Comparison between fixed-opening and variable-opening throttling on two-circuit evaporator performance under water maldistribution

Highlights: • Fixed-opening valve (FOV) and thermostatic expansion valve (TEV) are compared. • Two-circuit evaporator performance with water maldistribution is studied. • Water maldistribution led to random oscillation in FOV loop and hunting in TEV loop. • TEV amplified superheat oscillation in multi-circuit evaporator outlet. • TEV quickened capacity reduction of multi-circuit evaporator in hunting condition. -- Abstract: Water maldistribution in the two-circuit evaporator was found to induce instabilities, and further deteriorate capacity of the refrigeration system. As a follow-up study, this article presents a comparison between throttling types on the two-circuit evaporator performance under water maldistribution. The evaporator had two circuits of the same specifications, and a fixed-opening valve (FOV) and a thermostatic expansion valve (TEV) were accessed alternatively. Results show that water maldistribution aroused random oscillations in overall outlet temperature/superheat under FOV throttling, and further triggered hunting under TEV throttling and prompted system parameters into periodic oscillations. With water maldistribution increased, oscillation amplitude in overall outlet superheat increased first but decreased later under FOV throttling with the transition from unstable to stable refrigerant flow. However, it kept increasing under TEV throttling due to greater fluctuation in refrigerant supply during hunting conditions. Moreover, capacity of the circuit with larger water flowrate increased monotonously under FOV throttling, but increased first and decreased later after hunting occurred under TEV throttling, which dominated the faster reduction in overall capacity of the two-circuit evaporator in the TEV loop. The comparison suggests that in off-design conditions, variable-opening throttling might perform worse in stability and energy efficiency than fixed-opening throttling.