An Improved Analytical Model for Efficiency Estimation in Design Optimization Studies of a Refrigerator Compressor

The stack up of losses within a system gives an indication of its efficiency. In a refrigerator compressor, valves contribute to thermodynamic losses (pressure and cooling capacity) due to valve dynamics and mistiming. This paper proposes an improvement to an existing analytic closed form solution for efficiency estimation of such compressors by incorporating more detailed valve physics/ dynamics. For maximum energy efficiency ratio (EER), it is beneficial in certain variable capacity compressor architectures to drive the piston at a resonant frequency. In an oscillating system, this is typically determined by the mechanical spring-damper characteristics. This resonant frequency is usually a complex function of the geometry and operating conditions due to the gas-spring effect. A closed form solution for performance estimation of such a setup proposed earlier in literature (Choe & Kim) does not account for the effect of valve dynamics. However, the timing of valve operation influences the in-cylinder pressure build-up transients and thus modulates the gas-spring stiffness. While an accurate estimation of the resonant frequency requires a multi-physics simulation of the compressor, a detailed simulation of such complexity is time intensive especially when performing design optimization studies and hence analytical models would be preferred. The current work presents an improved equivalent analytic model for such optimization. Uncertainty analysis of the present approach is also discussed by comparing different performance parameters against full nonlinear model estimates, as relevant

Friction Model Development for a reciprocating compressor

Friction loss has a significant impact on the performance of a reciprocating compressor. Piston-cylinder friction is a major contributor compared to the other contributors like thrust bearing, piston pin and crank. In the present work,the piston–cylinder interaction inside a small hermetic compressor is modeled using the Reynolds equation which is solved using finite difference method. The model provides key compressor design parameters such as minimum oil film thickness, oil pressure distribution between piston-cylinder, normal forces and friction power loss. The model is validated against data from published literature. Using the above formulation, different concepts have been studied & compared against their friction loss characteristics. These include variable speed versus variable displacement for capacity control, piston-cylinder clearance for blow-by, cold start (high viscosity oil due to low temperature), lubricating fluids, viz. POE versus gas bearing

Development And Validation Of Integrated Design Framework For Compressor System Model

The performance of a refrigerator is largely guided by the efficiency of its drive unit, namely, the compressor & understanding the compressor behavior requires a detailed study of its dynamics, flow-thermals, electrical and controls aspects. Having a simulation model which captures these physics reasonably well is a critical part of the design and performance prediction of a compressor. The current paper describes a systematic approach of making a system level simulation framework by first developing individual models and then integrating them into a single framework to capture the multi-physics interaction of the different sub-components. This framework, developed in MATLAB/SIMULINK contains five modules, namely, Dynamics, Thermal, Motor, Controls and Post-processor. Dynamics is modeled as a spring-mass system with adjustable static equilibrium and head-crash prevention algorithm. The thermodynamics model essentially captures the valve physics. The valves in a reciprocating compressor contribute to pressure losses (pressure profile deviation from ideal suction & discharge processes, valve dynamics, leakages, pressure pulsations) and thermal losses (refrigerant back-flow caused by incorrect valve timing). The starting point of simulating these details is considering the gas dynamics and coupling it to the valve motion. The prediction of this model is validated against test data of a baseline compressor. As part of the integrated design framework, a permanent magnet motor is simulated as a resistance-inductance network with a series, velocity dependent voltage. To impress the desired operating conditions (capacity, stroke, clearance etc.) upon the integrated system model, a set of controllers were designed to control the motor