AIR DRYER

Abstract: An air dryer (13) for a vehicle braking system (14) having an inlet (16) for connection to an intermittent source of pressure (11), an outlet (23) for connection to a vehicle braking system (14), two independent air drying chambers (19, 20) for connection between said inlet and outlet, and control means (17, 18, 39) operative to alternately connect one of said chambers between said inlet and outlet, and the other of said chambers to permit regeneration thereof. The control means include a timer (39) operative to alternate said chambers after a pre-determined period, operation of said timer being suspended when, in use, the source of pressure is ceased.

Description: AIR DRYER This invention relates to an air dryer, and particularly to a dual or twin air dryer of a vehicle braking system. Vehicles having air braking system, typically heavy commercial vehicles, buses and the like, require dry air for safe and effective working of the system components . Accordingly an air dryer is provided immediately downstream of the usual air compressor. A typical air dryer comprises a chamber of desiccant material through which air under pressure is passed to the braking system. Periodically the desiccant is dried by passing a quantity of dry air from the system through the desiccant to atmosphere. Necessarily, such drying or regeneration must be carried out when the compressor is off load. Such dry air for regeneration is supplied from the system volume, for example from an air pressure reservoir. In an installation where repeated use of the braking system occurs at short intervals, for example in the system of a city bus, there may be little time available for regeneration. To solve this problem a dual chamber or twin air dryer system may be provided to permit separate drying chambers to alternate the system being controlled by, for example, a timer. However, a number of factors influence the need for regeneration, including for example the volume of air used by the braking system and the relative humidity of the incoming air. One problem with a dual or twin air dryer is that the timer alternates the drying chambers irrespective of the compressor load phase. In these circumstances the compressor and one dryer chamber might become locked in phase, causing saturation of one dryer chamber, and little or no use of the other dryer chamber. To minimise the possibility of saturation, some existing systems will regenerate the desiccant of both chambers when the compressor is off load, irrespective of the preceding drying phase and/or time, thus an inefficient waste of dry air can occur. According to the invention there is provided an air dryer for a vehicle braking system, said dryer having an inlet for connection to an intermittent source of pressure, an outlet for connection to said braking system, two independent air drying chambers for connection between said inlet and outlet and control means operative to alternately connect one of said chambers between said inlet and outlet, and the other of said chambers to atmosphere to permit drying thereof, where in said control means include a timer operative to alternate said chambers after a pre-determined period, operation of said timer being suspended when, in use the source of pressure is ceased. Such an arrangement permits effective use of each drying chamber, and effective drying of the inactive air dryer. Preferably the source of pressure is an engine driven compressor which is brought on and off load by a governor signal, the absence of which activates said timer, preferably via an electronic controller. Since drying is only required when the compressor is on load, the air for regeneration can be supplied directly from the compressor rather than from the system volume. In this way unnecessary depletion of system volume is avoided, and the compressor is not required to work when the braking system is not functioning, for example during continuous motoring on a highway, or when stationary with the engine running. In a preferred embodiment the governor signal is a fluid pressure signal, and the timer is inactivated by a piston operated actuator directly responsive to said signal. Preferably the actuator is resiliently biased to a condition in which the timer is active. Preferably the dryer includes electrically actuable pilot valves responsive to said timer to connect air under pressure downstream of said dryers to respective control valves operative to connect the source to a respective air drying chamber. In the preferred embodiment said control valves are resiliently biased to a condition whereby the respective air drying chambers are connected to the pressure source. Preferably the control valves are also operative to connect a respective drying chamber to atmosphere. In the preferred embodiment the downstream sides of said drying chambers are connected via a fluid restrictor. Preferably the downstream sides of the drying chambers are connected to the outlet via respective non-return valves. In an alternative embodiment the air dryer control means may be operable so as to connect both air drying chambers in parallel between the inlet and outlet. Such a mode of operation would be particularly useful where a large volume of dry air is needed at short notice. The control means could be configured to prevent both drying chambers from becoming saturated and thus enable the air dryer to revert to the alternating operating cycle. Other features of the invention will be apparent from the following description of a preferred embodiment shown by way of example only in the accompanying drawing which illustrates, somewhat schematically, part of a vehicle air braking system. The drawing illustrates an engine drive compressor 11 which receives atmospheric air represented by arrow 12, and delivers air under pressure to an air dryer 13 for use in a vehicle air braking system 14. A governor pressure signal 15, to be described later, brings the compressor on load when required. The air dryer 13 includes an inlet 16 from which air flows via respective control valves 17, 18 to drying chambers 19, 20 and thence via non-return valves 21. 22 to an outlet 23. The air dryers are connected immediately downstream thereof via a fluid restrictor 24. The control valves 17,18 are identical, and each has two flow conditions. The first condition is represented by valve 17, in which the air dryer 19 is connected to atmosphere, and the connection from inlet 16 is closed. The second condition is represented by valve 18, in which the inlet 16 in connected to the air dryer 20. The control valves 17,18 are biased to the second condition by respective springs 25, and are movable to the first condition by the presence or absence of fluid pressure acting in opposition to the spring via a respective pilot valve 26, 27. The pilot valves 26,27 control the admission of air under pressure to the control valves 17,18, and are supplied with air under pressure from a connection downstream of a respective non-return valve 21,22. The pilot valves 26,27 are identical and are movable between two conditions by a respective electrical solenoid 28. In the open condition, represented by valve 26, air under pressure is supplied to control valve 17, thus urging it against spring 25 to connect chamber 19 to atmosphere. In the closed condition, represented by pilot valve 27, the supply of air under pressure is closed off, and the signal line to control valve 18 is connected to exhaust. A governor pressure line 31 supplies air under pressure to a governor valve 32. The line 31 is shown connected to the outlet 23, but any convenient tapping of system pressure is suitable. The governor valve 32 determines whether the compressor is placed on load, and has two positions. As illustrated, the valve is biased to the right by return spring 33 to close off the supply of air under pressure from port 23, and to exhaust the governor pressure signal 15. In this condition the compressor is on load, and supplying air under pressure via drying chamber 20. When system demand is satisfied, for example by one or more pressure switches indicating that pressure in system 14 is at the desired level, the governor valve is shifted to the left, as viewed, by means not shown to connect the pressure line 31 to the compressor, thus giving a positive pressure signal 15 and bringing the compressor off load. The governor valve may for example by shifted by the action of system pressure in opposition to the spring 33, and a suitable snap-action spool valve. A timer actuator 34 comprises a piston 35 movable against a return spring 36 by the presence of pressure signal 15. An electrical power supply 37 provides power via a switch 38 to a timer 39, the output of which is connected to one of the solenoids 28 via a respective energisation line 40,41. As illustrated the switch 38 is closed by piston 35 under the action of return spring 36, the absence of a governor pressure signal 15 indicating that the compressor is on load due to system demand. In this condition the timer is running, and as illustrated drying chamber 19 is being regenerated by restricted air flow from downstream of drying chamber 20. After a predetermined period, with the compressor on load, the timer will cause pilot valve 26 to move to the closed condition, and pilot valve 27 to move to the open condition. As a consequence control valves 17,18 will switch over and air will be dried by drying chamber 19, thus permitting drying chamber 20 to regenerate. If the compressor is placed off load, by the presence of governor pressure signal 15, the piston 35 will move leftwards as viewed to open switch 38 and thereby suspend operation of the timer 39. In this way, the possibility of in phase locking of the compressor and one drying chamber is minimised, and the saturation of one air dryer chamber is thus avoided. The diagrammatic representation illustrated is schematic, and elements thereof can be combined in any suitable manner to provide a compact air dryer installation. In particular common components associated with each drying chamber can be incorporated in a mounting for a removable desiccant canister. The canister mountings are preferably identical save that formation to contain the governor valve and/or timer mechanism may be unused in one of the mountings. Alternatively, the control device(s) may be housed separately.

Claims: CLAIMS : 1. An air dryer for a vehicle braking system, said dryer having an inlet for connection to an intermittent source of pressure, an outlet for connection to a braking system, two independent air drying chambers for connection between said inlet and outlet and control means operative to alternately connect one of said chambers between said inlet and outlet, and the other of said chambers to atmosphere to permit drying thereof, wherein said control means include a timer operative to alternate said chambers after a pre-determined period, said timer being inactive when, in use, the source of pressure is ceased. 2. An air dryer according to claim 1 wherein the source of pressure is an engine driven compressor adapted to be brought on and off load by a governor signal, the absence of said governor signal activating said timer. 3. An air dryer according to claim 2 and further including a piston operated actuator responsive to said governor signal to inactivate said timer. 4. An air dryer according to claim 3 wherein said actuator is resiliently biased to a condition in which said timer is activated. 5. An air dryer according to any preceding claim and further including electrically actuable pilot valves and control valves, said pilot valves being responsive to said timer to connect air under pressure downstream of said dryers to respective control valves operative to connect the source to a respective air drying chamber. 6. An air dryer according to claim 5 wherein said control valves are resiliently biased to a condition in which the respective air drying chambers are connected to the pressure source. 7. An air dryer according to claim 6 wherein said control valves are operative to connect a respective drying chamber to atmosphere . 8. An air dryer according to any preceding claim wherein the downstream sides of said drying chambers are connected via a fluid restπctor. 9. An air dryer according to any preceding claim wherein the downstream sides of the drying chambers are connected to the outlet via respective non-return valves. 10. An air dryer substantially as described herein with reference to the accompanying drawings.

Inventors: TOWNSEND DAVID HUGH, TOWNSEND, DAVID, HUGH
Applicants: TOWNSEND DAVID HUGH, WABCO AUTOMOTIVE UK, TOWNSEND, DAVID, HUGH, WABCO AUTOMOTIVE U.K. LIMITED
Application year: 1999
Main classification: B60T17
IPC codes: B01D53, B60T17