A Simple Method of Designing Low Temperature Freezer Using Double Stage Refrigeration System For Small Capacity

Typically a double stage vapor compression refrigeration system is applied for low freezer temperature, because single compressor is not effective for this purpose due to low performance and overheating on the compressor. A prototype of a double stage refrigeration system using R22 as working fluid has been designed and constructed. The prototype uses two compressor, each having 0.25 HP (187.5 W) nominal power at 220 V. The evaporator made from copper tube of 5/16 inch (7.9 mm) and the liquid intercooler was made tube-in-tube form, with the outer tube 5/8 inch (15.8 mm). Design temperature of the condenser, evaporator and liquid intercooler are 40°C, -55°C and 12°C, respectively. In order to obtain the predesigned temperature, the appropriate length of evaporator and intercooler were determined based on the heat transfer analysis. The results from this analysis, the length of evaporator and liquid intercooler were 28.38 m and 0.55 m, respectively. The experimental results showed that the condensing temperature was lower than that of theoretical model, which is 33°C, the evaporating temperature was higher than that of model, that is -53°C and the liquid intercooling temperature was the same with the model. The coefficient of performance (COP) of a double stage prototype was 1.74, whereas the COP of single stage was 1.4 for the same operating conditions.