Design , Fabrication and Performance Evaluation of a Charcoal-Fired Tomato Dehydrator for Developing Countries

For the purpose of enhancing the food security and economies of developing countries, thereby meeting the sustainable development goals (SDGs 2030) especially SDG No. 2, which is concerned with hunger alleviation, achieving food security, improving nutrition and promoting sustainable agriculture before the year 2030, an energy-efficient charcoalfired tomatoes dehydrator having an efficiency of 88.6% and a drying rate of 0.51 kg/hour has been designed, fabricated and tested. It basically comprises of a combustion unit (CU) for heat energy generation through the combustion of charcoal, a heat exchanger unit (HEU) comprising of fins for efficient transfer of the generated heat through a 240V 0.15 HP DC centrifugal fan and a drying unit (DU). The results of its performance evaluation showed that, with a safe drying air temperature of 50 oC at 6 m/s fan speed, the dehydrator is capable of drying 5 kg of tomatoes per batch from an initial moisture content of 94% to 22% with a final weight of 1.4 kg over a drying period of about 7 hours. The microbiological analysis conducted on the post-drying tomato sample revealed a total bacterial counts (TBC) of 1.61 x 10 cfu/g and a total fungi counts (TFC) of 0.27 x 10 cfu/g which are both far below the allowable limits (10 cfu/g) for human consumption. The dehydrator has proved effective for extending the shelf life of tomatoes by mitigating the rate of spoilage due to microbial activities through drying, thereby enhancing the food security and economy of developing countries.


I. INTRODUCTION
In the quest to meeting the sustainable development goals (SDGs 2030) especially SDG No. 2, which is concerned with hunger alleviation, achieving food security, improving nutrition and promoting sustainable agriculture before the year 2030, food preservation had recently attracted global interest as it helps in bridging the gap of shortage during scarcity of food materials and of such food materials are tomatoes.Tomatoes are important items to human health and nutrition because of the reason that they contain essentials like vitamin C, potassium, vitamin K, fiber, carbohydrate and antioxidants, which are beneficial in mitigating the risk of cancer and heart diseases.Unfortunately, tomatoes are not only periodic but highly perishable.Consequently, there is always an unreasonable gap between harvest and off-season periods because they are often being consumed rapidly during harvest period due to lack of storage facilities especially in the rural communities of developing countries like Nigeria where there is limited access to electricity.According to [1], tomatoes are being cultivated virtually across the country but most predominantly grown in the northern Nigerian towns such as Zaria, Jos, Gombe, Kano, Maiduguri and the south west towns like Ogbomosho and Ibadan but unfortunately, the peasant farmers and traders incur a lot of losses during harvest peak owing to low prices which are traceable to surplus, poor storage facilities, lack of electricity, poor road network for transporting the items to urban areas and ultimately because the harvest periods are often characterized by low sun intensity cum high relative humidity which in addition to wind blow, insects and pests infestation makes direct/open sun-drying often inefficient.It is highly paradoxical that 29,000 metric tons of tomatoes are being wasted in three states of Northern Nigeria while Nigeria imports 26,000 metric tons of tomato paste annually [2], [3].Hence, it is highly imperative to source for alternative means of preserving this food material so as to provide a larger market which would afford tomatoes consumers and traders opportunities to purchase it on a year round basis and also return a higher profit for rural and suburban farmers of developing countries.
Drying process is very important in processing and preserving agricultural products cum many others particularly in developing countries.Many efforts have been made with respect to evaluation of drying process and development of some dryers for the improvement of some agricultural produce storage conditions.For the purpose of reducing wastage of vegetables and improving their storage conditions, Reference [4] developed a prototype of a hybrid solar dryer for drying of tomatoes.The collector efficiency of the solar dryer, whose capacity was 20 kg of half-cut fresh tomato to produce 2 kg of dried product per batch, was said to be 10% increased by using a solar reflector.The results of the tomatoes pretreatment revealed that, of all the pretreatments (UV radiation, acetic acid, citric acid, ascorbic acid, sodium metabisulphite and sodium chloride) used, only sodium metabisulphite (8 g/L) at a temperature lower than 45 o C was able to completely control microbial infestation of tomatoes.The authors therefore recommended that, pretreatment with 8 g/L of sodium metabisulphite should be carried out to prevent microbial growth if tomato is to be dried with continuous air temperature of 45 o C or less but no pretreatment is required if the drying is to be done with continuous air temperature above 45 o C.
Reference [3] designed and developed an industrial fruit Using tomato as the test material at an average drying chamber temperature of 50 o C, the dryer which was recommended for industrial users was found to improve the drying time of vegetables.
A small scale 1.0 HP single-phase electric motor powered column dryer for paddy rice drying and processing was designed, fabricated and tested by [5].The dryer was capable of drying the paddy rice from an initial moisture content of 22.36% to a final moisture content of 13.37 %.
Reference [6] conducted an analysis to compare the effect of three drying methods namely: natural sun drying, hot air oven drying and freeze drying on the dietary fiber contents in pumpkin, yard long bean, tomato, red cabbage and guava.The results of the comparative analysis showed that tomatoes of all the fruits and vegetables tested showed a higher dietary fiber content in hot air oven drying than other drying methods.
A double collector solar device for improved drying performance of four backyard agricultural products (carrots, chayote squash, tomatoes and oyster mushrooms) frequently used in the diet of Southwest region of Mexico was designed, built and validated by [7].The test results showed a drying efficiency range of 22.8-37.9%by reaching moisture values below 12.30%.
Though reasonable efforts have been made in the past, most of these dryers involve the usage of inaccessible electricity, uneconomical energy sources or in some cases a hybrid of solar energy with some others.In addition, the afore mentioned allied limitations of cheap open/direct sun drying method which is being practiced in many developing countries have necessitated the need to explore an alternative method of tomatoes preservation.For the purpose of reducing wastage, improving storage conditions, minimizing packaging requirements, improving the profits of farmers in the rural cum sub-urban communities of developing countries and meeting the all-year-round demand of healthy and cost-effective tomatoes and thereby enhancing food security and economies of developing countries like Nigeria, an energy-efficient tomato dehydrator with charcoal as the solid fuel for heat generation is designed, fabricated and evaluated for its performance.

A. Description and Operation of the Tomato Dehydrator
The tomato dehydrator as shown in Fig. 1 works on the principle of heat and mass transfer by forced convection.It comprises of three major sections namely: (1) A combustion unit (CU) is a conical container that is filled with the required quantity of charcoal which undergoes combustion and generates heat energy for the drying of the tomatoes.(2) A heat exchanger unit (HEU) located between the CU and the DU comprises of fins to aid the efficient transfer of the generated heat to the DU through a 240V 0.15 HP DC centrifugal fan installed at its side through an 8cm wide duct.(3) A two-shelves drying unit (DU) holding two food trays in each shelf.This DU is lagged around with fiber glass of 1cm thickness to provide insulation and prevent conduction and convection heat loss through the walls.In the DU, the conveyed hot dry process air from the HEU passes over the sliced tomatoes which are arranged on the tracked trays to effect drying by both heat and mass transfer after which the hot humid exhaust air exit to the surrounding through an air vent on top of the DU.The temperature variation in the DU is compensated for by regulating the speed of the fan which has sufficient impetus to overcome any back-up pressure that could be built up in the trays.The continuous repetition of this process leaves the tomatoes with less moisture content thereby effecting drying.

B. Design Analysis 1) Initial conditions and Design considerations
The tomatoes dehydrator design was based on the following initial cum design conditions: Safe drying temperature required for tomatoes,   = 50 o C [3]; initial mass of tomatoes to be dried per batch loading,  , = 5 kg; mean ambient temperature,   =31 o C; mean relative humidity (RH)=35%; minimum fan speed =6 m/s; Number of fins in the HEU= 4; the thickness (t) of each fin is small compared to its length (l) and breadth (b); the thermal conductivity of the fins material is constant throughout.The area of each fin was taken to be the cross sectional area (l x t) that is in contact with the metallic base (shelf).Hence the extended area of the heat exchanger was estimated as the cross sectional area of one fin multiplied by the number of fins.In addition, other factors considered for the design are: Human hygiene and health; dehydrator structural stability and weight; cheap and readily available heat generation method; ease of loading and retrieval of tomatoes; heat conservation for optimum drying; the ability of the air moving device (fan) to overcome pressure and resistance built up in the dehydrator.

2) Mass of moisture required to be removed from the tomatoes (𝑚 𝑤 )
This was estimated on wet basis from (1) according to [8]: Where   -Required mass of air for drying tomatoes, (kg)   -Mass of moisture required to be removed, (kg) (precomputed)  1 -Initial humidity ratio of air, (kg/kg dry air)  2 -Final humidity ratio of air, (kg/kg dry air)   was computed from the psychometric chart as thus: with the mean ambient WBT of 31 o C and DBT of 28 o C with 35% relative humidity (RH) as the air initial conditions, HR1 was calculated to be equal to 0.01 kg/kg dry air.It is expected that when this unknown quantity of hot air is supplied in the dehydrator, it will raise the temperature of the tomatoes to 50 o C (design condition) while its (hot air) temperature and final humidity ratio (HR2) is decreased and increased respectively.Therefore, working backward on the psychometric chart, HR2 was estimated under normal temperature and 101.325 kPa barometric pressure to be 0.028 kg/kg dry air.On substituting these data,   was estimated to be 254.4kg.

4) Volume of air required for drying, 𝑉 𝑎𝑖𝑟
This was calculated from (3), the conventional relationship between the density and volume as follows: Where   -Required volume of air for drying tomatoes, m 3   -Required mass of air for drying tomatoes, (kg) (precomputed)   -Air density, 1.115 kg/m 3 at 0 o C as given in [10]   was therefore calculated to be 228.2m 3

5) Air volumetric flow rate, 𝑄 ̇
The volume of the air flowing per time was estimated with (4).

6) Selection of fan
The satisfactory performance of dryer using forced convection depends greatly on the volumetric flow rate of the air supplied from the air moving device whose efficient performance is dependent on its design.The fan horsepower for the tomato dehydrator was estimated using (5) according to [11]: According to [11], fan efficiency oftentimes ranges from approximately 30 to 70% over its operating static pressure range and drying fans operating at peak efficiency will have an efficiency of about 65%.The static pressure was estimated with the data of the predetermined volumetric flow rate from fan characteristics curve to be 15 inches to which 0.5 inch was added since the air was delivered from duct as recommended by [11].Substitution of these values with the precomputed volumetric air flow rate in cfm (26.70 cfm) into (5) gave 0.1 HP.Hence 0.15 HP backward-curved centrifugal fan was selected.

7) Heat Energy required (used) to evaporate water from the tomatoes, 𝑞 𝑑𝑟𝑦𝑖𝑛𝑔
The amount of heat energy actually used to effect drying by evaporating (removing) the required moisture from the tomatoes was determined from the addition of the heat energy required to raise the temperature of 5 kg of tomatoes to the dehydrator design maximum temperature and that required to remove moisture which is mathematically represented in (6):   -Mass of moisture required to be removed, (kg) (precomputed)   was therefore estimated to be 11,346.1 kJ.

8) Mass of charcoal required for combustion, 𝑚 𝐶ℎ𝑎𝑟𝑐𝑜𝑎𝑙
The mass of the solid fuel (charcoal) required to be used in the CU was estimated using (7) according to [12]: Where  ℎ -Mass of charcoal required for combustion, (kg)   -Heat energy required for drying (predetermined)  ℎ -Higher calorific value of charcoal (29,600 kJ/kg as given by [13,14].
Therefore  ℎ was estimated to be 0.38 kg.

9) Energy supplied to the air during drying (𝑞 𝑠𝑢𝑝𝑝𝑙𝑖𝑒𝑑 )
The total amount of heat energy imputed to the required air mass during drying is computed from (8): Where   -Energy supplied to the air during drying, (kJ)   -Required mass of air for drying tomatoes, (kg) (precomputed)  , -Specific heat of air = 1.007 kJ/kg o C at 50 o C  ℎ -Temperature of hot air reaching the DU from the HEU ( o C) =     was then calculated to be 12,809.0kJ.

10) Heat transfer rate, 𝑞̇
This is estimated based on the convection heat transfer between the air and the heat exchanger coupled with the conduction heat transfer at the heat exchanger surface as given in (9): Basically, the efficiency of any machine can be estimated from the ratio of the desired or useful output to the total required input as given mathematically in (10): The thermal efficiency of the tomatoes dehydrator is indicative of how efficiently the hot air capability to absorb moisture is being utilized.Consequently, the thermal efficiency of the dehydrator was estimated from (11) according to [15].(11) Using the nomenclature in this study, the equivalence of ( 11) is presented in ( 12) On substituting the precomputed values of   and   , ŋ ℎ was computed to be 88.6%.

C. Material Selection and Fabrication
The specifications and quantities of the materials used are presented in Table I.Charcoal was chosen as the solid fuel for heat generation owing to the fact that it is cheap and readily available.The dehydrator frame with a dimension of 40 cm by 40 cm by 86 cm as shown in Fig. 2 was made by cutting a 20 mm by 20 mm square pipe into size and welded together.The body with the 40cm by 40 cm by 70 cm external dimension was made with mild steel of 1 mm thickness and lagged with fiber glass of 1cm thickness.This body was made by cutting the mild steel to size and welded it to the frame.The dehydrator door with dimension of 40 cm by 70 cm made with the same material was attached to the frame using hinges.Mild steel was preferred for the dehydrator body and its door because of its great strength and owing to the fact that it can be welded easily.Each of the two trays having a dimension of 35 cm by 35 cm with a height of 2 cm was made from stainless steel of thickness 1 mm.An 8 cm wide opening was made on one of the sides of the dehydrator to serve as an opening for the rectangular ductwork (6 cm (height) by 3.5 cm (width)) for the process air.Rectangular duct was preferred because of ease of connection and height requirement.For the purpose of providing healthy and hygienic dried tomatoes, stainless steel was chosen as a material for the trays because of its good machinability, high strength and its smooth cum nonporous surface which promotes its ease of cleaning and high corrosion resistance.The HEU comprises of four fins which are 6 cm spaced from one another.Each fin which is 30 cm long, 0.25 cm thick and 8 cm high was fabricated from galvanized steel and mounted on the mild steel-made shelf (as a base) through welding.Galvanized steel was preferred for the HEU fins owing to its toughness, high heat conduct and radiation potential.Backward-curved type of centrifugal fan was selected owing to the static pressure which was higher than 4 inches as recommended by [16].D. Performance Evaluation 5 kg of fresh fully ripe (red) tomatoes purchased from Oba market in Ado-Ekiti Nigeria were first washed, wiped with a clean dry cloth, thin sliced and then arranged in the two trays.After the initiation of the combustion process by igniting the solid fuel, the preheating of the dehydrator under no-load condition continued until the air temperature reaching the DU reached 50 o C which was maintained for the drying procedure with the aid of a heat control unit (HCU).The dehydrator was thereafter loaded with the time noted.The drying process continued while the weight of the tomatoes was constantly being monitored with the aid of an electronic balance.This procedure continued until there was no more noticeable variation in the weight of the dried tomatoes.At this point, the final weight of the dried tomatoes was recorded to be 1.4 kg and the total drying period was estimated to be approximately 7 hours (6 hours 54 minutes).Samples of both the dried tomatoes and the fresh ones were thereafter analyzed for their total bacterial counts (TBC), total fungal counts (TFC) and ash contents.

A. Physical characteristics
The comparison between the physical properties of the pre-drying (fresh) tomatoes and those of the post-drying (dried) ones is presented in Table II.It is evident from this data that the weight of the tomatoes decreased with increasing drying time.This trend which is consistent with those in the literature can be reasonably traced to the removal of water content (dehydration) during drying.

B. Drying rate, D
The average rate of drying in the DU of the dehydrator was estimated from (13) as the ratio of the change in mass of tomatoes to the drying time:

C. Microbiological analysis
The results of the microbiological analysis conducted on the samples of both the pre-drying and post-drying tomatoes has shown that the drying process has a great significance on reducing microbial activities in the tomatoes thereby extending its shelf life.As presented in Table III, the analysis revealed, for the post-drying tomatoes sample, a TBC of 1.61 x 10 2 cfu/g and a TFC of 0.27 x 10 2 cfu/g which are both far below the allowable limits (10 3 cfu /g) for food as recommended by [17].This shows that the dried tomatoes are safe for human consumptions as some bacteria are as well beneficial to human systems.Contrastingly, the TBC and TFC revealed for the sample of fresh tomatoes were both far higher than the acceptable limit for human consumption.Reference [18] obtained similar high values for fresh tomatoes.This microbial infestation and contamination might have occurred either in the process of harvesting, post-harvesting, handling, storage, transportation or exposure on benches and baskets in the open markets for customers as it is being practised in most developing countries [19], [20], [21].It could also be due to the presence of high water content and the succulent nature of tomatoes.

D. Ash contents evaluation
Ash content is a measure of the total amount of inorganic material, such as minerals, present within a food.The results of the chemical analysis conducted on the samples to test their ash contents revealed a value of 0.4% for the fresh tomatoes and 2.3% for the dried tomatoes.The lower ash content value revealed for fresh tomatoes may be due to the presence of high water content while the ash content in the dried ones may be due to the concentration of inorganic material such as mineral contents.Reference [6] obtained a similar trend in their work.

IV. CONCLUSION
An energy-efficient tomato dehydrator using charcoal as a solid fuel for heat generation, having a thermal efficiency of 88.6% and a drying rate of 0.51kg/hour has been designed, fabricated and tested.The microbiological analysis conducted on the post-drying tomatoes sample revealed a total TBC of 1.61 x 10 2 cfu/g and a TFC of 0.27 x 10 2 cfu/g which are both far below the allowable limits (10 3 cfu /g) for human consumption.The results of the chemical analysis conducted on the pre-drying and post-drying tomatoes samples revealed an ash content value of 0.4% and 2.3% respectively.The dehydrator has proved effective for extending the shelf life of tomatoes by mitigating the rate of spoilage due to microbial activities through drying, thereby enhancing food security and economy of developing countries.Scaling up of the tomatoes dehydrator to industrial level is recommended for further study

Fig. 1 .
Fig. 1.Conceptual design of the tomato dehydrator Heat transfer rate ℎ  -Convective heat transfer coefficient, (  2 ℃ ⁄ ) Nu -Nusselt's number Ra -Rayleigh number K -Thermal conductivity of heat exchanger material, (0.0305  ℃ ⁄ ) as reported by [3]  . -Characteristic length of fin.(0.3 m)  . -(4 × 0.00075 = 0.003  2 ).This is the extended area of the heat exchanger which was estimated as the cross sectional area of one fin multiplied by the number of fins. . -Temperature of heat exchanger =  = 50 o C  , -Temperature of incoming air from ambient =   = 31 o C ̇ was therefore estimated as 0.7 kJ/s 11) Thermal efficiency of the tomato dehydrator, ŋ ℎ

Fig. 2 .
Fig. 2. AUTOCAD sketch of the design: (a) Front view of dehydrator (b) Angle view of dehydrator (C) Plan view of dehydrator

TABLE I :
SPECIFICATIONS AND QUANTITIES OF THE MATERIALS USED

TABLE II :
COMPARISON BETWEEN THE PHYSICAL PROPERTIES OF FRESH

TABLE III :
MICROBIAL EVALUATION OF FRESH AND DRIED TOMATOES