The Development Of Calculation Techniques For HFC Blend â•fi Oil Behavior

Industry in the developed world has come to depend heavily on HFC refrigerants, all blends except for R134a. This dependency will increase when the last of the HCFCs, R22, becomes unavailable in the near future. Furthermore, the pressure for increased efficiency combined with reduced running costs and environmental impact is relentless, particularly since the Kyoto Protocol which has had the effect of exposing the HFC to competition from the “natural” refrigerants. The authors responded by searching for better blends by the use of calorimetric testing and mathematical modelling. This paper reports progress and illustrates features of the technique in the evaluation of a binary blend/POE oil combination suitable for centrifugal compressors. INTRODUCTION The intention of those who framed the Kyoto Protocol was the achievement of a reduction in man-made global warming. The net result is a threat to the availability of hydrofluorocarbon (HFC) refrigerants, which have high direct global warming indices, all blends apart from R134a. Work since Kyoto on alternatives, the “natural refrigerants”, i.e. ammonia, carbon dioxide, air, water, and hydrocarbons, has shown that for some applications the “total equivalent warming index” (TEWI), which takes into account direct effects and indirect effects due to the release of carbon dioxide caused by plant energy consumption, is increased by the use of natural refrigerants due to their inferior thermodynamic performance (Keogh 2002). There are signs that this view is gaining increasing acceptance, very importantly among legislators across a range of countries. The authors believe that the HFCs will remain in use for a significant period of time and retain a significant share of the refrigeration market. This belief has led to a program of research having the following objectives: 1. the discovery and evaluation of new HFC blends, lower in TEWI than current technology. 2. the provision of information and calculation methods for realistic HFC blend – oil combinations of value to plant designers. A particular obstacle to HFC blend – oil research is the lack of thermophysical measured data. The authors discuss a method of dealing with this problem with regard to a particular blend which has been proposed for a plant having a centrifugal compressor. The work reported here forms a part of a larger program. A TECHNIQUE FOR BLEND DEVELOPMENT The discovery of new HFC blends requires specialized facilities, in particular for the convenient and accurate assessment of trial blends. If calculations are too inaccurate (due usually to the lack of basic thermophysical data rather than the lack of an adequate conceptual model), then performance testing of trial blends is unavoidable. Unfortunately testing is time consuming and expensive. The time for one measurement for example, following a change in the blend composition is determined largely by the time to re-establish steady state conditions in the test equipment. Even in a small calorimeter like that used by the authors having a 3 kW maximum refrigerating effect, one to two hours is required followed by a further one hour of steady state running during which the measurements are made. The authors took the view that an approach which coordinates the use of relatively simple calculations dependent on available thermophysical data (Yan 2002) with a limited range of measurements, would yield useful results in a shorter timescale. This paper discusses this approach in relation to the evaluation of a binary blend (Pearson 1999) thought suitable for use in centrifugal compressors. motor torque transducer cooling water in compressor pressure regulator re fri ge ra nt cy lin de r hand expansion valve