MASS TRANSFER

Examples of mass transfer

Mass transfer requires
the presence of two
regions

at different

chemical compositions
→

→

→

→

movement of

chemical species from a
high concentration
region toward a lower
concentration (non-
homogeneous medium).

•

Note:

Fluid flow due to the
pressure difference.

dx

dC

DA

m

−

=

•

Mass diffusion

Concentration difference

is the driving force for mass transfer.

where

D

is the

diffusion coefficient

of the medium

• Note:

The diffusion rates are higher in gases than in liquids and in
solids.

Mass flow rate ∝

∝

∝

∝

Normal area

A

×

×

×

×

Concentration gradient

dC/dx

(kg/s)

Analogy Between Heat and Mass Transfer

Mass and heat – two
different forms of energy,
according to the
Einstein`s formula:

2

mc

E =

Radiation

dx

dT

A

cond

Q

Λ

−

=

•

dx

dC

A

D

A

AB

diff

m

−

=

•

Heat conduction

Mass diffusion

Concentration difference
is the driving force for
mass transfer

Convection

Steady mass diffusion through a wall

dw

dx

A

x =

=

0

0

An impermeable surface is a
surface that does not allow
any mass to pass through.

Thus at x = 0:

→

→

→

→

Analogy to an insulated

surface in heat transfer.

where

w

is the mass fraction.

Water vapour migration in buildings

Moisture – influence on the performance and durability of
building materials →

→

→

→

importance of moisture transmission

Example:

A 5% moisture content
can increase heat
transfer through wall
insulation by 25 %.

Moisture

– affects the

effective thermal
conductivity of porous
building materials →

→

→

→

linear increase of heat
transfer

Negative effects due to excess moisture – changes in the
appearance and physical properties of materials:

- corrosion and rusting in metals
- rotting in woods
- peeling of paint on the interior and exterior wall surfaces
- molds grow on wood surfaces at relative humidities above 85%
- damage of the porous material structure due to freezing

The effects of moisture in buildings

– migration of water vapour

through the walls and condensation on the inner side, releasing
the heat of vaporisation.

The vapour barriers (thick metal and plastic layers) and retarders
(thin metal, paper and plastic layers)

– control of moisture

migration in the walls, floors and ceilings.

Diffusion in a moving medium

Mass transfer - due to both diffusion and convection

Mass convection

∞

v

∞

v

Concentration boundary layer for species A during

external flow on a flat surface.

Evaporation from the free surface of water into air

Heat-mass convection analogy is not applicable when
the rate of mass transfer of species is high relative to
the flow rate of that species (boilers, condensers...)

Example of the fulfilled mass-heat analogy

Simultaneous heat
and mass transfer
(drying, evaporative
cooling, cooling by
dry ice, combustion
of fuel droplets,
transpiration cooling,
rain, snow) →

→

→

→

complex problem.