Experimental Investigation of Thermal Performance Characteristics of a Refrigerated Truck Cargo Box with Different Insulation Densities and Wall Thicknesses

This study experimentally investigated the thermal performance characteristics of refrigerated truck cargo boxes with different XPS insulation densities and wall thicknesses.Two refrigerated truck cargo boxes were evaluated using the same refrigeration system and under identical operating conditions.Cargo box #1 employed XPS insulation with a density of 30 kg/m³ and a total wall thickness of 65 mm, whereas cargo box #2 employed XPS insulation with a density of 34 kg/m³ and a total wall thickness of 78 mm.The inner and outer skin materials were identical for both cargo boxes.Pull-down tests and thermal retention tests were conducted to evaluate their cooling and insulation characteristics.In the pull-down test, the elapsed times required for the average internal temperature to decrease from approximately 29°C to 0°C and -18°C were measured.In the thermal retention test, the refrigeration unit was stopped and the elapsed time required for the internal temperature to increase from -16.5°C to 0°C was measured.The pull-down time from 29°C to 0°C decreased from 56 min for cargo box #1 to 33 min for cargo box #2, corresponding to a reduction of 41.1%.The time required to reach -18°C decreased from 339 min to 164 min, corresponding to a reduction of 51.6%.In addition, the thermal retention time from -16.5°C to 0°C increased from 93 min to 135 min, corresponding to an increase of 45.2%.Cargo box #2 also exhibited intermittent compressor operation after approaching the target temperature, suggesting a potential reduction in refrigeration load.However, because the insulation density and wall thickness were simultaneously changed in this study, their individual effects could not be independently quantified.Furthermore, compressor power consumption and vehicle fuel consumption were not directly measured; therefore, the energy-saving effect was interpreted qualitatively rather than quantitatively.The results demonstrate that the combined modification of insulation density and wall thickness can effectively improve the pull-down and thermal retention performance of refrigerated truck cargo boxes and provide useful experimental data for the design of high-performance insulation structures.