![]() The enthalpy and entropy are calculated using constant values of the specific heat for a given gas or gas mixture and the temperature differential from suction to discharge of the compressor.Īs the gas behavior becomes more complex, the perfect gas model needs to be modified with the addition of the specific heats becoming a func-tion of the temperature. The simplest, a perfect gas, requires only the pressure, temperature and gas molecular weight to obtain the specific volume, or its reciprocal the gas density, and the polytropic work. Thus, the ratio of the polytropic work to the total enthalpy increase of the gas defines the polytropic efficiency of the compression process.Īll thermodynamic properties needed to calculate compressor performance can be derived from three progressively complex thermodynamic models. The combined useful and lost work result in an increase in the total energy of the gas, also referred to as the increase in the thermodynamic property of enthalpy. Lost work manifests itself as an increase in the thermodynamic property of entropy of the gas or gas mixture and an additional increase in temperature above what would be expected for only the useful work also known as an isentropic process. However, the polytropic work (useful work) does not represent the total amount of work required by the compression process.Īn additional amount of work known as the lost work is consumed to raise the gas from suction to discharge pressure. These properties are all available from some equations of state for the gas. ![]() The average polytropic exponent, n, included in these relations is also derived from these parameters. This equation shows that the polytropic work (Wp) is only dependent upon the compressor suction and discharge values of pressure (P), temperature (T), specific volume (v), and the gas molecular weight (MW). It is expressed by the following equation based upon the work of Schultz and the definition of a polytropic process for an ideal gas: The polytropic work (or poly-tropic head) represents the useful work that is applied to the gas to raise its pressure from suction to discharge conditions. Industry standards, ASME PTC 10 and the ISO equivalent, ISO 5389, define some fundamental compressor performance measures that are universally accepted to describe their thermodynamic behavior. These include uncertainties associated with measured parameters of pressures and temperatures, composition of gas mixtures, and the calculated thermodynamic properties that are derived from these parameter measurements. ![]() The accurate determination of this performance is dependent upon a number of factors. Knowledge of centrifugal compressor thermodynamic performance is critical to initial acceptance and, once installed, periodic or continuous health monitoring to ensure that the operation is consistent with the original design expectations. Impact of Thermodynamic Property Accuracy on Compressor Performance
0 Comments
Leave a Reply. |