3 Drying Types Dryers.ppt
- 1. Grain Drying Drying refers to removal of moisture Dehydration refers to removal of moisture to almost bone dry state
- 2. XE
- 3. Typical Drying rate curve Kg/h
- 4. CRITICAL MOISTURE: The MC at which the drying rate is no more constant is known as critical moisture content of a solid (Xc). Depends upon: a) Surface Moisture Concentration ; % b) Bed thickness cm c) Drying Rate kg/h d) Shape and Size of the grains sphericity e) Drying conditions Temp ; RH
- 5. What are Drying Curves? CONSTANT RATE PERIOD : The rate of evaporation is essentially the same as the rate of evaporation from a free liquid surface under the same set of conditions. Drying takes place by surface evaporation & moisture moves by vapour pressure difference. • The plots of – Moisture Content Vs drying time – Drying rate Vs drying time – Drying rate Vs moisture content are known as Drying curves
- 6. FALLING RATE PERIOD: After the constant rate falling rate period starts because as: wetted surface continually decreases until at the end of this period the surface is dry. This cause of falling off in the rate of drying is due to the inability of the moisture to migrate from the centre of the body to the surface at a rate comparable with the moisture evaporation from its surface to the surroundings. The falling rate period of drying divided into 2 stages • Unsaturated surface drying • Drying where the rate of water diffusion within the product is slow and is the controlling factor
- 7. The psychrometric chart is a graphical representation of physical and thermal properties of atmospheric air, such as: • Dry bulb temperature, deg C • Wet bulb temperature, deg C • Dew point temperature, deg C • Absolute humidity or Humidity ratio, Kg of water vapour / Kg of dry air • Relative humidity, % • Humid volume or Specific volume, (cu.m/Kg of dry air ) • Enthalpy, (Kj/Kg dry air) OR (Kcal/ Kg of dry air) PSYCHROMETRIC CHART
- 8. Represents condition of the air
- 9. Humidity The absolute humidity, H is defined as Kg of water vapour present per kg of dry air under a given set of conditions. Relative humidity RH is the ratio of the partial pressure of water vapour in air to the partial pressure of water vapour in saturated air at the same temperature. Humid heat It is the kcal required to raise the temp of 1 kg of dry air and its accompanying water vapour through 1 deg C.
- 10. Enthalpy It is the total heat content of 1 kg of dry air + its accompanying water vapour in an air and water vapour mixture. Humid volume ‘v’ is the total volume in cu.m of 1 kg of dry air accompanying water vapour Dew point It is the temp to which a mixture of air and water vapour has to be cooled at constant humidity to make it saturated.
- 11. Wet bulb temperature Under adiabatic condition, if a stream of unsaturated air, of const. initial temp & humidity is passed over a wetted surface, then the evaporation of water from the wetted surface tends to lower the temp. of the liquid water. When water becomes cooler than the air, the sensible heat will be transferred from air to the water until a steady state condition is reached. Under such conditions, the temp of the water will remain const. and this const. temp. is called WB temp.
- 12. 1 2 1 2 2 1 1 100 100 M M M m m m W Wm / Where = Weight of moisture evaporated, kg W1 = Initial weight of wet material, kg W2 = Final weight of the dried product, kg m1, m2 = Initial and final moisture content , per cent wet basis M1, M2= Initial and final moisture content, percent dry basis.
- 13. • The air with an initial HR ratio of 0.0150 kg of water vapour/kg. of dry air was blown through 50 kg. of wheat with- moisture content as 22%(w.b) at the rate of 30cu.m / min. The temp of air at exit was 32.5 deg.C with a HR of 0.0188 kg of water vapour/kg of dry air. From psychro. Chart, Humid volume = 0.87 cu.m /kg. – Determine: Kg of dry air and also time in hour to reduce the moisture content of wheat from 22% to 13% (w.b)
- 14. A batch of 500 kg of ginger is to be dried from 85% to 5%MC(w.b)in a commercial spice dryer with 80% efficiency. Drying is to be accomplished in 8 hrs with a max. temp. of 60 deg C. What will be the power reqd/hr? 1 2 1 2 2 1 1 100 100 M M M m m m W Wm / Wt of water to be evaporated= 421 kg Drying rate = 421/8 =53 kg/hr From steam table: Latent heat of saturated steam at 60 Deg C= 564 K cal/kg There fore, 564 x 421 = 237444 K.Cal = 276 kW Since, 1 kW=14.34 K.Cal/min =860.58 K.Cal/hr ie. 1 kWh=860 K.Cal Hence, power reqd. per hr = 276/8 = 34.5 kW At 80% efficiency rate power reqd = 34.5/0.80 = 43 kW
- 15. Grain drying systems Convection drying Under fluidised state Under spouted bed Ordinary state Natural air drying With supplemental heat 5 to 10 deg C rise Heated air drying • Conduction drying • Convection drying • Radiation drying
- 16. Characteristics of Conduction drying • Heat transfer to the wet solid takes place by conduction through solid (metallic) surface • Surface temperatures may vary widely • Contact dryers can be operated under low pressure and in inert atmosphere • Dust & dusty materials can be removed effectively • With agitation, more uniform dried product and increased drying rate are achieved Example: Continuous Conduction dryers • Cylinder dryers, Drum dryers, Steam tube rotary dryers Batch Conduction dryers • Vacuum tray dryers, Freeze dryers, Agitated pan dryers
- 17. Characteristics of convection drying • Heat is transferred to the wet solid mainly by convection • Drying is dependent upon the heat transfer from the drying agent to the wet material • Steam, heated air, direct flue gases etc. can be used as drying agent • Drying temp varies widely • At gas temp’s below the boiling point, the vapour content of the gas affects the DR and final MC of the solid(grain) • If the atm. Humidity is high, natural air drying needs dehumidification • Fuel consumption per kg of moisture evaporation is always higher than that of conduction drying Example: Continuous : Tray, Sheeting, Pneumatic conveying, Rotary, Tunnel Batch : Tray & compartment, Batch through circulation
- 18. Processes used to dry cereal grains are divided into two broad categories: • Batch dyers : Dries grain on batches • Continuous dryers : Grain flows continuously Batch systems In-bin batch drying Column batch drying The dryer structure may serve as storage bin after drying After drying grains are stored in separate structure
- 19. In-bin drying system Full bin DS Layer drying DS Batch-In-Bin DS • Full bin & Layer DS dry and cool grain in a storage bin • The drying period extends over several days / weeks • Usually handles one batch per harvesting season • The grain bed is usually deep (up to 5 m) • Relatively low air flow rate is used • Natural air is generally used
- 20. Advantages Disadvantages • Grain can be harvested at any rate desired • Minimum grain handling • Grain is not over dried • Minimum Check cracking because of low drying air temp. • Grain cannot be harvested at high MC • Drying continues over an extended period of time – prolong management period Full bin drying
- 21. Layer drying system A grain layer is placed in the bin as initial batch & drying begins As Drying Zone moves through the grain other layers are added periodically- thus a depth of wet grain exists ahead of the DZ Bin is eventually filled – drying is continued – drying zone passes through the entire grain mass 1st layer have the highest initial MC – receives highest air flow as grain depth is shallow Last layer of grain added – lowest MC – receives lowest air flow because of greater depth Advantages are Grain with higher initial MC can be harvested to start with Drying period is shorter compared to Full Bin system • Disadvantages are – Careful management is required – A restricted harvesting schedule is essential
- 22. 1st batch 1st batch Drying Zone 2nd batch DZ Last batch DZ DZ moves from bottom to upwards Layer drying concept M M M M
- 23. Batch-In-Bin DS • Drying in batches within a bin and subsequently moving the grain to storage is called Batch-In-Bin DS. • This involves – Placing a layer of grain in the bin (< 1.2 m) – Forcing heated air through the grain until desired MC is reached – Cooling the grain with fan without heater – Moving the grain to a storage As dried grain is not stored in the drying chamber, separate batches may be dried – provides flexibility in harvesting schedule The main Dis-adv is the overdrying problem in the bottom layer Problem of over drying can be alleviated (minimized) by • Incorporating Grain stirring devices • Re-circulating the grain
- 24. Advantages of stirring devices in Batch systems are • Vertical moisture gradient is reduced • Air flow through the grain is increased • Drying rate enhances • Stirrers break-up areas where wet grain may be packed Recirculating means removing the layer of the grain next to the perforated floor at some pre-selected MC & place it on top of the batch. In other words, the DZ is kept stationary and wet grain is fed into it
- 25. Advantages of recirculating type Batch systems are • Heat is not wasted in drying grain below the desired MC • Grain is exposed to the hottest air while it is still wet • Grain is exposed to the high temp. for a shorter time than in a conventional batch system • The drying process can be made continuous flow by removing the grain from the dryer instead of depositing back on top
- 26. Column batch drying • Classified as Stationary bed dryers – portable • Drying chamber is a vertical column with perforated walls – Grain bed thickness <30 to 45 cm – Air flow rate is higher – Grain column is vertical and air passes through it from side to side – May be of mixing or Non-mixing type and in most cases recirculating type. – High rate of airflow across the narrow drying bed facilitates the DZ to extend completely across the bed by the time desired MC is reached. – Drying is completed within 2 to 3 hrs – Grain is cooled before it is unloaded
- 27. Continuous flow DS Cross flow Concurrent flow Counter flow Air Flow is perpendicular to the grain flow Air & grain move in the same direction Air & grain flow in opposite direction
- 31. Tempering • Grain is dried in stages with heated air & each stage is a pass through the dryer. Between passes, it is stored in bins for an equilibrium period known as “tempering” period. Moisture concentration equalizes between inner and outer of the kernel Why? • Shortens total in-dryer time • Prevents breakage during milling Aeration • Stored grain is ventilated with low air flow rates to maintain grain quality Why? • To prevent moisture migration by maintaining a uniform temp. throughout grain mass • To cool grain to reduce mold growth and insect activity • To remove storage odors • To distribute fumigants in the grain mass
- 32. Belt movement Heated air Cooling section Grain removed Grain fed in Continuous flow dryer, horizontal belt
- 34. Mixing type baffle dryer
- 35. Pictorial View of Recirculating Batch Dryer Coupled with Husk-fired Furnace for Paddy Drying (IIT, Kharagpur)
- 37. FAN AND HEATER COOLING FAN FILLING AUGER GRAIN COLUMN COOLING AIR PLENUM HEATED AIR PLENUM GRAIN MIXER UNLOADING AUGER WET GRAIN HOLDING BIN DOUBLE COLUMN DRYER
- 38. Drying systems….. On-the –floor driers : • It is low temperature drying, suitable for both drying and storage and can be carried out in any building with a level moisture proof floor and walls with adequate strength to resist the pressure of the grain. The drying air is blown through no. of ducts (Fig.1). • In such driers, a volume of 28cu.m / min of air at 60% RH will reduce 1 tone from 22 to 17 % moisture content in 24 hours. The system is mainly suitable for bulk handling installations.
- 39. Fig.1 On-floor system with ducting
- 40. In-bin-driers : These consist of a square or circular container with a capacity of up to 60 tones. Each has a perforated false floor above a plenum chamber (Fig.2). Grain can be dried up to a depth of 3 m provided its moisture content does not exceed 22%. Fig.2 Floor ventilated bin (on right : sectional perforated floor)
- 41. Tunnel Driers : In many areas, crops are handled in sacks and tunnel drying systems have been designed to dry bagged produce. A tunnel of sacks(Fig.3) is built, using a simple wooden frame for support. Using a “sack in the tunnel system’ keeps the units separate and allows the individual identity of each sack to remain. This system may well suit a co-operative drying group. It also has the advantage in requiring less expensive plant and can be set up in almost any building. Fig.3. Tunnel Drier
- 42. Diagram of a solar dryer 1. Solar panel; 2. Hot, dry air; 3. Grain to be dried.
- 43. A solar crop dryer developed by a UNSW photovoltaic and solar energy engineering student The dryer works with solar powered fans for forced convection, with products being laid out on mesh trays and air forced over them to extract moisture from the foods – See more at: http://www.engineering.u nsw.edu.au/mining- engineering/news/solar- crop-dryer-bears- fruit#sthash.ZmV8Ese2.d puf
- 44. Solar Crop Dryer is mainly consist of semi-cylindrical tunnel structure which covers with ultra violet (UV) stabilized polyethylene as collector material