Powder Flow Flashcards
Classification of powders is based on…
particle size e.g. coarse, fine, very fine
Powders are..
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)
Gravity
mechanical agitation
Examples of resistive forces (2)
cohesion/adhesion
friction
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:
large
spherical
small surface area
high density
Specific surface area equation
total surface area / mass
The larger the specific surface area…
more bonding
more cohesive
greater resistive forces
POORER FLOWABILITY
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.
Porosity is..
volume fraction of air
1 - k
Packing fraction is..
the volume fraction of solid particles (k)
What is bulk density?
Equation
Density of solid particles when the powder is allowed to settle naturally
mass / bulk volume
What is the tapped density?
Equation
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
less cohesive
better flowability
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
Method 1
measure amount of powder discharged over a fixed duration
Method 2
measure time taken to discharge a fixed amount of powder
Flow rate influenced by: (3)
- hopper shape
- office shape
- orfice diameter
Another way of measuring flowability: (3)
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
Hausner ratio calculation
Vf / Vt = Pf / Pt > 1
Vf = bulk volume vt = tapped volume Pf = bulk density Pt = tapped density
Compressibility index
100(Vf - Vt) / Vf = 100(Pf - Pt) / Pt < 100
Percentage difference
smaller value, better flowability
What is the Angle of repose?
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
Smaller angle of repose…
better flowability
Equation to work out angle of repose:
tan-1 (h/r)
