Development of a Lumped-Parameter Model for Hermetic Reciprocating Compressor with Thermal-Electrical Coupling

The design of high-efficiency reciprocating compressors requires good understanding of interactions between different phenomena inside the compressor. This paper describes a comprehensive model to predict the performance of reciprocating compressors with thermal-electrical coupling. The simulation of the compression cycle is based on an integral control volume formulation for mass and energy conservation. The thermal model follows steady state thermal energy balances applied to the compressor components by using global thermal conductances. Finally, the equivalent circuit method is employed to simulate a steady-state model of single-phase induction motor. The motor losses are used as heat generation in the energy equation of the thermal model, which in turn provides the motor temperature required to evaluate the windings resistances. Predictions are compared to experimental data under different operating conditions and reasonable agreement is observed

A NTU-Based Model to Estimate Suction Superheating In Scroll Compressors

Suction superheating plays a major role in determining the efficiency degradation of hermetic scroll compressors. Current models to predict superheating are usually experimentally calibrated and therefore can only be applied to existing compressors. This paper presents a thermal model to estimate suction superheating in scroll compressors, based on the NTU method for heat exchangers design. The model considers an isothermal surface exchanging heat with the gas in the suction path and in the discharge plenum. Compared to other models, the new approach described herein has the advantage of not requiring any experimental input data. The thermal model is coupled to a thermodynamic model and applied to evaluate the performance of a scroll compressor. The model was capable to predict the suction gas temperature in good agreement with experimental data, making it particularly useful for compressor design

Effects of Gas Compressibility and Piston Secondary Motion on Leakage in the Piston-Cylinder Clearance of Reciprocating Compressors

Leakage can significantly affect the performance of low-capacity reciprocating compressors, reducing the mass flow rate and increasing energy consumption. In reciprocating compressors, leakage takes place mainly in the piston-cylinder clearance and is brought about by the piston motion and pressure difference between the compression chamber and the shell internal environment. This paper reports a numerical analysis of leakage in the piston-cylinder clearance of a low-capacity reciprocating compressors based on the Reynolds equation for compressible fluid flow. A simulation model is developed and applied throughout the compression cycle to assess the effect of the clearance geometry, piston velocity and piston secondary motion on the leakage and compressor performance. A simplified version of the model considering the piston concentric in the cylinder is also adopted to assess the effect of the piston secondary motion on leakage. The results show that the compressibility effects are significant and have to be considered in the analysis and that the piston secondary motion can increase gas leakage by 90%

Modeling of Rolling-Piston Compressors with Special Attention to the Suction and Discharge Processes

The present paper describes a simulation model developed to predict the performance of rolling-piston compressors with special attention to the suction and discharge processes. The relevant input data required by the model, such as clearances between moving parts, valve stiffness and natural frequency and electric motor efficiency, were obtained experimentally. Correlations for effective flow and force areas associated with the suction and discharge processes were derived from flow simulations. It was found that the position of the rolling piston in relation to the suction and discharge ports must be included to fully characterize the effective flow and force areas. Numerical predictions of the thermodynamic inefficiencies associated with a R22 rolling-piston compressor were compared with measurements and good agreement was found at different operating conditions

A Heat Transfer Correlation for the Suction and Compression Chambers of Scroll Compressors

Heat transfer in the suction and compression chambers of scroll compressors has not been sufficiently studied and typical correlations available in the literature are based on simplified flow conditions. This paper presents the results of a model developed to predict fluid flow and heat transfer inside the suction and compression chambers of scroll compressors. Due to the particular geometry of scroll compressors, an algorithm was developed to automatically adapt the computational mesh for each orbiting angle. Convective heat transfer is strongly affected by the flow in the near- wall region and for this reason a low Reynolds number turbulence model was adopted in the simulations. The study covered a wide range of operating conditions and geometric parameters, allowing the proposal of a new heat transfer correlation for scroll compressors that is compared with other correlations available in the literature.

A Calibration Procedure for Compressor Simulation Models using Evolutionary Algorithm

Comprehensive models are widely adopted to predict the performance of compressors due to their low computational cost and acceptable prediction capability. In general, the accuracy of such models depends strongly on the correct adjustment of some parameters that are of difficult to determine both analytically and experimentally. However, due to the nonlinearities of the compressor model, the tuning of such parameters affects many output variables and hence can be very challenging and time consuming. In this paper, we consider this procedure of adjustment of parameters as a multiple objective optimization problem that can be solved by using an elitist non-dominated sorting genetic algorithm (NSGA-II). The parameters of two simulation models are adjusted following this new procedure. In the first model the suction muffler was neglected and a mass-spring-damper system was adopted to predict the suction valve dynamics. The second model solves the suction valve dynamics by the finite element method and the flow in the suction muffler with the finite volume method. In both models the clearance volume and parameters associated with the suction valve were chosen to be adjusted while the deviations between predictions and measurements for the mass flow rate, indicated power, and suction valve dynamics were defined as the objective functions to be minimized

A Neural Network to Predict the Temperature Distribution in Hermetic Refrigeration Compressors

The understanding of heat transfer interactions in refrigeration compressors is of fundamental importance to characterize their overall performance. Certain temperatures, such as those of the motor, oil, shell, and at suction and discharge chambers, have strong influence on the compressor electrical consumption and reliability. Experimental and numerical approaches have been successfully employed to characterize the thermal profile of compressors under different operating conditions. This paper presents a multi-layered feed-forward neural network developed to predict the main temperatures of a hermetic reciprocating compressor. Such a model can be used for different compressor layouts without major modifications, being a fast method for estimating temperatures without the solution of the compression cycle. Predictions of the neural network were compared with experimental data and numerical results from comprehensive thermodynamic simulations, and good agreement was observed in a wide range of evaporating and condensing temperatures. The neural network was found to predict the temperature distribution with sufficient accuracy for compressor analysis and development

Experimental Analysis of Refrigerant Flow in Small Clearances

An important source of thermodynamic and volumetric inefficiency in compressors is the leakage of gas through clearances. For instance, in reciprocating compressors this leakage may occur through the piston-cylinder gap or between the valve and its seat. Due to the fact that for some specific situations the characteristic length of these clearances can be of the same order of magnitude as the gas molecular mean free path, gas rarefaction must be taken into account in the modeling of the fluid flow. Under these conditions, the conventional continuum approach is not appropriate in order to predict the flow and alternative formulations on the basis of the kinetic theory of gases must be sought. Typical examples of non-equilibrium phenomena which cannot be predicted by the classical Navier-Stokes equations are slip, thermal transpiration and temperature jump in the proximities of the solid boundaries containing the fluid. In this work we propose an original compact experimental device that allows the characterization of gas flows in micrometric clearances. Through the aid of this device coefficients, such as the viscous slip coefficient to be introduced in the modified boundary conditions of the Navier-Stokes equations for slightly rarefied, will be obtained. These coefficients are necessary in order to model rarefied gas flows, and they can be as important as the coefficient of viscosity and thermal conductivity. Furthermore, the experimental results obtained will help us to validate a numerical model that predicts gas-leakages in compressors that was developed in-situ

A Combined Experimental-Numerical Procedure to Estimate Leakage Gap of Compressor Valves

Leakage through valves can significantly reduce the volumetric and isentropic efficiencies of compressors. Despite its importance for compressor design, the dimensional characterization of leakage gaps is not a trivial task. In this paper, we present a combined experimental-numerical method to estimate leakage gap of compressor valves. Measurements of leakage were carried out via the constant volume method, which is widely employed in the analysis of gas flow through microchannels. Additionally, numerical predictions were obtained with a one-dimensional flow model, taking into account viscous friction, slip at the walls, and gas compressibility. The leakage gap in the simulation model is adjusted so that predictions match the experimental results of leakage for different pressure differences. The procedure is applied for the analysis of three valve designs of refrigeration compressors