Conference Year
2016
Keywords
vapor injection, flash tank, simulation, ammonia, carbon dioxide
Abstract
Large scale refrigeration systems are used in many chemical processes to provide cold utilities which are essential in different stages of these processes. These refrigeration systems are great energy consumers and the costs of the compression and condensation processes have a significant impact on the cost of the final products. Therefore, it is very important that these refrigeration cycles operate in an optimized way, in order to reduce production costs. This work presents a study that combines an alternative cycle with a mixture refrigerant of low GWP, presenting a parametric analysis of some operating variables in a vapor injection refrigeration cycle with a flash tank (FTVI) using mixture refrigerant NH3/CO2, analyzing the influence of refrigerant mixture composition over the thermodynamic performance of this cycle, evaluating COP and also the refrigerant mass flow rate necessary to provide a cooling rate of 1 kW at the evaporator of the cycle. In this parametric analysis, the following operating variables were analyzed as a function of mixed refrigerant composition (wt%) and expansion ratio in the upper-stage valve: COP; compressor power; refrigerant mass flow rate; refrigerant temperature glide; mass flow ratio between vapor and feed streams in the flash tank; mass composition of liquid and vapor outlet streams from the flash tank and compression ratio. A basic vapor compression (VC) cycle was analyzed as well, by means of comparison. A mixture refrigerant NH3/CO2 gives a maximum COP at 30 wt% of NH3. Considering the expansion ratio in the upper-stage valve, it was verified that in the range of maximum COP the expansion ratio of 50% presents a slightly better COP. FTVI cycles present COPs that are 10 to 45% greater than the ones of the VC cycle, depending on the composition of mixture refrigerant and expansion ratio used. Â