INDUSTRIAL AIR DRYING PLANT

Abstract

This installation for air drying a wet product comprises:– a drying enclosure (1) through which the wet product to be dried (2) and the air ensuring the drying pass, said enclosure comprising at least one introduction circuit (4) for intake air and at least one discharge (5) for the air laden with moisture resulting from the exchange of water and energy with the wet product to be dried;– at least one boiler (7) operating with a working fluid, configured:. to recover the heat from one or more heat sources consisting of the air laden with moisture passing through said discharge (5) by means of a recovery exchanger (6); and.to generate steam using the heat thus recovered, at least one means capable of initiating the operation of the boiler(s) (7) by supplying heat;– at least one compressor (11) capable of increasing the pressure of the steam generated in said boiler (7);– at least one injector exchanger (12) capable of condensing the steam from the compressor (11), positioned within the introduction circuit (4) of the intake air, and of ensuring the heating of the latter upstream of the drying enclosure (1);– at least one collector (13) of the condensates from the condensation of the steam in said injector exchanger (12), said condensates being returned to the boiler (7). Figure for the abstract: Fig 1.

Description

Title of the invention: INDUSTRIAL AIR DRYING INSTALLATION Field of invention

[0001] The invention relates to the field of drying wet products by air in industry. It potentially targets any drying installation capable of being implemented such as spray dryers, belt dryers, heating cylinders, ovens, drying furnaces, flash dryers, fluidized bed dryers, paddle dryers, screw dryers, silos, rotary drums or tunnel dryers. These dryers are present in various industrial sectors such as food processing, chemicals, paper and cardboard, and non-metallic materials. This installation uses air as a drying fluid (a drying fluid is defined as a fluid that allows the evacuation of evaporated water from the dryer and the supply of heat to the dryer to facilitate this evaporation), which serves as a vector for transporting the evaporated water as well as a means for supplying calories to the dryer.

[0002] The invention aims more particularly at optimizing energy consumption in the context of such installations by recovering heat from the outgoing air (vapors) for reinjection into the intake air circuit. Prior art

[0003] Drying installations are very widespread equipment in industry and represent a significant part of industrial consumption: 10 to 20% of total energy consumption in industry in industrialized countries (according to the literature "Modern Drying Technology", Volume 4 by Evangelos Tsotsas and Arun S. Mujumdar - 2007) They generally consist of a drying chamber through which the wet product to be dried passes, said chamber being crossed by drying air, usually hot, which becomes loaded with moisture on contact with the wet product. The air thus loaded with moisture, conventionally called steam, is evacuated from the chamber. These steams have a relatively high temperature and humidity.

[0004] It is therefore easy to understand the loss in terms of energy, resulting from the temperature and humidity in the vapors, and consequently, the energy consumption necessary to ensure the effective evaporation of the moisture present in the product to be dried. It is in fact known that the enthalpy of vaporization at atmospheric pressure Ahvap is of the order of 630 kWh per ton of evaporated water, requiring in such a configuration where the vapors are simply released into the atmosphere, an energy consumption 2 to 4 times higher (according to the literature "Drying processes in industry”, ADEME, CETIAT, 2017), often gas or coal, possibly via a heat transfer fluid such as steam, superheated water or hot water.

[0005] In other words, this mode of operation is a source of energy and exergy inefficiency.

[0006] In order to reduce this consumption, solutions exist for recirculating part of the vapors at the level of the drying air introduction circuit, in order to transfer part of the relative heat of said vapors to it. This recirculation certainly allows an energy gain, however very limited.

[0007] Drying installations have also been described which combine the introduction of hot air on the one hand, as described previously, but also heating cylinders arranged within the heating enclosure. These heating cylinders are typically heated by steam or by hot air.

[0008] Other solutions allow heat to be recovered by preheating the intake air through the steam, via an air / steam heat exchanger.

[0009] Recently, there has been a growing interest in recovering heat from steam by means of high-temperature heat pumps (whose condenser has a temperature below 100°C), or even very high-temperature heat pumps (whose condenser has a temperature above 100°C), for reinjection into the intake air or into other means of supplying heat to the dryer (for example, heating cylinders).

[0010] Also, some manufacturers are working on closed-loop drying installations, in which a heat pump allows the water from the vapors to be condensed in the heat pump evaporator, then the dried vapors to be reheated via the heat pump condenser.

[0011] Finally, superheated steam dryers make it possible to approach the minimum energy required by the technological breakthrough, and can be coupled with mechanical steam compressors to approach the minimum exergy required by the drying process, with consumptions lower than 200 kWh / ton of evaporated water. These types of dryers and configurations are the most efficient, but their intrinsic characteristics limit their scope of application.

[0012] These configurations are intended to energetically optimize the drying process which has high potential for improvement given the low temperatures allowing the use of compressors and heat pumps.

[0013] It follows from these findings, particularly in a context where energy is becoming a particularly high cost item for manufacturers and where industry is strongly encouraged, or even forced, to reduce its carbon emissions to stop global warming, that an increasingly pressing need exists in terms of rationalization of such drying installations and more particularly in terms of reducing energy consumption at source.

[0014] This is the object of the present invention. Brief description of the invention

[0015] To this end, the invention relates to an installation for drying a wet product using a vector or drying fluid, and in particular using air, said installation comprising a drying enclosure within which the wet product to be dried and the air ensuring the drying pass, this enclosure being provided with at least one circuit for introducing intake air, and at least one evacuation of the air laden with moisture resulting from the exchange of water and energy with the wet product to be dried, referred to as steam.

[0016] According to the invention, the installation comprises: - at least one boiler operating with a working fluid (typically water), this boiler being configured: . to recover heat from one or more heat sources consisting of air laden with humidity (vapors) passing through the evacuation of air laden with humidity, this heat recovery being carried out using a recovery exchanger, and . to generate steam using the heat thus recovered, the boiling temperature of the working fluid within the boiler being adapted by means of the pressure in the boiler to the temperature of the heat source(s); - at least one means capable of initiating the operation of the boiler(s) by supplying heat within the latter(s), - at least one compressor, capable of increasing the pressure, and therefore the temperature, of the steam generated in the boiler; - at least one injector exchanger capable of condensing the steam from the compressor, this injector exchanger being positioned within the intake air introduction circuit, i.e. upstream of the drying enclosure, and ensuring the heating of this air upstream of the drying enclosure; - at least one collector of the condensates resulting from the condensation of the steam in the injector exchanger, these condensates being returned to the boiler.

[0017] In other words, the invention consists mainly in recovering the heat contained in the vapors, and, from this heat, in generating steam within the boiler, said steam undergoing an increase in its pressure and its temperature using a compressor so as to supply an injector exchanger to heat the air upstream of the drying enclosure.

[0018] For the purposes of the invention: - a boiler is defined as a reservoir containing a working fluid in a liquid / vapor phase equilibrium in which a heat input in the lower part allows the working fluid to boil; - a working fluid is defined as a fluid that carries heat from a source to a sink, through phase changes; - a recovery exchanger is defined as equipment allowing heat to be taken from the dryer extractions, in this case the fumes; - an injector exchanger is defined as equipment allowing heat to be supplied directly to the product to be dried or via the drying fluid, which is air.

[0019] According to a variant of the invention, the drying enclosure can itself integrate means for supplying additional heat, called additional injector exchangers, supplied with steam from the compressor mentioned above.

[0020] These means of providing additional heat are typically heating cylinders, double walls of drying enclosures, screws, pallets or even steam radiators.

[0021] In order to ensure the start of the boiling operation within the boiler, said means capable of initiating the operation of the boiler(s) by supplying heat consists of a heat source, i.e. an energy flow in which it is possible to recover additional heat. This additional heat source is located in the boiler, in the intake air or even in the enclosure itself, and may consist of any means of supplying calories, and in particular resistance or induction technology, by one or more electric immersion heaters, by one or more exchangers supplied by a heat transfer fluid of the hot water, thermal oil or steam type.

[0022] Furthermore, this additional heat source can also act as a backup or supplementary means in the event of a problem with the installation.

[0023] Correlatively, the installation may also provide a device for placing the boiler under vacuum in order to allow, here again, the start of the boiling operation within the latter.

[0024] In order to optimize the drying installation of the invention, the compressor(s) are regulated according to the drying needs defined by the drying kinetics of the different products: the installation allows an adaptation of the condensation temperatures of the steam by variation of the compression ratios of the compressor(s). It is thus possible to obtain an injection of steam at different pressure levels for the same injector exchanger, according to the drying needs of the product. This is done by means of measurements allowing an estimation of the drying kinetics in order to avoid over-drying. This regulation thus makes it possible to limit the consumption of electrical energy necessary for the operation of the compressor; when the drying temperature requirement is lower, the steam is compressed at lower pressure levels.

[0025] For this purpose, the electric motor equipping each compressor has a variator frequency to regulate the steam pressure setpoint at the outlet of said compressor. In doing so, electricity consumption is substantially limited by avoiding excessively high and unnecessary pressures. Thus, an online measurement of the variation in the temperature of the product to be dried, for example by infrared or using a thermal camera, makes it possible to control the drying kinetics of the product in question and avoid over-drying by defining the appropriate pressure / temperature levels.

[0026] Furthermore, with the aim of facilitating the operation of the boiler, the installation may also include a heat pump intended to raise the temperature level obtained by the recovery exchanger located in the steam discharge, particularly if the pressure at the temperature obtained is not achievable under satisfactory technical conditions. In the theoretical ideal, the evaporator of the heat pump is directly positioned in the steam flow, and the condenser in the boiler.

[0027] According to a particular configuration of the invention in a closed loop, the intake air comes from the vapors, after drying by means of a recovery exchanger then reheating by means of an injector exchanger.

[0028] In other words, the installation may provide for a closed loop assembly to allow total recycling of the vapors, if necessary by implementing means of air filtration and management of the fatal heat. Brief description of the figures

[0029] The manner in which the invention can be implemented and the advantages which result therefrom will become more apparent from the following examples of embodiment, given for informational and non-limiting purposes, with the support of the appended figures.

[0030] [Fig.l] is a schematic representation illustrating the basic drying installation of the invention.

[0031] [Fig.2] is a variant of [Fig.l] integrating an additional injector exchanger integrated into the drying enclosure itself.

[0032] [Fig.3] is a variant of [Fig.2] comprising two injector exchangers integrated within the drying enclosure.

[0033] [Fig.4] illustrates a more complex installation, for dryers with large evaporative capacity, integrating several (n) drying enclosures mounted in series, equipped with their respective air intake system.

[0034] [Fig.5] illustrates a closed loop drying installation, as described in the last paragraph of the state of the art, implementing the principle of the invention.

[0035] [Fig.6] illustrates a variant of [Fig.4] in closed loop mode.

[0036] [Fig.7] schematically illustrates a particular embodiment of the invention. wherein the low moisture content in the vapors requires the use of a heat pump to raise the temperature levels of the heat source. Detailed description of the invention

[0037] The various figures schematically illustrate different variants of the installation according to the invention. As mentioned in the preamble, this installation can be implemented for all types of dryers (except superheated steam dryers) and needs to be adapted in order to have the capacity to meet the needs of the wet products to be dried.

[0038] The implementation of the present invention coupled with other drying means (infrared, by dielectric losses such as microwaves or high frequencies) is possibly conceivable in order to ensure the best performance of the assembly and to meet the drying needs of the products.

[0039] Within [Fig.l], the drying enclosure (1) is intended to receive a wet product to be dried (2). Thus, the wet product (2) enters the enclosure, undergoes drying in the enclosure (1), and leaves it in dry form (3).

[0040] Within this drying enclosure (1), hot air is introduced via an introduction circuit (4). After exchange within the drying enclosure resulting from the phenomenon of convection and mass transfer between the hot air and the wet product to be dried, air highly charged with humidity, also called mist, leaves said enclosure via an outlet (5).

[0041] According to the invention, the vapors (5) emerge at a relatively high temperature and humidity, and part of the heat of said vapors (5) is recovered by means of a recovery exchanger (6) by recovering the energy in the form of sensible heat as well as the latent energy of the vapors by condensing water, coming from the evaporated product water, which can be recovered (23), this heat being transferred to the working fluid in the boiler (7).

[0042] This working fluid can be demineralized or softened water. The heat supplied via the recovery exchanger (6) will heat the working fluid to the saturation temperature of the pressure in the boiler. This working fluid in liquid / vapor phase equilibrium in the boiler thus generates steam at the saturation temperature.

[0043] However, and in order to initiate the process of boiling the water within the boiler, a heat input within the latter is necessary. This heat input can be achieved by a heat pump (8) (see [Fig.7]), taking the heat from the steam discharge (5), possibly by means of two intermediate heat transfer fluid loops (25.1 and 25.2).

[0044] Furthermore, still with the same objective of initiating the boiling process of water within the boiler, the pressure may have to be adapted to the temperature of the heat source. In this respect, the boiler can be connected to a vacuum device (9) (see [Fig.7]), in order to lower the pressure within the boiler during the start-up phases. This device is then stopped. The compressor (11) associated with the injector exchanger (12) ensure the maintenance of pressure in the boiler during normal operation, avoiding an excessive energy input by the recovery exchanger (6).

[0045] In stabilized operation, the steam produced in the boiler (7) is conveyed (10) to a main compressor (11) intended to increase the pressure, and therefore corollarily the temperature of the steam thus generated, according to the well-known laws of thermodynamics.

[0046] This steam is conveyed to an injector exchanger (12) positioned in the air intake circuit (4), and in order to raise its temperature.

[0047] Thus, once the boiling has started within the boiler (7), the only significant electrical consumption lies in the power supply to the main compressor (11), the possibility of regulation of which will be indicated below.

[0048] The liquid condensates, possibly sub-cooled, of the working fluid resulting from the heat exchange occurring at the level of the injector exchanger (12) placed within the intake circuit (4) are returned (13) to the boiler (7).

[0049] The condensed water (23) in the recovery exchanger (6) can be reused in the factory after possible water treatments depending on the case, these treatments being able to be that of the PH, the Biological Oxygen Demand, the Chemical Oxygen Demand. This recovered water (23) makes it possible to limit the consumption of industrial water and in fact constitutes an additional advantage of the invention, particularly interesting in a context in which water resources are increasingly under tension.

[0050] [Fig.2] illustrates another variant of the drying installation according to the invention.

[0051] In this, the drying of the wet product in the drying enclosure occurs not only due to the introduction of air, as illustrated in [Fig.l], but also by at least one additional heat supply means (14) positioned inside the drying enclosure (1).

[0052] For example, such a source of additional heat supply consists of one or more heating cylinders, the heating of which is provided by steam. And in fact in [Fig.2], part of the steam, produced by the boiler (7), and the pressure of which is increased by the compressor (11) is conveyed to an injector exchanger to be condensed there (14), then is sub-cooled in another sub-cooler exchanger (15) preheating the air upstream of the injector exchanger (12), the condensate joining (16) the supply circuit (13) of the boiler (7).

[0053] [Fig. 3] illustrates a variant of the installation of [Fig. 2]. In this, two injector exchangers are shown allowing an additional supply of heat, respectively (14.1) and (14.2), positioned within the drying enclosure (1). Part of the steam from the compressor (11.1), introduced at the first of these additional injector exchangers (14.1), is diverted and sent to a secondary or additional compressor (11.2), supplying the second means of additional heat supply (14.2). This secondary compressor (11.2) is intended to further increase the pressure (and therefore the temperature) of the steam, compared to that supplying the first means of additional heat supply (14.1). Indeed, in the example described, this second exchanger (14.2) is located downstream of the first exchanger (14.1) in the direction of the product flow on purpose, due to the effective progression of the wet product within the enclosure, configuration in which a higher heat source in terms of temperature is required downstream for the purpose of optimizing the drying process. The condensates from the injector exchangers (14.1) and (14.2) are returned to a tank (19), then sub-cooled in a sub-cooler exchanger (15) preheating the intake air before being returned (16) to the boiler feed circuit.

[0054] [Fig. 4] illustrates yet another embodiment of the invention. In this embodiment, the drying chamber is in fact subdivided into a number of chambers (1.1) to (1n), connected in series with each other, each of said chambers being supplied with hot air, as previously described.

[0055] However, in this embodiment, the evacuation of the vapors (5), that is to say in this case of the air most loaded with humidity, only occurs at the level of the most upstream enclosure (1.1). For an integer i between 1 and n-2, the enclosure (li) is supplied with air (4.i) coming from the heat and water exchange with the product to be dried from the enclosure immediately downstream (l.i+1), and possibly by additional intake air (2O.i). This air undergoes a rise in temperature by means of injector exchangers (12.i), themselves supplied with steam. The injector exchanger (12.1) of the intake circuit (4.1) of the most upstream enclosure (1.1) is supplied with steam coming from the main compressor (11.1). The injector exchangers (12.i) of the downstream intake circuits are supplied with steam taken from the upstream circuit, and undergoing a rise in pressure (and therefore temperature) by means of additional compressors (1 li).In fact, the further the wet product to be dried progresses within the enclosures, the hotter and drier the air becomes. The most downstream enclosure (ln) is supplied with fresh air (4.n), i.e. ambient air, and the previous enclosure (1.n-1) is supplied with slightly humid air heated by the product passing through the enclosure (ln); product which cools on contact with the fresh, dry air and absorbs . possibly air humidity depending on its hygroscopic capacities and its dry matter content.

[0056] It should be noted that the configuration presented in [Fig.4] could also include additional injector exchangers (14.i) in order to improve the supply of calories to the drying chambers, as presented in [Fig.3].

[0057] [Fig.5] illustrates the adaptation of the invention to a loop drying installation. closed. In this configuration, the recovery exchanger (6) is installed on the steam circuit to dry them by evacuating the water from the product (23). The dried steam is recirculated to the injector exchanger (12) in order to generate hot intake air (4) to supply the drying chamber (1).

[0058] This configuration potentially involves excess heat depending on the drying temperature regimes, compressor performance, product heat capacity and system losses. This excess heat, called waste heat, can be recovered in an excess heat exchanger (21) so that the condensates can be redirected to the boiler (7).

[0059] This configuration is particularly interesting in terms of the water evaporated from the product, which is thus recovered 100%, as well as the absence of discharges in the form of mist, only in liquid form.

[0060] [Fig.6] illustrates an application equivalent to the operation of [Fig.4] with closed loop air recirculation to reuse the vapors (5) dried in the recovery exchanger (6) to be redirected to the air intake (4.n) of the downstream enclosure (ln).

[0061] [Fig.7] illustrates a means of raising the temperature of the working fluid in the boiler (7), this means consisting of a heat pump (8). The latter makes it possible to raise the temperature level of the energy recovered from the vapors (5) by means of a vapor recovery exchanger (6.1), supplying the evaporator of the heat pump, and an exchanger (6.2) in the boiler (7). This equipment can be a palliative if the temperature level of the heat taken from the vapors is insufficient to allow the operation of the boiler (7). Note that this solution makes it easier to arrange using a standard technological brick (heat pump). However, it would be possible to install a compressor and an expansion valve between the exchangers (6.1) and (6.2) with another working fluid, the saturation temperatures of which at the recovery temperature in the exchanger (6.1) and at the saturation temperature of the working fluid at the secondary of the exchanger (6.2) in the boiler are adapted.

[0062] The heat pump can be partially or completely bypassed by control valves (24.1, 24.2) if the heat pump is not required or if the temperature rise required in the exchanger (6.2) is lower than the delta minimum temperature between the evaporator and condenser of said heat pump, involving a mixture between water from the condenser of the heat pump and water not heated by the heat pump, sent to the exchanger (6.2).

[0063] An advantageous characteristic of the invention lies in the ability to improve its performance at reduced loads: the main compressor (11.1) and the complementary compressors (11.2) to (11.n-2) are regulated according to the drying needs defined by the drying kinetics of the products entering the drying enclosure and the online measurements. The incoming humidity of said products can for example vary according to the seasons, or the recipes, implying a necessary online adaptation of the dryer to the drying need; which the invention allows by consuming less per unit of evaporated water when the drying installation is at low load by reducing the compression ratios of the steam compressors, and therefore their consumption per unit of flow rate.

[0064] Typically, the invention makes it possible to optimize the drying performance when the tonnage of evaporated water is lower than the sizing value of the dryer, which is very frequently the case in real life by initial oversizing and operation on various products of which the sized product is the most humid. Thus, a regulated pressure increase of the steam from the boiler (7) by means of the compressors (11.1 to 11.n-2) makes it possible: - to reduce air temperatures when the latter does not need too high a temperature; - to reduce the temperature in the means of additional heat supply by injector exchangers; - to obtain reduced energy consumption when drying needs are lower, since the lower steam pressures, generating lower electricity consumption of the compressors.

[0065] This management of the compressor(s) can be carried out by an online measurement, by variation of temperature of the wet product to be dried at different points for example. This measurement of the temperature can be carried out by any means of measurement and acquisition, for example infrared measurement or by means of thermal cameras with image processing, integrated in the drying enclosure, according to the configurations of dryers and the technologies adapted to the temperature, humidity and potential fouling, possibly with a presence outside the drying enclosure with a window if the technologies allow it.

[0066] The installation according to the invention constitutes an essential advance compared to those known from the prior art. It has a certain number of advantages, among which we can cite: - a very significant increase in the energy performance of drying without degrade the quality of drying, making it possible to move towards the Minimum Required Exergy through the electrification of dryers, making it possible to divide energy consumption by a factor of 2 to more than 10; - the extremely significant decarbonization potential of this arrangement, particularly in countries where electricity is low-carbon, such as France; - complete recovery (closed loop) or a large part of the water extracted from the products to be dried, making it possible to improve the water efficiency of drying operations; - the implementation of a limited number of components, and making it possible to limit the lengths of heat transport networks with a process self-powered by equipment present at the installation level, including start-up, emergency and backup equipment; - the implementation of technological building blocks that are now perfectly mastered and implemented to achieve this objective; - the use of a potential working fluid, water, already present as an energy vector in industry (therefore reassuring), facilitating filling in the event of a leak and non-polluting; - flexibility to approach the minimum drying requirement without overconsumption by a fixed supply temperature regardless of the types of products to be dried.

[0067] The installation thus allows substantial gains compared to known installations of the prior art, in particular in terms of energy consumption and therefore greenhouse gas emissions, and water consumption.

Claims

Claims

1. Installation for air drying of a wet product comprising a drying enclosure (1) within which the wet product to be dried (2) and the air ensuring the drying pass, said enclosure comprising at least one introduction circuit (4) for intake air and at least one discharge (5) for the air laden with moisture resulting from the exchange of water and energy with the wet product to be dried, characterized in that the installation comprises: - at least one boiler (7) operating with a working fluid, configured: . to recover the heat from one or more heat sources constituted by the air laden with moisture passing through said at least one discharge (5) by means of a recovery exchanger (6);and . to generate steam using the heat thus recovered, the boiling temperature of the working fluid being adapted by means of the pressure in the boiler (7) to the temperature of the heat source(s), possibly by means of an intermediate heat transfer loop; - at least one means capable of initiating the operation of the boiler(s) (7) by supplying heat; - at least one compressor (11) capable of increasing the pressure, and therefore the temperature, of the steam generated in said boiler(s) (7); - at least one injector exchanger (12) capable of condensing the steam from said at least one compressor (11), positioned within the introduction circuit (4) of the intake air, and of ensuring the heating of the latter upstream of the drying enclosure (1);- at least one collector (13) of the condensates resulting from the condensation of the steam in said injector exchanger(s) (12), said condensates being returned to said boiler(s) (7).;

2. Installation for air drying a wet product according to claim 1, characterized in that it further comprises means for supplying additional heat (14; 14.1; 14.2) positioned within the drying enclosure (1), supplied with steam, consisting of additional injector exchangers.

3. Installation for air drying a wet product according to claim 2, characterized in that the means for supplying additional heat are heating cylinders, double walls of the drying enclosure, screws, pallets and / or steam radiators.

4. Installation for air drying a wet product according to one of claims 1 to 3, characterized in that the means capable of initiating the operation of the boiler(s) (7) by supplying heat is an additional starting heat source chosen from the group comprising electrical induction technology, electrical resistors, direct combustion or exchangers supplied by a heat transfer fluid of the hot water, superheated water, thermal oil, hot air, fumes or steam type.

5. Installation for air drying a wet product according to claim 4, characterized in that the additional start-up heat source also acts as a backup or supplementary means in the event of a problem with the installation.

6. Installation for air drying a wet product according to one of claims 1 to 5, characterized in that the compressor(s) (11) are regulated according to the drying requirements defined by the drying kinetics of the different products.

7. Installation for air drying a wet product according to one of claims 1 to 6, characterized in that it comprises a vacuum member (9) for the boiler(s) (7) in order to allow the boiling operation to start within the latter.

8. Installation for air drying a wet product according to one of claims 1 to 7, characterized in that it comprises a heat pump (8) capable of raising the temperature level at the recovery exchanger (6.1), in particular when the pressure of the boiler (7) at the saturation temperature of the working fluid corresponding to the recovery temperature is not achievable under satisfactory conditions.

9. Installation for air drying a wet product according to one of claims 1 to 8, characterized in that the intake air (4) comes from the extraction air (5) of the drying enclosure (1), which is dried by means of a recovery exchanger (6), then reheated by means of an injector exchanger (12).

10. Installation for air drying a wet product, comprising a plurality of drying enclosures 1, 2, ..., n, mounted in series, the most upstream enclosure (1.1) receiving the product to be dried having the highest degree of humidity, and the most downstream enclosure (ln) receiving the product to be dried (2) after transit in the upstream enclosures, according to one of the claims indications 1 to 9, in which: - the intake air (4.i) conveyed into the enclosure (li) upstream relative to the direction of progression of the wet product to be dried comes from the air laden with moisture (4.i+l) from the drying enclosure (1.i+1) mounted immediately downstream of said upstream enclosure after reheating by an injector exchanger (12.i), i being an integer between 1 and n-2, - the steam from the boiler(s) and whose pressure (and therefore temperature) has been increased using the main compressor (11.1), is conveyed as a priority to an injector exchanger arranged in the air introduction circuit of the most upstream enclosure (1.1), - part of this steam is subjected to compression at the level of an additional compressor (11.2) to convey a higher pressure steam to the level of an injector exchanger (12.1) arranged within an air introduction circuit of said downstream enclosure with possible supply of new air for mixing (20.1); this compression process towards the downstream enclosure being likely to occur 1, 2, ..., n-2 times, - the air in the enclosure (ln-1) comes from the outlet air from the enclosure (ln), preheated by the hot product to be dried from the enclosure (ln-1), which has cooled in the enclosure (ln) by giving up its heat to the air and possibly drying the air (4.nl) by hygroscopicity of the product, - and the intake air from the enclosure (ln), furthest downstream, consists of dry air, possibly steam dried by the recovery exchanger (6) in the case of closed loop operation on the air.

Images