Method and apparatus for removing water from compressed air

Abstract

A method and apparatus for removing water from compressed air is disclosed. The method includes the steps of passing a stream of compressed air through a pressure swing adsorption (PSA) dryer. The dryer includes at least one vessel containing a desiccant material bound into pieces, for example tubes, using a polymer binder. The PSA dryer also has a control system for controlling the flow of the compressed air and switching between drying and purging modes. In particular the vessel and desiccant material contained therein are sized to produce a dew point suppression of less than 50° C.

Claims

1. A method of removing water from compressed air comprising, passing a stream of compressed air through a pressure swing adsorption dryer, the dryer including: at least one vessel containing at least one desiccant material formed into desiccant members using a polymer binder; and at least one control system for controlling the flow of said compressed air and switching the flow of said compressed air between a drying mode and a purging mode, wherein said vessel and said desiccant material contained therein are sized to produce a dew point suppression of less than 50° C., and wherein said desiccant members comprise a plurality of tubes extending along said vessel.

2. The method according to claim 1, wherein said vessel receives compressed air with a dew point of up to 50° C.

3. The method according to claim 1, wherein said desiccant material produces compressed air with dew points of greater than −10° C.

4. The method according to claim 1, wherein said dryer is not provided with means for heating the desiccant material during the purging mode.

5. A method of removing water from compressed air comprising, passing a stream of compressed air through a pressure swing adsorption dryer, the dryer including: at least one vessel containing a plurality of tubes extending along said vessel, the plurality of tubes formed from a desiccant material bound by a polymer binder; and at least one control system for controlling the flow of said compressed air and switching the flow of said compressed air between a drying mode and a purging mode, wherein said vessel and said desiccant material contained therein are sized to produce a dew point suppression of less than 50° C., and wherein said stream of compressed air flows through the plurality of tubes.

Description

(1) 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:—

(2) FIG. 1 is a sectional view of a drying device used in an embodiment of the present invention;

(3) FIG. 2 is a schematic representation of a compressed air system incorporating the device of FIG. 1;

(4) FIG. 3 is a graph showing the dew point recovery of an embodiment of the present invention;

(5) FIG. 4 is a schematic representation of a compressed air system including a drying apparatus used in another embodiment of the present invention;

(6) FIG. 5 is a schematic representation of the drying apparatus included in FIG. 4;

(7) FIG. 6 is a perspective view of a apparatus embodying the invention shown schematically in FIG. 5; and

(8) FIG. 7 is a graph illustrating the dew point suppression that can be achieved using the apparatus in FIGS. 4 to 6.

(9) Referring to FIGS. 1 to 3, a compressed air delivery system 10 includes a pressure swing adsorption (PSA) dryer 12. In the example shown the PSA dryer 12 is a single column dryer although it should be noted that the present invention is equally applicable to a two column dryer. The PSA dryer 12 includes a vessel 14 that contains a desiccant material that is formed into desiccant members using a polymer binder. In the embodiment presently described, the desiccant members are formed into tubes 16 that are arranged in a bundle that extend along the length of vessel 14. The tubes 16 are formed by combining a polymer with a desiccant material and extruding in the form of a tube. 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 2 mm diameter and a 0.9 mm bore extending therethrough.

(10) The compressed air system device also includes a control device 18 for controlling the flow of compressed air through the PSA dryer 12. In particular, the control device 18 switches the flow of compressed air between a drying mode indicated at 20 by the directional arrow and a purge mode indicated by arrow 22. The flow of compressed air is controlled by a series of valves contained within the PSA dryer and operate in a manner familiar to those skilled in the art.

(11) The compressed air system shown in FIG. 2 includes a compressed air source 24 and a compressed air reservoir 26. Measurements of temperature and pressure are taken throughout the system using pressure measuring devices 28, 30 and 32 and temperature measuring devices 34, 36, 38 and 40. After the compressed air source and pressure measuring device, a series of filters indicated at 42 remove oil, particulate matter and water droplets, reducing contamination of the PSA dryer. A mass flow meter 44 and pressure regulator 46 control the flow of compressed air through to a further pair of pressure and temperature sensors 30 and 34.

(12) The compressed air then enters the PSA dryer 12 and passes along desiccant tubes 16. The water contained in the air is quickly absorbed into the zeolite sieve that, in combination with the polymer, forms the desiccant tubes. The dry compressed air then passes through outlet valves and via temperature sensor 38 to the compressed air reservoir 26. Within the reservoir the pressure and temperature are measured by sensors 32 and 40 and the dew point is also measured by dew point measuring device 48. Once a required pressure has been reached, the flow of compressed air to the PSA dryer is cut off and purged air is returned from the reservoir 26 and passes back through the PSA dryer 12 in direction 22 at a significantly reduced pressure then passes through outlet valve 50. This quickly regenerates the desiccant to its dehydrated state. Because of the fast kinetics of the combination of desiccant with non-water absorbing polymer binder, the air returning from reservoir 26 does not need to have a dew point as low as it would if a standard bead desiccant of the prior art were being used. In the prior art, the difference between the dew point of the air entering the PSA dryer and exiting the PSA dryer needed to be more than 50° C. in order that the purging air would be able to sufficiently dry the desiccant beads before drying was recommenced.

(13) It is therefore possible for the apparatus to operate with a dew point suppression, that is the difference between the dew point of the air entering the PSA dryer and the air exiting the PSA dryer (of as little as 5° C.). It should be noted that any dew point suppression below this would be of very limited value since the change in dew point is so small as to be of little use. A dew point suppression of 10° C. may be of use in a medical situation where the air supplied to a device or patient needs to be slightly less than the ambient air. A dew point suppression of 20-30° C. may be of use on a small air compressor that operates in reasonably controlled environments. For example, such compressors are used to operate dentist's tools and only a small dew point suppression is required in order to ensure that no condensation forms in a device when there is a change in temperature in the room in which the device is located (for example when it cools overnight).

(14) A dew point suppression of 30-40° C. is appropriate for use in the rail industry where it is important to ensure that when a railway vehicle stops being used and is stored overnight, any drop in temperature is not sufficient to cause condensation of water within air lines. As a result, a dew point suppression of 30-40° C. is sufficient.

(15) FIG. 3 demonstrates dew point recovery from excessive duty where the inlet temperature increased to +57° C. The PDP was allowed to degrade to +30° C., full recovery to <+5° C. was achieved after a number of cycles following normal duty being resumed. For the 40° C. inlet temperature a DPS of 30-40° C. was achieved.

(16) There are many other uses to which the device of the present application may be put and these are not limited to those set out above. The present invention is applicable to any situation where a small dew point suppression (up to 50° C.) is required and is also applicable to situations where air is required to be supplied with a moisture content meeting the standards set out in ISO 8573.1 levels 2 to 6.

(17) Referring to FIGS. 4 to 6, an apparatus 60 for removing water from compressed air is designed to be located between a compressor device, which is preferably an oil free compressor 62 and a compressed air storage tank 64. The apparatus 60 includes a vessel 66 that defines a volume and contains a desiccant material 68. The desiccant material is bound together using a polymer binder and formed into desiccant members 70. These desiccant members are preferably in the form of tubes that extend along the length of vessel 66 and are typically formed with a 2 mm external diameter and a 0.9 mm bore extending therethrough. Although the above described tubes are preferable any suitable shape of desiccant member may be used including short lengths of the above described tubes randomly packed into the vessel 66 or desiccant members formed into beads. The desiccant can be any suitable desiccant but is preferably a zeolite molecular sieve, although other desiccant materials with fast kinetics may be used. The polymer can be any suitable durable polymer binder for example polyethersulphone (PES).

(18) The vessel is formed with a pair of end caps 72 and 74. The first end cap 72, located at the bottom of the vessel 66, has an inlet 74 to allow a stream of compressed air to enter the vessel. The inlet 73 has an inlet valve 76 that controls the flow of air through the inlet 73. The inlet valve 76 can be a simple open and shut valve and does not need to be a variable flow control valve to control the rate of flow of the compressed air into the vessel. The inlet valve 76 is preferably a non-return valve. End cap 72 is provided with a baffle 78 that directs the stream of compressed air down into the end cap before it changes direction to pass up through the desiccant members 70. This assists to ensure an even flow of compressed air entering the portion of the vessel 66 that contains the desiccant members 70. The baffle 78 also acts to capture droplets of water and direct them to the bottom of end caps 72.

(19) The second end cap 74, located at the top of vessel 66, has a first outlet 80 that is connected to tank 64. The first outlet 80 has a valve 82, preferably a non-return valve, and a purge orifice 84, which can be either fixed or variable, connected in parallel with the valve 82.

(20) A second outlet 86 is provided in the first end cap 72 and is located at the bottom of the end cap. The second outlet 86 has a second outlet valve 88. This second outlet valve controls the flow of purging gases that passed back through the vessel from the storage tank. The second outlet valve has a timer function which controls the period of time that the valve remains open and this is preferably a variable timer that can be changed by simple operation of a switch or dial for example. Second outlet valve 88 is also preferably a solenoid valve and further preferably opens in response to a signal from a pressure switch 90 on the storage tank 64 which also controls the operation of the compressor 62.

(21) Operation of the apparatus 60 will now be described. A stream of compressed air from compressor 62 passes through inlet valve 78 and into the inlet 73. The baffle 78 in end cap 72 forces the flow of compressed air down and any droplets of water in the compressed air hit the baffle 78 and run down towards the bottom of the end cap 72 from where they will be exhausted with any purge air through second outlet 86. The stream of compressed air passes up along the desiccant members 70 and water from the compressed air is adsorbed into the desiccant material resulting in a dryer air passing into end cap 74 and out of the first outlet 80. The dryer compressed air passes through valve 82, with a small volume passing through the purge orifice 84, before entering storage tank 64.

(22) When sufficient air to fill tank 64 to a predetermined required pressure has passed through the apparatus 60 the pressure switch 90, associated with the tank 64, causes compressor 62 to switch off. This same signal causes solenoid valve 88 to open. This opening of valve 88 allows some of the compressed air from tank 64 to flow back through purge orifice 84 back into end cap 74 and down through the desiccant members 70. Because the first outlet valve 82 is a non-return valve, the air from tank 64 must passed through the orifice 84 which limits the flow of air ensuring that this flow of purge air is at reduced and near atmospheric pressure thereby allowing the purge of the moisture trapped in the desiccant material 68 from the desiccant members 70.

(23) Solenoid valve 88 remains open for a predetermined period of time during which this purge takes place. When the predetermined period of time has ended the valve 88 closes. This predetermined period of time might typically be 20 seconds. The period of time that the valve 88 remains open can be used to determine the dew point suppression that the apparatus 60 provides. The longer that the valve 88 remains open the more complete the purge of moisture that takes place. A very short purge results in not all of the water being removed and some water remaining adsorbed in the desiccant material. Because the desiccant is bound by a polymer binder it is able to operate under these circumstances without damage to the desiccant members. Indeed in the event of some mechanical failure, for example a leak from the tank which causes the compressor to run continuously which would result in saturation of the desiccant members, the desiccant material can recover once normal operation is resumed. As a result, a single size of the apparatus of the present invention can be used to provide a variety of the dew point suppressions by simply varying the period of time that the purge valve is open. Therefore, if solenoid valve 88 is provided with a variable timer, which allows out operator to alter the period of time that the valve remains open once the signal is received from the pressure switch 90, the apparatus can act to offer variable dew point suppression from a single apparatus. It is therefore possible, for example, for a manufacturer of air compressors to use a single size of the compressed air drying apparatus of the present invention on a variety of different sized compressors and storage tanks by simply varying the purge time by altering the time on the variable timer valve 88 to obtain the dew point suppression required.

(24) At any point during the process compressed air can be drawn from the storage tank 64 for use. When the pressure in storage tank 64 drops below a predetermined value the compressor 62 restarts and the process of recharging the tank 64 begins repeating the steps set out above. In the event that the pressure in tank 64 reduces to below the predetermined value during the twenty second purge time that the solenoid valve 88 is open the solenoid valve 88 closes to prevent the compressed air from the compressor 62 escaping through outlet valve 88. Although the device in FIGS. 4 to 6 is shown aligned vertically the apparatus is equally capable of operating if aligned horizontally. In this instance it is preferable that the second outlet is located at the lowest point of the apparatus so that any excess droplet water tends to run towards second outlet where it is quickly removed during the purge cycle.

(25) Referring to FIG. 7, the graphs shown thereon demonstrate how the device of the present invention can be used to select a dew point suppression. The graph shows that for a given inlet pressure dew point the conversion to outlet pressure dew point is substantially linear across the full range of percent rated flows at a given pressure and that the lines representing the inlet pressure dew points only marginally converge meaning that the pressure dew point suppression is fairly constant across a range of inlet pressure dew points. For example at a 100% rated flow and inlet PDP of 50° C. gives an outlet PDP of 10° C. meaning a dew point suppression of 40° C. At an inlet PDP of 40° C. there is an outlet PDP of −1° C. giving a dew point suppression of 41° C. At an inlet PDP of 30° C. the is an outlet PDP of −14° C. giving a dew point suppression of 44° C. At an inlet PDP of 20° C. there is an outlet PDP of −25° C. giving a dew point suppression of 45° C. Finally, at an inlet PDP of 20° C. there is an outlet PDP of −36° C. giving a dew point suppression of 46° C. It can therefore be seen that the dew point suppression only varies by 6° C. over the range of inlet PDP from 10 to 50° C.

(26) It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only and not in any limitative sense, and that the various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims. For example, the desiccant material need not necessarily be formed in tubes extending along the length of a vessel. The PSA dryer would also work if filled with the desiccant dryer tubes cut into short lengths, formed in pellets or formed into beads. However, the fixed nature of the elongate tubes provides advantages of non-settling of the desiccant material and greater resistance to vibration therefore leading to no dust formation. It should also be noted that the example shown in FIG. 2 is an R&D test set up which, while is designed to replicate actual operating conditions, has instrumentation suitable for such testing. In normal use such instrumentation would not be used and the system would consist of the compressed air source (outside of our scope of supply) the inlet filtration, the dryer and a controller. The receiver and distribution system are also not part of the device of the present invention.