Powder Flow

Question 1

Classification of powders is based on…

Answer 1

particle size e.g. coarse, fine, very fine

Question 2

Powders are..

Answer 2

a complex fluid comprising a mixture of solid particles and air.

Question 3

What is the flowability of a powder?

Answer 3

How well a powder flows.

Question 4

How do powders flow?

Answer 4

A tug of war between driving forces that promote flow and resistive that inhibit in.

Driving force needs to exceed resistive force.

Question 5

Examples of driving forces (2)

Answer 5

Gravity

mechanical agitation

Question 6

Examples of resistive forces (2)

Answer 6

cohesion/adhesion

friction

Question 7

What is the difference between adhesion and cohesion?

Answer 7

Both are attractive forces between two forces.

Adhesion is when 2 chemically different surfaces

Cohesion is when 2 chemically similar surfaces

Question 8

Gravitational force equation

Answer 8

W = m x g

m - mass of particle
p - particle acceleration

Question 9

Van der Waals forces equation, FVdw =

Answer 9

A x R1 x R2 / 6d2 (R1+R2)

A = Hamaker constant
d = surface-to-surface distance between particles
R1 = radius of one particle
R2 = radius of other

Question 10

Electrostatic forces equation: Fe =

Answer 10

Fe = k Q1 x Q2 / d2

k - constant
Q1 - charge on one particle
Q2 - charge on other particle
d2 = distance between particles

Question 11

What does mechanical interlocking do?

Answer 11

physically obstructs movement of other particle.

Question 12

How does friction inhibit flow?

Answer 12

Acts in opposite direction of the movement, inhibiting particle movement.

Question 13

Friction equation =

Answer 13

Ff = uFr

u = coefficient of friction
Fr = reaction force

Question 14

Condition that favours powder flow:

Answer 14

large
spherical
small surface area
high density

Question 15

Specific surface area equation

Answer 15

total surface area / mass

Question 16

The larger the specific surface area…

Answer 16

more bonding
more cohesive
greater resistive forces

POORER FLOWABILITY

Question 17

Do spherical particles flow well?

Answer 17

yes because has the smallest specific surface area than any other shape.

Question 18

Packing geometry is…

Answer 18

the position of solid particles relative to one another.

Question 19

Porosity is..

Answer 19

volume fraction of air

1 - k

Question 20

Packing fraction is..

Answer 20

the volume fraction of solid particles (k)

Question 21

What is bulk density?

Equation

Answer 21

Density of solid particles when the powder is allowed to settle naturally

mass / bulk volume

Question 22

What is the tapped density?

Equation

Answer 22

density of solid particles with minimal amount of entrapped air

mass / tapped volume

Question 23

True density equation

Answer 23

Bulk density / packing fraction

Question 24

How does density (true density) affect flowability?

Answer 24

greater mass at same volume
greater gravitational force

lower specific surface area
smaller resistive forces
less cohesive

better flowability

Question 25

How does moisture content affect flowability?

Answer 25

too much will cause them to stick

moderate provides balance between lubrication and cohesion.

Question 26

What’s mass flow vs funnel flow?

Answer 26

The way the powder exists a hopper. Mass flow is an example of powder with good flowability while a funnel flow is not.

Question 27

Flow rate equation

Answer 27

Amount of powder discharged / discharge duration

Question 28

2 methods of determining flow rate

Answer 28

Method 1
measure amount of powder discharged over a fixed duration

Method 2
measure time taken to discharge a fixed amount of powder

Question 29

Flow rate influenced by: (3)

Answer 29

• hopper shape
• office shape
• orfice diameter

Question 30

Another way of measuring flowability: (3)

Answer 30

measure bulk volume in a graduated cylinder

consolidate powder on a tapped volume tester to a standardised set of parameters

measure tapped volume in the graduated cylinder

compressibility index and harsher ratio

Question 31

Hausner ratio calculation

Answer 31

Vf / Vt = Pf / Pt > 1

Vf = bulk volume
vt = tapped volume
Pf = bulk density
Pt = tapped density

Question 32

Compressibility index

Answer 32

100(Vf - Vt) / Vf = 100(Pf - Pt) / Pt < 100

Percentage difference

smaller value, better flowability

Question 33

What is the Angle of repose?

Answer 33

The angle between slope and horizontal.

Steepness of the slop represents equilibrium.

Cohesion increases this by causing particles to pile up
Gravity reduces it by causing particles to roll down

Question 34

Smaller angle of repose…

Answer 34

better flowability

Question 35

Equation to work out angle of repose:

Answer 35

tan-1 (h/r)