Drying Processing Presentation SNV PUM

Under mechanical dehydration the overall speed of drying depends on - The relative humidity of the air - The air speed and direction - The temperature of air passing over the fruit.

Relative humidity Relative humidity control is the most important factor for efficient dehydration. Air in a dehydrator continually circulated without replacement would rapidly become saturated with water vapour. Evaporation would stop. To avoid this it is necessary to 'bleed off' some of the moist air and replace it with dry air from outside. The aim is to keep the relative humidity below 40 per cent.

Air-speed and direction Air has two functions in the drying of fruit. Firstly it conveys heat from the heating device to the fruit to provide the energy required to vaporise the water. Secondly, it serves as a means for transferring moisture to the outside atmosphere. Airspeeds of around 240 metres per minute (or 4 m per second) are recommended for fruit dehydration. Direction: vertical if a product bed must be dried, otherwise: horizontal

Air temperature Air temperature is increased to supply the heat required to evaporate fruit moisture and to increase the moisture-carrying capacity of the air. Air at 60° C can carry five times more moisture than air at 32° C.

Dryer types A dryer can work either on a batch system or a continuous flow system. In the batch system the dehydrator is filled with product and will run until the entire load is dry. Another complete batch is then loaded and the cycle repeated. Batch can be using trays above each other or tunnel dryer. The continuous flow system using lorries and trays is started with a partial batch normally filling one-third to half the capacity of the dehydrator. Trucks of product are routinely added to the dehydrator until it is full or almost full. Then, when the first truck has dried sufficiently it is removed and another put in at the other end, and so on. Another type of continuous drying is based on drying on a belt, with a continuous in-feed and out-feed of product. We will not consider this type of drier since these are normally used for shorter drying times. A belt dryer with the same drying surface as a tray dryer would also be much more expensive.

Batch system A problem with the batch system is uneven moisture content. This problem can be remedied by switching the lorries or trays during the drying cycle. This switching will result in some heat loss. Another problem associated with the batch system is the continual adjustment of shutters required to control relative humidity during the drying cycle. This problem can be overcome with proper attention or automatic relative humidity control. An automated system allows the dehydrator to be operated overnight without supervision.

Continuous flow system with trolleys product must be loaded and unloaded periodically while the dehydrator is operating, requiring labour around the clock This regular removal and replacement of trolleys from the dryer results in some heat loss when the doors are opened.

Belt dryer With a continuous in-feed and out-feed of product. We will not consider this type of drier since these are normally used for shorter drying times. A belt dryer with the same drying surface as a tray dryer would be much bigger and also be much more expensive. 20 m2 could be one trolley with 20 trays of 1 m2 or a belt dryer with belt width of 1 m and a belt length of 20 m. Based on 10 hours drying: with a belt speed of 20 m in 10 hours (or 2 m/hr. or 3.3 cm /minute)

Heating the air Solar heating Biomass heating Fossile fuel heating Electrical heating Combinations of the above

Solar heat collector Solar air heating No heat, so no drying when there is no sun Solar water heating. Advantage of solar water heating: Heat can be stored in the water to continue drying while solar air heating would not be able to furnish hot air anymore. Solar water heating must be combined with a water to air heat exchanger to heat the drying air

In case of solar air heating Seen: limited number of solar panels added to a roof Better: make the roof one big solar panel Remark: solar panels can get stolen, a roof cannot System: paint your roof black for optimum heat collection Insulate it, with space between roof and insulation Blow air through the space between roof and insulation. That air will be heated and can be used for drying. If the fan has the proper speed, it can also make sure the air speed inside the dryer is sufficient.

SOLAR roof - system

Solar boiler dryer - system

Simple solar boiler system Black hose on a black roof. Tested in Kenya on a black tar road

Air speed and direction Air speed: 4 m/sec. or 240 m/min Speeds below 200 m/min. will prolong the drying time. Speeds much higher than 300 m/min may even hamper drying because it may cause a film of dried tissue on the outside of the product to be dried. And will require bigger fans/more power. Optimal contact of air with product. Vertical: no open spaces on the tray. Air must go through a product bed of free rolling products. Otherwise air will not go through product bed, but through the openings Horizontal: all other products inclusive slices/pieces of fruit

Drying phases There are two distinct phases of drying: - An initial fast rate of moisture loss followed by a slower second phase. Initially, when the fruit surface is wet, water evaporates from the fruit forming a thin boundary layer of high-humidity air. -The thickness of this layer determines the rate of drying in the first phase of drying. Positive air movement, for example forced-air movement from a fan, over the fruit surface reduces the thickness of the high-humidity layer increasing the evaporation rate. During the second phase of drying, the rate of moisture loss decreases. The second phase begins when the rate of moisture movement to the surface of the fruit is less than the rate of evaporation from the surface - that is, the speed of drying is limited by the rate at which moisture can move through the fruit tissue. Important for drying speed and efficiency is that the layer of moisture in the initial phase is evaporated (by heat) and removed (by air flow) quickly

Determining factor in the first phase Determining in the first phase is whether water is evaporated and moisture is transferred Evaporation is related to heat, temperature Moisture transfer is related to how much moisture can be picked up by the air. In both cases: the higher the air speed, the faster the process Because air temperature and humidity are determined and fixed by what maximally is possible for the product or what can be achieved under Ugandan climatic circumstances, it are air speed and direction that are determining

Air Flow Direction Vertical: where do you think the drying air will go through the tray?

Determining factor in the second phase The limiting factor for drying speed at this phase is the flow of water from inside to outside of the product to be dried Air can pick up more moisture than will be evaporated Therefore air can be recirculated with averagely 20% bleeding of air. This way energy is re-used and not wasted by removing 100% of the heated air without using the moisture pick-up capacity of that air. Essential for effective and efficient drying is control of humidity.

Example of a configuration for horizontal air-flow Exhaust is also called bleeding Heat exchanger can be based on any heating system, based on warm water from a solar boiler, otherwise heated water by one source or a hybrid system or just represent the hot air from the burner And the housing can be a container, covered with stainless steel cladding inside for quality

Picture of such a dryer

Trolleys and trays for free rolling products

Based on existing dryers in Uganda Think about heating and heating sources. Make optimal use of solar/optimal use of the entire roof and/or heat water to store heat; Use hybrid because that gives the highest efficiency and lowest costs if solar is not sufficient Make sure the air speed is as high as required Make sure air humidity is controlled and a bleeding system is in place. Automation is better than manual

Dryer adaptations starting point:

Dryer adaptations

Dryer adaptations The dryer can be a short version of the container dryer and be designed for just one or two or three stacks of trays. Important is to make sure the air flow is the same over every tray. Otherwise trays must be moved during drying. Even airflow: Airflow must be bigger than the total open space between all trays easily can accept, so there will be a plenum between heat exchanger and trays, forcing air over every tray. Can be arranged by limiting the distance between the trays or a screen between heat exchanger and trays, creating that plenum. Screen against the trays. Door can be at an angle of 90 degrees towards the air direction for easy filling and emptying the dryer.

Improvements explained Product on trays heated from top and bottom since trays also are heated by warm air from below) Quick removal of water from product and moisture from air inside dryer Drying times clearly below 8 hours for the existing standard thin product slices. So: Higher quality end-product Considerably higher hourly capacity with the same number of trays Better use of solar Lower other energy requirement Probably, may be even considerably, lower cost than investing in more of the existing dryers for the same increased capacity.

Solar drying cabinets

Solar drying cabinet remarks Compared with drying in the sun, solar dryers can generate higher air temperatures and lower relative humidities. This results in shorter drying times and lower product moisture contents, and reduced spoilage during the drying process and in subsequent storage. The higher temperatures attained in solar drying also act as a deterrent to insect and microbial infestation. Although drying for more than 12 hours implies lower temperatures at night. Protection of the drying fruit against rain, dust, insects and other pests is improved when drying in an enclosed structure. All of these factors contribute to improving quality and providing a more consistent product compared to open sun drying. Compared to artificial or mechanical drying, solar drying is not capable of such high throughputs or such consistent product quality. Quality basically is too low for export and since the local market for dried pineapple is stagnant investing in such dryers may not pay off. Even although simple solar dryers such as the modified Kawanda solar cabinet dryer can be built locally and do not require motive power or other forms of energy to operate. The other side of the same medal is that the simplicity also implies there is no control of temperature, air speed nor air humidity.

Evaluation solar drying cabinet No control of temperature, air speed and air humidity Long drying times Low quality end-product Not hygienic Very much depending on manpower for control. Inexpensive