Dryer cartridge

Abstract

A dryer cartridge 220 for installation in an air drying system (200, Fig 8) is provided. The cartridge comprises an internal chamber 222 having chamber walls 224 and containing a drying material for r

Description

Field of the Invention The present invention relates to a dryer cartridge for installation in an air drying system and an air drying system comprising said cartridge. Background Compressed air is used in many applications such as for medical breathing air, dentistry, vacuum packaging, automotives, pneumatic conveyancing, food and beverage production, manufacturing, etc. It is important to ensure that a compressed air supply system delivers compressed air of a quality which is suitable for the end application, including ensuring that the compressed air supply is clean, dry and contaminant free. Any form of moisture or contamination in a compressed air line has the potential to cause costly downtime, machine damage, and product spoilage. Air drying and filtration systems are typically used to remove moisture and impurities from compressed air. Such systems typically comprise desiccant beds for removing moisture from air. However, the desiccant beds must be regularly regenerated whereby moisture is removed from the desiccant bed in order to extend the life of the air drying system. Typically, regeneration of the desiccant bed is performed by passing dry air through the desiccant bed. Some systems are configured to use bleed air from a first desiccant cartridge to regenerate the desiccant bed of a second cartridge and vice-versa. The flow rate of the bleed air is typically restricted to a predetermined flow rate which is tailored to the target application of the air drying system and to the flow rate of the incoming air being dried. However, adjusting the bleed flow rate of such systems (and replacing parts when necessary) is typically a complex and timeconsuming task requiring an expert to dismantle the system and replace components. Additionally, inefficiencies in the airflow of existing air drying systems can result in increased energy consumption, reduced reliability, and increased running costs of such systems. The present invention has been devised in light of the above considerations. Summary of the Invention Broadly, the present invention relates to a removable dryer cartridge for an air drying system, the cartridge comprising an integral flow restriction device for selectively restricting the flow rate of incoming bleed air for regenerating the drying material in the cartridge. The flow restriction device is configured to allow an outgoing main airflow to exit the cartridge after drying at a first flow rate, whilst limiting the flow rate of incoming bleed air. By providing a respective flow restriction device inside the cartridge itself, the flow restriction device is more easily accessible for adjustment and results in improved airflow characteristics. Accordingly, in a first aspect of the present invention there is provided a dryer cartridge for installation in an air drying system, the cartridge comprising: an internal chamber having chamber walls and containing a drying material for removing moisture from air, and first and second openings in the chamber walls for permitting air to pass into and out of the internal chamber. The cartridge is operable in: a drying mode: in which a main airflow is provided through the first opening for drying in the internal chamber and exhaustion through the second opening, and a regeneration mode: in which a secondary airflow is provided through the second opening for removal of moisture from the drying material in the internal chamber. The cartridge further comprises a flow control device configured to selectively control a flow rate through the second opening. The flow control device may be configured to permit the secondary airflow to pass from the second opening directly into the internal chamber along a linear airflow path. Advantageously, by providing a respective flow control device in the cartridge, the flow control device is more easily accessible for adjusting the flow rate. Consequently, the dryer system is more adaptable. In contrast to existing drying systems, the cartridge of the present invention comprises a respective flow control device as opposed to existing systems in which a cartridges typically share a flow control device. The present inventors have found that providing the flow control device in the cartridge results in less undesirable pressure loss. By locating the flow control device in the cartridge, in a same air flow path as the main airflow (i.e., in the second opening of the internal chamber), and ensuring that the secondary airflow follows a linear airflow path, the secondary airflow is subject to a less arduous path than if the flow control device were located in a separate air flow path outside of the cartridge. Thus, the secondary airflow may experience less pressure loss, resulting in improved energy consumption of the air drying system. The first and second openings may be lower and upper openings of the dryer cartridge and may also be referred to as inlets and outlets. For example, the first opening may be referred to as a main airflow inlet for permitting the main airflow to enter the cartridge for moisture removal and the second opening may be referred to as a main airflow outlet for permitting the main airflow to exit the cartridge after moisture removal. However, when the dryer cartridge is operating in the regeneration mode the second opening may be referred to as an inlet for the secondary airflow. In this mode, the second opening may be an outlet for the secondary airflow. The term opening is used herein to refer to one or more ports or apertures enabling a fluid (e.g., a gas such as compressed air) to pass from one side of a chamber wall to another. Each opening, may for example, comprise a plurality of individual apertures or slots for providing such fluid passage. For example, the first and / or second openings may be covered by a grill comprising a plurality of apertures. The first opening may be located in a base portion of the cartridge and be covered by a grill and / or a filter plate configured to permit airflow therethrough whilst containing the drying material in the internal chamber. The cartridge may be cylindrical, the internal chamber being an interior portion of the cartridge for containing the drying material. The first and second openings may be therefore provided in respective planar end faces of the cylindrical cartridge. The drying material may comprise or be a desiccant or another sorbent material configured to remove moisture from air. Accordingly, the drying material may also be referred to herein as a desiccant bed. The main airflow provided through the first opening, when the cartridge is in the drying mode, may comprise moist air for drying. Conversely, the secondary airflow provided through the second opening, when the cartridge is in the regeneration mode, may comprise dry air. The drying mode may refer to an operating state of the cartridge wherein a (moist) airflow is being provided through the first opening, through the drying material, and expelled through the second opening. The regeneration mode may refer to a restoring state of the cartridge wherein a (dry) airflow is being provided through the second opening, through the drying material, for expulsion through the first opening. The cartridge may comprise an exhaust port for exhausting the secondary airflow (after passage through the drying material). The first opening may be the exhaust port of the secondary airflow such that the first opening is for permitting the main airflow to enter the internal chamber and the secondary airflow to exit the internal chamber. Alternatively, the cartridge may comprise a separate exhaust port for expelling the secondary airflow. The secondary airflow may be a bleed airflow which has been diverted from a main airflow exiting a second cartridge. Therefore, the secondary airflow may also be referred to herein as bleed air or purge air. The linear airflow path may be considered to be a direct airflow path comprising no direction changes to the airflow which deviate from a direction parallel to an axis connecting the flow control device and the internal chamber by more than a threshold amount. Accordingly, the secondary airflow passing from the second opening to the internal chamber may follow and airflow path which is substantially parallel to an axis connecting the first and second openings. That is, when the first and second openings are locating in upper and lower faces of the dryer cartridge as discussed below, the secondary airflow path may be absent of any radial components. The linear airflow path between the second opening and the internal chamber may comprise a total airflow path deviation of no more than 20 degrees, no more than 30 degrees, no more than 50 degrees, no more than 90 degrees, no more than 100 degrees, no more than 120 degrees, or no more than 180 degrees from a direction parallel to an axis connecting the first and second openings. In this context, the airflow path deviation may be understood to mean a total magnitude of direction changes undertaken by air following the airflow path. Restricting the total amount of airflow path deviation in this way, for example, by reducing number of direction changes to airflow in the cartridge and providing a straighter airflow path, reduces a pressure loss of the secondary airflow in the cartridge. That is, an orifice of the flow control device may be fluidly connected directly to the internal chamber for allowing the secondary airflow to pass therethrough. The direct fluid connection between the flow control device and the internal chamber may be absent of any airflow diverter which may cause a direction change to the secondary airflow, e.g., of more than a threshold amount (e.g., 90 degrees). The dryer cartridge may therefore exhibit improved air pressure characteristics than dryer cartridges with more arduous airflow paths. In some examples, the secondary airflow path between the second opening and the internal chamber, or between the first and second openings may contain a total airflow direction change (i.e., airflow path deviation) of no more than 20 degrees, no more than 30 degrees, no more than 50 degrees, no more than 90 degrees, no more than 100 degrees, no more than 120 degrees, or no more than 180 degrees. The flow rate through the second opening, as controlled by the flow control device, may be dependent on the operating mode of the cartridge. Accordingly, the flow control device may be operable in a first configuration, when the cartridge is in the drying mode, in which the main airflow is permitted to exit the internal chamber at a main airflow rate. Additionally, the flow control device may be operable in a second configuration, when the cartridge is in the regeneration mode, in which the secondary airflow entering the internal chamber is restricted to a predetermined bleed airflow rate. The flow rate through the second opening may be dependent on a direction of airflow through the second opening. The flow control device may be configured to provide a larger restriction to the secondary airflow entering the internal chamber than to the main airflow exiting the internal chamber. For example, the flow control device may be configured to provide no restriction to the flow rate of the main airflow (in addition any restriction which is provided by the size of the second opening or the chamber walls etc). For example, the first configuration may be an open configuration so as to permit unrestricted (or substantially unrestricted) flow of the main airflow. The second configuration may be a closed configuration wherein the secondary airflow is permitted to enter the internal chamber only by flowing through the flow control device. Accordingly, the bleed airflow rate may be less than the main airflow rate. The flow control device may comprise a restriction orifice for permitting the secondary airflow to pass through the flow control device. The orifice size (i.e., a bore size, the size of the orifice diameter, etc) may be configured to restrict the secondary airflow to the bleed airflow rate. Accordingly, the orifice size may be smaller than a size of the second opening. The flow control device may be movable between: a first position and a second position, the first position corresponding to the first configuration and the second position corresponding to the second configuration. In the first position, the main airflow may be permitted to exit the internal chamber by flowing around the flow control device and, optionally, through the restriction orifice. When the air flow control device is in the second configuration, the secondary airflow may be permitted to pass into the internal chamber through only the restriction orifice. The flow control device may be movable between the first and second positions by a pressure differential provided across the flow control device. Accordingly, the flow control device may be configured to move automatically into the first or second configuration depending on if a main airflow for drying or a secondary airflow for regeneration is being provided. In this way the configuration of the flow control device may be automatically adjustable according to the operating mode of the dryer cartridge. For example, when an internal pressure of the internal chamber is higher than an external pressure - adjacent to the flow control device -, the flow control device may be configured to move into the first position. When the internal pressure of the internal chamber is lower than the external pressure, the flow control device may be configured to move into the second position. Thus, when the cartridge receives no main airflow through the first opening and a secondary airflow is provided to the second opening, the flow control device may be configured to move into the second position under the fluid pressure of the secondary airflow being provided to the second opening. Conversely, when a main airflow for drying is provided through the first opening to the internal chamber, the resultant pressure inside the internal chamber may act to push the flow control device into the first position thereby permitting the main airflow to exit the internal chamber and preventing air flow back into the internal chamber. The flow control device may be located further away from the drying material (and the first opening) in the internal chamber when it is in the first position than when it is in the second position. When the flow control device is in the first position, a gap may be provided between the flow control device and a perimeter of the second opening thereby permitting the main airflow to exit the internal chamber via the gap and, optionally, through (the restriction orifice of) the flow control device. When the flow control device is in the second position, a seal may be formed between the flow control device and the perimeter of the second opening, such that the secondary airflow is permitted to pass into the internal chamber through the flow control device. Specifically, the secondary airflow may be permitted to pass into the internal chamber through the restriction orifice of the flow control device. In some examples, the second opening may be located in a planar face of the internal chamber. The internal chamber (and the cartridge) may be cylindrical. The second opening may therefore be located in a planar end face of the cylinder. The flow control device may be arranged in the second opening and configured to permit the secondary airflow to pass into the internal chamber in a direction which is perpendicular to the planar face. In this way, the flow control device may be positioned in-line with a direction of the main airflow. Accordingly, the secondary airflow may follow the same flow direction (in reverse) as the main airflow, i.e., from the second opening directly into the inner chamber without a direction change or significant flow deviation. By arranging the flow control device and secondary airflow path in this way, the secondary airflow has been found to experience less pressure loss. The first and second openings may be located in opposing planar faces of the internal chamber with the drying material therebetween. Therefore, when the cartridge is in the drying mode the main airflow may flow from the first opening to the second opening through the drying material in a main airflow direction defined by an axis extending between the openings. The flow control device may be configured to permit the secondary airflow to pass into the internal chamber from the second opening in a direction which is towards the second opening. The secondary airflow may therefore follow a flow direction which is parallel to the axis extending between the first and second openings. As mentioned above, the flow control device may be arranged so that the flow direction of the secondary airflow follows a linear airflow path. Accordingly, the flow direction of the secondary airflow may deviate from the direction parallel to the to the axis extending between the first and second openings by no more than a threshold amount (e.g., 20, 30, 40, 50, 60, 90 degrees). The first opening may be located in a base of the cartridge and the second opening may be located in an upper portion of the cartridge such that, in use, the main airflow moves (vertically) upwards between the first and second openings, and the flow control device is configured to permit the secondary airflow to descend (vertically) downwards into the internal chamber through the second opening. Accordingly, the first and second openings may also be referred to as lower and upper openings, respectively. The flow control device may be adjustable to modify a size of the restriction orifice, so as to modify the flowrate of the secondary airflow. The flow control device may be manually adjustable so that the size of the restriction orifice may be modified by hand. Specifically, the flow control device may be reversibly adjustable. In this way, the flow rate of the secondary airflow may be modified easily without permanently changing the structure of the flow control device. The flow control device may comprise a plurality of restriction orifices. The plurality of orifices may comprise a plurality of different orifice sizes. Each of the plurality of orifices may have a respective, distinct orifice size such that each orifice is a different size to the others. The flow control device may further comprise a cover element for selectively covering one or more of the plurality of orifices and exposing one or more of the orifices for permitting air to pass therethrough. The cover element may be a rotatable cover element for concealing one or more of the plurality of orifices. The rotatable cover element may comprise a selection aperture for selectively exposing one or more of the restriction orifices. In this way, the flow rate of the secondary airflow may be configured by adjustment of a position of the cover element relative to the plurality of orifices. The selection aperture may therefore expose one of the restriction apertures for permitting airflow to pass through the flow control device at a desired flow rate. In some examples, the cover element may comprise a void, a segment, or a cut-out, etc for selectively exposing one or more of the restriction orifices. The cartridge may further comprise an exhaust port for exhausting the secondary airflow from the cartridge. Alternatively, the first opening may be for exhausting the secondary airflow. In this example, the first opening may be configured to connect to an exhaust valve for controlling exhaust of the secondary airflow through the first opening. The flow control device may be referred to as a bleed valve. In a second aspect of the present invention, there is provided an air drying system comprising a dryer cartridge according to the first aspect. The air drying system may comprise an air inlet port for receiving air to be dried, an air outlet port for expelling dried air, wherein in the air inlet port is in fluid communication with the first opening for providing the main airflow for drying and the air outlet port is in fluid communication with the second opening for exhausting the main airflow after drying. The secondary air flow for regenerating the dryer cartridge when the cartridge is operating in regeneration mode may be provided by a dry air source. For example, the dry air source may be a second dryer cartridge or a second air drying system. The dryer system may comprise a control unit and a valve system under the control of the control unit for selectively permitting airflow from the inlet, and / or the dry air source to flow towards the dryer cartridge in order to operate the dryer cartridge in the dryer or the regeneration mode. The dryer cartridge may be a first dryer cartridge and the air drying system may further comprise a second dryer cartridge. The drying system may consist of only these two dryer cartridges. The second dryer cartridge may be configured to provide air to the second opening of the first cartridge for regenerating the drying material of the first cartridge when the first cartridge is in regeneration mode. The secondary airflow may be reversible so that the first cartridge may provide air for regeneration of the second cartridge. The second dryer cartridge may be a dryer cartridge according to the first aspect of the present invention. Thus, the drying system may be configured to operate one of the first or second cartridges in the regeneration mode by providing a portion of the main airflow which is exiting one of the first or second cartridges to the other of the first or second cartridges as a secondary airflow. Accordingly, in this example, the air inlet port may be in fluid communication with the first opening of the second cartridge, and the air outlet port may be in fluid communication with the second opening of the second cartridge. The second openings of the first and second cartridges may be in fluid communication (with each other) such that a portion of the main airflow exiting the second cartridge may be divertible, as a secondary airflow, to the first cartridge. The second openings being in fluid communication may thereby enable bleed air from the main airflow which is exiting one of the first and second cartridges after drying to pass into the other of the first and second cartridges, via the flow control device, as a secondary airflow. The provision of bleed air to each of the cartridges from the other may be reversible so that each of the first and second cartridges may be operated alternately in the regeneration mode. The dryer system may comprise a control valve arranged to divert incoming air from the system inlet to the first openings of the first and / or second cartridges. The control valve may be switchable (e.g., the control valve may be a shuttle valve) so as to selectively operate one (or both) of the first and second cartridges in drying mode to dry the incoming air. The control valve may be located in a T-junction between the system inlet and the first openings of the first and second cartridges. Accordingly, the control valve may be movable between a first position in which the incoming air is provided to only the first cartridge, and an optional middle position in which the incoming air is provided to both the first and second cartridges, and a third position (or second position if the middle position is not present) in which the incoming air is provided to only the second cartridge. As mentioned above the air drying system may further comprising a control unit for controlling the control valve. Accordingly, when the incoming air for drying is diverted to one of the cartridges the respective flow control device of that cartridge may be configured to move, under the internal pressure of air in the internal chamber into a first configuration in which the main airflow is permitted to exit the internal chamber via the second opening at the main airflow rate (i.e., without substantial restriction by the flow control device). Conversely, when the control valve is preventing the incoming air from entering one of the cartridges, a resulting lack of airflow pressure in the internal chamber may act to move the flow control device into a second configuration for restricting the flow rate of secondary airflow (the secondary airflow being bleed air exiting the other cartridge). The dryer system may further comprise an exhaust port for exhausting secondary air from the or each of the dryer cartridges. The dryer system may comprise an exhaust port for each of the first and second cartridges, for exhausting the secondary airflow from the respective cartridge when it is operating in regeneration mode. Each of the exhaust ports may be in fluid communication with the first opening of the respective cartridge such that the secondary airflow is exhaustible via the respective first opening. Accordingly, the secondary airflow may exit the internal chamber of the respective cartridge via the second opening. The control valve mentioned above for directing the incoming air may be configured to enable the secondary airflow to exit the second opening for exhaustion by the exhaust port. Alternatively, the cartridge may comprise a separate exhaust outlet for expelling the secondary airflow, the separate exhaust outlet being in fluid communication with the system exhaust port. Each exhaust port may comprise an exhaust valve for opening (and closing) the respective exhaust port for enabling airflow to flow therethrough. Thus, each cartridge may be configured to operate in the regeneration mode when the respective exhaust valve is open therefore enabling the secondary air flow to be expelled from the cartridge, and lowering an internal pressure of the internal chamber. The lowered internal pressure may act to move the air control device into the second position thereby restricting the flow rate of the incoming secondary air. Opening the exhaust valve may act to lower the internal pressure of the internal chamber thereby causing the flow control device to move into the second position and the secondary airflow to be diverted into the internal chamber via the flow control device and be exhausted from the system via the respective exhaust port. The exhaust valves may be under the control of the control unit (e.g., using solenoids). The air drying system may further comprise one or more silencers for deadening audible sound of secondary airflow being expelled from the system. Each of the exhaust ports may comprise a respective silencer. The system may comprise a user interface communicatively coupled to the control unit. The control unit may be configured to (periodically) automatically switch the operation of each dryer cartridge between the drying and regeneration modes after a predetermined period of time. For example, the control valve and, optionally, the exhaust valves, may be solenoid valves. Therefore, the control unit may be configured to operate each of the dryer cartridges in the drying mode or in the regeneration mode by providing control signals to the solenoids. In a third aspect of the present invention there is provided a method of operating the dryer cartridge of the first aspect. The method comprises operating the dryer cartridge in the drying mode by providing a main airflow to the first opening, and operating the dryer cartridge in the regeneration mode by (ceasing provision of the main airflow and) providing a secondary airflow to the second opening such that the flow control device in the dryer cartridge selectively controls the flow rate through the second opening. The method may further comprise operating the dryer cartridge in the dryer mode by ceasing provision of the secondary airflow and restarting provision of the main airflow to the first opTening. The method of the third aspect may be a computer-implemented method. For example, the method may be performed by the control unit of an air drying system, wherein the control unit is configured to control a valve system, the valve system being configured to selectively provide the main airflow and the secondary airflow to the dryer cartridge. In a fourth aspect of the present invention there is provided a method of operating an air drying system according to the second aspect. The method comprising the steps of the third aspect. When the air drying system comprises first and second dryer cartridges according to the first aspect, the method may comprises operating each of the first and second dryer cartridges alternatingly in the drying mode and the regenerating mode by performing the method of the third aspect on each of the cartridges, wherein the secondary air flow for each of the cartridges is bleed air exiting the other of the cartridges. For example, operating the first cartridge in the drying mode (and the second cartridge in the regeneration mode) may comprise sending a signal to the control valve, the signal being configured to cause the control valve to move into a first position whereby incoming air for drying is directed through the first opening of the first cartridge. This step my further comprise sending a signal to an exhaust valve corresponding to the second cartridge, the signal being configured to cause the exhaust valve to open thus permitting the expulsion of secondary airflow from the second cartridge and therefore operating the second cartridge in regeneration mode. Likewise, operating the second cartridge in the drying mode (and the first cartridge in the regeneration mode) may comprise sending a signal to the control valve, the signal being configured to cause the control valve to move into a second position whereby incoming air for drying is directed through the second opening of the second cartridge. This step my further comprise sending a signal to an exhaust valve corresponding to the first cartridge, the signal being configured to cause the exhaust valve to open thus permitting the expulsion of secondary airflow from the first cartridge and therefore operating the first cartridge in regeneration mode. The method of the fourth aspect may be a computer-implemented method. For example, the method may be performed by the control unit of the air drying system, wherein the control unit is configured to control a valve system, the valve system being configured to selectively operate the first and second cartridges in the drying and regeneration modes as discussed above. Additional aspects of the invention may provide a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of a computer-implemented method of the third or fourth aspects of the present invention. Further aspects of the invention may provide a computer-readable storage medium, having stored thereon the computer program of the previous aspects of the invention. The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. Summary of the Figures Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which: Fig. 1 shows a functional diagram of an air drying system according to the prior art; Fig. 2 shows a section view an air drying system according to the prior art; Fig. 3 shows a perspective air drying system according to aspects the present invention; Fig. 4 shows a perspective view of the air drying system of Fig. 3 in which the outer housing is removed; Fig. 5A shows a perspective view of a dryer cartridge according to aspects of the present invention; Fig. 5B shows an exploded view of the dryer cartridge of Fig. 5A; Fig. 5C shows an exploded view of an upper portion of dryer cartridge of Fig. 5A; Figs. 6A-B shows perspective views of the flow control device; Fig. 7 shows a section view of an upper portion of the dryer cartridge of Fig. 5A installed in the air drying system of Fig. 3; Fig. 8 shows a section view of the air drying system of Fig. 3; Fig. 9 shows a functional diagram of an air drying system according to aspects of the present invention; and Figs. 10A-B show graphs of comparative results of differential pressures of air drying systems. Detailed Description of the Invention Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. Fig. 1 shows a functional diagram of an air drying system 1 for drying air according to the prior art. The air drying system 1 comprises a first dryer cartridge 14a and a second dryer cartridge 14b, each comprising an internal chamber 20 housing a drying material for removing moisture from incoming air. Typically, the drying material is a desiccant and may be referred to as a desiccant bed. The air drying system 1 comprises a main airflow inlet 10 for receiving incoming air and a main airflow outlet 12 for expelling air after drying in one of the dryer cartridges 14a, 14b. Each cartridge 14a, 14b comprises a lower opening 22a, 22b for permitting the main airflow from the system inlet 10 to pass into the internal chamber 20 of the respective cartridge 14a, 14b, and an upper opening 23a, 23b for permitting the main airflow to exit the internal chamber 22, after drying by the desiccant bed, for expulsion by the system outlet 12. Thus the main airflow flows into the base of the cartridge 14a, 14b through the lower opening 22a, 22b (which may therefore be referred to as a main airflow inlet), upwards through the desiccant bed toward the upper portion of the cartridge 14a, 14b, and through the upper opening 23a, 23b (which may therefore be referred to as a main airflow outlet). However, airflow can also be provided to the first 14a and second 14b cartridges in the opposite direction in order to restore the desiccant bed. Airflow provided in this direction, which may be referred to as a secondary airflow, a purge airflow, or a bleed airflow, is typically dry air which is provided through the upper opening 23a, 23b and exhausted through the lower opening 22a, 22b in order to remove moisture from the desiccant in a process known as regeneration of the desiccant bed. Thus, the first 14a and second 14b cartridges are each operable in: a drying mode in which the main airflow is provided through the lower opening 22a, 22b for drying in the internal chamber 20 and exhaustion through the upper opening 23a, 23b, and a regeneration mode in which the secondary airflow is provided through the upper 23a, 23b opening for removal of moisture from the drying material in the internal chamber 20. In Fig. 1 the first cartridge 14a (left) is shown as operating in the regeneration mode wherein the arrows indicate a bleed airflow descending through the desiccant bed of the first cartridge 23a, 23b from the upper 23a to the lower opening 22a. In contrast, the second cartridge 14b (right) is operating the drying mode wherein the arrows indicate main airflow ascending through the desiccant bed of the first cartridge from the lower 22b to the upper opening 23b. The secondary airflow provided to the first cartridge 14a is bleed air diverted from the main airflow which is exiting the second cartridge 14b after drying. The bleed air is guided towards the upper opening 23a of the first cartridge 14a through a bleed flow path 30 which provides fluid communication between the upper openings 23a, 23b of the first 14a and second 14b cartridges. The flow rate of the bleed air is controlled by a restriction orifice 32 located in the bleed airflow path 30 between the first 14a and second 14b cartridges. The restriction orifice 32 limits a flow rate of the bleed airflow being syphoned off the exiting main airflow to a predetermined bleed airflow rate. Typically, the restriction orifice 32 is a predrilled hole of a predetermined size provided in a brass plug located between the two cartridges 14a, 14b. Thus, to change the orifice size of the restriction orifice 32, and hence the bleed airflow rate, the brass plug must be removed and replaced or re-drilled. The airflow system 1 comprises a first control valve 28 for selectively diverting the incoming air from the system inlet 10 to the lower opening 22a, 22b of the first cartridge 14a or of the second cartridge 14b. The first control valve 28 is a shuttle valve movable between a first position in which the incoming air is permitted to flow towards the first cartridge 14a and second position in which the incoming air is permitted to flow towards the second cartridge 14b. The airflow system 1 comprises a second control valve 34 (which is also a shuttle valve) for selectively enabling the main airflow which is exiting the first 14a or second 14b cartridge to flow towards the system outlet 12 after drying. The bleed airflow path 30 described above is circumnavigates the second control valve 34 thereby enabling the bleed air from the main airflow to flow around the second control valve 34 towards the upper opening of the cartridge which is not in the drying mode. The first 28 and second 34 control valves are controlled by a control unit 16 which is configured to operate the first 14a and second 14b cartridges alternatively in the drying or regeneration mode by switching the configurations of the first 28 and second 34 control valves. Finally, the air drying system 1 also comprises first 25a and second 25b exhaust lines in fluid communication with the lower opening 22a, 22b of the first 14a and second 14b cartridge, respectively. The exhaust lines 25a, 25b are for expelling the secondary airflow after regeneration of the desiccant bed. The exhaust lines 25a, 25b comprise respective exhaust valves 24a, 24b for opening and closing the exhaust lines depending on if the respective cartridge is operating in the regeneration mode or not. When one of the exhaust valves 24a is open (as shown in Fig. 1), a secondary airflow is drawn from the main airflow exiting one of the cartridges 14b, through the bleed flow path 30 and the desiccant bed of the cartridge 14a which is connected to the open exhaust valve 24a, before being expelled from the system 1 through the exhaust line 25a. The exhaust lines 25a, 25b are each connected to a silencer 26 for muting the sound of the secondary airflow being expelled. Fig. 2 shows a section view, from above, of an air drying system 100 according to the prior art which is configured to operate as described above for Fig. 1. The system 100 comprises first 114a and second 114b cartridges, which are configured to operate alternatively in a drying mode and in a regeneration mode, as described above for Fig. 1, wherein dry bleed air exiting one of the cartridges is provided to the other cartridge for regeneration of the desiccant bed. The bleed air flow path between the first 114a and second 114b cartridges is indicated by a dashed line in Fig. 2, and contains a flow restriction device 132 in the form of a brass plug comprising a permanent restriction orifice. The restriction orifice is predrilled to a predetermined size in order to fix the flow rate of the bleed airflow. The first 114a and second 114b cartridges therefore share a common flow restriction device 132 which is located between the cartridges 114a, 114b in a separate flow path to that which is followed by the main airflow. The present inventors have found that the tortuous path followed by the bleed airflow in this system 1 can result in undesirable pressure losses and energy inefficiencies. Further, the flow restriction device 132 is difficult and time consuming to access for adjustment or replacement which means that the system 1 is complex to adapt for different target applications. Fig. 3 shows an air drying system 200 according to aspects of the present invention. The system 200 comprises a main housing 208, a first air filtration module 204 and a second air filtration module 206 connected to an airflow outlet 202 and an airflow inlet (hidden from view), respectively. Also shown are first and second silencers 240 for muting the noise of exhausting bleed air. Fig. 4 shows the air drying system 200 of Fig. 3 with the main housing 208 and other components removed. The air drying system 200 comprises first 220a and second 220b cylindrical dryer cartridges each comprising an internal chamber 222 having internal walls 224 for containing a drying material. As described above in relation to the system of Fig. 1, each cartridge 220a, 220b is configured to operate in a drying mode for removing moisture from a main airflow and in a regeneration mode for removing moisture from the drying material using a secondary (purge) airflow. In Fig. 4 the first cartridge 220a (left) is transparent to show the internals of the cartridge 220a and the second cartridge 220b (right) is shown from the exterior only. Each cartridge 220a, 220b comprises a lower opening (which is concealed by a filter plate 262 in Fig. 4) and an upper opening 226 for permitting airflow to pass into and out of the inner chamber 222. In contrast to the air drying system Fig. 1, each dryer cartridge 220a, 220b comprises a respective, integral flow control device 250 for selectively controlling and restricting the flow rate of the air flowing through the upper opening 226. Thus, the flow control device 250 is positioned in the main airflow path, rather than in a separate bleed line. The flow control device 250 is movable between an open position wherein the main airflow is permitted to exit the internal chamber 222 of the cartridge 220a, 220b at a main airflow rate, and a closed position in which the airflow entering the internal chamber 222 via the upper opening 226 (i.e., a secondary airflow) is directed through the flow restriction device 250 thereby limiting the secondary airflow to a bleed airflow rate which is lower than the main airflow rate. Figs.5A-C show a dryer cartridge 220 according to aspects of the present invention for installation in the air drying system 200. The cartridge 220 is cylindrical and comprises an upper handle 236 for ease of installation and removal from the system 200. A lower opening of the cartridge is covered by a grill 264 and a filter plate 262 upon which rests the drying material for drying moist air, the drying material being contained by the chamber walls 224. An upper opening 226 is provided in a ring member 246. In the drying mode, a main airflow is permitted to exit the internal chamber 222 via the upper opening 226 and in the regeneration mode bleed air is permitted to enter the internal chamber 222 via the upper opening 226. The upper opening 226 is separated from the drying material by an upper grill 242 and spring 224. The upper grill may comprise a filter. The spring 224 is provided to ensure that the drying material inside the internal chamber 222 is compacted to reduce movement of the drying material in the cartridge 220. The flow control device 250 is located above the ring member 246 and positioned for closing the upper opening 226. In the regeneration mode, when the bleed air is permitted to enter the internal chamber 222 via the upper opening 226 the bleed air follows a linear airflow path from the upper opening 226 towards the internal chamber 222, and through the desiccant towards the lower opening. The linear airflow path is substantially parallel to an axis connecting the upper 226 and lower openings. The upper grill 242 and the internal chamber are located directly below and in fluid communication with the flow control device 250. Therefore, in the regeneration model the secondary airflow does not experience a significant deviation in direction in the cartridge. Similarly, in the drying mode, when the main airflow is permitted to exit the internal chamber 222 via the upper opening 226, the main airflow follows a linear airflow path from the internal chamber 222 towards the upper opening 226 owing to the direct fluid communication between the internal chamber 222 and flow control device 250. The linear airflow paths followed by the main and bleed airflows has been found 13 to result in improved pressure differential characteristics than dryer cartridges with more arduous airflow paths as discussed below in relation to Figs. 10A-B. The flow control device 250 is an adjustable flow restriction valve which is manually adjustable for adjusting a flow rate of air through the flow control device 250 when it is in the close position. With reference to the exploded view of Fig. 5C, the flow restriction device 250 comprises three parts: a valve body 258 comprising a valve handle 259, a circular orifice plate 252 and a circular cover element 254. The orifice plate 252 comprises a plurality of restriction orifices of various diameters positioned around a common PCD (pitch circle diameter), i.e. with each of their centres set at the same radial displacement from the centre of the orifice plate. The cover element 254 is rotatable with respect to the orifice plate 252 and comprises a selection aperture 256 for selectively exposing one or more of the plurality of restriction orifices. The cover element 254 is fixedly attached to the valve body 258 which is also rotatable relative to the orifice plate 252. The flow control device 250 is therefore manually adjustable by turning the valve handle 259 of the valve body 258 thereby rotating the cover element 254 with respect to the orifice plate and selectively exposing one of the restriction orifices through the selection aperture 256 for enabling air to flow therethrough. Figs. 6A-6B show assembled views of the flow restriction device 250. Fig. 7 shows a section view of the flow control device 250 installed in the upper portion of a dryer cartridge 220, the dryer cartridge 220 being installed in the dryer system 200 of fig. 3. The flow control device 250 is configured to move between a first position, when the cartridge 220 is in the drying mode, in which the main airflow is permitted to exit the internal chamber 222 at a main airflow rate, and a second position, when the cartridge 220 is in the regeneration mode, in which the secondary airflow entering the internal chamber 222 is restricted to a predetermined bleed airflow rate. In Fig. 7 the flow control device 250 is shown in the first position. In the first position, the flow control device is in its lowermost position, closer to the internal chamber 222 than in the second position, such that a seal is formed between the flow control device 250 and a perimeter of the upper opening 226. Note that in Fig. 7 the white element is not a void but is the ring element 246 which surrounds the upper opening 226. In the second position the flow control device is held flush against an O-ring 247 surrounding the upper opening 226. Thus, when the flow control device 250 is in the second position, airflow flowing into the chamber 222 must flow through the exposed restriction orifice 251 of the flow control device 222. In the first position, when the dryer cartridge 220 is operating in the drying mode, the flow control device 250 moves upwards so that a gap is provided between the flow control device 250 and a perimeter of the upper opening 226 (defined by the ring element 246) thereby permitting the main airflow to exit the internal chamber 222 via the gap and, optionally, through the exposed restriction orifice 251. Also shown in Fig. 7 is a top plate 211 of the air dryer 200 system of Fig. 3, the main airflow inlet 203 through which the incoming air for drying is provided, and the main airflow outlet 202 through which the main airflow is expelled after drying. The main airflow outlet 202 is formed by a pipe running through the 14 top plate 221 which provides a connection between the upper openings 226 of the first 220a and second 220b cartridges. The main airflow outlet 202 therefore acts as an inlet for the secondary airflow which is flowing from one the upper opening 226 of one cartridge to the other depending on the operating mode of each cartridge 220a, 220b. The flow control device 250 is configured to move automatically between the first and second positions in the presence of a pressure differential provided across the flow control device 250. Specifically, when a main airflow is provided through the lower opening of one of the cartridges 220, an internal pressure in the internal chamber 222 is higher than an external pressure above the flow control device 250 thereby causing the flow control device 250 to move upwards, into the first position. However, when there is no main airflow being provided through the lower opening and a secondary airflow is being provided through the upper opening 226 then the internal pressure of the internal chamber 222 is lower than the external pressure above the flow control device 250 thereby causing the flow control device 250 to move downwards into the second position and form a seal with the permitter of the upper opening 226. The only flow of air through the flow control device 250 is therefore through the exposed restriction orifice 251. Fig. 8 shows a section view of the air dryer 200 system of Fig. 3 including first 220a and second 220b cartridges having respective flow control devices 250 as described above in relation to Figs. 5 to 6. A centre bore or extrusion 274 descending between the first 220a and second 220b cartridges guides incoming air into a base unit 270 of the air drying system 200. The base unit 270 comprises a control valve 272 for selectively diverting the incoming air towards the lower opening of the first 220a or the second 220b cartridge. The control valve 272 is a switchable shuttle valve and controllable, using a solenoid, by a control unit of the air drying system 200 so as to selectively operate one or both of first 220a and second 220b cartridges in the drying mode. The dryer system 200 further comprises respective exhaust ports 280a, 280b for each of the first 220a and second 220b cartridges, for exhausting the secondary airflow from the respective cartridge 220 when it is operating in regeneration mode. In this example, the exhaust ports 280a, 280b are connected to the lower openings of the first 220a and second 220b cartridges, respectively. Each exhaust port 280a, 280b comprises an exhaust valve for opening (and closing) the respective exhaust port 280a, 280b. As mentioned above, the system 200 comprises of a pipe running through the upper plate 212 of the system 200 fluidly connecting the upper openings 226 of each cartridge 220a, 220b to each other and also to the system outlet 202. Therefore, in use, the control unit may simultaneously operate the first cartridge 220a in the regeneration mode and the second cartridge 22b in drying mode by opening the exhaust valve for the first cartridge 220a and configuring the control valve 272 to divert the incoming main airflow through the lower opening of the second cartridge 220b. The main airflow therefore flows through the second cartridge 220b for moisture removal in the desiccant bed and out of the upper opening 226 of that cartridge 220b for expulsion by the main airflow outlet 202. The combination of the fluid pressure of the exiting main airflow and the open exhaust valve under the first cartridge 220a causes the internal chamber 222 of the first cartridge 220b to have a lower internal pressure than an external pressure above the upper opening 226 of first cartridge 220a. This pressure differential therefore acts to pull a portion of the main airflow which is exiting the second cartridge 220b through the upper opening 226 of the first cartridge 220a, as a secondary airflow, and to push the flow control device 250 of the first cartridge 220a downwards into the flow restriction position. The flow rate of the secondary airflow into the first cartridge 220a is therefore restricted to the bleed airflow rate by the flow restriction device 250. The secondary airflow flows through the desiccant bed of the first cartridge 220a, removing moisture from the desiccant, before being expelled from the system 200 through the respective exhaust port 280a. Each of the exhaust ports 280a, 280b is connected to a respective silencer 240 for muting the sound of the secondary airflow being exhausted. The airflow directions may be switched by the control unit using the control valve 272 and the exhaust valves in order to change the configuration of each dryer cartridge 220a, 220b so that the first dryer cartridge 220a is operated in dryer mode and the second dryer cartridge 220b is operated in regeneration mode. Furthermore, in some examples, the control valve 272 may be moved to a middle position in which the incoming air is directed to both cartridges 220a, 220b in order to operate both cartridges in drying mode. This operation is illustrated diagrammatically in Fig. 9 in which the left hand cartridge 220a is operating in regeneration mode and the right hand cartridge 220b is operating in drying mode. Note that the pipes shown in Fig. 9 are representative illustrations for showing the airflow directions of the main airflow and the bleed airflow (secondary airflow) in this particular configuration of the dryer system 200 and do not show the full piping structure inside the system 200. By providing respective flow control devices 250 for each removable cartridge 220a, 220b as described herein it is easier for users to access and service the flow control devices 250 resulting in a more adaptable air drying system 200. Furthermore, the present inventors have found that by providing respective flow control devices 250 in each cartridge 220a, 220b the air drying system 200 is subject to fewer pressure losses and energy inefficiencies. In particular, by locating the flow control devices 250 in the same path as the main airflow, the flow control device 250 is configured to permit the secondary airflow to flow into the internal chamber 222 in a direction which is directly towards the desiccant bed. Thus, the secondary airflow may enter the internal chamber 222 in a direction which is substantially parallel to an axis connected the lower opening to the upper opening of the cartridge 220a, 220b thus presenting a less arduous flow path for the secondary airflow after the flow control device 250. Figs. 10A and B show comparisons of the differential airflow pressure measured across air drying systems comprising cartridges according to the present invention in comparison to air drying systems according to the prior art. The differential pressure represents an amount of pressure loss of compressed air entering the system compared to that exiting the system. A higher pressure loss can be indicative of inefficiencies and leakages in the system causing higher energy consumption. As shown in these diagrams, the air drying system of the present invention (represented by the bar on the left of each diagram) results in a significantly lower pressure loss than the systems of the prior art owing to the more direct airflow path through the cartridge. Thus, an air drying system including dryer cartridges according to the present invention has significantly improved flow rate characteristics and energy efficiency than existing air drying systems. *** The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations. Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / -10%.

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