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Halogen free (natural) refrigerants The compressors, for example, must be properly designed because of the high vapour density and pressure levels (particu- larly on the suction side). There are also specific requirements with regard to materi- als. Furthermore only highly dehydrated CO2 must be used. High demands are made on lubricants as well. Conventional oils are mostly not misci- ble and therefore require costly measures to return the oil from the system. On the other hand, if miscible and highly soluble POE are used, the viscosity is strongly reduced. Further information to lubricants see page 41 and Fig. 37, page 45. For subcritical CO2 applications BITZER offers two series of special compres- sors. an intermediate pressure receiver. Depend- ing on the temperature curve of the heat sink, a system designed for transcritical operation can also be operated subcritically ‒ with higher efficiency. In this case, the gas cooler becomes the condenser. Another feature of transcritical operation is the necessary control of the high pressure to a defined level. This "optimum pressure" is determined as a function of gas cooler outlet temperature by means of balancing between the highest possible enthalpy dif- ference and at once minimum compression work. It must be adapted to the relevant operating conditions using an intelligent modulating controller (see system example, Fig. 32). As described before, under purely thermo- dynamic aspects, the transcritical operating mode appears to be unfavourable in terms of energy efficiency. In fact, this is true for systems with a fairly high temperature level of the heat sink on the high pressure side. However, additional measures can improve efficiency, such as the use of parallel com- pression (economiser system) and/or ejec- tors or expanders for recovering the throt- tling losses during expansion of the refrigerant. Apart from that, there are application areas in which a transcritical process is advanta- geous in energy demand. These include heat pumps for sanitary water or drying pro- cesses. With the usually very high tempera- ture gradients between the discharge tem- perature at the gas cooler intake and the heat sink intake temperature, a very low gas temperature outlet is achievable. This is facilitated by the temperature glide curve and the relatively high mean temperature difference between CO2 vapour and sec- ondary fluid. The low gas outlet temperature leads to a particularly high enthalpy differ- ence, and therefore to a high system COP. Low-capacity sanitary water heat pumps are already manufactured and used in large quantities. Plants for medium to higher capacities (e.g. hotels, swimming pools, drying systems) must be planned and realised individually. Their number is there- fore still limited, but with an upward trend. Apart from these specific applications, there is also a range of developments for the classical areas of refrigeration and air-con- ditioning, e.g. supermarket refrigeration. Installations with parallel compounded com- pressors are in operation to a larger scale. They are predominantly booster systems where medium and low temperature circuits are connected (without heat exchanger). The operating experience and the calculat- ed energy costs show promising results. However, the investment costs are still higher than for conventional plants with HFCs and direct expansion. On the one hand, the favourable energy costs are due to the high degree of opti- mized components and the system control, as well as the previously described advan- tages regarding heat transfer and pressure drop. On the other hand, these installations are preferably used in climate zones permit- ting very high running times in subcritical operation due to the annual ambient tem- perature profile. For increasing the efficiency of CO2 super- market systems and for using them in warmer climate zones, the technologies described above using parallel compres- sion and/or ejectors are increasingly used. Therefore, but also because of very demanding technology and requirements for qualification of planners and service person- nel, CO2 technology cannot be regarded as a general replacement for plants using HFC refrigerants. Resulting design criteria Detailed information on this topic would go beyond the scope of this publication. In any case, the system and control techniques are substantially different from conventional plants. Already when considering pressure levels as well as volume and mass flow ratios specially developed components, controls, and safety devices as well as suit- ably dimensioned pipework must be pro- vided. Supplementary BITZER information con- cerning compressor selection for sub- critical CO2 systems (see also https://www.bitzer.de) o Brochure KP-120 Semi-hermetic reciprocating compres- sors for subcritical CO2 applications (LP/HP standstill pressures up to 30/53 bar) o Brochure KP-122 Semi-hermetic reciprocating compres- sors for subcritical CO2 applications (LP/HP standstill pressures up to 100 bar) o Additional publications upon request Transcritical CO2 applications Transcritical processes are characterized in that the heat rejection on the high pressure side proceeds isobar but not isotherm. Contrary to the condensation process dur- ing subcritical operation, gas cooling (desu- perheating) occurs, with corresponding temperature glide. Therefore, the heat exchanger is described as gas cooler. As long as operation remains above the critical pressure (74 bar), only high-density vapour will be transported. Condensation only takes place after expansion to a lower pres- sure level – e.g. by interstage expansion in 35PDF Image | REFRIGERANT REPORT 21
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