Conference Year

2016

Keywords

operational stability; sensor dynamics; variable speed; hunting; EEV

Abstract

The instability in a refrigeration system, conventionally known as hunting, is the phenomena of the oscillation of certain system operational parameters such as the degree of refrigerant superheat (DS), refrigerant mass flow rate and evaporating pressure. Hunting has been noticed in not only the refrigeration systems controlled by thermostatic expansion valves (TEVs), but also those controlled by electronic expansion valves (EEVs), and hence attracted a lot of research interest. However, no previous studies on the impacts of both the operational parameters of an EEV and the simultaneous variation of compressor and supply fan speed in a variable speed (VS) direct expansion (DX) air conditioning (A/C) system may be identified. Â This paper reports on a modeling study on the operational stability of a VS DX A/C system. An existing detailed, physical based dynamic model for a VS DX A/C system has been further developed by adding equations for both the temperature sensor in an EEV and the air side of the evaporator. Using this further developed model, a modeling study on the stability of a VS DX A/C system based on the classical minimal-stable-signal (MSS) theory has been carried out. The effects of the dynamics of the temperature sensor, Proportional-Integral (PI) control settings of the EEV and variable speed operation of both compressor and supply fan on the operational stability of the VS DX A/C system have been investigated. The modelling results demonstrated that increasing the time constant of the temperature sensor would help mitigating the system instability, and increasing the compressor speed or decreasing the supply fan speed would lead to the movement of a mixture-vapor transition point towards evaporator exit, leading to potential instability of the VS DX A/C system.Â

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