In positive displacement compressor, leakages of refrigerant can occur from the compression chamber and affect significantly its efficiency. In reciprocating compressors, thee leakages occurs through the piston-cylinder gap and through the valves. In general, the dimensions of the gaps through which the leakages occur are on the order of micrometer and in some conditions the flow becomes rarefied, that is, the mean free path of the gas molecules is of the same order of dimension of the gap, invalidating the description of the gas as a continuous medium. In this case, the classical Fluid Mechanics theory does not apply and the gas-surface interaction plays a determinant role on the flow, being commonly described via coefficients obtained experimentally and founded on the kinetic theory of gases. Therefore, to identify the coefficients that characterize the gas-surface interaction is fundamental for the correct prediction of leakages that occur under rarefied conditions.