Drying Systems PDF

CLASSIFICATION OF DRYING SYSTEMS CLASSIFICATION ON DIFFERENT BASIS 1. Based on Technology a.SUN DRYING b.MECHANICAL DRYING 2. Based on mode of heat transfer a.CONDUCTION DRYING b.CONVECTION DRYING c.RADIATION DRYING 3. Based on contact of he...

CLASSIFICATION OF DRYING SYSTEMS CLASSIFICATION ON DIFFERENT BASIS 1. Based on Technology a.SUN DRYING b.MECHANICAL DRYING 2. Based on mode of heat transfer a.CONDUCTION DRYING b.CONVECTION DRYING c.RADIATION DRYING 3. Based on contact of heating medium and grain a.BATCH DRYING b.CONTINUOUS FLOW DRYING Mixing Type Non-Mixing Type 4. Based on type of flow a.COUNTER-CURRENT FLOW b.CON-CURRENT FLOW c.CROSS FLOW SUN DRYING Sun drying is the most popular traditional method of drying. Sun drying is an example of radiation drying based on the absorption of the radiant energy of the sun and its transformation in to heat energy by the grain. The effectiveness of sun drying depends open temperature and relative humidity of the atmospheric air , speed of the wind , type and condition of the grain. MECHANICAL DRYING It is the process of utilising mechanical means for drying of grains by ventilating natural or heated air through the grain mass to accomplish removal of moisture from it. Rate of drying of grains can be controlled by controlling the temperature of hot air ventilating through the grain mass.  SUN DRYING MECHANICAL DRYING  The process required no fuel  The process requires or mechanical energy. fuel/electrical/mechanical  Operation is very simple power.  Completely dependent on  The process is automatic and weather requires a small number of  Not possible round the clock trained person. round the year  Grain can be dried in all  Excessive losses due to seasons and during night. shattering, birds, rodents  Losses avoided  Requires specially  It requires a machine room constructed large drying area  It is hygienic  Entire process is unhygienic  Cost of drying per unit mass is comparatively higher than sun drying CONDUCTION DRYING When the heat for drying is transferred to RADIATION DRYING the wet solid mainly by conduction through a solid surface(usually metallic) the Heat energy can be supplied to phenomenon is known as conduction or wet products by electromagnetic contact drying. (Rotary,drum,freeze,tray dryer) waves. (sun, infrared,microwave) The rate of heat transfer is dependent upon the thermal conductivity of the heated surface The radiation within the & also depends on the heat transfer wavelength range lying between 0.76-400 micro metre are called coefficient from the heating medium to the as infra-red radiation. surface. A continuous conduction dryer can not dry Microwave radiation also causes heat-sensitive solutions, slurries, pastes, and rapid heating. granules containing pigments and salts Commonly used for drying thin materials and drying of food grains and powders.  3 1. Ambient Air 2. Heater 6 3. Wet Material 4. Dry Material 5 5. Exit Air 6. Drying Chamber 4 CONVECTIVE DRYING 2 In this method the sensible heat of heated air is transferred to the wet products by convection. Heated gaseous medium (usually air) is ventilated through a mass of wet materials and carries with it the water vapour evaporated from the material. 1 Natural air, heated air and with supplementary heat (just by 5 to 100C to reduce RH). Used for drying of grain, potato, banana, guava, carrot, etc. Advantages of Conduction Drying: Advantages of Convection Drying:  Continuous operation.  Allows accurate control of  Thermally efficient. temperature  Low operating & maintenance cost.  Provides uniform heating  Least supervision. regardless of product size,  Least mould growth. colour or shape Disadvantages of Conduction Drying:  Doesn’t require contact to heat  Uneven drying rate.  Steam heated air, direct flue  Drying device is expensive gases of bio waste can be used as drying agent Advantages of Radiation Drying: Disadvantages of Convection Environment friendly. Drying: High productivity due to instantaneous drying.  Achieves a lower rate of heat Energy and space saving transfer than other heating No waste products during this process. methods Ability to work with heat sensitive materials.  Requires increased product Uniform heating exposure time Disadvantages of Radiation Drying:  Fuel consumption per kg of Possibility of health hazards. moisture evaporated is always In infrared drying, surface temperature should be within limit to avoid denaturation or browning. higher than that of conduction High maintenance cost. drying BATCH DRYER  A certain volume of grain is being dried at a time. The grain is placed on perforated floor & the heated air is forced through the deep bed of grain.  After drying of grains to the desired moisture level, they are discharged manually. CONTINUOUS FLOW DRYER In Continuous dryer the dried grains continuously flow from inlet to outlet. Generally used in relatively large grain complexes.  BATCH DRYER CONTINUOUS FLOW DRYER Batch dryer is a system ADVANTAGES ADVANTAGES Drying is uniform. Mostly used for on-farm drying. Rate of drying is faster. Intermittent drying can also be used. Require less drying time as compared to Price is less. batch dryer. Operation is very simple. DISADVANTAGES It can be manufactured locally by using Intermittent drying can’t be used. various type of material Price is comparatively more than batch DISADVANTAGES dryer. Drying is non-uniform drying which results The operation is not so simple and the higher percentage of broken grains. operator must have knowledge about grain Require more drying time. drying management. Rate of drying is slow. It requires specialized unit. The temperature of the heated air should be limited to 45⁰ c. Mixing and non-mixing dryers are two types of continuous flow dryer. Schematic diagram MIXING DRYERS  Mixing dryers are available in several designs-Baffle type and L S U type  In baffle type dryers moist grain flows downward in zig-zag path by means of baffle provided in the way.  Then the heated air is forced through the grains and mixed vigorously hence named as mixing type dryers.  These dryers permit a low flow rate of air i.e. 50-95m³/min-tonne & high drying temp. of 65˚c. NON-MIXING DRYERS  The wet grains are descended between two parallel screens and Heated air is blown through the screen.  After drying is done dried grains are collected from the discharge.  They are distinguished from the mixing type due to the flow of grain in a straight path.  Drying air temperature of 54ºc is used.  These dryers can resist a high air flow rate of 125-250m³/min-tonne.  A mixing-type baffle dryer A non-mixing columnar type dryer Schematic of the drying chamber of a mixing-type LSU grain dryer.  Different modes of airflow in a thin-layer grain-drying system: (a) countercurrent-flow, (b) concurrent-flow, (c) cross-flow, and (d) mixed-flow.  CROSS FLOW Cross flow dryers are the most popular dryer type, they have a plenum surrounded by a relatively thin grain column; hot air traverses the grain perpendicular to the downward flow of the grain. Advantages  The grains which come in contact with heated air dries early. Disadvantages  The grain layer which is at farther point remains comparatively cooler.  It do not dry grain uniformly.  Concurrent flow The direction of flow of grains and heated air is parallel to each other. The cool and wet grains meet the heated air first. Therefore,during evaporation of moisture the sensible heat for drying is absorbed as latent heat, as a result cooling of air takes place and this restricts the elevation of grain temp. to a level of drying air temp. Advantages: The ability to employ different air temperatures in different stages. Uniformity of drying process. Disadvantages: High technology is used. So initial cost is high  Counter-Current flow Here the direction of flow of grains and heated air is opposite to each other, thus the hottest grains come in contact with heated air. Since, at this point drying is almost complete, therefore, very little amount of latent heat is absorbed for evaporation of moisture. Advantages: heat transfer more efficient. Disadvantages:  Typically higher initial and long term cost Water activity or aw is the partial vapor pressure of water in a substance divided by the partial vapor pressure of pure water at the same temperature. Water in food that is not bound to food molecules can support the growth of bacteria, yeast, and mold. The term water activity (aw) refers to this unbound water. The water activity of a food is not the same thing as its moisture content. Spoilage aw Food microorganism 0.90- Bacteria Cottage cheese, meat 1.00 0.85 - Margarine, condensed Bacteria, molds, yeasts 9.0 milk, whipped butter 0.80 - Yeasts Fruit syrups 0.85 0.75 - Xerophilic molds, molds Dried figs, jams 0.80 and yeasts 0.70 - Yeasts Confections 0.75 Water activity versus moisture content plot 0.65 - for different types of food. 0.70 Osmophilic yeasts Molasses 0.60 - Xerophilic molds, Dried fruit 0.65 osmophilic yeasts SALTS AND THE RESPECTIVE RELATIVE HUMIDITY GENERATION