Solar Dryer


In many countries of the world, the use of solar thermal systems in the agricultural area to conserve vegetables, fruits, coffee and other crops has shown to be practical, economical and the responsible approach environmentally.  Solar heating systems to dry food and
other crops can improve the quality of the product, while reducing wasted produce.
Under the module of Advance topics in mechanical engineering projects, we were assigned to design the low cost vegetable dryer. The design of dryer consists of several steps and sequence procedure.
This report consists of back ground about project and the first chapter covers the introduction about drying and our project. Second chapter covers the over view, applications of solar dryer and benefits of solar drying foods. The third chapter consist with literature review and description of dryers. The forth chapter is written on materials and methods and fifth chapter gives cost estimation for the dryer. sixth chapter covers conclusions and recommendations and final chapter consist with references.
This project report gives a great guideline about designing of low cost vegetable dryer compare with the other competitive industrial food processing technique.


"Drying is an excellent way to preserve food and solar food dryers are an appropriate food preservation technology for a sustainable world."   Actually, solar food drying is one of the oldest agricultural techniques related to food preservation, but every year, millions of dollars worth of gross national product is lost through spoilage.  Reasons include, ignorance about preservation of produce, inadequate transportation systems during the harvest season (mostly climate related), and the low price the rural farmer receives for products during the harvest season. 
Drying of crops can change this trend and is useful in most areas of the world, especially those without a high humidity during the harvesting season.  If drying of produce were widely implemented, significant savings to farmers would be achieved.  These savings could help strengthen the economic situation of numerous developing governments as well as change the nutritional condition in these same countries.
Drying generally refers to the removal of moisture by evaporation rather than by pressure or other physical means. The factures that are affecting the drying can be identified as the temperature, humidity, pressure, velocity of air and the size and the shape of the wet surface and the air moment with respect to it. There are various mechanical drying methods that are available at present. Some of these are operating utilize fans for air circulation, and electric or gas heater for heating of drying air. But they are very expensive drying methods so we have introduced a high efficiency low cost vegetable dryer for our advance topic in mechanical engineering module.
This dryer has been designed to construct from material available in the market at low cost. This especially to over come the problems encountered in the sun drying such as over drying occur, low drying rate, risk of spoilage, contaminating with impurities. It uses solar energy for heat generation. Evaluation of a prototype of this dryer is important to reduce the drying losses and optimize the utilization. This dryer consists of main parts namely the body of the dryer, the cabinet, chimney, and solar unit.
1. To study about solar energy.
2. To obtain knowledge about industrial applications of solar dryers.
3.To Understanding the way of selecting required components.
4. To familiarize with the workshop environment.
5. To get Familiarize with the way of project presentation.
6. To get the maximum benefit of the teamwork.
7. To obtain knowledge about report writing.

The project is carried out in order to get out side knowledge and involve in practical applications beyond in our day-to-day academic studies under in the module of “Advanced Topics in Mechanical Engineering”. Designing of the solar dryer minimizing shortcomings associated with than low efficiency, cost not portable solar dryer

2.1       OVERVIEW
Drying preserves foods by removing enough moisture from food to prevent decay and spoilage. Water content of properly dried food varies from 5 to 25 percent depending on the food. Successful drying depends on:
• Enough heat to draw out moisture, without cooking the food;
• Dry air to absorb the released moisture; and
• Adequate air circulation to carry off the moisture.
When drying foods, the key is to remove moisture as quickly as possible at a temperature that does not seriously affect the flavor, texture and color of the food.  If the temperature is too low in the beginning, microorganisms may grow before the food is adequately dried.  If the temperature is too high and the humidity too low, the food may harden on the surface. This makes it more difficult for moisture to escape and the food does not dry properly.  Although drying is a relatively simple method of food preservation, the procedure is not exact.
Food drying is a very simple, ancient skill. It requires a safe place to spread the food where dry air in large quantities can pass over and beside thin pieces. Sun is used to provide the hot dry air. Dry, clean air including dry cold air from any source will dehydrate food. Draping food over branches or spreading it on wide shallow baskets on the roof is an old widespread tradition still in use around the world. Many other arrangements have been used to support a thin spread of food pieces
For centuries people of various nations have been preserving dates, figs, apricots, grapes, bananas, pineapples, other fruits, herbs, cassava, yams, potatoes, corn, peas, onions, garlic, carrots, peppers, milk, coffee, meat, and fish by drying. But drying is also beneficial for hay, copra (kernel of the coconut), tea and other income producing non-food crops.  

"Dried foods are tasty, nutritious, lightweight, easy-to-prepare, and easy-to-store and use. The energy input is less than what is needed to freeze or can, and the storage space is minimal compared with that needed for canning jars and freezer containers.
"The nutritional value of food is only minimally affected by drying. Vitamin A is retained during drying; however, because vitamin A is light sensitive, food containing it should be stored in dark places. Yellow and dark green vegetables, such as peppers, carrots, winter squash, and sweet potatoes, have high vitamin A content. Vitamin C is destroyed by exposure to heat, although pretreating foods with lemon, orange, or pineapple juice increases vitamin C content.
"Dried foods are high in fiber and carbohydrates and low in fat, making them healthy food choices. Dried foods that are not completely dried are susceptible to mold. "Microorganisms are effectively killed when the internal temperature of food reaches 145 degrees Fahrenheit (F)."


Drying is the removal by evaporation of volatile substances (e.g. moisture) from a wet material in order to obtain a dry product.
When a wet solid is subjected to thermal drying, two process occur simultaneously, namely,
1) Transfer of heat to raise the temperature of the wet solid and to evaporate the moisture.
2) Transfer of mass in the form of internal moisture to the surface of the solid and its subsequent evaporation.
The rate at which drying is accomplished is governed by the rate at which these two processes proceed. Heat transfer in the drying operation will occur throw the flow of heat as a result of convection, conduction and radiation.
Mass transfer in the drying of a wet solid will depend on two mechanisms
1) Evaporate of moisture from the surface
2) Migration of moisture from the interior of a particle to the surface
In a drying operation, either of these mechanisms may be a limiting factor on the rate of drying, although they will both proceed simultaneously through out the drying cycle. With some materials, at a certain stage of the drying operation, the rate of movement of the moisture within the solid to the surface may be controlling factor. In the approach to a drying problem and to obtain optimum drying conditions, it is important to bear this in mind and to understand the processes. They are equally important from an energy efficiency point of view.
The action of applying heat to material in order to dry does not merely remove the moisture but can affect the quality of the dried product. These affect are varied from only those phenomena commonly encountered will be describe herein.
Browning is the discolouration of the material during drying, which can be caused by either the physical processes or chemical reactions. It is depend upon the combustion effect of time and temperature on moisture content of the drying material.
This is characterized by the material surface becoming drying and relatively impermeable to further flow of moisture but with the interior remaining at somewhat higher moisture content. But with drying of most vegetable and fruit, case hardening is not commonly observed.
It is not be through that rehydration is a complete reversal of the dehydration process. Rehydration in terms of producing rehydrated product similarly in appearance to the original form is not so important as the dried product is utilized in soup or stews by adding directly to the cook pot.


Wide varieties of dryer designs have evolved over the years with a view to conduct the unit operation of drying. Several methods have been suggested as to how best to classify them but before considering the methods of classifications seriously, it is necessary to define, on broad principles,
a) How the necessary thermal requirements are supplied
b) The mode of operations of dryers
In the first place, heat must be transferred to the wet material to promote the drying operation. Heat can be applied by one or more of the following methods.
a) Convection, whereby the heating medium, usually air or the products of combustion, is in direct contact with the wet material.
b) Conduction, where heat is transmitted indirectly by contact of wet material with a heated surface.
c) Radiation, where heat is transmitted directly and solely from a heated body to the wet material by radiation of heat.
In a drying operation, heat supplied by radiation alone is exceptional, and is usually restricted to so-called infrared dryer installations, which will be described later.
However, radiation may have quite an important bearing on the performance of a dryer designed for convection or conduction effects to predominate.
The majority of industrial drying installations rely on the heat transfer and drying effect through convected heat supplied directly to the wet material. This heat is supplied by a mass of heated air or combustion gases, forced induced or circulated in contact with the material by means of fans.
The conduction of heat to the material through a retaining enclosure, heated externally from source of heat, is another commonly used technique. Water vapour released in the drying operation is removed by a mild exhaustion of humid air.
Therefore industrial dryers fall into two broad categories, namely, convection dryers and conduction dryers, with additional special types involving almost entirely radiation or some other specialized form of heat generation or application.
Such dryer will also take the form of intermittent or ‘batch’ type dryers or alternatively, continuous dryers. Therefore, there are two categories of convection dryers and two categories of conduction dryer. For the reader to be familiar at this stage with the wide variety of industrial drying plant falling within these categories, the fallowing is a brief description of each type/category with illustrations of typical application.
For both batch and continuous dryers, a distinction can be made between conduction and convection drying. Convection dryers (e.g., flash and spray dryers) often require relatively large solid gas separating equipment. Thermal efficiency of convection dryers increases with increasing inlet temperature of the drying gas. There are no such effects with conduction dryers. Further more conduction dryers may be preferred for dusty products.
The difference between two methods is the relationship between product temperature and heating medium temperature. However, in conduction dryers, products in contact with hot metal will attain its temperature.
The residence item in conduction dryers can be up to several hours. The residence time in flash and spray dryers can be quite short ten seconds or less, both time and temperature are important with regard to thermal degradation.
A radiation oven is much faster and requires less energy than convection drying, since only the surfaces to be dried are heated. However only the surfaces exposed to radiation can be dried and this limits the possible shape of the material. Another disadvantage is that different colors, pigments and surface structures are not heated to the same degree. Convection drying does not have these product restrictions, but a big thermal mass has to be heated.
Drying usually occurs in two different ways.
a) In radiation drying. The surface is wet during most of the drying time and the resistance to transfer is mainly in the air film surrounding the material to be dried. The drying is usually performed with acceptable energy economies and 50%-80% of the energy is used for evaporation.
b) In convection drying, the rate of drying is mainly dependent on the resistance of the embedded moisture to transfer in the material. Energy utilization is often poor and it is not unusual that only 10%-20% of the energy is used for evaporation.
For energy management and efficiency improvement, it is always important to calculate the mass and energy balances of the dryer in order to check the efficiency.

Some examples of dryers are presented below.
Tray dryers (figure 3. 1) are classified as batch type and can dry almost everything. However, because of the labors required for loading and unloading, they are expensive to operate. They find most frequent application when valuable products like dyes and pharmaceuticals are involved. This type of dryer is frequently used for drying of wood and various agricultural products.
Figure 3.1-Schemitic diagram of a Tray dryer
A band dryer (figure 3. 2) is preferable if the particles to be dried are rather coarse (i.e. between 5 to 10 mm). The particles spread evenly into slowly moving, e.g. 5mm/s, perforated belt. The belt moves into a drying cabinet and warm gas passes downward through the layer. This type of dryer is chosen when it is not possible to suspend the particles in the drying gas. The dryer must offer a residence time, say 15min, because bound moisture must defuse through the pellet.
Many band dryers are used to dry pre-formed particles. The wet particle material is mixed with additives granulated and dried. One reason for doing this is that direct drying of the wet material may yield a dusty material, whereas the granules are less dusty. Band dryers are also used in food applications, for example, in the drying of diced carrots.
Exhaust air damper
clip_image003 Figure 3.2 –Schematic diagram of a Band dryers
Rotary dryers consist of a cylindrical shell, horizontal or slightly inclined toward the outlet. They are heated by
a) Direct contact of air or gas with the solids,
b) By hot gasses passing through an external jacket on the shell, or,
c) By steam condensing in a set of longitudinal tubes mounted on the inner surface of the shell.
The product is fed to the upper orifice and transported during mixing through the cylinder. Regardless of the method of the heating, the water is removed with the air. The disadvantage of rotary dryers is the big power losses, which occur if the product is fine grained. The installation of the particle/ gas separation at the moist air outlet (e.g. cyclones) may reduce the product losses significantly.

A roller dryer consist of a cylinder, heated from the inside by stream. A thin film of the product is sprayed on the outside of the cylinder and it is heated while the cylinder rotates. The rate of drying and the final water content in the product is affected by the
a) Rotational spread of the roller,
b) Steam pressure,
c) Thickness of the film, and
d) Properties of the product.
Owing to the sort contact time in combination with high drying temperature, the roller dryer is well adapted to heat sensitive products. Typical applications are fabric drying in textile industries and paper band drying in paper mills.
In a fluidized –bed dryers, the particles are fluidized by air or gas in a boiling bed unit. The average time a particle stays in a bed is usually between 30and 60s. If fine particles are present, there may be considerable solids carried over with the exit gas, and cyclones and bag filters are needed for there recovery.
In a spray dryer (figure3.3), a slurry or a liquid solution is dispers3ed in to a stream of hot gas in the form of a mist of fine droplets. The chief advantage of spry dryer is the very short drying time, of the order of 2 to 20s, which permits drying of highly heat sensitive materials, and the production of solid or hollow spherical particles. They can also yields a dry product from a solution, slurry or thin paste, and thus simplify the overall manufacturing process. When considered solely the dryers, spray dryers are not highly efficient.
Dry Material
Figure 3.3 -Schematic diagram of a Spray Dryer


Though food preservation is the main objective of the solar dryers, diversified shortcoming can be observed which are associated with them. The following are the shortcomings.
· Low efficiency.
· High cost.
· Not portable enough.
But, the solar dryer model, which we suggest, has over come the above shortcomings.



The body of the dryer is a locally constructed by simple design with portable size. Because main parts of the body such as chimney, cabinet, solar unit can be separated. UV stabilized polythene is used to cover the whole the dryer body externally. Frame of the body is made of L- iron.
This cabinet consists of four walls. One wall is opened into the solar collector. Which bring air into the cabinet and is moved up through the chimney and out the vent at the top. Air circulation is of utmost importance, as the moisture must be able to escape the cabinet, and the entire product must be exposed to circulation, in order for proper drying. There are seven trays in the cabinet so capacity of the dryer is high. And there is a 36” height chimney top of the cabinet and it is help to the natural convection process of the dryer. Instead of normal bend, used smoothes bend to increase the flow rate of air inside the cabinet as shown in Figure No: 4.3

4.1.3 TRAYS
The frames of the Seven trays which is 18”x18”, made by using steel rod(1/4”) and nylon wire mesh fixed into it.
Figure No: 4.4

The frame of the solar unit is made using steel rods and it is covered with UV stabilized polythene except two sides. Both of them, one side which air entering side is covered with a wire mesh in order to avoid entering numerous particles and insects. Other side is connected to the cabinet using two spring locks. Here the joint is sealed with rubbers in order to minimize the leakage of the airflow to the environment. Black colored galvanized plate is located middle in the solar unit. Due to this the solar unit absorbs more amount of solar energy. There is an inclination of about 2 0 for the purpose of gather solar energy through out the day.
The frame of the chimney is made using steel rods and it is covered using UV stabilized polythene. Which is connected the top surface of the cabinet using spring locks. Here also joint is sealed with rubber in order to avoid airflow leakage from the system. Inside the chimney, there exist a unit as shown in the figure no 4.5 made using black colored galvanized steel plates. Which is absorbed more solar energy. Due to this air flow of the top of the chimney is heated and move away. So it provides better ventilation for the system.


During the drying process the humidity ratio changes from 0.0104 to 0.0140 i.e. about 0.0036 kg of vapour per kg of dry air is absorbed. Now by using solar energy, the air is heated to 45oC with a relative humidity of 17 per cent and is passed over drying material. During the drying process, this air is cooled adiabatically along the 24oC wet bulb line, and then the final humidity ratio will be 0.0189. thus the moisture evaporated with the heated air will be 0.0075 kg of vapour per kg of dry air which is almost double the water evaporated compared to when air was too heated.
The initial moisture content, the final moisture content and the maximum temperature at which product should be dried are very important and the values for a variety of products are given in Table.
Table: 3.1 Maximum temperature allowable for drying and the initial and final moisture contents of various products.
Product Moisture content
Maximum temperature
allowable for
the drying ( oC )
Initial Final
Rice 24 11 50
Carrots 70 5 75
Green beans 70 5 75
Onions 80 4 55
Potatoes 75 13 75
Chilies 80 5 65
Pineapple 80 10 65
Yams 80 10 65

Table5.1 Material cost

Spring lock Nos 4 150.00 600.00
Nylon mesh Ft2 16 10.00 160.00
Wire mesh Ft2 1/2 150.00 75.00
Nuts and Bolts Nos 100 2.00 200.00
Flat iron (1/4”) Ft 42 7.00 294.00
L- iron (1/2 " * 1/2" *1/4") Ft 37 25.00 925.00
Galvanize plate(1/8”) Ft2 9 70.00 630.00
UV stabilized polythene Ft2 40 5.00 200.00
Total material cost 3084.00

Table:5.2 Labour cost
Welder 1 400.00 400.00
Labour 2 300.00 600.00
Total labour cost 1000.00

Material cost 3084.00
Labour cost 1000.00
Sub total 4084.00
Over head (5%) 154.00
Estimated cost 4238.00
Total cost for the dryer 4238.00


  • Under the module Advance Topics in Mechanical Engineering Projects on process engineering was to design a low cost vegetable dryer all over the semester five we got lot of effort to reach our targets according to our time schedule.
  • Data collection was the difficult test on this project and we had to refer Internet and lot of standard books about dryers and chemical engineering.

Chapter 07

· IPHT lecture notes
Coulson JM
Coulson and Richerdson’s Chemical Engineering: Vol-5
2nd Edition 1979 (1997)
· Oxford: Butterworth
· Energy Audit manual module 5: Furnaces Kilns and Dryers
Jasiewicz, Jan. (comp.)
Siyambalapitiya, Thilak (ed.)
Colombo: Sri Lanka Energy Manager’s Association, 1999.
· Internet