Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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pressure drop has to be developed across the bed to

form the spout. Once this is formed the pressure drop

decreases to a value less than that which develops

across a conventional fluidized bed. A spouted-bed

drier featuring tangential air inlet has also been

reported (Figure 5). The air enters through slits at

the bottom of the chamber. An open screw conveyer

is located in the spout to control the upward move-

ment of the particles, independently of the air vel-

ocity. This enables smaller particles to be dried as

compared with a conventional spouted-bed drier. A

high early air pressure drop is not required to initiate

spouting. Wheat, corn, and diced carrot are among

the foods which have been successfully dried in

spouted-bed driers.

Toroidal-bed Drier

0011 This is another novel design of fluidized-bed drier

(Figure 6). A high-velocity stream of heated air enters

the base of the process chamber through blades or

louverswhichimpartarotarymotiontotheair.This

creates a compact rotating bed of particles, which may

vary in depth from a few millimeters to 50 mm. High

rates of heat and mass transfer are attainable in this

bed, resulting in high initial rates of drying. This drier

can accommodate a wide range of particle sizes and

shapes and can be operated on a batch or continuous

basis. Not much information on the performance of

this equipment for drying foods has been published.

However, preliminary studies, in which this author has

participated, indicate that it has considerable potential

for drying small particulate foods and liquid foods on a

similar principle to the spouted bed above. It can be

used for roasting products such as coffee beans and

nuts, expanding pasta pellets, and possibly cooking or

sterilizing other particulate. In a current study it is

being used to produce puffed potato cubes.

Air

Wet material

Dry product

Air

fig0005 Figure 5 Spouted-bed drier with tangential air inlet and central

conveyor screw: 1, tangential air inlet; 2, conveyor screw; 3, drier

body. From Mujumdar AS (ed.) (1995) Handbook of Industrial

Drying, 2nd edn, by courtesy of Marcel Dekker: New York.

Product

Feed

fig0006Figure 6 The toroidal-bed drier: 1, rotating disk distributor to

deliver raw material evenly into the processing chamber; 2,

rotating bed of particles; 3, fixed blades with hot air passing

through at high velocity; 4, burner assembly. From Brennan JG

(1994) Food Dehydration ^ A Dictionary and Guide. Oxford:

Butterworth-Heinemann with permission.

1926 DRYING/Fluidized-bed Drying

Centrifugal Fluidized-bed Drier

0012 This consists of a cylindrical chamber rotating about

a horizontal axis (Figure 7). The cylinder wall is

perforated and a high-velocity (up to 15 m s

1

) stream

of air flows through these perforations and across the

drying chamber. The feed enters at one end of the

chamber and the dried product leaves over a weir at

Product

Discharge weir

Rotation

Product

Cross air flow

Perforated cylinder

feeder

fig0007 Figure 7 Centrifugal fluidized-bed. From Mujumdar AS (ed.) (1995) Handbook of Industrial Drying, 2nd edn, by courtesy of Marcel

Dekker: New York.

Solids

discharge

Atomizing nozzle

Sliding mesh

Regulating

valve

Pitot

tube

Air

Liquid

Heater

Com

ressor

Fine mesh

Spout

deflector

Perspex

window

Spouted bed

drier

Cyclone

fig0008 Figure 8 Experimental spouted-bed drier for liquid foods. From Ochoa-Martinez AE, Brennan JG and Niranjan K (1993) Spouted bed

dryer for liquid foods. Food Control 4: 41 – 45 with permission.

DRYING/Fluidized-bed Drying 1927

the other end. Some 10–20% of the chamber volume

is occupied by the food particles. These are made to

fluidize during part of each revolution and form a

stationary bed during the rest of the cycle. This drier

has been used to dry sliced and shredded vegetables

which are difficult to dry in conventional fluidized-

bed driers and for puffing vegetable pieces, including

rice grains, to produce a quick-cooking product.

Novel Applications for Fluidized and

Spouted Beds

0013 Fluidized beds of inert particles have been used to dry

solid and liquid food materials. A whirling fluidized

bed of glass beads has been used to dry diced vege-

tables and sticky products such as cooked rice. Mush-

rooms, carrot, beef, and shrimp have been dried

at low temperature in a fluidized bed of activated

alumina. Paste and liquid foods have been dried in

spouted beds of inert particles. Blood has been dried

in this way. In a recent study, model liquid foods were

sprayed on to polypropylene beads in an experimen-

tal spouted bed drier (Figure 8). Drying occurred by a

combination of convected heat from the air and con-

ducted heat from the beads. The performance of this

drier was compared with that of a laboratory spray

drier. Early results indicated that the drying process in

the spouted bed was rather more severe than in the

spray drier. However, the evaporative capacity of the

spouted bed was some six times that of the spray

drier, which was a comparable size.

0014Fluidized beds may be used to produce agglomer-

ated or granulated products. A liquid is sprayed on to

a fluidized bed of particles, coating them. Collisions

between particles cause them to adhere together to

form agglomerates. When the agglomerates reach the

desired size they are dried to a stable moisture content

by raising the temperature of the fluidizing air. Both

batch and continuous agglomerating systems are

available (Figure 9).

0015A relatively new application for fluidized beds is

the puffing of fruit and vegetable pieces. The pieces

Police filters

Spraying system

Inlet air

filter

Inlet air

filterHumidifier

Heater Cooler

HEPA

Product filters

Filter housing

Expansion zone

Product container

Air distributor

Inlet air plenum

Control panel

Fan

fig0009 Figure 9 Batch fluidized-bed agglomerator. Courtesy of Niro Aeromatic AG.

1928 DRYING/Fluidized-bed Drying

are predried in a conventional hot-air drier such as a

cabinet or tunnel, to form an impervious layer on

their surface. They are then introduced into a fluid-

ized bed, operating at a relatively high temperature.

The water evaporates internally within the pieces,

causing them to expand and producing an open

porous structure. The puffed product is then dried

down to a stable moisture content in a cabinet or

tunnel drier. This has the effect of reducing the overall

drying time and produces a product which reconsti-

tutes rapidly. Alternatively, the dry product may be

used as a snack food, low in fat, compared with fried

products.

Drying Difficult Materials

0016 To contain hazardous materials or unpleasant odors

semiclosed-cycle fluidized-bed systems are available.

In such systems a large proportion of the exhaust air

is recycled. The air from the cyclones goes to a con-

denser/scrubber which removes the water from it as

condensate. Only a small amount of air is released to

the atmosphere; the rest is recycled through the heater

and drier. The air vented from the system may be

burnt off if necessary. Similar systems are available

for spray driers. Application of these specialized types

of driers to food dehydration is limited at the present

time. However, as regulations relating to pollution of

the environment become more stringent, the use of

such systems may increase.

See also: Drying: Theory of Air-drying; Physical and

Structural Changes; Chemical Changes

Further Reading

Brennan JG (1994) Food Dehydration – A Dictionary and

Guide. Oxford: Butterworth-Heinemann.

Brennan JG Butters JR, Cowell ND and Lilly AEV (1990)

Food Engineering Operations, 3rd edn. London: Else-

vier Applied Science.

Filkova I and Mujumdar SS (1995) Industrial spray drying

systems. In: Mujumdar AS (ed.) Handbook of Industrial

Drying, 2nd ed, pp. 263–307. New York: Marcel Dekker.

Hovmand S (1995) Fluidized bed drying. In: Mujumdar AS

(ed.) Handbook of Industrial Drying, 2nd edn, pp.

195–248. New York: Marcel Dekker.

Ochoa-Martinez LA, Brennan JG and Niranjan K (1993)

Spouted bed dryer for liquid foods. Food Control 4:

41–45.

Pallai E, Szentmarjay T and Mujumdar AS (1995) Spouted

bed drying. In: Mujumdar AS (ed.) Handbook of Indus-

trial Drying, 2nd edn, pp. 425–488. New York: Marcel

Dekker.

Varnalis A, Brennan JG and MacDougall DB (1998) Puffing

of foodstuffs – a review. European Food and Drink

Review winter 23–26, 28–31.

Spray Drying

J G Brennan, University of Reading, Reading, UK

Principles

0001This is the most commonly used method for drying

food liquids and some slurries. The feed is converted

into a fine mist or spray which is then mixed with

heated air. Very rapid drying takes place, converting

the liquid droplets into powder particles. Because

of the small size of the particles – diameters usually

in the range 10–200 mm – a very large surface area is

available for drying. Also the distance that moisture

has to migrate within the particles to the drying

surface is relatively small. Thus, short drying times,

1–20 s, are a feature of this method of drying. There is

also a significant evaporative cooling effect so that

the surface temperature of the droplets does not rise

much above the wet bulb temperature of the drying

air until drying nears completion. Provided the par-

ticles, once dry, are swiftly removed from the drying

chamber, heat damage to the product can be limited.

0002The main features of a single-stage spray-drying

plant are shown in Figure 1. Inlet fan A draws air in

through a filter B and pushes it through the air heater

C into the drying chamber D. By means of pump E the

feed material is pumped from feed tank F to the spray-

forming device G. It is converted into spray which

mixes with the drying air in the drying chamber D.

The bulk of the dry powder thus formed is removed

from the drying chamber via valve H and is carried

pneumatically to a storage bin through duct I. The air

is removed from the drying chamber via ducting J

through one or more air/powder separators K and is

exhausted to the atmosphere by outlet fan L. The fine

powder recovered by the separators K may be added

to the main product stream via valve M or recycled

into the wet zone of the drying chamber via duct N.

Spray-drying Equipment

0003Air heating may be indirect using steam, fuel oil, or

gas. Direct heating would obviously improve the ther-

mal efficiency of a drier and in recent years natural

gas has been used directly to heat the incoming air.

There is some concern over the possible contamin-

ation of the dried products with nitrate and nitrite

compounds and N-nitrosodimethylamine. The intro-

duction of low NO

(nitrogen oxide) burners has

reduced this problem. However, work is still in pro-

gress to establish the level of contamination, if any, of

food products dried by directly heated air.

DRYING/Spray Drying 1929

0004 The formation of a spray of the feed liquid within a

specified size range is an essential feature of spray

drying. Too wide a droplet size range can lead to

nonuniform drying and/or build-up of powder on

the inner wall of the chamber. Droplet size can influ-

ence important properties of the dried product such

as its bulk density and reconstitution characteristics.

0005 Spray-forming devices, also known as atomizers,

are of three general types: (1) centrifugal or wheel

atomizers; (2) pressure nozzles or jets; and (3) two-

fluid or pneumatic nozzles.

0006A centrifugal atomizer consists of a disk or bowl on

the end of a rotating shaft. The liquid feed is intro-

duced on to the disk near its center of rotation. It

accelerates to the peripheral velocity of the disk and

is spun off, in the form of a spray, into the drying

chamber. There are numerous different designs of

spinning disk, some of which are depicted in Figure 2.

B AC

fig0001 Figure 1 A typical single-stage spray drier, featuring recycling of fines. A, air inlet fan; B, filter; C, air heater; D, drying chamber; E,

feed pump; F, feed tank; G, atomizer; H, rotary valve; I, product conveyor; J, duct for outlet air; K, cyclone, L, air outlet fan; M, rotary

valve; N, duct for returning fines to chamber. From Brennan JG (1994) Food Dehydration ^ A Dictionary and Guide, Oxford: Butterworth-

Heinemann with permission.

Circular vanes Rectangular curved vanes Oval vanes

fig0002 Figure 2 Different designs of centrifugal atomizers. Adapted from Mujumdar AS (1995) Handbook of Industrial Drying, 2nd edn. New

York: Marcel Dekker with permission.

1930 DRYING/Spray Drying

Disk diameters range from 50 to 300 mm, they rotate

at speeds of 50 000–10 000 revolutions per minute,

respectively, and produce droplets with mean diam-

eters in the range 1–600 mm. Centrifugal atomizers

can produce uniform sprays within a wide range of

droplet sizes. The size distribution of the droplets can

be altered by changing the speed of rotation. Such

atomizers are not subject to blocking or abrasion due

to insoluble solid particles in the feed and can handle

viscous materials at low pumping pressures.

0007 A pressure nozzle (Figure 3a) features a small ori-

fice, with a diameter in the range 0.4–4 mm, through

which the feed is pumped at high pressure, in the

range 5–50 MN m

2

. A grooved insert, located

behind the orifice, imparts a spinning motion to the

liquid, producing a hollow cone of spray. Pressure

nozzles can produce quite uniform droplets with

mean diameters ranging from 10 to 800 mm, which

usually results in dried particles with hollow centers.

The orifices in such nozzles may be abraded or

blocked by insoluble solid particles in the feed. Very

high pumping pressures are required to atomize

viscous liquids.

0008 In a two-fluid or pneumatic nozzle (Figure 3b) the

energy in a high-velocity stream of gas is used to

atomize the feed. The gas exits from the nozzle

through an annular opening while the liquid feed

exits through a circular opening concentric with the

annular one. A Venturi effect is created and the liquid

is converted into a spray. Such nozzles are subject to

blocking and abrasion by solid particles in the feed.

The size of the droplets can be changed by varying the

gas–liquid ratio. They have relatively small capacities

and can produce a spray with a broad spectrum of

droplet sizes, especially with high-viscosity feeds.

0009 A recent development involves the use of sonic

energy to atomize highly viscous materials. A sonic

resonance cup is located in front of a two-fluid

nozzle. This creates a field of high-frequency sound

waves that breaks up the liquid into droplets. This

system is not yet being used commercially.

0010 In the drying chamber of a spray drier the spray of

feed contacts the heated air and drying takes place.

There must be sufficient dwell time in the chamber for

the particles to reach a suitably low moisture content

but, once that is attained, the dry particles must be

removed quickly from the chamber to avoid unneces-

sary heat damage. There are many designs of drying

chamber available. These may be classified into three

groups based on the direction of flow of the feed

relative to the air, i.e., concurrent, countercurrent,

and mixed flow. Countercurrent chambers are seldom

used for food applications because of the risk of heat

damaging the product. Concurrent chambers usually

consist of a cylindrical body with a conical base.

When drying large droplets of a heat-sensitive feed,

in particular if the dried particles are tacky and tend

to adhere to the wall of the chamber, a tall cylindrical

body is used (Figure 4a). The feed is introduced at the

top of the chamber, usually through a nozzle atom-

izer, and follows a fairly straight-line flow path to the

bottom of the chamber. The wall of the conical

section of the chamber may be cooled to facilitate

removal of the powder.

0011Other concurrent chambers feature a shorter cylin-

drical body (Figure 4b). The air enters at the top of

the chamber tangentially and follows a downward,

spiral flow path. The liquid feed enters at the top of

the chamber through a centrifugal atomizer. This

design of drying chamber is widely used for drying

foods which are not very heat-sensitive or tacky. A

mixed-flow chamber features both concurrent and

countercurrent flow patterns (Figure 4c). Such cham-

bers find limited application in the food industry. The

risk of heat damaging the product is greater than in

concurrent chambers. Many other types of chamber

are available, including flat-bottomed cylindrical and

horizontal box-like designs.

0012In industrial-sized spray driers the bulk of the dried

powder is removed directly from the chamber through

a rotary valve or vibrating device. Some food powders,

mainly those with high sugar or fat contents, become

sticky when hot and adhere to the wall of the cham-

ber. Various devices are available to assist in the

removal of such products. Hammers may be made

to strike the outside wall of the chamber at intervals

(a) (b)

Liquid

Air

fig0003 Figure 3 (a) Pressure nozzle; (b) two-fluid or pneumatic nozzle. Adapted from Brennan JG, Butters JR, Cowell ND, Lilly AEV (1990)

Food Engineering Operations, 3rd edn. London: Elsevier Applied Science with permission.

DRYING/Spray Drying 1931

to loosen deposits. Brushes, chains, or air brooms

may sweep the inner surface of the chamber. Redu-

cing the temperature of the chamber wall may help.

This can be achieved by removing some of the insula-

tion from the wall or by drawing cool air in through a

jacket covering some or all of the chamber wall

(Figure 4a). This warm air may then be introduced

into the main air supply to the heater, thus recovering

some of the heat lost from the chamber. Fine powder

entrained in the exhaust air from the chamber may be

recovered with the use of dry cyclone separators,

filters, or wet scrubbing devices, singly or in various

combinations. If some of the feed is used to wash the

outgoing air in a scrubber and is then recycled as feed

into the drier, savings in energy can result.

0013 Semiclosed or closed spray-drying systems are

available in which some or all of the drying gas is

recycled. A scrubber-condenser is used to remove the

water vapor from the gas. These systems may be used

for drying materials with strong, unattractive odors

or where gases other than air are used as the drying

medium. Aseptic spray-drying systems are also avail-

able in which all the air entering the system is pre-

sterilized, usually by filtration, and the product is

discharged into a sterile packing zone. Such systems

are mainly used in the pharmaceutical industry but

may find applications in food dehydration as hygienic

requirements become more stringent.

Safety Considerations

0014There are fire and explosion hazards associated with

spray drying of liquid foods. A dust explosion may

occur when a fine combustible powder is dispersed in

air and ignited. For an explosion to occur there must

be a sufficient concentration of powder in the air and

sufficient oxygen present to support combustion.

Such conditions may exist in spray driers under

normal operating conditions. Self-ignition may

occur in food powders. Oxidative reactions are exo-

thermic and the generation of heat by such reactions

can be very fast at the temperatures encountered in

spray drying. In thick layers of powder, heat gener-

ated in this way cannot escape quickly enough and

the temperature rises rapidly, causing ignition. The

critical thickness for ignition depends on the tempera-

ture and the powder characteristics. Obviously, to

minimize the occurrence of self-ignition, the build-up

Product out

Air out

Cooling jacket

(optional)

Feed in

Air in Air in

Feed in

Air in

Air out

Product out

Air out

Feed in

Air in

(a) (b) (c)

fig0004 Figure 4 Different designs of spray-drying chambers: (a) concurrent with straight-line flow path; (b) concurrent with spiral flow path;

1932 DRYING/Spray Drying

of layers of powders in spray-drying systems should

be avoided. Regular and efficient cleaning of the plant

(see below) and the use of devices, such as those

mentioned above, to remove powder deposits can

reduce the hazard. There may be other causes of

powder ignition such as heat generated by friction

or electrical sparks. In spray driers featuring direct

flame heating, very hot particles entering the drier in

the heated air may ignite the powder. Good design

and maintenance of equipment should minimize the

chances of ignition occurring by these means. The

fitting of weaker doors or panels, which are designed

to fail at relatively low pressures, relieving the explo-

sion, is the most common precaution taken to limit

damage due to dust explosions in spray driers. Other

measures such as containment or suppression of such

explosions are not usually feasible in high-capacity

driers.

0015Efficient cleaning of spray drier installations is

essential. This may be done manually using high-pres-

sure water jets. Rotating nozzle heads may be strategic-

ally located throughout the system so that all parts of

the equipment are contacted by the cleaning fluid.

Most modern plants are equipped for cleaning-in-

place (CIP). Such a system is shown in Figure 5.

Energy Considerations

0016Spray drying is an energy-intensive method of remov-

ing moisture. In a straight-through spray drier, up to

fig0005 Figure 5 Cleaning-in-place (CIP) system for spray drier layout. From Masters K (1991) Spray Drying Handbook, 5th edn. New York:

Longmans with permission.

DRYING/Spray Drying 1933

6000 kJ of energy may be required to evaporate 1 kg

of water. By comparison, in a six-stage multiple-effect

evaporator with mechanical recompression, as little

as 200 kJ may do the same task. Therefore, to con-

serve energy, liquid foods should be concentrated to

as high a solids content as possible prior to spray

drying. The upper limit of solids concentration is

usually determined by the viscosity of the concentrate

which, if too high, may result in poor atomization.

The use of direct flame heating of the air to the spray

drier is more energy-efficient than indirect methods

(see above). The higher the air inlet temperature and

the lower the air outlet temperature, the more ther-

mally efficient the drying operation. The upper limit

on air inlet temperature is usually determined by the

heat sensitivity of the feed. For many foods 200



Cis

this upper limit. Some can tolerate 250



C. It is now

possible to go as high as 300



C for some foods when

using modern, multistage spray-drying systems (see

below). A low outlet temperature may lead to too

high a moisture content in the product. Few driers

operate well below 80



C. Up to 50% of the exhaust

air from the drying chamber may be recycled under

certain circumstances and can lead to up to a 20%

saving in energy. However, this must be balanced

against a reduction in evaporative capacity and is

really only feasible when high air outlet temperatures

are used. Thus, it is not suitable if the product is heat-

sensitive. Recovery of heat from the exhaust air leav-

ing the drying chamber offers another opportunity to

conserve energy. This may be achieved using air/air

heat exchangers so that the incoming air is pre-

warmed with heat recovered from the exhaust air.

There have been considerable improvements in the

design of such heat exchangers in recent years. Alter-

natively, heat recovered from the exhaust air may be

used to prewarm the liquid feed to the drier. This can

be achieved by the use of wet scrubbers to wash the

fines from the exhaust air (see above).

Finishing Operations

0017 In the case of some powders, better control over

product quality may be achieved by removing it

from the main spray-drying chamber at a moisture

content higher than the desired final value and finish-

ing the drying in another type of drier. Vibrating

fluidized-bed driers are most commonly used for

this purpose. In this secondary drier some agglomer-

ation of fine powder particles may occur, reducing the

amount of very fine particles in the end product. If the

fine powder recovered from the cyclones is recycled

into the wet zone of the drying chamber, this will

further reduce the amount of very fine particles in the

product (Figure 1). Such a reduction usually facili-

tates the handling of the dry powder and improves its

reconstitution characteristics. The use of such second-

ary driers can also lead to savings in energy as com-

pared with single-stage drying. The powder leaving

the secondary drier may be cooled in another vibrat-

ing fluidized bed.

0018Spray driers are also available which permit multi-

stage drying in one unit. One design features a built-in

fluidized bed (Figure 6). This bed is in the shape of a

ring at the bottom of the chamber. Heated air is

introduced at the top of the chamber and also at the

bottom, fluidizing the particles in the annular bed.

Drying takes place partly in the main chamber and

partly in the fluidized bed. A further supply of air is

introduced tangentially into the chamber. This sweeps

the wall of the chamber, reducing powder build-up

and improving air/powder separation. In the Filter-

mat drier (Figure 7) part of the drying takes place in

the main chamber and the semidried powder falls on

to a perforated, moving belt where it is dried to the

required moisture content in two further stages, with

a short holding period between, by a through flow of

warm air.

0019Spray-dried products containing very fine particles

may be difficult to reconstitute. When added to hot

coffee, for example, dried milk powder may form

clumps which float on the surface of the liquid and

require vigorous and prolonged stirring to disperse.

The most common solution to this problem is to

Static

fluid bed

fig0006Figure 6 A two-stage layout with integrated static fluid bed in

drying chamber: A, air flow; F, feed; P, dried product. From

Masters K (1991) Spray Drying Handbook, 5th edn. New York:

Longmans with permission.

1934 DRYING/Spray Drying

increase the size of the powder particles. This may be

achieved by adjusting the drying conditions, by recyc-

ling of fines, or by agglomeration in a secondary drier

(see above). When these measures are inadequate,

instantizing may be effected by rewetting the surface

of the particles with steam, warm humid air, or a fine

mist of liquid, promoting interparticle collisions by

swirling the wetted powder in a vortex and redrying

the agglomerated powder, often in a fluidized-bed drier.

Control of Spray Driers

0020 Most modern spray driers are equipped with auto-

matic control systems. The main parameter which is

controlled is the outlet air temperature. In the case of

driers equipped with rotary atomizers, the outlet air

temperature is controlled by adjusting the rate at

which the feed enters the chamber. The air inlet tem-

perature is separately controlled by adjusting the fuel

supply. When nozzle atomizers are in use, the feed

rate needs to be constant, and the outlet air tempera-

ture is controlled by adjusting the combustion rate of

the fuel. In both cases, an additional safety device

must be fitted to avoid a rapid rise in outlet tempera-

ture should there be an unexpected drop in the feed

rate. In addition to the above controls, the starting-

up, shutting-down, and cleaning of the plant may be

controlled by timing devices.

Spray-Dried Food Products

0021A very wide range of spray-dried food products is

produced worldwide. A summary of typical operating

conditions and some properties of a selection of dried

powders is presented in Table 1.

Dairy Products

0022Skim milk is spray dried on a very large scale:

some plants handle more than 1 million liters of

milk per day. It is not a difficult material to spray

dry with no tendency to adhere to the chamber

wall and not being very hygroscopic. Many designs

of drying chamber are used, featuring both centri-

fugal and nozzle atomizers. Skim milk powder

produced in a simple single-stage drier contains a lot

of fine particles, is dusty, and is difficult to handle

and reconstitute. These properties can be improved

by recycling fines, promoting agglomeration in

secondary driers or integral fluidized beds or by

rewetting.

0023Whole milk is rather more difficult to dry. It tends

to be rather sticky when hot and can build up a

deposit on the chamber wall. Hammers or other

devices (see above) to assist in the removal of such

deposits can be used. Handling and reconstitution

characteristics can be improved by agglomeration.

In addition, the dried particles may be coated with

fig0007 Figure 7 Filtermat spray dryer (DEC): 1, feed to nozzles; 2, primary drying air; 3, ceiling cooling air; 4, secondary drying air; 5,

cooling air; 6, textile mesh belt conveyor; 7, powder layer; 8, exhaust air to cyclone; 9, product outlet. From Mujumdar AS (1995)

Handbook of Industrial Drying, 2nd edn. New York: Marcel Dekker with permission.

DRYING/Spray Drying 1935