Low-consumption apparatus and method for drying construction panels

Abstract

The present invention concerns an apparatus for drying construction panels comprising,  a conveyor (40) for conveying wet construction panels (60) along a drying path through a number of high temperature chambers (1, 2) (= HT chambers) located upstream along the drying path of a number of low temperature chambers (N) (= LT-chambers),  a distribution system (5) comprising a LT-distribution section configured for distributing a heated drying gas into the LT-chambers coupled in series by a liaising duct (5L) to a HT-distribution section configured for distributing the heated drying gas into the HT-chambers, thus heating the main surfaces of the construction panels and for removing moisture therefrom as they travel through the chambers,  an exhaust system (7) configured for evacuating out of the apparatus exhaust fluid along an exhaust flowing path extending from the HT-chambers (1, 2) to outside the apparatus, wherein the fluid flowing into the HT-chambers is heated by heating elements (11) and wherein the fluid flowing into the LT-chambers is heated by a LT-heat exchanger (X95N) heated by the exhaust gas, and wherein a heat pump / MVR-system comprising a heat pump (8) transferring heat from the exhaust fluid to an MVR-cycle, which transfers the heat to the drying gas in the liaising duct (5L) pre-heats the drying gas prior to reaching the heating elements (11).

Description

LOW-CONSUMPTION APPARATUS AND METHOD FOR DRYING CONSTRUCTION PANELSTECHNICAL FIELD

[0001] The present invention concerns an apparatus and method for drying construction panels made of gypsum (= plasterboards or gypsum fibreboard) or panels made of cement or calcium silicate during the formation thereof, consuming less energy than state of the art dryers and methods for achieving a same level of drying with lower energy consumption. The construction panels to be dried are typically driven on a conveyor through N chambers of a drying apparatusBACKGROUND OF THE INVENTION

[0002] Construction panels such as panels made of gypsum (= plasterboards or gypsum fibreboard) or panels made of cement or fibre cement or calcium silicate are formed by mixing and reacting water with a base composition. For example panels made of gypsum (CaSO4.2H2O) referred to as plasterboards are formed by mixing and reacting plaster of Paris (CaSO4.% H2O) with water. The panels thus formed must be heated to accelerate the reaction and eliminate excess water. For example, the apparatus illustrated in Figure 1 comprises a distribution system (5) configured for blowing a warm drying gas, typically air, over both main surfaces of the panels to raise the temperature thereof and reduce the moisture content, as they pass through N chambers. Before the air reaches the panels in the chambers, it is heated with heating elements (11), which can typically be air-fuel blowers. The N chambers are divided into high temperature (HT-) chambers (1 ,2) distributed in a HT-drying zone wherein the air is heated at a high temperature, followed by low temperature (LT-) chambers (N) distributed in a LT-drying zone. An exhaust system (7) is provided for evacuating out of the apparatus the cooled gas charged with moisture after it contacted the surfaces of the panels in each chamber.

[0003] Even if efforts are continuously made for reducing the amount of water added to the base composition, drying costs still represent a substantial portion of the total manufacturing costs. In view of the importance of reducing CO2 emissions and of the rising cost of energy, efforts have been made to increase the efficacy of the drying apparatuses. For example, as illustrated in Figure 1 , a heat exchanger (X75) is used to recover some heat from the exhaust air flowing in the exhaust system (7) before being released in the atmosphere and to transfer it to the fresh air flowing in the distribution system (5) prior to reaching the chambers and for feeding with pre-heated air the air-fuel burners used for heating the flowing air.

[0004] GB1562031 describes a drying apparatus wherein the air-fuel burners in the LT-drying zone are replaced by a heat exchanger extracting heat from the exhaust gas from the HT-chambers and transferring the heat to a fresh air supply blown into the LT-chambers. To increase the heat transfer from the exhaust air, the air blown onto the articles in the HT-drying zone has a high temperature and high moisture content. This has the effect of increasing the moisture content in the exhaust air, but it also reduces the amount of water removed from the articles to be dried.

[0005] WO2019105888 describes a dryer and method for drying plasterboards wherein heat from exhaust gas removed from the HT-chambers is extracted by a heat exchanger and transferred to fresh air supplies via heat exchangers arranged in series, each air supply being coupled to a LT-chamber. This method is very efficient, but it is difficult to upgrade an existing dryer of the type illustrated in Figure 1 to a dryer according to WO2019105888 as substantial extra room is required to achieve it.

[0006] . US2010299956 describes a dryer using oil-to-air heat exchangers instead of air-fuel burners for heating the air blown into the chambers. The oil is heated in a furnace, circulated into the various oil-to-air heat exchangers, and returned to the furnace. The dryer is equipped with an organic rankine cycle system (ORC) to use the heat either from the exhaust air from the last LT-chamber or from the furnace to produce electricity.

[0007] CN103708697 describes a mechanical vapour recompression (= MVR-) heat pump sludge drying system comprising a drying machine and a vapour compressor. Wet sludge is dried by the drying machine generating secondary vapour, which is compressed in the vapour compressor to become recompressed vapor. The recompressed vapour flows back into the drying machine. Thus, energy is saved by recovering the latent heat of the secondary vapor. Similarly, CN109282615A describes an MVR band dryer for drying articles.

[0008] It can be seen that various options have been explored in the art, all aiming at reducing energy consumption of the drying process. The present invention proposes an apparatus and method for drying construction boards such as plasterboards and cement boards during production thereof, which is making most use of the calorific energy available in the apparatus to substantially reduce energy consumption. A great advantage of the present invention is that it is easy and economically efficient to upgrade existing drying apparatuses of the type illustrated in Figure 1 to yield a drying apparatus according to the present invention without reducing the capacity of the apparatus.

[0009] These and other advantages of the present invention are described in continuation.SUMMARY OF THE INVENTION

[0010] The present invention is defined in the appended independent claims. Preferred embodiments are defined in the dependent claims. In particular, the present invention concerns an apparatus for drying construction panels comprising a conveyor for conveying wet construction panels along a drying path through a number N of chambers distributed in series and in fluid communication with one another along the drying path. The dryer comprises one or more high temperature chambers (= HT chambers) located in a high temperature drying zone (HT) in an upstream portion of the drying path, and one or more low temperature chambers (= LT-chambers) located in a low temperature drying zone (LT) downstream of the high temperature drying zone (HT) along the drying path.

[0011] A distribution system in fluid communication at one end with a source of drying gas, preferably air, and, at other ends, with the N chambers. The distribution system is configured for circulating a flow of the drying gas along a drying gas flowing path extending from the source of drying gas into the N chambers. Heating elements are provided and are configured for heating the drying gas in thedistribution system (5) prior to penetrating into the respective HT-chambers,

[0012] An exhaust system is configured for evacuating out of the apparatus exhaust gas along an exhaust flowing path extending from the HT-chambers through return exhaust ducts leading to corresponding LT-heat exchangers (X75N) provided in the LT-chambers, and outside the apparatus. The LT-heat exchangers (X75N) are provided in each LT-chamber and configured for transferring heat from the exhaust gas flowing in the corresponding return exhaust ducts to the drying gas as it flows through the corresponding LT-chambers,

[0013] The distribution system comprises a LT-distribution section for distributing drying gas into the LT-chambers and a HT-distribution section for distributing drying gas into the HT-chambers, and coupled in series and downstream of the LT-distribution section by a liaising duct, wherein• the LT-distribution section comprises one or more drying gas feeding ducts leading to the corresponding one or more LT-chambers in parallel, and wherein• the HT-distribution section comprises one or more drying gas feeding ducts leading to the corresponding one or more HT-chambers in parallel,

[0014] The apparatus comprises a first heat pump-heat exchanger (X78) configured for transferring heat from the exhaust gas flowing in the exhaust system to a low boiling point gas of a heat pump. The heat pump comprises a heat pump compressor configured for compressing and increasing a temperature of the low boiling point fluid, and for driving a flow of the low boiling point fluid from the first heat pump-heat exchanger (X78) to a second heat pump-heat exchanger (X89).

[0015] The second heat pump-heat exchanger (X89) is configured for transferring the heat thus captured by the low boiling point fluid to a heating fluid having a boiling temperature higher than the low boiling point fluid of the heat pump and configured for circulating in a mechanical vapour recompression (MVR-) cycle comprising an MVR-compressor configured for compressing the heating fluid, and thus increasing a temperature of the heating fluid, and for driving a flow of the heating fluid from the second heat pump-heat exchanger (X89) to an MVR-heat exchanger (X95L) . The MVR-heat exchanger (X95L) is configured for transferring heat from the heating fluid of the MVR-cycle to the drying gas in the distribution system downstream of the LT-distribution section and upstream of the HT-distribution section.

[0016] The heat pump and MVR-cycle together form a heat pump I MVR system characterized by a coefficient of performance (= COP) preferably comprised between 2 and 5, preferably between 2.5 and 4. The COP is defined as a ratio (Q / W) of a useful heat (Q) supplied by the heat transfer combination to a work (W) required to operate the heat pump compressor and the MVR-compressor.

[0017] In a preferred embodiment, the dryer comprises a pre-heating heat exchanger (X75) configured for transferring heat from the exhaust gas in the exhaust system to the drying gas in the distribution system to increase the temperature of the drying gas flowing in the distribution system prior to reaching the LT-chambers. The pre-heating heat exchanger (X75) is located,• In the distribution system (5), upstream of the LT-distribution section, and• in the exhaust system (7), downstream of the LT heat exchanger (X75N) and preferably upstream of the first heat pump-heat exchanger (X78), wherein ‘upstream’ and ‘downstream’ are defined herein relative to a flow direction of a fluid in the corresponding distribution or exhaust systems.

[0018] If the heating elements are air-fuel burners, it is preferred that the source of drying gas be a source of air, and a part of the air flowing in the distribution system be fed to the air-fuel burners. Alternatively, the heating elements can be electrical heaters or high temperature fluid heat exchangers.

[0019] The distribution system can comprise distribution fans configured for driving the flow of drying gas from the source of drying gas towards the N chambers. The HT-chambers are preferably equipped with chamber fans configured for driving a drying gas flow cycle flowing through the heating element and into the corresponding HT-chambers and back to the heating element for a next cycle or out of the drying gas flow cycle into the exhaust system. The fans are configured for driving a drying gas flow through the LT-chambers between radiation surfaces of the LT-heat exchangers (X75N) to collect heat and the main surfaces of the construction panels to transfer heat thereto and collect moisture therefrom and out of the LT-chambers into the liaising duct and through the MVR-heat exchanger (X95I) prior to reaching the HT-distribution section.

[0020] The temperature in the HT-chambers can vary between, e.g., 120 and 260°C, preferably between 150 and 250°C. The temperature in the one or more LT-chambers has an average value lower than an average temperature in the HT-chambers and can vary between 90 and 170°C, preferably between 100 and 160°C. The temperature of the low boiling point fluid of the heat pump in the second heat pump-heat exchanger (X89) can be comprised between 90 and 110°, and is preferably equal to 100°C + 5°C. The temperature of the heating fluid of the MVR-cycle (9) in the MVR-heat exchanger (X95L) can be comprised between 120 and 180°C, preferably between 135 and 170°C, and is preferably equal to 150°C + 10°C. The heating fluid in the MVR-cycle is preferably water / steam.

[0021] The heat pump I MVR system can be provided with additional heat exchangers to increase the temperature of the drying gas in the distribution system. In an embodiment, the MVR-cycle comprises an MVR-distribution system heating loop branching off the MVR-cycle downstream of the MVR-compressor and joining back the MVR-cycle upstream of the MVR-compressor after passing through a second MVR-heat exchanger (X95b) configured for transferring heat from the heating fluid in the MVR-distribution system heating loop to the drying gas in the distribution system. A gas heat exchanger (X85) can be provided configured for transferring heat from the low boiling point fluid of the heat pump to the drying gas of the distribution system.

[0022] In an embodiment, a branched duct branches off the return exhaust ducts, wherein the branched duct is configured for flowing exhaust gas into the LT-chambers admixed with drying gas from the LT-distribution section, and out of the LT-chambers. The branched duct and exhaust ducts are provided with valves to control a ratio of an amount of exhaust gas from the exhaust system to anamount of drying gas from the LT-distribution section flowing into the LT-chambers.

[0023] The present invention also concerns a method for drying construction panels comprising,• providing an apparatus as defined supra,• driving a wet construction panel along the drying path through the chambers of the HT-drying zone followed by through the chambers of LT-drying zone,• flowing the drying gas through the distribution system into each of the N chambers,• heating with the heating elements the drying gas to desired temperatures prior to penetrating into the HT-chambers,• exhausting the exhaust gas from each of the N chambers through the exhaust system,• heat is exchanged in the first heat pump-heat exchanger (X78) from the exhaust gas in the exhaust system to the low-boiling point fluid in the heat pump and after compression of the low boiling point fluid,• heat is exchanged in the second heat pump-heat exchanger (X89) from the thus compressed low-boiling point fluid to the heating fluid in the MVR-cycle and after compression of the heating fluid,• heat is exchanged in the MVR-heat exchanger (X95L) from the thus compressed heating fluid to the drying gas in the distribution system flowing out of the LT-distribution section and before the drying gas is heated by the heating elements prior to penetrating into the respective HT-chambers (.

[0024] The construction panels can be panels made of a material comprising as main component in an amount of at least 50 wt.%, gypsum, or cement, or calcium silicate.BRIEF DESCRIPTION OF THE FIGURES

[0025] For a fuller understanding of the nature of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings in which:Figure 1 : shows a typical drying apparatus according to the prior art.Figure 2 shows an embodiment of drying apparatus according to the present invention.Figure 3 shows an alternative embodiment of the present invention.Figure 4 shows yet an alternative embodiment of the present invention.Figure 5 shows yet an alternative embodiment of the present invention.Figure 6 shows yet an alternative embodiment of the present invention.Figure 7 shows yet an alternative embodiment of the present invention.Figure 8 shows an example of heat pump coupled to an MVR-cycle according to the present invention.DETAILED DESCRIPTION OF THE INVENTION

[0026] The present invention concerns an apparatus for drying construction panels such as made of gypsum (= plasterboards or gypsum fibreboards) or panels made of cement or calcium silicate or fibre cement, during their production. The apparatus comprises a conveyor (40) for conveying the construction panels (60) along a drying path through a number N of chambers (1 , 2... N). A distribution system (5) distributes a heated drying gas into each chamber, with heating elements (11) provided for heating the drying gas in the distribution system before the drying gas reaches the chambers. An exhaust system (7) is provided for evacuating the exhaust gas out of the chambers.

[0027] The present invention is not limited to any specific type of conveyor (40) as long as it allows conveying wet construction panels (60) along the drying path through the chambers exposing both main surfaces of the panels to the drying gas. For example, the conveyor (40) can comprise rollers or a foraminous sheet configured for supporting and transporting the panels, while exposing both main surfaces of the panels to a flow of hot drying gas, typically hot air.

[0028] The drying unit comprises a total number N of chambers (1 , 2 ... N) distributed in series and in fluid communication with one another along the drying path. The N chambers are composed of a number i of high temperature chambers (1 , 2) (= HT chambers) located in a high temperature drying zone (HT) in an upstream portion of the drying path, and of a number j of low temperature chambers (N) (= LT-chambers) located in a low temperature drying zone (LT) downstream of the high temperature drying zone (HT) along the drying path, wherein 1 < i < N — 1 , and j = N - i. Figures 2 to 6 illustrate a total number N = 3 chambers, a number i = 2 HT-chambers (1 , 2) and a number j = 1 LT-chamber (N), but it is clear that the numbers N, i, and j of chambers is not limited and can vary depending on the specific applications.

[0029] The distribution system (5) is in fluid communication at one end with a source of drying gas (5s), generally air and, at other ends, with the N chambers (1 , 2 ... N). It is configured for circulating a flow of the drying gas along a drying gas flowing path extending from the source of drying gas into the N chambers. Heating elements (11) configured for heating the drying gas in the distribution system (5) are arranged in the distribution system (5) before the drying gas penetrates into the respective HT-chambers (1 , 2). The LT-chambers (N) do not need a heating element for the stationary use of the dryer. Usually, shutdown heaters are provided for use only during the shutdown of a panel drying session (cf. shutdown burner (11s) in Figures 3 to 6. The distribution system (5) comprises a LT-distribution section for distributing drying gas into the LT-chambers (N) and a HT-distribution section for distributing drying gas into the HT-chambers (1 , 2), and coupled in series and downstream of the LT-distribution section, wherein

[0030] The LT-distribution section comprises one or more drying gas feeding ducts (5N) leading to thecorresponding one or more LT-chambers (N) in parallel. The HT-distribution section comprises one or more drying gas feeding ducts (51 , 52) leading to the corresponding one or more HT-chambers (1 , 2) in parallel.

[0031] The exhaust system (7) is configured for evacuating out of the apparatus exhaust gas along an exhaust flowing path extending from the N chambers (1 , 2, ... N) to outside the apparatus.

[0032] The exhaust system (7) comprises return exhaust ducts (7h) leading in parallel to corresponding LT-chambers. A LT-heat exchanger (X75N) is provided in each LT-chamber (N) and is configured for transferring heat from the exhaust gas flowing in the corresponding return exhaust ducts (7h) to the drying gas as it flows through the corresponding LT-chambers (N).

[0033] The gist of the present invention is the provision of a heat pump I MVR system composed of the heat pump (8) and MVR-cycle (9). The “heat pump / MVR system" is used to recover heat from the exhaust gas flowing in the exhaust system (7) to substantially raise the temperature of the drying gas flowing in the distribution system (5) before it reaches the drying gas feeding ducts (51 , 52) of the HT-distribution section. The heat pump / MVR system comprises a heat pump (8) and a mechanical vapour recompression (MVR-) cycle (9), coupled to respective heat exchangers (X78, X89, X95L) for efficiently transferring calorific energy from the exhaust gas to the drying gas of the HT-distribution section of the distribution system (5). In particular, the MVR-heat exchanger (X95L) configured for transferring heat from the heating fluid of the MVR-cycle (9) to the drying gas in the distribution system (5) after the drying gas flowed out of the LT-distribution section and flowing towards or in the HT-distribution section, prior to penetrating into the respective HT-chambers (1 , 2).

[0034] The MVR-heat exchanger (X95L) downstream of the LT-distribution section and upstream of the HT-chambers (1 , 2) raises the temperature of the drying gas flowing in the distribution system (5) towards the HT-chambers (1 , 2) prior to reaching the corresponding heating elements (11). This has the effect that less energy must be provided by the heating elements (11) to heat the drying gas to the desired temperatures (T1 , T2) required in the HT-chambers (1 , 2). The exhaust gas flowing out of the HT-chambers in the exhaust system (7) still retains considerable heat which is used to heat the drying gas flowing in the LT-chambers (N) as follows.

[0035] A first heat pump-heat exchanger (X78) is configured for transferring heat from the exhaust gas in the exhaust system (7) to a low boiling point fluid in the heat pump (8). The heat pump (8) comprises a closed loop duct for circulating the low boiling point fluid and comprises a heat pump compressor (8c) configured for compressing and increasing a temperature of the low boiling point fluid, and for driving a flow of the low boiling point fluid in the closed loop duct from the first heat pump-heat exchanger (X78) to a second heat pump-heat exchanger (X89). The LT-heat exchanger (X75N) is preferably located upstream of the first heat pump-heat exchanger (X78) relative to the flow direction of the exhaust gas in the exhaust system (7).

[0036] The second heat pump-heat exchanger (X89) is configured for transferring the heat thuscaptured by the low boiling point fluid to a heating fluid, preferably steam, having a boiling temperature higher than the low boiling point fluid of the heat pump (8). The heating fluid is configured for circulating in the MVR-cycle (9). The MVR-cycle (9) comprises an MVR-compressor (9c) configured for compressing the heating fluid, and thus increasing a temperature of the heating fluid, and for driving a flow of the heating fluid from the second heat pump-heat exchanger (X89) to an MVR-heat exchanger (X95L).

[0037] The MVR-heat exchanger (X95L) is configured for transferring heat from the heating fluid of the MVR-cycle (9) to the drying gas in the distribution system (5), downstream of the LT-distribution section and upstream of the HT-distribution section. The MVR-heat exchanger (X95L) is configured for transferring considerable heat to the drying gas flowing towards the HT-chambers (1 , 2), which is beneficial, as heating element (11) of smaller dimensions and power can be used to heat an already warm drying gas to the desired temperature required in the HT-chambers (1 , 2). Tests have shown that the temperature of the drying gas reaching the inlet of the heating elements (1 1) of the HT-chambers in a dryer according to the present invention comprising an MVR-heat exchanger (X95L) as defined supra, was 30 to 40°C higher than in a dryer of the prior art, without MVR-heat exchanger (X95L), as illustrated in Figure 1. For example, if the drying gas temperature required in the first chamber (1) is T1 = 260°C, and the temperature of the drying gas reaching the corresponding heating element (11) is 110°C without MVR-heat exchanger and 140°C with MVR-heat exchanger (X95L), 20% less heat must be transferred to the air to reach the set temperature of 260°C, than without the MVR-heat exchanger.

[0038] The heat can be transferred from the drying gas to the construction panels at least by forced convection of the drying gas but also by radiation. In this embodiment, the drying gas is preferably air, which flows in contact over the two main surfaces of the construction panels (60) directly after being heated to the corresponding predefined temperatures by the heating elements (11) In the HT-chambers (1 , 3) and by the LT-heat exchanger (X75N) in the LT-chambers (N).DISTRIBUTION SYSTEM (5) AND EXHAUST SYSTEM (7)

[0039] The distribution system comprises ducts configured for circulating a flow of the drying gas along a drying gas flowing path extending from the source of drying gas (5s) into the N chambers. The drying gas being circulated is preferably air which, after heating to desired temperatures is blown over the two main surfaces of the panels as they travel through the successive chambers. Heat is thus transferred from the drying gas to the main surfaces of the panels by forced convection and moisture is removed from the panels.

[0040] The distribution system comprises an upstream end coupled to the source of the drying gas (5s). If the drying gas is air, the source of air can be atmospheric air at room temperature. A blower or fan (5f) is provided for flowing the drying gas from the source of drying gas (5s) into the distribution system (5). The distribution system (5) is divided into a LT-distribution section for distributing drying gas into the LT-chambers (N) coupled downstream of the LT-distribution section in series with aHT-distribution section for distributing drying gas into the HT-chambers (1 , 2). The LT-distribution section comprises one or more drying gas feeding ducts (5N) leading to the corresponding one or more LT-chambers (N) in parallel. Similarly, the HT-distribution section comprises one or more drying gas feeding ducts (51 , 52) leading to the corresponding one or more HT-chambers (1 , 2) in parallel.

[0041] The LT-distribution section comprises j drying gas feeding ducts (N) leading in parallel to the corresponding j LT-chambers (N), and the HT-distribution section comprises i drying gas feeding ducts (51 , 52) leading in parallel to the corresponding i (= (N - j)) LT-chambers (1 , 2). A liaising duct (5L) connects in series the HT-distribution section to the LT-distribution section. The MVR-heat exchanger (X95L) is positioned on the liaising duct (5L), between the LT- and HT-distribution sections.

[0042] The drying gas feeding ducts (5N) of the LT-distribution section lead directly downstream of each LT-chamber (N) to allow the drying gas to flow counter-current relative to the drying path of the construction panels (60) in heat exchange contact with the LT-heat exchanger (X75N). As shown in Figures 2 to 7, the drying gas can be preheated in the pre-heating heat exchanger (X75) between the exhaust system (7) and the distribution system, upstream of the LT-chambers (N). After reaching an upstream end of the corresponding LT-chamber (N) (upstream relative to the drying path), the drying gas exits the LT-chambers and joins the liaising duct (5L) and the MVR-heat exchanger (X95L) to acquire some heat from the heat pump I MVR system, prior to reaching the HT-distribution section.

[0043] Contrary to the drying gas feeding ducts (N) of the LT-distribution section, as the drying gas feeding ducts (51 , 52) of the HT-distribution section reach the corresponding HT-chambers (1 , 2), they each form a drying gas flow cycle comprising a chamber fan (1f, 2f) for driving the drying gas flow (e.g., air) along the drying gas flow cycle. The drying gas flow cycle in the HT-chambers is configured for flowing the drying gas (e.g., air) through a heating element (11) such as an air-fuel burner, and into the corresponding HT-chamber (1 ,2) to lick both main surfaces of the panels to increase their temperature and remove most moisture therefrom.

[0044] In the HT-distribution system, all or a fraction of the drying gas (e.g., air) can run one or more cycles from the chamber to the heating element (11), until the moisture content thereof is considered too high, at which point it can be driven out of the drying gas flow cycle into the exhaust system (7). Valves (5v, 7v) are provided in both distribution system (5) and exhaust system (7) to control the moment and the fraction of drying gas (e.g., air) to be introduced into and exhausted from the drying gas flow cycle. Heating elements (1 1) are used in a stationary use of the dryer in the HT-chambers (1 , 2) only. As mentioned supra, the LT-chambers can be equipped with shutdown burners (11s) (illustrated with dotted lines in the Figures), which are used exclusively during a shutdown operation of the dryer at the end of a drying session after the heating elements (11) in the empty HT-chambers have been turned off. At the end of a drying operation, as the last panels have run through the HT-chambers, the corresponding heating elements (11) are turned off and the temperature of the exhaust gas in the exhaust system (7) is not sufficient any more to heat to the predefined temperature the drying gasflowing into the LT-chambers (N). The shutdown burners (11s) are used exclusively to this end and are not used to heat the LT-chambers during “normal” or stationary use of the apparatus. Alternatively, the shutdown burners (11s) can be used to heat the drying gas flowing into the LT-chambers (N) during stationary drying operation at a fraction only of their full heating capacity, such as not more than 25%, preferably not more than 10% of the full heating capacity of the shutdown burners (11s). It is, however, preferred that no heating element be used to heat the LT-chambers in a stationary use of the apparatus. The apparatus may comprise a processing unit configured for controlling that the shutdown burners (11s) remain off or at a fraction only (e.g., not more than 25%, or not more than 10%) of their full heating capacity during stationary use of the apparatus and the drying gas flowing in the LT-distribution section and into the LT-chambers is heated exclusively by means of heat exchangers, including the preheating-heat exchanger (X75) upstream of the LT-chambers (N) relative to the drying gas flow direction in the distribution system (5).

[0045] When the drying gas flowing several times into the corresponding HT-chambers (1 , 2) along the drying gas flow cycle as removed sufficient moisture from the construction panels (60), thus reaching a high moisture content, it is exhausted from the cycle through the exhaust system (7). The exhaust gas still retains considerably heat. For example, the temperature of the exhaust gas outside of the HT-chambers can be of the order of 170°C to 200°C, typically 180°C + 10°C. Each HT-chamber has an exhaust gas duct (71 , 72) exiting the corresponding drying gas flow cycle and all joining in at a central exhaust duct leading to return ducts (7h). Each return duct (7h) joins a corresponding LT-heat exchanger (X75N) of a corresponding LT-chamber (N). After leaving the LT-heat exchanger (X75N), the exhaust gas preferably flows through the pre-heating-heat element (X75) to pre-heat the drying gas flowing in the distribution system (5) prior to entering into the LT-chambers (N). The gist of the present invention is that at this stage, instead of releasing the exhaust gas in the atmosphere, the exhaust gas is flowed through the first heat pump-heat exchanger (X78) where it releases part of its calorific energy to the low boiling point fluid of the heat pump (8). At this stage, substantial amounts of condensation water (8w) are formed by condensation of the cooling high moisture content exhaust gas. The condensation water (8w) can be used for forming new construction plates. Before being added to the base composition, the heat retained in the condensation water (8w) is preferably driven through the condensation heat exchanger (X8w5) to heat the drying gas flowing directly from the source of drying gas (5s) priorto reaching eitherthe pre-heating heat exchanger (X75) orthe LT-heat exchanger (X75N).

[0046] As shown in Figures 2, 3, and 7, valves (7v) allow an accurate control of the amount of exhaust gas introduced from the return exhaust ducts (7h) into the LT-heat exchangers (X75N) to control the temperature transferred to the drying gas in the LT-heat exchangers (X75N). In an embodiment illustrated in Figure 7, the return exhaust ducts (7h) each comprise a branching duct (7h5) joining in fluid communication the distribution system (5), upstream of the entry points of the corresponding LT-chambers (N) and configured for flowing exhaust gas into the LT-chambers (N) admixed with fluid from the distribution system (5). The drying gas exhaust ducts (7h) and branching ducts (7h5) areprovided with valves (7v) to control a ratio of amount of exhaust gas to amount of drying gas. This is important, since the exhaust gas is loaded with substantial amounts of moisture, and careful balance of the moisture content of the resulting gas mixture blown onto the construction boards travelling through the LT-chambers (N) is required to optimize the drying operation of the boards. The valves (5v, 7v) of the distribution system (5) and of the exhaust system (7) can be controlled by a processor. Preferably the processor is configured for controlling in a closed loop the flowrates through the control of the fans (1f, 2f, 5f, 7f) and the valves (5v, 7v) as a function of the temperature of the drying gas flowing through the corresponding chambers (1 , 2...N).

[0047] If the drying gas is air, the heating elements (11) are preferably air-fuel burners fed with a fraction of the air flowing through the drying gas feeding ducts (51 , 52) leading to the HT-chambers. The fuel (generally a gas such as acetylene, natural gas, or propane) is fed to the air-fuel burners via ducts (9g) as shown in Figures 2 to 6. Alternatively, the heating elements (11) can be electrical heaters or heat exchangers receiving a flow of hot fluid, such as a hot oil to increase the temperature of the drying gas. In these cases, the drying gas can be any gas, although air is readily available and is preferred.

[0048] To increase the temperature of the drying gas flowing in the distribution system (5) before it reaches the LT-chambers (N), it is preferably preheated in a pre-heating heat exchanger (X75) configured for transferring heat from the exhaust fluid in the exhaust system (7) to the drying gas in the distribution system (5). The pre-heating heat exchanger (X75) is preferably located upstream of the LT-chambers (N) relative to the flow direction in the distribution system (5), and preferably upstream of the first heat pump-heat exchanger (X78) relative to the flow direction in the exhaust system (7).

[0049] Unless otherwise defined, the terms “upstream” and “downstream” are defined herein relative to the flow direction of a fluid in the corresponding fluid system, such as the drying gas in the distribution system (5), the exhaust gas in the exhaust system (7), the low boiling point fluid in the heat pump (8), or the heating fluid in the MVR-cycle (9).

[0050] Preheating the drying gas prior to reaching the LT-chambers is advantageous as the sole remaining source of heat for bringing the drying gas flowing through the corresponding LT-chambers (N) to the required temperature is the corresponding LT-heat exchangers (X75N). The preheating heat exchanger (X75) can heat a flow of drying gas from room temperature to about 100 + 10°C before it penetrates into the corresponding LT-chambers (N) and be further heated to the desired temperature by contact with the LT-heat exchanger (X75N) to reach a temperature of fer example about 110 + 10°C upon exiting the LT-chambers.

[0051] The temperature in the LT-chambers (N) can be controlled by controlling the drying gas flowrate in the distribution system (5) by means of the fan (5) in the distribution system, and by controlling the temperature of the drying gas flowing through the LT-chambers (N). The latter can be achieved by controlling the exhaust gas flowrate in the preheating heat exchanger (X75) and LT-heat exchangers(X75N). The temperature in the HT-chambers can be controlled via the heating elements (11). Smaller heating elements (11) can be used thanks to the heat pump I MVR system, which raises the temperature of the drying gas flowing in the distribution system (5) prior to reaching the heating elements (11).

[0052] For drying plasterboards, the temperature (T1 ; T2) in the HT-chambers (1 , 2) can vary between a low temperature (T1 L, T2L) and a hight temperature (T1 H, T2H) which can range between 120 and 260°C, preferably between 150 and 250°C, with preferably a maximum temperature in a mid-section of the HT-drying zone. The temperature (TN) in the one or more LT-chambers (N) has an average value lower than an average temperature in the HT-chambers and can vary between a low temperature (TNL) and a hight temperature (TNH) which can range between 90 and 170°C, preferably between 100 and 160°C, with a maximum temperature at an upstream section of the LT-drying zone, wherein upstream is defined relative to the direction of the drying path.HEAT EXCHANGERS (X75, X78, X75N)

[0053] As shown in Figures 2 to 7, the exhaust system (7) comprises gas exhaust ducts (71 , 72) for extracting the drying gas from the HT-chambers when the temperature or moisture contents of the drying gas flowing into the HT-chambers along the drying gas flow cycles is considered out of predefined boundaries. The gas exhaust ducts (71 , 72) are coupled to the corresponding drying gas flow cycles at a position between the chamber fan (1f, 2f) and the heating elements (11) of the HT-chambers (1 , 2).

[0054] The exhaust gas exiting the HT-chambers through the exhaust system (7) is colder and more humid than the drying gas entering the chambers through the distribution system (5) but still retains substantial heat. The exhaust gas can be at a temperature of the order of 180 + 20°C depending on the temperatures (T1 , T2) required in the HT-chambers (1 , 2). The aim of the present invention is to recover as much calorific energy as possible from the exhaust gas by means of different heat exchangers (X75, X78, X75N).

[0055] Unlike the HT-chambers (1 , 2), the drying gas flowing into the LT-chambers (N) is not heated by a heating element (11), (at least not in stationary use), but is exclusively heated by heat transfer from the exhaust gas in the optional pre-heating-heat exchanger (X75) located upstream of the LT-chambers (N) relative to the flow direction in the distribution system (5), and as it flows through the LT-chambers (N) with the LT-heat exchanger (X75N). As mentioned supra, the LT-chambers (N) can be equipped with shutdown heating elements (11s) used only during shutdown of a drying operation. This can be controlled by a processor. Each LT-heat exchanger (X75N) comprises a channel configured for circulating exhaust gas releasing heat to a radiation surface located inside the corresponding LT-chamber (N), heating the drying gas flowing through the LT-chamber. The drying gas flows through the LT-chambers (N) counter-currently relative to the drying path followed by the construction panels (60), so that the drying gas contacts the substantially fully dried panels when it enters into a LT-chamber(N) with its lowest temperature, and builds up heat as it flows through the LT-chamber to leave it at its highest temperature, where the moisture contents of the panels is low, but higher than at the exit of the LT-chamber. For example, if the drying gas was pre-heated in the pre-heating heat exchanger (X75), it can enter into the different LT-chambers with a temperature of the order of 100°C + 10°C, and leave the chambers with a higher temperature of the order of 110°C + 10°C. The heating of the drying gas is moderate, as much of the calorific energy acquired from the radiation surface of the LT-heat exchanger (X75N) is transferred to the panels (60).

[0056] After flowing through the LT-heat exchanger (X75N) and optionally the pre-heating-heat exchanger (X75), the exhaust gas has lost considerable heat, but is still relatively warm. For example, the exhaust gas can still be at a temperature above 70°C, such as 75 to 85°C. Releasing the exhaust gas in the atmosphere at such temperatures is a waste of energy. Part of the calorific energy still transported by the exhaust gas is transferred to the heat pump I MVR system by flowing it through the first heat pump-heat exchanger (X78) described below.HEAT PUMP (8) AND MVR-CYCLE (9)

[0057] The gist of the present invention is the heat pump I MVR system used to recover heat from the exhaust gas flowing in the exhaust system (7) to substantially raise the temperature of the drying gas flowing in the distribution system (5) before the drying gas reaches the HT-chambers (1 , 2). It comprises the heat pump (8) in heat conductive contact with the MVR-cycle (9) via the second heat pump-heat exchanger (X89), as illustrated in Figure 8.Heat Pump (8)

[0058] As shown in Figure 8, the heat pump (8) comprises a closed circuit circulating a low-boiling point fluid, driven by a heat pump compressor (8c). The low boiling point fluid can have a boiling temperature of not more than -10°C, preferably not more than -20°C. For example, the low boiling point fluid can be a hydrofluorocabon (HFC) such as tetrafluoroethane, a halogenated fluorocarbon (PFC), a halogenated fluorine olefin (HFO), or a natural gas such as propane. The low-boiling point fluid collects in the first heat pump-heat exchanger (X78) heat from the exhaust gas flowing in the exhaust system (7). As the thus heated low boiling point fluid passes through the heat pump compressor (8c) the pressure is increased, and the temperature follows the same trend. The heat thus accumulated by the low boiling point fluid is transferred to a heating fluid, preferably steam, flowing in the MVR-cycle (9) through the second heat pump-heat exchanger (X89). The pressure of the thus cooled low boiling point fluid is reduced by an expansion device (8e) to prevent or restrict condensation before flowing back to the first heat pump-heat exchanger (X89) and resuming the foregoing cycle.

[0059] During the transfer of heat in the first heat pump-heat exchanger (X78), the temperature of the moist exhaust gas drops and condensation water (8w) is formed which has a higher temperature than the drying gas at the source of drying gas (5s). As shown in Figures 3 to 7, to profit of the heattransported by the condensation water (8w), the apparatus can comprise a condensed heat exchanger (X8w5) configured for transferring heat from the condensation water (8w) to the drying gas flowing from the source of drying gas (5s) into the distribution system (5). Since the temperature of the condensation water is not very high, the condensed heat exchanger (X8w5) is preferably positioned upstream of the pre-heating heat exchanger (X75) relative to flow direction in the distribution system (5). Beside preheating the drying gas, the use of the condensed heat exchanger (X8w5) has the advantage of also lowering the temperature of the condensate water (8w) which can be used as such for other operations, such as the formation of new construction boards (60), by adding the condensation water (8w) to the mixer upstream of the production line and well upstream of the drying apparatus. This substantially reduces the water consumption of the board manufacturing process as a whole.

[0060] As illustrated in Figure 6, downstream of the second heat pump-heat exchanger (X89) and upstream of the expansion device (8e) (not shown for sake of clarity) and of the first heat pump-heat exchanger (X78), the heat still remaining in the low boiling point fluid can be transferred to the drying gas flowing in the distribution system (5) by flowing the low boiling point fluid through a fluid heat exchanger (X85) in heat contact with the drying gas in the distribution system (5).

[0061] The temperature of the low boiling point fluid of the heat pump (8) in the second heat pump-heat exchanger (X89) can typically be comprised between 90 and 110°, and is preferably equal to 100°C ± 5°C.MVR-Cycle (9)

[0062] The heating fluid, preferably water I steam, of the MVR-cycle captures heat from the low boiling point fluid of the heat pump (8) at the second heat pump-heat exchanger (X89).

[0063] The flow of the heating fluid is driven by the MVR-compressor which increases the pressure of the heating fluid, and thus the temperature too. The thus compressed and heated heating fluid flows through the MVR-heat exchanger (X95L), which is configured for transferring heat from the heating fluid of the MVR-cycle (9) to the drying gas in the distribution system (5) at the level of the liaising duct (5L) joining the upstream LT- distribution section to the downstream HT-distribution section. The MVR-cycle can be an open cycle requiring no expansion device (9e) and producing condensation water which can be recovered. In this case, however, the MVR-cycle must be replenished with heating fluid at regular intervals, which is not readily available from the process. For this reason, the MVR-cycle (9) of the present invention is preferably a closed cycle wherein any condensation liquid (generally water) formed during the transfer of heat from the heating fluid to the drying gas flowing in the distribution system (5) is vaporized by flowing the heating fluid through an expansion device (9e) as illustrated in Figure 8 (not shown in Figures 2 to 7 for sake of clarity).

[0064] The MVR-heat exchanger (X95L) is located in the distribution system (5) upstream from the entry point of the drying gas into the respective HT-chambers (1 , 2) at the level of the liaising duct (5L). Tests have shown that as the temperature of the heating fluid flowing through the MVR-heat exchanger(X95L) can be comprised between 120 and 180°C, preferably between 135 and 170°C, and is preferably equal to 150°C + 10°C, the MVR-heat exchanger (X95L) can increase the temperature of the drying gas flowing in the distribution system (5) of 30 to 40°C. This can contribute to from 20 to 75% of the heat to be transferred to the drying gas to reach the required temperature before flowing into the HT- chambers (1 , 2). This is a considerable saving of energy, allowing heating elements (11) of smaller dimensions to be used to heat the thus pre-heated drying gas to its required temperature. Furthermore, if the drying gas is air and the heating elements (1 1) are air-fuel burners, feeding pre-heated air to the air-fuel burners increases the efficacy of the latter.

[0065] The energy required for operating a heating element (11) of the type of an air-fuel burner or an electrical heater or other heating elements (11) to heat the drying gas by an extra 30 to 40°C is replaced by the energy required for operating the heat pump / MVR-system, i.e., for operating the heat pump compressor (8c) and the MVR-compressor (9c). Operating the heat pump / MVR system consumes energy, but considerably less than the energy required for operating the heating elements (1 1), as long as the coefficient of performance (= COP) discussed below remains greater than unity.

[0066] The coefficient of performance (= COP) is defined as a ratio (Q / W) of a heat (Q) supplied by the MVR-heat exchanger (X95L) to a work (W) required to operate the heat pump-MVR-system namely, to operate the heat pump compressor (8c) and the MVR-compressor (9c). It is an excellent indicator of the efficacy of the heat pump / MVR-system. Tests have shown that the COP of the heat pump I MVR-system can be comprised between 2 and 5, preferably between 2.5 and 4 (to be efficient, heat pump I MVR-system must have a COP > 1).

[0067] In an embodiment illustrated in Figure 6, the distribution system (5) can comprise a bypass duct (5b) bypassing the LT-chambers (N) and leading the drying gas directly to the liaising duct (5L). Valves (5v) are provided to control the fraction of drying gas flowing through the bypass duct (5b) and through the LT-chambers (N). As shown in Figure 6, the bypass duct (5b) can start downstream of any number of heat exchangers (X8w5, X85, X75), but in all cases upstream of the LT-heat exchanger (X75N). The MVR-loop can comprise an MVR-distribution heating loop (95) branching off the MVR-cycle (9), downstream of the MVR-compressor (9c) and joining back the MVR-cycle (9) upstream of the MVR-compressor (9c). In between, the MVR-distribution heating loop (95) passes through a second bypass-heat exchanger (X95b) configured for transferring heat from the heating fluid in the MVR-cycle (9) to the drying gas in the bypass duct (5b). Since the heating fluid enters into the MVR- distribution heating loop (95) downstream of both MVR-compressor (9c) and MCR-heat exchanger (X95L) but upstream of the expansion device (9e), the heating fluid has a high pressure and still a rather high temperature, lower than at the inlet of the MVR-heat exchanger (X95N), since the heating fluid has already transferred part of its heat to the drying gas flowing in the liaising duct (5L) through the MVR-heat exchanger (X95N). The MVR-heating loop (95) is preferably provided with a valve (9v) to control the fraction of heating fluid flowing through the MVR-heating loop (95). The valve (9v) can be controlled by a processor.

[0068] As shown in Figure 6, the drying gas in the distribution system (5) can also collect heat directly from the heat pump (8) via a fluid heat exchanger (X85) as discussed supra.

[0069] The preferred heat exchangers discussed herein can be selected and used in any combination. When positioning several heat exchangers in a same duct of the distribution system (5), or of the exhaust system (7), or of the heat pump I MVR system, it is important to ensure that the temperature difference between the high temperature fluid and the low temperature fluid is high enough to transfer substantial amounts of heat from the high temperature fluid to the low temperature fluid.

[0070] Upstream of the heating elements (11), the temperature of the drying gas flowing in the distribution system (5) is lowest at the level of the source of drying gas (5s), where it is generally at room temperature, and increases each time it passes through a heat exchanger (X8w5, X85, X75, X75N, X95b, X95L) to be highest when reaching the drying gas feeding ducts (51 , 52). Inversely, the temperature of the exhaust gas is highest when leaving the drying gas flow cycles of the HT-chambers (1 , 2) and flows through the corresponding exhaust gas ducts (71 , 72) and drops every time it flows through a heat exchanger (X75N, X75, X78).

[0071] The heat exchanger whose high temperature fluid has the lowest temperature is therefore preferably positioned most upstream of the distribution system (5) where the drying gas is cooler to maintain a substantial temperature gradient between hot and cold sources. In the present case, as illustrated in Figure 6, the ducts of the distribution system (5) can pass through one or more of the condensed heat exchanger (X8w5), the fluid-heat exchanger (X85), the pre-heating heat exchanger (X75), and the second MVR-heat exchanger (X95), before reaching the drying gas feeding ducts (51 , 52) of the HT-distribution section leading to the HT-chambers (1 , 2). They are preferably positioned along the distribution system in the following sequence,• the condensation water (8w) produced in the first heat pump heat exchanger (X78) flowing into the condensed heat exchanger (X8w5) has the lowest temperature of all high temperature fluids. Consequently, if present, the condensed heat exchanger (X8w5) is preferably positioned most upstream of the distribution system (5).• The low boiling point fluid flowing through the heat pump (8) has a temperature when passing through the fluid heat exchanger (X85) which is lower than when it passes through the second heat pump-heat exchanger (X89) where the temperature can be comprised between 120 and 180°C. As the low boiling point fluid still carries sufficient heat, if present, the fluid heat exchanger (X85) can therefore safely be positioned downstream of the condensed heat exchanger (X8w5).• The pre-heating heat exchanger (X75) is generally positioned upstream of the first heat pump-heat exchanger (X78) relative to the flow direction in the exhaust system (7). The temperature of the exhaust gas flowing through the pre-heating heat exchanger (X75) is therefore higher than the one flowing through the first heat pump-heat exchanger (X78).Depending on whether the temperature of the exhaust gas in the first heat pump-heat exchanger (X78) is higher or lower than the temperature of the low boiling point fluid flowing through the fluid heat exchanger (X85), the pre-heating heat exchanger (X75) is preferably positioned downstream or upstream of the first heat pump-heat exchanger (X78), respectively. In the example of Figure 6, the fluid heat exchanger (X85) is positioned upstream of the pre-heating heat pump (X75) relative to the flow direction in the distribution system (5), suggesting that the temperature of the exhaust gas flowing through the pre-heating heat exchanger (X75) is higher than the temperature of the low boiling point fluid flowing through the fluid heat exchanger (X85). But this must be assessed from the design of the dryer.• The temperature of the heating fluid in the MVR-distribution heating loop (95) is quite high, of the order of 100 and 240°C and the bypass-heat exchanger (X95b) can therefore be positioned most downstream of the LT-distribution section, before reaching the liaising duct (5L) and MVR-heat exchanger (X95L) which is the last heat exchanger on the distribution system (5) prior to reaching the HT-chambers (1 , 2).Heat Transfers

[0072] As discussed supra, heat is recovered from the exhaust gas flowing in the exhaust system (7) to be transferred to the drying gas of the distribution system (5) through a number of heat exchangers. The heat transfer can be achieved,• through direct conduction with both drying gas and exhaust gas flowing in heat transfer communication through separate channels of the heat exchanger, as in the pre-heating heat exchanger (X75), or• by radiation and forced convection with the drying gas flowing against the radiating surfaces of the LT-heat exchangers (X75N), heated by exhaust gas flowing through a channel of the LT-heat exchanger (X75N), or• the heat of the exhaust gas can be transferred indirectly to the drying gas through the warm condensation water (8w) produced in the condensation heat exchanger (X8w5) or, as is unique of the present invention,• the heat of the exhaust gas can be transferred indirectly to the drying gas through the heat pump I MVR system.

[0073] The drying gas flowing through the LT-chambers (N) is heated exclusively by the heat transferred directly or indirectly from the exhaust gas both upstream of the LT-chambers with heat exchangers (X8w5, X85, X75) and in the LT-chambers (N) with the LT-heat exchanger (X75N). No heating element (11) is used to heat the drying gas flowing in the LT-chambers (N) (other than during shutdown of the line, wherein shutdown heating elements (11s) can be used to heat the drying gas flowing in the LT-chambers (N) once the heating elements (11) in the HT-chambers have been switchedoff after the last panels (60) left the HT-chambers (1 , 2). This already results in a substantial saving of energy. The present invention pushes the energy saving even further with the heat pump I MVR system.

[0074] The gist of the present invention is to transfer heat from the exhaust gas flowing in the exhaust system (7) to the heat pump I MVR-system, when the exhaust gas would normally be released in the atmosphere. The heat pump I MVR-system allows increasing the temperature of the heating fluid in the MVR-heat exchanger (X95L) to sufficiently high values to heat the drying gas flowing in the liaising duct (5L) to higher temperatures. The temperature increase can be of the order of 30 to 40°C, preferably of 33 to 37°C, above the drying gas temperature when flowing out of the LT-chambers (N). The temperature increase is beneficial at the moment of heating the drying gas with the heating elements (11) to the required temperatures prior to entering into the HT-chambers (1 , 2).

[0075] For example, if the drying gas leaving the LT-chambers and reaching the MVR-heat exchanger (X95L) iae at a temperature of the order of 1 10 + 10°C. The drying gas temperature can pick up 30 to 40°C by flowing through the MVR-heat exchanger (X95L), such that the temperature of the drying gas downstream of the MVR-heat exchanger (X95L) can be of the order of 130 to 140°C + 10°C.

[0076] The drying gas reaching the HT-chambers is heated to high temperatures with the heating elements (1 1) in the corresponding drying gas flow cycles. The required drying gas temperature before flowing into the HT-chambers (1 , 2) can be comprised between 120 and 260°C, preferably between 150 and 250°C, with a maximum temperature in a mid-section of the HT-drying zone. Referring to the drying gas temperature examples supra, in a prior art dryer, the drying gas would have to be heated from e.g., 110°C to about 250°C, i.e., the drying gas temperature would have to be increased of 250 - 110 = 140°C. With a dryer according to the present invention, the drying gas can reach the heating elements (1 1) with a temperature of say 140°C, reducing the temperature increase required to reach the required temperature to 1 10°C, i.e., 79% of the temperature increase required with the prior art dryer. This represents a considerable energy saving.

[0077] After being heated to the required temperatures (T1 , T2) by the heating elements (11), the drying gas is circulated in the drying gas flow cycles against the main surfaces of the panels (60) as they travel through the HT-chambers (1 , 2) along the drying path to increase the temperature thereof and remove moisture therefrom. At each cycle, a fraction of the thus cooled and moistened drying gas can be exhausted and the rest recirculated, mixed with fresh and preheated drying gas from the source of drying gas (5s). Alternatively, the whole drying gas is recirculated during various cycles in a closed circuit until it is exhausted to be replaced by a fresh drying gas admitted in the drying gas flow cycle. These operations can be controlled by means of the valves (5v, 7v). In both cases, the exhaust gas extracted out of the HT-chambers (1 , 2) is still quite warm, with temperatures which can be of the order of 180°C + 20°C, depending on the temperature in the HT-chambers (1 , 2) and the amount of moisture contained in the panels. Like in previous attempts of the prior art, it is mostly the heat stored in the exhaust gas that is to be recovered, but the apparatus of the present invention takes substantially moreprofit of it than prior art apparatuses.

[0078] As shown in Figure 8, the heat pump / MVR-system requires three heat exchangers:• The first heat pump-heat exchanger (X78) is configured for transferring heat from the exhaust gas in the exhaust system (7) to the low boiling point fluid in the heat pump (8), when the exhaust gas would normally be disposed of in the prior art dryers,• The second heat pump-heat exchanger (X89) is configured for transferring the heat of the low boiling point fluid at high temperature and pressure to the heating fluid of the MVR-cycle (9) at low temperature and pressure, and• The MVR-heat exchanger (X95L) is configured for transferring heat from the heating fluid of the MVR-cycle (9) at high temperature and pressure to the drying gas flowing in the liaising duct (5L) of the distribution system (5).

[0079] As illustrated in Figures 2 to 7, the apparatus can comprise a pre-heating heat exchanger (X75) for directly transferring part of the heat of the exhaust gas flowing in the exhaust system (7) to the drying gas flowing in the distribution system (5). This is straightforward and is clearly a preferred embodiment. But the pre-heating heat exchanger (X75) alone is not sufficient to make without heating elements (11) in the LT-chambers (N), and the LT-heat exchangers (X75N) located in heat transfer contact with the LT-chambers (N) are essential to make without heating elements (11) in the LT-chambers (N).First Heat Pump-Heat Exchanger (X78)

[0080] The low boiling point fluid flowing in the heat pump (8) is in a gaseous state at a low temperature and low pressure when it enters into heat transfer contact with the exhaust gas in the first heat pump-heat exchanger (X78). As the low boiling point fluid exits the first heat pump-heat exchanger (X78), it has a higher temperature from the heat collected from the exhaust gas and maintains the low pressure. Pressure and temperature are increased as it passes through the heat pump compressor (8c) and reaches the second heat pump-heat exchanger (X89) at a higher temperature and higher pressure. When the low boiling point fluid exits the second heat pump-heat exchanger (X89), it is at lower temperature because of the heat released to the heating fluid, and at the high pressure, and part of it may have condensed. The low boiling point fluid is then expanded through the expanding device (8e) shown in Figure 7 (not shown in Figures 2 to 7 for sake of clarity), prior to being reintroduced into the first heat pump-heat exchanger (X78) at the low temperature and low pressure.Second Heat Pump-Heat Exchanger (X89)

[0081] The heating fluid flowing in the MVR-cycle (9) is at least partly in a gaseous state at a low temperature and low pressure when it enters into heat transfer contact with the low boiling fluid (at high temperature and high pressure) in the second heat pump-heat exchanger (X89). As the heating fluid exits the second heat pump-heat exchanger (X89), it has a higher temperature and maintains the low pressure. Pressure and temperature are increased as it passes through the MVR-compressor (9c) andreaches the MVR-heat exchanger (X95L) at a higher temperature and higher pressure. When the heating fluid exits the MVR-heat exchanger (X95L), it is at the low temperature and at the high pressure, and part of it may have condensed. The condensation liquid (generally water) is vaporized by flowing the heating fluid through an expansion device (9e) as illustrated in Figure 8 (not shown in Figures 2 to 6 for sake of clarity).MVR-Heat Exchanger (X95L)

[0082] Driven by the MVR-compressor (9c), the gaseous heating fluid reaches the MVR-heat exchanger at a higher temperature and higher pressure to transfer heat to the drying gas flowing in the liaising duct (5L) downward of the LT-chambers (N) relative to the flow direction in the distribution system (5). As discussed supra, heating the drying gas flowing through the MVR-heat exchanger in the liaising duct (5L) decreases the energy to be supplied by the heating elements (11) to heat the drying gas to the required temperatures (T 1 , T2) prior to entering into the HT-chambers (1 , 2).

[0083] In the preferred embodiment wherein, the drying gas is air, and the heating elements (11) are air-fuel burners, a part of the air flowing in the drying gas feeding ducts (51 , 52) can be diverted at (9g) to feed the air-fuel burners. Increasing the temperature of the air in the MVR-heat exchanger (X95L) prior to reaching the air-fuel burners is beneficial as it increases the efficacy of the air-fuel burners compared with feeding them with colder air.METHOD FOR DRYING CONSTRUCTION PANELS

[0084] The present invention also concerns a method for drying construction panels, using an apparatus as discussed supra. After being formed and cut into panels, the still wet construction panels (60) are driven along the drying path sequentially through the chambers (1 , 2) of the HT-drying zone followed by through the chambers (N) of LT-drying zone. As the construction panels travel through the chambers (1 , 2... N), the drying gas, preferably air, flows through the distribution system (5) into each of the LT-chambers first, followed by the HT-chambers (1 , 2) via the liaising duct (5L) and the MVR-heat exchanger (X95L). The drying gas flows through the N chambers (1 , 2 ... N) over the main surfaces of the construction panels (60) increasing the temperature and decreasing the moisture contents thereof. Before the drying gas reaches the main surfaces of the construction panels in the HT-chambers (1 , 2), the drying gas is heated with the heating elements (11) to desired temperatures. The exhaust gas is then removed from each of the HT-chambers through the exhaust system (7).

[0085] At least during a stationary drying operation, the drying gas blown into the LT-chambers (N) is heated using exclusively heat recovered from the exhaust gas via the LT-heat exchangers (X75N) and, preferably, upstream of the LT-chambers (N), by one or more of the pre-heating-heat exchanger (X75) or the condensation heat exchanger (X8w5).

[0086] This method yields the same quality of drying as prior art drying apparatuses but consuming considerably less energy. Table 1 compares the performance of a dryer of the prior art according toFigure 1 with the performance of a dryer of the present invention according to Figure 2, with same components and same temperatures in the chambers. In both cases, the drying gas is air, and the heating elements (11) are air-fuel burners. The dryer of the invention comprises a heat pump I MVR-system which is not included in the dryer of the prior art. Both dryers are provided with a pre-heating heat exchanger (X75). The values are nominal values as they were calculated and not measured. With the components selected for the present simulation, the COP of the apparatus of Figure 2, the heat pump / MVR-system had a COP = 2.5.Table 1: comparison of the performance of a dryer of the prior art according to FIG. 1 with a dryer according to the invention (FIG.2) (calculated)

[0087] It can be seen that by increasing the temperature of the gas in the MVR heat exchanger prior to feeding it to the heating elements (1 1), the dryer of the present invention drops the consumption of natural gas by 8%! Of course, the heat pump compressor (8c) and MVR compressor (9c) consume electrical power, but as long as the heat pump I MVR system has a COP > 1 , the total power consumption drops. In the present example, with a COP = 2.5 of the heat pump I MVR system, the dryer of the invention consumes 5% less energy than the prior art dryer. In times of increasing energy costs, this is a spectacular drop.

[0088] With a lower consumption of natural gas by using smaller air fuel burners, the CO2 emissions have dropped by about 8%. With global warming, it was a priority of the inventors to reduce the CO2 emissions. The amount of condensation water recovered from the various heat exchangers is an indirect indicator of the efficacy of the heat transfer from the exhaust gas (air) to the gas (air) flowing in the distribution system (5). It can be seen in Table 1 , that more than two and half times higher amounts of condensation water were recovered from the dryer of the present invention compared with the prior art dryer, showing the superior level of heat transfers thus achieved.

[0089] The dryer of the present invention decreases the energy requirements for drying wet construction panels (60). CO2 emissions are lower for similar results.

[0090] Constructions panels are cement boards, calcium silicate boards, fibre cements boards and preferably gypsum boards.

Claims

CLAIMS1 . An apparatus for drying construction panels comprising,• a conveyor (40) for conveying wet construction panels (60) along a drying path through a drying unit; the drying unit comprising,• a number N of chambers (1 , 2 ... N) distributed in series and in fluid communication with one another along the drying path, comprising one or more high temperature chambers (1 , 2) (= HT chambers) located in a high temperature drying zone (HT) in an upstream portion of the drying path, and one or more low temperature chambers (N) (= LT-chambers) located in a low temperature drying zone (LT) downstream of the high temperature drying zone (HT) along the drying path,• a distribution system (5) in fluid communication at one end with a source of drying gas (5s), preferably air, and, at other ends, with the N chambers (1 , 2 ... N) and configured for circulating a flow of the drying gas along a drying gas flowing path extending from the source of drying gas into the N chambers,• heating elements (11) configured for heating the drying gas in the distribution system (5) prior to penetrating into the respective HT-chambers (1 , 2),• an exhaust system (7) configured for evacuating out of the apparatus exhaust gas along an exhaust flowing path extending from the HT-chambers (1 , 2) through return exhaust ducts (7h) leading to corresponding LT-heat exchangers (X75N) provided in the LT-chambers, and finally outside the apparatus, wherein• the LT-heat exchangers (X75N) are provided in each LT-chamber (N) and configured for transferring heat from the exhaust gas flowing in the corresponding return exhaust ducts (7h) to the drying gas as it flows through the corresponding LT-chambers (N),Characterized in that,• the distribution system (5) comprises a LT-distribution section for distributing drying gas into the LT-chambers (N) and a HT-distribution section for distributing drying gas into the HT-chambers (1 , 2), and coupled in series and downstream of the LT-distribution section by a liaising duct (5L), wherein o the LT-distribution section comprises one or more drying gas feeding ducts (5N) leading to the corresponding one or more LT-chambers (N) in parallel, and wherein o the HT-distribution section comprises one or more drying gas feeding ducts (51 , 52) leading to the corresponding one or more HT-chambers (1 , 2) in parallel,• the apparatus comprises a first heat pump-heat exchanger (X78) configured for transferring heat from the exhaust gas flowing in the exhaust system (7) to a low boiling point gas of a heat pump (8)• the heat pump (8) comprises a heat pump compressor (8c) configured for compressing and increasing a temperature of the low boiling point fluid, and for driving a flow of the low boiling point fluid from the first heat pump-heat exchanger (X78) to a second heat pump-heat exchanger (X89) configured for transferring the heat thus captured by the low boiling point fluid to• a heating fluid having a boiling temperature higher than the low boiling point fluid of the heat pump (8) and configured for circulating in a mechanical vapour recompression (MVR-) cycle (9) comprising an MVR-compressor (9c) configured for compressing the heating fluid, and thus increasing a temperature of the heating fluid, and for driving a flow of the heating fluid from the second heat pump-heat exchanger (X89) to,• an MVR-heat exchanger (X95L) provided at the level of the liaising duct (5L) and configured for transferring heat from the heating fluid of the MVR-cycle (9) to the drying gas in the distribution system (5) downstream of the LT-distribution section and upstream of the HT-distribution section.

2. Apparatus according to claim 1 , wherein a heat pump / MVR system composed of the heat pump (8) and MVR-cycle (9) is characterized by a coefficient of performance (= COP) comprised between 2 and 5, preferably between 2.5 and 4, wherein the COP is defined as a ratio (Q / W) of a useful heat (Q) supplied by the heat transfer combination to a work (W) required to operate the heat pump compressor (8c) and the MVR-compressor (9c).

3. Apparatus according to claim 1 or 2, comprising a pre-heating heat exchanger (X75) configured for transferring heat from the exhaust gas in the exhaust system (7) to the drying gas in the distribution system (5) to increase the temperature of the drying gas flowing in the distribution system (5) prior to reaching the LT-chambers (N), wherein the pre-heating heat exchanger (X75) is located,• In the distribution system (5), upstream of the LT-distribution section, and• in the exhaust system (7), downstream of the LT heat exchanger (X75N) and preferably upstream of the first heat pump-heat exchanger (X78), wherein ‘upstream’ and ‘downstream’ are defined herein relative to a flow direction of a fluid in the corresponding distribution or exhaust systems (5, 7).

4. Apparatus according to any one of the preceding claims, wherein• the heating elements (11) are air-fuel burners,• the source of drying gas (5s) is a source of air, and a part of the air flowing in the distribution system (5) is fed to the air-fuel burners.

5. Apparatus according to any one of claims 1 to 3, wherein the heating elements (11) are selected among electrical heaters and high temperature fluid heat exchangers.

6. Apparatus according to any one of the preceding claims, wherein• the distribution system (5) comprises distribution fans (5f) configured for driving the flow of drying gas from the source of drying gas towards the N chambers (1 , 2 ... N),• the HT-chambers are equipped with chamber fans (1f, 2f) configured for driving a drying gas flow cycle flowing through the heating element (11) and into the corresponding HT-chambers (1 , 2) and out of the drying gas flow cycle into the exhaust system (7), and• one or more of the fans (5f) are configured for driving a drying gas flow through the LT-chambers (N) between radiation surfaces of the LT-heat exchangers (X75N) to collect heat and the main surfaces of the construction panels to transfer heat thereto and collect moisture therefrom and out of the LT-chambers (N) into the liaising duct (5L) and through the MVR-heat exchanger (X95N) prior to reaching the HT-distribution section.

7. Apparatus according to any one of the preceding claims, wherein,• a temperature (T1 , T2) in the HT-chambers (1 , 2) varies between 120 and 260°C, preferably between 150 and 250°C, and I or• a temperature (TN) in the one or more LT-chambers (N) has an average value lower than an average temperature in the HT-chambers and varies between 90 and 170°C, preferably between 100 and 160°C.

8. Apparatus according to any one of the preceding claims, wherein a temperature of the low boiling point fluid of the heat pump (8) in the second heat pump-heat exchanger (X89) is comprised between 90 and 110°, and is preferably equal to 100°C + 5°C.

9. Apparatus according to the preceding claim, wherein a temperature of the heating fluid of the MVR-cycle (9) in the MVR-heat exchanger (X95L) is comprised between 120 and 180°C, preferably between 135 and 170°C, and is preferably equal to 150°C + 10°C.

10. Apparatus according to anyone of the preceding claims, comprising an MVR-distribution system heating loop (95) branching off the MVR-cycle (9), downstream of the MVR-compressor (9c) and joining back the MVR-cycle (9) upstream of the MVR-compressor (9c) after passing through a second MVR-heat exchanger (X95b) configured for transferring heat from the heating fluid in the MVR- distribution system heating loop (95) to the drying gas in the distribution system (5).11 . Apparatus according to any one of the preceding claims, comprising a gas heat exchanger (X85) configured for transferring heat from the low boiling point fluid of the heat pump (8) to the drying gas of the distribution system (5).

12. Apparatus according to any one of claims 1 to 11 , wherein a branched duct (7h5) branches off the return exhaust ducts (7h), wherein the branched duct is configured for flowing exhaust gas into the LT-chambers (N) admixed with drying gas from the LT-distribution section, and out of the LT-chambers (N); wherein the branched duct (7h5) and exhaust ducts (7h) are provided with valves (7v) to control a ratio of an amount of exhaust gas from the exhaust system to an amount of drying gas from theLT-distribution section.

13. Apparatus according to any one of the preceding claims, wherein the heating fluid in the MVR-cycle is water / steam.

14. Method for drying construction panels comprising,• providing an apparatus according to any one of the preceding claims,• driving a wet construction panel (60) along the drying path through the chambers (1 , 2) of the HT-drying zone followed by through the chambers (N) of LT-drying zone,• flowing the drying gas through the distribution system (5) into each of the N chambers (1 , 2 ... N),• heating with the heating elements (11) the drying gas to desired temperatures prior to penetrating into the HT-chambers (1 , 2),• exhausting the exhaust gas from each of the N chambers through the exhaust system (7),Characterized in that,• heat is exchanged in the first heat pump-heat exchanger (X78) from the exhaust gas in the exhaust system (7) to the low-boiling point fluid in the heat pump (8) and after compression of the low boiling point fluid,• heat is exchanged in the second heat pump-heat exchanger (X89) from the thus compressed low-boiling point fluid to the heating fluid in the MVR-cycle (9) and after compression of the heating fluid,• heat is exchanged in the MVR-heat exchanger (X95L) from the thus compressed heating fluid to the drying gas in the distribution system (5) flowing out of the LT-distribution section and before the drying gas is heated by the heating elements (11) prior to penetrating into the respective HT-chambers (1 , 2).

15. Method according to claim 14, wherein the construction panels (60) are fibre cement panels or panels made of a material comprising as main component in an amount of at least 50 wt.%, gypsum, or cement, or calcium silicate.

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