SCREW CONVEYOR BASIC DESIGN CALCULATION
CEMA (Conveyor Equipment Manufacturer Association) Approach
HISTORY & APPLICATION
HISTORY: 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 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.
APPLICATION: Screw conveyor s are bulk material transporting devices capable of handling a great variety of materials which have relatively good flowability. 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 Material Characteristics Included Density Bulk Density, loose Size Very Fine No. 200 sieve (0.0029") and under
Abrasiveness
A200
No. 100 sieve (0.0059") and under
A100
No. 40 sieve (0.016") and under Fine
A40
No. 6 sieve (0.132") and under
Granular
B6
1/2" and under
C1/2
3" and under
D3
7" and under
D7
over 16" to be spesified
D16
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 Extremely abrasive
index 1 - 17 index 18 - 67 index 68 - 416
5 6 7
Lumpy
Flowability
Code Designation Actual, lb/ft3
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 Material Code Component Series Mat'. Factor Vertical lb/ft3 105
105B635
1A-1B-1C
2.0
-
Ashes, coal, dry __ 1.2"
35 - 45
40C1/246TY
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
68A10026M
2D
1.4
V
Coal, Anthracite, sized__1/2"
49 - 61
55C1/225
2A-2B
1.0
-
Coal, Bitumious, mined
40 - 60
50D335LNXY
1A-1B
0.9
-
Coal, Bitumious, slack
43 - 50
47C1/245T
2A-2B
0.9
-
Dolomite, crushed
80 - 100
90C1/236
2D
2.0
-
Dolomite, lumpy
90 - 100
95Dx36
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
75A4046MY
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 24
Pipe OD (inches)
Radial Clearance (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 4 1/2
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 10 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 3/4 1/2 2 1/4 1 1/2 3/4 2 1/4 1 1/2 3/4 2 3/4 2 1 2 3/4 2 1 2 3/4 2 1 3 1/4 2 1/2 1 1/4 3 1/4 2 1/2 1 1/4 3 3/4 2 3/4 1 1/2 3 3/4 2 3/4 1 1/2 4 1/4 3 1 3/4 4 1/4 3 1 3/4 4 3/4 3 1/2 2 4 3/4 3 1/2 2 6 3 3/4 2 1/2
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
𝟎𝟎. 𝟕𝟕𝟕𝟕𝟕𝟕𝟕𝟕 𝑫𝑫𝑫𝑫𝟐𝟐 − 𝑫𝑫𝑫𝑫𝟐𝟐 𝑷𝑷 𝑲𝑲 𝟔𝟔𝟔𝟔 𝑪𝑪 = 𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏
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:
𝑵𝑵 =
𝑸𝑸. 𝑪𝑪𝑪𝑪𝟎𝟎 . 𝑪𝑪𝑪𝑪𝟏𝟏 . 𝑪𝑪𝑪𝑪𝟐𝟐 . 𝑪𝑪𝑪𝑪𝟑𝟑 𝑪𝑪𝟏𝟏
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 Pitch Capacity Factor, CF1 Pitch Description Standard Pitch = diameter of screw Short Pitch = 2/3 diameter of screw Half Pitch = 1/2 diameter of screw Long Pitch = 1 1/2 diameter of screw
CF1 1 1.5 2 0.67
Special Conveyor Flight Capacity Factor, CF2 Conveyor Loading Type of Flight 15% 30% Cut Flight 1.95 1.57 Cut & Folded Flight NR 3.75 Ribbon Flight 1.04 1.37
45% 1.43 2.54 1.62
Special Conveyor Mixing Paddle Capacity Factor, CF3 Standard paddles per pitch set at 45o reverse pitch None 1 2 3 4 Factor CF3 1 1.08 1.16 1.24 1.32 Compiled by Masda Ehsan
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 A-25 B-15 B-25 C-15 C-25
45%
6 9 12 14 16 18 20 24
165 155 145 140 130 120 110 100
368 1,270 2,820 4,370 6,060 8,120 10,300 16,400
2.23 8.20 19.40 31.20 46.70 67.60 93.70 164.00
Material Class Degree of Screw Dia. Max. Capacity (ft3/hour) Code Trough Loading (inches) RPM at max. RPM at one RPM A-16 A-26 A--36 A-46 B-16 B-26 B-36 B-46 C-16 C-26 C-36 C-46
D-16 D-26 D-36 D-46 E-16 E-26 E-36 E-46
30%
6 9 12 14 16 18 20 24
60 55 50 50 45 45 40 40
90 300 645 1,040 1,400 2,025 2,500 4,360
1.49 5.45 12.90 20.80 31.20 45.00 62.50 109.00
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-35 A-45 B-35 B-45 C-35 C-45 D-15 D-25 D-35 D-45
E-15 E-25 E-35 E-45
30%
6 9 12 14 16 18 20 24
120 100 90 85 80 75 70 65
180 545 1,160 1,770 2,500 3,380 4,370 7,100
1.49 5.45 12.90 20.80 31.20 45.00 62.50 109.00
Material Class Degree of Screw Dia. Max. Capacity (ft3/hour) Code Trough Loading (inches) RPM at max. RPM at one RPM A-17 A-27 A-37 A-47 B-17 B-27 B-37 B-47 C-17 C-27 C-37 C-47
D-17 D-27 D-37 D-47 E-17 E-27 E-37 E-47
15%
6 9 12 14 16 18 20 24
60 55 50 50 45 45 40 40
45 150 325 520 700 1,010 1,250 2,180
0.75 2.72 6.46 10.40 15.60 22.50 31.20 54.60
COMPONENT GROUP
NORMAL SERVICE Component Group 1A, 1B and 1C Regular flights and regular trough Thickness, US standard Screw Dia. Coupling Dia. gauge or inches (inches) (inches) Trough Cover 6 1 1/2 16 ga. 16 ga. 9 1 1/2 14 ga. 14 ga. 9 2 14 ga. 14 ga. 12 2 12 ga. 14 ga. 12 2 7/16 12 ga. 14 ga. 14 2 7/16 12 ga. 14 ga. 16 3 12 ga. 14 ga. 18 3 10 ga. 12 ga. 20 3 10 ga. 12 ga. 24 3 7/16 10 ga. 12 ga.
HEAVY SERVICE Component Group 2A, 2B, 2C and 2D Regular flights and heavy trough Thickness, US standard Screw Dia. Coupling Dia. gauge or inches (inches) (inches) Trough Cover 6 1 1/2 14 ga. 16 ga. 9 1 1/2 10 ga. 14 ga. 9 2 10 ga. 14 ga. 12 2 3/16 14 ga. 12 2 7/16 3/16 14 ga. 14 3 3/16 14 ga. 16 3 3/16 14 ga. 18 3 3/16 12 ga. 20 3 3/16 12 ga. 24 3 7/16 3/16 12 ga.
EXTRA HEAVY SERVICE Component Group 3A, 3B and 3D Regular flights and heavy trough Thickness, US standard Screw Dia. Coupling Dia. gauge or inches (inches) (inches) Trough Cover 6 1 1/2 14 ga. 16 ga. 9 1 1/2 10 ga. 14 ga. 9 2 10 ga. 14 ga. 12 2 3/16 14 ga. 12 2 7/16 3/16 14 ga. 14 3 3/16 14 ga. 16 3 3/16 14 ga. 18 3 3/16 12 ga. 20 3 3/16 12 ga. 24 3 7/16 3/16 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 (HPm) 𝑯𝑯𝑯𝑯𝒇𝒇 = 𝑯𝑯𝑷𝑷𝒎𝒎 = 𝑯𝑯𝑯𝑯 =
Where: C : Equivalent design capacity [ft3/h], L : Total length of conveyor [ft] Fb : Hanger bearing factor Ff : Flight Factor Fd : Conveyor diameter factor
𝑳𝑳. 𝑵𝑵. 𝑭𝑭𝒅𝒅 . 𝑭𝑭𝒃𝒃 𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏
𝑪𝑪. 𝑳𝑳. 𝑾𝑾. 𝑭𝑭𝒇𝒇 . 𝑭𝑭𝒎𝒎 . 𝑭𝑭𝒑𝒑 𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏𝟏 𝑯𝑯𝑯𝑯𝒇𝒇 + 𝑯𝑯𝑯𝑯𝒎𝒎 𝑭𝑭𝑭𝑭 𝒆𝒆
e : Drive efficiency N : operating speed [rpm] Fm : Material factor Fo : Power overload factor
HORSEPOWER REQUIREMENT (HORIZONTAL SCREW CONV.)
Hanger Bearing Factor, Fb 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
Flight Factor, Ff
Fb 1.0 1.7
2.0
Type of Flight
Standard Cut Flight Cut & Folded Flight Ribbon Flight
Compiled by Masda Ehsan
Fd 12 18 31 37 55 78 106 135 165 235
95% 1.0 1.30 2.20 -
Paddle Factor, Fp 4.4 Fp
Screw Diameter Factor, Fd Screw Dia. (inch) 4 6 9 10 12 14 16 18 20 24
15% 1.0 1.10 NR 1.05
Conveyor Loading 30% 45% 1.0 1.0 1.15 1.20 1.50 1.70 1.14 1.20
Standard paddles per pitch set at 45o reverse pitch None 1 2 3 4 1 1.29 1.58 1.87 2.16
Mechanical Efficiencies of Speed 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 Gigh ratio (60:1 to 100:1 ratio range) enclosed worm gear speed reducers Note : Contact Gear Reducer Manufacturer for the exact Mechanical Efficiencies
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 with 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: 𝑯𝑯𝑯𝑯𝒉𝒉 = 𝑯𝑯𝑯𝑯 =
𝑪𝑪 . 𝑾𝑾 . 𝒉𝒉 𝟑𝟑𝟑𝟑𝟑𝟑𝟑𝟑𝟑𝟑 . (𝟔𝟔𝟔𝟔)
𝑯𝑯𝑯𝑯𝒇𝒇 + 𝑯𝑯𝑯𝑯𝒎𝒎 + 𝑯𝑯𝑯𝑯𝒉𝒉 𝑭𝑭𝑭𝑭 𝒆𝒆
Where: HPh: Horsepower to lift the material [HP] C : Equivalent design capacity [ft3/h] h : actual height of lift [ft] e : Drive efficiency
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 25o, 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
