Energy and Function Analysis of Hot Gas Defrost in Ammonia Refrigeration Systems

摘要

Ammonia has over decades proven its value as an effective refrigerant,but choosing the right-and correctly sized-defrost and control methods is important to ensure high efficiency.Traditionally one of two methods for controlling drainage of the evaporator during hot gas defrost is used: pressure control,which keeps the pressure in the evaporator constant during defrost or liquid drain control,which uses a float valve to drain condensed liquid from the evaporator.Each method’s energy consumption is quite different as the pressure-control method bypasses a certain amount of hot gas during the defrost period.This paper is based on results from a research project focusing on energy savings potential during hot gas defrost in ammonia refrigeration systems(ELFORSK project 347-030J.In the ELFORSK project an ammonia pumped circulation system was built at the Danish Technological Institute,enabling detailed measurements of the defrost system.Two methods of hot gas defrost were tested and analyzed(pressure control and liquid drain method),as were three evaporator designs(bottom feed,top feed,and side/bottom feed).A simulation model was also developed and validated using the measurements.This paper will focus on the design requirements of the two most common defrost methods for ammonia systems(pressure control and liquid drain method)and describe the design requirements for both systems to obtain the highest efficiency.The efficiency of the Mo defrost systems will be analyzed and compared.