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For high temperature heat pumps in pro- cess technology and special applications at high temperatures, the low pressure refriger- ants based on HFO and HCFO developed primarily for systems with turbo compres- sors are also potentially suitable (page 24). They are characterised by very high critical temperatures (> 130°C), which enable eco- nomical operation at condensing tempera- tures of sometimes well above 100°C. How- ever, only purpose-built compressors and system components can be used here. Another advantage is their very low GWP and the classification in safety group A1 (non-flammable, non-toxic). A detailed evaluation of these refrigerants is not yet possible with respect to the chemi- cal stability of the refrigerants and of the lubricants at the very high operating tem- peratures and the long service life required for industrial systems. Special applications also include cogenera- tion systems – "Organic Rankine Cycle" (ORC), which become increasingly impor- tant. In addition to the potentially suitable substances listed in the table above, a series of other fluids are possible, depend- ing on the temperature level of the heat source and heat sink. They include the so far mostly used R245fa (GWP 1050) with a critical temperature of 154°C and a boiling temperature of 15.1°C. Solvay offers further refrigerants for ORC applications, containing the base compo- nent R365mfc. A product with the trade name Solkatherm® SES36 already pre- sented several years ago contains perfluoro- polyether as a blend component. It is an azeotropic blend with a critical temperature of 178°C. Meanwhile two zeotropic blends containing R365mfc and R227ea have been developed whose critical temperatures are 177°C and 182°C, due to different mixing ratios. They are available under the trade names Solkatherm® SES24 and SES30. In ORC systems zeotropic behavior may be advantageous. In the case of single-phase heat sources and heat sinks, the tempera- ture difference at the so-called "pitch point" can be raised by the gliding evaporation and condensation. This leads to improved heat transmission due to the higher driving average temperature difference. Refrigerants for special applications Screw and scroll compressors can be adapted by construction as an expander for ORC systems. BITZER has been involved in various projects for several years and has already been able to gain important insights with this technology, which have been implemented in the design of screw expanders and their application. An individual expander design is available upon request. A comprehensive description of ORC systems would go beyond the scope of this Refrigerant Report. Further informa- tion is available upon request. 09.20 Refrigerant Type Composition (with blends) GWP AR4 (AR5) Safety Group Boiling temperature [°C] Critical temperature [°C] R1224yd(Z) HCFO 4 (1) A1 14.6 156 R1233zd(E) HCFO 5 (1) A1 18.3 166 R1336mzz(E) HFO – (7) A1 7.6 130 R1336mzz(Z) HFO 9 (2) A1 33.5 171 R514A HFO R1336mzz(Z)/R1130(E) 7 (2) A1 28.8 178 Tab. 6 Characteristics of HFO (HCFO) low-pressure refrigerants 38PDF Image | REFRIGERANT REPORT 21
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