Dryer design

The design of the dryer is based on some meteorological conditions on the day of the trial and experimental characteristics of the Ataulfo mango. For the conditions of the trial day (April 21), an actinometric station was used at the Center for Research and Development of Renewable Energies (CIDTER) of the University of Sciences and Arts of Chiapas (UNICACH), where the dryer was developed solar. This station registers a large number of ambient parameters, only interested for this study the solar radiation, the ambient temperature and relative humidity, through its WatchDog Data Logger 450 recorder. This recorder includes clock and internal memory to record the time and day of the readings. Measurements were made to the variables every half hour. Registers temperatures between -20 °F to 70 °F and relative humidity between 20% to 100%, with accuracies of ±0.7 °F and ±3% respectively. Solar radiation was recorded with a pyranometer attached to it, with a range between 300 and 1100 W m^-2 and an accuracy margin of ± 5%. The data that was useful for the design were.

Average solar radiation: I_t= 500 W m^-2; average ambient air temperature: T_a= 298 K; average relative humidity of the ambient air HR= 47%. Mango data were obtained from the bibliography referenced below: initial humidity of mango: M_iwb= 80% (fresh product) (^{Buitrago, 2014}); final humidity of mango: M_fwb= 10% (equilibrium moisture) (^{Buitrago, 2014}); density of the ataulfo mango: ρ=1 050 kg m^-3 (^{González et al., 2015}). Porosity of mango: ξ= 0.9 (^{Ramírez et al., 2010}).

The handle goes through a pre-treatment consisting of washing, peeling and slicing. These slices are submerged in a solution with 2% citric acid to avoid oxidation, obtaining a clean initial mass of mango: W_w= 100 kg.

Calculation of the air flow required for drying

^{Hematian et al. (2012)} stated that the increase in ambient air temperature when passing through a solar collector is:

Where: ΔT is the temperature difference between the air leaving the collector and the environment; β is a dimensionless parameter that is between 0.14 - 0.25 and the value of 0.2 was assumed; T_b is the boiling temperature of water at atmospheric pressure (100 ℃); T_c; is the freezing temperature of water at atmospheric pressure(0 ℃); I_t is the intensity of the average incident solar radiation on the collector plane and I_o is the solar constant (1 367 W m^-2).

The amount of water that must be extracted to the product to bring it to the final humidity is according to ^{Forson et al. (2007)}.

Where: W_w is the initial mass; M_iwb is the initial moisture base humidity of the mango; M_fwb is the final moisture of the wet base mango.

^{Forson et al. (2007)} proposes that the total volume of air needed (V_a), to eliminate the amount of water to the product is evaluated from the equation.

Where: R_a= 283 kJ kg^-1 K^-1 (constant of the ideal gases); P_a is the partial pressure of the dry air in the atmosphere; C_pa= 1.012 kJ kg^-1 K^-1 (specific heat of air at constant pressure), T_f is the average temperature of the air that comes out of the drying chamber; L_t is latent heat of evaporation. T_f= T_a + 0.25(ΔT), where: T_a a is the average temperature of the ambient air and T_o= 323 K (collector air output temperature) (^{Cengel and Boles, 2011}).

The average temperature of the product during drying (T_pt)was estimated as the weighted average of the temperatures T_o and T_a: T_pt= 0.25(3T_o+T_a)= 316.5 K (^{Aquino et al., 2009}).

^{Chavez (2012)} proposes that the value of the latent heat of evaporation of equation (3) can be estimated by the following expression.

Where: R_g is the gas constant for water vapor (461 J kg^-1 K^-1); T_b is the boiling point of water (373 K); P_c is the critical water pressure (22.1 MPa); T_c is the critical water temperature (647.4 K) (^{Hernández, 2014}).

The air flow necessary for drying the product was obtained by:

Where: t is the time needed to dry the mango to its equilibrium moisture, according to the experimental tests it was t=8 h.

Calculation of the drying area inside the chamber

^{Forson et al. (2007)}states that the effective area for drying (A) can be calculated by:

Where: ρ= density of the product in wet conditions, h_l= thickness of the product layer in the tray (0.01 m); ξ= porosity of the product; ε_v= fraction of the empty tray determined experimentally (ε_v= 0.3).

The loading density of the trays is determined by:

Calculation of the catchment area of incident solar energy

The catchment area of a solar collector is related to the efficiency of the drying system (η_s)that is given by the equation: (^{Duffie and Beckman, 1980}).

Where: A_c is the total catchment area; I_t is the energy incident on the dryer.

Determination of the drying curve

To trace the curve of the drying kinetics of a product it is necessary to measure its loss of weight at fixed intervals of time. The drying ratio was determined with the following expression (^{García et al., 2015}; ^{Doymaz, 2004}).

DR shows the drying curve (kg_agua kg^-1 _seco); (M_t+dt - M_t) is the variation of the mass of the product (kg) and dt is the variation of time.

Agricultural products are hygroscopic, so they always have a residual moisture content, which for each storage environment is called “equilibrium moisture” (^{Ekechukwu, 1999}).

Description of the experiment

First a vacuum evaluation was carried out (without product) to test the thermodynamic behavior of the drying installation, where thermodynamic performance parameters of the dryer were obtained. For this evaluation, instruments that are detailed below were placed.

Thermocouples for measuring temperature, placed in: drying chamber, upper adsorber plate, lower adsorber plate, damp air outlet door of the chamber to the environment, in the product, to the air inlet to the collector and to the collector air outlet. They are “K” type thermocouples that are connected to a STEREN multimeter hook type MUL - 100 (with range up to 750 ^oC). Relative humidity sensor, in the drying chamber, brand HANNA HI-6838.

For the experimentation with loading (with product), a pre-preparation of the mango was carried out, consisting of washing, peeling and cutting in 0.01 m thick flakes, these were submerged in a 2% citric acid solution to avoid oxidation, so I’m ready to put it in a single layer in the dryer trays. These were built with the wire mesh background. They were allowed to drain for half an hour before placing it inside the dryer. Once the entire handle was placed inside the dryer the entire mango began recording the weight loss of a 21.2 g sample of mango with an American Weigh electronic scale. For the processing of data a personal computer was used.

Design considerations of the solar dryer

With the results of the design calculations the type of dryer is selected and it is decided to make a dryer with two collectors one of natural convection (primary) in the lower part and one convection blanket (secondary) in the top or ceiling of the dryer. This configuration is to give you more flexibility in its operation, since this way you can work the dryer only by natural convection in places where electrical power is lacking in the network just by leaving open the compound ones (6) and in case if there is electrical energy, the dryer can use the two collectors to heat the air and only the composite ones open (6) when the air in the chamber is saturated.

In addition, this secondary collector in the ceiling of the dryer substantially reduces heat losses through the roof, where they are usually significant, even in the case that forced air does not circulate through this, also contribute to increase the thermal efficiency of the dryer as it would heat its absorber sheet and transmit that heat by natural convection to the drying chamber and thus constitutes a thermal barrier.

According to the result of the catchment area (A_c= 5.8 m²), it was decided to make a dryer with 1.5 m width, with this starting measure the areas between the two collectors were distributed. Taking into account that the heat transfer in forced fluids is more intense than in convective fluids, an area greater than 3.6 m² (2.4*1.5 m) was assigned to the secondary collector and to the primary collector 1.25 m² (1.5*1.5 m).

Between both they add 5.85 m² of collection area, which represents 8.6% greater than the calculated catchment area, which is assumed as security rank. Respecting that the dryer has a width of 1.5 m and that the effective drying area is A= 15.12 m², the dimensions of the chamber will be 1.5 m*2.3 m and height 1 m on the front and 1.65 m on the back. This guarantees the inclination of the secondary collector (ceiling of the dryer) of 18°, which gives an approximate volume of the drying chamber of 3.8 m³. With these dimensions it will be possible to accommodate 12 trays of 1.26 m² each, with dimensions of 0.9*1.4 m, arranged in two rows of 6, separated 0.1 m between them. The general dimensions are presented in Figure 1.

Figure 1 Main dimensions of the solar dryer. 

To ensure its transportation and assembly of the mango solar dryer is built in such a way that the parts can be coupled by means of screws. The assembled parts are the two collectors, the chamber is formed by screwing the walls to a frame, the doors are assembled by the hinges. Once formed in this way, the guides for trays or tray carriers previously assembled by screws are introduced. Figure 2.

Figure 2 Photos of the solar dryer.  

To guarantee the separation of the floor and the appropriate inclination of the primary collector, a base is available in the lower part of the chamber with a height of 0.35 m. To allow access to the interior of the drying chamber, two side doors are designed in the form of two sheets.

Description of the dryer

In the Figure 3 shows a diagram of a side cut of the solar dryer. It is composed of the primary (1) and secondary (9) collector and the drying chamber (4). It also has an axial fan (7) on the back, which has the function of recirculating the air forcedly through the secondary collector and the drying chamber. The primary collector is in the lower part where the ambient air passes through it under a thermosyphonic regime. Solar energy for through the covers of both collectors (3) and (8).

Figure 3 Side cut of the solar dryer. 

In the back part there are two openings (6) for the renewal from the saturated air chamber and ambient air inlet with diameter of 0.1 m, made of 4” PVC pipes, with their plugs. The product is placed in the trays (5). The entire body of the collectors and the drying chamber is made of insulated multimide panels (2). The movement of the air inside the dryer is described with arrows.

In the Figure 4 shows the results of the vacuum evaluation of the dryer, the variation of solar radiation over time is observed with a maximum radiation value of 910 W m^-2, between 1:00 pm to 2:00 pm. The average solar radiation in the hours with the highest solar incidence (10:00 and 16:00 h) is 500 W m^-2, this being a very good value from the spring season in which the research was carried out (April 21, sunny day with few clouds).

Figure 4 Results of the vacuum evaluation of the dryer. 

The variation of the temperatures of the ambient air, of the air in the drying chamber and of an average of the two absorber plates of the two collectors are also appreciated. The temperature of the air (drying agent) increases during the day to the same extent that solar radiation increases. The average room temperature was 25 °C and in the drying chamber it reached 52 °C. This increase of 27 °C is very appropriate for this type of installation and suitable for the process of drying the handle. The upper absorber plate reaches maximum temperatures of 70 °C in the hours of highest radiation, this indicates that there are few heat losses in the solar collectors, a good hermeticity and a suitable design, since it is a high temperature for the spring time. These values are similar to those of ^{Iglesias et al. (2011)}.

In the Figure 5 shows the variation of the relative humidity of the ambient air and inside the drying chamber as a function of time. This drops from 52% to a minimum of 1.5% (no load) at noon, according to the variation of the incident radiation. An average value of 5.5% is obtained between 10:00 and 16:00 h, a very appropriate value for drying agricultural products, since this air exhibits a great avidity for the humidity of said product.

Figure 5 Variation of the relative humidity of the air in the drying chamber and the environment.  

In the Figure 6 shows the variation of the weight of a sample of 21.2 g of Ataulfo mango on the day of experimentation. The weight begins to decrease gradually. As of 2:00 pm, the decreases are smaller, until at 5:00 pm equilibrium humidity is reached and the weight remains constant.

Figure 6 Variation of the weight of the Ataulfo mango sample. 

In the Figure 7 shows the variation of mango moisture, from its initial humidity of 80% to 8%, which is 1.6% below its equilibrium humidity, according to ^{Buitrago (2014)}.

Figure 7 Variation of humidity percent of Ataulfo mango. 

As shown in Figures 6 and 7, the curves behave very similar to the curve because the product (mango) when dehydrating is losing water (moisture) and therefore weight. Since that is the process of drying, in the extraction of water from a product that is part of its mass.

A typical drying curve is obtained by plotting the free moisture content (X) (kg of water/kg of dry solids) versus the drying time as shown in Figure 8. In equation (13) the form is exposed to calculate this free moisture (^{Aviara et al., 2002}).

Figure 8 Variation of the free humidity of the Ataulfo mango.  

Where: X_t is the humidity that is registered in time and X_eq is the equilibrium humidity.