RACA Journal June 2016 - Page 93

Technical of alternative refrigerants and system com­ponents for these areas has not been so great. In the meantime a group of alternatives for the CFC R114 and Halon R12B1 (high temperature), R13B1, R13 and R503 (extra low temperature) were offered as the replacements. With closer observations it has been found that the thermodynamic properties of the alternatives differ considerably from the previously used substances. This can cause costly changes especially with the conversion of existing systems. Alternatives for R114 and R12B1 R227ea and R236fa are considered suitable substitutes even though they may no longer be used in new installations in the EU from 2020, due to their relatively high GWP. R227ea cannot be seen as a full replacement. Recent research and field tests have shown favourable results, but with normal system technology the critical temperature of 102°C limits the condensing temperatures to about 85-90°C. R236fa provides the more favourable conditions at least in this regard – the critical temperature is above 120°C. A disadvantage, however, is the smaller volumetric refrigerating capacity. This is similar to R114 and with that 40% below the performance of R124 which is widely used for extra high temperature applications today. The low-GWP refrigerant R1234ze(E) can also be regarded as a potential candidate for extra high temperature applications. Refrigerant R600a (Isobutane) will be an interesting alternative where the safety regulations allow the use of hydrocarbons (safety group A3), With a critical temperature of 135°C, condensing temperatures of 100°C and more are within reach. The volumetric refrigerating capacity is almost identical to R124. The low-GWP refrigerant R1234ze(E) can also be regarded as a potential candidate for extra high temperature applications. Compared to R124, its cooling capacity is higher by 10-20% and its pressure level by about 25%. At an identical refrigerating capacity, the mass flow differs only slightly. Its critical temperature is 107°C, which would enable an economical operation up to a condensing temperature of about 90°C. However, like R1234yf, R1234ze(E) is mildly flammable and therefore classified in the new safety group A2L. The corresponding safety regulations must be observed. However, until now no sufficient operating experience is available, which is why an assessment of the suitability of this refrigerant for long-term use is not yet possible. For high temperature heat pumps in process technology and special applications in the field of high temperatures, DuPont www.hvacronline.co.za has presented an HFO-based refrigerant called DR-2. The critical temperature is at 171°C, the boiling temperature at 33.4°C. This enables an operation at condensing temperatures far above 100°C for which only purpose-built compressors and system components can be used. DR-2 has a GWP < 10 but is not flammable according to tests carried out so far. Therefore a classification in safety group A1 can be expected. A more detailed evaluation is not yet possible with respect to the chemical stability of the refrigerant and of the lubricants at the very high temperatures and the usually very long operating cycles of such systems. The special applications also include systems for power-heat coupling – the so-called ‘Organic Rankine Cycle (ORC)’, which has become increasingly important. In addition to DR-2 as a potentially suitable operating fluid, a series of other substances are also possible, depending on the tem­ perature level of the heat source and the heat sink. They include R245ca (GWP100 = 1 030) having a critical temperature of 154°C, which like DR-2 is also suitable as refrigerant for chil lers with large centrifugal compressors. In addition Solvay offers suitable refrigerants containing the base component R365mfc for ORC applications. A product with the trade name Solkatherm SES36 already presented several years ago contains perfluoropolyether as a blend component. It is an azeotrope having 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 behaviour may be advantageous. In the case of single-phase heat sources and heat sinks the temperature 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. As an expander for ORC systems screw and scroll compressors can be adapted in their construction accordingly. For several years Bitzer has been involved in various projects and has already gained important knowledge with this technology and experience in design and application. A comprehensive description of ORC systems would go beyond the scope of this Refrigerant Report. Alternatives for R13B1 Besides R410A, ISCEON M089 (DuPont) can be regarded as potential R13B1 sub­stitute. With R410A a substantially higher discharge gas temperature is to be con­sidered when compared to R13B1 which restricts the application range even in two-stage RACA Journal I June 2016 91