Page 9 of catalogue Diaphragm Compressors of Andreas Hofer Hochdrucktechnik GmbH

Hofer Diaphragm compressors

page 9 of 20

Fig. 6: Compensating pump This is ensured by a compensation pump. This oil pump is directly driven by the crankshaft, and at every suction stroke of the piston, it is feeding a small quantity of oil into the chamber behind the diaphragm. In every case, the so injected oil quantity has to exceed the oil loss of the leakage, which means, that before the compression stroke, the diaphragm head is slightly overfilled. The excess oil has to be removed from the system now. For this purpose, an oil overflow valve is provided at the highest point of the oil chamber on top of the flange. This overflow valve feeds back the excess oil, pumped in by the compensation pump. When the compressor is operating with a constant or nearly constant discharge pressure, the opening pressure of this overflow valve is firmly adjusted by a pre-tightened spring (Fig. 7). In this case, the oil pressure has to be always approx. 10 % higher than the maximum allowable gas discharge pressure. If the application of the compressor foresees a variable discharge pressure, e.g. when filling pressure vessels, an overflow valve is being used, which is controlled by the gas pressure (Fig. 8). As a function of the varying discharge pressure, the valve spring is pre-tightened by a stem and thus the oil pressure is limited to approx. 10 % above the momentary gas pressure. This ensures, that the oil pressure is steadily adapted to the slowly increasing gas pressure; a measure which contributes to increase the life length of all components, since it avoids to overload the compressor by operating it over a longer period with a an unnecessarily high oil pressure. With the forward movement of the piston - already before reaching the front dead point - the diaphragm is pressed against the diaphragm cover; the oil quantity which is compressed with the remaining stroke of a few millimeters flows back through the oil overflow valve into the oil tank. Simultaneously, all air will be entrained, which has gathered at the highest point from degasing of the Hydraulic oil, and thus is reducing the efficiency of the compressor. The excess oil quantity flows into the surge oil tank, where it degases and from where also the compensation pump sucks in oil. This particular arrangement ensures, that the hydraulic system is always filled up to the optimum oil quantity and that the oil chamber is totally airbleeded.