An Integrated Model for an Oil Free Carbon Dioxide Compressor Using Sanderson-Rocker Arm Motion Mechanism

The multi-piston axial reciprocating compressor using the Sanderson-Rocker Arm Motion (S-RAM) mechanism is expected to have high volumetric efficiencies due to the application of a new type of seal to prevent the in-cylinder refrigeration gas leaking through the clearance between piston and cylinder wall. The stroke of the compressor can be controlled by adjusting the inclination angle between the connecting shaft and machine driving shaft. This allows the control of the delivered refrigerant mass flow rate to match the capacity requirement in the field. A comprehensive simulation model to predict the performance of a prototype reciprocating compressor using the SRAM mechanism has been developed. The natural refrigerant carbon dioxide (CO2) is used as the working fluid. The comprehensive model is comprised of a kinematics model, compression process model and an overall energy balance model. In the kinematics model, the movement of the piston is given including its displacement, velocity and acceleration. It is found that the moving path of the center of the ball in the ball-socket joint is moving around a corresponding cylinder centerline with a ‘figure 8’ motion instead of moving along the cylinder centerline. In the compression process model, the system of governing equations is solved, which incorporates a valve sub-model, leakage sub-model and gas pulsation sub-model. The classical 4th order Runge-Kutta method and Broyden’s method are employed to solve the non-linear system of equations to find the in-cylinder refrigerant state (temperature, pressure) at each rotational angle of the machine driving shaft. The variations of suction and discharge valve movements with respect to drive shaft rotational angle are also given. The values of the cylinder wall temperature, the actual suction and discharge temperatures in the connecting pipes are required to initiate the solving of the compression process model. These temperatures are solved simultaneously by incorporating the overall energy balance model with the compression process model. A lumped temperature assumption is employed in the overall energy balance model to assume there is no temperature gradient in each compressor component at steady-state.