## Posted tagged ‘center’

### Design of Screw Conveyor

September 8, 2011

The size of screw conveyor depends on two factors

1. The capacity of the conveyor

2. The lump size of the material to be conveyed (Maximum dimensions of the particle)

Usually there are three ranges of lump sizes which are considered for selection of screw size. These are:

· A mixture of lumps and fines in which not more than 10% are lumps ranging from maximum size to one half of the maximum, and 90% are lumps smaller than one half of the maximum size.

· A mixture of lump and fines in which not more than 25% are lumps ranging from the maximum size to one half of the maximum, and 75% are lumps smaller than one half of the maximum size.

· A mixture of lump only in which 95% or more are lumps ranging from maximum size to one half of the maximum size and 5% or less are lumps less than one tenth of the maximum 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, as well as the proportion of the lumps in the mixture.

The lump size of the material affects the selection of screw diameter which should be at least 12 times larger than the lump size of a sized material and four times larger than the largest lumps of an un-sized material.

Example, if screw diameter is 250mm means radial clearance is 105mm, & Maximum lump size is 60mm of 10% lumps.

Capacity of Screw Conveyor:

The capacity of a screw conveyor depends on the screw diameter, screw pitch, speed of the screw and the loading efficiency of the cross sectional area of the screw. The capacity of a screw conveyor with a continuous screw:

Q = V. ρ

Q = 60. (π/4).D2.S.n.ψ.ρ.C

Where,

Q = capacity of a screw conveyor

V = Volumetric capacity in m3/Hr

ρ = Bulk density of the material, kg/m3

D = Nominal diameter of Screw in m

S = Screw pitch in m

N = RPM of screw

C = Factor to take into account the inclination of the conveyor

Screw Pitch:

Commonly the screw pitch is taken equal to the diameter of the screw D. However it may range 0.75 – 1.0 times the diameter of the screw.

Screw Diameter:

 Nominal Size D Trough height from center of screw shaft to upper edge of the trough Trough width C Thickness of Tough Tubular shaft (d1 * Thickness) Outside diameter of solid shaft Coupling diameter of shaft Heavy Duty Medium Duty Light Duty 100 63 120 – 2 1.6 33.7*2.5 30 25 125 75 145 – 2 1.6 33.7*2.5 30 25 160 90 180 5 3.15 1.6 42.4*2.5 35 40 200 112 220 5 3.15 2 48.3*3.5 40 40 250 140 270 5 3.15 2 60.3*4 50 50 315 180 335 5 3.15 – 76.1*5 60 50 400 224 420 5 3.15 – 76.1*5 60 75 500 280 530 5 3.15 – 88.9*5 70 75

RPM of Screw:

The usual range of RPM of screw is 10 to 165. It depends on the diameter of screw and the type of material (Max RPM of screw conveyor is 165)

The value of loading efficiency should be taken large for materials which are free flowing and non abrasive, while for materials which are not free flowing and or abrasive in nature, the value should be taken low:

Ψ = 0.12 to 0.15 for abrasive material

= 0.25 to 0.3 for mildly abrasive material

= 0.4 to 0.45 for non abrasive free flowing materials

Inclination Factor:

The inclination factor C is determined by the angle of screw conveyor with the horizontal.

 Angle of screw with the horizontal 0° 5° 10° 15° 20° Value of factor C 1 0.9 0.8 0.7 0.65

Types of screw flight:

The screw of the conveyor may be right hand or left hand, the right hand type being the usual design. The threads of the screw may be single, double or triple.

The flight of the screws may be made in either of the two ways:

1. As Helicoids

2. As Sectional flight

Helicoids Flight:

They are formed from a flat bar or strip into a continues helix. The threads are thinner at the outer edge and thicker at the inner edge.

Sectional flights:

Sectional flights are formed from a flat disc and the thickness of the thread is uniform throughout. A continuous helix is made by joining a number of sectional flights together on a piece of pipe and butt welded them. Various styles of screw flights are in use, depending on the service required.

Some of the typical configurations are:

1. Short pitch or continuous flight:

If the conveyor is required to handle dry granular or powdered materials that do not pack, this style of flight may be selected. It is of regular construction and recommended for inclined conveyors having a slope of 20 or more, including vertical conveyors. This style is extensively used as feeder screw.

2. Ribbon flight:

If the conveyor is to handle lumpy, clinging, sticky, gummy or viscous substances, this type flight may be selected. It consists of continuous helical flight formed from steel bar and secured to the pipe by supporting lugs.

3. Cut flight:

In this type of flight screws have notches cut in the periphery of the flight. These notches supplement the conveying with moderate mixing action. They are recommended for conveyors required to handle light, fine, granular or flaky materials.

4. Cut and folded flights:

This type of flight is characterized by notches as in cut flight, together with folded segments. This type of flight creates agitation and aeration resulting in better mixing. This type of flight is used to handle light or medium weight materials having fine, granular or flaky materials.

5. Some screw conveyors have cut flight with paddles mounted at regular intervals. The paddles counteract the flow of material past the flight resulting in greater agitation and mixing.

6. Sometimes screws are made of stainless steel to suit special requirements, like the sanitation requirements for handling food, drugs and other hygienic materials.

### BELT DRIVES

August 23, 2011

Types and Selection of Drives:

• Single Unsnubbed Bare / Lagged pulley Drive
• Snubbed Bare / Lagged Pulley Drive
• Tandem Drive
• Special Drives

Single Unsnubbed Bare / Lagged Pulley Drive:

This is the simplest drive arrangement consisting of a steel pulley connected to a motor and the belt wrapped round it on an arc of 180°. This can be used for low capacity short center conveyors handling non-abrasive material. The pulley may be lagged to increase the coefficient of friction.

Snubbed Bare / Lagged Pulley Drive:

Here the angle of wrap is increased from 180° to 210° or even up to 230°, by providing a snub pulley to the driving pulley. In majority of medium to large capacity belt conveyors, handling mild abrasive to fairly abrasive materials, 210° snub pulley drive with load pulley lagged with hard rubber is adopted.

Tandem drive:

Here belt tension estimated to be high; the angle of wrap is increased by adopting tandem drives. Both of tandem pulleys are driven. The tandem drive with arc of contact from 300° to 480° or more can operate with one or two motors. The location of such drive is usually determined by the physical requirements of the plant and structural constraints.

Special Drive:

Special drives with snub pulleys and pressure belts used in heavy and long conveyors.

### MAGNETIC BEARING TECHNOLOGY

August 22, 2011

Magnetic bearings have been utilized by a variety of industries for over a decade with benefits that include non-contact rotor support, no lubrication and no friction.

Conventional mechanical bearings, the kind that physically interface with the shaft and require some form of lubrication, can be replaced by a technology that suspends a rotor in a magnetic field, which eliminates friction losses.

There are two types of magnetic bearing technologies in use today – passive and active.  Passive bearings are similar to mechanical bearings in that no active control is necessary for operation. In active systems, non-contact position sensors continually monitor shaft position and feed this information to a control system.  This in turn, based on the response commanded by the system, flows to the actuator via current amplifiers.  These currents are converted to magnetic forces by the actuator and act on the rotor to adjust position and provide damping.

Additional benefits of magnetic bearings include:

• No friction
• No lubrication
• No oil contamination
• Low energy consumption
• Capacity to operate within a wide temperature range
• No need for pumps, seals, filters, piping, coolers or tanks
• Environmentally friendly workplace
• Impressive cost savings

In practice, these attractions are balanced in order to maintain a gap between the shaft (rotor) and static parts (stator). The function of the magnetic bearing is to locate the shaft’s rotation axis in the center, reacting to any load variation (external disturbance forces),

Floating rotors could boost compressor efficiencies

Traditional centrifugal compressors are based on low-speed drives, mechanical gears and oil-film bearings, resulting in high running costs because of their high losses, wear, and need for maintenance.

This new compressor drive (above) uses a permanent magnet motor, operating at an efficiency of around 97%, to drive a rotor “floating” on magnetic bearings, which spins the compressor impeller at speeds of around 60,000 rpm. These drives experience almost no friction or wear, and need little maintenance. They also minimize the risk of oil contamination, and result in compressors that are about half the size of traditional designs.

How they work

Magnetic bearings are basically a system of bearings which provide non-contact operation, virtually eliminating friction from rotating mechanical systems. Magnetic bearing systems have several components. The mechanical components consist of the electromagnets, position sensors and the rotor. The electronics consist of a set of power amplifiers that supply current to electromagnets. A controller works with the position sensors which provide feedback to control the position of the rotor within the gap.

The position sensor registers a change in position of the shaft (rotor). This change in position is communicated back to the processor where the signal is processed and the controller decides what the necessary response should be, then initiates a response to the amplifier. This response should then increase the magnetic force in the corresponding electromagnet in order to bring the shaft back to center. In a typical system, the radial clearance can range from 0.5 to 1 mm.

This process repeats itself over and over again. For most applications, the sample rate is 10,000 times per second, or 10 kHz. The sample rate is high because the loop is inherently unstable. As the rotor gets closer to the magnet, the force increases. The system needs to continuously adjust the magnetic strength coming from the electromagnets in order to hold the rotor in the desired position.