by at least 9 degrees. Thereby, depending on process parameters of specific drying systems, space requirements of the drying system 1 may be optimised although the same energy efficiency may be obtained as compared to known systems.

[0062] In an embodiment, the drying system 1 is adapted to pass a volume rate of process gas through the dehumidifier 5 which volume rate is between 5 and 35 per cent of a volume rate of process gas circulated through the heater 3. Preferably, according to this embodiment, the cooler 6 is adapted to reduce the dew point of the process gas entering the dehumidifier 5 by at least 6 degrees Celsius, more preferred by at least 7 degrees Celsius, even more preferred by at least 8 degrees Celsius, and most preferred by at least 9 degrees. Thereby, depending on process parameters of specific drying systems, space requirements of the drying system 1 may be optimised although the same energy efficiency may be obtained as compared to known systems.

[0063] In an embodiment, the efficiency of the gas-to-gas heat exchanger is below 85 per cent, preferably below 80 per cent, and most preferred below 75 per cent.

[0064] As seen, in the embodiment illustrated in Fig. 2, a process gas inlet conduit 12 for the dehumidifier 5 is branched off the heating flow conduit 9 between the fan 4 and the process gas inlet 11 of the drying chamber 2. Thereby, a pressure rise caused by the fan 4 in the heating flow conduit 9 in order to urge process gas through the heater 3 may also urge process gas through the dehumidifier 5. Consequently, a separate fan for the dehumidifier 5 is not necessary.

[0065] Furthermore, in the embodiment illustrated in Fig. 2, a process gas outlet conduit 13 for the dehumidifier 5 is connected to the heating flow conduit 9 between the fan 4 and the process gas outlet 10 of the drying chamber 2. Alternatively, the process gas outlet conduit 13 for the dehumidifier 5 could be connected directly to the drying chamber 2.

[0066] Furthermore, in the embodiment illustrated in Fig. 2, the heater 3 is arranged after the fan 4 along the heating flow conduit 9, and wherein the process gas inlet conduit 12 for the dehumidifier 5 is branched off the heating flow conduit 9 between the fan 4 and the heater 3. Thereby, process gas entering the dehumidifier 5 has not yet passed the heater 3 on its way from the fan 4, and therefore, it may be avoided cooling process gas in the dehumidifier 5 which has just been heated in the heater 3. Consequently, better overall efficiency of the drying system 1 may be obtained. Furthermore, according to this embodiment, pipe connections of the dehumidifier 5, that is both an inlet connection 18 for process gas and an outlet connection 19 for process gas, may fit directly into known traditional convection ovens not being provided with a thermal energy system. Thereby, a dehumidifier 5 according to the present invention may easily be retrofitted into such known convection ovens.

[0067] Furthermore, in the embodiment illustrated in Fig. 2, a damper 14 is arranged in series with the dehumidifier 5, before the process gas inlet connection 18. The damper 14 could also be arranged in series with the dehumidifier 5, after the process gas outlet connection 19. Alternatively, a damper 14 could be arranged integrated in the dehumidifier 5. Thereby, the flow rate of the process gas passing through the dehumidifier 5 relative to the flow rate of the process gas passing through the heater 3 may easily be regulated and / or controlled in a cost effective manner.

[0068] In the embodiment illustrated in Fig. 3, in which the flow direction through the dehumidifier 5 is reversed as compared to Fig. 2, a separate fan 15 is arranged in series with the dehumidifier 5. The separate fan 15 could also be arranged in series with the dehumidifier 5, after the process gas outlet connection 19. Alternatively, the separate fan 15 could be arranged integrated in the dehumidifier 5. Thereby, the flow rate of the process gas passing through the dehumidifier 5 relative to the flow rate of the process gas passing through the heater 3 may be regulated and / or controlled even more precisely and more independently of the flow of process gas through the heating flow conduit 9.

[0069] It is further noted that, in the embodiment illustrated in Fig. 3, the process gas inlet conduit 12 for the dehumidifier 5 is branched off the heating flow conduit 9 between the process gas outlet 10 of the drying chamber 2 and the fan 4. This is possible, because the separate fan 15 is arranged in series with the dehumidifier 5 in order to urge the process gas through the dehumidifier 5. Alternatively, the process gas inlet conduit 12 for the dehumidifier 5 could be branched off the heating flow conduit 9 after the fan 4 and before the heater 3 or even after the heater 3, or it could be branched off directly from the drying chamber 2. Furthermore, in the embodiment illustrated in Fig. 3, the process gas outlet conduit 13 for the dehumidifier 5 is connected to the heating flow conduit 9 between the fan 4 and the heater 3. Alternatively, the process gas outlet conduit 13 for the dehumidifier 5 could be connected to the heating flow conduit 9 after the heater 3 or it could be connected directly to the drying chamber 2.

[0070] In the embodiments illustrated in Fig. 7 and 8, a filtering system 16A is arranged in the dehumidifier 5 downstream the cooler 6 and upstream the cold side of the gas-to-gas heat exchanger 7. Thereby, a relatively high filtration efficiency, in particular of fats, may be obtained, because the filtering system 16A according to this embodiment is arranged where the lowest temperature in the drying system 1 is obtained. According to this embodiment, in particular, fouling of the secondary side of the gas-to-gas heat exchanger 7 may be avoided.

[0071] In the embodiment illustrated in Fig. 8, a filtering system 16B is arranged upstream or at the entrance of the dehumidifier 5. Thereby, in particular, large debris, such as a ball pin from laundry, may be catched before the process gas enters the gas-to-gas heat exchanger 7 of the dehumidifier 5. This embodiment may therefore be particularly advantageous for laundry dryers.

[0072] In Fig. 8, further possible filtering systems 16C, 16D, 16E are illustrated. A filtering system 16C arranged upstream the cooler may mitigate fouling of the cooler. Slightly higher temperature as compared to the arrangement downstream the cooler means lower filtration efficiency in the case of oil or fat. A filtering system 16E arranged upstream the heater 3 outside the dehumidifier 5 may mitigate fouling of the heater, fan and even also the dehumidifier (three different versions shown). However, the filtering system 16E may have reduced filtration efficiency of fats due to high temperature. Furthermore, additional pressure loss may be experienced, and more fan power may be needed resulting in higher running cost, as well as more maintenance / service. The various filtering systems 16A-E may include an electrostatic precipitator.

[0073] In the embodiment illustrated in Fig. 5, the dehumidifier 5 is arranged in the drying system 1 as a separate exchangeable unit 17 having an inlet connection 18 for process gas to be dehumidified, an outlet connection 19 for dehumidified process gas, and pipe con- nections 20 for the heat pump system 8. Thereby, the dehumidifier 5 has been retrofitted into existing traditional baking oven and a heat pump system 8 with heater 3 according to the present invention has been connected to the dehumidifier 5. The heater 3 of the heat pump system 8 has also been retrofitted into the existing system in that the existing heater, such as a gas heater or an electrical heater, has been replaced by the heater of the heat pump system 8 which may typically fit into the confinements of the existing system. Existing connections for humid air exhaust 28 and fresh air intake 29 has now been rendered obsolete.

[0074] Furthermore, in the embodiment illustrated in Fig. 5, the dehumidifier 5 in the form of the separate exchangeable unit 17 has a hot water outlet 25 for condensed water resulting from condensation of the evaporated water from drying process. Thereby, condensate and condensate heat may possibly be reused as cleaning water and heating source for a Cleaning in place (CIP) system arranged for cleaning and / or maintenance of the drying system 1.

[0075] Fig. 7 illustrates possible arrangements of a Cleaning in place (CIP) system 21A-21C in the drying system 1 according to the present invention for the removal of collected particulates. The following possibilities of Cleaning-in-place to increase time between manual cleaning are suggested: CIP 21C on primary side of Air-to-air heat exchanger, CIP 21B on filtering system 16A, and CIP 21A on cooler 6.

[0076] In the embodiment illustrated in Fig. 6, a drying system 1 according to the present invention has the form of a convection oven including several sections 27 each provided with one or more dehumidifiers 5 and one or more heaters 3, and wherein the drying system is provided with a central heat pump system 8.

[0077] Fig. 9 illustrates an embodiment of the present invention being simple in terms of components and operation in that there is not provided any possibility of regulation or control of the flow through the dehumidifier. Fig. 10 illustrates an embodiment of the present invention providing possibilities of enhanced regulation or control of the flow through the dehumidifier in that both a damper 14 and a separate fan 15 are arranged in series with the dehumidifier 5.

[0078] Fig. 11 illustrates an embodiment of the present invention also providing possibilities of enhanced regulation or control of the flow through the dehumidifier 5 in the same way as the embodiment illustrated in Fig. 10. However, in this case, the process gas inlet conduit 12 for the dehumidifier 5 is branched off the heating flow conduit 9 before the fan 4 and the process gas outlet conduit 13 for the dehumidifier 5 is also connected to the heating flow conduit 9 before the fan 4. In fact, according to this embodiment, because the dehumidifier 5 is provided with a separate fan 15, the process gas inlet conduit 12 and the process gas outlet conduit 13 for the dehumidifier 5 may be connected to any suitable point along the heating flow conduit 9 or even to any suitable point of the drying chamber 2. According to this embodiment, the arrangement of the dehumidifier 5 is more independent of the remaining drying system 1.

[0079] Fig. 12 illustrates an embodiment of the present invention particularly well suited for convection ovens. There is a sufficient pressure difference across the fan 4 to create a necessary flow through the dehumidifier 5, and the damper 14 may control the flow rate. There is a high filtration efficiency of fats due to low temperature in the dehumidifier 5.

[0080] Fig. 13 illustrates an embodiment of the present invention particularly well suited for laundry dryers. There is a sufficient pressure difference across the fan 4 to create a necessary flow through the dehumidifier 5, and the damper 14 may control the flow rate. There is a filter 16B to catch large debris, such as a ball pin in laundry. Blocking of the gas-to-gas heat exchanger 7 due to fin dust may be avoided.

[0081] According to the present invention is disclosed a method of drying a product, whereby moisture is evaporated from the product in a drying chamber 2, whereby process gas is heated by circulating the process gas through the drying chamber 2 and a heater 3 by means of a fan 4, whereby process gas from the drying chamber 2 is passed through a dehumidifier 5 and back to the drying chamber 2, whereby the process gas is dehumidified in the dehumidifier 5 by means of a cooler 6 by cooling the process gas to a temperature at or below its dew point, whereby thermal energy is transferred from the cooler 6 to the heater 3 by means of a heat pump system 8, whereby the process gas passing through the dehumidifier 5 is cooled in a gas-to-gas heat exchanger 7 before being cooled by the cooler 6, and whereby the process gas passing through the dehumidifier 5 is heated in the gas-to-gas heat exchanger 7 after being cooled by the cooler 6. The method is characterised in that a volume rate of process gas passing through the dehumidifier 5 is smaller than a volume rate of process gas circulating through the heater 3.

[0082] In an embodiment of the method, the dew point of the process gas leaving the dehumidifier 5 is at least 6 degrees Celsius less than, preferably at least 7 degrees Celsius less than, more preferred at least 8 degrees Celsius less than, and most preferred at least 9 degrees Celsius less than the dew point of the process gas entering the dehumidifier 5.

[0083] In an embodiment of the method, process gas is passed through the dehumidifier 5 by means of the fan 4.

[0084] In an embodiment of the method, the volume rate of process gas passing through the dehumidifier 5 is controlled or regulated by means of a damper 14.

[0085] Although the present invention has in particular been explained by means of an industrial convection oven for baking bread and an industrial laundry dryer, it is evident that the drying system according to the present invention may be applied in various different industries and applications. For instance, the drying system according to the present invention may be applied within the food industry, such as in the production of bread, biscuits, pasta, petfood, dried fruit, rice, etc. Furthermore, it may be applied within the materials industry, such as in the production of bricks, drywalls, plasterboards, etc. In addition, it may be applied within the paper and pulp industry, such as in the production of paper, post print and press processes. However, it may also be applied within various other industries than those mentioned just above, such as in trash and garbage handling, fly ash handling, agriculture food production, biowaste treatment, battery slurry handling, etc.

[0086] Comparative example

[0087] In the following example, an example of process parameters has been calculated for a known full flow dehumidifying solution as illustrated in Fig. 14 (corresponding to that of Fig. 1) and for a partial flow dehumidifying solution according to the present invention as illustrated in Fig. 15, respectively. The results illustrate that the same energy saving is obtained for both said solutions as compared to a traditional solution without a thermal energy system recovering energy, but that a lot of space is saved by means of the partial flow dehumidifying solution according to the present invention as compared to the known full flow dehumidifying solution.

[0088] In following table a comparison is given between relevant process variables at the indicated points of the Figs. 14 and 15: In following table a comparison is given between volume size of relevant components of the system of Figs. 14 and 15:

[0089] In particular, it is noted that the same energy saving of 50% is obtained for both said solutions as compared to a traditional solution without a thermal energy system recovering energy and that the total space of the known full flow dehumidifying solution is 5.2 times larger than the total space of the partial flow dehumidifying solution according to the present invention.

[0090] In the comparative example is considered an oven running a baking process needing 185 kW power, and of this, 82 kW is used for evaporation of water from baked products at 220 degC setpoint temperature, and there is a constant dewpoint in the baking chamber of 64.3 degC.

[0091] This example lists the calculated process of a full flow and a partial or bypass system according to the present invention with ~22% flow of the full flow system wherein both solutions realise similar performance in terms of system savings of 50% on such an oven.

[0092] Lower air volume flow through the partial flow solution or bypass solution according to the present invention opens up the possibility to optimise the design of all the heat exchangers with regards to volume size. The necessary exchanger sizes to achieve the performance at the flows and powers shown in the tables above are identified through supplier data. Comparing the two solutions reveals a significant difference in overall exchanger volume sizes in favour of the partial flow solution or bypass solution with a volume reduced by a factor of more than

[0093] 5. The partial or bypass concept will from the smaller volume of heat exchangers compared to the known full flow solution be easier to implement due to less footprint and it saves a lot of material.

[0094] Internal Heat Exchanger (EX)

[0095] The function of the internal air-to-air heat exchanger 7 is to remove as much sensible heat as possible before the cooler 6 (cold side of the heat pump system 8) and bringing back the energy to the air returning to the heater 3.

[0096] Qsensible_heat=rhprocess_air *CP* (T return "T dew_retu rn )

[0097] Cpand process temperatures Treturn and Tdew_return are the same in both solutions. The formula reveals that less airflow (mProcess_air) means less sensible heat (Qsensibie_heat ).

[0098] Lower air volume flow through the partial flow solution or bypass solution means that a lot less sensible heat needs to be transferred in the air-to-air HX before reaching dew point temperature for water condensing. The outcome is a much higher fraction of the heat extracted by the cooler 6 is from latent heat, not sensible heat which improves overall efficiency of the system since there is a minimum waste transport of energy by the heat pump.

[0099] The efficiency requirement of HX will also be of less importance. Even with a less efficient internal heat exchanger 7 in the partial flow solution or bypass solution, the residual sensible heat at the cooler 6 is less than that of the known full flow solution. Additionally, the fan 4 will have a less adverse effect on the performance on the partial flow solution or bypass solution since only a fraction of the heating of air added by the fan 4 will flow into the solution as additional sensible heat to be transferred before the cooler

[0100] 6. Cooler

[0101] The partial flow solution or bypass solution utilizes the exponential nature of maximum amount of water vapor that can be carried by air vs. air temperature as illustrated in Fig. 16. Having a process with a dew point in the steep part of the curve has a significant benefit for the partial flow solution or bypass solution.

[0102] A small reduction in air temperature e.g. from 60 degC to 50 degC (as indicated on the curve of Fig. 16) will almost halve the maximum amount of water vapour that can be carried by the air and excess water vapour condenses.

[0103] Removing the exact amount of water from the air as evaporated during drying equals a reduction in dewpoint temperature of e.g. 2-3 degC in the full air flow.

[0104] In the partial flow solution or bypass solution, the same amount of water can be removed in much less air flow, e.g. 20 % of full flow, by only needing to reduce the dewpoint e.g. 10 degC, by having a marginally lower cooler temperature. The lower cooler temperature will increase needed temperature lift from the heat pump system by a few degrees thus a small decrease in the COP.

[0105] Heater and Heat pump system

[0106] The temperature before the heater 3 is higher for the partial flow solution or bypass solution, resulting from enhanced transfer of sensible heat by internal HX, thus primarily latent heat is being extracted from the air in the cooler 6. Less power is then required for the heater 3 to heat the air up to the setpoint temperature.

[0107] A combination of less power to be transferred to the air by the heater 3 and a lower mass flow rate of air results in a possible optimisation with less heater volume size. Using the partial flow solution or bypass solution, the heat pump system 8 has a minor reduced COP from the lower cooler temperature, resulting in a higher required temperature rise from the heat pump system. But this is equalized by a better system efficiency from much less energy being transferred from the cooler 6 to the heater 3 and less power being needed to the heater 3, thus almost the same power being needed to the heat pump system. Overall, the heat pump compressor power is very similar between the two concepts, thus same saving may be obtained.

[0108] List of reference numbers

[0109] TLtemperature lift of heat pump system

[0110] Tsource temperature of cooler (Heat source)

[0111] Tsink temperature of heater (Heat sink)

[0112] Qsource energy flow from cooler (Heat source)

[0113] Qsink energy flow to heater (Heat sink)

[0114] 1 drying system

[0115] 2 drying chamber

[0116] 3 heater

[0117] 4 fan

[0118] 5 dehumidifier

[0119] 6 cooler

[0120] 7 gas-to-gas heat exchanger

[0121] 8 heat pump system

[0122] 9 heating flow conduit

[0123] 10 process gas outlet of drying chamber

[0124] 11 process gas inlet of drying chamber

[0125] 12 process gas inlet conduit for dehumidifier

[0126] 13 process gas outlet conduit for dehumidifier

[0127] 14 damper

[0128] 15 separate fan

[0129] 16A-16E filtering system

[0130] 17 separate exchangeable unit

[0131] 18 inlet connection for process gas of separate exchangeable unit

[0132] 19 outlet connection for process gas of separate exchangeable unit

[0133] 20 pipe connections for heat pump system

[0134] 21A-21C CIP (Cleaning in place) system

[0135] 22 compressor of heat pump system

[0136] 23 valve of heat pump system

[0137] 24 working fluid circuit of heat pump system

[0138] 25 hot water outlet 26 electrical power for compressor 1 section of convection oven

[0139] 28 humid air exhaust of traditional system

[0140] 29 fresh air intake of traditional system 30 process gas inlet conduit for open cooler

[0141] 31 open cooler

[0142] 32 fluid circulation pump

[0143] 33 filtering system for open cooler

[0144] 34 fluid-to-fluid heat exchanger for open cooler 35 process gas outlet conduit for open cooler

[0145] 36 heat pump system flow conduit to fluid-to-fluid heat exchanger

[0146] 37 heat pump system flow conduit from fluid-to-fluid heat exchanger

[0147] 38 spray head of open cooler

[0148] 39 fluid mixture 40 condensate drain

Claims

Claims1. A drying system (1) for removing moisture from a product, the drying system including a drying chamber (2) in which moisture may be evaporated from the product, a heater (3) for heating process gas and a fan (4) for circulating process gas through the drying chamber (2) and the heater (3), the drying system (1) including a dehumidifier (5) for dehumidifying process gas, the dehumidifier (5) being arranged for dehumidifying process gas from the drying chamber (2) and sending dehumidified process gas back to the drying chamber (2), the dehumidifier (5) including a cooler (6) for cooling process gas to a temperature at or below its dew point, the dehumidifier (5) including a gas-togas heat exchanger (7) arranged to cool process gas before entering the cooler (6) and to heat process gas after exiting the cooler (6), and the drying system (1) including a heat pump system (8) having a thermal energy source arranged as the cooler (6) of the dehumidifier (5) and a thermal energy sink arranged as the heater (3), characterised in that the drying system (1) is adapted to pass a volume rate of process gas through the dehumidifier (5) which volume rate is smaller than a volume rate of process gas circulated through the heater (3).

2. A drying system according to claim 1, wherein the drying system (1) is adapted to pass a volume rate of process gas through the dehumidifier (5) which volume rate is between 1 and 80 per cent, preferably between 2 and 70 per cent, more preferred between 3 and 60 per cent, even more preferred between 4 and 40 per cent, and most preferred between 5 and 35 per cent of a volume rate of process gas circulated through the heater (3).

3. A drying system according to claim 1 or 2, wherein the cooler (6) is adapted to reduce the dew point of the process gas entering the dehumidifier (5) by at least 6 degrees Celsius, preferably by at least 7 degrees Celsius, more preferred by at least 8 degrees Celsius, and most preferred by at least 9 degrees.

4. A drying system according to any one of the preceding claims, wherein a heating flow conduit (9) connects a process gas outlet (10) of the drying chamber (2) with a processgas inlet (11) of the drying chamber (2), wherein the fan (4) and the heater (3) are arranged in series along the heating flow conduit (9), and wherein a process gas inlet conduit (12) for the dehumidifier (5) is branched off the heating flow conduit (9) between the fan (4) and the process gas inlet (11) of the drying chamber (2).

5. A drying system according to claim 4, wherein a process gas outlet conduit (13) for the dehumidifier (5) is connected to the heating flow conduit (9) between the fan (4) and the process gas outlet (10) of the drying chamber (2).

6. A drying system according to claim 4 or 5, wherein the heater (3) is arranged after the fan (4) along the heating flow conduit (9), and wherein the process gas inlet conduit (12) for the dehumidifier (5) is branched off the heating flow conduit (9) between the fan (4) and the heater (3).

7. A drying system according to any one of the preceding claims, wherein a damper (14) is arranged in series with or is integrated in the dehumidifier (5).

8. A drying system according to any one of the preceding claims, wherein a separate fan (15) is arranged in series with or is integrated in the dehumidifier (5).

9. A drying system according to any one of the preceding claims, wherein a filtering system (16A) is arranged in the dehumidifier (5) downstream the cooler (6) and upstream the gas-to-gas heat exchanger (7).

10. A drying system according to any one of the preceding claims, wherein a filtering system (16B) is arranged upstream or at the entrance of the dehumidifier (5).

11. A drying system according to any one of the preceding claims, wherein the dehumidifier (5) is arranged in the drying system (1) as a separate exchangeable unit (17) having an inlet connection (18) for process gas to be dehumidified, an outlet connection (19) for dehumidified process gas, and pipe connections (20) for the heat pump system (8).

12. A method of drying a product, whereby moisture is evaporated from the product in a drying chamber (2), whereby process gas is heated by circulating the process gas through the drying chamber (2) and a heater (3) by means of a fan (4), whereby process gas from the drying chamber (2) is passed through a dehumidifier (5) and back to the drying chamber (2), whereby the process gas is dehumidified in the dehumidifier (5) by means of a cooler (6) by cooling the process gas to a temperature at or below its dew point, whereby thermal energy is transferred from the cooler (6) to the heater (3) by means of a heat pump system (8), whereby the process gas passing through the dehumidifier (5) is cooled in a gas-to-gas heat exchanger (7) before being cooled by the cooler (6), and whereby the process gas passing through the dehumidifier (5) is heated in the gas-to-gas heat exchanger (7) after being cooled by the cooler (6), characterised in that a volume rate of process gas passing through the dehumidifier (5) is smaller than a volume rate of process gas circulating through the heater (3).

13. A method of drying a product according to claim 12, wherein the dew point of the process gas leaving the dehumidifier (5) is at least 6 degrees Celsius less than, preferably at least 7 degrees Celsius less than, more preferred at least 8 degrees Celsius less than, and most preferred at least 9 degrees Celsius less than the dew point of the process gas entering the dehumidifier (5).

14. A method of drying a product according to claim 12, wherein process gas is passed through the dehumidifier (5) by means of the fan (4).

15. A method of drying a product according to any one of the claims 12 to 14, wherein the volume rate of process gas passing through the dehumidifier (5) is controlled or regulated by means of a damper (14).

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