First Law Analysis of a Compressar Using a Computer Simulation Model

To perform a second law analysis for a compressor, basic thermodynamic variables must be known. This data set can be generated by an analysis requiring use of the first law. This aspect is the focus of the paper. In developing a computer simulation model considerable attention has been given to the proper evaluation of cylinder thermodynamics and cyclic heat transfer. This model compares well with the polytropic process model and with a benchmark simulation except for the cylinder temperature-time history. This is because the instantaneous temperature prediction strongly depends on mass flow and heat transfer rates. INTRODUCTION For many industrially important machines, including reciprocating compressors which are the focus of the present paper, design modifications leading to more efficient operation can be inferred by consideration of a rank-ordered list of availability (exergy) losses and destructions experienced over typical operating cycles. The development of such a list is the result of a second law analysis. second law analyses of machines modeled as control volumes reported in the literature are generally for operation at steady-state. A steady-state analysis is not appropriate for reciprocating compressors and this introduces some computational and theoretical complexities due to the often strongly time dependent heat transfer and mass flow rates. Moreover, to perform second law analyses requires that thermodynamic variables such as pressure, temperature, enthalpy and entropy be known at various points of the operating cycle. Accordingly, the objective of the current phase of the research project being reported upon here is not on the ultimate goal of second law analysis but rather on the intermediate, but essential, step of developing adequate means for accurately predicting, via conservation of mass, momentum and energy, the needed operating variables. This paper presents a progress report of the current phase of the project, including typical results and a brief outline of future directions. 577 SIMULATION MODELS First Law Analysis For thermodynamic analysis of the cylinder we choose the control volume as shown in Figure 1. We assume the following: (i) uniform gas properties throughout the cylinder, (ii) ideal gas (pv = RT) with constant specific heats, (iii) no leakage past the piston, (iv) uniform running speed (8 = constant), (v) negligible oil and friction effects, (vi) negligible kinetic and potential energies associated with mass flux, and (vii) no heat exchange between the cylinder and suction (or discharge) gas. Using ideal gas property relations, the first law of thermody~amics yields the following equation: m c t + c m T = Q p V + m c T mdc Td c v c v c c c c s p s p ( 1 )