GB2522877A

GB2522877A - Apparatus for drying a stream of compressed gas

Info

Publication number: GB2522877A
Application number: GB1402090.3A
Authority: GB
Inventors: Colin Billiet
Original assignee: Nano Porous Solutions Ltd
Current assignee: Nano Purification Solutions Ltd
Priority date: 2014-02-07
Filing date: 2014-02-07
Publication date: 2015-08-12
Also published as: WO2015118349A1; GB201402090D0

Abstract

An apparatus for drying a stream of compressed gas comprises one or more vessels 12, 14 that contain a drying media 16 of desiccant bound with polymer material and formed into tubes. The tubes are arranged so that the compressed air entering the vessel passes along the tubes. There is also an after-cooler 18 for reducing the temperature of the compressed air and an arrangement of pipes and valves connecting the vessel to the after-cooler. The valves 30, 32 are arranged so that the vessel can work in a purge mode where the hot compressed air passes through the vessel without having passed through the after-cooler and works in a drying mode where the after-cooler cools the air before it passes through the vessel. Ideally the apparatus is arranged with a pair of vessels where the hot compressed air passes through a first vessel, through the after-cooler then through the second vessel before switching to pass through the second vessel, after-cooler then the first vessel.

Description

Apparatus for Drying a Stream of compressed Gas The present invention relates to an apparatus for drying a stream of compressed gas and relates particularly, but not exclusively, to an apparatus for drying compressed air for small scale continuous use applications.

compressed air is widely used throughout industry as a safe and reliable source of energy. However, the quality of the compressed air delivered by the compressor is unsuitable for use without treatment to improve its purity, compressed air contains contaminants such as water, oil and particulate which must be removed before use. Treating compressed air has generally involved filtering it, to remove oil/water aerosols and dirt, and drying it to remove water vapour.

The dryness of the compressed air is crucial since free water will affect the operation of pneumatic components such as valves, regulators, actuators etc. Wet air lines promote corrosion and bacterial growth and are for example unsuitable for medical air, pharmaceutical use and contact with food and beverages.

Typically there are many applications where the dryness of the compressed is required to meet 1S08573.l Quality classes, class 2 for humidity, -40°c (-40°F) Pressure Dew Point (PDP) and various drying techniques are employed to achieve this standard.

Heatless desiccant dryers are cothmonly used due to their simplicity and hence low cost. A heatless twin tower dryer operates by removing moisture through adsorption onto a granular desiccant bed from the feed air (typically at 7-10 barg) as it flows up through a packed bed of desiccant, formed into a column and referred to as column A. Column B (having been previously used in drying the inlet air) is at atmospheric pressure and dry purge air from the outlet of column A is fed through a control orifice, expanded to near atmospheric pressure, and flowed in contra flow direction down through column B to effect the regeneration of *its granular desiccant bed. When the desiccant in column A becomes saturated with water vapour (usually determined by a simple timer controller) the feed air is switched back to column B, after it has been pressurised, and the cycle continues.

Typically such adsorption dryers are used with the adsorption columns oriented vertically. However, these dryers are used extensively on railway carriages to control the operation of doors and the like and since dryers are body mounted, height is a particular concern as is damage from ballast should the columns protrudes too low. It is therefore more convenient for dryers to be mounted horizontally to avoid such issues.

Granular adsorbents are widely used to remove contaminants from gases including air. The properties of ouch technologies are well defined and their use limited to certain applications. These limitations include orientation, vibration, high pressure loss, attrition, channelling, by-pass, high regeneration energy.

Furthermore, exposure to bulk water is a common problem in granular adsorbent beds and results in the breakdown of desiccant beads due to the fact that the clay binder (bentonite) is soluble in water. Such conditions result when condensed water vapour in droplet form is not adequately removed prior to the dryer.

Once this exposure occurs the entire adsorption bed needs to be replaced. The resultant dust washes through downstream filters with potential downstream consequences.

As a result it is typical for desiccant material to be changed every 6 months. This is costly and disruptive particularly to rail operators. Desiccant dust shed from the adsorbent is about 2 microns in size and is carried throu9hout the compressed air distribution system. This dust is hard and when combined with moisture can damage pneumatic components and prevent their efficient operation.

An alternative to the heatless desiccant dryer are so called "Heat of Compression dryers" which utilise the heat in the air that is compressed to facilitate the purge cycle.

Such dryers can only be deployed with non-lubricated compressors since the hot air used for regeneration must be free of oil vapour which would, if used, soon destroy the adsorbents used for drying.

One form of such dryers employs two columns which are used alternately to adsorb moisture from the feed air while the second column is being regenerated with hot air from the compressor or from an external heat source. Such products are large, complex and expensive. Regeneration requires the saturated column containing granular adsorbent to be heated to well in excess of 100°C to drive the adsorbed moisture from the adsorbent beads. In the process the entire column, which is typically a welded steel pressure vessel, has to be heated to ensure full regeneration. This is a very large thermal mass so the heating and cooling cycle typically takes 6-8 hours. It is important that all of the granular adsorbent, in particular the adsorbent located around the walls of the vessel which take longest to reach the regeneration temperature due to their proximity to the steel wall of the pressure vessel, is completely regenerated as excess water remaining in the granular adsorbent will lead to degradation of the adsorbent in subsequent cycles.

consequently the adsorption column has to be made much larger than say a heatless dryer (which is typically operated on a 2-5 minute half-cycle) . The advantage of heat of compression dryers is only in the fact that little or no air is used for purging, unlike heatless dryers, which typically require up to 20% purge air although this is at the expense of the size and expense of the system. However, due to the size, complexity and expense they are typically limited to use on large compressor systems of 150 1KW (200bhp) power and above.

Preferred embodiments of the present invention seek to overcome the above described disadvantages of the prior art.

According to an aspect of the present invention there is provided an apparatus for drying a stream of compressed gas, the apparatus comprising:-at least one vessel containing drying media comprising at least one desiccant bound by a polymer material and formed into tubes, said tubes arranged in said vessel such that a stream of compressed gas entering said vessel passes along said tubes; at least one gas stream cooling device to reduce the temperature of said stream of compressed gas; and a plurality of valves the opening and closing of which determines the flow of said compressed gas through said vessel and said cooling device, wherein said apparatus operates in a purge mode in which said compressed gas passes through at least one said vessel without passing through said cooling device and in a drying mode in which said compressed gas passes through at least one said cooling device before passing through at least one said vessel.

By providing a heat of compression dryer in which the desiccant material is formed into tubes and bounded using a polymer material, the advantage is provided that a much smaller drying apparatus can be operated with much shorter cycle times. This has the further advantage, common to heat compression dryers, of complete utilisation of the compressed gas with no losses required for purge air in a small scale drying apparatus. The heat of compression drying technique is able to work on a small scale using a polymer bound adsorbent formed into tubes in part because the tubes are easily held away from engagement with the walls of the vessel with the gas between the tubes and the vessel acting as insulation. Furthermore, because the compressed gas passes up the tubes of desiccant the majority of the adsorption takes place on the inner surface of the tubes with only a small volume of the compressed gas making it through the porous wall of the tube. As a result, the majority of the water to be desorbed is also located in the wall of the tubes adjacent the inner surface allowing it to be easily desorbed by the hot gas from the compressor. Because the adsorbent is bound by a polymer the kinetics relating to the adsorption I desorpt ion process are much quicker than those for desiccant beads, with for example a clay binder, aiding the quicker desorption and further reducing the time required to complete the purge cycle-The polymer binder further S assists in stabilisation of the adsorbent resulting in a material for which it is less critical that the purge cycle removes all water thereby allowing the adsorbent material to completely recover from even total and repeated saturation of the adsorbent. This also has the advantage of increasing the lifespan of the adsorbent further reducing the cost of maintenance of the drying apparatus.

In a preferred embodiment the at least one vessel comprises a plurality of vessels including at least one first vessel and at least one second vessel, said apparatus having: -a first operating condition in which said stream of compressed gas passes through the or each first vessel thereby operating in said purge mode and then passes through said cooling device and passes through the or each second vessel thereby operating in said dryin9 mode; and a second operating condition in which said stream of compressed gas passes through the or each second vessel thereby operating in said purge mode and then passes through said cooling device and passes through the or each first vessel thereby operating in said drying mode.

By having at least a pair of vessels with a cooling device located between them, the advantage is provided that hot compressed gas from the compressor can pass through a first vessel, thereby operating in a purge mode, before entering the cooling device, reducing the temperature and removing the majority of the water from the compressed gas before final drying in the other vessel, which is operating in the drying mode. This therefore allows compressed gas to be dried by simple switching of the valves and without the * loss of any compressed gas through purging. As a result, this system is very energy efficient requiring little or no additional energy and simple to operate by the opening and closing of valves.

* In another preferred embodiment at least one said vessel comprises a plurality of ports to allow said stream of compressed gas to enter or exit the or each said vessel, at least one first port located at a first end of said vessel and at least one second port located at a second end of said vessel.

In a further preferred embodiment the first end of said vessels has a plurality of ports, at least one port acting as an inlet and at least one other port acting as an outlet and said cooling device is connected between said second ends of said vessels.

In a preferred embodiment the second end of said vessels has at least one port acting as an inlet and an outlet.

By having a pair of ports at one end of a vessel and a single port at the other end of the vessel provides the advantage that the minimum number of tubing connections is required simplifying the construction of the device and the space required for all of the components of the dual column system.

The apparatus may comprise a single vessel and operating in a drying mode followed by a purge mode.

According to another aspect of the present invention there is provided a method of drying a stream of compressed air, comprising the steps:-controlling a plurality of valves so as to pass a stream of compressed gas through at least one gas stream cooling device to reduce the temperature of said stream of compressed gas and then to pass said stream of compressed gas though at least one vessel containing drying media comprising at least one desiccant bound by a polymer material and formed into tubes, said tubes arranged in said vessel such that said stream cf compressed gas entering said vessel travels along said tubes and exits said vessel and thereby operating in a drying mode; and controlling said plurality of valves so as to pass said stream of compressed gas through at least one said vessel without passing through said cooling device and thereby operating in a purge mode.

preferred embodiments of the present invention will now be described, by way of example only, and not in any limitative sense, with reference to the accompanying drawings in which:-Figure 1 is a schematic representation of a first embodiment of the present invention; and Figure 2 is a schematic representation of a second embodiment of the present invention.

Referring to figure 1, an apparatus 10 is provided for drying a stream of compressed gas, for example compressed air. This embodiment of the invention includes a pair of vessels, being first vessel 12 and second vessel 14, each of which contains a drying media 16. The vessels 12 and 14 are generally tubular and preferably have a circular cross-section. The drying media is formed from a desiccant bound by a polymer material and is formed into tubes which are arranged coaxially with the vessels 12 and 14. An example of the polymer is polyethersulphone (PES) although other durable polymer binders may also be used and most preferably binders that do not absorb any water. The desiccant material used is most preferably zeolite molecular sieve although other desiccant materials with fast kinetics are also acceptable.

The tubes are typically formed with a 2mm diameter and a 0.9mm bore extending therethrough. The tubes of drying media 16 are arranged in a bundle and are sealed with a potting material at each end and between the tubes so that air entering the vessels 12 and 14 is able to pass along the central bore of the tubes but is unable to pass axially along the space between the tubes.

The apparatus 10 also includes a gas stream cooling device, in the form of after-cooler 18 that is used to reduce the temperature of the stream of gas passing therethrough. A series of compressed air carrying pipes connect the vessels 12 and 14 and the after-cooler 18 and a series of valves control the flow of the compressed gas through the pipes and therefore the vessels and the after-cooler. A main inlet pipe 20, which comes from an oil free compressor (not shown), divides into a first inlet pipe 22 and a second inlet pipe 24 that enter the first and second vessels 12 and 14 via first and. second inlet ports 26 and 28 respectively. The flow of the compressed air through the first and second inlet pipes 22 and 24 is controlled by first and second inlet valves 30 and 32. As a result, the inlet valves 30 and 32 control the flow of compressed air into the first ends 34 and 36 of the vessels 12 and 14.

At the second ends 38 and 40 of the vessels 12 and 14 single inlet/outlet pipes are provided with a first inlet/outlet pipe 42 extending from a first inlet/outlet port 44 and a second inlet/outlet pipe 46 extending from a second inlet/outlet port 48. Both the first and second inlet/outlet pipes 42 and 46 are connected to the after-cooler 18.

The first inlet/outlet pipe 42 split into a pair of pipes which includes a first after-cooler inlet pipe 50 which includes a first after-cooler inlet valve 52. After passing through the first after-cooler inlet pipe and valve 50 and 52 the compressed air passes through the after-cooler 18 leaving the after-cooler via a first after-cooler outlet pipe 54 which includes a first after-cooler outlet valve 56. The first after-cooler outlet pipe 54 is connected to the second inlet/outlet pipe 46.

The second inlet/outlet pipe 46 is also connected to a second after-cooler inlet pipe 58 which includes a second after-cooler inlet valve 60. After passing through the second after-cooler inlet pipe and valve 58 and 60 the compressed air passes through the after-cooler 18 leaving the after-cooler via a second after-cooler outlet pipe 62 which includes a second after-cooler outlet valve 64.

Returning to the first ends 34 and 36 of vessels 12 and 14, the first vessel 12 has a first outlet pipe 66 which includes a first outlet valve 68 and the second vessel 14 has a second outlet pipe 70 which includes a second outlet valve 72. The first and second outlet pipes 66 and 70 join together to form a main outlet pipe 74 from which the stream of compressed air is exhausted after treatment by the apparatus 10.

Operation of the apparatus 10 will now be described. A stream of compressed gas (typically air) enters the main inlet pipe 20. This compressed air coming from an oil free compressor is hot, typically at a temperature greater than 100° C. Under the control of a control processor (not shown) the first and second inlet is arranged such that the first inlet valve 30 is open and the second inlet 32 is closed. As a result, the stream of compressed air is prevented from entering the second vessel 14 and instead enters the first vessel 12 through first inlet port 26. The air passes along the inner bore of the tubes of drying media and exits the vessel 12 by the first inlet/outlet port 44.

Although the air from the compressor has not been dried the raised temperature of the air coming straight from the oil free compressor causes any water present in the drying media 16 is desorbed and enters the stream of compressed air.

As a result, the first column 12 is acting in a purging mode in which water is driven from the drying media 16.

The stream of compressed air being exhausted from the first vessel 12 via the first inlet/outlet port 44 then enters the first inlet/outlet pipe 42. The first inlet/outlet pipe 42 splits into the first after-cooler inlet pipe 50 and the second after-cooler outlet pipe 62. However, the stream of compressed air is prevented from passing along the second after-cooler outlet pipe 62 because second after-cooler outlet valve 64 is a one-way valve arranged so as to allow a stream of air only to pass through the valve if it is coming from the after-cooler 18. In contrast the first after-cooler inlet valve 52 is arranged in the other orientations so that air passing along the first after-cooler inlet pipe is able to pass through the valve 52 and into the after-cooler 18. Air coming from the first after-cooler inlet pipe 50 and passing through after-cooler 18 is only able to exit the after-cooler via first after-cooler outlet pipe 54. The after-cooler 18 acts to cool the stream of compressed air which results in water vapour held in the stream of compressed air condensing out of the air as it passes through the after-cooler 18. That condensing water is collected and exhausted from the after-cooler 18 in a manner

familiar to those skilled in this field.

The stream of cold compressed air exiting after-cooler 18 via first after-cooler outlet pipe 54 passes through the first after-cooler outlet valve 56 which is also a one-way valve orientated to allow the stream of compressed air to exit the after-cooler 18. The stream of cold compressed air then enters the second inlet/outlet pipe 46, passes through the second inlet/outlet port 48 and enters the second vessel 14 at the second end 40. The stream of cold compressed air then passes up of the tubes of drying media 16 and the desiccant in the drying media adsorbents the moisture in the compressed air resulting in a stream of cool and dry compressed air exiting the second vessel 14 via the second outlet pipe 70 and through the second outlet valve 72 which is maintained in an open condition. As a result, the second column 14 is acting in a drying mode in which water is adsorbed into the drying media 16. As can be seen from the preceding description the apparatus 10 is operating such that the first column 12 is operating in a purging mode whilst the second column 14 is operating in a drying mode. From the second outlet pipe 70 the stream of cool and dry compressed air enters the main outlet pipe 74 which is attached to downstream apparatus that make use of the compressed air.

After a predetermined period of time operating in the above described first condition (the first column 12 purging whilst the second column 14 is drying) the apparatus 10 switches, under the control of the processor, into a second condition (the second column 14 purging whilst the first column 12 is drying) by altering the open/closed orientation of the inlet valves 30 and 32 and the outlet valves 68 and 72. In the first condition, described above, the first inlet valve 30 is open and the first outlet valve 68 is closed. At the same time, the second inlet valve 32 is closed and the second outlet valve 72 is open. In the second condition the valve orientations switch so that the first inlet valve 30 is closed and the first outlet valve 68 is open whilst the second inlet valve 32 is open and the second outlet valve 72 is closed. This switching of the valves orientation and the running of the apparatus 10 in the second condition results in the switching of the first and second vessels 12 and 14 from the purge and drying modes to drying and purging modes respectively.

Looking at this second condition in detail the hot compressed air entering the main inlet pipe 20 passes into the second inlet pipe 24, through the second inlet valve 32, the second inlet port 28 and into the first end 36 of second vessel 14. The hot compressed air purges the drying media 16 is it passes along the walls of the bores of the tubes of drying media in vessel 14 before exiting at the second end via second inlet/outlet port 48 and into second inlet/outlet pipe 46. The first after-cooler outlet valve 56 prevents any air entering the after-cooler 18 via first after-cooler outlet pipe 54 but the compressed air enters after-cooler 18 via the second after-cooler inlet valve 60 and second after-cooler inlet pipe 58. The cooled and somewhat dewatered compressed air exits the after-cooler 18 via second after-cooler outlet valve 64 and second after-cooler outlet pipe 62 before joining the first inlet/outlet pipe 42. The compressed air then enters the second end 38 of first vessel 12 via first inlet/outlet port 44 and passes along the drying media tubes 16. The cooled and dried compressed air exits the first end 34 of vessel 12 via the first outlet pipe 66 and first outlet valve 68 before joining the main outlet pipe 74.

It can therefore be seen that the apparatus 10 is able to switch from the first condition to second condition and continuously produce a stream of dry compressed air with water being removed and exhausted from the apparatus via the after-cooler 18. It will be apparent from the above description to persons skilled in the art that the after-cooler 18 includes two separate and unconnected cooling conduits that separately connect the first after-cooler inlet pipe 52 the first after-cooler outlet pipe 54 and connect the second after-cooler inlet pipe 58 to the second after-cooler outlet pipe 62.

It will also b apparent to persons skilled in the art that the after-cooler valves 52, 58, 60 and 64, which are shown as one-way valves, could be replaced with standard valves that are opened and closed under the control of the same processor that opens and closes the inlet and outlet valves 30, 32, 68 and 72.

An alternative embodiment of the present invention is shown in figure 2, in which parts in common with those in figure 1 have been shown with like reference numerals increased by 100. The apparatus 110 has a single vessel 112 that contains drying media 116 of the type described in relation to the embodiment shown in figure 1. The apparatus 110 also includes an after-cooler 118. However, the arrangement of pipes and valves that connect the main inlet pipe 120 with the vessel 112 and after-cooler 118 is such that the apparatus 110 operates either in a drying mode or in a purge mode but not both at the same time as with the previous embodiment. As a result, the apparatus 110 is designed to produce a stream of dry compressed air on demand and not on a continuous basis as seen with the embodiment in figure 1, typically charging an air receiver.

The main inlet pipe 120 splits into two pipes, one of which is an after-cooler inlet pipe 180 which includes an article inlet valve 182 that directs and controls the flow of air coming from the oil free compressor (not shown) into the main inlet pipe 120 through to the after-cooler 118. The other pipe, which splits from the main inlet pipe 120, is a vessel purge inlet pipe 184 that includes a vessel purge inlet valve 186. The vessel purge inlet pipe 184 is connected to the vessel 112 at the first end 134 allowing a stream of compressed air to enter the vessel directly from the compressor. Also connected to vessel 112, at the first end 134, is the dry outlet pipe 190 that includes a dry outlet valve 192.

Returning to the after-cooler 118, the outlet from the after-cooler is after-cooler outlet pipe 188 which enters the vessel 112 at the second end 138. Also in the second and 138 of vessel 112 is a purge outlet pipe 194 with a purge outlet valve 196.

Operation of the apparatus 110 is as follows. Working firstly in a drying mode, the after-cooler inlet valve 182 is opened and the vessel purge inlet valve 186 is closed. This results in air passing along after-cooler inlet pipe 180 and into the after-cooler 118 where it is cooled resulting in the condensation of some of the water vapour held in the hot air coming from the compressor. On exiting the after-cooler 118 the cooled air passes along the after-cooler outlet pipe 188 and into the second end 138 of vessel 112. The purge outlet valve 196 is closed thereby preventing air from exhausting through the purge outlet pipe 194. The compressed air passes along the internal bore of the tubes of drying media 116 and the dried air exits the vessel 112 at the first end 134 via the dry outlet pipe 190 and through the dry outlet valve 192 which is open.

upon exhausting the apparatus 110, the dried air passes onto downstream apparatus to be used. Typically this use may be via a storage tank (not shown) which is filled with the dried compressed air until it reaches a predetermined pressure. Once the storage tank is reached this pressure a signal is sent from a sensor on the tank which indicates that no more compressed air is required in the tank. This results in the apparatus 110 switching from the drying mode to a purge mode. The purge mode will typically run for a predetermined period of time and uses air from the compressor which continues to run for that predetermined time.

In the purge mode the after-cooler inlet valve 182 is closed and the vessel purge inlet valve 186 is opened. This results in no air passing through the after-cooler 118 and the stream of compressed air from main inlet pipe 120 entering the vessel 112 at the first end 134. The hot compressed air passes down the bores of the tubes of drying media 116 causing the desorption of water that had been adsorbed into the desiccant material of the drying media 116.

The dry outlet valve 192 is closed preventing air from lb passing along the dry outlet pipe and the purge outlet valve 196 is opened causing the stream of compressed air to pass through the vessel 112 out of the second end 138 via the purge outlet pipe 194. This purging results in regeneration of the drying media 116 and once the purging is complete, after the predetermined period of time, the compressor shuts down.

When air has been used from the dry air storage tank and the pressure therein decreases below the predetermined value the compressor restarts and the valves 182, 186, 192 and 194 switch modes to return to the previously described drying mode.

It will be appreciated by persons skilled in the arz that the above embodiments have been described by way of example only and not in any limitative sense, and that various alterations and modification are possible without departure from the scope of protection which is define by the appended claims.

Claims

Claims 1. An apparatus for drying a stream of compressed gas, the apparatus comprising:-at least one vessel containing drying media comprising at least one desiccant bound by a polymer material and formed into tubes, said tubes arranged in said vessel such that a stream of compressed gas entering said vessel passes along said tubes; at least one gas stream cooling device to reduce the temperature of said stream of compressed gas; and a plurality of valves the opening and closing of which determines the flow of said compressed gas through said vessel and said cooling device, wherein said apparatus operates in a purge mode in which said compressed gas passes through at least one said vessel without passing through said cooling device and in a drying mode in which said compressed gas passes through at least one said cooling device before passing through at least one said vessel.
2. An apparatus according to claim 1, wherein said at least one vessel comprises a plurality of vessels including at least one first vessel and at least one second vessel, said apparatus having:-a first operating condition in which said stream of compressed gas passes through the or each first vessel thereby operating in said purge mode and then passes through said cooling device and passes through the or each second vessel thereby operating in said drying mode; and a second operating condition in which said stream of compressed gas passes through the or each second vessel thereby operating in said purge mode and then passes through said cooling device and passes through the or each first vessel thereby operating in said drying mode.
3. An apparatus according to claim 1 or 2, wherein at least one said vessel comprises a plurality of ports to allow said stream of compressed gas to enter or exit the or each said vessel, at least one first port located at a first end of said vessel and at least one second port located at a second end of said vessel.
4. An apparatus according to claim 3, wherein said first end of said vessels has a plurality of ports, at least one port acting as an inlet and at least one other port acting as an outlet and said cooling device is connected between said second ends of said vessels.
5. An apparatus according to claim 4, wherein said second end of said vessels has at least one port acting as an inlet and an outlet.
6. An apparatus according to claim 1, comprising a single vessel and operating in a drying mode followed by a purge mode.
7. A method of drying a stream of compressed air, comprising the steps:-controlling a plurality of valves so as to pass a stream of compressed gas through at least one gas stream cooling device to reduce the temperature of said stream of compressed gas and then to pass said stream of compressed gas though at least one vessel containing drying media comprising at least one desiccant bound by a polymer material and formed into tubes, said tubes arranged in said vessel such that said stream of compressed gas entering said vessel travels along said tubes and exits said vessel and thereby operating in a drying mode; and controlling said plurality of valves so as to pass said stream of compressed gas through at least one said vessel without passing through said cooling device and thereby operating in a purge mode.