The Research Of TXV Selection Method And The Impact On System Reliability

From 2010 only air conditions with environmentally friendly refrigerants and higher efficiency than today can be sold in USA, so North-America companies are busy with replacement of refrigerants and efficiency upgrade in order to put the new products on the market before 2010. As a new environmentally friendly refrigerant with high efficiency R410a will be used in new unitary products of Trane. These R410a unitary products will work at a wide running condition and can endure a large fluctuant load. Although thermostatic expansion valves (TXVs) can adjust the superheat well only at a narrow running condition, these air conditions are required to use TXVs considering a balance between the price and performance. This thesis is formed with the work of system design such as selection,application and effect on system reliability of TXVs. It is the main purpose to prevent hunting, reduce power consumption and improve system reliability by utilizing the advantages of TXVs in controlling capability. The following are main content of this thesis:The theoretical analysis indicates that the TXVs selected by using the conventional method can not full play their superheat-controlling capability under wide running condition. Considering the effect of hunting, an improved method is introduced. Finally an experiment designed to verify this method is demonstrated, and the experimental results show that the expansion valve selected by using new method can control the superheat of an air-conditioning system at most running points efficiently, moreover, the hunting is avoided.Due to the lag in research on TXVs used to R410a, even the minimum TXV can't meet the system requirement and makes the system hunting at some running conditions with low TXV loading. In this thesis, the way using TXV plus orifice to increase TXV loading is proposed. The experimental results show that this way can solve the hunting problem very well.With the observation on the practical action of a TXV in running condition, when the TXV loading ratio≥120% (approximatively), this TXV is full open acting as a fixed orifice and can't control superheat correctly. By using simulation tools T-Rex & MiniTab, key parameters, such as suction pressure, discharge pressure, discharge temperature, superheat and subcooling, are calculated and predicted, and the effect on system reliability of these parameters is evaluated by analyzing their trend.