Ezio Fornasieri

Optimisation of the throttling system in a CO2 refrigerating machine

This paper is an answer to the need of finding the optimal solution for the throttling system in refrigerating machines using CO2 as working fluid; such a solution must combine reliability, low installation cost and high energy efficiency. To this purpose, different expansion systems are compared by means of a simulation programme, including a new one, operating with a differential valve, a liquid receiver and a thermostatic valve. The typical compression refrigerating cycle performed by CO2 involves transcritical operations and therefore the upper pressure needs to be adjusted to the optimal value, that, unlike the traditional cycle, is not determined by heat transfer. The innovative system here proposed shows an intrinsic self-adjusting capability that leads to COP values quite close to the maximum ones when a fixed suitable value of the differential pressure is chosen, even if the temperature of the secondary fluid varies to a large extent.

Energy efficiency evaluation of CO 2 transcritical refrigeration cycles for telecommunication and data centres

There is no need to stress the importance of telecommunication industry in the global economy. Web hotel applications pose a peculiar problem for compensating very high thermal power inside rather restricted spaces. Distinctive features of this refrigerating installations are high evaporation temperature and high flow rate of cooling air, whereas latent heat load is absent. The typical system used in this Held is a direct expansion refrigerating unit, with air cooled condenser and operates on R407C refrigerant. Typical values for the evaporation temperature are around 10degC, with the process air cooled down to 26degC, but applications exist with high density of heat load that can operate with 30divide35degC air temperature at the evaporator inlet. Nowadays synthetic refrigerants, such as R407C, are increasingly under pressure from environmentalists and from a greater part of the society in industrialised countries since they raise great concern about the future of the planet, because of their high GWP potential. Natural refrigerants, such as ammonia, hydrocarbon and CO2 are seen as the ultimate solution for this environmental problem, but the first two fluid hardly can be considered as viable alternative to synthetic refrigerants because their unfavourable safety characteristics. CO2 is indeed safe, cheap, easily available, its ODP is zero and its GWP is negligible, even zero, if the fluid is recovered from waste of industrial processes; the dark side of CO2 can be energy efficiency because of its low critical temperature, that often makes the refrigerating machine operate according to a transeritieal cycle; this means that high temperature heat rejection does not involve two phase transformation (condensation) but only gas cooling (consequently the heat exchanger is named gas cooler). Nevertheless, according to common opinion, this thermodynamic penalisation can be, at least partially, counterbalanced by its very good characteristics of heat transfer and pressure drop. Another strong point of CO2 is the possibility of running with a rather low flow rate of the cooling medium at the gas cooler. In any case, the penalisation in energy efficiency with respect to traditional compression vapour inverse cycle is decreasing when the temperature lift from the lower to the upper cycle temperature is shrinking. Since high evaporation temperature is typical of the refrigeration systems in use in web hotel, CO2 transeritieal cycle can be considered an effective alternative to traditional installations. In the paper a theoretical analysis is carried out, aimed at comparing the energy efficiency characteristics of CO2 transeritieal cycle with the ones of the application on the market. Different transeritieal cycles are analysed. The effects of independent variable, such as upper and lower cycle pressures and the mass flow rate of air at the heat exchangers are investigated.