A retarding valve at a single-tower air drier

Abstract: A retarding valve at a single-tower air drier for obtaining a controlled time for the flow of regenerating air from an air tank to a desiccant cartridge has a movable valve stem (7), which is provided with a piston (14) having a slightly smaller diameter than a chamber (13), in which it is movable in hydraulic oil (15), so that a narrow circumferential slit for the oil is formed. The valve stem with its piston is biassed by a spring (16) from a position - at the beginning of each regenerating cycle - permitting flow through the valve to a position closing off flow through the valve.

Description: This invention relates to a retarding valve at a single-tower air drier for obtaining a controlled time for the flow of regenerating air from an air tank to a desiccant cartridge, wherein a valve stem, movable by a spring from a position - at the beginning of each regenerating cycle - permitting air flow through the valve to a position closing off air flow through the valve, is provided with a piston having a slightly smaller diameter than a chamber, in which it is movable in hydraulic oil, so that a narrow circumferential slit for the oil is formed. It is well known that the desiccant in an air drier, preferably for use on a vehicle, is regenerated by dry air that is passed backwards through the desiccant and out in the atmosphere thereby removing the moisture gathered by the desiccant, which thereafter will be ready for drying more air coming from the compressor in the system. The regenerating occurs when the compressor has been shut off because no more air is needed in the system. In a single-tower air drier system there are different principles to govern the regeneration. It is presently preferred to regenerate during a certain time, say 20-30 seconds, after each drying sequence. This controlled time for passing dry air from the air tank through a retarding valve (by-passing a check valve) to the desiccant cartridge may be obtained in different ways. One method of governing the regeneration time is disclosed in DE-A-3 139 682, where the opening of the valve is electrically controlled by a timer or the like. Another method is disclosed in EP-A-36 569, where a completely pneumatic retarding is obtained with a relatively complex and sensitive system having a spring biassed piston, chambers and orifices. A retarding device operating according to the principle with a piston movable in hydraulic oil is known through DE-A-39 12 969. The general object of the present invention is to obtain a retarding valve, which gives a certain regeneration time with satisfactory accuracy in a simple and reliable way without necessitating any electrical means. Based on the principle with a piston movable in hydraulic oil, the specific object is to obtain a retarding valve with a regenerating time which is relatively independent of the temperature. This is according to the invention attained in that the hydraulic is silicon oil with high viscosity, i.e. in the order of 30 000 centistoke (mm²/s), and in that the piston is made of a material with a higher coefficient of thermal expansion than the housing forming the chamber. An important advantage with silicon oil is that its viscosity is relatively independent of the temperature. However, in order to compensate also for the still existing change of viscosity with temperature the piston is made of a material with higher coefficient of thermal expansion, for example acetal plastic, than the housing forming the chamber, which may be made of aluminium. By these means it is possible to obtain a regeneration time which is relatively independent of the temperature. It is even possible to obtain a somewhat longer regeneration time at higher temperature, which may be desirable. The invention will be described in further detail below reference being made to the accompanying drawing, in which Fig 1 is an enlarged view, mostly in cross-section, of a valve according to the invention, shown in two operational positions, and Fig 2 is a diagram of an air drier arrangement including a valve according to the invention. A retarding valve 1 according to the invention for a single-tower air drier has a housing 2 with an inlet port 3 from an air tank 4 on a vehicle and an outlet port 5 to a desiccant cartridge 6 to be regenerated by air from the air tank. A valve stem 7 is axially movable in a bore 8 in the housing 2, and its upper part is sealed by means of a O-ring 9 arranged in the housing. The upper end of the valve stem 7 is provided with a rubber cap 10 for sealing contact with a valve seat 11 around a restricted bore 12 from the inlet port 3. Below the O-ring 9 the bore 8 widens into a chamber 13. At its lower end the valve stem 7 is provided with a piston 14. The outer diameter of the piston 14 is only slightly less than the inner diameter of the chamber 13, so that a narrow circumferential slit is left. The lower part of the chamber 13 around the piston 14 is filled with silicon oil 15. The valve stem 7 is biassed upwards to contact with the valve seat 11 by a spring 16. There is a sealing rubber boot 17 above the level of the silicon oil 15 between the housing 2 and the valve stem 7. From the rest position of the valve stem 7 shown to the left in Fig 1 with the rubber cap 10 in contact with the valve seat 11, a certain pressure in the outlet port 5 presses the valve stem down to the position shown to the right in Fig 1 against the force of the spring 16 and under transfer of the silicon oil 15 past the piston 14. This "loading" of the retarding valve 1 occurs during the operational stage of the system when air from a compressor 18 is supplied to the air tank 4 through the desiccant cartridge 6 of the air drier. When the regeneration of the desiccant is to start, the valve stem 7 is in its bottom position shown to the right in Fig 1. Dry air from the air tank 4 flows through the inlet port 3, through the restricted bore 12, which has a diameter of for example 1.3 mm in order to control the air flow through the desiccant for obtaining optimal regeneration, and out through the outlet port 5. The normal conduit between the cartridge 6 and the air tank 4 is closed in this flow direction by a check valve 19. It may be desired to have a regenerating time of for example 20-30 seconds, i.e. the valve stem 7 shall get in closing contact with the valve seat 11 after the desired time under the action of the spring 16 and controlled by the transfer of silicon oil 15 past the piston 14. However, in order to compensate for the lower viscosity of the silicon oil at rising temperature the housing 2 is made of a material with a lower coefficient of thermal expansion than the piston 14, so that at rising temperature the slit between the housing and the piston becomes narrower. In this way it is possible to make the regeneration time virtually independent of the temperature or even to increase the time somewhat at higher temperatures, which may be desirable. The housing 2 may for example be made of aluminium and the piston 14 of acetal plastic; other material combinations are also feasible. The retarding valve according to the invention may be built into the air drier itself or be provided as a separate unit. It has to be mounted in the shown upright position.

Claims: A retarding valve at a single-tower air drier for obtaining a controlled time for the flow of regenerating air from an air tank (4) to a dessicant cartridge (6), wherein a valve stem (7), movable by a spring (16) from a position - at the beginning of each regenerating cycle - permitting air flow through the valve to a position closing off air flow through the valve, is provided with a piston (14) having a slightly smaller diameter than a chamber (13), in which it is movable in hydraulic oil (15), so that a narrow circumferential slit for the oil is formed, characterized in that the hydraulic oil (15) is silicon oil with high viscosity, i.e. in the order of 30 000 centistoke (mm²/s), and in that the piston (14) is made of a material with a higher coefficient of thermal expansion than the housing (2) forming the chamber (13). A valve according to claim 1, character ized in that the piston (14) is made of acetal plastic and the housing (2) of aluminium.

Inventors: LARSSON SVEN-OLOF, LUNDSTROEM BO, SEVERINSSON LARS MATTIS, LARSSON, SVEN-OLOF, LUNDSTROEM, BO, SEVERINSSON, LARS MATTIS, Lundström, Bo
Applicants: HALDEX AB
Application year: 1991
Main classification: B01D53
IPC codes: B60T17, B01D53