Pneumatic Conveying

Question 1

What is pneumatic conveying?

Answer 1

Transport of solids in a gas stream

Question 2

Important considerations when designing a pneumatic conveying system?

Answer 2

1. particle size (what few micron - 8 cm)
   - need to be large enough to avoid cohesion, but small enough to not be too power intensive to move
2. conveying line (pipe) size (often 0.25 - 24 inches)
3. Conveying line length: few m to a few km (size depends on process)
4. Capacity (amount of solids moved): 1-400 tonnes solids/h
5. Inner diameter of pipe ( at least 3X larger than particle diameter)

Question 3

Advantages of Pneumatic conveying

Answer 3

1. Clean since it totally enclosed
2. dust free transport
3. high security for moving high value products
4. low maintenance and manpower costs (no operator needed)
5. easy to automate and control

Question 4

Disadvantages of pneumatic conveying

Answer 4

1. high power consumption (for gas stream blower/fan/compressor)
   - smaller particles = lower delta P = lower energy needed
2. shorter distance (delta P limits possible distance that can be travelled)
3. pipe erosion
   - solids sliding along and colliding with pipe creates erosion
   - if pipe material is abrasive, then might have high maintenance costs (deal with this with a special lining or just replacing the pipes)

Question 5

Necessary information for pneumatic converying

Answer 5

1. average particle size + size distribution
2. % of particles less than 200 microns (these particles are very cohesive due to electrostatic forces and changes delta P due to clustering)
3. Stickiness (particle stick together or to pipe) - loose particles
4. moisture content (wet makes powder a bit cohesive)
5. Particle density and bulk density
6. Consistency of product

Question 6

Components of pneumatic conveying

Answer 6

1. Conveying line (pipe) - avoid bends as much a possible
2. Conveying gas
3. Controller (located where gas/solids are fed): fan/blower/compressor/screw
4. Feeder (solids entering from this unit)
5. Collector (downstream unit)

Question 7

Bends in pneumatic converying

Answer 7

• particles slow down as enter bed
• particles then accelerate as they leave bend
• slowdown can cause accumulation of particles and could cause plugging

Question 8

Important items to choose for pneumatic converying

Answer 8

1. pipe size
2. gas velocity
3. delta P from inlet to outlet
4. chosen gas (often pick an inert)

Question 9

Modes of transportation in pneumatic converying

Answer 9

1. dilute phase
   - large volume of gas + low amount of solids
   - epsilon = 0.98-0.99
   - particles move depending on gas flow regime (will swirl in turbulent regime)
   - dilute is mass flow ratio is 0-15
2. Dense phase
   - small volume of gas, high amount of solids
   - mass flow ratio is > 15

**mass flow ratio = mass solids/mass conveying gas

Question 10

Saltation velocity

Answer 10

velocity at which solids start to deposit on ground
- occurs in horizontal conveying
- should operate at a velocity slightly higher than Usalt (1.5X greater minimum)
- greater mass of solids increases Usalt

Question 11

Pick up Velocity

Answer 11

velocity required to pick up particles from bottom of pipe
- Usalt often 0.5X Ugpu
- for very small particles, Ugpu will increase (rather than decrease) as small particles stick to wall - and start to agglomerate
- occurs in horizontal converying

Question 12

Choking velocity

Answer 12

velocity where no pneumatic conveying occurs (slugging bed is formed in conveying line)