CA1057950A - Continuous air drying in alternate dessicating towers - Google Patents

Abstract

A B S T R A C T

The present invention relates to a method and apparatus for continuously removing moisture from especially compressed air.
The compressed air is brought to pass a moisture-absorbing re-generatable agent in a plurality of drying towers. Said drying towers being alternatively connectable to said flow of air so that drying towers not in use could be subjected to a regeneration process and cooled before reuse.

Description

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The present invention relates to a method of and equipment for con-tinuously removing moisture from a fluid such as compressed air.
Drying plants for compressed air are known and generally consist of drying towers containing a moisture-absorbing agent through which the com-pressed air flows.
~ According to this invention there is provided a method of con-tinuously removing moisture from a fluid, said method comprising the steps of continuously supplying the fluid to an apparatus comprising at least two dry-ing towers each containing a moisture absorbing reactivatable drying agent, lQ causing the fluid to flow through at least one of the towers so that the fluid is dried and simultaneously subjecting the remaining tower or towers to : a regeneration and cooling process comprising the steps of causing air to flow into the or each tower being regenerated, through a heating zone which is located within the or each tower being regenerated and subsequently to flow through the drying agent in the or each tower being regenerated, the regener-. ated tower or towers being cooled with flow of cooling fluid, the pressure in - the or each cooled tower finally being equalised with that of the tower or towers performing the drying process, said method also comprising the step of changing the flow of fluid from the tower or towers effecting the drying pro-2Q cess to the regenerated and cooled tower or towers, said change being effected withcut pressure surges, the original tower or towers effecting the drying process then being subjected to a corresponding regeneration and cooling pro-cess.
According to another aspect of this invention there is provided an : apparatus for continuously removing moisture from a fluid said apparatus com-prising means for continuously supplying to the fluid to at least two drying towers each containing a moisture absorbing reactivatable drying agent, means to cause the fluid to flow through at least one of the towers so that the ~k . ~ -2- ~
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fluid is dried and means for simultaneously causing air to flow into the removing tower or towers, there being heating means wlthin each tower to heat - such air before the air flows through the drying medium, means being provided for subsequently directing a flow of cooling fluid through each tower and means being provided to equalize the pressure of the or each regenerated and cooled tower with the or each tower performing the drying process, means also being provided for changing the flow of fluid from the tower or towers effect-ing the drying process to the regenerated and cooled towers without pressure surges and means for leading away dried fluid from the apparatus.
~ In a preferred embodiment of the invention continuous operation is achieved, i.e. there is no need for the actual drying process to be inter-rupted.
In order that the invention may be more readily understood and so that further features thereof may be more readily appreciated the invention will now be described by way of example with reference to the accompanying drawings in which:
Figure 1 is a diagrammatic sketch of a drying equipment in accord-ance with the invention;
, Figure 2 is a view, partly in section, of a drying tower included in the equipment shown in Figure l;
Figure 3 is a detail of the inlet and outlet valves included in the equipment shown in Figure l;
Figure 4 shows an electrical circuit diagram for the equipment shown in Figure l;
Figure S shows a time schedule for the programming unit used in the equipment, and Figure 6 shows an electrical circuit for the equipment shown in Figure 1, with flow regulation.

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The equipment or plant shown in Figure 1 comprise~ two drying towers 1, la, respectively. A fluid to be dried, for ' ~; ~
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example compressed air, enters the equipment through an inlet valve 2a and leaves through an outlet valve 2b. rhe inlet and outlet valves 2a, 2b are three-way valves. The inlet valve 2a is connected via inlet pipes 3, 3a to inlet openings 4, 4a in the lower part of the drying towers 1 and la, , respectively. The inlet valve 2a is such that fluid can be directed to the tower 1 or to the tower la. The outlet valve 2b communicates with both the drying towers 1, la through outlet pipes 5, 5a.
Supply pipes 6, 6a for drying air, i.e. for air used to regenerate the drying medium, also lead into the outlet pipes 5, 5a, and the pipes 6, 6a are connected by a common pipe 7 to a fan 8. A damper 9 is also provided in the pipe 7 to control the flow of air from the fan, and a pressure control device or pressostat 10 for maintaining the pressure of the air blown by the fan at a constant value is also arranged in the pipe between the fan 8 and the damper 9 and is adapted to control the fan 8. Valves 11, lla are arranged in the supply pipes 6, 6a and between said valves 11, lla and the respective out-:,, let pipes S, Sa are further pressostats 12, 12a.
Each of the pipes S, Sa passes through the outer casing 13 of the respective drying tower 1, la and extends to approximately the centre of the drying tower 1, la. From there each pipe S, Sa leads into a somewhat larger central pipe 14, 14a respectively extending towards the upper part of each tower. Inside said pipes 14, 14a, or to be more precise, along the inner wall of each such pipe extends a helical member lS, lSa. A number of heating elements 16, 16a also protrude into said pipes 14, 14a and cooperate with a thermostat 17, 17a which is provided with overheating protection means. The heating elements 16, 16a and the associated thermostat 17, 17a with overheat-ing protection means are attached to a centrally arranged flange 18, 18a pro-vided with a cap, in respective drying towers, 1, la. In the region located nearest to the flange 18, 18a the pipe 14, 14a is provided with perforations , . : , .
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1~5 iJ~S3 in the form of a filter 19, l9a which is pervious to the fluid being dried, e.g. air.
As can be seen more clearly from Figure 2, the drying tower 1, la is filled with drying agent 20, preferably in the form of beads of silica gel, activated alumina or other conventional drying agents, or mixtures of such agents. There is also a filter arrangement 21 at the bottom of the drying tower 1, la, said filter having a smaller mesh than the diameter of the beads of drying agent used. A flange 22, 22a with a cap is provided for emptying the drying tower of beads when the beads are exhausted, and a flange 23, 23a, with a' cap, is provided at the top of each drying tower 1, la through which fresh beads can be poured. The upper part of the drying tower is preferably also provided with internal or external insulation. At the bottom of the dry-ing tower 1, la is an outlet 24, 24a for air utilised to regenerate the beads, and the outlet is provided with a control valve 25, 25a. If the drying beads are of silica gel type, the beads, as they are being introduced into the towers may conveniently be sifted in such a way that water proof drying beads of type W (grain size 2 - 6 mm) are used for filling the lower part of the drying tower and standard drying beads of type S (grain size 2 - 6 mm) are used for filling the upper part of the drying tower. The filter means 21 conveniently comprises a conventional outwardly biased spring ring 26 with a filter cloth 27, supported by a grid cloth 28.
For synchronised switching of the valves 2a, 2b a common actuating unit 29 is provided in the form of an air operated piston and cylinder arrange-ment which moves a control member which actuates the valves simultaneously.
The equipment also includes a pressure equalising valve 30 which, by way of pipes 31 and 32, is connected, via a filter, to each drying tower. The valve 30 is a three-way valve with a T-tap and a silencer is provided at the outlet so that each drying tower can be emptied of compressed air slowly and without -.~ .

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the generation of disturbing noise. The valve 30 is associated with two piston and cylinder arrangements 52, 53 which are arranged to control the position of the valve 30.
A thermometer 33, 33a is inserted in the upper part of the drying tower, as well as a control thermostat 34, 34a and a function control thermo-stat 35, 35a.
Figure 3 shows how a synchronised operation of the valves 2a and 2b is achieved by means of the air operated piston and cylinder arrangement 29.
A piston and rod 39 protrudes from the cylinder and is pivotally attached to a lever 40, the upper end of which is pivotally connected to a yoke 41 having two shafts 42, 43 of which are attached to and cooperate with valve housings 44, 45, respectively, in the pipes 5, 5a. The other end of the lever 40 is connected in the same manner to a yoke 46 which, by way of two shafts 47, 48, cooperates with valve housings 49, 50 in the pipes 3, 3a. When the lever 40 is moved in the direction of the arrow 51, a synchronised changeover of the valve functions in the valves 2a and 2b is achieved.
Figures 4, 5 and 6 show the electrical devices provided on the equipment, a time schedule for the programming unit concerned, and an electric circuit diagram. Electric components illustrated are designated as follows:
A denotes a programming unit constituting a central control member; Bl - B6 denote thermostats, B21 - B25 pressostats, El and E2 heating elements; Fl -F4 denote fuses, Fll a motor protection means, H an indicating lamp, Kl - K3 - and Kll - 13 contactors such as solenoid operated switches or relays, Ml denotes the electric motor for the fan 8; Q denotes a main switch, Sl and S2 denote limit switches, Sll a push-button, T a transformer and U a central control means; and Yl - Y6 denote electrically controlled fluid flow regulat-ing valves.
The equipment shown in the drawings operates in the following manner:

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The fluid to be dried, for example compressed air from a compressor, enters the equipment or plant through the inlet valve 2a (dark arrow 36) and then flows through the inlet pipe 3 to the lower part of the lefthand drying tower 1 and from there, in the direction of the arrow 37, up towards the upper part of the tower where the air flow - as is shown by the arrows 38 -passes the filter 19 and flows down in the pipe 14 and from there out through the outlet pipe 5 and outlet valve 2b.
This continues until the adsorption capacity of the drying beads in the tower is almost exhausted, after which the air to be dried is instead directed to the drying tower la where the drying beads have been sub~ected to regeneration and subsequent cooling while the drying tower 1 has been in use.
The regenerated drying tower is, however, first cooled by the fan 8 blowing air through the tower for a few minutes after the heating has been turned off.
No adsorption of moisture can occur since the fan air is heated by residual heat present in the heating elements 16, 16a and the adjacent parts of the tower. Just before the changeover the regenerated drying agent should be further cooled in order to achieve full adsorption capacity immediately after the changeover. This is attained by allowing compressed air from the tower, " i.e. the tower in use, to expand in the other tower, i.e. the tower coming into use, this air then flowing out through the bottom valve in the respective tower.
The transfer of the flow of compressed air from one tower to the other should take place without pressure surges, partly to avoid pressure fluctuations in the output pipes of the equipment, partly so as not to damage the drying agent in the towers and partly also for environmental reasons since any rapid change of pressure may generate undesirable noise, such as a blowing sound. To satisfy these requirements a pressure equalisation must occur before the flow is switched from one tower to the other. Furthermore, J;

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the valves 2a, 2b must be connected together in order to avoid any interrup-tion in the continuous drying process.
The function of the devices controlling the equipment will now be described with reference to Figures 4 to 6.
The necessary switching of the main valves 2a, 2b is accomplished pneumatically, under the control of electrically actuated valves such as solenoid valves. The central control member for the whole plant is the pro-gramming unit A. The programming unit A gives start and stop signals for all movements and controls such elements as the heating elements and is provided with means for actuating an appropriate number of micro-switches. These means may comprise a number of disco or cams provided with appropriately shaped peripheries and arranged to rotate in synchronism. Each cam or disc is located so that the associated switch is actuated for a predetermined part or parts of each cycle of rotation of the cam or disc. According to the time schedule of the programming unit - the time schedule is shown in Figure 5 -one complete turn of the cams corresponds to 60 "markings", which normally covers a period of 12 hours. Between 0 - 30 a transfer of compressed air takes place to the righthand drying tower la and a regeneration in the left-hand drying tower 1 and between 30 - 60 a transfer to the lefthand drying tower 1 and regeneration in the righthand tower la.
Figure S shows the times when various microswitches are closed and Figure 6 shows the circuitry associated with each microswitch.
Let us assume that the equipment is in a condition with pressure in lefthand drying tower 1.
When cam I reaches position 0 current flows to the electric valve Yl and the piston and cylinder arrangements 52, 53 associated with the pressure equalising valve 30 are displaced so that the valve allows compressed air to flow from the lefthand tower 1 to the righthand tower la. The valves lla and f~

25a are open, and the transferred compressed air passes out into the open air - through these valves so that cooling of the drying tower la on the right which has become somewhat warm after the regeneration is achieved. At position 1 the cam II connects the current to the electric valve Y2 which then closes valves lla, 25a. A pressure equalisation is thereby achieved. At position

2 cam III connects current via a pressostat B24 operating at 5 bars to electric valve Y3 and the piston cylinder arrangement 29 activates valves 2a, 2b so that the flow of compressed air is switched to the righthand tower la. The piston cylinder arrangement 29 influences a pneumatic limit sensing device SPl wh~en it reaches its end position, which device then completes a supply of air to the piston cylinder arrangement 53 associated with the pressure equalising valve 30. Due to the movement of the piston cylinder arrangement, the valve 30 is displaced so that the compressed air in the lefthand tower flows out into the open air. When the pressure in the lefthand tower has dropped to about 1 bar the pressostat B21 closes and the electric valve Y5 receives current, thus opening the valve 25 at the bottom of the lefthand tower 1, which is thus completely emptied. When the valve 25 has opened entirely a pneumatic limit sensing device SP3 is activated which allows oper-ating air to flow to the piston cylinder arrangement for the valve 11 adja-cent the fan 8. When in its open position the valve 11 actuates a limit position Sl and the changeover is complete.
The transfer of pressure from the righthand tower to the lefthand tower takes place in accordance with the above, but instead of cam III it is cam IV which takes part in the process, together with electric valves and ; control functions actuated thereby.
In brief, therefore, cooling is obtained using expanded compressed air via the valve 30; pressure equalisation via the valve 30 after the bottom valve 25, 25a and valve 11, lla have been closed and changeover after pressure - . . ..: ....

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equalisation by the use of plain valves 44, 45, 49, 50 connected together with a common control member so that a continuous through-flow of air through the driers is obtained.
As soon as the valves 44, 45, 49, 50 are switched, the common control member 40 actuates a pneumatic limit sensing valve SP2, SPl respectively, and control air flows to the piston cylinder arrangements 52, 53, respectively, which switches valve 30 so that compressed air from the drying tower previous-ly used for adsorption flows out slowly into the open through a silencer in valve 30. This prevents the formation of rapid air currents and this prevents any movement in the drying bed. Only when the pressure in the drying tower has dropped to about 1 bar over-pressure will the bottom valve in the drying tower open and when this has fully opened a pneumatic limit position sensing valve is actuated which in turn opens the large valve lla to the fan 8. The silencer used may suitably consist of a tubular extension of the exhaust valve in which a sound-dampening mass has been arranged, such as mineral fibres, steel wool, or sponge rubber. Practical experiments have shown that good sound reduction is achieved with a 1/2 " exhaust valve if a tubular extension about 30 cm long is used having an outer diameter of about 100 mm and an inner tube of sound-dampening material is inserted having a central through-flow channel with a diameter of 35 mm.
When the cam V reaches position 3, contactor K3 receives current and the fan 8 is started. The fan air pressure activates the pressostat B25 which supplies current to a relay K12. Current flowing via cam III, B 21 operating at 1 bar and Sl actuates contactor Kl and the elements in the left-hand tower are heated. The regeneration is thus initiated. Regeneration cannot therefore start before an electric limit sensor Sl, S2, respectively, associated with the means controlling the valves 11, lla has been actuated.
The regeneration temperature can be regulated in two ways - with .

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thermostats or by regulating the flow of air from the fan.
When the regulation is thermostatic, the thermostat Bl, B2 is placed in the circuit after Sl (S2) and disconnects contactor Kl (K2) when the regeneration temperature set is exceeded. When the temperature has drop-ped somewhat the contactor is connected again and thus keeps the regeneration temperature at the correct value.
When regulation by means of the fan air flow is used, a transducer is placed in each drying tower which, via a central control means U in the apparatus cabinet, actuates a motor Mll which in turn adjusts a damper 9 to give the correct quantity of fan air for providing the regeneration tempera-ture required.
If a fault should occur and the regenerating temperature rises about 30 C above the normal temperature, the over-heating protection means B3 (B4) disconnects, via a relay Kll, the operating current to the contactors for the heating means, and the lamp H in the apparatus cabinet lights up. In order to start the regeneration process again after a power cut or after over-heating, a re-setting key Sll must be pressed in by hand.
Between terminal blocks 101 and 102 is an output for alarm signals with a separate current source. The signal is emitted if there is a power cut, over-heating, if regeneration fails to take place or is incomplete or if there is a fault in the actual changeover process. An alarm signal for power cut and over-heating is obtained via a closing contact on relay Kll. If regeneration is incomplete or non-existent or if there is a fault in the changeover, an alarm signal is obtained via the thermostats B5 and B6 which sense the temperature in the lefthand or righthand tower, respectively.
B5 and B6 are set to switch at a temperature of about 25 C below the regeneration temperature set. When the cam VI (VII) at position 8 (38) connects the current to the relay K13, the regeneration temperature in the - ~ .

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, drying tower where the control thermostat is located is normally higher and B5 (B6) has consequently already disconnected the current so that the alarm signal will not be sent. If, on the other hand, there is a fault in the regeneration process or the changeover, the control thermostat will not dis-connect the current and the relay K13 will close its contact between the terminal blocks 101 and 102 and an alarm signal will be emitted.
During the regeneration process the drying agent i5 heated from above and moisture departs through the valves in the bottom of the tower.
The regeneration temperature may suitably lie between +130 - +200C. depending on the dew point desired for the subsequent adsorption.
After regeneration the heating elements and drying agent are cooled briefly by air from the fan. Due to the construction of the system the air provided by the fan will be heated during the cooling period by the heat distributor in the drying agent so that no adsorption of moisture from the fan air will occur.
Whilst the invention has been described with specific reference to compressed air it will be appreciated that the invention may also be applied to the drying of other gases or fluids. Also, whilst the use of two towers has been described it will be appreciated that three or more towers may be used successively.

Claims (16)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of continuously removing moisture from a fluid, said method comprising the steps of continuously supplying the fluid to an apparatus comprising at least two drying towers each containing a moisture absorbing reactivatable drying agent, causing the fluid to flow through at least one of the towers so that the fluid is dried and simultaneously sub-jecting the remaining tower or towers to a regeneration and cooling process comprising the steps of causing air to flow into the or each tower being regenerated, through a heating zone which is located within the or each tower being regenerated, and subsequently to flow through the drying agent in the or each tower being regenerated, the regenerated tower or towers being cooled with flow of cooling fluid, the pressure in the or each cooled tower finally being equalised with that of the tower or towers performing the dry-ing process, said method also comprising the step of changing the flow of fluid from the tower or towers effecting the drying process to the regenerated and cooled tower or towers, said change being effected without pressure surges, the original tower or towers effecting the drying process then being subjected to a corresponding regeneration and cooling process.

2. A method according to claim 1 wherein the air introduced into the or each tower being regenerated is an adjustable flow of air.

3. A method according to claim 1 wherein the air flowing into the or each tower being regenerated flows downwardly through the drying agent.

4. A method according to claim 1, 2 or 3 wherein the fluid to be dried flows upwardly through the drying agent in the or each tower perform-ing the drying process.

5. A method according to claim 1, 2 or 3 wherein after the flow of fluid has been changed from one or more towers originally performing the drying process to one or more towers that have been regenerated and cooled, the tower or towers originally performing the drying process being emptied through a valve provided with an exhaust silencer so that rapid air currents and movement in the drying bed are avoided.

6. A method according to claim 1, 2 or 3 wherein, during the cooling of the or each regenerated tower prior to changeover, expanded compressed air is used, a part of the cooling air being brought to pass through the or each tower and from there to a fan, and another part of the compressed air being brought to pass downwardly through valves so that the drying agent is cooled down at the point where the fluid to be dried will enter the drying agent so that full adsorption can occur immediately after changeover.

7. A method according to claim 1, 2 or 3 wherein said fluid is com-pressed air.

8. A method according to claim 1, 2 or 3 wherein only two drying towers are used, one tower being regenerated and cooled whilst the other effects the drying process, the changeover between the drying towers being performed by two valves which operate synchronously with each other, one being arranged in an inlet pipe to the said apparatus for the fluid to be dried, and the other being arranged in the outlet pipe from the said apparatus for the dried fluid, said valves being operated so that one is gradually opened to the same extent that the other is gradually closed.

9. An apparatus for continuously removing moisture from a fluid said apparatus comprising means for continuously supplying to the fluid to at least two drying towers each containing a moisture absorbing reactivatable drying agent, means to cause the fluid to flow through at least one of the towers so that the fluid is dried and means for simultaneously causing air to flow into the remaining tower or towers, there being heating means within each tower to heat such air before the air flows through the drying medium, means being provided for subsequently directing a flow of cooling fluid through each tower and means being provided to equalise the pressure of the or each regenerated and cooled tower with the or each tower performing the drying process, means also being provided for changing the flow of fluid from the tower or towers effecting the drying process to the regenerated and cooled towers without pressure surges, and means for leading away dried fluid from the apparatus.

10. An apparatus according to claim 9 comprising two towers, the arrangement being such that one tower will effect the drying process whilst the other is regenerated and cooled.

11. An apparatus according to claim 10 wherein synchronously operating valves are connected to the means for supplying fluid to the apparatus and means for leading dried fluid from the apparatus, the valve connected to the supplying means being operable to cause the fluid to flow selectively to one of a pair of inlets, each inlet being formed in the lower region of a respective tower, and the valve connected to the means for leading dried fluid from the apparatus being operable to cause fluid to flow selectively from one of a pair of outlets, each outlet being formed in a respective tower, the said valves being provided with a switching mechanism that is permanently joined to them.

12. An apparatus according to claim 11 wherein the said switching mechanism comprises a pneumatically operated cylinder with a displaceable piston rod connected to a lever, one end of the lever being connected to a bracket mounted on the housing of the valve connected to the means for leading fluid from the apparatus, the opposite end of the piston rod being joined, in a similar manner, to a bracket mounted on the valve housing of the valve con-nected to the supply means so that synchronised changeover of the valve functions in the valve is achieved when the lever is moved.

13. An apparatus according to claim 9 wherein supply pipes for regener-ating air lead into central pipes in the drying towers at a distance from the bottoms of the towers, these central pipes extending towards the top of the towers and being provided close to the top with perforations in the form of a filter.

14. An apparatus according to claim 13, wherein heating elements are arranged inside said central pipes.

15. An apparatus according to claim 13 or 14, wherein in that the inner wall of the central pipe is provided with helical members.

16. An apparatus according to claim 9, 10 or 11, comprising control members which, if a technical fault occurs in the equipment, result in a closed circuit at a common output to indicate an alarm.