METHOD AND MEANS TO PREVENT CONDENSATION IN MONOCOQUE STRUCTURES

Abstract: A method and means to prevent water vapour in humid air (Ac) in an inner space (1) of a monocoque structure, which shows an outer, at least an essentially hermetic shell (2) and an inner partition (3) which is devised with an intermediate space (4) against the hermetic shell (2), e.g. a pressure cabin of an aeroplane, from condensating into water and possibly freeze into ice on the inside of the outer, hermetic shell (2) and in the space (4), and also to remove existing water, if any, from this space (4). Dry air (Ad, Ab) with a relative humidity of less than 50 % preferably less than 10 % and particularly less than 5 % is directed into the space (4) between the outer shell (2) and the inner partition (3) with the help of means (6) for directing the dry air (Ad, Ab) with a pressure which exceeds the pressure of the intermediate space (4), which means are connected to at least one inlet opening (8.2, 8.4) in the outer shell (2) and/or the inner partition (3) of the monocoque structure.

Description: METHOD AND MEANS TO PREVENT CONDENSATION IN MONOCOQUE STRUCTURES TECHNICAL FIELD This invention relates to a method for preven¬ ting water vapour in humid air in an inner space of a monocoque structure, which shows an outer, at least an essentially hermetic structure, and an inner partition which is devised with a space against the hermetic shell, e.g. a pressure cabin of an aeroplane, from condensating into water and possibly freezing into ice on the inside of the outer hermetic shell which constitutes the mono¬ coque structure and in the space by the shell, and for removing existing water, if any, from this space. Even if the invention primarly is intended to be applicable to aeroplanes, it does not exclude that the invention is even applicable to stationary as well as transportable structures, e.g. constructions, containers and ground vehicles. TECHNICAL BACKGROUND A pressure cabin of an aeroplane constitutes an essentially hermetic monocoque structure which is pressu¬ rized through a flow of heated air from the aeroplane ' s engine compressors and is directed into the cabin after the regulation of pressure, temperature and humidity. The engines of the aeroplane must therefore be operating, wherefore this type of pressurisation normally is done only during flight. A similar system, which is however preferably meant for being stationed on ground, is de- scribed in the american patent publication US 3 623 332. Systems of this kind are intended for creating a condi¬ tioning of air which is blown into the aeroplane ' s pres¬ sure cabin in order to give a good comfort to the aero¬ plane ' s passengers and crew without considering the possible condensation of the water vapour which exists in the air. These systems have consequently not been utili¬ sed in order to eliminate damage to the monocoque structure of the aeroplane or to other vital parts of the aeroplane which are caused by water which condensates in the aeroplane. Concurrently with the increasing costs of new produced aeroplanes, it has become an aim among the air¬ lines to extend the aeroplane ' s life expectancy beyond the originally projected life expectancy. Components such as engines and other equipment therefore can be overhau¬ led and exchanged continuously during the use of the aeroplane, whereas worn and damaged parts of the aero¬ plane body have turned out to be more difficult and time-consuming to repair and also contribute to extra weight. Damage to the aeroplane body is mostly caused by corrosion, mainly on the aeroplane ' s pressure cabin, which is comprised of a hermetic shell, which on the in¬ side maintains an environment with a high humidity ari¬ sing from i.a. the water each passenger emits during the stay in the aeroplane, and which on the outside is being exposed to very low air temperatures. The outer shell of the pressure cabin is further complemented with a heat- insulated inner partition being extended along the men¬ tioned shell. There is no diffusion barrier against water vapor between the inner partition and the outer shell, wherefore the humid air in the cabin reaches the cold outer shell unimpededly, where the water in the air con¬ densates and possibly also freezes into ice. In a known aeroplane accident, where the roof of the pressure cabin was torn off in the air, the reason for the lacking strenght was that the pressure cabin had been exposed to very severe corrosion, which had contri¬ buted to decrease the fatigue strength of the pressure cabin. Beside damage to parts of the aeroplane body itself, the condensated water also causes damage to other components and foremost to electrical apparatuses. Even mould and fungus assaults can be found in humid areas of an aeroplane. To prevent the uprise of such damage, one has been forced to enclose these components in water- -proof covers and similar devices, which has made not on¬ ly the cost of the structures higher but it has also in¬ creased the aeroplane ' s weight. Because of the pressure changes in the aeroplane, humid air penetrates into the components in spite of these measures and causes damage. Experience has also shown that an aeroplane of the conventional type for 120-150 passengers contains at least 500 kg water in free form on free surfaces and in cavities and similar places, as well as absorbed in the insulation of the pressure cabin and in hygroscopic mate¬ rials. Even considerably larger amounts of water can du¬ ring a flight be collected in the aeroplane, e.g. in the form of ice, which when melting must be drained or be removed in another way before the aeroplane can be taken into traffic again. The increase of the aeroplane ' s fu¬ tile load in the form of condensed water, which can not be removed during the ground stops, is therefore a considerable negative factor. In order to solve the condensation problem in aeroplanes, attempts have been made to use ground-based dehumidifying equipment which is connected to the aero¬ plane when stationed on ground. During the dehumidifica- tion on ground, which also is very time-consuming, the cabin must be entirely closed, which implies that the dehumidification is difficult to carry out at the same time with technical work being carried out inside the aeroplane. Another solution which has been applied to dimi- nish the corrosion damage from condensated water, is to treat the surfaces being exposed to corrosion with water- -proof or water-repelling materials. However, up to now these methods have not lead to any success, instead in¬ spections and repairs must be carried out in a continous- ly increasing frequency the older the aeroplanes become. A general known procedure to make water in humid air to condense is to let the humid air pass cold surfaces with a temperature below dew point. This proce¬ dure is applied in e.g. known freon-based airconditioning equipment in housing, vehicles etc. DESCRIPTION OF THE INVENTION The purpose of the present invention is to pre¬ vent water vapour in humid air in an inner space of a monocoque structure, which shows an outer, at least an essentially hermetic shell, and an inner partition which is devised with a space against the hermetic shell, e.g. a pressure cabin of an aeroplane, from condensating into water and possibly freeze into ice on the inside of outer hermetic shell and in the space between, and to remove existing water, if any, from this space. This water might for example have been created earlier through condensa- tion of water vapour in humid air under a situation where no dehumidification has taken place. By preventing con¬ densation to occur mainly on the hermetic shell of the structure, corrosion is avoided on the bearing parts of the structure. The purpose is further to avoid damage to e.g. existing electrical components and to reduce the amount of water to be found inside of the structure, e.g. absorbed by hygroscopic materials, and increasing its weight. Another purpose of the invention is to regulate the humidity of the cabin, as the air that normally is directed into the cabin from the engines of the aeroplane is very dry. The purpose has been fulfilled with a method being characterized of dry air with a relative humidity of less than 50 %, preferably less than 10 % and parti- cularly less than 5 % being directed into the space be¬ tween the outer shell and the inner partition of the mo¬ nocoque structure. The dry air is preferably directed in¬ to the space at the shell with a pressure that is higher than in an inner space, which is enclosed by the inner partition, whereafter this air is made to enter into the inner space after having absorbed humidity from the sur¬ faces surrounding the space. Furthermore the dry air is preferably heated before being directed into the space at the shell to a temperature of at least 10°C, preferably 20°C and particularly 30°C. In a specific application of the invention a pressure cabin constitutes the inner space of the mono¬ coque structure of an aeroplane. The dry air, which is directed into the space between the outer shell and the inner partition of the pressure cabin, can thereby be created by directing heated dry outer air from at least one of the engine compressors of the aeroplane via regu¬ lation devices for pressure and temperature into the space between the outer shell and the inner partition before being directed into the inner space which is en¬ closed by the inner partition. Preferably the dry air is however created by making humid air from the cabin to pass through at least one dehumidifying device, before being directed into the space between the outer shell and the inner partition. In order to maintain the cabin pres¬ sure, dry heated air from the outside from at least one of the engine compressors of the aeroplane is directed into the cabin simultaneously with humid air being redi¬ rected into the cabin from the dehumidifying device. The surplus of humid air from the dehumidifying device is lead away from the cabin through an outlet valve of the aeroplane. It is an advantage to split the pressure cabin of the aeroplane into a passenger compartment and a freight compartment, whose spaces are separated by trans¬ verse partitions in the aeroplane or by the floor of the aeroplane, on which the passenger seats of the aeroplane are placed. Therefore air from the passenger compartment of the cabin can be made- to pass through a first dehumi¬ difying device, whereafter dry air from this one is di¬ rected into the space between the outer shell and the inner partition which is adjacent to the passenger com¬ partment, and air from the freight compartment of the ca¬ bin is made to pass through a second dehumidifying device, whereafter dry air from this one is directed into the space between the outer shell and the inner partition adjacent to the freight compartment. In this way the two dehumidifying systems can accordingly be kept separated so that any smoke fumes from a fire in the freight com¬ partment are avoided to penetrate into the passenger com¬ partment via the space between the outer shell and the inner partition. The space between the outer, hermetic shell and the inner, possibly heat-insulated partition can be of a very varied size. In an aeroplane for ca 150 passengers this space can be the form of a compartment of ca 0,01 - 1,0 m along the length of the aeroplane body, but also constitute a larger space as for a component compartment, which is not provided with an inner partition against the outer shell. The inner partition can in the latter case constitute the floor of the passenger compartment or a transverse partition in the aeroplane. It has turned out to be advantageous to direct the dry air flow to parts of the monocoque structure especially sensitive to corrosion. In an aeroplane it is particularly important to prevent corrosion-causing con¬ densation in bearing parts of the structure such as e.g. attachment points of the wings, landing gear and engine attachments. The present invention is also comprising a de¬ vice in order to carry out the method according to what is described above, which is characterized by means to direct dry air with a pressure which exceeds the pressure in the intermediate space, which means are connected to at least one inlet opening in the outer shell and/or the inner partition of the monocoque structure, whereby pre¬ ferably at least one dehumidifying device is permanently mounted in connection with the monocoque structure, e.g. within an aeroplane, and that inlet openings in the inner partition and/or in the outer shell are devised for di¬ recting dry air into the space between the outer shell and the inner partition. The invention is, which can be concluded from the above mentioned, not limited to aircraft pressure ca¬ bins, but is applicable to all kinds of monocoque struc- tures which show a space between an outer hermetic shell and an inner partition being extended along this shell. Further details concerning the application of the in¬ vention is described in connection with the enclosed drawings. DESCRIPTION OF FIGURES The invention is described below in the form of a model in connection with the enclosed drawings. Figure 1 shows diagrammaticly an aeroplane in a plan view, in which a pressurization system for a pres- sure cabin is shown diagrammaticly. Figure 2 shows an aeroplane diagrammaticly in a longitudinal cross section through its pressure cabin. Figure 3 shows diagrammaticly an aeroplane in a transverse cross section through a pressure cabin which shows an upper passenger compartment and a lower freight compartment. Figure 4 shows diagrammaticly in a transverse cross section an absorption dehumidifier existing on the market. Figure 5 shows a diagram with the water content in the air at different temperatures and relative humi¬ dity. The aeroplane in the example is equipped with a pressure cabin showing an inner space 1 , which is sur- rounded by an outer hermetic shell 2 and an inner, heat- insulated partition 3 extending along the shell 2 with a space 4 against the shell 2. The inner space 1 in the cabin on figure 3 is divided into two compartments, one passenger compartment 1.1 and one freight compartment 1.2, which are kept separated by an essentially hermetic floor 2.1. Through this floor 2.1 the space 4 between the outer shell 2 and the inner partition 3 is also divided into two sections 4.1, 4.2 in connection with the two compartments 1.1, 1.2 of the cabin. Heated external air from the aeroplane ' s engines 5 is directed into the ca¬ bin in a known way, which is however not shown on the figure. The heated external air is tapped at respective engine from a compressor 5.1 in a known way at a tempe¬ rature of ca 200°C and is thereafter made to pass a regulator mixer 5.2, in which the air is cooled in one ore more steps to a temperature of ca 20 C before under pressure being blown into the inner space 1 of the pres¬ sure cabin. The air being blown in is named Ab on figure 1. In the regulator mixer 5.2 the air humidity is also regulated possibly through the mixing in of cabin air, named Ac on figure 1 , or condensation of water depending on the humidity of the external air. A number of dehumidifying devices 6 for cabin air, named Ac on figure 2, are placed along the length of the cabin, with whose help dry air is created and made to flow into and through the space 4 between the outer shell 2 and the inner partition 3- These air flows are named Ad on figure 2. Humid air from the dehumidifying devices 6 is tapped to a first, for several dehumidifying devices 6 common outlet tube 7, and is made to leave the cabin to the external air. The released air flow is named Aw on figure 1 and 2. On figure 3 a first dehumidifying device 6.1 is shown placed in the cabin ' s 1 passenger compartment 1.1 and a second dehumidifying device 6.2 placed in its freight compartment 1.2. The first dehumidifying device 6.1 is connected to the first outlet tube 7 for humid air Aw via a connection tube 7.1, as well as connected to the upper space section 4.1, which is located along the pas¬ senger compartment 1.1, via a first inflow tube 8.1 and a first inlet opening 8.2 for dry air Ad. The connection tube 7.1 shows also a redirection 7.2 of humid air Ar to the cabin ' s passenger compartment 1.1. The dry air Ad is directed into the upper space section 4.1 with a pressure which is higher than the air pressure in the passenger compartment 1.1 and penetrates therefore into the pas¬ senger compartment 1.1 through the not hermetic inner partition 3 after having absorbed humidity from those surfaces in the upper space section 4.1 on which conden¬ sation has been formed. The second dehumidifying device 6.2 is connected to a second common outlet tube 9 for humid air Aw via a connection tube 9.1, as well as to the lower space sec- tion 4.2, which is located along the freight compartment 1.2, via a second inflow tube 8.3 and a second inlet opening 8.4 for dry air Ad. The upper space section 4.1 is kept separated from the lower space section 4.2 as the floor 2.1 also extends through the space 4. Air from the lower space section 4.2 is redirected to the freight com¬ partment 1.2 through the not hermetic inner partition 3 as at the passenger compartment 1.1. The dehumidifying device 6 which is meant for being used in order to create the dry air Ad is known in itself and constitutes a so called absorption dehumidi¬ fier. An example of a known and on the market existing such dehumidifier is shown on figure 4 in order to com¬ plete the description of the invention. The invention is however not restricted to the use of such a dehumidifier. This device contains a rotor 10, which is equipped with a humidity-absorbing material, which rotates slowly inside the device and thereby is passed by two separate air flows. The humid cabin air Ac, which is to be dehumidi¬ fied, is made, with the help of a first fan 11, to pass the rotor 10 in a distinct direction, so that its humidi¬ ty is absorbed by the rotor 10. Pre-heated air, so called regulation air Ah is, with the help of a second fan 12 simultaneously made to flow through the rotor 10 at ano¬ ther section of it, so that the humidity in the rotor 10 is absorbed by the regulation air Ah which thereafter is led away entirely from the construction in form of humid air Aw or is partly redirected as regulation air Ar in the way described above. The regulation air Ah is pre¬ heated with the help of an air pre-heater 13, through which the regulation air Ah is made to pass. In the diagram on figure 5 the situation is shown in the space 4 between the outer shell 2 and the inner partition 3 of a cabin in an aeroplane concerning the Ac temperature, relative humidity RH and dew point of the cabin air as well as without and with the use of de¬ humidifying devices according to the invention. In an aeroplane with passengers and a closed cabin, the cabin air Ac has in this example a temperature of 20 C and 100 % RH when stationed on ground. This point is named A in the diagram. After taxing and start the relative hu¬ midity of the cabin air Ac has been reduced to ca 50 % RH through dry air Ab being directed from the engine com¬ pressors at unchanged temperature. This point is named B in the diagram. After ca 20 minutes flight at an external temperature of ca -20 C, the relative humidity of the cabin air Ac has been reduced to ca 30 % RH. Point C in the diagram. Without the use of dehumidifying devices, the cabin air Ac has in this position a dew point of ca 0°C. The temperature in the space between the outer shell 2 and the inner partition 3 of the cabin is however lower than the dew point, wherefore the humid cabin air Ac which penetrates into the space 4, condensates and creates water which usually also freezes into ice. With the help of dehumidifying devices 6 accor¬ ding to the invention, cabin air Ac is drawn in directly at the inner partition 3, where the air temperature is ca 10°C, point X in the diagram, to the dehumidifying de¬ vices, in which the relative humidity is reduced to 4 % RH and its temperature is increased to 20°C. Point Y in the diagram. The dry air Ad which then is blown into the space 4 between the outer shell 2 and the inner partition 3, then has a dew point of -23 C and no condensation takes place in the space 4. The above mentioned alternative method, at which heated external air from the aeroplane ' s engine compres¬ sors is directed into the space 4 between the outer shell 2 and the inner partition 3 in order to prevent water va- pour from condensation into water and also to remove any existing water from the space 4, is not shown in the diagram on figure 5.

Claims: PATENT CLAIMS 1. A method to prevent water vapour in humid air (Ac) in an inner space (1) in a monocoque structure, which shows an outer, at least essentially hermetic shell (2) and an inner partition (3), which is deviced with an intermediate space (4) against the hermetic shell (2), e.g. a pressure cabin of an aeroplane, from condensating into water and possibly freezing into ice on the inside of the outer, hermetic shell (2) and in the space (4), and also to remove existing water, if any, from this space (4), c h a r a c t e r i z e d in that dry air (Ad, Ab) with a relative humidity of less than 50 %, pre¬ ferably less than 10 % and particularly less than 5 % is directed into the space (4) between the outer shell (2) and the inner partition (3). 2. A method according to claim 1, c h a r a c - t e r i z e d in that dry air (Ad, Ab) being directed into the space (4) by the shell (2) with a pressure which is higher than the air pressure in the structure ' s inner space (1), which is enclosed by the inner partition (3), whereby this air is made to enter the inner space (1) first after having absorbed humidity from the surfaces which surround the space (4). 3. A method according to any of the claims 1-2, c h a r a c t e r i z e d in that the dry air (Ad, Ab) being heated before being directed into the space (4) by the shell (2) to a temperature of at least 10°C, pre¬ ferably 20°C and particularly 30°C. 4. A method according to any of the claims 1-3, c h a r a c t e r i z e d in that the dry air (Ad) being created through humid air (Ac) from the inner space (1) is made to pass at least one dehumidifying device (6) be¬ fore being directed into the space (4) between the outer shell (2) and the inner partition (3). 5. A method according to claim 4, whereby the monocoque structure constitutes a pressure cabin of an aeroplane, c h a r a c t e r i z e d in that dry, heated external air (Ab) from at least one of the aeroplane ' s engine compressors via regulation devices for pressure and temperature is directed into the cabin and the humid air (Ar) is redirected into the inner space (1) from the dehumidifying device (6), and/or that a surplus of humid air (Aw) from the dehumidifying device (6) is led away from the outer shell (2) of the aeroplane. 6. A method according to claim 5, c h a r a c ¬ t e r i z e d in that air (Ac) from the cabin ' s pas- senger compartment is made to pass a first dehumidifying device (6.1), whereafter dry air (Ad) from this one is directed into the space (4.1) between the outer shell (2) and the inner partition (3) which is adjacent to the pas¬ senger compartment (1.1), and air (Ac) from the cabin ' s freight compartment (1.2) is made to pass a second dehu¬ midifying device (6.2), whereafter dry air (Ad) from this one is directed into the space (4.2) between the outer shell (2) and the inner partition (3) which is adjacent to the freight compartment (1.2). 7. A method according to claim 6, c h a r a c ¬ t e r i z e d in that all humid air (Aw) from the second dehumidifying device (6) is completely lead away from the outer shell (2) of the aeroplane. 8. A method according to any of the claims 1-4, whereby the inner space (1) of the monocoque structure is located in a pressure cabin of an aeroplane, c h a - r a c t e r e i z e d in that heated, dry air (Ab) from at least one of the aeroplane ' s engine compressors being directed into the space (4) between the outer shell (2) and the inner partition (3) before being directed into the cabin ' s passenger compartment (1.1) respective freight compartment (1.2). 9. A device for carrying out the method accor¬ ding to the claims 1-8 in order to limit water vapour in humid air (Ac) in an inner space (1) of a monocoque structure, e.g. a pressure cabin of an aeroplane, which shows an outer, at least essentially hermetic shell (2) and an inner partition (3) which is devised with an in¬ termediate space (4) against the hermetic shell (2), from condensating into water and possibly freeze into ice on the inside of the outer, hermetic shell (2) and in the space (4), and also to remove existing water, if any, from this space, c h a r a c t e r i z e d by means (6) for directing dry air (Ad, Ab) with a pressure which ex¬ ceeds the pressure in the intermediate space (4), which means are connected to at least one inlet opening (8.2, 8.4) in the outer shell (2) and/or in the inner partition (3) of the monocoque structure. 10. The device according to claim 9, c h a ¬ r a c t e r i z e d by means for heating the dry air (Ad, Ab) before being directed into the intermediate space (4). 11. The device according to claim 9 or 10, c h a r a c t e r i z e d in that said means constitute at least one dehumidifying device (6) for cabin air (Ac), preferably an absorption dehumidifier is permanently mounted in connection with the monocoque structure. 12. The device according to claim 9 or 10, c h a r a c t e r i z e d in that said means constitute outlet connections in an aeroplane ' s pressure cabin sy¬ stem for dry, heated air (Ab) with a higher pressure than the cabin pressure and with at least one tube which via an inlet opening connects the pressure cabin system with the intermediate space (4). 13- The device according to any of the claims 9-12, c h a r a c t e r i z e d in that at least one inlet opening (8.2, 8.4), being devised in connection with corrosion-sensitive sections of the monocoque struc¬ ture and/or components in the intermediate space (4), so that the dry air (Ad, Ab), after having passed the inlet opening (8.2, 8.4), first passes the mentioned section and/or components before spreading in the intermediate space.

Inventors: AXELSSON TOMAS, NORDSTROEM CHRISTER, WHITE THOMAS, AXELSSON, TOMAS, NORDSTROEM, CHRISTER, WHITE, THOMAS
Applicants: CTT SYSTEMS HB
Application year: 1991
Main classification: B64D13
IPC codes: B64D13, F24F3