Abstract:
In order to maintain high efficiency close, to isothermy despite high frequencies in a pneumo-hydraulic converter with reciprocating pistons, pipe cluster-heat exchange pipes (38) are provided in the gas working chambers of the converter and the exchange fluid in the pipes is kept at approximately ambient temperature. For this the gas working chambers must be arranged axially next to one another and, in order to eliminate dead space, connected in pairs by conical exchange valves (12a/12b) which take in the entire wall thickness of the valve flange (5a/5b) dividing the air chambers.

Description:
BACKGROUND OF THE INVENTION 
     A pneumo-hydraulic converter with reciprocating double piston which connects a compressed air storage and a hydraulic circuit at maximum efficiency in such a way that energy can flow into the storage (charging) or can be removed from the storage (discharging) is known. 
     The good efficiency of isothermal processes is obtained in the above system by stabilizing the temperature in the working chambers (piston spaces) during each stroke by means of the operating medium, i.e., oil. This will result in relatively slow working processes, since the limited velocity of the heat transfer from the lateral surface of the cylinder to the air during the working stroke cannot compensate the temperature fluctuations at increased cycle frequency. As a consequence, the structual units employed are comparatively large in relation to the power involved. 
     It is the object of this invention to achieve good efficiency while increasing the cycle frequency at the same time. 
     SUMMARY OF THE INVENTION 
     According to the invention tubular heat exchangers pass through some of the working chambers of the converter and an exterior circuit maintains the exchange fluid approximately at ambient temperature. 
     This heat exchanger may either be carried along by the set of reciprocating pistons, or remain stationary. Since the heat exchanger moving along with the pistons will require fewer sliding sealings (approximately by one third), and the bundle of tubes will considerably increase the buckling and deflection strength of the piston set, the present description will be restricted to presenting the converter with movable heat exchanger. To achieve the desired increase in cycle frequency, an arrangement of working chambers is called for which involves a dramatic reduction of dead volumes and will hence generate high buckling forces. As a consequence, buckling strength will become an extremely important structural factor which must also be allowed for when deciding on the arrangement of the valves. 
     As the converter is designed to operate as both compressor and decompressor, the valve sets on each side--each consisting of high-pressure valve, exchange valve, low-pressure valve--must be subject to forced control; under certain conditions it is possible to pair off the movements of exchange valve and low-pressure valve. The configuration of these valves must fulfill the topological requirements of the heat exchanger as well as the strict demand for the smallest possible dead volumes. The solution of these tasks and the operation of the device proposed by this invention will now be explained by means of the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal section through the axis of the four cylindrical working chambers, 
     FIG. 2 is a section transversely to the axis in FIG. 1, through the high-pressure chamber and through the tube bundle of the heat exchanger, 
     FIG. 3 illustrates the same section as FIG. 2, though with a bridge across the tubes of the bundle. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In its high-pressure variant the converter includes three coaxial and approximately equal lengths of cylindrical pipe: the pre-pressure pipe 1 and the high pressure chamber pipes 3a, 3b, the pre-pressure pipe 1 containing the pre-pressure piston 2 and having a significantly larger diameter than the two high-pressure chamber pipes 3a, 3b which are symmetrically arranged vis-a-vis the pre-pressure pipe 1 and contain the equally symmetrical high-pressure pistons 4a, 4b. Since both movable and stationary parts are mirror-symmetrical relative to the longitudinal centre plane, the pre-pressure pipe 1 is connected via valve flanges 5a, 5b to the two screwed-in high-pressure chamber pipes 3a, 3b, which are closed off on the other ends by fitting covers 7a, 7b fastened by screw caps 6a, 6b. Axially sliding in the cylindrical pipes are a set of three pistons which are rigidly connected by the tubular rod 8 and will thus define 2×3 working chambers, i.e., oil chambers 9a, 9b between covers 7a, 7b and high-pressure pistons 4a, 4b; air high-pressure chambers 10a, 10b between high-pressure pistons 4a, 4b and valve flanges 5a, 5b; and air pre-pressure chambers 11a, 11b between valve flanges 5a, 5b and pre-pressure piston 2. The air high-pressure chambers 10a, 10b are connected to the air pre-pressure chambers 11a, 11b via the exchange valves 12a, 12b; communication between the pre-pressure chambers 11a, 11b and the exterior is established via the low-pressure valves 13a, 13b; air from the air storage 14 is admitted into the air high-pressure chambers 10a, 10b via the high-pressure valves 15a, 15b, which are supplied from the air storage 14 via feed lines 16a, 16b and fittings 17a, 17b. 
     One variant of hydraulic pilot control is shown employing the high-pressure valves 15a, 15b in FIG. 1, where the pressure chambers 18a, 18b are either depressured or pressured by electric two-way pilot valves 20a, 20b connected to a pressure source 19, such that the valve pistons 21a, 21b are set into motion, which are connected to the high-pressure valves 15a, 15b via rods 22a, 22b and nuts 23a, 23b. Similar devices may be provided for the exchange valves 12a, 12b and the low-pressure valves 13a, 13b, whose actuating rods 24a, 24b and 25a, 25b are shown only. 
     For better understanding of the functional principle of the converter, a possible working environment for the converter is included in FIG. 1, beginning at the oil fittings 26a, 26b, with feed lines 27a, 27b leading to a four-way valve 28 acting on a variable hydrostatic unit 29 with flywheel 30 and electromotor/generator 31. The exchange circuit begins at the feed pump 32, which delivers the exchange fluid through the external exchanger 33 via fitting 34b in cover 7b and via feeder pipe 35b to the tubular rod 8. As the tubular rod 8 is stopped by a conical plug 36 in the plane of the pre-pressure piston 2, the exchange fluid is pushed back through the annular space between feeder pipe 35b and tubular rod 8 towards the high-pressure piston 4b, where the fluid is delivered to the bundle of heat exchange pipes 38 (and thus to the piston 4a itself) via radial bores 37b, and where the tubular rod 8 is reached in turn via radial bores 37a; the loop back to the feed pump 32 is closed via feeder pipe 35a and fitting 34a in cover 7a. 
     Like the high-pressure piston sliding sealings 39a, 39b and the exchange valve sliding sealings 40a, 40b, the exchanger sealings 41a, 41b and 42a, 42b are subject to the full pressure difference throughout the entire period of piston movement. This is the actual technological challenge of the design, in particular if the configuration of the tube bundle includes a bridge 43 as shown in FIG. 3, in order to increase buckling strength and improve heat transfer. It is only the sliding sealing 44 of the pre-pressure piston 2 that is not exposed to the high pressures, as it is only subject to the pre-pressure. The remaining sealings, which are not referred to in detail, are only subject to static pressures or short-stroke movements. 
     The functional principle of the converter will now be discussed with reference to a decompression (discharge) cycle corresponding to the position of valves shown here, where the pistons move towards the right: at the moment shown in the drawing the air high-pressure chamber 10b is directly connected to the air storage 14 through the open air high-pressure valve 15b. The pressure force acts on the oil chamber 9b and is transmitted via the oil column in line 27b and the four-way valve 28 to the pressure side of the hydrostatic unit 29 acting as a motor, which in turn will actuate the flywheel 30 and the generator 31. Moreover, due to this movement to the right decompressed air in chamber 11b is pushed out into the open by the pre-pressure piston 2 through the open low-pressure valve 13b; at the same time the air from the previous movement which has remained under pre-pressure in the high-pressure chamber 10a, will assume discharge pressure via the open exchange valve 12a due to the expanding pre-pressure chamber 11a. By the same movement the oil emerging from the hydrostatic unit is forced into the oil chamber 9a. The force picked up by the cushion in the oil chamber 9b is thus generated not only by the exposure to high pressure in the air high-pressure chamber 10b, but also by the thrust produced by the pre-pressure at the large surface of the pre-pressure piston 2, which is transmitted via the tubular rod 8 and pipes 38 of the tube bundle. This is the very site where the danger of buckling is encountered. At a certain moment of this movement to the right, which is to be determined by computer, the high-pressure valve 15b must be closed, for the decompression of the thus defined volume to yield at the end of the stroke precisely that pre-pressure which will produce the discharge pressure due to expansion after the beginning of reverse movement, by pushing the volume of the air high-pressure chamber 10b into the pre-pressure chamber 11b. At the beginning of the reverse movement, 15a, 13a and 12b must be opened and 12a and 13b must be closed simultaneously with the switchover of 28 (13b being forced into closing position by the oncoming pre-pressure piston 2). The switchover may be initiated by a proximity switch. 
     It should be emphasized here that the specific topological configuration is part of the invention and is particularly well suited for the repetitive thermodynamic process described; the special arrangement of pressure chambers and exchanger will permit the shuttle valve design avoiding dead volumes, which is essential to the principle of maximum efficiency conversion. 
     It should be pointed out finally that the pressure of the oil penetrating from the converter during each stroke is subject to variations at a ratio of about 1:30 (at 200 bar in the air storage 40), which will be an obstacle to the direct use of the converter in many applications, as the hydrostatic units have a displacement volume control range of 1:10 at most. If the converter is to operate at constant power the addition of a flywheel is recommended, which can bridge a wide range of cycle frequencies; the hydrostatic unit would only have to follow effective changes in load in that case. 
     If the converter is employed exclusively as a compressor, the forced control of the valves may be omitted, but the four-way switchover valve 28 must be synchronized with the stroke of the converter, either automatically (by the pressure peak at the stop) or by means of a proximity switch. In the instance of simple compression tasks (e.g., for cooling circuits) the compressor need not include a pre-pressure cylinder; the tubular heat exchanger may be chosen to be either stationary or movable in this case, as no buckling forces will arise.