Feasibility Investigation Of Ammonia Refrigeration System In Cold Storage

Ammonia is a natural refrigerant with good thermodynamic properties and zero GWP and ODP values, so it is widely applied in refrigeration system for cold storage. However, ammonia has fatal defects of flammability and toxicity, which can lead to harmful accidents when the leakage occurred. It is considered as an effective method to ensure the safety by reducing the ammonia amount charged in the system. In this thesis, the rationed ammonia refrigeration system is presented to keep the thermal performance and decrease the ammonia amount in the system.Firstly, the rationed ammonia refrigeration system is compared with the traditional ammonia refrigeration system and direct-expansion ammonia refrigeration system. We can see that the rationed ammonia refrigeration system can lessen the high pressure liquid line between the reservoir and the evaporator and can reduce the fluid supply to the evaporator. So the amount of ammonia in this system can be reduced.Secondly, three void fraction models, Tom model, Zivi model and Smith model, are used to calculate the average density of two-phase ammonia in the evaporator coil. The results show that the amount of ammonia in the evaporator coil is reduced more than 60% when the circulating ratio is reduced from 4 to 1. Above all, the rationed ammonia refrigeration system can significantly reduce the amount of ammonia in the evaporator and the related supply/return lines and the related low pressure receiver by reducing the circulating ratio.Thirdly, the numerical model of the evaporator is built with the heat transfer experience formula of forced convection in tube. The influences of mass flow rate, circulating ratio, frost layer thickness on heat transfer were all analyzed by numerical simulation. The results show that the heat transfer rate is only 7.1% decrease when the circulation rate decreases from 4 to 1.Lastly, the test bench is built to experimentally investigate the effect of circulating ratio on ammonia amount and heat transfer performance in ammonia refrigeration system. The results also show that the ammonia amount can largely decreased when the circulation rate decreases and the heat transfer rate is 6.6% decrease when the circulation rate decreases from 4.7 to 1.3, which validates the numerical model. And both the numerical results and experimental data indicate that the circulating ratio has little effect on the heat transfer performance when the circulation ratio is higher than 1. Then, the rationed ammonia refrigeration system is applied in Xijiao Food Cold Storage Plant and the system is running well.In conclusion, the rationed ammonia refrigeration system significantly reduced the amount of ammonia in the refrigeration system while the heat transfer performance has little changes. The above results have great theoretical and practical values to improve the safety of ammonia refrigeration system.