TECHNICAL TRAINING IN DRYING WITH BIOMASS BASED DRYERS INTRODUCTION TO DRYING AND BIOMASS DRYERS Technology Informatics Design Endeavour (TIDE) #19, 9 th Cross, Malleswaram, Bamgalore 560 003 Phone: 080-23315656; Email: tide@vsnl.com;
Introduction Drying is the oldest method of preserving food. For thousands of years people have dried vegetables and meats to preserve them for leaner times. Preserving foods by drying is useful, convenient, inexpensive, and needs little storage space. What you should know about drying: 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 Water content of properly dried food varies from 5 to 25 percent depending on the food. Dried Onion Slices Onion Slices Other useful information: Once drying is begun, it should not be interrupted. Lack of care in temperature control can lead to poor quality of dried product. Fruits, vegetables and flowers, meats, fish etc can all be dried and stored for use or sale in the future. Dried foods keep well because the moisture content is so low that spoilage organisms cannot grow. Drying, like all methods of preservation, can result in loss of some nutrients. For best retention of nutrients in dried foods, they should be stored in a cool, dark, dry place and used within a year. Drying is not difficult, but it does take time and a lot of attention.
DRYERS You are all familiar with drying food in the sun. However, drying in a dryer has several advantages over sun drying. These are: o It is hygienic, as the food to be dried is not exposed to dust, birds, insects and other animals. o The quality of dried product is better o The drying is much faster. There are different kinds of dryers, based on the kind of fuel used. The most commonly used dryers are Solar dryers- uses solar energy to dry food. Electric dryers- uses electricity to heat the air to dry the food Biomass dryers- uses biomass like wood, agricultural waste to heat the air in the dryer to dry the food. The table below shows a comparison between solar, electric and biomass dryers. S.No. Characteristic Solar dryer Electric dryer Biomass dryer 1 Energy to heat air Sun s heat Electricity Biomass like wood/ agricultural residues 2 Energy cost No cost for fuel Electricity is Cost of biomass is low. expensive 3 Availability of Only on sunny days For a few hours Available in plenty energy each day 3 Usage Only on cloudless, sunny days Only when electricity is supplied at 220 V Can be used on all days, including during monsoon 4 Temperature in Depends on weather Can go upto 120 o C Can be raise upto 100 o C the dryer and intensity of sun 5 Capacities of Range from 5 kg to Range from Range from 25 kg- 1 dryers 6 Control of temperature Can be controlled by thermostat. tonne Can be controlled by controlling the quantity of biomass burnt. 7 Cost of dryer Highest Same as biomass dryer 9 Emissions No emissions No emissions Combustion gases released into atmosphere Same as electrical dryer The biomass dryer is ideal as the fuel for it is available in plenty and is cheaper than electricity, it can be used on all days, including rainy days.
Drawing of a Biomass based Cabinet Dryer This is how a cabinet dryer looks.in the following pages, each part indicated here is explained briefly, so that you know all that you have to know about the dryer.
PARTS OF A DRYER 1. Combustion chamber: This is a box-like structure where the fuel is burnt. It is usually present at the bottom at the left side of the dryer. It consists of a grate on which the fuel is placed and burnt. The combustion chamber has a door that should be kept closed when the dryer is in operation. The grate inside the combustion chamber allows primary air to circulate in the chamber, so that the fuel burns well. through the chimney. 2. Ducting: This is an important part of the dryer. It is a 4 pipe that is bent in such a way that it runs all along the base of the dryer. The ducting is made of mild steel in cabinet dryers. Air which is drawn from below the dryer passes over the ducting, gets heated, rises over the trays and carries with it the moisture contained in the products arranged on the trays. The gasses in the ducting then cool down and pass out of the dryer 3. Chimney: The chimney is an ACC pipe of 6 diameter that carries the cooled gasses from the ducting out into the air. It is advisable to fix a cap for the chimney to prevent rain from entering the dryer. The chimney also plays a role of creating an air draught, which causes gasses to be pulled into the ducting and then into the chimney. The chimney should be cleaned regularly to prevent it from getting clogged with soot. 4. Dryer Chamber: is the large space inside the dryer, where the drying takes place. The size of the dryer chamber would vary
according to the capacity of the dryer. The walls of the dryer chamber are fitted with runners, on which trays are arranged. The ducting, gravel and moisture vent are located inside the dryer chamber. The air circulating inside the dryer chamber comes in contact with the hot ducting, and becomes hot. This hot air passes over the food to be dried and removes the moisture from it. The moist air then goes out of the moisture vent. 5. Trays: The number of trays in a dryer depend on its capacity. The trays in a cabinet dryer are arranged in two columns. Each column has one stack of 10 trays in front and another stack of 10 trays towards the back of the dryer chamber. The size of the trays in a 100 kg, 40- tray capacity dryer is 2ftx3ft. Wood slats or stainless steel screen mesh are the best materials to use for the trays. Stainless steel mesh does not rust over time. Each tray has a wooden frame, with stainless steel mesh fixed within this frame It is not advisable to use solid metal trays or paper sheets to dry food because air must circulate all around the food so that drying can take place from the bottom and the top at the same time. The wood that is used should be of good quality that will not burn, crack or warp with high temperatures. 6. Moisture exhaust: is a pipe that carries moist air out from the dryer chamber and releases it outside. The moist air enters the moisture exhaust through the moisture vent, located on the roof of the dryer chamber. 7. Ashpit: is present below the grate in the combustion chamber. After the fuel burns, the ash collects the ashpit. An opening is provided to clean the ash and to allow entry of primary air when kept open. It has to be cleaned out after every time that the dryer is fired. 8. Priming hole: This is a small hole provided at the base of the chimney. It is usually kept closed. However, when the weather is wet and cold, fire is placed in the priming hole for about 5 minutes to allow the chimney to heat up and create air draught.
When the dryer is fired after this, the temperature in the dryer chamber will go up fast. 9. Air vents are provided in the bottom of the dryer on all sides to provide for air circulation below the ducting inside the dryer. 10. Dryer doors: The dryer has double doors made of steel with insulation. The doors are kept closed during the drying process. If the doors are left open during drying, there will be a loss of heat and the quality of drying will be bad and will take a very long time. 11. Gravel: Gravel of 40mm size is spread on the floor of the dryer all around the ducting, upto the level of the ducting. The function of the gravel is to retain the heat inside the dryer chamber. 12. Thermometer: The thermometer is placed in a slot provided in the outer body of the dryer. It is generally placed just below the door of the dryer. The thermometer maybe fixed to the dryer or can be detached and placed in the slot during the drying process. It is an important part of the dryer, because it indicates the temperature inside the dryer chamber.