B angladesh J. Agri. Res. 12(1) : 15-25

OPTIMIZING THE DESIGN OF A BOX TYPE DIRECT SOLAR DRYER

M. AKHTARUZZAMAN

Abstract

This paper contains the res u lts of a stu dy in w hich the solar energy potential for dry ing has been assessed, and the effects of som e im portant design param eters on the perform ance of a box ty pe direct solar dry er hav e been inv estigated w ith an objectiv e to optim iz e the design for m ax im u m dry ing rate. From hou rly v alu es of solar radiation m eas u red at B AU in 1984, m onthly av erage of daily total insolations w ere calcu lated w hich ranged from 7.74 to 19.30 M J/m 2/day . These am ou nts of solar energy w ere capable of rem ov ing at least 0.7 to 1.72 kg w ater per m 2 per day . The av erage dry er tem peratu re and the tem peratu re rise in a box ty pe solar dry er w ere fou nd to v ary w idely w ith the area of air openings. M ax im u m tem peratu res w ere obtained w hen the area of air openings w ere below 2% of the collector area. M ax im u m ev aporations w ere observ ed at 4 to 5% air openings; below and abov e this range, the ev aporation rates declined. A box ty pe dry er, w hen prov ided w ith a chim ney , increased the air tem peratu re by 3.20C and increased the dry ing rate by 2.3% as com pared to those of a dry er w ithou t a chim ney .

Introduction The primary objective of optimizing the design of a solar dryer is to maximize the rate of drying and to improve the quality of a dried product. A number of factors are involved in the process of solar drying. These factors (Table 1) interact in a complicated manner, and it is really very difficult to quantify the extent of these interactions and their effect on the performance of the dryer. In this study, 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. Design of a solar dryer and its optimization require a clear understanding of the heat and mass transfer process that takes place during solar drying. In a box type direct solar dryer (a cross-section of the dryer is shown in Fig. 1), short wave solar radiation is transmitted through the transparent cover into the dryer box. This solar radiation is then absorbed by the black surfaces of the dryer panels, and the short wave solar radiation is converted into heat. The ambient air, entering into the dryer through the air inlet, is heated. The food materials, spread in a tray on the bottom panel of the dryer, receive heat from the warm air inside the dryer and the food moisture is evaporated. The solar-heated air A ssistant P rofessor, D epartm ent of Farm P ow er and M achinery , B angladesh Agricu ltu ral U niv ersity , M y m ensingh, B angladesh

16 AK HTARU ZZAM AN Table 1. Factors affecting the rate of heat and mass transfer during solar drying Factors related to food products

1. Su rface area 2. C ellu lar stru ctu re

3. P orosity 4. B inding of w ater

5. Solu te concentration 6. Tem pertu re

7. Heat condu ctiv ity 8. Absorptiv ity of solar radiation

9. Initial m oistu re content 10. D epth of food lay er

Factors related to drying air

1. Tem peratu re 2. Relativ e hu m idity

3. Air flow rate 4. Air ex pos u re tim e

5. M ode of air circu lation

a. natu ral flow (free conv ection)

b. forced flow by fans

Factors related to heat source and conversion mechanism 1. G lobal radiation as affected by 2. C ov er m aterial

a. tim e of the y ear a. short-w av e transm ittance

b. tim e of the day b. long-w av e em ittance

c. site of the installation in relation to latitu de d. ex tent of clou d cov er

3. Absorber m aterial 4. C ollector siz e

a. short-w av e absorptance

b. long-w av e em ittance

5. Inclination of cov er 6. Inclination of absorber

7. Alignm ent of collector in relation 8. Ins u lation m aterial

to solar altitu de and az im u th angle a. heat resistiv ity

b.thickness

absorbs the evaporated moisture and escapes through the air outlet. This process of heat and mass transfer during solar drying is explained by a schematic diagram shown in Fig. 2. Theoretical Design Considerations Heat Transfer 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)

B O X -TY P E SO LAR D RY ER 17

Fig. 1 Cross-section of the solar dryer

Fig. 2 Heat and mass transfer

during solar drying 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) Evidently, 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)

18 AK HTARU ZZAM AN 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) The temperature rise (TR) inside the dryer is proportional to the total insolation. The intensity of solar radiation (GR) depends on the time of the year, time of the day and the extent of cloud cover in the sky. The efficiency factor (eth) is dependent upon (i) the short wave transmittance and long wave emittance of the transparent cover, (ii) the short wave absorptance and long wave emittance of the black absorber surfaces and (iii) the inclination of the cover and the black surfaces. Maximum radiation is transmitted through the dryer cover when the beams of solar radiation fall normally on the transparent cover. The cyclic variation of solar altitude and azimuth angle necessitates the use of a sun-following surface for interception of maximum energy but since its construction and use is very difficult, fixed collectors are always preferred. In northern hemisphere, fixed collectors generally receive the most energy per day if they face due south. Deviations up to 15o from due south make little difference in the total energy received (MPWS, 1980). On the other hand, a tilt angle equal to latitude is the best if the collector is used all the year round (MWPS, 1980). Bangladesh lies between 20o34′ and 26o38′ N latitude. So a tilt angle of 20 – 26o is alright for all places in the country. The values of Cp and L can be assumed constant. The value of Cp at atmospheric pressure equals 1.02 kW/kg.oC. Within the temperature range of 20 to 60oC, L for free water varies from 2.45 to 2.35 MJ/kg with a maximum variation of ± 4%. L for food products is slightly higher depending upon the kind of product and its composition. For approximate calculations, L can be taken as 2.8 MJ/kg (Exell et al., 1980). Now the equation (8) indicates that the drying rate (mw/t) is proportional to the product of temperature rise (TR) and the mass flow rate of air (ma) in general, increased air flow and temperature increase the drying rate but increased air flow decreases the dryer air temperature. The mass flow rate of air across the dryer depends on the area of air opening, difference between air densities inside and outside the dryer and the difference in height between the air inlet and the air outlet. If the dryer is wide open to atmosphere, it approaches the open air drying conditions; the air inside and outside the dryer will have the same temperature and humidity. Thus the main purpose of using the dryer enclosure (to increase the temperature for increased drying rate) is not served. On the other hand, if the air-passage is completely closed, there will be no air flow across the dryer. If the air is still in the dryer, it will create a saturated atmosphere which would slow down subsequent moisture removal. So it is important that the size of air opening should be optimum for maximum drying rate.

B O X -TY P E SO LAR D RY ER 19 The air flow across the dryer due to natural convection is caused by a pressure drop resulting from the difference between the density of relatively cool ambient air and the warm air inside the dryer. Let P1 and P2 be the air pressures at the level of air inlet and air outlet respectively and let ρ′ be the density of air inside the drier, then the pressure drop across the dryer is 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. This height can be increased by adding a chimney with the dryer as shown in Fig. 3. This results in increased air flow without substantial decrease in air temperature. Mass Transfer The amount of moisture that can be removed from a product by drying depends not only on the heat transfer from the air to the product but also on the capability of the air to absorb moisture. Increased temperature increases the moisture absorbing capacity of the air. This is explained in Fig. 4. The unheated air at 30oC with 75% relative humidity (RH) has a maximum capacity of absorbing 1 gm water per kg dry air but if this air is heated to 45oC (resulting in 15oC temperature rise), its RH is reduced to 33% and its moisture absorbing capacity increases to 6.5 g/kg dry air. The amount of moisture that can be absorbed by air flowing through the dryer in time t is given by mw = 0.001 (∆W. ma. t) (kg) ………..(10) Therefore, the moisture removal rate based on mass transfer is mw/t = 0.00l (∆W. ma) (kg/hr) ………..(11) The moisture content of a product at which the vapour pressure of the product moisture counteract the vapour pressure of air moisture (resulting in stop of mass transfer from the product to the air) is called the equilibrium moisture content (EMC) of the product. In a solar dryer, temperature increases and as a result, EMC decreases; on the other hand, RH decreases and as a result, EMC decreases. So higher is the temperature rise in the dryer lower will be the EMC and higher will be the drying rate. Materials and methods In order to determine the solar energy potential for drying, the intensities of solar radiation were measured at the Bangladesh Agricultural University, Mymensingh, on different dates in different months in 1984. An indicating type solar meter was used to measure the solar radiation. The hourly values from 9:00 AM to 4:00 PM on each date were integrated to determine the daily total insolation. Daily total insolation on different dates in a month were averaged to get the monthly average of daily total insolation.

20 AK HTARU ZZAM AN

Fig. 3 Pressure drop across the dryer

Fig. 4 Drying potential of solar- heated air

Fig. 5 Variation of temperature and evapo- ration ratio with air opening area

B O X TY P E SO LAR D RY ER 21 Several box type direct solar dryers were constructed using bamboo mats for the box panels, wood for frame, dry paddy straw for heat insulation, charcoal paste for black surfaces and polyethylene sheet for dryer cover. The dryers were provided with adjustable air opening. Hinged doors were provided on the rear panel for loading and unloading the dryer. 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. Results and Discussion From the solar radiation data measured at the BAU, monthly average of daily total insolation was computed for each month. These are shown in table 2. The table indicates clearly that there was wide variation in daily total insolation depending on the time of the year and the presence of cloud in the sky. During the months from May to September, insolations were low because of cloud cover. Daily total insolations were found to vary from 7.74 MJ/m2/day in September to 19.3 MJ/m2/day in March. A plot of hourly solar radiation with time on a typical day is shown in Fig. 7. The evaporative potential of available solar energy were computed assuming 25% efficiency, which are presented in table 2. Table 2. Monthly average of daily total insolation measured at BAU, Mymensingh in 1984

M onth A v erage daily total insolation Ev aporativ e potential

kW /m 2/day M J/m 2/day kg w ater /m 2/day

Janu ary Febru ary M arch April M ay Ju ne Ju ly A u gu s t Septem ber O ctober N ov em ber D ecem ber

4.47 4.42 5.36 5.13 3.56 2.47 2.55 2.81 2.15 3.47 3.60 3.41

16.09 15.91 19.30 18.47 12.82 8.89 9.18

10.12 7.74

12.49 12.96 12.28

1.44 1.42 1.72 1.65 1.14 0.79 0.82 0.90 0.69 1.12 1.16 1.10

A v erage 3.62 13.02 1.16

22 AK HTARU ZZAM AN

In a study, the variation of dryer air temperature with the size of air opening was investigated. The areas of air openings were varied from 1 to 6 percent of the dryer cover area. The dryer air temperature and the temperature rise in the dryers were found to increase as the area of air openings were decreased. The dryers developed maximum temperatures when the areas of the air passages were 1 and 2 percent. The results are shown in table 3. A plot of the dryer-to-ambient temperature ratio with percent air opening is shown in Fig. 5. The temperature ratio gradually decreased as the air-openings were increased. The temperature of air inside the dryer was found to vary widely at different regions as shown in Fig. 6. The air temperatures were gradually high from front to the rear part of the dryer and from lower to the upper part. At the outlet, the temperature was maximum and at the inlet it was minimum. Table 3. Variation of dryer temperature and rate of evaporation with the size of air opening D ry er N o.

O pening area as % of collector area

A v erage dry er tem p. (0C )

A v erage am bient tem p. (0C )

Tem p. rise (0C )

D ry er-to am bient tem p. ratio

Ev apo- ration of w ater in the dry er (kg)

Ev apo- ration of w ater in open floor (kg)

Ev apo- ration ratio

D 1 1.0 50.5 28.7 21.8 1.76 1.593 1.378 1.16

2.0 49.3 27.4 21.9 1.80 1.624 1.441 1.13

3.0 43.1 24.7 18.4 1.75 1.576 1.410 1.12

4.0 40.3 24.8 15.5 1.62 1.388 1.246 1.11

5.0 37.3 23.4 13.9 1.59 1.597 1.354 1.18

6.0 35.5 24.8 10.7 1.43 1.110 0.988 1.12

C ollector Siz e = 122 cm × 95 cm

D 2 1.0 48.7 23.2 25.5 2.10 1.662 1.401 1.19

2.0 48.7 26.7 22.0 1.83 1.851 1.679 1.10

3.0 44.5 24.1 20.4 1.84 1.517 1.347 1.13

4.0 44.0 24.5 19.5 1.80 1.559 1.304 1.20

5.0 40.0 24.6 15.4 1.63 1.549 1.267 1.22

6.0 35.8 23.2 12.6 1.54 1.005 0.957 1.05

C ollector Siz e = 124 cm × 97 cm

D 3 1.0 49.7 28.7 21.0 1.73 1.545 1.378 1.12

2.0 48.2 27.4 20.8 1.76 1.531 1.441 1.06

3.0 43.5 26.4 17.1 1.65 1.482 1.400 1.06

4.0 39.2 24.1 15.1 1.63 1.409 1.123 1.25

5.0 38.0 24.1 13.9 1.57 1.353 1.092 1.24

6.0 34.7 23.9 10.8 1.45 1.328 1.177 1.13

C ollector Siz e = 122 cm × 99 cm

B O X TY P E SO LAR D RY ER 23

Fig. 6 Dryer cross-section showing locations

of thermocouples and temperatures

Fig. 7 Variation of dryer temperature and reduction in moisture content of spinach with time of the day

24 AK HTARU ZZAM AN In a study, the area of air passage was varied from 1 to 6 percent of the cover area to investigate the effect of this variation on the rate of moisture evaporation. Daily totals of evaporated moisture from the dryer and the open floor were measured. In every test, the amount of water taken for evaporation and the surface area from which evaporation took place were kept constant. The results of some of the tests are presented in table 3. The maximum evaporations were obtained when the air opening size was within the range of 4 to 5 percent of the cover area. Below and above this range, evaporations were declined. A plot of dryer-to-open floor evaporation ratio with the percent air opening is shown in Fig. 5. The test results revealed that the optimum size of air opening was 4-5 percent of the dryer cover size. The performance of two dryers, one with a chimney and the other with air holes on the rear panel, were compared. Several drying tests were conducted to study the improvement of dryer performance due to a chimney. The area of air outlets were the same in both the dryers. Temperature rise in the dryer with chimney was higher than that with air holes. On average, the temperature difference between these two dryers was 3.2oC. When tested with spinach, the chimney increased the drying rates by 2.25 percent, and drying time was reduced by 3 hours. Fig. 7 shows the temperature and moisture curves obtained from the results of one of the tests. The box type solar dryers with floor area of 240 cm × 120 cm and 135 cm × 90 cm and with length-width ratio of 2:1 and 1.5:1 respectively were compared in terms of drying rates. But these two sizes did not show any substantial differences in drying rates. Conclusion Monthly average of daily total insolations 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 3.2oC 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.

B O X TY P E SO LAR D RY ER 25

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1. N o. 2, Janu ary . 1980 M idw est P lan Serv ice (M W P S). 1980. Low Tem peratu re and Solar G rain D ry ing Handbook. Iow a State

U niv ersity , Am es. Iow a 50011 P otter, N .N . 1978. Food Science, Third Ed., AV I P u blishing C o. Inc., W estport. C onnecticu t.