Flashcards in Powder Flow Deck (35)
Classification of powders is based on...
particle size e.g. coarse, fine, very fine
a complex fluid comprising a mixture of solid particles and air.
What is the flowability of a powder?
How well a powder flows.
How do powders flow?
A tug of war between driving forces that promote flow and resistive that inhibit in.
Driving force needs to exceed resistive force.
Examples of driving forces (2)
Examples of resistive forces (2)
What is the difference between adhesion and cohesion?
Both are attractive forces between two forces.
Adhesion is when 2 chemically different surfaces
Cohesion is when 2 chemically similar surfaces
Gravitational force equation
W = m x g
m - mass of particle
p - particle acceleration
Van der Waals forces equation, FVdw =
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
Electrostatic forces equation: Fe =
Fe = k Q1 x Q2 / d2
k - constant
Q1 - charge on one particle
Q2 - charge on other particle
d2 = distance between particles
What does mechanical interlocking do?
physically obstructs movement of other particle.
How does friction inhibit flow?
Acts in opposite direction of the movement, inhibiting particle movement.
Friction equation =
Ff = uFr
u = coefficient of friction
Fr = reaction force
Condition that favours powder flow:
small surface area
Specific surface area equation
total surface area / mass
The larger the specific surface area...
greater resistive forces
Do spherical particles flow well?
yes because has the smallest specific surface area than any other shape.
Packing geometry is...
the position of solid particles relative to one another.
volume fraction of air
1 - k
Packing fraction is..
the volume fraction of solid particles (k)
What is bulk density?
Density of solid particles when the powder is allowed to settle naturally
mass / bulk volume
What is the tapped density?
density of solid particles with minimal amount of entrapped air
mass / tapped volume
True density equation
Bulk density / packing fraction
How does density (true density) affect flowability?
greater mass at same volume
greater gravitational force
lower specific surface area
smaller resistive forces
How does moisture content affect flowability?
too much will cause them to stick
moderate provides balance between lubrication and cohesion.
What's mass flow vs funnel flow?
The way the powder exists a hopper. Mass flow is an example of powder with good flowability while a funnel flow is not.
Flow rate equation
Amount of powder discharged / discharge duration
2 methods of determining flow rate
measure amount of powder discharged over a fixed duration
measure time taken to discharge a fixed amount of powder
Flow rate influenced by: (3)
- hopper shape
- office shape