There is renewed interest in developing cost-effective, energy efficient and environment friendly ultra low temperature refrigerators for long term preservation of biological specimens. These refrigerators are indented to replace the conventional cascade refrigerators and are capable of reaching lower refrigeration temperatures, retaining higher COP and exergy efficiency.
Linde-Hampson refrigerators, working with refrigerant mixtures, are capable of achieving ultra low and cryogenic temperatures. Considerable literature exists on Linde-Hampson
refrigerators operating with nitrogen-hydrocarbon mixture for providing refrigeration below 100 K. However, there are very few works in the open literature on Linde-Hampson refrigerators operating in the ultra low temperature region between −120 °C and −60 °C that
is of interest for the preservation of biological tissues including blood plasma. The main objective of this study is to identify suitable refrigerant mixture compositions and to develop an ultra low temperature refrigerator working with these refrigerant mixtures.
R14−hydrocarbon mixtures exhibit vapour-liquid-liquid-equilibria (VLLE) at the low temperatures required in ultra low temperature refrigerators. The refrigerant glide is close to
zero when VLLE occurs. High COP or exergy efficiency can also be achieved with mixtures that exhibit VLLE. R14−hydrocarbon mixtures are identified in this study as candidate refrigerant mixtures for Linde-Hampson refrigerators working between −120 °C and −60 °C.
The PC-SAFT equation of state has been used to analyse the performance of the refrigerator and also optimize the mixture compositions. The necessary binary interaction parameter of
the mixing rule have been regressed from low pressure vapour-liquid equilibria (VLE) data available in the literature. Both VLE and VLLE have been estimated and compared with experimental data, including high pressure data not used to regress the binary interaction
parameters. The results are also compared with other thermodynamic models available in the literature. R14 − hydrocarbon mixture compositions have been optimized for use in the Linde-Hampson
refrigerator working between −120 °C and −60 °C. Formation of vapour-liquid-liquid equilibria has been observed in the optimized refrigerant mixtures. The theoretical performance of the refrigerator operating with these mixtures has been determined at different
operating conditions. Two experimental setups were used in this work. The first setup is a refrigeration system and has been used to validate the thermodynamic model proposed for R14−hydrocarbon mixtures. The second setup is a practical 195 litre originally designed for use with a two-stage cascade refrigerator. The freezer was retrofitted with a single stage Linde-Hampson refrigerator
operating with R14−hydrocarbon mixtures. The system achieved a minimum temperature of −90 °C and consumed much lower power than the original cascade refrigerator. The results of the thermodynamic modelling of R14−hydrocarbon mixtures, optimisation of R14−hydrocarbon mixtures for use in Linde-Hampson refrigerator, and the performance of the freezer with different mixtures are presented in the thesis.