EVALUATION THE PERFORMANCE OF A HORIZONTAL A PROTOTYPE NATURAL DRAUGHT SOLAR DRYER

Abstract

In the present work, the experiments were carried out at the Mechanical Power Department, Faculty of Engineering, El-Mansoura University, in the year 2008. An experimental study on a horizontal prototype solar dryer with natural draught of water vapor. The experiments were performed some days during four menthes from March to June. Experimentation started at 9 a.m. and ended at 4 p.m. The present work is aimed to examine the thermal performance of a prototype solar dryer with natural draught under Egyptian climate conditions. In this process, solar radiation striking the wetted black coated bed surface causes evaporation to occur. The vapor transfers from the bed, by convection through the outlet duct to the atmosphere. The solar dryer under investigation is a simple rectangular basin containing a wetted black wick and covered with a glass layer. A vertical duct is used to remove the water vapor from the solar dryer enclosure. The ambient air is drawn to the system by heating the black wick and the hot and humid air and then vented via a vertical draught tube. Experiments are carried out to examine the variation of system parameters with time. The objective of the experimental work is to study the performance of the dryer and evaluate the effects of ambient conditions on the rate of evaporation of water from the wetted wick. Measurements of solar radiation, ambient air temperature and humidity of inlet and outlet air through the duct are taken during the experimental testes.The mass transfer potential which is defined as the vapor pressure difference between the hot wick and the ambient air inside the system is also evaluated and presented graphically with time. Also, system drying efficiency is defined and plotted with time for a sample of the experimental data. Results have shown that as follows: 1- The wetted black wick surface temperature increase with time and reaches a maximum limit in the afternoon, then decreases with time again at the end of the day. 2- The wetted black wick surface temperature increase with time and reaches a maximum limit in the afternoon, then decreases with time again at the end of the day. 3- The values of absorber surface temperature are higher than that of outlet and inlet streams of air. 4- The potential for mass transfer of vapor from the wetted black coated surface to air is proportional to the vapor pressure difference between black coated surface and air stream. 5- In most cases, the average of vapor pressure on the wetted black wick reaches a maximum of 41 kN/m2 at the noon, which is about 34 kN/m2 higher than that of the air stream at dryer exit 6- Augmentation of the natural draught can be obtained with higher values of vapor pressure on the black coated surface. 7- The variations of drying air temperature at the inlet and outlet of the dryer and the corresponding relative humidity at the inlet and outlet of the dryer was well as the relative humidity of the ambient air. 8- The hourly values of outlet air humidity can be used to evaluate the cooling rate of the system, which equals the product of latent heat of water and the mass of water evaporated. 9- The solar radiation increases gradually to a maximum value of 734.38W/m2 near noon time, and then decreases again to the end of the day 10- The heat transfer rates qr, qc, qe and qia increases with time and reaches a maximum limit near the noon time, then decreased again to the end of the day. 11- The dryer efficiency increases with the increase of solar radiation and decreases with the decrease in the air flow rate. The maximum dryer efficiency 21.95% has been obtained. Finally, it must be recorded that no condensation of water vapor on the glass surface observed during experiments. This is a proof that the vapor pressure difference between air and glass is kept at maximum level by the draught of air. The long term performances of the considered solar air dryer clearly prove that these systems can be used as a source of the hot air for space heating and drying applications for different agriculture crops all over the year.