Energy Efficiency, Air Curtain, Research and Testing Development, Supermarket, Cold Storage Facility
Air curtains are often used to reduce the energy transfer across high traffic doorways of cold storage facilities and cold food sections of warehouse type supermarkets. A test method and facility were developed to quantify and visualize the performance of air curtains covering a doorway between a simulated cold storage building and the simulated surrounding ambient conditions. This method was used to determine the effectiveness of horizontal air flow recirculatory air curtains compared to an open doorway. Tracer gas methods have been used in the past to determine losses but this method is designed to measure the actual energy flow through the doorway. Two environmental calorimeters were configured side-by-side with a 2.1m by 2.1m doorway separating high temperature and low temperature spaces. The higher temperature calorimeter contained PID controlled heaters and steam generators to maintain temperature and humidity conditions. The power input to these and all other electrical devices as well as the power loss through the calorimeter walls were measured. The difference at steady state between these values yields the energy transfer through the doorway. The lower temperature side contained a cooling coil located in a wind tunnel and PID controlled trim heaters to finely control the temperature. The coolant flow rate and temperature difference across the coil were measured along with the power consumption of all electrical devices and heat transfer through the walls. The difference between these values yields the heat transfer through the doorway which was used as a check for the calculation from the higher temperature calorimeter. The heat transfer was first measured through the open doorway with no air curtain with the warm side controlled to 24ºC and 60% relative humidity and the cold side controlled to 4ºC. The horizontal air flow, recirculatory air curtain was then installed, optimized, and tested at the same ambient conditions. The air curtain reduced the heat transfer between the calorimeters from 35.7kW to 10.3kW yielding an effectiveness of 71%. There is clear visual evidence that was measured by air thermocouple grids and shown in temperature gradient plots which exemplifies the effectiveness of the air curtain at creating a barrier between the warm and cold sides. The flow of warm air through the top of the doorway and the return flow of cold air through the bottom of the doorway with no air curtain was clearly visible and the temperature gradients became diagonal to horizontal. Upon air curtain activation, the temperature gradients became vertical showing that there was little energy transfer from side to side. In addition to energy savings, there is a comparable benefit in the reduction of humidity transfer which helps to prevent icing and condensation on products and on the floor. The reduction in humidity transfer also helps to prevent frosting of the cooling coils which results in fewer defrost cycles being necessary. Reduced frosting of cooling coils improves performance and further reduces energy consumption.