Screw Conveyor Engineering Guide

Screw Conveyor Capacity

Calculation Of Conveyor Speed Capacity Factors for Special Pitches Capacity Factors for Modified Flight Capacity Table

Capacity is defined as the weight or volume per hour of a bulk material that can be safely and feasibly conveyed using a screw conveyor. Screw conveyor diameter is determined by multiple factors including capacity.

The following steps are required for proper screw conveyor selection –

  1. Calculate required capacity in cubic feet per hour (ft3/hr).
  2. Select the recommended trough loading percentage from the Bulk Material Table for the specific bulk material to be conveyed.
  3. Select the screw conveyor diameter that corresponds with the recommended trough loading and is less than the capacity at maximum RPM from the Capacity Table.
  4. Calculate actual conveyor speed by dividing the required capacity by the capacity at 1-RPM from the Capacity Table.

NOTE: Maximum speeds shown in the Capacity Table are not intended for every application and were developed for non-abrasive, free-flowing bulk materials. KWS recommends lower screw conveyor speeds for most industrial applications. A larger screw conveyor diameter will be required for lower screw conveyor speeds.

Maximum lump size must also be considered when designing a screw conveyor. Please refer to the Bulk Material Lump Size section for further information.

The Capacity Table is only intended for designing control fed screw conveyors. Conversely, screw feeders are flood loaded at the inlet and require special design considerations. Please refer to the Screw Feeder section for further information.

Calculation Of Conveyor Speed

The maximum recommended capacity for each screw diameter is listed in the Capacity Table for reference and should not be exceeded. Using the formula below, the exact conveyor speed (S) can be calculated:

Calculation of Conveyor Speed
Nomenclature:
S = Conveyor Speed
CFH = Capacity in Cubic Feet per Hour (ft3/hr)

KWS Screw Conveyor Speed Conveying Material

Capacity Factors for Special Pitches

Screw conveyors transfer bulk materials volumetrically and capacity is calculated in cubic feet per hour. Screw conveyor capacity is affected when the flight pitch is reduced from standard full pitch. Reducing the flight pitch reduces the capacity. For example, a 1/2 pitch screw carries 1/2 the capacity of a full pitch screw at the same level of trough loading and speed. Capacity Factors for Special Pitches are shown below.

Selection Capacity (SC) is the capacity adjusted for special pitch or modified flights used in the process of selecting the screw conveyor diameter for the application.

Calculation of special screw conveyor capacities is as follows:

Calculation of Special Screw Conveyor Capacities
Nomenclature:
SC = Selection Capacity
CFH = Required Capacity in Cubic Feet per Hour (ft3/hr)
CF = Capacity Factor

Capacity Factors for Special Pitches
Pitch Description Capacity Factor
Standard Pitch = Diameter 1.00
Short Pitch = 2/3 Diameter 1.50
Half Pitch = 1/2 Diameter 2.00
Long Pitch = 1-1/2 Diameters 0.67
Screw Pitch & Screw Diameter

Capacity Factors for Modified Flight

Screw conveyor capacity is also affected by using modified flight types such as cut flight, cut-andfolded flight, ribbon and paddles. The use of modified flights reduces conveying capacity. Capacity Factors for Modified Flights are shown below for various flight types and trough loading

Capacity Factors for Cut or
Cut and Fold Flights
Flight Type Conveyor Loading
15% 30% 45%
Cut flight 1.92 1.57 1.43
Cut & folded flight X 3.75 2.54
Capacity Factors for Paddles
Paddles Per Pitch 1 2 3 4
Factor 1.08 1.16 1.24 1.32
Cut, Cut and Fold, and Paddles Flight Screw Conveyors
Capacity factors for ribbon flights
Ribbon Screw dia. Conveyor Loading
15% 30% 45%
4 X X X
6 1.32 1.52 1.79
9 1.34 1.54 1.81
12 1.11 1.27 1.50
14 1.27 1.45 1.71
16 1.55 1.69 1.90
18 1.33 1.53 1.80
20 1.60 1.75 1.96
24 2.02 2.14 2.28
30 2.16 2.29 2.44
36 3.27 3.37 3.70
Ribbon Flight Screw Conveyor

Example

A screw conveyor is required to transport 10 tons per hour of unslaked lime with a bulk density of 60 lbs/ft3. The unslaked lime also needs to be mixed in transit using cut and folded flights. The conveying distance is 15-feet, so short (2/3) pitch screws will be used to ensure proper mixing. The recommended trough loading percentage from the Bulk Material Table is 30A.

Actual calculated capacity is:

Actual Calculated Capacity

For proper selection of screw conveyor diameter and speed, the actual capacity must be adjusted for the cut and folded flights and short pitch using the Capacity Factors for Special Pitches and Modified Flights. As stated above, the adjusted capacity is defined as Selection Capacity (SC).

The Capacity Factor for short (2/3) pitch is 1.50.

Capacity Factors for Special Pitches
Pitch Description Capacity factor
Standard Pitch = Diameter 1.00
Short Pitch = 2/3 Diameter 1.50
Half Pitch = 1/2 Diameter 2.00
Long Pitch = 1-1/2 Diameters 0.67

The Capacity Factor for cut and folded flights with 30-percent trough loading is 3.75.

Capacity Factors for Cut or
Cut and Fold Flights
Flight Type Conveyor Loading
15% 30% 45%
Cut flight 1.92 1.57 1.43
Cut & folded flight X 3.75 2.54

Therefore:
SC = 1.50 X 3.75 X 333 ft3/hr
SC = 1,873 ft3/hr

The Selection Capacity will be used to select the proper screw conveyor diameter and speed from the Capacity Table. With a recommended trough loading of 30A, the Selection Capacity must be less than the maximum capacity given in the Capacity Table. A 16-inch diameter screw conveyor will convey 2,496 cubic feet per hour at the maximum recommended speed of 80-rpm.

The actual screw conveyor speed is calculated by dividing the Selection Capacity by the capacity at 1-rpm.

1,873/31.2 = 60-rpm

60-rpm is the correct speed for a 16-inch diameter screw conveyor with cut and folded flights and short pitch for conveying and mixing 333 cubic feet per hour.

Capacity Table

Trough Loading Screw Dia. (in.) Max. RPM * Capacity in ft3/hr
At Max. RPM At 1 RPM

15%
4 69 14.5 0.2
6 60 45 0.8
9 55 150 2.7
12 50 323 6.5
14 50 520 10.4
16 45 702 15.6
18 45 1,012 22.5
20 40 1,248 31.2
24 40 2,184 54.6
30 35 3,728 106.5
36 30 5,532 184.4

30% A
4 139 57 0.4
6 120 179 1.5
9 100 545 5.5
12 90 1,161 12.9
14 85 1,768 20.8
16 80 2,496 31.2
18 75 3,375 45.0
20 70 4,375 62.5
24 65 7,085 109.0
30 60 12,798 213.3
36 50 18,440 368.8

30% B
4 69 28 0.4
6 60 90 1.5
9 55 305 5.5
12 50 645 12.9
14 50 1,040 20.8
16 45 1,404 31.2
18 45 2,025 45.0
20 40 2,500 62.5
24 40 4,360 109.0
30 35 7,465 213.3
36 30 11,064 368.8

45%
4 190 116 0.61
6 165 368 2.2
9 155 1,271 8.2
12 145 2,813 19.4
14 140 4,368 31.2
16 130 6,071 46.7
18 120 8,112 67.6
20 110 10,307 93.7
24 100 16,400 164.0
30 90 28,800 320.0
36 75 41,490 553.2

NOTE: Maximum speeds shown in the Capacity Table are not intended for every application and were developed for non-abrasive, free-flowing bulk materials. KWS recommends lower screw conveyor speeds for most industrial applications. A larger screw conveyor diameter will be required for lower screw conveyor speeds.


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