Gas separation: processes – Heat exchanging – Cold wall-hot wall thermal diffusion
Reexamination Certificate
2000-03-15
2001-11-06
Simmons, David A. (Department: 1724)
Gas separation: processes
Heat exchanging
Cold wall-hot wall thermal diffusion
C096S221000
Reexamination Certificate
active
06312506
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a device and a method for housing moisture sensitive components of the type mentioned in the preambles of the independent claims.
DESCRIPTION OF RELATED ART
Moisture sensitive components, especially sensitive electronic components, are often protected from the effects of humidity in the air by being placed in air-tight housings. There are however situations when it is not desirable to have a completely sealed casing around such components. For example there are components and devices which have to work in an operating range of from −55° C. to +65° C. If they are housed in air-tight housings then the pressure changes caused by the expansion of the air inside said housings when the temperature changes from the lower temperature to the upper temperature require that the housings have a strong and comparatively expensive construction Furthermore any water vapour contained in the housing when the housing was sealed at, for example, room temperature can condense out at lower temperatures. If the housing is not sealed then external temperature falls lead to a temporary underpressure in the housing until air has flowed into the housing. When the external temperature rises then there is a temporary overpressure in the housing and external air flows out of the housing until this pressure difference between the inside and the outside of the housing no longer exists. The air flow into the housing entrains water vapour with it and this vapour can condense out to form water droplets inside the housing when the temperature falls the next time.
Another way of reducing the influence of water vapour on components in a housing is to leave small vent holes in the housing so that it is no longer airtight and to provide heating means inside the housing. If there are no heating means in the housing then as the temperature drops the relative humidity of the air in the housing rises and eventually condensation forms on the enclosed components. The heating means prevents the temperature in the housing from dropping to such a low level that the dew point is reached and thereby prevents moisture condensing on the enclosed components. This however consumes energy.
SUMMARY
The object of the present invention is to provide a method and a device which can overcome the problem of how to protect sensitive components from water vapour in the air when the components are housed in protective housing which are not airtight.
Normally if a housing is outdoors or is in a normal unsealed room or space which undergoes diurnal changes in temperature then there is a continuous exchange of air from the surroundings to the inside of a ventilated housing through the vent holes due to differences between the ambient temperature and ambient pressure and the temperature and pressure inside the housing. The vent holes are only intended to even out the pressure differences which occur between the inside of the housing and the surroundings and the flow of air though them is small. However the random motion of water molecules in the water vapour in the air allows them to enter or leave the casing independently of any air flow through the vent holes.
When a housing at a certain temperature containing air with a certain relative humidity is cooled, for example when night falls, the air inside the casing may cool to below its dew-point temperature, thereby causing water vapour to condense out on the components inside the housing. When the casing warms up again this moisture can evaporate as the warmer air is able to absorb more water vapour. Therefore there is only a risk of condensation occurring when the temperature of a casing falls. If the water vapour inside a casing can be removed at a rate equal to, or faster than, that required to keep the relative humidity low enough to prevent condensation then condensation in the housing during falling temperatures can be avoided.
There can occur times when the number of molecules of water vapour per unit volume inside the housing is the same as the number of molecules of water vapour per unit volume outside the housing. If the water molecules inside the housing are at the same temperature as the water molecules outside the housing then they will have the same average energy and if the number of molecules of water vapour per unit volume inside the housing is the same as the number of molecules of water vapour per unit volume outside the housing then there will be no nett exchange of molecules between the interior and exterior of the housing.
If there is a difference in the number of molecules of water vapour per unit volume inside the housing and the number of molecules of water vapour per unit volume outside the housing, and both the housing and its surroundings are at the same temperature then there will be a nett flow which will tend to reduce the difference until a steady state is reached in which the average flow in and out of the housing is the same.
If the housing and its surroundings are at different temperatures then when the steady state of water molecule flow is reached then there will be a difference in the number of molecules per unit volume inside the housing and the number of molecules per unit volume outside the housing. This is because, if, when staring from a state where there are equal numbers of water molecules per unit volume both inside and outside the housing, a temperature difference arises between the interior and exterior of the housing then there will be a nett flow of water molecules from the warmer region to the colder region. This is because more of the higher energy water molecules in the warmer region will pass through the vent holes than the lower energy molecules in the colder region. This will lead the warmer region becoming relatively less humid than the colder region. Thus if the ambient temperature outside a housing falls then there should theoretically be a nett flow of molecules out of the housing until the temperature of the housing falls and a new equilibrium state is reached. If in the new equilibrium state the housing is still warmer than the surroundings due to, for example, heat given off by components contained in it, then the number of molecules per unit volume inside the housing will be less than the number of molecules per unit volume outside the housing. In other words it will be less humid inside the housing.
In practice the difference in the number of molecules per unit volume inside the housing and outside the housing caused by cooling of the surroundings is less than the theoretical maximum because of heating of the air in the immediate vicinity of the vent holes by the walls of the casing. This problem is caused by the water molecules on the outside of the housing near to the vent holes have substantially the same average energy as the molecules on the inside of the casing near to the vent holes and the nett flow of water molecules through the vent holes becomes zero.
An object of the present invention is to overcome this problem.
According to the present invention the above stated problem is solved by providing a housing which has means, preferably passive means, for producing a difference in the average velocity of water molecules inside and outside of the housing in the vicinity of vent holes provided in the housing wherein the water molecules inside the housing have a higher average velocity than the molecules outside the housing. In this way there will be a nett transfer of water molecules from inside the casing to outside the casing thereby resulting in a reduction in the humidity inside the casing.
In a preferred embodiment of the invention the passive means for producing a difference in the average velocity of water molecules inside and outside of the housing in the vicinity of vent holes provided in the housing comprises that the wall of the housing in the vicinity of the vent hole or holes is constructed so that it has a lower thermal inertia than the rest of the housing and is thermally insulated from the rest of the housing. In this way when the
Hopkins Robert A.
Nixon & Vanderhye P.C.
Simmons David A.
Telefonaktiebolaget LM Ericsson (publ)
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