INDUSTRIAL AIR DRYING INSTALLATION

Abstract

This air-drying installation for a wet product comprises: – a drying chamber (1) through which the wet product to be dried (2) and the drying air pass, said chamber comprising at least one intake air circuit (4) and at least one exhaust (5) for the moisture-laden air 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 heat from one or more heat sources consisting of the moisture-laden air passing through said exhaust (5) by means of a heat recovery exchanger (6); and.to generate steam using the heat thus recovered, at least one means suitable for initiating the operation of the boiler(s) (7) by supplying heat; – at least one compressor (11) suitable for increasing the pressure of the steam generated in said boiler (7); – at least one injection heat exchanger (12) suitable for condensing the steam from the compressor (11), positioned within the intake air supply circuit (4), and for heating the latter upstream of the drying chamber (1); – at least one condensate collector (13) for the condensate from the condensation of the steam in said injection heat exchanger (12), said condensate being returned to the boiler (7). Figure for the abbreviation: Fig 1.

Description

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

[0001] The invention relates to the field of air drying of wet products in industry. It potentially covers any drying installation that can be 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 found in various industrial sectors such as food processing, chemicals, paper and cardboard, and non-metallic materials. This installation uses air as the drying fluid (a drying fluid is defined as a fluid that allows the evaporated water to be removed from the dryer and provides heat to the dryer to facilitate this evaporation), which serves as a transport medium for the evaporated water as well as a means of supplying heat to the dryer.

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

[0003] Drying systems are very common in industry and represent a significant portion of industrial energy consumption: 10 to 20% of total industrial energy consumption 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. Drying air, usually hot, flows through this chamber and becomes saturated with moisture upon contact with the wet product. This saturated air, commonly referred to as vapor, is then expelled from the chamber. This vapor has a relatively high temperature and humidity level.

[0004] The energy loss resulting from the temperature and humidity in the vapors is therefore easily understood, as is the corollary to the energy consumption required to ensure the effective evaporation of the moisture present in the product to be dried. It is indeed known that the enthalpy of vaporization at atmospheric pressure Ahvap is on the order of 630 kWh per tonne of water evaporated, requiring, in such a configuration where the vapors are simply released into the atmosphere, an energy consumption 2 to 4 times greater (according to the literature "Processes of drying 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 exergetic inefficiency.

[0006] To reduce this consumption, solutions exist for recirculating some of the vapors back into the drying air intake circuit, thereby transferring some of the relative heat from said vapors. While this recirculation does provide some energy savings, these savings are quite limited.

[0007] Drying installations combining the introduction of hot air, similar to that described previously, and heating cylinders located within the heating chamber have also been described. These heating cylinders are typically heated by steam or hot air.

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

[0009] Recently, there has been growing interest in heat recovery from vapors 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 heat input in the dryer (for example, heating cylinders).

[0010] Also, some manufacturers are working on closed loop drying systems, in which a heat pump allows the water from the vapors to condense at the heat pump evaporator, and then to reheat the dried vapors 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 consumption below 200 kWh / tonne of water evaporated. 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 optimize the energy efficiency of the drying process, which has significant potential for improvement given the low temperatures that allow the use of compressors and heat pumps.

[0013] It follows from these observations, particularly in a context where energy is becoming a particularly high expense for manufacturers and where industry is strongly encouraged, or even forced, to reduce its carbon emissions to curb global warming, that an increasingly pressing need exists in terms of streamlining such drying facilities and more specifically in terms of reducing energy consumption at the source.

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

[0015] To this end, the invention relates to a drying installation by means of a drying vector or fluid, and in particular by air, of a wet product, said installation comprising a drying chamber within which the wet product to be dried and the air ensuring the drying pass, this chamber being provided with at least one intake air circuit, and at least one exhaust of the air charged with moisture resulting from the exchange of water and energy with the wet product to be dried, referred to as vapors.

[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 the humid air (mists) passing through the humid air exhaust, this heat recovery being carried out using a heat 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 suitable for initiating the operation of the boiler(s) by supplying heat to 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 vapor from the compressor, this injector exchanger being positioned within the intake air supply circuit, i.e. upstream of the drying chamber, and to ensure the heating of this air upstream of the drying chamber; - at least one condensate collector from the condensation of steam in the injection exchanger, this condensate being returned to the boiler.

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

[0018] In the sense 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 at the bottom 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 heat recovery exchanger is defined as equipment that allows heat to be taken from the dryer's exhausts, in this case the vapors; - an injection exchanger is defined as equipment that allows 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 chamber can itself integrate additional heat supply means, called additional injector exchangers, supplied with steam from the previously mentioned compressor.

[0020] These means of additional heat supply are classically heating cylinders, double walls of drying chambers, screws, pallets or even steam radiants.

[0021] To ensure the start of the boiling operation within the boiler, the means for initiating the operation of the boiler(s) by supplying heat consists of a heat source, that is to say, an energy flow from which additional heat can be recovered. 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 heat, and in particular resistance or induction technology, one or more electric immersion heaters, or one or more heat exchangers supplied with a heat transfer fluid such as hot water, thermal oil, or steam.

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

[0023] As a corollary, the installation may also provide for a device for putting the boiler under vacuum in order to allow, once again, the start of the boiling operation within the latter.

[0024] In order to optimize the drying system of the invention, the compressor(s) are regulated according to the drying requirements defined by the drying kinetics of the different products: the system allows for the adaptation of the vapor condensation temperatures by varying the compression ratios of the compressor(s). This makes it possible to obtain vapor injection at different pressure levels for the same injection exchanger, depending on the drying requirements of the product. This is achieved through measurements that allow for an estimation of the drying kinetics in order to avoid over-drying. This regulation thus makes it possible to limit the electrical energy consumption required for the operation of the compressor; When the drying temperature requirement is lower, the steam is compressed to lower pressure levels.

[0025] To this end, the electric motor equipping each compressor has a frequency converter that allows the vapor pressure setpoint at the compressor outlet to be regulated. This substantially limits electrical consumption by preventing excessively high and unnecessary pressures. Thus, online measurement of the temperature variation of the product to be dried, for example by infrared or thermal imaging, makes it possible to control the drying kinetics of the product and prevent over-drying by defining the appropriate pressure / temperature levels.

[0026] Furthermore, to facilitate the operation of the boiler, the installation may also include a heat pump designed to raise the temperature level obtained by the heat exchanger located in the steam discharge, particularly if the pressure at the obtained temperature cannot be achieved under satisfactory technical conditions. In the theoretical ideal, the evaporator of the heat pump is positioned directly in the steam flow, and the condenser in the boiler.

[0027] According to a particular closed-loop configuration of the invention, the intake air is taken from the mist, after drying by means of a recuperating exchanger and then reheating by means of an injecting exchanger.

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

[0029] The manner in which the invention can be implemented and the resulting advantages will be more apparent from the following implementation examples, given by way of illustration and not limitation, in support of the attached figures.

[0030] Fig. 1 is a schematic representation illustrating the basic drying installation of the invention.

[0031] The [Fig.2] is a variant of the [Fig.1] incorporating an additional injector exchanger integrated into the drying chamber itself.

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

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

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

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

[0036] Figure 7 schematically illustrates a particular embodiment of the invention in which the low moisture content in the vapors necessitates 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 requires adaptation to meet the needs of the wet products to be dried.

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

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

[0040] Hot air is introduced into this drying chamber (1) via an inlet circuit (4). After exchange within the drying chamber resulting from the phenomenon of convection and mass transfer between the hot air and the wet product to be dried, air heavily laden with moisture, also referred to as vapors, exits said chamber through 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 recuperative exchanger (6) by recovering the energy in the form of sensible heat as well as the latent energy of the vapors by condensing water, from the water of the evaporated product, 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 heat exchanger (6) will heat the working fluid to the saturation pressure temperature in the boiler. This working fluid, in liquid / vapor phase equilibrium in the boiler, thus generates steam at the saturation temperature.

[0043] However, in order to initiate the boiling process of the water within the boiler, a heat input is necessary. This heat input can be carried out by a heat pump (8) (see [Fig.7]), taking heat from the evacuation (5) of vapors, possibly by means of two intermediate loops of heat transfer fluid (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 need to be adjusted to the temperature of the heat source. In this regard, the boiler can be connected to a vacuum device (9) (see [Fig. 7]) to lower the pressure within the boiler during the start-up phases. This device is subsequently switched off. The compressor (11) associated with the injection heat exchanger (12) ensures pressure is maintained in the boiler during normal operation, preventing excessive energy input through the heat recovery heat 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 consequently the temperature of the steam thus generated, according to the well-known laws of thermodynamics.

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

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

[0048] The liquid condensates, possibly subcooled, 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 heat recovery exchanger (6) can be reused in the plant after any necessary water treatment, such as pH adjustment, Biological Oxygen Demand (BOD), or Chemical Oxygen Demand (COD). This recovered water (23) helps limit industrial water consumption and is therefore an additional advantage of the invention, particularly relevant in a context of increasing water resource scarcity.

[0050] Figure 2 illustrates another variant of the drying installation conforming to the invention.

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

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

[0053] Figure 3 illustrates a variant of the installation in Figure 2. In this figure, two injector heat exchangers (14.1) and (14.2) are shown, providing supplementary heat input, positioned within the drying chamber (1). Part of the steam from the compressor (11.1), introduced at the first of these supplementary injector heat exchangers (14.1), is diverted and sent to a secondary or supplementary compressor (11.2), which supplies the second supplementary heat input (14.2). This secondary compressor (11.2) is designed to further increase the pressure (and therefore the temperature) of the steam, compared to that supplying the first supplementary heat input (14.1). Indeed, in the example described, this second heat exchanger (14.2) is located downstream of the first heat exchanger (14.1).1) in the direction of the product flow intentionally, due to the effective progression of the wet product within the enclosure, a configuration in which a higher temperature heat source 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 subcooled in a subcooler exchanger (15) preheating the inlet air before being returned (16) to the boiler feed circuit.

[0054] Figure 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 (ln), mounted in series with each other, each of said chambers being supplied with hot air, as described previously.

[0055] However, in this embodiment, the removal of the vapors (5), i.e., in this case, the air with the highest humidity content, occurs only at the level of the upstream chamber (1.1). For an integer i between 1 and n-2, the chamber (11) is supplied with air (4.i) from the heat exchanger and water containing the product to be dried from the chamber immediately downstream (1.i+1), and optionally with additional intake air (20.i). This air is heated by means of injector heat exchangers (12.i), which are themselves supplied with steam. The injector heat exchanger (12.1) of the intake circuit (4.1) of the upstream chamber (1.1) is supplied with steam 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 pressure (and therefore temperature) increase by means of additional compressors (1 li).In fact, the more humid the product is. Drying progresses within the enclosures, and the warmer and drier the air. The lowermost enclosure (ln) is supplied with fresh air (4.n), i.e. ambient air, and the previous enclosure (ln-1) is supplied with slightly humid air heated by the product passing through the enclosure (ln); the product cools down in contact with the fresh, dry air and may absorb moisture from the air according to its hygroscopic capacities and its dry matter content.

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

[0057] Figure 5 illustrates the adaptation of the invention to a closed-loop drying system. In this configuration, the heat recovery exchanger (6) is installed on the vapor circuit to dry the vapors by removing the water from the product (23). The dried vapors are recirculated to the injection exchanger (12) to generate hot intake air (4) to supply the drying chamber (1).

[0058] This configuration potentially implies 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 a waste heat exchanger (21) so that the condensate can be redirected to the boiler (7).

[0059] This configuration is particularly interesting with regard to the evaporated water of the product, which is thus recovered at 100% as well as the absence of discharges in the form of vapors, 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 dried vapors (5) in the recuperator exchanger (6) to be redirected to the air intake (4.n) of the downstream enclosure (ln).

[0061] Figure 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 raises the temperature of the energy recovered from the steam (5) by means of a heat exchanger on the steam (6.1), which supplies the evaporator of the heat pump, and a heat exchanger (6.2) in the boiler (7). This equipment can be a temporary solution if the temperature of the heat extracted from the steam is insufficient to allow the boiler (7) to operate. It should be noted that this solution simplifies the installation using a standard technological component (heat pump). However, it would be possible to install a compressor and an expansion valve between the heat exchangers (6.1) and (6.2) with a different working fluid, whose saturation temperatures are at the recovery temperature in the heat exchanger (6.1) and the saturation temperature of the working fluid in 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 minimum temperature delta between the evaporator and condenser of said heat pump, implying a mixing 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 feature of the invention lies in its ability to improve performance at reduced loads: the main compressor (11.1) and the auxiliary compressors (11.2) to (ll.n-2) are regulated according to the drying requirements defined by the drying kinetics of the products entering the drying chamber and online measurements. The incoming moisture content of said products may, for example, vary depending on the season or the recipe, requiring online adaptation of the dryer to the drying needs. The invention achieves this by reducing consumption per unit of evaporated water when the drying system is operating at low load through a decrease in the compression ratios of the steam compressors, and therefore their consumption per unit flow rate.

[0064] Typically, the invention optimizes drying performance when the tonnage of evaporated water is lower than the dryer's design value, which is very frequently the case in real-world applications due to initial oversizing and operation on various products, the most humid of which is the designed product. Thus, a regulated pressure increase of the steam from the boiler (7) via the compressors (11.1 to 1ll.n-2) allows: - to lower air temperatures when the latter does not need to be too high; - to reduce the temperature in the means of supplementary heat supply by injection exchangers; - to obtain reduced energy consumption when drying needs are lower, since lower vapor pressures result in lower electricity consumption of compressors.

[0065] This compressor control can be achieved through online measurement, for example, by varying the temperature of the wet product to be dried at different points. This temperature measurement can be carried out by any measurement and acquisition method, for example, infrared measurement or by means of thermal imaging cameras integrated into the drying chamber, depending on the dryer configurations and technologies adapted to the temperature, humidity, and potential fouling, possibly with the presence of an outdoor drying chamber with a viewing window if the technologies allow it.

[0066] The installation according to the invention constitutes a significant improvement over those known in the prior art. It offers a number of advantages, including: - a very significant increase in the energy performance of drying without degrading the quality of drying, allowing us to move towards the Minimum Energy Required through the electrification of dryers, allowing us 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-based, such as France; - the complete (closed loop) or large part recovery of the water extracted from the products to be dried, allowing to improve the water efficiency of the drying operations; - the implementation of a limited number of components, and allowing to limit the lengths of heat transport networks with a process self supplied by equipment present at the installation level, including start-up, backup and supplementary 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, namely water, already present as an energy carrier in industry (therefore reassuring), facilitating refilling in case of leaks and non-polluting; - the flexibility to get closer to 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 prior art installations, particularly in terms of energy consumption and therefore greenhouse gas emissions, and water consumption.

Claims

Demands

1. An air-drying installation for a wet product comprising a drying chamber (1) through which the wet product to be dried (2) and the drying air pass, said chamber comprising at least one intake air circuit (4) and at least one exhaust (5) of 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, adapted to produce steam in a conveying circuit (10), comprising: O a tank containing a working fluid in a liquid / vapor phase equilibrium in which a heat input at the bottom allows boiling of the working fluid; O a recuperating exchanger (6, 6.2) adapted to heat the working fluid of the boiler (7) to the saturation temperature of the pressure in the boiler, this exchanger being mounted on a steam circuit to dry them, and disposed in said tank, 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); - at least one means suitable for initiating the operation of the boiler(s) (7) by supplying heat; - at least one compressor (11, 11.11.3) arranged on the conveying circuit (10) capable of increasing the pressure, and therefore the temperature, of the steam; - at least one injector exchanger (12) capable of condensing the steam from said at least one compressor (11), positioned within the intake air supply circuit (4), and of ensuring the heating of the latter upstream of the drying chamber (1); - at least one collector (13) of the condensate from the condensation of the steam in said injector exchanger(s) (12), said condensate being returned to said boiler(s) (7).

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

3. Air drying installation for a wet product according to claim 2, characterized in that the means for additional heat supply are heating cylinders, double walls of the drying chamber, screws, paddles and / or steam radiants.

4. Air drying installation for a wet product according to any one of claims 1 to 3, characterized in that the means suitable for initiating the operation of the boiler(s) (7) by supplying heat is an additional start-up heat source selected from the group comprising electrical induction technology, electrical resistances, direct combustion or heat exchangers supplied by a heat transfer fluid such as hot water, superheated water, thermal oil, hot air, fumes or steam.

5. Air drying installation for 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 case of a problem with the installation.

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

8. Air drying installation for a wet product according to any one of claims 1 to 7, characterized in that it comprises a heat pump (8) capable of raising the temperature level at the level of the recuperating 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 attainable under satisfactory conditions.

9. An air-drying installation for a wet product according to any one of claims 1 to 8, characterized in that the intake air (4) is taken from the exhaust air (5) of the drying chamber (1), which is

10. dried by means of a recuperating exchanger (6), then heated by means of an injecting exchanger (12). An air-drying installation for a wet product, comprising a plurality of drying chambers 1, 2, ..., n, connected in series, the upstream chamber (1.1) receiving the product to be dried with the highest moisture content, and the downstream chamber (ln) receiving the product to be dried (2) after passing through the upstream chambers, according to any one of claims 1 to 9, wherein: - the inlet air (4.i) supplied to the upstream chamber (li) relative to the direction of flow of the wet product to be dried comes from the moisture-laden air (4.i+l) from the drying chamber (1.i+1) mounted immediately downstream of said upstream chamber after heating by an injector heat exchanger (12.i), i being an integer between 1 and n-2, - the steam from the boiler(s) whose pressure (and therefore temperature) has been increased by means of the main compressor (11.1), is conveyed as a priority within an injector exchanger provided in the air supply circuit of the most upstream enclosure (1.1), - part of this vapor is subjected to compression at the level of an additional compressor (11.2) to convey a higher pressure vapor to the level of an injector exchanger (12.1) provided within an air supply circuit of said downstream enclosure with possible supply of fresh air for mixing (20.1); this compression process towards the downstream chamber being likely to occur 1, 2, ..., n-2 times, - the air of the chamber (ln-1) comes from the air exiting the chamber (ln), preheated by the hot product to be dried from the chamber (ln-1), which has cooled in the chamber (ln) by giving up its heat to the air and possibly by drying the air (4.nl) by hygroscopicity of the product, - and the air entering the chamber (l.n), the most downstream, consists of dry air, possibly of vapors dried by the recuperating exchanger (6) in the case of closed loop operation on air.

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