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Optimizing the Design of a Box Type Direct Solar Dryer

M. Akhtaruzzaman
Assistant Professor
Department of Farm Power and Machinery, Bangladesh Agricultural University, Mymensingh, Bangladesh.

Abstract/ Executive Summary:

This paper contains the results of a study in which the solar energy potential for drying has been assessed, and the effects of some important design parameters on the performance of a box type direct solar dryer have been investigated with an objective to optimize the design for maximum drying rate. From hourly values of solar radiation measured at BAU in 1984, monthly average of daily total insolation were calculated which ranged from 7.74 to 19.30 MJ/m2/day. These amounts of solar energy were capable of removing at least 0.7 to 1.72 kg water per m2 per day. The average dryer temperature and the temperature rise in a box type solar dryer were found to vary widely with the area ofair openings. Maximum temperatures were obtained when the area of air openings were below 2% of the collector area. Maximum evaporations were observed at 4 to 5% air openings; below and above this range, the evaporation rates declined. A box type dryer, when provided with a chimney, increased the air temperature by 3.2⁰C and increased the drying rate by 2.3% as compared to those of a dryer without a chimney.

Keywords: Solar dryer, Drying, Improve the quality, Dried product.

Location:

Start Date:

End Date:

Program Area: Farm Mechanization

Commodity: Drier

Objectives:

To optimize the design of a solar dryer is to maximize the rate of drying and to improve the quality of a dried product.

Methodology:

The total insolation available on the dryer cover is given by, IT = GR AC (MJ/hr) ………..(1). where, GR = global radiation falling on the dryer cover (MJ/m2/hr), AC = collector (cover) area (m2), Now let eth is the efficiency with which solar radiation is collected and converted into heat, then the solar energy available for sensible heat gain by air inside the dryer is eth IT = eth GR AC (MJ/hr) ………..(2). On the other hand, the sensible heat gain by air inside the dryer is given by Ha = ma Cp t (Td - Ta) = ma Cp t TR (MJ) ………..(3), Where, Ha = sensible heat gain by air (MJ), ma = mass flow rate of air through the dryer (kg/hr), Cp = specific heat of air at constant pressure (MJ/kg 0C), Td = air temperature inside the dryer ( 0C), Ta = ambient temperature ( 0C), TR = temperature rise ( 0C), t = total drying time (hr). ma Cp t TR = eth GR AC t ………..(4). If mw kg water is evaporated from the product being dried in time t hours, then the heat required for vaporization of this moisture is Hw = mw L (MJ) ………..(5). where, L = specific latent heat of vaporization (MJ/kg). During drying, the latent heat of vaporization of food moisture is exchanged for sensible heat of the drying air. Then the basic energy balance equation for the drying process is given by mw L = ma Cp t TR ………..(6). Now combining the equations (4) and (6) we get, mw L = ma Cp t TR = eth GR AC t ………..(7). Therefore, mw /t = (ma Cp TR)/L =( ethGRAC)/L ………..(8). P1 – P2 = h ρ′g ………..(9). where h is the height between the air inlet and air outlet and g is the acceleration due to gravity. mw = 0.001 (ΔW. ma. t) (kg) ………..(10), Therefore, the moisture removal rate based on mass transfer is mw/t = 0.00l (ΔW. ma) (kg/hr) ………..(11). The performances of the dryers were evaluated on the basis of temperature rise in the dryer and the rates of drying. A potentiometer equipped with copper-constantan thermocouples was used to measure the temperature inside the dryer. Hourly readings from 18 thermocouples, set at strategic positions in the dryer, were averaged to get the average hourly values of dryer temperature. Dry-and wet-bulb temperature were measured by a hygrometer. A triple beam balance was used to determine the weight loss due to evaporation. Air-oven method was used to determine the moisture contents of the drying materials.

Source of Information: Bangladesh J. Agri. Res. 12(1) : 15-25

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

Monthly average of daily total insolation measured at BAU in 1984 ranged from 7.74 to 19.3 MJ/m2/day with capabilities of removing 0.7 to 1.72 kg water per day. The average dryer air temperature and the temperature rise in a box type solar dryer were found to vary with the size of air opening. Maximum temperatures were obtained when the area of air openings were below 2 percent of the dryer cover area. The optimum size of air passage for maximum drying rate was 4 to 5 percent of the cover size. A box type dryer, when provided with a chimney instead of air outlet holes, increased the air temperature in the dryer by 3.2^oC and increased the drying rate by 2.3 percent. Dryer sizing, with 240 cm × 120 cm and 135 cm × 90 cm floor area and with length-width ratio of 2:1 and 1.5:1 respectively, did not show any substantial differences in drying rates.

Knowledge Management: Journal