Effects of PWM valve unsteadiness on the performance of a CO2 commercial display cabinet

Steady state condition is a common assumption during the design of refrigeration systems and display cabinets. This assumption is reasonably accurate when Step Motor valves are considered, as they enforce a quasi-steady condition in the evaporator due to the fact that the valve continuously modulates its opening in relation to the external boundary conditions. On the other side, the use of Pulse-Width-Modulation valves prevents the realization of a steady condition, leading instead to a periodic cycle based on the valve modulation period, also when subject to constant boundaries conditions. In this paper, the effects of the pressure gradients and the mass flow fluctuation inside the refrigeration circuit will be experimentally investigated on a CO2 medium temperature display cabinet. The time averaged data using Pulse-Width-Modulation valves will be compared to the ones obtained using Step Motor valves to assess the effect of the periodic modulation on the overall system performances.