No abstract

Different methods have been proposed in the literature for the calculation of phase envelopes of mixtures. The phase envelopes of even simple mixtures have at least two stationary points on the P–T plane [∂P/∂T = 0 at the point of maximum pressure (cricondenbar) and ∂T/∂P = 0 at the point of maximum temperature (cricondentherm)]. Even more stationary points exist for complex mixtures. Therefore, a monotonic variation of either pressure or temperature cannot be used to trace the full phase envelope. Different methods have been proposed in the literature to help choose the right variable at the right location of the phase envelope to automatically trace the entire phase envelope. Although most methods work well for simple mixtures, many of them do not work well with wide-boiling mixtures or with mixtures that exhibit open-ended dew lines. In this article, we compare phase envelopes with space curves and show that the use of an independent variable helps trace complex phase envelopes automatically. It is shown that density is the ideal independent variable. A density-based method is developed to determine saturation points and automatically construct complex phase envelopes. The proposed method has immediate application in property programs and process simulators and was implemented in version 9.1 of NIST REFPROP property program released in May 2013.

No abstract

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.

No abstract