Grain Dryer Design

7/22/2019 Grain Dryer Design

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Louisiana State University

(LSU) Type dryer


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LSU Type dryer


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LSU Type dryer


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LSU Type dryer

Rectangular drying

chamber fitted with airports and the holdingbinSquare section 1.2, 1.5,

1.8, 2.1 mTop holding bin

Bottom drying chamber

Air blower with duct

Grain dischargingmechanism with ahopper bottom

Air heating system


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LSU Type dryer

Inverted V-pots. Same size, arranged in equal

spacing.One end of each air channel has an opening and

the other end is sealed.

Alternate layers are air inlet and air outlet channels.

In the inlet layers, the channel openings face the airinlet plenum chamber but are sealed at theopposite wall.

The outlet layers, the channel openings face theexhaust but are sealed other side.

The inlet and outlet ports are arranged one belowthe other in an offset pattern.


6/41

LSU Type dryer

Advantages:

Uniformly dried product can be obtained if

the dryer is designed properly.

The dryer can be used for different types ofgrains.

Disadvantages:

High capital investment.High cost of drying if oil is used as fuel.


7/41

MASS AND HEAT BALANCE IN GRAIN DRYING

HeaterDRYERAmb Air

Qa,T0,H0

Heated air

Qa,T1,H0

Grain

WG1, TG1, m1(wb)

Grain (dried)

WG2, TG2, m2(wb)

Exhaust air

Qa,T2,H1


8/41

Mass balanceLet

W =WG1WG2 (1)

or

WG1 =WG2 + W

Amount of dry material entering into the dryer =(1m1/100)WG1 , kg/hr

Amount of dry material leaving the dryer =(1m2/100)WG2 , kg/hr

But

(1m1/100)WG1 =(1m2/100)WG2 , kg/hr

WG2 =WG1[(100m1)/(100m2) (2)

Put equation (2) in equation (1)

W =WG1WG1[(100m1)/(100m2)=WG1(m1m2)/(100m2)

Similarly

W =WG2(m1m2)/(100m1)

WG1(m1m2)/(100m2) =WG2(m1m2)/(100m1) (3)


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Heat balance

Sensible heats (Grain + Water) + Latent heat

WdxCPGx(TG2TG1) + WdxM1xCPWx(TG2TG1)

+ Wd(M1M2) (5)

=Qa(0.24+0.45H0)(T1T2) x t (6)


10/41

Design of LSU type dryer


11/41

Design a LSU dryer of 2.0 tonnes holding

capacity with paddy at 18% m.c.

Air Density of air= 1.13kg/m3,

Ambient air temperature= 30oC,

RH of ambient air= 70%,

Absolute humidity of ambient air= 0.019,Heated air temperature= 85oC,

Exhaust air temperature= 35oC,

Latent heat of water vapour= 600 kcal/kg,


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Paddy Angle of repose= 45o,Specific heat of the grain= 0.4 kcal/ kg oC,

Bulk density of paddy at 18% m.c.= 600kg/m3,

Initial m.c. of paddy= 30%(w.b.),

Final m.c. of paddy= 18%(w.b.),

Grain inlet temperature= 30 oC,

Grain outlet temperature= 70 oC,

Grain residence time= 30 minutes


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Dryer Cross-section of the dryer= 1.2x1.2m2

Air velocity in the air ports= 5m/sec

Pitch of the air ports= 40 cm

Row to row spacing = 20 cmHeight of the grain holding bin= 35 cm,

Height of the hopper bottom= 60 cm,

Diameter of the discharge roll= 2.5 cm,Diameter of flute= 7.5 cm,

Discharge hopper angle= 50o.

3


14/41

Air Density of air= 1.13kg/m3,

Ambient air temperature= 30 oC,


Absolute humidity of ambient air= 0.019,

Heated air temperature= 85 oC,

Exhaust air temperature= 35 oC,Latent heat of water vapour= 600 kcal/kg,

Paddy Angle of repose= 45o,

Specific heat of the grain= 0.4 kcal/ kg oC,


Initial m.c. of paddy= 30%(w.b.), Final m.c. of paddy= 18%(w.b.),

Grain inlet temperature= 30oC,

Grain outlet temperature= 70oC,

Grain residence time= 30minutes

Dryer Cross-section of the dryer= 1.2x1.2m2

Air velocity in the air ports= 5m/sec

Pitch of the air ports= 40 cm, Row to row spacing = 20 cm

Height of the grain holding bin= 35 cm,

Height of the hopper bottom= 60 cm,

Diameter of the discharge roll= 2.5 cm,

Diameter of flute= 7.5 cm, Discharge hopper angle= 50o.


15/41

Air requirement Bone dry paddy

= 2000 (10.18) = 1640 IMC = 30% w.b. 42.86% d.b.

FMC= 18% w.b. 21.95% d.b.

Weight of moisture evaporated

Wt. of bone dry paddy x (FMC-IMC) 342.92 kg From psychrometric chart

Specific heat

S=0.24 + 0.45 x 0.019 kg/kg0.248 kcal/kgoC

Let G be the rate of air supply in kg/min

Heat supplied by the air in 30 min

GS(85-35oC) 30 G 0.248 kcal/kgoC (85-35 oC) 30 min373 G


16/41

Air requirement

Heat Utilized

1. As sensible heat ofgrain

= BD paddy x Sp heat Grain x Temp rise

= 1640 kg x 0.4 kcal/ kg oC x (70-30) oC

= 26,240 kcal

2. As sensible heat ofwater

= Total wt of water x Sp heat Grain x Temp rise

= (1640 x 0.42857 kg) x 1.0 kcal/ kg oC x (70-30) oC

= 28,114.2 kcal3. As latent heat of water vapour

= Water evaporated x latent heat

= 342.86 kg x 600 kcal/ kg

= 2,05,715.04 kcal


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Air requirement

Total Heat Utilized

= Sum of 1, 2 and 3

= 26,240 + 28,114.2 + 2,05,715.04

= 2,60,069.24 kcal

Suppose, heat loss = 10 percent

Net heat requirement = {260069.24 kcal / (1-loss)} x 100

= 260069.24 kcal / 0.90

= 2,88,965.82 kcal

Hence,Heat supplied = Net heat required

373 G = 2,88,965.82 kcalG = 774.71 kg/min


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Drying chamber

G air supply in kg/min m3/min

From psychrometric chart, humid volume of the ambient air= 0.88 m3/kg

G = 774.71 kg/min x 0.88 m3/kg

=681.75 m3/min

Since velocity of air inside the air port or duct is 5m/secTotal cross-sectional area of ducts required

= (681.75 m3/min) / (5 x 60 m/min)

= 2.27 m2

Therefore, volume of drying chamber

V = Drying chamber volumeDucts volume

= 1.2 m x 1.2 m x h m2.27 m2 x1.2 m

= (1.44 h2.724) m3


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Power requirement

Air requirement per m2

As calculated: Air requirement 681.75 m3/min

Total cross-sectional ducts area is 2.27 m2

681.75 m3/min

2.27 m2

= 300.33 m3min-1 / m2

From Shedds curve, static pressure drop for 32.12 m3min-1 /

m2=8.13 cm of water per 30.48cm grain depth.

Depth of grain = 20cm

So pressure drop = 8.13 x 20 / 30.48 = 5.42 cm of water

Pressure drop in terms of air column= 0.0542x(1000kgm-3/1.13kgm-

3)

= 47.95 m


20/41

Airflow in Packed BedsDrying, Cooling, etc.

Source: ASABE D272.3, MWPS-29

Design Values for Airflow Resistance in Grain

0.1

1

10

100

0.001 0.01 0.1 1 10

Pressure Drop per Foot, inH2O/ft

Airflow,cfm/ft2 Corn (PF=1.5)

Sunflower (PF=1.5)

Soybeans (PF=1.3)

Barley (PF=1.5)

Wheat (PF=1.3)

Milo (PF=1.3)

0.027

1.3


21/41

Power requirement

HP req = Height of air column, m x air flow rate, kg/min

4500

= 47.95 m x 774.71 kg/min

4500

= 8.25 hp


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Aeration Fan Selection

SSStttaaatttiiiccc PPPrrreeessssssuuurrreee,,, iiinnn HHH222OOO

MMMooodddeeelll 000""" 000...555""" 111""" 111...555""" 222...555""" 333...555"""111222""" 333///444 hhhppp 111999000000 111666777555 111222999000 888111555 333222555 000111222""" 111 hhhppp 222333000888 111999666333 111444666000 888777666 333000555 000111444""" 111...555 hhhppp 333111333222 222888555222 222555222666 222111222666 111000444000 000

Axial Flow Fan Data (cfm):


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Aeration Fan Selection

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 500 1000 1500 2000 2500 3000

Airflow, cfm

StaticPressure,

inH2O

System

Fan


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Standards, Codes, & Regulations

Standards

ASABE

ASAE D245.5 and D272.3ASAE D243.3 Thermal properties of grain and

ASAE S448 Thin-layer drying of grains and crops

Several others

Others not likely for unit operations


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Dryer Fan Selection

1. Select highest airflow (cfm/ft2) for drying rate

2. Airflow: cfm/ft2

3. Pressure drop: DP = (inH2O/ft)

4. Total airflow: cfm = (cfm/ ft2) x (floor area)

5. Select fan to deliver flow & pressure (fan data)

Cross section of the dryer


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Cross-section of the dryer

1.2x1.2 m2

Ht of the grain holding bin= 35 cm,

Pitch of the air ports = 40 cm

Row to row spacing = 20 cm

Dia. of the discharge roll = 2.5 cm,

Dia. of flute = 7.5 cm,

Ht of the hopper bottom = 60 cm,

Discharge hopper angle = 50o.


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Drying chamber

Holding capacity of the dryeris 2000 kg at 18% m.c. and

Bulk density of paddy at 18% m.c. = 600 kg/ m3Hence

V = 2000 kg / 600 kg m-3

= 3.33 m3

V = (1.44 h2.724) m

3

h = (3.33 + 2.724) / 1.44

= 4.21 m

Height of grain holding bin = 35 cm (assumed)

Height of hopper bottom = 60 cm (approx)

According to spacing (40 cm),number of ducts in a row = 1.20 m/ 40 cm = 3 and

number of row in 4.21 m = 4.21 m/ 20 cm = 21

Total number of ducts = 20x3 = 60(leaving 1 row for discharge rolls)


28/41

Drying chamberCross sectional area of each duct (b2/4)

a = 2.27 m2 / 60

= 37.83 cm2

b = 12.3 cm

Let there be 3 discharge rolls having shaft diameter 2.5

cm and flute diameter7.5 cm

Vol discharged by each roll in one revolution

= (7.52 2.52) 120 / 4

= 4712.39 cm2Vol discharged by 3 rolls, m3/rev

= 0.014137m3/rev

Wt of paddy discharged, kg/rev

= 0.014137 x 600 kg/m3

Since, grain retention time is 30 min

Grain discharge rate=2000/30

=66.67 kg/minRoller speed=66.67kg min-1/ 8.129kg rev min-1

=8 rpm


29/41

LSU dryer specificationTotal height of dryer

Height of drying chamberHeight of holding bin

Height of hopper bottom

Number of discharge rolls

Grain outlet diameterSpace between inlet and outlet duct

Pitch of ducts in a row

Blower capacity (CFM, CMM with static pr)

Blower motor

Duct dimension

Speed of discharge rolls


30/41

LSU dryer

Air requirement

Bone dry paddy


Heat requirement considering heat loss

Blower power

Dryer dimensions

Volume of drying chamber

Cross sectional area of each duct

Roller speed


31/41

Design of PHTC- re circulating

batch dryer


32/41

D i f PHTC i l ti f 2 0


33/41

Design of PHTC- re circulating of 2.0 tonnesholding capacity with paddy at 18% m.c.

Air Density of air= 1.13kg/m3,Ambient air temperature= 30oC,


Absolute humidity of ambient air= 0.019,Heated air temperature= 85oC,

Exhaust air temperature= 35oC,

Latent heat of water vapour= 600 kcal/kg,

D i f PHTC i l ti f 2 0 t


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Paddy Angle of repose= 45o,Specific heat of the grain= 0.4 kcal/ kg oC,


Initial m.c. of paddy= 30%(w.b.),

Final m.c. of paddy= 18%(w.b.),

Grain inlet temperature= 30 oC,

Grain outlet temperature= 70 oC,

Grain drying time= 180 minutes

D i f PHTC i l ti f 2 0 t


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Dryer Thickness of grain bed to be dried= 20cm,Diameter of plenum chamber= 135cm,

Diameter of dryer= 175cm,

Diameter of grain outlet= 15cm,

Hopper angle= 50o,

Drying time= 3 hr,

Static pressure drop for 32.12m3/min m2= 8.13cm of

water per 30.48cm grain depth.


36/41

Thickness of grain bed to be dried= 20cm,

Diameter of plenum chamber= 135cm,

Diameter of dryer= 175cm,

Diameter of grain outlet= 15cm,

Hopper angle= 50o,

Drying time= 3 hr,

Static pressure drop for 32.12m3/min m2=

8.13cm of water per 30.48cm grain depth.


37/41


38/41

Air requirement Bone dry paddy

= 2000 (10.18) = 1640 IMC = 30% w.b. 42.86% d.b.

FMC= 18% w.b. 21.95% d.b.


Wt. of bone dry paddy x (FMC-IMC) 342.92 kg From psychrometric chart

Specific heat

S=0.24 + 0.45 x 0.019 kg/kg0.248 kcal/kgoC

Let G be the rate of air supply in kg/min

Heat supplied by the air in 180 min

GS(85-35oC) 30 G 0.248 kcal/kgoC (85-35 oC) 180 min2232 G


39/41

Air requirement

Heat Utilized

1. As sensible heat ofgrain

= BD paddy x Sp heat Grain x Temp rise

= 1640 kg x 0.4 kcal/ kg oC x (70-30) oC

= 26,240 kcal

2. As sensible heat ofwater

= Total wt of water x Sp heat Grain x Temp rise

= (1640 x 0.42857 kg) x 1.0 kcal/ kg oC x (70-30) oC

= 28,114.2 kcal3. As latent heat of water vapour

= Water evaporated x latent heat

= 342.86 kg x 600 kcal/ kg

= 2,05,715.04 kcal


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Air requirement

Total Heat Utilized

= Sum of 1, 2 and 3

= 26,240 + 28,114.2 + 2,05,715.04

= 2,60,069.24 kcal

Suppose, heat loss = 10 percent

Net heat requirement = {260069.24 kcal / (1-loss)} x 100

= 260069.24 kcal / 0.90

= 2,88,965.82 kcal

Hence,Heat supplied = Net heat required

2232 G = 2,88,965.82 kcalG = 129.46 kg/min


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Air requirement

G air supply in kg/min m3/min

From psychrometric chart, humid volume of the

ambient air

= 0.88 m3/kg

G = 129.46 kg/min x 0.88 m3/kg =113.93 m3/min

Grain Dryer Design

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