Effects of water mal-distribution on thermostatic-expansion-valve stability

Abstract:

To explore the hunting behavior of the thermostatic-expansion-valve (TEV), dynamic characteristics of the TEV (TEX2 externally balanced type) control-loop ranged from stability to vibration were studied under the selected 7 sets of water mal-distribution experimentally. Results showed that as F₂ dropped from 36.9% to 27.0%, the TEV control-loop was stable and the overall evaporator capacity decreased by only 3.4%. As F₂ dropped from 27.0% to 9.1%, evaporator overall outlet temperature changed from stability to vibration. Its vibration amplitude increased gradually with the phase transformation from vapor to mist, indicating that hunting behavior of TEV control-loop occurred, and the overall evaporator capacity decreased by 14.1%. The superheated refrigerant gas from Circuit 1 outlet only provided sensible heat and couldn't evaporate the liquid refrigerant out of Circuit 2 into superheated steam. Consequently, refrigerant in the evaporator overall outlet was of a smaller superheat than the necessary minimal stable signal (MSS) of the TEV, even at two-phase state, and would inevitably give rise to the hunting of the TEV -controlled system according to the MSS theory. These experimental results might provide references for the design of the heat exchanger of multi-circuit refrigerating system.