In this paper, a comprehensive review of the principles of different refrigeration methods covering the temperature range from 4 K to 300 K is presented. The methods covered are based on steady state systems, such as the Carnot cycle, the vapor compression cycles: basic, cascade, and mixed gas refrigeration cycles, and the recuperative type cryocooler cycles: Joule–Thomson cycle, Brayton cycle, and Claude cycle, and periodic systems such as the regenerative type cryocooler cycles: Stirling cycle, pulse tube cycle, and Gifford–McMahon cycle. The current state of technology and challenges for further improvements are briefly summarized. Some comparisons and assessments are provided for these methods. It is seen that among other things, the selection of a proper refrigeration method is dependent on the following principal factors: (i) the refrigeration capacity required, (ii) the temperature level, and (iii) the application environment. Even though more than one refrigeration method may be suitable for a given application, the selection is further guided by considerations such as cost, reliability, size/compactness, and unit power. An attempt has been made in this paper to (1) present in-depth relevant details to understand the current state of engineering and technology, (2) provide a handy document for refrigeration designers in the industry, and (3) present the guiding principles in the selection of refrigeration methods.

This paper presents the design, selection of equipment, testing, and analysis of a 2 TPD (76 kg/hr) prototype oxygen liquefier that employs a mixed gas refrigeration cycle. Small scale oxygen plants (30–60 TPD) based on VPSA systems (Vapor Pressure Swing Adsorption) periodically require liquid back-up to provide uninterrupted supply of O2 gas to customers during planned plant maintenance. Supply of liquid for back-up, especially to customers in remote locations, is expensive and difficult. Economically designed MGR liquefiers will fulfill this market need. The 2 TPD prototype O2 liquefier is based on the dual loop MGR Rankine cycle (see Figure 1). The forecooler loop provides refrigeration at the warm end (233K). R507 is used as a refrigerant for the forecooler in the warm end loop. The main refrigeration loop uses a mixture of R218, R14 and N2. The main advantage of separating the refrigerants into two different loops is to avoid freezing of high boiling point refrigerants at liquid O2 temperatures in the main refrigeration loop. The process and mixture composition were optimized using the HYSYS process simulation package. Very useful insights were gained in terms of reducing the irreversibilities in the heat exchanger. Low cost innovative designs were adopted for the heat exchangers. For example: (1) plate-and-frame heat exchangers were successfully used for multiple gas streams, (ii) a spirally wound coiled heat exchanger was used to liquefy oxygen. Similarly, the compressors used in the forecooler and main cycle were low cost, off-the-shelf items used in conventional refrigeration systems. The liquefier unit was initially demonstrated and a liquid making capacity of 1.5 TPD at a unit power of 44 kW/TPD was achieved. Subsequent modifications to the heat exchangers resulted in meeting the design expectations at a unit power of 37 kW/TPD.

This paper summarizes results of a study conducted to minimize total cost of ownership of multistage air compressors by integrating it with compact and efficient off-the-shelf organic rankine power cycle units to recover low grade waste heat from inter-stage coolers with subsequent conversion to power. The paper also highlights challenges faced by the integration and provides guidance for future cost and technology targets for key components to make it a commercial scale reality. Various schemes for vaporization of the working fluid including direct and indirect as well as full or partial were explored. Also, in order to better understand interaction between cycle efficiency and capital cost of key components, design as well as operating parameters including evaporator approach temperature, compression stage suction temperature, number of compression stages and cooling water supply temperature were investigated. Configuration, size and hence the cost of evaporator/ inter-stage cooler was found to be one of the major factors governing the overall cost. Impact of various operating modes including turn-down and seasonal variations were also studied. Air flow and final discharge pressure from the multistage air compressor were kept constant throughout the study to facilitate a fair comparison.