Drying of milk

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DRYING OF MILK
Drying Principles
E Refstrup, Niro A/S, Soeborg, Denmark
Copyright 2002, Elsevier Science Ltd. All Rights Reserved
Introduction
Drying is de®ned as the removal of a liquid, usually
water, from a product by evaporation, leaving the
solids in an essentially dry state.
A number of different drying processes are in use in
the dairy, food, chemical and pharmaceutical indus-
tries, such as:
spray-drying
¯uid bed drying
drum-drying
batch drying in trays
freeze-drying
microwave drying
superheated steam drying.
Due to drying economy and ®nal product quality
considerations, the only processes of signi®cance in
milk powder manufacture are spray-drying, ¯uid bed
drying (the two most often in combination), and
drum drying, although the latter is in only limited use
nowadays. Only these three drying processes will be
discussed here.
Drum-Drying
The principle of drum-drying (or roller-drying) is that
preconcentrated product is applied as a thin ®lm on
Contents
Drying Principles
Dryer Design
860 DRYING OF MILK/Drying Principles

the outer surface of an internally heated rotating
metal drum. A vapour hood and exhaust system are
placed above the drum. The milk ®lm is scraped off
the drum surface as a sheet of dry product by
stationary knives located opposite the point of milk
concentrate application. The product sheet or ¯akes
falls into an auger trough, which partly disintegrates
it and conveys it to a pneumatic cooling and con-
veying system, often with integrated milling, and to
storage and packaging.
Types of Drum Driers
Several types of drum dryers exist. They can be
characterized by the combination of:
1. Number of drums
(a) Single-drum dryers
(b) Double-drum dryers.
2. Method of product application
(a) Sump between two closely positioned drums
(b) Spray with nozzles
(c) Immersed applicator roll system.
Drum-Drying Process
The main process parameters affecting the plant
capacity and product properties are:
1. Drum surface temperature: saturated steam, at up
to 0.5 MPa pressure, corresponding to about
150
C, is used as a heating medium.
2. Feed temperature: feed temperature may vary
from about 10 to 80
C, depending on the type of
product: the higher the feed temperature, the
higher the plant capacity.
3. Feed solids content: a total solids content up to
45% is usually used; the higher the solids content,
the higher the product drying rate.
4. Drum rotational speed: the time of exposure to
the hot drum surface and hence the ®nal moisture
content is controlled by the rate of rotation of the
drum. At a given drum temperature and feed
solids content and viscosity, the drum rate of
rotation also affects the thickness of product ®lm.
5. Distance between drums and/or applicator rolls:
the gap between drums also controls the thickness
of product ®lm, which is usually `100 mm.
6. Area of heat transfer surface: the plant capacity is
proportional to the effective area of heat transfer.
Generally, drum-drying suffers from a number of
serious disadvantages compared to spray-drying:
1. Severe heat damage and protein denatura-
tion during the slow drying and relatively long
residence time on the hot drum (3±6 s), result-
ing in:
poor solubility
cooked or burned ¯avour.
2. Relatively low evaporative capacity: the highest
capacity of a single unit is about 1000 kg hÀ1
water evaporation.
3. In¯exible in relation to control of powder proper-
ties ± no possibilities of making agglomerated or
instant products.
Despite these disadvantages, drum dryers are still
in use in niche productions, where the special func-
tional properties of drum-dried powders are desir-
able. For instance, the high free fat content of
drum-dried whole milk is advantageous in the choc-
olate industry and the high water-binding capacity of
drum-dried skim milk in the meat-processing industry.
Drum dryers are also used to dry off highly viscous
cereal- or starch-based product blends that cannot
easily be atomized.
Spray-Drying
The basic principle of spray-drying is the exposure of
a ®ne dispersion of droplets, created by means of
atomization of preconcentrated milk products, to
a hot air stream. The small droplet size created, and
hence large total surface area, result in very rapid
evaporation of water at a relatively low temperature,
whereby heat damage to the product is minimized.
The spray-drying process for milk comprises
essentially ®ve subprocess stages: (1) atomization of
the feed, (2) mixing of spray-drying air, (3) evapora-
tion, (4) separation of product from the drying air,
and (5) cooling of the powder. Each of these process
stages can be carried out in different ways, depending
on plant design. The plant design in combination with
operational conditions and other features, such as
integrated and/or external ¯uid beds and destination
of ®ne powder from powder separators, in turn
determine the characteristics and properties of the
®nal powder.
Figure 1 shows the basic type of a conventional
spray-dryer with a conical chamber base for one-
stage drying. New dryers of this type are only rarely
installed nowadays. New types of more ef®cient
dryers, as discussed later, are mostly chosen now.
Atomization of the Feed
Atomization of the feed, i.e. formation of a spray,
is the characteristic feature of spray-drying. The
purpose of atomization is to create a large number
DRYING OF MILK/Drying Principles 861

of small-diameter particles. Assuming a completely
homogeneous spray, the total droplet surface area is
inversely proportional to the droplet diameter, and
the number of particles is inversely proportional to
the square of the droplet diameter.
Rotary atomizers and pressure nozzles are in use
in the milk powder industry. Rotary atomizers
use centrifugal energy and the pressure nozzles use
pressure energy in the atomization process.
Rotary atomizers (wheel atomization) In rotary atom-
izers, the feed is accelerated to the applied peripheral
speed of the wheel, which is typically in the range of
150±165 m sÀ1
. The feed is introduced centrally
around the atomizer shaft through a liquid distri-
bution device. A number of different liquid distributor
designs have been developed over the years, and this
emphasizes the importance of this component for
optimum and trouble-free operation of the atomizer
(no vibrations and product deposits).
The ef®ciency of atomization (droplet size) depends
on a number of factors:
1. Higher peripheral speed, vane height and number
of vanes reduce the mean droplet size.
2. Higher density, rate, viscosity and surface tension
of the feed increase the mean droplet size.
The power consumption of an atomizer is directly
proportional to the feed rate and to the square of the
peripheral speed of the wheel.
Numerous atomizer wheel designs are available.
Figure 2 shows a curved vane wheel type, which is
widely used in the milk powder industry. The special
vane design reduces the amount of occluded air
(vacuoles) in the powder particles. This is of great
1
2
3
4
5
6
7
8
9
Figure 1 Conventional spray-dryer with pneumatic cooling and conveying system. 1, Drying chamber; 2, drying air supply and
heating system; 3, air disperser; 4, atomizing device; 5, cyclone; 6, pneumatic cooling system; 7, conveying cyclone; 8, feed system;
9, exhaust system.
Figure 2 Curved vane atomizer wheel (top cover removed).

importance in the manufacture of high bulk density
products as well as instant products, because of the
higher particle density of the powders.
The main advantages of rotary atomizers are:
1. Very ¯exible with respect to feed rate and feed
viscosity.
2. Higher feed solids can be handled, hence there is
higher product capacity and better economy.
3. There are no fouling or blockage problems.
4. High-capacity units are available (up to 200 t hÀ1
,
although this is not relevant in the milk powder
industry).
5. Can handle abrasive, crystal-containing feed.
6. Different powder properties are achievable with
different wheel designs.
Nozzle atomization The nozzles used in spray-
drying are of the centrifugal-pressure type, in which
pressure energy is converted into kinetic energy of
a thin (0.5±4 mm), moving liquid sheet with a partly
rotational motion, which causes the spray pattern to
be of the `hollow cone' type. Examples of pressure
nozzle designs are shown in Figure 3.
Pressures in the range 18±25 MPa are used for most
products. However, pressures as high as 50±60 MPa
have occasionally been applied for highly viscous
feeds, such as Na-caseinate.
The volumetric ¯ow rate of a nozzle is directly
proportional to the square root of the pressure:
Q ˆ A Á N Á F Á

p
p
where Q ˆ ¯ow rate (m3
hÀ1
), A ˆ nozzle capacity
factor for water, F ˆ viscosity factor (0.9 is used
for most milk concentrates) and p ˆ nozzle pressure
(MPa).
The effects of operating parameters on the
ef®ciency of nozzle atomization (droplet size) are:
1. Higher capacity of nozzles, higher viscosity and
surface tension of the feed and larger ori®ce
diameter (other parameters constant) will increase
the droplet size.
2. Higher pressure and wider spray angle will reduce
the droplet size.
The power consumption of an atomizer is directly
proportional to the feed rate and nozzle pressure.
The main advantages of nozzle atomization are:
1. Minimum aeration of the feed during atomiza-
tion, hence virtually air-free particles and higher
particle density. Typical particle densities of
whole milk powder produced using different
types of atomization are:
straight vane wheel atomization: 1.14 g cmÀ3
curved vane wheel atomization: 1.18 g cmÀ3
nozzle atomization: 1.23 g cmÀ3
.
2. Improved powder ¯owability.
3. Possibility of individual directions of sprays from
each nozzle in multinozzle installations. Improved
agglomeration may be achieved.
4. Less fouling of dryers producing dif®cult-to-dry
products.
The main disadvantages of nozzle atomization are:
1. In¯exible to variation in throughput, as it affects
the nozzle pressure and hence the atomization
ef®ciency.
2. Fairly low capacity per nozzle, ideally not more
than 2000 kg hÀ1
and preferably lower.
3. Multinozzle arrangements required for larger
plants, resulting in more complicated plant start/
stop procedures.
4. Fouling with deposited milkstone, particularly at
higher feed temperatures, causing gradually in-
creased nozzle pressure at constant throughput.
5. Wear of ori®ce and swirl chamber/core grooves,
causing limited lifetime. Depending on operating
conditions and type of product, the nozzle insert
parts (made of tungsten carbide) should be
renewed after 400±800 h of operation.
5
4
3
2
1
3
2
1
(A) (B)
Figure 3 Centrifugal-pressure nozzles. (A) 1, Nozzle body;
2, ori®ce insert; 3, swirl chamber; 4, end plate; 5, screw pin.
(B) 1, Ori®ce insert; 2, nozzle cap; 3, grooved core insert.

Mixing Of Spray and Drying Air
The air disperser and the atomizing device are the
most vital components in a spray-dryer. It has been
metaphorically claimed that they are the lungs and
the heart of a spray-dryer. Consequently, the design
of the air disperser has to be in unison with the at-
omizing device and the desired air ¯ow pattern in the
drying chamber. Air ¯ow pattern in different types of
dryers is discussed elsewhere (see Drying of Milk:
Dryer Design).
Basically, two types of air dispersers are used in
milk powder plants:
1. Air dispersers which create rotational air ¯ow are
used in combination with rotary or nozzle
atomization (Figure 4).
2. Air dispersers which create a vertical downward
air ¯ow are used with nozzle atomization only
(Figure 5).
Most air dispersers are equipped with adjustable
devices for control of the drying air velocity pro®le
and direction, and when nozzle atomization is used,
the nozzle positioning is usually adjustable as well.
The adjustable devices enable control of the
impingement areas of moist product and hence optim-
ization of the drying process with respect to heat
economy and capacity without jeopardizing overall
plant performance due to excessive product depos-
itions in the drying chamber. Figure 6 illustrates an
example of the effect of air disperser adjustments on
the shape and impingement of the spray cloud.
In recent years, the development of computerized
¯uid dynamic (CFD) software and the powerful com-
puters required have provided a new tool for the study
of air ¯ow and particle paths in spray-dryers. Despite
the shortfalls of the software presently available
which, for instance, does not take into account the
hygroscopic or desorption properties of products in
the calculations, CFD simulations have already proven
to be a useful tool in design and troubleshooting.
Figure 7 shows examples of CDF simulations.
Evaporation
Following the intimate mixing of droplets and
drying air, dramatic changes in the states of both take
Cooling air
Concentrate
Drying air
Figure 4 Ceiling air disperser with adjustable guide vanes for rotational air ¯ow.
Drying
air
Fines
Concentrate
Cooling
air
Cooling
air
Figure 5 Air disperser for vertical downward air ¯ow.

place within a fraction of a second after spray-drying/
air mixing.
Change of state of drying air Properties of the
humid air can be shown in a humidity chart (or
Mollier diagram), which is also useful for illustrating
the changes that the air undergoes during the spray-
drying process (Figure 8).
In Figure 8, which is only valid for an atmospheric
pressure of 101.3 kPa, the water vapour content, as
g kgÀ1
dry air, is plotted on the x-axis and the tem-
perature on the y-axis. The parallel sloped lines
are curves of enthalpy, as kcal kgÀ1
dry air, and
®nally the curved lines show the relative humidity
(rh). When two of the four parameters are known
(for instance, the temperature and rh, which are
easily measured), the other two can be found from
Figure 8.
If a change of state occurs without any heat ex-
change with the surroundings, the enthalpy of the
system will not change and hence follow a line
parallel with the enthalpy lines, and the change is said
to be adiabatic. The drying process shown in Figure 8
is assumed to be adiabatic. However, in a real situ-
ation in a spray-dryer, this is not quite the case. Heat
will be added to the system with the warm concen-
trate (T Tˆ 0
C), but on the other hand, heat is re-
moved from the system by transmission loss and with
the warm powder leaving the dryer. Any additional
air ¯ow to the dryer, such as air disperser and atom-
izer cooling air or integrated ¯uid bed air, will also
have an effect, but for the situation in a conventional
dryer, as shown in Figure 1, the illustration of the
drying process in Figure 8 is reasonably accurate.
If the ¯ow rate of the drying air in a plant is known,
the evaporative capacity can be estimated under any
given drying conditions (ambient humidity, inlet and
outlet temperature). In the example in Figure 8, the
ambient humidity is 5 g kgÀ1
dry air and the outlet air
humidity is 40 g kgÀ1
, i.e. the evaporation is 35 g kgÀ1
dry air which, multiplied by the air rate, yields the
evaporative capacity of the plant.
Change of state of droplets When pure water is
dried, the water droplets will reach the wet-bulb
temperature in the initial stage of drying. However,
the presence of dissolved and/or dispersed solids in
the droplets causes the water activity (aw) of the
drying product to decrease as the drying proceeds.
The driving force in the drying process is the differ-
ence (roughly) between the aw of the product and
the rh of the drying air, and any decrease in aw or
increase in rh will reduce the driving force. Further,
the diffusion coef®cient of water decreases with
increasing solids content. Both in combination will
result in reduced drying rates.
The relation between aw and moisture content
at constant temperature is called a sorption isotherm.
As hysteresis effects are quite common, it is important
to differentiate between absorption and desorption
isotherms in connection with drying. A typical shape
of a sorption isotherm together with some common
drying terms is shown in Figure 9.
In practice, the minimum outlet temperature of
a spray-dryer (the highest rh of the outlet air) and
the highest capacity and drying economy without
forming excessive deposits depends mainly on the
corresponding aw. If the particle and gas residence
time were inde®nite, equilibrium between product
and drying air would be reached. However, this is
obviously not the case and the outlet air rh must
be kept well below the product aw to achieve the
desired powder moisture content. In Figure 8 it can
Air
A
B
Figure 6 Impingement area control by air disperser
adjustments. (A) Impingement of roof and upper wall due to
excessive air rotation and insuf®cient spray cloud depression.
(B) Impingement of lower wall and cone due to excessive spray
cloud depression and insuf®cient air rotation.

be seen that the rh at a given absolute humidity
is very dependent on temperature, so the lower rh
is achieved by operating at increased outlet
temperatures.
Figure 10 shows the drying characteristics of
a droplet during spray-drying. Initially, the drying
rate is (nearly) constant and the particle temperature
is near the wet-bulb temperature of the surrounding
air. At a certain solids content, the critical point is
reached, beyond which the drying rate decreases.
Moisture diffusion is now drying rate-limiting.
Simultaneously the particle temperature increases.
The outlet temperature from the dryer corresponds
to the equilibrium moisture content, whereas the
droplet temperature only reaches the temperature
corresponding to the actual residual moisture. Sim-
ilarly, aw (or partial pressure) of the dried particle
will correspond to the residual moisture, whereas
(A)
(C)
(B)
(D)
Figure 7 Computerized ¯uid dynamic simulations of a Compact dryer. (A) Air ¯ow pro®le; (B) evaporation pro®le; (C) particle
population density pro®le; (D) temperature pro®le.

the vapour pressure in the exhaust air (or rh) cor-
responds to the equilibrium moisture content.
The outlet temperature is a very important process
parameter and, to cope with smaller changes in other
key parameters and still maintain constant product
moisture content, the following guideline can be given:
ÁTout ˆ ÁTina10 ‡ ÁTS ‡ Áxamba2X8 À K Á ÁH2O
where ÁTout is the required change in outlet
temperature; ÁTin is the change in inlet temperature,
i.e. a change of 10
C in inlet temperature should be
compensated by 1
C in outlet temperature; this can
be read from the Mollier diagram; ÁTS is the change
in % total solids in the concentrate; this is purely
empirical; Áxamb is the change in ambient humidity in
g kgÀ1
dry air, i.e. a change of the ambient humidity
of 2.8 g kgÀ1
dry air should be compensated by 1
C in
outlet temperature; K is a product-dependent factor,
which is about 5 for skim milk powder and 6 for
whole milk powder; and ÁH2O is the change in
powder moisture content.
100 kcalkg–1 DA 150
100
%RH
100
50
20
10
5
50
0
300
250
200
150
100
50
0
50
180
87
Temperature(°C)
0 50 1005 40
Water vapour gkg–1
dry air
Figure 8 Humidity chart showing a spray-drying process.
Ambient air with 5 g moisture kgÀ1
dry air is heated to 180
C.
Assumed adiabatic drying to a relative humidity (RH) of 10% in
the exhaust air, assumed to give the required ®nal product
moisture content, results in an outlet temperature of 87
C.
Equilibrium
moisture
Evaporated (free)
water
Spray
particles
kg H2Okg–1
TS
Equilibrium curve
Bound
water
Unbound
water
100
ϕ
0
Figure 9 Sorption isotherm and common drying terms.
Temperature
Equilibrium
moisture
Residual
moisture
ex dryer
Feed to
dryer
% H2O
Partial
pressure
Constant drying rate
Partial pressure
Decreasing
drying rate
Tout
Critical point
Vapour pressure
in exhaust air
Temperature
Figure 10 Drying characteristics during spray-drying.

Separation of Product from the Drying Air
Inevitably, some powder, mainly smaller particles
(®nes), will be entrained in the exhaust air from
the drying chamber. Traditionally, these ®nes were
collected in cyclones only. However, the stricter
environmental demands to minimize powder emis-
sion from dryers have in many places necessitated
installation of secondary and more ef®cient separa-
tors such as bag ®lters or wet scrubbers.
In recent years, there has been signi®cant progress
in the development of cleanable-in-place bag ®lters,
which replace the cyclones and other traditional
secondary separators (see Drying of Milk: Dryer
Design).
Cooling of the Powder
Cooling of powder can take place in a pneumatic
cooling and conveying system, as shown in Figure 1.
However, the passage of powder through the ducts
and cyclone imposes a signi®cant mechanical im-
pact on the product, which causes attrition and de-
struction of desirable properties of the product,
such as agglomerate structures and instant proper-
ties. In order to avoid that, more lenient cooling
methods must be applied, and this can be achieved in
¯uid beds.
Fluid-Bed Drying/Cooling
A ¯uid bed is basically a box, divided by a perfor-
ated air distributor plate in a lower air inlet and
distribution section (air plenum) and an upper
product section. Different types of ¯uid beds are used:
back-mix or plug-¯ow ¯uid beds
stationary or vibrated ¯uid beds
external or integrated ¯uid beds in the drying
chamber.
Stationary plug-¯ow beds are used for products that
are easily ¯uidizable at the inlet conditions to the bed.
On the other hand, back-mix beds are used for prod-
ucts that are not directly ¯uidizable, but may be so
when mixed and conditioned with the (partly) dry
powder already present in the ¯uid bed.
Integrated ¯uid bed dryer design and technology
are discussed elsewhere (see Drying of Milk: Dryer
Design).
Extensive developments of ¯uid bed plates have
taken place in recent years. The requirements for the
plate are:
ability to control powder movements and self-
emptying properties
nonsifting, i.e. products should not fall through
the plate and down into the air plenum
easily cleanable and ideally without crevices and
sharp corners (sanitary design).
The challenge has been to combine these three re-
quirements in one design. The development of the
Bubble PlateTM
seems to ful®l the requirements.
A selection of different plates, including the Bubble
PlateTM
, is shown in Figure 11.
Originally, the external, vibrating ¯uid beds
were used for cooling purposes only, but the advant-
ages of using them for drying as well were soon real-
ized. When the drying takes place in one stage in
the drying chamber, the outlet temperature has to
be kept fairly high to maintain the required driving
force to achieve the desired ®nal moisture content
during the fairly short residence time in the drying
chamber.
Two-stage drying Two-stage drying implies spray-
drying to a moisture content 2±4% higher than the
required ®nal moisture. The ®nal drying takes place
in the drying section of a plug-¯ow ¯uid bed with
a long product residence time of 5±10 min rather
than the total of 20±30 s in the spray-drying
chamber. The higher outlet moisture content is
achieved by operating the dryer at 10±20
C lower
outlet temperature. This has a number of advantages
over one-stage drying:
less overall heat damage of the product
higher plant capacity
lower speci®c energy consumption
improved product quality.
If a product quality comparable to that of one-stage
dried product is acceptable, even further increases
in plant capacity can be obtained by operating at
an increased inlet temperature to the spray-dryer
(Table 1).
Two-stage drying also proved to be suitable for
the production of agglomerated products, where the
®nes return to the atomization zone and the higher
powder moisture content inside the drying chamber
facilitated enhancement of the agglomeration pro-
cess and stabilization of the agglomerates formed.
Further, the external ¯uid bed also functions as
a classi®er, meaning that nonagglomerated ®nes are
selectively blown off and reintroduced into the drying
chamber for further agglomeration.
Integrated fluid bed drying The limitation of the
two-stage drying process is mainly set by the inabil-
ity to handle the very moist powder leaving the drying

Figure 11 Examples of perforated air distribution plates for ¯uid beds.
Table 1 Comparison of drying processes
Drying system Unit SDP SD2 SD2
high Ti
CDI MSDTM
Spray-dryer
Drying air
C 200 200 230 230 260
Drying air kg hÀ1
31 500 31 500 31 500 31 500 31 500
Skim milk, 8.5% TS kg hÀ1
12 950 16 150 19 800 24 000 31 300
Concentrate 48% TS kg hÀ1
2 290 2 860 3 510 4 250 5 540
Evaporation chamber kg hÀ1
1 150 1 400 1 720 2 010 2 620
Powder from chamber kg hÀ1
3.5% moisture kg hÀ1
1140
6% moisture kg hÀ1
1460 1790
9% moisture kg hÀ1
2 240 2 620
Fuel oil consumption kg hÀ1
175 175 205 205 230
Power consumption kW 120 125 130 140 150
Energy consumption Mcal 1 818 1 823 2120 2 130 2 380
per kg powder Kcal 1 595 1 250 1184 950 820
Fluid bed
Drying air kg hÀ1
3 430 4 290 6 750 11 500
Drying air kg hÀ1
100 100 115 120
Evaporation SFB/VF kg hÀ1
40 45 125 165
Powder, 3.5% H2O kg hÀ1
1 420 1 745 2 115 2 755
Steam consumption kg hÀ1
135 167 290 400
Power consumption kW 20 22 25 35
Energy consumption Mcal 95 115 195 265
Drying total
Energy consumption, total Mcal 1 818 1 918 2 235 2 325 2 645
per kg powder Kcal 1 595 1 350 1 280 1 038 960
Energy consumption, relative to SDP % 100 88 80 65 60
SDP, spray-dryer with pneumatic transport; SD2, two-stage drying; SD2 high Ti, SD2 with high inlet temperature; CDI, compact dryer; MSDTM
, multistage
dryer. SFB/VF, static ¯uid bed/Vibro-Fluidizerr
; TS, total solids.

7
6
9
5
8
3
12
4
Figure 13 Multistage (MSDTM
) dryer. 1, Drying chamber; 2, drying air supply and heating system; 3, air disperser; 4, atomizing
devices: nozzles; 5, cyclones; 6, static ¯uid bed with air supply; 7, Vibro-Fluidizerr
system with air supplies; 8, exhaust system; 9, ®nes
return system.
1
2
3
4
5
6
7
8
9
10
Figure 12 Compact dryer with Vibro-Fluidizerr
. 1, Drying chamber; 2, drying air supply and heating system; 3, air disperser;
4, atomizing devices: rotary atomizer and nozzles; 5, cyclone; 6, static ¯uid bed with air supply; 7, wall sweep system; 8, vibro-¯uidizer
system with air supplies; 9, exhaust system; 10, ®nes return system.

chamber in an external ¯uid bed without lump
formation or even blockage of powder outlet from the
chamber. In some cases impingement of wet particles
on the plant surfaces and deposit formation can also
be a problem.
When an annular, back-mix ¯uid bed forms the
bottom part of the drying chamber, as in the Com-
pactTM
dryer (Figure 12), a higher moisture content
of the powder from the primary drying stage can
be handled. With the circular back-mix ¯uid bed
and different air ¯ow pattern in the multistage
(MSDTM
) dryer (Figure 13), still higher moisture
contents can be handled.
A comparison of the different drying processes is
presented in Table 1.
See also: Drying of Milk: Dryer Design. Milk Powders:
Types and Manufacture; Physical and Functional
Properties of Milk Powders. Recombined and
Reconstituted Products.
Further Reading
CaricÂ M (1994) Concentrated and Dried Dairy Products:
General Production. New York: VCH Publishers.
Masters K (1991) Spray-Harlow Drying Handbook.
Harlow: Longman Scienti®c and Technical.
Pedersen PJ (1985) In: Hansen R (ed.) Evaporation,
Membrane Filtration, Spray Drying, Roller Drying.
Vanlùse: North European Dairy Journal.
PõÂsecky J (1997) Handbook of Milk Powder Manufacture.
Copenhagen: Niro.
Walstra P, Geurts TJ, Noomen A, Jellema A and van
Boekel MAJS (1999) Dairy Technology: Concentration
Process. New York: Marcel Dekker.
Westergaard V (1994) Milk Powder Technology: Evapora-
tion and Spray Drying. Copenhagen: Niro.
Dryer Design
V Westergaard, Niro A/S, Soeborg, Denmark
Copyright 2002, Elsevier Science Ltd. All Rights Reserved
Introduction
For the purposes of this article, dryers are de®ned as
spray-dryers, although other means of drying are
possible. By de®nition, spray-drying is the trans-
formation of feed from a ¯uid state into a dried form
by spraying the feed into a hot drying medium. The
feed can be either a solution, suspension or a paste,
depending on which dairy product has to be dried.
The dried product forms a powder consisting of single
particles or agglomerates, depending on the physical
and chemical properties of the feed and the dryer
design and operation.
Principles of Drying
A spray-dryer operates in the following way. The feed
is pumped from the product feed tank to the atom-
izing device, which is located in the air disperser in
the top of the drying chamber. The drying air is
drawn from the atmosphere via a ®lter by a supply fan
and is passed through the air heater to the air dis-
perser. The atomized droplets meet the hot air and
evaporation takes place, cooling the air at the same
time. After drying of the spray in the chamber, the
majority of the dried product falls to the bottom
where it is collected for further processing. The ®nes,
which are the particles with a small diameter, will
remain entrained in the air, and it is therefore neces-
sary to pass the air through powder collectors, such as
cyclones. The air passes from the cyclone to the
atmosphere via an exhaust fan. The two fractions of
powder are, for example, collected in a pneumatic
system for conveying and cooling and are then passed
through a cyclone for separation, after which they are
bagged off.
A conventional spray-dryer consists of the follow-
ing main components (Figure 1):
1. Drying chamber.
2. Hot air system and air distribution.
3. Feed system.
4. Atomizing device.
5. Powder separation system.
6. Pneumatic conveying and cooling system.
7. Integrated ¯uid bed.
8. Fluid bed after drying/cooling.
Drying Chamber
Various designs (Figure 2) of drying chambers are
available on the market. The most common one is the
cylindrical chamber with a cone of 40±60
, so that the
powder can leave the chamber by gravity. This
chamber may also have a ¯at bottom, in which case
a scraper or suction device is needed to remove the
powder from the chamber. A horizontal box-type
drying chamber is also available and this, too,
DRYING OF MILK/Dryer Design 871

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