UNIVERSITI TEKNOLOGI MARA
FACULTY OF CHEMICAL ENGEENERING
CPE591 ( PROCESS ENGINEERING I )
NAME GROUP MEMBER :
NAME GROUP MEMBER STUDENTS ID
1.SUDINI BINTI ABDUL KAHAR
2. NOORFARAHANIS BT ILIAS
3. SITI NURSHAHIRA BT AZIMAN
4. FAIZ SHAFIQ BIN ZAILI
5. NURFARAHIN FATIHA BINTI SUHAIMI
GROUP : EH 221
ASSIGNMENT 2 : CONTINUOUS TUNNEL DRYERS
DATE SUBMIT :
SEMESTER : 4
PROGRAMME CODE : EH221 BACHELOR (HONS) OF CHEMICAL AND
PROCESS AND ENGINEERING
SUBMIT TO : MADAM PROF DR NORAZAH BINTI ABD RAHMAN
[Continuous Tunnel Dryers] Page 3
Continuous Tunnel Dryer
Drying is a method to removal of water from process materials and other substances by
removal of relatively small amounts of water from material. Heat may be supplied by convection
(direct dryers), by conduction (contact or indirect dryers). Drying is used to remove water from
foods for two reasons so that to prevent (or inhibit) micro-organisms and hence preserve the food
and to reduce the weight and bulk of food for cheaper transport and storage. For effective drying,
air should be hot, dry and moving. These factors are inter-related and it is important that each
factor is correct (for example, cold moving air or hot, wet moving air is unsatisfactory).
Difference between Drying & Evaporation
Removal of small amounts of water from
Removal of large amounts of water from
Water is removed as a vapor by air Water is removed as vapor as its boiling point
Drying can be described by three processes operating simultaneously:
1. Energy transfer from an external source to the water or organic solvent
Direct or Indirect Heat Transfer
2. Phase transformation of water/solvent from a liquid-like state to a vapour state
Mass Transfer (solid characteristics)
3. Transfer vapour generated away from the API and out of the drying equipment
[Continuous Tunnel Dryers] Page 4
Figure 1: The figure shown that types of dryer categorized based on direct heat and indirect
Purpose of drying is to reduce the moisture level of wet granules. Besides that, to improve or
keep the good properties of a material such as compressibility .Drying also not only to reduce the
cost of transportation of large volume materials(liquids) but to preservative and to make the
material easy or more suitable for handling.
What are the
What are the equipmentReasonNeedDrying
Over drying (bone
Possible fire hazard
Continuous Tunnel Drying
Direct Heating Moving Solids
Fluid Bed Dryer
Indirect Conduction Dryers
To keepthe residual
moisture low enough
(preferably as a
range) to prevent
Ensure free flowing
[Continuous Tunnel Dryers] Page 5
Firstly, drying method can be classified in several different ways, such as a batch, where
the material is inserted into the drying equipment and drying proceeds for a given time or as
continuous, where the material is continuously added to the dryer and dried material is
continuously removed. There are many type of dryer which is tray dryer, vacuum-shelf indirect
dryers, continuous tunnel dryer and other.
Classified of dryer processing:-
Continuous: large quantities/small residence time
Batch: small quantities/ long residence time
Secondly, dryers can be classified according to heat transferring methods by direct or
Direct method: - Fluidised,Tray ,Tunnel ,Spray ,Rotary Dryers, and other
Indirect method:-Cone, Tumble, Pan Dryers and Other.
Most commercial dryers are insulated to reduce heat losses, and they recirculate hot air to
save energy. Many designs have energy-saving devices, which recover heat from the exhaust air
or automatically control the air humidity. Computer control of dryers is increasingly sophisticated
and also results in important savings in energy.
Thus, for the continuous tunnel dryers are often batch truck or tray compartments operated
in series , as shown in Figure1. Tunnel dryer is a direct continuous type of dryer. It is a largest
[Continuous Tunnel Dryers] Page 6
Figure 2:-Continuous tunnel dryer:- tunnel dryer trucks with countercurrent air flow.
Based on this simple dryer concept, cabinets, trucks or trolleys containing the
material to be dried are transported at an appropriate speed through a long insulated chamber (or
tunnel) while hot drying gas is made to flow in concurrent, counter current, cross-flow or mixed
flow fashion. In the concurrent mode, the hottest and driest air meets the wetted material and hence
results in high initial drying rates but with relatively low product temperature (wet-bulb
temperature if surface moisture is present). For the product heat sensitive, the inlet drying gas
must be at a lower temperature if we use higher gas temperature in counter-current dryers. If the
material to be dried is not heat-sensitive and low residual moisture content is a requirement, one
may employ higher gas temperatures in the countercurrent arrangement as well. For combination
flow or cross-flow arrangements are used less commonly. The latter offer high drying rates but the
tunnels must be designed to fit the trolleys snugly so the drying gas flows through the material
much like a through-circulation packed bed dryer. Standard for operation total drying times can be
handled range from 30 minutes to 6 hours.
Figure 3:- Continuous tunnel dryer :- through-circulation screen conveyor dryer.
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Basic Mechanisms of drying:
There are two basic mechanics is:-
Migration of moisture from the interior of an individual grain to the surface.
Evaporation of moisture from the surface to the surrounding air.
In this continuous tunnel dryer, the materials to be dried are sent to the air heated tunnel for
drying purpose. The material is entered at one end and the dried material is collected at the other
end of the tunnel. The solids are placed on trays or on trucks which move continuously through a
tunnel with hit gases passing over the surfaces of each tray. The outgoing material met the
incoming air to ensure maximum drying and the out going air contacted the wettest material so
that the air was as nearly saturated as possible.
3.0 Mechanism of action
One of the doors of the tunnel is opened and the materials to be dried are placed to the
trolleys and trucks are pushed slowly in the tunnel and then door is closed. The hot air flow can be
counter current, cocurrent or combination. Hot air is circulated and passed through the rail truck
and perforated trolleys. Many foods are dried in this way. The hot air then followed are recirculate
with the help of fans and the material becoming dried. The moist air is passed out through the
exhaust after completion of drying. The door is opened and the trolleys are taken out of the funnel
and some new trolleys with the wet materials are introduced into the trucks and the process is
Basically, for continuous conveyor dryers are up to 20m long and 3 m wide. After that,
when the food is enter it will undergoes drying on a mesh belt in beds 5-15 cm deep. Then, from
Figure 2 and 3, wet material will supplied to the spreader device at the left-hand end is loaded
evenly and in a relatively deep layer on the surface of a slowly moving conveyor belt. The bed of
food will undergoes through by the air initially directed upwards and then downwards the last 1 or
2 sections so that to prevent light-weight, nearly dry pieces from blowing out of the bed. Thus, to
[Continuous Tunnel Dryers] Page 8
improve the uniformity of drying of a thick layer of wet material, sometimes the drier is designed
so that to produce up-through and down-through flow in alternate sections.
Sectionalising the drier is function to control air temperature, humidity, and velocity
independently in several stages to give optimum output and quality. At the first section, air is
supplied with high temperature and moderate humidity because rapid evaporation from the wet
material keeps its temperature down. Meanwhile, at the final stage dryer can be operated with very
dry air at a low enough temperature to avoid damage during that time by slow approach to the
desired final level of moisture content. Mostly, the foods moisture content is dried to 10-15 % and
then the food will transferred to bin dryers for finishing. For the most important feature,
construction of the dryer as two separate conveyors in series makes it possible to discharge the
partly dry material at the end of the first stage. Not only does the mixing aid in making a uniform
product, but also the repiling makes possible a great saving in the flour space required.
The conveyor dryer use centrifugal fan wheels are ordinarily used. A large proportion of
the air passing through the layer of moist material recirculates into the fan, and then to dampers
into the next section, to lattes into fan, may divert part of it; dampers into the next section, to be
replaced by heated fresh air, may divert part of it. The fact hot air temperature is uniform makes it
necessary to control this temperature at a level which is safe to apply to the moist material leaving
the first section and already partly dry. Conveyor dryer equipment has good control over drying
conditions and high production rates. This is because due to used for large scale drying of foods
such as fruits and vegetables are dried in 2 hour to 3.5 hour at up to 5.5 hour). Thus, if we use
some soft, such as starchy or sugary materials may benefit by being subjected first to very rapid
surface drying in such equipment as the belt through drier so that the working surfaces of the
conveyor drier will remain clear and free of sticky build -up. Besides that, conveyor drier has
independently controlled drying zones and is automatically loaded and unloaded, which reduces it
can reduce labour costs. As a result it has largely replaced the tunnel drier.
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4.0 a)Advantages and Disadvantages of Continuous Tunnel Dryer
1. 1. Comparing with the compartmental dryers
tunnel dryers has the advantage of continuous
2. 2.A large amount of materials can be dried.
3. 3. Tunnel dryers are used for drying of paraffin
wax, gelatin, soap, pottery, etc.
1. High labor cost for loading and
2. Thermolabile substances can’t be
3. Drying rage is slow, so time
4. It is not suitable for small scale
5. It is a non-agitated process.
6. Drying of liquid materials is not
7. There is a chance of accident when
doors are opened before stopping the
hot air circulation.
4.0 b) Selection of Drying Equipment
◦During selections of drying equipment depend to;
1. Physical or Chemical properties of the product.
2. Production Capacity (Kg/hr)
3. Initial Moisture Content and final moisture required.
4. Particle Size Distribution.
5. Temperature and Drying Characteristics.
6. Explosion and Toxicological Characteristics.
[Continuous Tunnel Dryers] Page 11
The figure shown that characteristics of dries based on the food for each type of equipment.
[Continuous Tunnel Dryers] Page 12
5.0) Drying concept
To understand the theoretical foundations of drying, there are some definitions of basic concept:
The moisture content of a solid can be expressed on dry or wet basis, but the drying solid
is more appropriate to refer the moisture on dry basis (mass of water that accompanies the
2. Moisture equilibrium (X*):
When a wet solid is in contact with air temperature, humidity and certain constants, the
conditions of equilibrium achieved between the air and the wet solid. The equilibrium
conditions are reached when the partial pressure of water that accompanies the wet solid is
equal to the vapor pressure of water in the air.
3. Free moisture:
Is the difference between the moisture of a solid and moisture equilibrium with the air in
the given conditions. It is the moisture that can be lost after some time of contact with air
given and constant conditions.
4. Critical moisture:
The humidity is critical to a solid point that separates the two periods of drying critical
period ahead and critical period post.
6.0) Drying rate
Drying rate is defined by the loss of moisture from the wet solid per each unit of time, and more
specifically by the differential quotient (-dX/dθ) operating in constant drying conditions, when
are air-conditions (temperature, pressure, humidity and velocity) are constant along time.
Analytically, drying rate refers to the unit area of surface drying, according to the equation:
[Continuous Tunnel Dryers] Page 13
𝑅 = −
Ls is weight of dry solid use (kg)
A is exposed area for drying (m2)
dX is change in moisture content (kg H20/ kg dry solid)
dt is change in time (h)
Figure 9.5-1: rate of drying curve vs. free moisture (Geankoplis, Fourth edition, page 577)
Drying occurs in three different periods, or phases, which can be clearly defined.
[Continuous Tunnel Dryers] Page 14
The first phase, or initial period, is where sensible heat is transferred to the product and the
The second phase, or constant rate period, is when the free moisture persists on the surfaces
and the rate of evaporation alters very little as the moisture content reduces.
The third phase, or falling rate period, is the phase during which migration of moisture
from the inner interstices of each particle to the outer surface becomes the limiting factor
that reduces the drying rate.
Constant rate period (B to C)
In constant rate period, the surface of the solid is initially very wet and a continuous of film of
water exists on the drying surface. This water is entirely unbound water and it acts as if the solid
were not present.
The rate of evaporation under the given air a condition is independent of the solid and is essentially
the same as the rate from a free liquid surface.
Falling rate period (C to E)
At the critical free moisture content Xc, there is insufficient water on the surface to maintain a
continuous film of water. The entire surface is no longer wetted, and the wetted area continually
decrease in the first falling rate period until the surface is completely dry at point D.
Second falling rate period begin at point D where the surface is completely dry. Heat for the
evaporation is transferred to the solid to the zone of vaporization. Vaporized water moved through
the solid into the air stream.
[Continuous Tunnel Dryers] Page 15
Tunnel dryer possesses all the advantages of tray dryer. Moreover, they have a semi-
continuous operation. This type of dryer, which may be up to 24m long with square or rectangular
cross section about 2x2m, consist of a cabinet equipped with rails to move the tray racks along the
drying chamber. The wetted materials are loaded in a tray that is stacked on trolley. The trolleys
are introduced periodically into the end of the tunnel. They advance through the tunnel and are
removed at the other end. A typical tunnel dryer can be operated in counter current or co current
flow of air and trolleys.
In the counter current systems, the air and the product move in opposite direction while in
co-current systems, they move in same directions. Drying times are similar to those obtained in
cabinet tray dryers, while the air flow rate is linked to the total number of trays. Dimensions of the
tunnel are calculated based on the required drying output, drying time and the capacity of a single
[Continuous Tunnel Dryers] Page 16
7.0) Equation of continuous tunnel dryer
Humidification involves the transfer of water from the liquid phase into a gaseous mixture
of air and water vapor .Dehumidification involves the reverse transfer, whereby water vapor is
transferred from the vapor state to the liquid state. Humidity is defined as the humidity H of an air
where the water vapor mixture is defined as the kg of water vapor contained in 1 kg of dry air.
Humidity depends on partial pressure pA of water vapor in the air and total pressure P (assume to
be 101.325 kPa, 1.0 atm abs, or 760 mm Hg).
Equation involve in determine the humidity:
Saturation humidity, Hs
Percentage humidity, Hp :
100 times the actual humidity H of the air divided by the humidity Hs if the air were saturated at
the same temperature and pressure,
[Continuous Tunnel Dryers] Page 17
Percentage relative humidity, HR:
The amount of saturation of an air-water vapor mixture is also given as percentage relative
humidity HR using partial pressures,
Humid heat of an air-water vapor mixture,Cs
The humid heat cs is the amount of heat in J (or kJ) required to raise the temperature of 1 kg of
dry air plus the water vapor present by 1 K or 1°C.
The heat capacity of air and water vapor can be assumed constant over the temperature ranges
usually encountered at 1.005 kJ/kg dry air•K and 1.88 kJ/kg water vapor•K,respectively.
Humid volume of an air-water vapor mixture,VH
The humid volume vH,. is the total volume in m3 of 1 kg of dry air plus the vapor it contains at
101.325 kPa (1.0 atm) abs pressure and the given gas temperature. Using the ideal gas law,
[Continuous Tunnel Dryers] Page 18
Constant rate drying
In the constant-rate period moisture movement within the solid is rapid enough to maintain
a saturated condition at the surface, and the rate of drying is controlled by the rate of heat
transferred to the evaporating surface. Drying proceeds by diffusion of vapor from the saturated
surface of the material across a stagnant air film into the environment and as the rate of mass
transfer balances heat transfer, the temperature of saturated surface remains constant. If the heat
supplied for drying is solely by convection, the surface temperature approaches the boiling point
temperature rather than the wet bulb temperature.
Typical drying rate curve for constant drying conditions
(a) Plot data as free moisture versus time
To obtain the rate-of-drying curve from this plot, the slopes of the tangents drawn to the curve in
figure a) can be measured, which give values of dX/dt at given values of t.
The drying rate R is calculated for each point by
[Continuous Tunnel Dryers] Page 19
R = drying rate in kg H2O/h.m2 (lbm H2O/h.ft2)
LS = kg of dry solid used (lbm)
A = exposed surface area in m2 (ft2)
b)The drying rate curve is then obtained by plotting R versus the moisture content
[Continuous Tunnel Dryers] Page 20
Method using Experimental Drying Curve
The time required for the constant-rate period can be determined directly from the drying
curve of free moisture content versus time.
Method using rate of drying curve for constant rate period
The drying rate is defined as:
The equation can be rearranged and integrated over the time interval from drying from X1 at t1
= 0 to X2 at t2 = t:
Integrating Eq. (9.6-1) for the rate constant period (where R = Rc = constant),
[Continuous Tunnel Dryers] Page 21
8.0) Applications of Tunnel Dryers
There is several application of tunnel dryer. Tunnel dryer are widely used in research
laboratories and Quality Control department for drying glass wires and small apparatus. It also
used in drying of packaging materials plastic caps, spoons, injectable vials, glass containers, etc.
Other than that, it used in sterilization of containers. Last but not least, tunnel dryer used for dry
food in food production.
As we all know, the tunnel dryer is widely used in so many industries because of tunnel
dryer has the advantage of continuous operation compare to other compartmental dryers. Besides
that, large amount of materials can be dried using continuous tunnel dryer. Tunnel dryers are also
used for drying of paraffin wax, gelatine, soap, pottery, etc. which can’t be dried using other type
Even though, there are a lot of advantages of the using of continuous tunnel dryer. It also
has disadvantages. The disadvantages of using tunnel dryer are it requires a lot of manpower for
loading and unloading the materials which increase the cost and not everything can be dried using
continuous tunnel dryer such as thermolabile substances can’t be dried using the dryer.
Furthermore, continuous tunnel dryer through slow drying rate which can drag the time for
consuming the product. It is not suitable for small scale production and non-agitated process. It
also can’t perform the drying of liquid materials.