FR2789629A1 - Heating and/or air conditioning unit for an automobile, comprises vertically mounted heat exchange and evaporator assemblies - Google Patents

Abstract

The unit comprises of a main feed conduit (6) that is connected to the fresh air intake. An evaporator (5) is mounted in a vertical orientation within this conduit prior to a division where the airflow can be directed through three chambers (25,26,27), top, middle and lower respectively. The junctions between conduits are controlled via butterfly valve assemblies dependent upon the operational mode of the unit.

Description

The invention relates to a heating and / or installation

  air conditioning of the passenger compartment of a motor vehicle.

  It relates more particularly to an installation of the air mixing type, comprising a fresh air supply duct which is divided into a first duct for the transmission of a fresh air flow, a second U-shaped duct located below the first duct and in which is placed a heat exchanger and which opens into the first duct by a hot air outlet, a first mixing flap to distribute the fresh air entered between the first duct and the second duct, said mixing flap being movable between an "all cold" position in which it closes the inlet and the outlet of the second duct, and a "all hot" position in which the first duct is closed and all the air passes through the second duct, and means for distributing the mixed air between a defrost outlet, a ventilation outlet and an additional outlet supplying at least one heating outlet of the

  feet, these means being connected to a distribution control.

  In these so-called air mixing installations, the incoming air which can be cooled beforehand by an evaporator is divided into two streams, one of which passes through the radiator, and is then mixed with that which has not passed by the radiator in order to reach the desired temperature. Once the air is at the desired temperature, it is distributed by a system of shutters to the outputs for heating the feet, the ventilation outputs and the defrost outputs. In air mixing systems, the outlets are arranged in the upper part of the installation, and the outlet for heating the feet is connected to air down ducts located either on the rear face of the installation. rating

  the passenger compartment, either on the side faces of the installation.

  In some cases, the space available in the bulkhead separating the front compartment of the vehicle from the passenger compartment is insufficient to allow the presence of air down ducts on the rear face or

  the side faces of the installation.

  It has already been proposed, in heating devices of the type comprising a variable-temperature heat exchanger, to have the heating outlet of the feet in the rear face of the installation opposite the heat exchanger. In this case, it is the air having passed through the heat exchanger which is delivered to this outlet, and it is necessary to provide an adjustable tap in the hot water circuit of the exchanger to regulate the temperature of the air sent to the feet. This provision is notably disclosed by documents FR-A-2 738 532 and

EP 0 781 673.

  The object of the invention is to propose a heating and / or air conditioning installation of the air mixing type which occupies little space. The invention achieves its object by the fact that the supply duct is further divided into a third fresh air duct situated under the second duct and communicating with the second duct downstream of the heat exchanger, and by the fact that the additional outlet is arranged in the extension of said third conduit, a second mixing flap being provided at the junction of the third conduit and the second conduit, this second component being movable between a "hot" position in which it closes the third conduit upstream of the junction and an "all cold" position in which it closes the communication between the second conduit and the

third conduit.

  The air for the additional outlet is therefore tapped in the lower part of the installation in a mixing chamber separate from that which manages the temperature of the defrosting and aeration air. The installation therefore includes two mixing zones which supply specific outputs. One of the mixing zones is located in the upper part of the installation and supplies the defrost and ventilation outlets which diffuse air into the middle or upper part of the passenger compartment. The other mixing zone is located in the lower part of the installation and delivers heated air to the additional outlet, from which heated air is supplied to the lower zone of

  the passenger compartment and in particular the foot area of the front passengers.

  To control the mixing flaps, it is possible to couple these two flaps to a mechanical control system, or to activate them independently by micromotors controlled by

  a temperature regulating member.

  To avoid a descent or an ascent of cold air downstream of the heat exchanger in the operating modes of foot heating, or defrosting, or aeration, or combined defrosting / aeration, means are provided for selectively closing the communication. of

  second conduit with the first conduit and with the third conduit.

  In the case where the mixing shutter control system is mechanical, the selective shutter means comprise either two shutters, one of which is in the open position, or a single shutter. When the mixing flaps are controlled by micromotors, the first mixing flap is put in the "all hot" position in the foot heating operating mode, and the second mixing flap is put in the "all cold" position in the modes of

  ventilation or defrost operation.

  In order to ensure that the temperature of the air supplied by the ventilation outlet is lower than the temperature of the air supplied by the foot heating outlet in the combined ventilation / foot heating operating mode, the installation comprises in addition to means for increasing the flow of fresh air in the ventilation outlet. These means preferably include a channel disposed in the first duct to bring a flow of fresh air directly to the ventilation outlet. The output of this channel may be blocked by a

  air distribution flap to the ventilation outlet.

  Other advantages and characteristics of the invention will become apparent from

  reading the following description given by way of example and

  reference to the accompanying drawings in which: FIG. 1 is a sectional view along a vertical plane of symmetry of an air conditioning installation for the passenger compartment of a motor vehicle, according to the invention; Figure 2 shows the arrangement of the shutters of the installation of Figure 1 in the ventilation mode of operation; FIG. 3 shows the arrangement of the flaps of the same installation in the combined foot heating / ventilation mode; FIG. 4 shows the arrangement of the flaps in the foot heating operating mode; FIG. 5 shows the arrangement of the flaps in the combined foot heating / defrosting operating mode; FIG. 6 shows the arrangement of the flaps of the same installation in the defrosting operating mode; Figures 7 to 12 show alternative embodiments of the partial insulation flaps of the heating duct in the modes of ventilation, defrosting and heating of the feet, in the case where the mixing flaps are coupled to a mechanical control system ; FIGS. 13 to 15 show the arrangement of the mixing flaps actuated by micromotors respectively in the foot heating, ventilation and defrost operating modes; and Figures 16 to 18 show alternative embodiments of the invention. The air conditioning installation 1, shown in the drawings, comprises a box 2 which has in its front part an air inlet chamber 3, supplied laterally with air taken from outside or inside the passenger compartment 4 of the vehicle fitted with it, and delivered by an air blower not shown in the drawings. From the inlet chamber 3, the air delivered by the blower passes through a vertical evaporator 5 arranged transversely in the supply duct 6 and serving to cool the incoming air flow. Downstream of the evaporator 5, the cooled air enters a chamber located in the rear part of the housing 2. The housing 2 is delimited by a front wall 7, a rear wall 8, an upper wall 9 and a lower wall 10 which extend perpendicular to the planes of the drawings and which connect two walls

  lateral parallel to the planes of the figures.

  The lower wall 10 and the rear wall 8 are connected to a duct 11 suitable for transmitting a flow of heated air for heating the feet, and possibly other vents, and in which is pivotally mounted about a transverse axis 12 , a butterfly flap 13 in position

  adjustable suitable for regulating the flow of heated air delivered by the duct 11.

  The upper wall 9 and the rear wall 8 are connected to an aeration duct 14 opening through an aeration outlet 15 by

  which an air flow is delivered for the ventilation of the passenger compartment 4.

  A butterfly flap 16 with adjustable position is pivotally mounted around a transverse axis 17 in the duct 14. This butterfly flap 16

  is intended to regulate the air flow delivered by the ventilation outlet 15.

  Above the upper wall 9 and at the front of the duct 14, there is a defrost duct 18 which opens to the outside by a defrost outlet 19 and to the inside of the housing 2 by an opening in the upper wall 9 in the upper part of the part

back of housing 2.

  A butterfly flap 20 with adjustable position is pivotally mounted around a transverse axis 21 in the defrost duct 18. The function of the flap 20 is to regulate the air flow delivered by the outlet of

defrost 19.

  The duct 11, the ventilation duct 14, the defrost duct 18, and the butterfly flaps 13, 16 and 20 constitute the means for distributing the air having passed through the interior of the housing 2. The butterfly flaps 13, 16 and 20 are mechanically coupled to a

  distribution control located on the vehicle dashboard.

  Each of these flaps can in particular take a closed position in

  which the associated duct is closed and no longer delivers air.

  The defrost outlet 19 is capable of delivering air to the windshield, the ventilation outlet 15 delivers air into vents located in the upper part of the dashboard, and the duct 11 is capable of delivering normally hot air in the lower area of the passenger compartment 4, in particular towards the feet of the front passengers and

  possibly to other outlets.

  The side walls of the housing 2 are further interconnected by two spaced intermediate walls 22, 23 which extend transversely through the chamber located in the rear part of the housing 2. The first intermediate wall 22 is located below the connection the rear wall 8 and the air duct with the ventilation duct 14. The second intermediate wall 23 is in the form of a flared U and is interposed between the first intermediate wall 22 and the bottom wall 10 of the housing 2. A heat exchanger 24 is

  installed between the two intermediate walls 22 and 23.

  The chamber located in the rear part of the housing 2 is thus separated into three conduits or passages through the intermediate walls 22 and 23: an upper conduit 25 through which fresh air from the evaporator is supplied to the aeration conduit 15 and the defrosting duct 18, a central heating duct 26 in which the heat exchanger 24 is placed and a lower duct 27 delimited by the lower wall

  and the second intermediate wall 23.

  The chamber 28 located between the heat exchanger 24 and the rear wall 8 of the housing 2 communicates with the lower duct 27 and the duct 11 for heating the feet by an orifice 29, and with the upper duct 25 by a delimited duct 30 by the first intermediate wall 22 and the upper part 31 of the rear wall 8, this upper part connecting to the ventilation duct 14 at the point referenced 32. This duct 30 has an inlet orifice 33 opening into the chamber 28 and a outlet 34 opening into the conduit

higher 25.

  A first mixing flap 40 of the butterfly type is pivotally mounted around a transverse axis 41 located near the upstream edge of the first intermediate wall 22. This mixing flap 40 comprises an upstream wing 40a capable of selectively closing the entry of the median conduit 26 or the inlet of the upper conduit 25, and a downstream wing 40b capable of selectively closing the outlet 34 of the conduit 30. The first mixing flap 40 is movable between an "all cold" position and a "all hot" position ". In the “all cold” position, the upstream wing 40a closes the inlet of the median duct 26, and the downstream wing closes the outlet 34 of the duct 30. In this “all cold” position, no air flow circulates at through the heat exchanger 24. In the "all hot" position of the first mixing flap 40, the upstream wing 40a closes the inlet of the upper duct 25 and the downstream wing 40b is pressed against the first intermediate wall 22. In this “all hot” position of the first mixing flap 40, no cold air circulates in the upper duct 25. In an intermediate position of the first mixing flap 40, a first flow of cold air circulates in the upper duct 25, a second air flow circulates in the median duct 26, is heated through the radiator and is capable of going up in the upper duct 25 by the duct 30. In the junction zone of the two air streams, the flow of cold air and the heated air flow delivered by the outlet 34 s e mix before being distributed by the ventilation outlet 15 and the defrost outlet

  19 as a function of the respective positions of flaps 16 and 20.

  When fresh air passes through the median duct 26 where it is heated by the heat exchanger 24, part of the heated air is evacuated through the orifice 29 in the lower duct 27 or it mixes with the fresh air delivered through the lower duct 27, in order to supply the duct

  11 for heating the feet with mixed air.

  A second mixing flap 50 of the drum type is pivotally mounted about a transverse axis 51 located at the junction of the bottom wall 10 and the conduit 11. This flap 50 has a cylindrical wall 52 capable of selectively closing the orifice 29 and the lower duct 27. This second mixing flap 50 is movable between an extreme “all hot” position in which the wall 52 closes the lower duct 27, the orifice 29 being open and in communication with the duct 11, and a position "all cold" in which the orifice 29 is closed by the cylindrical wall 52 and the lower duct 27 is in

communication with conduit 11.

  In FIG. 1, the "hot" position of the two mixing flaps 40 and 50 has been shown in solid lines, and in dotted line the

  "all cold" position of these two components.

  In the upper part of the upper duct 25, a channel 60 is provided which places the outlet of the evaporator 5 in communication with the ventilation duct 14. The outlet 61 of this duct 60 is closed by the ventilation shutter 16 in closed position. The channel 60 makes it possible to send a forced flow of fresh air to the ventilation duct 14, so that the temperature of the air supplied by the duct 11 for heating the feet is slightly higher than the temperature of the air d ventilation in the foot heating / ventilation combined operating mode. The temperature difference depends on the section of the channel 60 and can be adjusted by the position of an additional flap installed in the channel 60 and not

shown in the drawings.

  The two mixing flaps 40 and 50 are controlled by a temperature control device, installed in the dashboard of the vehicle. The transmission of the command can be carried out mechanically, in this case the two mixing flaps are coupled together. The mixing flaps 40 and 50 can also be moved by independent micromotors under the control of

  the temperature control member.

  In all cases of control of the two mixing flaps 40 and 50, the law of opening of the two flaps 40 and 50 is carried out so as to ensure, in the combined operating mode foot heating / defrosting, a temperature evolution almost identical, or with a small difference, if required by the specifications, between the air at the defrost outlet 19 and the air in the duct 11 which supplies the additional outlet. In the combined foot heating / ventilation operating mode, the presence of the channel 60 makes it possible to have an air temperature lower at the ventilation outlet 15 than in the duct 11 which

feeds the additional output.

  When the two mixing flaps 40 and 50 are in an intermediate position, and to prevent fresh air from rising upstream in the chamber 28 behind the heat exchanger 24, when the butterfly flap 13 is in the closed position or when the two butterfly flaps 16 and 20 are in the closed position, means are provided for selectively closing the orifice 29 or the inlet 31 of the duct 30. According to a first embodiment of these means, shown in FIG. 1, a first butterfly flap 70 is pivotally mounted around a transverse axis 71 in the orifice 29 which puts the chamber 28 in communication with the lower duct 27, and a second butterfly flap 80 and pivotally mounted around a transverse axis 81 in inlet 31 of duct 30. For simple distribution methods (ventilation, defrosting, foot heating), these two flaps 70 and 80 can only take

  two extreme positions, an open position or a closed position.

  So they work in all or nothing. But in all operating cases, at least one of these flaps 70 or 80 is in the open position. In the two combined operating modes foot heating / ventilation and foot heating / defrosting, the two flaps 70 and 80 are in the open position or in intermediate positions defined to comply with the evolution curves of

  temperature or a temperature difference between the outlet vents.

  Figures 2 to 6 show the positioning of the different components of the installation described above with reference to Figure 1, that is to say, respectively: 1- the ventilation operating mode; 2- the combined operating mode foot heating / ventilation; 3- the foot heating operating mode; 4- the combined operating mode foot heating / defrosting; and - the defrost operating mode. In all these modes, the two mixing flaps 40 and 50 are in intermediate positions, that is to say the upper duct 25, the middle duct 26 and the lower duct 27 receive fresh air from

the evaporator 5.

  In 1- ventilation mode (see Figure 2), the foot heating flap 13 and the defrosting flap 20 are closed. The ventilation flap 18 is open. The first butterfly flap 70 is open. The second butterfly flap 80 is in the closed position. No air flow circulates in

the lower duct 27.

  In the combined mode 2- foot heating / ventilation (see Figure 3), only the defrosting flap 20 is in the closed position, the

  other flaps 13, 18, 70 and 80 are in the open position.

  In the 3- foot heating mode, shown in FIG. 4, the defrosting flap 20, the ventilation flap 18, and the first flap 70 are

  in the closed position. The flaps 13 and 80 are however open.

  No fresh air flow circulates in the upper duct 25, nor in

the conduit 30.

  In the combined mode 4- foot heating / defrosting (see Figure 5), the flaps 70, 80 and 13 are in positions similar to those shown in Figure 3. The ventilation flap 18 is in the

  closed position and the defrost flap 20 is in the open position.

  The 5- defrost mode (figure 6) is identical to the 1- mode

  ventilation shown in Figure 2, with the defrost flap 20 in the open position, and the ventilation flap 18 in the closed position. Here

  also, no fresh air circulates in the lower duct 27.

  In a second embodiment, shown in FIG. 7, the flaps 70 and 80 consist of flag-type flaps mounted pivoting around transverse axes 71 and 81 located in the vicinity of the rear wall 8. The numbers 1-5 indicated on the flaps 80 and 81 correspond to the positioning of these flaps 70 and 80 in the modes

  operating modes 1- to 5- listed above.

  Figure 8 shows another alternative embodiment of the flaps 70

  and 80 which are here produced in the form of drum-type shutters.

  In the embodiment shown in FIG. 9, the two flaps 70 and 80 are replaced by a single flap 90 of the flag type mounted pivoting about a transverse axis 91 located in the rear wall 8. This flap 90 is capable of close the orifice 29 in operating modes 1 and 5-, close the inlet 31 of the conduit 30 in mode 3 - foot heating. The flag flap 90 takes an intermediate position in the combined modes 2- and 4- and shares the air flow

  hot delivered by the heat exchanger 24 in two flows.

  The embodiment shown in FIG. 10 differs from the embodiment shown in FIG. 9, in that the single flap 90 of the flag type is replaced by a single flap 90 of the drum type

  mounted pivoting about a transverse axis 91 located in the chamber 28.

  The cylindrical wall 92 of the flap 90 can seal either the orifice 29 or the inlet 31. In the combined modes 2- and 4-, the cylindrical wall 92

  is attached to the rear wall 8 of the housing 2.

  The flag flap 90 shown in FIG. 9 can be replaced by a butterfly flap 95 as can be seen in the

  realization shown in figure 11.

  The selective closing of the orifice 29 and of the inlet 31 in the conduit 30 can also be carried out by non-return valves 97 and

  98, as shown in Figure 12.

  When the two mixing flaps 40 and 50 are actuated by independent micromotors, it is possible to prevent the phenomena of bypassing the fresh air in the chamber 28, without the presence of the additional flaps 70 and 80, by acting on the positioning of one of the mixing flaps 40, 50, as is

shown in Figures 13 to 15.

  In 3- foot heating mode, the defrost flap 20 and the ventilation flap 16 are in the closed position. No fresh air should circulate in the conduit 30. In this mode 3-, the micromotor which actuates the first mixing flap 40, positions the latter in the "all hot" position, as shown in FIG. 13, while that the second mixing flap 50 remains in an intermediate position. When changing the mode, the first mixing flap returns to the normal position displayed by the temperature control unit. In ventilation or defrost operating mode, on the contrary, the second mixing flap 50 is put in the "all cold" position, as shown in FIGS. 14 and 15. The first mixing flap 40 keeps its intermediate position. When changing the mode, the corresponding micromotor will return the second mixing flap 50 to an intermediate position corresponding to the position

  of the temperature control member.

  FIG. 16 shows an alternative embodiment of the second mixing flap 50. The drum type flap 50 visible in FIGS. I to 15 is replaced by a flap 50 of the butterfly type pivotally mounted in the lower duct 27. This flap 50 manages the quantity of cold air admitted into the mixing zone below the inlet of the duct 11. The flow of hot air is controlled by the first mixing flap 40. In the operating modes 1 - ventilation and 5 - defrosting, l orifice 29 is closed by the second butterfly flap 80. The advantage of this variant is to offer the possibility of reducing the volume of the installation in the lower part and therefore better responding to the constraints of

compactness.

  According to two other alternative embodiments shown in FIGS. 17 and 18, the inlet of the median duct 26 is divided into two entrances by means of an additional intermediate wall 101, the upper inlet alone being capable of being closed by the upstream wing 40a of the first

mixing flap 40.

  In the variant embodiment shown in FIG. 17, a flag-type flap is pivotally mounted around an axis 102 adjacent to

  wall 101. In simple distribution modes 1 - ventilation, 3 -

  foot heating, 5 - defrosting, shutter 100 closes the inlet

  lower. In the combined modes, 2 - feet / ventilation, 4 -

  feet / defrost, the flap 100 is directed towards the heat exchanger 24.

  The shutter 80, of flag type conforms to that of FIG. 7. The numbers 1 - 5 indicated on the shutter 80 of FIG. 17 correspond to the positioning of this shutter 80 in the operating modes 1 - to 5 listed above. In the combined operating modes 2 - and 4 -, the air flow passing through the heat exchanger 24 is separated into i2 two streams, the upper stream being directed, after reheating, to the upper mixing zone and the lower stream warmed up being directed to the lower mixing zone. The flaps 80 and 100 are

  coupled with distribution shutters.

  In the simple operating modes 1 - ventilation, 5 - defrosting, the shutter 80 closes the passage 29 towards the lower mixing chamber. We thus recover the entire power of the heat exchanger

  heat 24 for ventilation or defrosting.

  In operating mode 3 - foot heating, the flap 100 is retracted by pressing against the wall 23, which makes it possible to obtain all of the power of the heat exchanger 24 for heating the feet. The descent of cold air from the upper mixing chamber is prevented by the anti-bypass shutter 70 which

  controls the passage between the chamber 28 and the conduit 30.

  As can be seen in FIG. 17, the flaps 80 and 100 play the role of flaps for separating the lower and upper veins in the heat exchanger 24 in the combined operating modes 2 feet / ventilation and 4 - feet / defrost . These shutters 80 and 100 have a different position in these combined modes, which results in a different partition of the heat exchanger 24. In the simple operating modes, on the other hand, the lower inlet is closed by

  the flap 100, which avoids parasitic heating.

  In the embodiment shown in FIG. 18, the upstream separation flap 100 is eliminated, but the additional intermediate wall 101 extends to the vicinity of the heat exchanger 24. The lower inlet is here permanently supplied with fresh air,

  which creates parasitic heating in certain configurations.

  This variant has the advantage of being less complex. It is obviously possible to replace the second mixing flap 50 of FIGS. 17

  and 18 by a butterfly type flap 50, such as that

shown in figure 16.

  In all the embodiments described above, the installation can be separated in two by means of a partition positioned in a median plane. We then obtain a separate right / left device or two zones allowing to differentiate the temperature and the distribution between two parts of the passenger compartment, for example, the right part and the left part. The installation described in detail above relates to air conditioning for a vehicle. It is obvious that the evaporator 5 could be omitted, the installation then relating to a device for

heating ventilation.

Claims (13)

  1. The installation for heating and / or air conditioning of the passenger compartment of a motor vehicle, of the air mixing type, comprising a supply duct (6) for fresh air which is divided into a first duct (25 ) for the transmission of a fresh air flow, a second U-shaped duct (26) located below the first duct (25) and in which is placed a heat exchanger (24) and which opens into the first conduit (25) by a hot air outlet (34), a first mixing flap (40) for distributing the fresh air entered between the first conduit (25) and the second conduit (26), said mixing flap ( 40) being movable between an "all cold" position in which it closes the inlet and outlet (34) of the second duct (26), and a "all hot" position in which the first duct (25) is closed and all the air passes through the second duct (26), and means for distributing the mixed air between a defrost outlet (19), a ventilation outlet (15) and an outlet additional supplying at least one foot heating outlet (11), these means being connected to a distribution control, characterized in that the supply duct is further divided into a third duct (27) for fresh air located under the second conduit (26) and communicating with the second conduit (26) downstream of the heat exchanger (24), and by the fact that the outlet (11) for heating the feet is arranged in the extension of said third conduit (27), a second mixing flap (50) being provided at the junction of the third conduit (27) and the second conduit (26), this second mixing flap (50) being movable between a "hot" position in which it closes the third conduit (27) upstream of the junction and an "all cold" position. 2. Installation according to claim 1, characterized in that the second conduit (26) comprises downstream of the heat exchanger (24) means (70, 80) for selectively closing the communication (29) of the second conduit ( 26) with the first conduit (25) and with the third   conduit (27), these means being coupled to the distribution control.   3. Installation according to claim 2, characterized in that said shutter means comprise two independent flaps (70,   ), at least one of which is in the open position.   4. Installation according to claim 2, characterized in that   said sealing means comprise a single flap (90, 95).   5. Installation according to any one of claims 2 to 4,   characterized in that the first mixing flap (40) and the second mixing flap (50) are coupled by a mechanical system. 6. Installation according to claim 1, characterized in that the first mixing flap (40) and the second mixing flap (50) are each actuated by a micromotor, and by the fact that in foot heating operating mode , the first mixing flap (40) is put in the "all hot" position, and, in ventilation or defrosting operating mode, the second mixing flap (50) is put in the "all cold" position.   7. Installation according to any one of claims 1 to 6,   characterized in that it further comprises means for increasing the flow of fresh air in the ventilation outlet (15) in the mode   of combined ventilation / heating of the feet.   8. Installation according to claim 7, characterized in that said means comprise a channel (60) disposed in the first duct (25) to bring a flow of fresh air directly to the outlet ventilation (15).   9. Installation according to claim 7, characterized in that the outlet (61) of said channel (60) is capable of being closed by a flap of distribution (16).   10. Installation according to any one of claims 1 to 9,   characterized in that the second mixing flap (50) is a drum type flap, which, in the "all cold" position, closes the communication (29) between the second conduit (26) and the third conduit (27).   11. Installation according to any one of claims 1 to 9,   characterized in that the second mixing flap (50) is a   butterfly type shutter disposed in the third conduit.   12. Installation according to any one of claims 10 or 11,   characterized in that the inlet of the median duct (26) is divided into two inlets by means of an additional intermediate wall 101, the upper inlet alone being capable of being closed by the first mixing flap (40).   13. Installation according to claim 12, characterized in that the lower entrance is capable of being closed by a flag flap 100.