Screw Conveyor Basics
Primary considerations for the selection of a screw
conveyor are:
- Type and condition of the bulk material to be conveyed including maximum particle size and specific bulk density.
- Capacity or feed rate of bulk material to be conveyed expressed in pounds per hour, tons per hour, or cubic feet per hour.
- Required distance and incline the bulk material is to be conveyed.
- Design
conditions such as materials of construction, inlet feed conditions and
operating temperature.
Screw Conveyor
The necessary information for selecting a screw
conveyor in a series of five steps. These steps are arranged in logical order
and are divided into separate sections for simplicity.
The five steps are:
- Establish characteristics of the bulk material to be conveyed.
- Determine conveyor size and speed based on capacity.
- Calculate horsepower requirements.
- Verify torque rating of components.
- Select conveyor components.
Types of Screw Conveyors
Horizontal Screw Conveyors
Horizontal screw conveyors are the most widely used type of screw
conveyor. Used to convey bulk materials from one part of a process to another,
horizontal screw conveyors are available in a wide range of sizes, lengths,
configurations and materials of construction.
Screw conveyors are typically designed to convey bulk
materials at 15, 30 or 45-percent trough loading, depending upon material
characteristics of the specific bulk material. As a general rule, trough
loading of 45-percent can be used for light, free-flowing and non-abrasive bulk
materials. Trough loading of 15 and 30-percent are typically used for denser,
sluggish and more abrasive bulk materials.
Inclined Screw Conveyors
Inclined screw conveyors typically operate from slightly
above the horizontal position to 45-degrees from the horizontal position. Above
45-degrees an inclined screw conveyor is considered a vertical screw conveyor.
As the degree of incline increases, conveying efficiency is reduced and
horsepower requirements increase due to the effects of gravity and bulk
material fall back. Conveying efficiency is affected by angle of incline,
characteristics of the specific bulk material, type of screw conveyor trough and
screw pitch. designing screw conveyors using the lowest possible degree of
incline for maximum efficiency.
The following are design and construction features to
consider when designing an inclined screw conveyor:
· Incline Up to
10-Degrees –
Loss
in conveying efficiency is minimal on inclines up to 10-degrees. A screw
conveyor with U-trough and full pitch screw is sufficient for most
applications. Loss in efficiency can be overcome by increasing the speed of the
screw conveyor, increasing the diameter of the screw conveyor or reducing the
pitch of the screw.
· Incline between 10
and 20-Degrees –
Loss
in conveying efficiency is typically between 10 and 40-percent on inclines up
to 20-degrees. A screw conveyor with U-trough and 2/3-pitch screw is sufficient
for most applications. Loss in efficiency can also be overcome by increasing
the speed or the diameter of the screw conveyor. Additional horsepower is
required to overcome gravity and bulk material fall back.
· Incline Between 20
and 30-Degrees –
Loss
in conveying efficiency is typically between 10 and 70-percent on inclines up
to 30-degrees. A screw conveyor with tubular housing and reduced pitch screw
(1/2 or 2/3) is recommended for most applications. Loss in efficiency can also
be overcome by increasing the speed or the diameter of the screw conveyor.
Additional horsepower is required to overcome gravity and bulk material fall
back.
· Incline Between 30
and 45-Degrees –
Loss
in conveying efficiency is typically between 30 and 90-percent on inclines up to
45-degrees. A screw conveyor with tubular housing and reduced pitch screw (1/2
or 2/3) and larger diameter is recommended for most applications. Increasing
the speed of the screw conveyor is also required. Additional horsepower is
required to overcome gravity and bulk material fall back.
Pitch Efficiency
· The Pitch
Efficiency chart shows the relative conveying efficiency at different degrees
of incline and pitch configurations. As the degree of incline increases,
reduced pitch screws (1/2 and 2/3) are more efficient than full pitch screws.
The combination of reduced pitch screws (1/2 and 2/3) and tubular housings
provide the highest conveying efficiency.
Horsepower Requirements
The horsepower requirements for inclined screw conveyors
increase with the degree of incline. The Horsepower Factor (Fi) is incorporated
into the standard screw conveyor horsepower calculations to compensate for the
additional horsepower required to overcome gravity and bulk material fall back.
TSHP(i) = Total
Shaft HP for Inclined Screw Conveyor
MHP =
Material HP (HP required to move
the material
FHP =
Friction HP (HP required to drive the conveyor empty
Fi = Incline Factor
e = Drive Efficiency (Typical
value of 0.88 is used for a shaft mount reducer/motor)
* If calculated Material
Horsepower is less than 5HP it should be corrected for potential overload. Use
the Corrected Material HP Chart.
Upset
Conditions
Screw conveyors located on inclines over 10-degrees must
be designed to start and operate under upset conditions. An upset condition is
caused when normal flow in an inclined screw conveyor is interrupted and the
bulk material inside the conveyor slips back to the lower end, filling up the
conveyor. Additional horsepower is required to restart and convey the bulk
material because the conveyor will temporarily experience 100-percent trough
loading.
Shaft-less Screw Conveyors
Bulk materials discharged from centrifuges, filter
presses or mixers can easily be conveyed using a Shaft-less Screw Conveyor. Shaft-less design provides a non-clogging conveying surface that allows
difficult-to-convey materials to become easy-to-convey. The perfect solution
for handling bulk materials with high moisture content is the Shaft-less Screw
Conveyor.
Advantages of Shaft-less Screw Conveyors
- Ideal for handling sticky and sluggish bulk materials
- Improved conveying efficiency when compared to other types of conveyors
- Allows greater flexibility for plant layout due to configurations available
- Internal bearings are eliminated
Shaft-less Screw Conveyors are successfully used
throughout the chemical, food, minerals processing and wastewater treatment
industries for conveying everything from catalysts to de-watered biosolids.
Vertical Screw Conveyors
Vertical screw conveyors are a very efficient method for
elevating a variety of bulk materials at very steep inclines or completely
vertical. Any screw conveyor located on an incline over 45-degrees is
considered as a vertical screw conveyor. The compact design allows for the
vertical screw conveyor to fit into almost any plant layout. With a minimum
number of moving parts, the vertical screw conveyor is a cost-effective and
dependable component of any bulk material handling process.
Advantages
of Vertical Screw Conveyors
- Ideal for handling dry to semi-fluid materials.
- Capacities up to 6,000 cubic feet per hour.
- Ability to elevate bulk materials up to 30-feet without use of internal bearings.
- Totally enclosed design for dust and vapor-tight requirements.
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