PROPERTIES OF CO2 AS A REFRIGERANT - Centro Studi Galileo

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In all cases, not only with reference to an ideal simple cycle, the trend of the cycle coefficient of performance as a function of the gas cooler pressure for p>p opt is rather flat. Therefore a slight overpressure in the gas cooler with respect to the optimum, does not penalise too much the cycle efficiency. On the other hand, a gas cooler pressure below the optimum value can sometimes drastically penalise the cycle efficiency. A proper control of the high-pressure level is indispensable to take best advantage of CO 2 refrigeration technology. A further important difference of CO 2 transcritical cycles, as compared to traditional refrigerating cycles, is given by the much higher pressure levels (see data reported in Tab. 1), at equivalent working conditions as far as temperatures of the external source and sink are concerned. It is no doubt that high pressure levels penalise the structural design of components of a refrigerating circuit, and in particular of the compressor; nevertheless their reduced sizes counterbalance this drawback. As compared to traditional refrigerants, the reduced volume flow rate of CO 2 necessary to yield a certain refrigerating power is evidenced by the value of the volumetric latent heat reported in Tab. 1. This is six times larger for CO 2 than for R-22, even if one must also take into account the high vapour quality of CO 2 at the evaporator inlet, as compared to that of R-22 in comparable situations, proper to the transcritical cycle. The high pressure level with CO 2 also brings about a reduced penalty due to the fluid pressure loss when designing the refrigerating circuit appropriately. From what said above it can be concluded that equipment with CO 2 as the refrigerant, in spite of the higher working pressure, is not necessarily heavier or bulkier or potentially more dangerous as compared to equivalent equipment operated with the traditional working fluids. This is due to the smaller flow cross sections of the components, consequent on the reduced refrigerant volume flow rate. Moreover, at equivalent working conditions, the compressor pressure ratio with CO 2 is notably lower than with traditional refrigerants. Together with a higher pressure level, this fact makes it possible to get higher compression isoentropic efficiencies, with advantages in energy consumption. On the other hand, CO 2 compressors work with much higher differential pressures than traditional refrigerating compressors, and this fact may enhance backflow losses. Another peculiarity of carbon dioxide is evident in the diagram of Fig. 1: the triple point pressure (p t =5.18 bar) is above that of the natural environment. At atmospheric pressure the transition from solid to gas (sublimation process) takes place at temperature t t =-78.9 °C, and the cooling effect is exploited in the so-called dry ice. In mechanical refrigeration this means that, in the event of collapse of a circuit, residual CO 2 solidifies in the plant at around –79 °C, sublimating successively into the atmosphere. 3. THE CO 2 TRANSCRITICAL REFRIGERATING CYCLE Energy performance of a CO 2 transcritical cycle is now discussed and compared with that of a traditional refrigerating cycle operated with conventional working fluids. To make the discussion concrete and to be able to argue about definite numerical values, two refrigerating cycles are directly analysed: a transcritical one operating with CO 2 , and a traditional one operated with R-22; this last refrigerant is in fact often taken as a bench-mark. The operative conditions for the two cycles, illustrated in Tab. 2, are fixed in such a way that they can be considered approximately equivalent with respect to external constraints. The situation considered refers to an outside air temperature t a =28 °C, with an air temperature increase in the R-22 condenser equal to 10 °C, and condensation of the refrigerant 5 °C above the condenser air exit temperature. For the CO 2 transcritical cycle, a 3
°C temperature approach at the counter-current gas cooler cold end has been assumed; further, the optimum gas cooler pressure p opt =78 bar (refer to relationship (1)) is assumed. Table 2. Vapour compression refrigerating cycles Point(s) Description Refrigerants CO 2 R-22 4 1 Temperature of isobaric evaporation t e =-10 °C t e =-10 °C 1 2 Isoentropic efficiency of adiabatic compression ic =0.80 ic =0.80 3 Temperature of isobaric condensation (R-22) t c =43 °C 2 3 Pressure (constant) in the gas cooler (CO 2 ) p gc =78 bar 3 Outlet gas cooler temperature (CO 2 ) t gce =31 °C 1 Vapour superheat at compressor inlet 0 °C 0 °C 3 Liquid sub-cooling at condenser outlet (R-22) 0 °C Temperature (constant) of the cooled source t r =-5 °C t r =-5 °C CYCLE COEFFICIENT <strong>OF</strong> PERFORMANCE COP 2.52 3.04 It can be noticed that for the theoretical cycles compared in the Table, in spite of the fact that a close temperature approach has been assumed for the CO 2 gas cooler, carbon dioxide COPis penalised by 20% as compared to the benchmark refrigerant R -22. To examine the reasons for the lower energy efficiency of the CO 2 transcritical cycle (at least at the considered working conditions), one can find, for any single process making up the cycle, the thermodynamic loss. This is defined as the individual contribution to the increase of the needed compression work, with respect to the one strictly necessary, with ideal thermodynamic processes, to obtain the same cooling effect, still complying with the external constraints. The computation of the thermodynamic losses of the individual cycle processes (referred to the unit mass of refrigerant) can be easily performed by recurring to the thermodynamic function exergy, once a suitable value for the ambient temperature t a is recognised. In this example, this is clearly the temperature at which the external cooling agent (air, in this case) is available for rejecting the condenser (R-22 cycle) or gas cooler (R-744 transcritical cycle) heat, that is t a =28 °C (that is, T a =301.15 K). Without going into the details of the theory of exergy (to be found in any Basic Applied Thermodynamics textbook), here below the expressions of the specific thermodynamic losses (that is, referred to a unit mass of refrigerant) are reported. Reference is made to the four processes of the CO 2 transcritical cycle represented in Fig. 3, conforming to operative conditions given in Tab. 2: - Adiabatic compression loss com = T a ( s 2 s 1 ) (2) - Gas cooler loss gc = h2 h3 Ta ( s2 s3 ) (3) - Adiabatic throttling loss = ( ) (4) - Evaporator loss t T a s 4 s 3 Q0 ev = Ta s 1 s4 (5) Tr In the expressions above, h and s are specific enthalpy and specific entropy of refrigerant respectively, while Q 0 =h 1 -h 4 is the cycle refrigerating effect. Capital T is temperature in absolute units.
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PROPERTIES OF CO2 AS A REFRIGERANT - Centro Studi Galileo

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