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Refrigerant blends Refrigerant blends Refrigerant blends have been developed for existing as well as for new plants with pro- perties making them comparable alternatives to the previously used substances. It is necessary to distinguish between three categories: 1. Transitional or service blends which mostly contain HCFC R22 as the main constituent. They are primarily intended as service refrigerants for older plants with view on the use ban of R12, R502 and other CFCs. Correspond- ing products are offered by various manu- facturers, there is practical experience covering the necessary steps of conver- sion procedure. However, the same legal requirements as for R22 apply to the use and phase-out of these blends (see page 8). 2. HFC blends These are substitutes for the refrigerants R502, R22, R13B1 and R503. Above all, R404A, R507A, R407C and R410A, are being used to a great extent. One group of these HFC blends also con- tains hydrocarbon additives. The latter exhibit an improved solubility with lubri- cants, and under certain conditions they allow the use of conventional oils. In many cases, this permits the conversion of exist- ing (H)CFC plants to chlorine-free refriger- ants (ODP = 0) without the need for an oil change. 3. HFO/HFC blends as successor generation of HFC refriger- ants. It concerns blends of new "Low GWP" refrigerants (e.g. R1234yf) with HFCs. The fundamental target is an addi- tional decrease of the global warming po- tential (GWP) as compared to established halogenated substances (see page 24). Blends of two and three components already have a long history in the refrigeration trade. A difference is made between the so called "azeotropes" (e.g. R502, R507A) with ther- modynamic properties similar to single sub- stance refrigerants, and "zeotropes" with "gliding" phase changes (also see next chapter). The original development of "zeotropes" mainly concentrated on special applications in low temperature and heat pump systems. Actual system construction, however, remained the exception. A somewhat more common earlier practice was the mixing of R12 to R22 in order to improve the oil return and to reduce the dis- charge gas temperature with higher pres- sure ratios. It was also usual to add R22 to R12 systems for improved performance, or to add hydrocarbons in the extra low tem- perature range for a better oil transport. This possibility of specific "formulation" of certain characteristics was indeed the basis for the development of a new generation of blends. At the beginning of this Report (see chapter Refrigerant developments and legal situa- tion, page 3) it was already explained that no direct single-substance alternatives (on the basis of fluorinated hydrocarbons) exist for the previously used and current refriger- ants of higher volumetric refrigeration cap- acity than R134a. This is why they can only be "formulated" as blends. However, taking into account thermodynamic properties, flammability, toxicity and global warming potential, the list of potential candidates is strongly limited. For the previously developed CFC and HCFC substitutes, the range of substances was still comparably large, due to the fact that substances of high GWP could also be used. However, for formulating blends with significantly reduced GWP, in addition to R134a, R1234yf and R1234ze(E), primarily refrigerants R32, R125 and R152a can be used. Most of them are flammable. They also exhibit considerable differences with respect to their boiling points, which is why all "Low GWP" blends of high volumetric refrigerating capacity have a substantial temperature glide (see next chapter). BITZER has accumulated extensive experience with refrigerant blends. Laboratory and field testing was com- menced at an early stage so that basic information was obtained for the opti- mizing of the mixing proportions and for testing suitable lubricants. Based on this data, a large supermarket plant – with 4 BITZER semi-hermetics in par- allel – could already be commissioned in 1991. The use of these blends in the most varied systems has been state-of- the-art for many years – generally with good experiences. General characteristics of zeotropic blends As opposed to azeotropic blends (e.g. R502, R507A), which behave as single sub- stance refrigerants with regard to evapora- tion and condensing processes, the phase change with zeotropic fluids occurs in a "gliding" form over a certain range of tem- perature. This "temperature glide" can be more or less pronounced, it depends mainly on the boiling points and the percentage propor- tions of the individual components. Certain supplementary definitions are also used, depending on the effective values, such as "near-azeotrope" or "semiazeotrope" for less than 1 K glide. Essentially, this results in a small tempera- ture increase already in the evaporation phase and a reduction during condensing. In other words: At a certain pressure level, the resulting saturation temperatures differ in the liquid and vapour phases (Fig. 11). To enable a comparison with single sub- stance refrigerants, the evaporating and condensing temperatures have been often defined as mean values. As a consequence the measured subcooling and superheating conditions (based on mean values) are unrealistic. The effective difference – based on dew and bubble temperature – is less in each case. These factors are very important when assessing the minimum superheat at the compressor inlet (usually 5 to 7 K) and the quality of the refrigerant after the liquid receiver (vapour bubbles). 13PDF Image | REFRIGERANT REPORT 21
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