Experimental Investigation of the Efficiency of Heat Pump Drying System with Full Air Recirculation

Abstract

The paper presents results of an experimental investigation of convective drying of apples var. Idared, potatoes var. Desiree and bananas var. Grand Nein, in full recirculation laboratory-scale dryer. The experiments were conducted with cube-shaped apple samples (side length 10 and 13mm), potato chips (slice thickness 2 and 3mm) and banana cylinders (slice thickness 6 and 7mm), without pre-treatment. The drying experiment was conducted at fully controlled air temperature (35-45-55 degrees C), relative humidity (10-20-30%) and air velocity (1-1.5-2m/s). Results are discussed through their comparison with other heat pump drying system scenarios in order to find the most efficient one. The efficiency of the heat pump drying (HPD) system was analyzed using the several indicators: specific moisture extraction rate (SMER), specific energy consumption (SEC), heat pump dryer efficiency (HPDE) and moisture extraction rate (MER). The average values of the full air recirculation HPD system efficiency indicators were obtained in range: SMERHP=0.030-1.317kg/kWh, SMERHPD=0.047-0.651kg/kWh, SEC=0.211-9.358 MJ/kg, HPDE=0.426-20.7 MJ/kg and MER=1.362-17.194 kWh/kg. The results were evaluated in order to provide better information about the benefits of full air recirculation usage in HPD systems in regards to the other similar dryer configurations. Practical ApplicationsThis study uses full air recirculation to provide better control of the main drying parameters and to minimize impact of the surroundings. The novelty of this study lies in the fact that RH of the drying air was precisely controlled during all experiments. The evaluation of the relevance of full air recirculation HPD system is rare in literature and therefore the present study is directed toward quantification of influence of HPD system configuration on drying process efficiency. Dependence of several energy efficiency indicators of HPD system performance: SMER, SEC, HPDE and MER on HPD system air recirculation rate were investigated. The results were evaluated in order to provide better information about the benefits of full air recirculation usage in HPD systems in regards to the other similar dryer configurations.