The temperature of frozen foods in open display freezer cabinets: simulation and testing

Frozen foods require strict temperature control throughout the so-called "freezer chain". A particular storage temperature is needed for each link in the chain, according to the interval involved. For delivery vehicles and retailing cabinets, a storage temperature of -18°C is specified by European Union regulations, which allow for fluctuations of up to -15° or even -12°C. Most frozen-food cabinets in retail stores are forced-air open display cabinets. Though improvements have been made in the last 20 years, it is still hard to maintain correct storage temperatures in such freezers. In fact, their performance in terms of the temperature of the contents is strongly affected by ambient conditions. Slight changes in the room temperature, and particularly in the air circulation and raditive heat flux, cause considerable differences in the temperature of the load. A higher ambient air temperature and radiative heat mostly lead to a higher temperature in the upper layers of frozen foods, while greater air movement leads to early frosting of the evaporating coil and air ducts due to the infiltration of moisture through the cabinet opening. In the European Union the prEN 441 Standard is now being completed. It deals with the performance testing of open display freezer cabinets and defines five climate classes depending on room temperature and relative humidity, for all of which the specified room air velocity is 0.2 ± 0.1 m/s in the direction of the longitudinal axis of the cabinet. Each cabinet should be certified to comply with the Standard in force for a particular climate class and, once installed in the retail store, it should be kept at the climate conditions defined in the Standard. However, lighting or forced-air heating and conditioning can induce changes in the performance of the cabinet and therefore in the temperature of its contents. A great deal of literature has become available since the first papers in 1960-70 /1,2/, but a deeper knowledge of heat exchanges in open cabinets is still necessary. The availability of methods for predicting performance is also very useful for estimating the operating condition ranges within which proper food storage can be ensured. This paper presents a simple application of a two-dimensional finite element method for establishing the load temperature distribution in an open display freezer cabinet. Some experimental data are also reported to illustrate the considerable influence of room climate conditions on the temperature of the load.