different dryers

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

28. x x
Grain
Air
Cross flow

29. x x
Grain
Concurrent flow
Air

30. x x
Grain
Air
Counter flow

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

33. CIAE MULTIPURPOSE DRYER

34. Mixing type baffle dryer

35. Pictorial View of Recirculating Batch Dryer Coupled with Husk-fired
Furnace for Paddy Drying (IIT, Kharagpur)

36. Continuous Column dryer

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

45. Commercial tray dryer