In the present study, a simplified quasi-steady state reduced order model is utilized to investigate moving boundary characteristics in a narrow tube of heat exchanger that carries organic refrigerant as working fluid in the medium solar thermal applications (similar to 200 degrees C). The present model provides the variation in mass flow rates with varying temperature of the heat transfer fluid that exchanges energy with the narrow tube containing organic refrigerant and estimates suitable mass flow ranges for different steady state bulk heat transfer fluid temperatures. The study also brings forth the changes in wall segment temperatures of the subcooled, two-phase and superheated regions for the tube carrying the refrigerant, with changing average heat transfer fluid (HTF) temperature that changes with the solar radiation incident on the solar collectors. Computational results indicate very high wall segment temperatures as well as the required working fluid mass flow rates for higher HTF temperature. Thus, the model could be useful in determining the viability of the operating conditions based on the changing values of the driving parameters, such as solar power level, which varies over time during a day as well as from one day to another, and heat transfer fluid temperature, which essentially would help in modelling the transient characteristics in narrow tube of heat exchanger. (C) 2018 Elsevier Ltd. All rights reserved.