Tracer gas technique: A new approach for steady state infiltration rate measurement of open refrigerated display cases

Abstract

A new approach, combining experimental and analytical methods, for finding the infiltration rate of outside warm air into open refrigerated display cases is introduced. This method is based on direct measurements and is more reliable and accurate than conventional techniques. Although a typical display case usually benefits from an air curtain at its opening to reduce the infiltration of the warm ambient air into the system, still the infiltration accounts for up to 75% of the total cooling load of their refrigeration system. In 1999, for example, about 69,000 of such display cases were operating in the supermarkets and grocery stores of the USA with annual energy consumption more than 1000 GW-hr (California Energy Commission (CEC) and Southern California Edison (SCE) Co., (2003). Private Communications.). Therefore, determining the exact amount of their infiltration rate and improving their performance are of great importance. In this method, a tracer gas is released into the air inside the duct of a display case and after mixing, the mixture discharges into the display area and ambient through the air curtain and/or the perforated back panel of the display case. This technique is based upon the concentration measurements of the tracer gas at three locations: Discharge duct, return duct and the ambient. Finding the percentage of the tracer gas that is lost (by escaping into the ambient) between the Discharge and the return ducts forms the fundamental of this approach. By analyzing the mass conservation around the air curtain of the display case, a relationship between the tracer gas concentration and the infiltration rate can be established.