Conference Year
2016
Keywords
Reciprocating compressor, reed valve, measurement, valve dynamics, FSI
Abstract
In principle, a reciprocating compressor stage consists of a cylinder, the working volume which varies periodically due to a piston moving inside a liner. Latter is sealed by two sets of valves. On the one hand the suction valve for admitting the gas to be compressed and on the other the discharge valve for allowing the high pressure gas to be delivered to the downstream process. The typical type of valves used within compressors for refrigeration plants are reed valves. The key feature of these mentioned valves - simultaneously affecting both their sealing performance as well as the reliability - is that they are not actuated. They are held close by elastic forces and they open and close automatically, in accordance to the balance of gas pressure forces as well as the previously mentioned elastic ones. The former is primarily characterized by the static pressure difference across the closed valve and the aerodynamic drag brought about by the gas flow impinging onto the sealing reed in a partially or fully open valve. Evidently, a high degree of coupling between the gas flow and the sealing element motion, today referred to as fluid-structure interaction (FSI), can be expected. The reed and the absence of a reliable guide that would provide for the plan-parallel motion give rise to non-parallel impacts with other parts of the valve assembly, leading to dynamic stress effects. This not only complicates the prediction of the reed valve dynamics, it also causes premature sealing element fracture, decreases the compressor efficiency and ultimately leads to unscheduled maintenance shutdowns. Consequently pursuing the optimization of the reed valve movement is a necessity. The measurements of reed valve dynamics as well as their movement in compressors can be performed by commercial proximity sensors, strain gauges or in house developed sensors. Although timing and lift information is available through these methods, other modes of fluttering such as torsional movement are more difficult to be observed. This work tries to address these aspects by providing real operation data obtained in a test rig. An in-depth comparative analysis of three measurement methodologies, represented by an optical, resistive as well as an eddy current sensor system will be carried out regarding their possible adoption in a state-of-the-art refrigeration compressor. Furthermore, the acquired data will be evaluated with respect to its potential application for collecting mentioned torsional movements. In the end a comparison against a numerical approach shall be conducted.