SCREW CONVEYOR BASIC DESIGN CALCULATION
CEMA (Conveyor Equipment Manufacturer Association) Approach
HISTORY & APPLICATION
HISTORY:
APPLICATION:
The first conveyor as such as designed by ARCHIMEDES (287 – 212 BC) for removing water from the ship and to raise water from river to farm land
Screw conveyor s are bulk material transporting devices capable of handling a great variety of materials which have relatively good flowability .
The next technological advancement of importance in screw conveyor design was patented on march 29, 1898 by Frank C. Caldwell.
This construction is now known as the helicoid flight.
Flowability is defined in the CEMA material classification standard and denotes: “the degree of freedom of individual material particles to move past each other”
This characteristic is important in a screw conveyor operation as the screw helix.
The many advantages of screw conveyor is feasibility of nomerous feed and discharge of the material to be conveyed.
Another used is the blending of several ingredients to make a finished product or to make a pre-mix for some product
Screw conveyors may be operated horizontally, on an incline or vertically.
MATERIAL CODE & BULK MATERIAL CHARACTERISTICS
MATERIAL CODE
A study has been made to define the characteristic of bulk materials in terms which are readily recognized.
Table 2-1 Material Classification Code Chart Major Class Density Size
Material Characteristics Included Bulk Density, loose Very Fine
Abrasiveness
Actual, lb/ft3 No. 200 sieve (0.0029") and under
A 200
No. 100 sieve (0.0059") and under
A 100
No. 40 sieve (0.016") and under
A 40
Fine
No. 6 sieve (0.132") and under
B6
Granular
1/2" and under
C1/2
3" and under
D3
7" and under
D7
over 16" to be spesified
D 16
X = actual maximum size
Dx
Irregular
Stringy, fibrous, cylindrical, slabs,etc
E
Very free flowing
flow function > 10
1
free flowing
4 < flow function < 10
2
average flowability
2 < flow function < 4
3
sluggish
flow function < 2
4
Midly abrasive Moderately abrasive
index 1 - 17 index 18 - 67
5 6
Extremely abrasive
index 68 - 416
7
Lumpy
Flowability
Code Designation
MATERIAL CODE & BULK MATERIAL CHARACTERISTICS
BULK MATERIAL CHARACTERISTIC:
A study has been made to define the characteristic of bulk materials in terms which are readily recognized. The table shows in the first column the range of density that is usually experienced in handling material The material table is a guide only. The material code and the material factor (Fm) are based on experience of several conveyor manufacturers.
Table 2-2 Material Characteristic Material Ash, black ground
Weight lb/ft3
Material Code Component Series Mat'. Factor Vertical
105
105B635
1A-1B-1C
2.0
-
Ashes, coal, dry __ 1.2"
35 - 45
40C 1/246TY
3D
3.0
V
Ashes, coal, dry __3"
35 - 40
38D346T
3D
2.5
-
Cement, Clinker
75 - 95
85D336
3D
1.8
-
94
94A10026M
2D
1.4
V
Cement, Aerated (Portland)
60 - 75
68A 10026M
2D
1.4
V
Coal, Anthracite, sized__1/2"
49 - 61
55C 1/225
2A-2B
1.0
-
Coal, Bitumious, mined
40 - 60
50D 335LNXY
1A-1B
0.9
-
Coal, Bitumious, slack
43 - 50
47C1/245T
2A-2B
0.9
-
Dolomite, crushed
80 - 100
90C 1/236
2D
2.0
-
Dolomite, lumpy
90 - 100
95D x36
2D
2.0
-
Fly Ash
30 - 45
38A4036M
3D
2.0
-
Gypsum, raw__1"
70 - 80
75D325
2D
2.0
Limestone, crushed
85 - 90
88Dx36
2D
2.0
Limestone, dust
55 - 95
75A 4046MY
2D
1.6 - 2.0
Cement, Portland
SELECTION OF CONVEYOR SIZE AND SPEED – 1
MAXIMUM LUMP SIZE
The Allowable size of a lump in a screw conveyor is a function of the radial clearance between the outside diameter of the central pipe and the radius of the inside of the screw trough.
Table Maximum Lump Size Screw dia. (inches)
6 9 9 12 12 12 14 14 16 16 18 18 20 20
Pipe OD (inches)
2 3/8 2 3/8 2 7/8 2 7/8 3 1/2 4 3 1/2 4 4 4 1/2 4 4 1/2 4 4 1/2
Radial Clearance (inches)
2 5/16 3 13/16 3 9/16 5 1/16 4 3/4 4 1/2 5 3/4 5 1/2 6 1/2 6 1/4 7 1/2 7 1/4 8 1/2 8 1/4
Class 1
Class 2
Class 2
10% lumps
25% lumps
95% lumps Ratio
Ratio R=1.75
Ratio R=2.5
R=4.5
Max. Lump (inch.) Max. Lump (inch.) Max. Lump (inch.)
1 1/4 2 1/4 2 1/4 2 3/4 2 3/4 2 3/4 3 1/4 3 1/4 3 3/4 3 3/4 4 1/4 4 1/4 4 3/4 4 3/4
3/4 1 1/2 1 1/2 2 2 2 2 1/2 2 1/2 2 3/4 2 3/4 3 3 3 1/2 3 1/2
1/2 3/4 3/4 1 1 1 1 1/4 1 1/4 1 1/2 1 1/2 1 3/4 1 3/4 2 2
SELECTION OF CONVEYOR SIZE AND SPEED – 1 CONVEYOR SIZE & SPEED In order to determine the size and speed of a screw conveor, it is necessary first to establish the material code number. This Code number controls the cross-sectional loading that should be used. The capacity table (below) gives the capacities in ft3/hour at one revolution per minutes for various sized and various cross-sectional loading. The basis for the Capacity Table is as follows
= Where: C : Capacity [ft3/hour.rpm] Ds : Diameter of screw [inches] Dp : Diameter of pipe [inches] P : pitch of screw, [inches] K : percent trough loading
. −
SELECTION OF CONVEYOR SIZE AND SPEED – 2
In order to determine the conveyor speed can be calculated by the formula:
=
. . . .
Where: N : Conveyor speed [rpm], (≤ recommended max. Rpm)
Q : Required capacity [ft3/h] C1 : Conv. Capacity at one rpm (see table below) CFo : Overload capacity factor [110% - 120%] CF1 : Conveyor pitch factor CF2 : Type of flight factor CF3 : Mixing paddle factor
Special Conveyor Pi tch Capacity Factor, CF1 Pitch
Standard Short Half Long
Description
CF1
Pitch = diameter of screw Pitch = 2/3 diameter of screw Pitch = 1/2 diameter of screw Pitch = 1 1/2 diameter of screw
1 1.5 2 0.67
Special Conveyor Fli ght Capacity Factor, CF2 Conveyor Loading
Type of Flight
Cut Flight Cut & Folded Flight Ribbon Flight
15%
30%
45%
1.95 NR 1.04
1.57 3.75 1.37
1.43 2.54 1.62
Special Conveyor Mixi ng Paddle Capacity Factor, CF3 o
Standard paddles per pitch set at 45 reverse pitch
Factor CF3 Compiled by Masda Ehsan
None 1
1 1.08
2 1.16
3 1.24
4 1.32
SELECTION OF CONVEYOR SIZE AND SPEED – 3 SCREW CONVEYOR CAPACITIES Material Class Degree of Screw Dia. Max. Capacity (ft3/hour) Code Trough Loading (inches) RPM at max. RPM at one RPM A-15
45%
A-25
6 9
165 155
368 1,270
SCREW CONVEYOR CAPACITIES Material Class Degree of Screw Dia. Max. Capacity (ft3/hour) Code Trough Loading (inches) RPM at max. RPM at one RPM
2.23
A-35
E-15
6
120
180
8.20
A-45
E-25
30%
9
100
545
1.49 5.45
E-35
12
90
1,160
12.90
E-45
B-15
12
145
2,820
19.40
B-35
B-25
14
140
4,370
31.20
B-45
14
85
1,770
20.80
6,060
46.70
C-35
16
80
2,500
31.20 45.00
C-15
16
C-25
130
18
120
8,120
67.60
C-45
18
75
3,380
20
110
10,300
93.70
D-15
20
70
4,370
62.50
164.00
D-25
24
65
7,100
109.00
24
100
16,400
D-35 D-45
Material Class Degree of Screw Dia. Max. Capacity (ft3/hour) Code Trough Loading (inches) RPM at max. RPM at one RPM A-16
D-16
30%
6
60
90
Material Class Degree of Screw Dia. Max. Capacity (ft3/hour) Code Trough Loading (inches) RPM at max. RPM at one RPM
1.49
A-17
D-17
15%
6
60
45
0.75 2.72
A-26
D-26
9
55
300
5.45
A-27
D-27
9
55
150
A--36
D-36
12
50
645
12.90
A-37
D-37
12
50
325
6.46
D-47
14
50
520
10.40
A-46
D-46
14
50
1,040
20.80
A-47
B-16
E-16
16
45
1,400
31.20
B-17
E-17
16
45
700
15.60
B-26
E-26
18
45
2,025
45.00
B-27
E-27
18
45
1,010
22.50
E-37
20
40
1,250
31.20
E-47
24
40
2,180
54.60
B-36
E-36
20
40
2,500
62.50
B-37
B-46
E-46
24
40
4,360
109.00
B-47
C-16
C-17
C-26
C-27
C-36
C-37
C-46
C-47
COMPONENT GROUP NORMAL SERVICE
HEAVY SERVICE
Component Group 1A, 1B and 1C
Component Group 2A, 2B, 2C and 2D
Regular flights and regular trough
Regular flights and he avy trough
Screw Dia.
Coupling Dia.
(inches)
(inches)
6 9 9 12 12 14 16 18 20 24
1 1/2 1 1/2 2 2 2 7/16 2 7/16 3 3 3 3 7/16
Thickness, US standard gauge or inches Trough Cover
16 ga. 14 ga. 14 ga. 12 ga. 12 ga. 12 ga. 12 ga. 10 ga. 10 ga. 10 ga.
16 ga. 14 ga. 14 ga. 14 ga. 14 ga. 14 ga. 14 ga. 12 ga. 12 ga. 12 ga.
Screw Dia.
Coupling Dia.
(inches)
(inches)
6 9 9 12 12 14 16 18 20 24
1 1/2 1 1/2 2 2 2 7/16 3 3 3 3 3 7/16
EXTRA HEAVY SERVICE Component Group 3A, 3B and 3D Regular flights and heavy trough Screw Dia.
Coupling Dia.
(inches)
(inches)
6 9 9 12 12 14 16 18 20
1 1/2 1 1/2 2 2 2 7/16 3 3 3 3
Thickness, US standard gauge or inches Trough Cover
14 ga. 10 ga. 10 ga. 3/16 3/16 3/16 3/16 3/16 3/16
16 ga. 14 ga. 14 ga. 14 ga. 14 ga. 14 ga. 14 ga. 12 ga. 12 ga
Thickness, US standard gauge or inches Trough Cover
14 ga. 10 ga. 10 ga. 3/16 3/16 3/16 3/16 3/16 3/16 3/16
16 ga. 14 ga. 14 ga. 14 ga. 14 ga. 14 ga. 14 ga. 12 ga. 12 ga. 12 ga.
HORSEPOWER REQUIREMENT (HORIZONTAL SCREW CONV.)
The horsepower required to operate a horizontal screw conveyor is based on proper installation, uniform and regular feed rate.
The horse power requirement is the total of the horsepower to overcome conveyor friction (HPf ) and the horsepower to transport the material at specified rate (HP m)
=
=
=
. . .
. . . . . +
Where: C : Equivalent design capacity [ft3/h],
e : Drive efficiency
L : Total length of conveyor [ft]
N : operating speed [rpm]
Fb : Hanger bearing factor
Fm : Material factor
Ff : Flight Factor
Fo : Power overload factor
Fd : Conveyor diameter factor
HORSEPOWER REQUIREMENT (HORIZONTAL SCREW CONV.) Hanger Bearing Factor, Fb
Flight Factor, Ff
Component Group Bearing Type Group A Ball Group B Babbit Bronze Wood Group C Plastic Nylon Teflon Group D Chilled hard iron Hardened alloy sleeve
Fb
1.0 1.7
2.0
Type of Flight
Standard Cut Flight Cut & Folded Flight Ribbon Flight
Conveyor Loading 15%
1.0 1.10 NR 1.05
30%
1.0 1.15 1.50 1.14
45%
1.0 1.20 1.70 1.20
95%
1.0 1.30 2.20 -
Paddle Factor, Fp
4.4
Standard paddles pe r pitch set at 45o reverse pitch
Fp
None
1
2
3
4
1
1.29
1.58
1.87
2.16
Screw Diameter Factor, Fd Screw Dia. (inch)
Fd
4 6 9 10 12 14 16 18 20 24
12 18 31 37 55 78 106 135 165 235
Compiled by Masda Ehsa n
Mechanical Efficiencies of Spe ed Reduction Mechanisms Type of Speed Reduction Mechanism
V-belts and sheaves Precision roller chain on cut tooth sprockets, open guard Precision roller chain on cut tooth sprockets, oil tights casing Single Reduction helical or herringbone eclosed gear reducer or gearmotor Double Reduction helical or herringbone eclosed gear reducer or gearmotor Triple Reduction helical or herringbone eclosed gear reducer or gearmotor Low ratio (upto 20:1 range) enclosed worm gear speed reducers Medium ratio (20:1 to 60:1 ratio range) enclosed worm gear speed reducers Gi gh rati o ( 60:1 to 100:1 rati o range ) e ncl ose d w orm ge ar spe ed re duce rs Note : Contact Gear Reducer Manufacturer for the exact Mechanical Efficie ncies
Approximate Efficiencies 0.94
0.93 0.94 0.95 0.94 0.93 0.9 0.7 0.5
HORSEPOWER REQUIREMENT (HORIZONTAL SCREW CONV.)
HORSEPOWER REQUIREMENT (HORIZONTAL SCREW CONV.)
HORSEPOWER REQUIREMENT (INCLINED SCREW CONV.)
ADVANTAGES: Inclined screw conveyor is often very desirable as it may solve a conveying problem with a minimum of equipment and occupy a minimum space
DISADVANTAGES:
The capacity, or the maximum available capacity of a given screw conveyor decreases wit h increase of incline
The horse power per-unit capacity increases.
ADDITIONAL POWER to lift the material, the formula to calculate horsepower to lift the material as follow:
=
=
HPh: Horsepower to lift the material [HP] h : actual height of lift [ft] e : Drive efficiency
. ()
+ +
Where: C : Equivalent design capacity [ft3/h]
. .
PROBLEMS ASSOCIATED WITH INCLINED SCREW CONVEYOR
Several things can be done to overcome many of the problems associated with inclined screw conveyor:
Limit the use of standard screw components to inclines of less than 25 o, preferably not over 15o
Use close clearance between trough and screw
Increase the speed over that applicable for a horizontal screw conveyor of the same size
Use short pitch screws, 2/3 or ½ pitch
