GB2369070A

GB2369070A - Regenerating desiccant humidity vent

Info

Publication number: GB2369070A
Application number: GB0028208A
Authority: GB
Inventors: Geoffrey Russel-Smith
Original assignee: RUSSELL SMITH GEOFFREY
Current assignee: RUSSELL SMITH GEOFFREY
Priority date: 2000-11-18
Filing date: 2000-11-18
Publication date: 2002-05-22
Also published as: AU2002223809A1; GB2369070A8; WO2002041094A1; GB0028208D0

Abstract

A moisture regulator for a container, comprises a closed box 2 with vent 4 to the interior of the container and vent 5 to the external atmosphere. Desiccant holder 3 is suspended by spring 9 and comprises metal cage to hold a desiccant bag and a spigot 3a. Slide member 45b, 4a, 5a is arranged to close off either vent 4 or vent 5 in response to actuator 6, the action of which is controlled by thermal actuator 7, pawls 10 and spring bias 11. In use, a low ambient temperature causes the slide member to assume its lower position, thus opening vent 4. The interior atmosphere of the container is now in communication with and dried by the desiccant. Once the desiccant has absorbed sufficient moisture, holder 3 descends against the spring bias 9 and spigot 3a pushes out pawls 10 to couple the slide member to actuator 6. Hereafter an increase in ambient temperature beyond a certain level will cause thermal actuator 7 to expand thus raising the slide member, closing off the interior of the container and opening the moisture regulator to the external atmosphere in order to allow the desiccant to regenerate.

Description

REGENERATING DESICCANT HU1\lIDITY VENT The device relates to a

regenerating desiccant humidity vent, which can s control moisture levels inside a closed container. The device operates according to the temperature and humidity levels inside and outside the container. The background to the device is the dilemma faced by persons packaging

lo goods susceptible to damage by humidity or moisture for shipment. If the packaging container is sealed, changes in temperature causes damaging condensation from humidity contained within the package. This effect is commonly referred to as the "sweating" effect. However, if the container is permanently vented, air with high humidity can enter the container and Is again cause damage. This phenomenon can occur in plastic bags, wooden boxes, metal containers, mobile vehicles and mobile homes and such like.

Natural ventilation is often used to combat the "sweating" effect and is normally achieved through a grill or louver, which normally has no moving to parts. When there are moving parts these are normally only manually adjustable to give a set airflow rate over a long period of time. To use natural ventilation as a means to control humidity requires that a sufficient area of duct is open to get a large enough air exchange. If a traditional air vent is too large it will be detrimental, as there is little control if undesirable Is atmospheres are entering the container.

Materials that have flap like mechanisms that respond to ambient temperature and humidity changes have also been proposed.

Desiccants of both adsorptive and absorptive varieties have been used extensively for controlling moisture levels in a closed container. These suffer from two main disadvantages.

s 1. Once the desiccant has reached its capacity to absorb moisture then no further air-drying is possible.

2. If high temperatures occur inside the container then adsorptive desiccant will expel moisture and raise the humidity inside the container.

There are desiccants which do not release moisture until the temperature is lo higher than might naturally occur but these are normally more expensive.

Thus, an objective of this device is to provide a method of controlling moisture levels in a container with desiccants but having an automatic mechanism, which prevents moisture flowing in the opposite direction to the Is control condition being sought. Advantageously, the desiccant can be dried or desorbed, giving it a new capability to control the air inside the container again. The invention provides a humidity control device for a closed container 20 comprising a hydroscopic desiccant medium contained within a chamber, the chamber including at least two closeable vents controlled in response to ambient temperature such that one of the vents is openable to allow the desiccant medium to adsorb moisture from air entering the chamber via that vent when ambient temperature is below a first predetermined temperature 25 value and the other of the vents is openable to allow the desiccant medium to expel moisture via that vent when ambient temperature exceeds a second predetermined temperature value.

The device is designed to give users a low cost but permanent solution to the problem of controlling condensation in containers. With this in mind the initial cost of the device is very low in comparison to traditional electrically operated dehumidifiers and the desiccant can be automatically regenerated 5 or dried without any human intervention given that certain ambient atmospheric temperature conditions prevail. Studies have shown that the sweating effect is greatest when large daytime/night- time temperature swings take place driven by the heating and cooling caused by the sun. It is in these situations that the device will be most effective in adsorbing and lo expelling humidity.

Accordingly, the device is a regenerating desiccant humidity vent comprising a chamber which is placed on the wall or surface of a closed container. The chamber contains a desiccant or hydroscopic material in a holder, which allows good air circulation. The chamber has two vents, which can be opened and closed. One vents to the outside of the container and one vents to the inside of the container. These vents can be mechanically linked and are designed to move together so that when one vent opens the other shuts and vice versa. This is achieved by having the 20 vent holes in an offset fashion. It is important to note this simultaneous action as it means that the desiccant can only exchange moisture with either the air inside the container or outside the container but not both. In some situations it may be advantageous for the operation of the vents also to include a fully closed position in which both vents are closed before the next :5 one is opened.

The desiccant will adsorb or desorb according to ambient conditions of temperature and relative humidity. The desiccant will seek to move to equilibrium with the surrounding atmosphere. So given time, the desiccant

will adsorb or desorb a set amount of moisture for the relative humidity it is seeking equilibrium with. Given the venting arrangement described this can be controlled to exchange air through one vent either from the inside or outside of the container.

The ambient temperature level is the overriding factor in the equilibrium curve of when the desiccant adsorbs or expels moisture. Firstly it has the greatest impact on the drying capability of the desiccant and secondly as relative humidity is inversely proportional to the temperature at a specific lo glM3 the air will naturally accept more moisture at higher temperature and vice versa.

To understand if the desiccant is adsorbing or desorbing two factors are pertinent. 1. The conditioned Relative Humidity % of the desiccant 2. The Relative Humidity % of the surrounding ambient air.

The term "conditioned relative humidity % of the desiccant" relates to the to relative humidity that a desiccant or hydroscopic medium will attain over a period of time when it is exposed to air. This is because when a desiccant or hydroscopic material is exposed to air, at a constant temperature, it will adsorb or desorb moisture from the air until it attains equilibrium with the air around it. If the ambient conditions of the air change then the desiccant 5 will seek to move to a new equilibrium. The equilibrium position is when the vapour pressure of the moisture in the desiccant and the surrounding air are equal.

The possibility for regenerating the desiccant comes because the conditioned relative humidity % of the desiccant does not change as fast as the change in the relative humidity % of the ambient air conditions. This creates opportunities where the ambient air temperature rises quickly giving s a corresponding fall in relative humidity %. This causes a large difference between the conditioned Relative Humidity % of the desiccant and the Relative Humidity % of the ambient air. Where this condition exists the desiccant will seek equilibrium with air and the desiccant will desorb or adsorb depending on if the conditioned relative humidity % of the desiccant lo is higher or lower than the relative humidity % of the ambient air.

Figures 1 and 2 are graphs showing curves of specific humidity and adsorption/desporption of the desiccant, respectively.

5 Using the information from Figure 1 the following example illustrates this action. Internal container conditions External Container conditions Desiccant and air in equilibrium - Air not in contact with desiccant and contact to Figure 1 Point 1 Figure 1 Point 2 Specific humidity lOg/M3 15g/M3 Temperature 12C 12C Condition RH% desiccant 85% RH% Air 100% as If the ambient temperature rises dramatically and the mechanical device operates the following conditions would apply.

Desiccant in contact with the exterior air Figure 1 Point 3 s

Specific humidity lOg/M3 15g/M3 Temperature 40C 40C Condition RH% desiccant 85% RH% Air 30% s The fact that the desiccant is still at a level of 85% conditioned relative humidity is because it takes a longer time for the desiccant to seek equilibrium with the surrounding air.

Thus in this simple example, even though the exterior air actually contains a lo greater level of specific humidity it can dry the desiccant at 85% conditioned RH% as the RH% of the surrounding air has fallen to 30% due to the higher temperature.

Thus, in this example the desiccant can be dried during the periods of high is exterior air temperature and the according lower relative humidity %. The external relative humidity % at higher temperatures will normally be lower than the conditioned relative humidity % of the desiccant at lower temperatures meaning a drying action will normally occur if the vents allows air from the interior in cold conditions and to the exterior in hot 20 conditions.

Choosing a temperature at which the device changes from being open to the interior of the container to the exterior of the container or vice versa is important. Important factors, for optimal performance that should be taken 25 into account when choosing this temperature are: 1. expected levels of temperatures 2. expected levels of specific humidity g/M3 3. expected swings in maximum and minimum temperatures.

Selecting this temperature is important because the drying action described may not always take place. It should be pointed out that in certain extreme conditions, particularly very hot and humid climates the desiccant could actually transfer moisture into the container when a drying action is 5 required.

For example, again using data from Figure 1, if the specific outside atmospheric humidity level were 20 /M3 or very wet and the ambient air temperature rises to a level of 35 degrees centigrade, then external relative lo humidity would become around 50 RH%. If the interior of the container and the desiccant was very dry, say at a conditioned level of 40% relative humidity, then if the desiccant were then exposed to the exterior atmosphere then the desiccant would actually adsorb moisture from the wetter air. If the temperature then dropped and the desiccant had conditioned to the 50% 5 relative humidity this would then re-release back into the container. This would be obviously undesirable if the intention was to dry the container.

To avoid this and to improve performance, a further embodiment of the device that incorporates a mechanism which takes into account the to condition relative humidity % of the desiccant can be used.

A resilient member or calibrated spring can suspend the desiccant from the chamber roof. As the desiccant loses or gains weight due to levels of adsorption of moisture it will move up or down respectively under the force :5 of gravity. This motion can be used to either position the sliding vents directly or to engage or disengage a power source, which actuates the sliding vents. Thus a preferred embodiment of the device can be advantageously set to operate to regenerate when the desiccant is at a selected level of conditioned relative humidity %, say 85%. If the device

operated at a set temperature of 30C then it would be almost impossible for the desiccant to adsorb significant moisture.

Given that the adsorbing and desorbing process is largely dependent on s temperature, a preferred embodiment of the device operates automatically using the thermal expansion of materials, to give the necessary power and movement to operate the synchronized and connected air vents. Thermally expanding actuators are commercially available and can easily give the motion of around 20mm required to open and shut the vents.

For the device to give an effective performance in terms of the quantity of moisture it can remove from or introduce to the container, the characteristics of the desiccant are paramount.

Firstly the capacity of the desiccant must be as high as possible. This is normally measured by the increase in weight a desiccant can achieve from a low conditioned relative humidity % to a high conditioned relative humidity %. so Secondly, the desiccant must be responsive so that it can quickly change from: - adsorbing moisture where the relative humidity % of the surrounding ambient air is above its conditioned relative humidity % to - desorbing when the relative humidity % of the surrounding ambient 25 air is below its conditioned relative humidity.

And vice versa.

This information is best shown on adsorption / Resorption curves, which plot the adsorption capacity % of the desiccant, normally by weight against

different relative humidity % values. Figure 2 shows charts for the adsorption / Resorption curves of two desiccant materials, with Figure 2. 1 showing good capacity and responsiveness characteristics and Figure 2.2 showing inferior characteristics.

s The term hysteresis is used to describe the condition where the desiccants adsorption curve lies below its desorption curve. For a desiccant to be able to respond quickly as desired then the adsorption curve should lie as close as possible to the desorption curve, as in Figure 2.1. Where the adsorption lo curve lies some way from the desorption curve as shown in Figure 2.2 then conditioning to a change in the surrounding air will result in a smaller increase in weight gain.

As an indication of a good hysteresis curve for a desiccant material the Is vertical difference between the adsorption curve and desorption curve should not be more than 12% of the weight gain possible. By this definition Figure 2.1 shows good characteristics and Figure 2.2 shows poor characteristics. to This is illustrated in that in Figure 2.1 a change in the relative humidity of the ambient air from 30% to 80% results in a net increase in weight of 50% of the desiccants initial weight. Whereas in Figure 2.2 a corresponding increase in relative humidity only results in an 18% increase in weight.

as In the desorptive phases the decrease in weight is also accordingly less.

The key to the device therefore, is that the simultaneous opening and shutting of the vents, powered by the expansion or contraction of the thermal material, is calibrated at a temperature which in a preferred

embodiment of the device for drying containers will allow the desiccant to adsorb in colder conditions from the interior of the container and expel moisture in hotter conditions from the exterior of the container, where the setting of the air temperature, taking into account the inversely proportional s relationship between temperature and relative humidity % and the conditioned relative humidity % of the desiccant, will take advantage of the good hysteresis properties of the desiccant according to the prevailing atmospheric conditions.

to The use of a thermal actuator means that no other power sources such as batteries, kinetics, electrical supply is necessary. This is seen as an advantage over devices, which require battery power for three reasons.

Firstly, batteries can fail in service and secondly they require periodic replacement with new batteries and finally batteries substantially increase IS running costs.

Devices, which use a fixed electrical power such as a mains supply or based on a ship or motor vehicles power source are not considered equivalent.

This is because the device is intended for use in containers used in shipping, 20 storage and transit where portability, negligible human input and handling requirements limit the practicality for making electrical connections.

An embodiment of the device will now be described, by way of example only, with reference to the accompanying drawings in which: 2s Figure 3 shows a representation of a container in cross section with a regenerating desiccant humidity vent device positioned through the wall of the container;

Figure 4 shows a pictorial representation of an embodiment of the device showing the device positioned on the wall of an otherwise sealed container; Figure 5 shows a diagrammatic representation in cross section of an embodiment of the device drying the air inside the container with the s desiccant in a dry condition; Figure 6 shows a diagrammatic representation in cross section of an embodiment of the device with the desiccant heavy with moisture actuating the pawl mechanism; and Figure 7 shows a diagrammatic representation in cross section of an lo embodiment of the device drying the desiccant to the exterior of the container effectively in regeneration mode.

As shown in Figure 3 of the drawings, the chamber device 2 is positioned through the wall of an otherwise closed container 1.

In a preferred embodiment of the device shown in Figure 4 the chamber 2 is made from a solid material such as mild steel or a polymer type plastic. The chamber 2 is securely fixed over an orifice in an otherwise closed container 1. A seal 13 is used to ensure that no air exchange takes place other than so through the designated vent openings in the chamber 2. This seal 13 is held under compression by the fixing screws 14 that secure the chamber 2 to the container 1.

As Figure 5 shows, the chamber 2 supports the weight of the desiccant 3 and Is desiccant holder with a spigot 3A, which is suspended from the chamber roof 2A by a spring or resilient member. This allows the desiccant holder with a spigot to move up and down according to the degree of hydration of the desiccant 3.

The spigot of the desiccant holder with a spigot 3A is located in a hole in a bush in the cross-member 45B which rigidly connects the two sliding vents 4A and 5A. Thus the spigot is free to slide up and down within the bush as the position of the desiccant holder and spigot 3A changes due to differing s levels of hydration. The desiccant holder with a spigot 3A is designed so that the desiccant gains maximum air contact from the air entering the chamber. The desiccant 3 is contained in a bag with holes a little smaller than the smallest size of the desiccant crystals and the walls of the desiccant holder and spigot 3A have a wire mesh type construction with the holes as lo large as possible.

The position of the holes of the sliding vents 4A & 5A are staggered in relation to each other whereas the holes in the vents 4 & S are opposite each other. This has the effect that one vent opens when the other vent closes.

15 Vents 4A and 5A sit flush against the vented walls of the chamber 4 and 5 and are coated in a low friction material to allow a non-sticking action when sliding movement is required.

A pawl arrangement 10 is situation in the bush centrally positioned as part so of and below the cross-member 45B. The pawls are spring loaded so that they will always try to move towards the centre line of the chamber. Thus as the degree of hydration of the desiccant 3 changes it moves the desiccant holder with a spigot 3A up and down changing the position of the pawl arrangement 10.

The thermal expanding cylinder or actuator 7 is fixed to the base of the chamber 2B. When it expands and contracts due to temperature changes two possibilities can occur.

If the pawls 10 are in their Coronal, spring loaded position unaffected by the desiccant holder with a spigot 3A (As shown in Figure S) then the mechanical link will move up and down without affecting the position of the sliding vents 4A and SA. If the pawls 10 have been pushed into a 5 protruding position, overcoming the force of the pawl spring loading, by the desiccant holder with a spigot (As shown in Figure 6) then the mechanical link 6 will move the sliding vents 4A and SA up and down.

The full cycle of this embodiment of the design adsorbing moisture and then lo regenerating itself can be described as follows: In Figure 5, the chamber device 2 is shown with the desiccant 3 adsorbing moisture from inside the container 1 through the air vents 4, which has its sliding vent 4A in the open position. The exterior sliding vent 5A is consequently in the closed position. The pawl arrangement 10 is in a non-

protruding position for the mechanical link 6. Thus, any movement of thermally expanding cylinder 7 in the direction of the adjacent arrow does not result in any movement of the sliding vents 4A and 5A. The temperature at which the thermally expanding cylinder 7 expands or 20 contracts is set according to the moisture adsorption and desorption properties of the desiccant 3. With the vents set in this position the desiccant 3 will gradually adsorb moisture from within the container and as it gets heavier the desiccant 3 and desiccant holder with a spigot 3A will move down towards the pawl arrangement 10.

Figure 6 shows the desiccant 3 has become so heavy with moisture that it has dropped enough for the desiccant holder with a spigot 3A to push past the spring-loaded pawls 10. This has the effect that the pawls will now be

in the path of the mechanical link 6 if the thermally expanding cylinder 7 expands. The desiccant 3 is now ready to be regenerated.

Once the atmospheric conditions are sufficiently warm enough for the set s temperature point of the thermally expanding cylinder 7 this will expand moving the mechanical link 6 towards the protruding pawls 10. The mechanical link 6 continues to move pushing the protruding pawls 10, the connected sliding vents 4A and 5A and the desiccant 3 upwards into a new position. The stroke length of the thermally expanding actuator 6 is set so lo that it corresponds with the movement required to change the vents 4A and SA from open to closed and closed to open respectively.

As Figure 7 shows the chamber is now set to dry the moist desiccant 3 to the exterior of the container 1. This is because the movement of the sliding vent 4A has now closed vent hole 4, and accordingly the sliding vent 5A has uncovered the holes in vent S. Thus the device is now in regeneration mode drying the desiccant to the exterior of the chamber and container.

As the ambient temperatures drop and the possibilities for drying the so desiccant 3 recede then the thermally expanding cylinder 7 contracts. The return springs 11 will move the sliding vents 4A and SA to their original positions so the air change is with the interior of the container 1 again. If the desiccant 3 has dried sufficiently during the regeneration phase the spring 9 will pull the desiccant 3 away from the pawl 10 arrangement. The :5 device will now be back in the same position as that shown in Figure 5.

The above actions described in this embodiment of the device have the effect that at lower temperatures, the device would adsorb high RH% levels inside the container until the desiccant 3 reaches its capacity, and then at

higher temperatures the device is expelling moisture, drying and effectively regenerating the desiccant ready to adsorb further moisture in further cycles.

Thus the device will regenerate the desiccant 3 when two conditions are true. Firstly the desiccant has sufficient weight from adsorbed moisture for 5 the mechanism to engage the pawls 10 and then there is sufficient temperature to actuate the thermally expanding cylinder 7. Both of these parameters can be set according to the expected conditions.

Calibration of the thermally expanding cylinder 7 is possible by adjusting a lo screwed knob 8. By adjusting the screwed knob 8 the volume in which the thermal material inside can expand can be increased or decreased. This lowers or raises respectively the set temperature at which the thermally expanding cylinder 7 operates. A typical set temperature would be 30C.

15 The conditioned relative humidity % of the desiccant, at which the desiccant 3 and desiccant holder with a spigot 3A operates the pawl 10 mechanism, can be calibrated through the selection of the spring 9 and would be engineered to engage the pawls 10 mechanism, when it is at a condition relative humidity % of around 85 RH%.

The ability to set the actuation point of the changeover from adsorbing to desorbing according to the ambient temperature and the conditioned relative humidity % of the desiccant is highly advantageous. Hygroscopic materials and desiccants normally take hours to adsorb significant amounts of 25 moisture and days to show any significant physical changes. Any mechanical devices linked directly to changes in humidity normally have a slow incremental response. This is not very satisfactory when it is desirable to have a vent change position rapidly in response to changes in atmospheric conditions.

This disadvantage is overcome in the embodiment of the device described above through the action of the pawl arrangement. As the pawls can be calibrated to engage the mechanical link at a set level of conditioned relative s humidity % level of the desiccant this gives a clear and controllable signal concerning the regeneration of the desiccant. The time cycle for the thermal actuator is less than 10 minutes. Thus, if it is desirable for the desiccant to regenerate, indicated by the protruding pawls then the actuation time of the opening and shutting of vents is less than 10 minutes. In terms of using lo non-electrical responses to humidity and temperature conditions this is a digital type response.

If it was desirable to have high moisture levels inside the container then in a different embodiment of the device, the chamber could be positioned so that IS moisture was adsorbed from the outside of the container and expelled inside the container. The device would still function in the same manner except that the vent to the exterior of the chamber would be open to the exterior of the container when the ambient conditions are colder. The temperature actuation devise would be in the contracted position and the chamber vents so to the interior of the container closed. In this position the desiccant would adsorb moisture from the exterior of the container. When the temperature rises past a predetermined set temperature and the desiccant is sufficiently hydrated to operate the pawl arrangement, the position of the inner and exterior vents would reverse due to the expansion of the temperature as actuation device allowing additional water vapour to desorb into the container. This would be useful in trying to keep a cargo of a product such as tobacco, for example, moist in hot conditions.

Given also that the device may be used in all weather conditions, materials are selected to withstand extremes of temperature and humidity. For a mild steel construction a coating of a dry film lubricant such as MOS2, which is widely available, would be advantageous on the sliding surfaces. It should be noted that a flexible breathable waterproof PTFE membrane 12 may cover the exterior vent 5 to stop any ingress of rain or water form the exterior of the container. This material is at its most breathable when temperatures are highest and moisture will move from the higher temperature area, the chamber 2, to the exterior of the chamber.

Claims

1. A humidity control device for a closed container comprising a hydroscopic desiccant medium contained within a chamber, the chamber 5 including at least two closeable vents controlled in response to ambient temperature such that one of the vents is openable to allow the desiccant medium to adsorb moisture from air entering the chamber via that vent when ambient temperature is below a first predetermined temperature value and the other of the vents is openable to allow the desiccant medium to lo expel moisture via that vent when ambient temperature exceeds a second predetermined temperature value.

2. A humidity control device as claimed in Claim l wherein the first

and second predetermined temperature values are the same.

3. A humidity control device as claimed in any preceding claim wherein opening and closing of the vents is controlled such that one of the vents is open when the other of the vents is closed.

so 4. A humidity control device as claimed in any preceding claim wherein opening and closing of the vents is affectable by expansion and contraction of an actuating body in response to changes in the ambient temperature.

5. A humidity control device as claimed in any preceding claim wherein :5 opening and closing of the vents is further controlled by the degree of hydration of the desiccant medium.

6. A humidity control device as claimed in Claim 5 wherein a resilient member supports the desiccant medium such that the degree of hydration of

the desiccant medium determines the relative position of the desiccant medium within the chamber.

7. A humidity control device as claimed in Claim 4 and Claim 5 or 5 Claim 6 wherein the actuating body only effects opening and closing of the respective vents, upon expansion in response to an increase in the ambient temperature above the second predetermined temperature value, when the degree of hydration of the desiccant medium is greater than a predetermined hydration value.

8. A humidity control device as claimed in any preceding claim wherein the actuating body is calibrated according to ambient temperature in order to synchronize opening and closing of the vents.

15 9. A humidity control device as claimed in any of Claims 4 to 8 wherein the actuating body is a thermally expandable cylinder.

10. A humidity control device as claimed in any of Claims 6 to 9 wherein the resilient member is a spring.

11. A humidity control device as claimed in Claims 7, 9 and 10, wherein the desiccant medium includes a spigot member that engages and actuates spring-biased pawls when the degree of hydration of the desiccant medium exceeds the predetermined hydration value such that an actuating portion of 25 the thermally expandable cylinder engages the actuated pawls when the thermally expandable cylinder expands in response to an increase in the ambient temperature above the second predetermined temperature value in order to open and close the respective vents.

12. A humidity control device as claimed in any preceding claim wherein the vent that is openable in order to expel moisture from the desiccant medium includes a flexible membrane that prevents ingress of water into the chamber while allowing moisture to leave the chamber.

13. A humidity control device as claimed in Claim 12 wherein the flexible member is a breathable PTFE membrane.

14. A humidity control device as claimed in any preceding claim wherein lo a hysteresis curve for adsorption and desorption of the desiccant medium is such that the difference between the adsorption and Resorption curves is not greater than 15% of the possible mass increase at any value of relative humidity. Is IS. A humidity control device as claimed in any preceding claim wherein operation of the device is controlled by ambient temperature and humidity changes alone.

16. A humidity control device for a closed container comprising a 20 hydroscopic desiccant medium contained within a chamber, the chamber including at least two closeable vents controlled in response to the degree of hydration of the desiccant medium such that one of the vents is openable to allow the desiccant medium to adsorb moisture from air entering the chamber via that vent when the degree of hydration is below a first :5 predetermined hydration value and the other of the vents is openable to allow the desiccant medium to expel moisture via that vent when the degree of hydration exceeds a second predetermined hydration value.

17. A humidity control device generally as herein described with reference to and/or as illustrated in Figures 3 to 6 of the accompanying drawings. s 18. A closeable container including a humidity control device in accordance with any preceding claim.

19. A closeable container as claimed in Claim 18 wherein one of the vents is openable to the interior of the container and the other of the vents is lo openable to the exterior of the container.

20. A closeable container as claimed in Claim 18 or Claim 19 wherein the container includes a plurality of interlinked walls and the humidity control device is located within one of the walls.