THERMAL DESIGN OF ROTARY DRIER

Contents
DRYING.....................................................................................................................................................3
1. INTRODUCTION.................................................................................................................................3
2. PHYSICAL MECHANISM OF DRYING...........................................................................................4
3. CLASSIFICATION OF DRYERS...................................................................................................5
3.1 Batch Type Dryers...........................................................................................................................6
3.2Continuous Dryer.............................................................................................................................8
Design objective:........................................................................................................................................9
CALCULATION.....................................................................................................................................11
THERMAL DESIGN OF ROTARY DRIER........................................................................................11

1
THERMAL DESIGN OF ROTARY DRIER

2
 THERMAL DESIGN OF ROTARY DRIER

DRYING

1. INTRODUCTION
The drying of materials is often the final operation in a manufacturing process, carried out
immediately prior to packaging or dispatch. The term drying refers to the removal of moisture
from a substance. It is one of the oldest, most commonly used and most energy consuming unit
operation in the process industries.

Three basic methods of drying are used today:

1) Sun drying, a traditional method in which materials dry naturally in the sun,
2) Hot air drying in which materials are exposed to a blast of hot air and
3) freeze drying, in which frozen materials are placed in a vacuum chamber to draw out the
water. The fundamental nature of all drying process is the removal of volatile substances (mainly
moisture) from mixture to yield a solid product.

In general drying is accomplished by thermal techniques and thus involves the application of
heat, most commonly by convection from current of air. Throughout the convective drying of
solid materials, two processes occur simultaneously namely, transfer of energy from the local
environemnt in the dryer and transfer of moisture from within the solid. Therefore this unit
operation may be considered as simultaneous heat and mass transfer operation. Drying processes
and equipment may be categorised according to several criteria, incuding the nature of material
and the method of heat supply and the method of operation.

In some cases, drying is an essential part of the manufacturing process, as for instance in paper
making or in the seasoning of timber, although, in the majority of processing industries, drying is
carried out for one or more of the following reasons:
(a) To reduce the cost of transport.
(b) To make a material more suitable for handling as, for example, with soap powders, dyestuffs
and fertilizers.
(c) To provide definite properties, such as, maintaining the free-flowing nature of salt.

3
 THERMAL DESIGN OF ROTARY DRIER

(d) To remove moisture this may otherwise lead to corrosion.

2. PHYSICAL MECHANISM OF DRYING

Drying does not mean only removal of the moisture but during the process, physical structure as
well as the appearance has to be preserved. Drying is basically governed by the principles of
transport of heat and mass. When a moist solid is heated to an appropriate temperature, moisture
vaporizes at or near the solid surface and the heat required for evaporating moisture from the
drying product is supplied by the external drying medium, usually air or a hot gas.

Drying is a diffusional process in which the transfer of moisture to the surrounding medium
takes place by the evaporation of surface moisture, as soon as some of the surface moisture
vaporizes, more moisture is transported from interior of the solid to its surface. This transport of
moisture within a solid takes place by a variety of mechanisms depending upon the nature and
type of the solid and its state of aggregation.

Different types of solids may have to be handled for drying crystalline, granular, beads, powders,
sheets, slabs, filter-cakes

The mechanism of moisture transport in different solids may be broadly classified into

(i) transport by liquid or vapour diffusion

(ii) (ii) capillary section, and
(iii) (iii) pressure induced transport.

The following term are commonly used in designing of drying systems.

Moisture content of a substance which exerts as equilibrium vapour pressure less than of the
pure liquid at the same temperature is refered to as bound moisture.

Moisture content of the solid which exters an equillibrium vapour pressure equal to that of pure
liquid at the given temperature is the unbound moisture.

4
 THERMAL DESIGN OF ROTARY DRIER

The moisture content of solid in excess of the equilibrium moisture content is refered as free
moisture. During drying, only free moisture can be evporated. The free moisture content of a
solid depends upon the vapour concentration in the gas.

The moisture contents of solid when it is in equilibrium with given partial pressure of vapour in
gas phase is called as equilibrium moisture content.

the moisture content at which the constant rate drying peroid ends and the falling rate drying
period starts is called critical moisture content.

3. CLASSIFICATION OF DRYERS
Drying equipment is classified in different ways, according to following design and operating
features. It can be classified based on mode of operation such as batch or continuous, In case of
batch dryer the material is loaded in the drying equipment and drying proceeds for a given period
of time, whereas, in case of continuous mode the material is continuously added to the dryer and

5
 THERMAL DESIGN OF ROTARY DRIER

dried material continuously removed. Some dryers can handle almost any kind of material,
whereas others are severely limited in the style of feed they can accept. Drying processes can
also be categorized according to the physical state of the feed such as wet solid, liquid, and
slurry. Type of heating system i.e. conduction, convection, radiation is another way of
categorizing the drying process. Heat may be supplied by direct contact with hot air at
atmospheric pressure, and the water vaporized is removed by the air flowing. Heat may also be
supplied indirectly through the wall of the dryer from a hot gas flowing outside the wall or by
radiation. Dryers exposing the solids to a hot surface with which the solid is in contact are called
adiabatic or direct dryers, while when heat is transferred from an external medium it is known as
non-adiabatic or indirect dryers. Dryers heated by dielectric, radiant or microwave energy are
also non adiabatic. Some units combine adiabatic and non adiabatic drying; they are known as
direct-indirect dryers.
Classification according to mode of operation
1. Batch drying
2. Continuous drying

3.1 Batch Type Dryers

1. Tray Dryer

Schematic of a typical batch dryer is shown in figure 1.1. Tray dryers usually operate in batch
mode, use racks to hold product and circulate air over the material. It consists of a rectangular
chamber of sheet metal containing trucks that support racks. Each rack carries a number of trays
that are loaded with the material to be dried. Hot air flows through the tunnel over the racks.
Sometimes fans are used to on the tunnel wall to blow hot air across the trays. Even baffles are
used to distribute the air uniformly over the stack of trays. Some moist air is continuously vented
through exhaust duct; makeup fresh air enters through the inlet. The racks with the dried product
are taken to a tray-dumping station.

6
 THERMAL DESIGN OF ROTARY DRIER

These types of dryers are useful when the production rate is small. They are used to dry wide
range of materials, but have high labor requirement for loading and unloading the materials, and
are expensive to operate. They find most frequent application for drying valuable products.
Drying operation in case of such dryers is slow and requires several hours to complete drying of
one batch.

2. Pan Dryer

The atmospheric pan drier has a jacketed round pan in which a stirrer or mill revolves slowly,
driven from below. The slow moving stirrer exposes fresh surfaces and thereby raises the rate of
evaporation and, hence, of drying. The pan drier is a batch machine and is limited to small
batches. Pan driers may be used first to evaporate a solution to its crystallizing concentration and
then can function as a crystallizer by sending cold water instead of steam into the jacket. The
effect of the stirrer during crystallization prevents the growth of large crystals and promotes
formation of small, uniform crystals. The mother liquor is then drained off and the crystals dried
in the same apparatus.

3 Agitated Vacuum Dryer

7
 THERMAL DESIGN OF ROTARY DRIER

The dryer essentially consists of a jacketed cylindrical vessel arranged for hot water, steam or a
suitable thermal fluid flow through the jacket for heating. Doors are provided on the shell, at the
top for loading the feed material and at the bottom for discharging.

3.2Continuous Dryer

1. Rotary Dryer
The rotary drier is basically a cylinder, inclined slightly to the horizontal, which may be rotated,
or the shell may be stationary, and an agitator inside may revolve slowly. In either case, the wet
material is fed in at the upper end, and the rotation, or agitation, advances the material
progressively to the lower end, where it is discharged.

Figure 1.2: Counter current direct heat rotary dryer

The shell is supported at a small angle to the horizontal so that material fed in at the higher end
will travel through the dryer under gravity, and hot gases or air used as the drying medium are
fed in either at the upper end of the dryer to give co-current flow or at the discharge end of the
machine to give countercurrent flow. One of two methods of heating is used:
A) Direct heating, where the hot gases or air pass through the material in the dryer.
B) Indirect heating, where the material is in an inner shell, heated externally by hot
gases. Alternatively, steam may be fed to a series of tubes inside the shell of
the dryer.

8
 THERMAL DESIGN OF ROTARY DRIER

The other types of continuous dryer are listed below ;-

1. Rotary Dryer
2. Drum Dryer
3. Flash Dryer
4. Fluidized Bed Dryer
5. Screen Conveyor Dryer
Design objective:

 Design the length and diameter of the rotary drier.

Wet-bulb temperature is the dynamic equilibrium temperature attained by a water surface

when the rate of heat transfer to the surface by convection equals the rate of mass transfer away
from the surface. At equilibrium, if negligible change in the dry-bulb temperature is assumed, a
heat balance on the surface is given by:

kg λ(ps − p) = hc(t − tw) ……………………………………..*

where kg = mass-transfer coefficient, lb/(h*ft2*atm); λ = latent heat of vaporization, Btu/lb; ps =
vapor pressure of water at wet-bulb temperature, atm; p = partial pressure of water vapor in the
environment, atm; hc = heat-transfer coefficient, Btu/(h*ft2*°F); t = temperature of air-water
vapor mixture (dry-bulb temperature), °F; and tw = wet-bulb temperature, °F. Under ordinary
conditions the partial pressure and vapor pressure are small relative to the total pressure, and the
wet bulb equation can be written in terms of humidity differences as
Hs − H = (hc /λk′) (t − tw) …………………………………………..**(φ )
Where k′ = lb/(h*ft2) (unit humidity difference) = (Ma /Mw)kg = 1.6kg. It can be also written as
follow:
h G(T G−T W )
ȹ = HmG-HmW=
(mƛW PK G)

18.02∗PA
HmG is humidity of air at temperature T G0F = , where PA and p were known. pA =
28.97 P−PA
partial pressure of water in the air, P = total pressure = 101.325 kPa in this psychometric chart.
HmW is humidity of air at temperature TW0F

9
 THERMAL DESIGN OF ROTARY DRIER

TG is temperature of inlet air 0F; TW =Wet bulb Temperature0F

Ma is Molecular weight of air; λw= Latent heat of vaporization@ Tw0F

Hg is the heat transfer coefficient air,

Kg is the mass transfer coefficient of air and the following ratio is for the constant and it also
constant.

hG/(mPKG) = 0.26 for air at T W and here m=29. If some of the above information is given we can
search from the following Psychometric chart.

10
 THERMAL DESIGN OF ROTARY DRIER

Chart 1.1The graph of temperature and humidity at different percent humidity

11
 THERMAL DESIGN OF ROTARY DRIER

CALCULATION
THERMAL DESIGN OF ROTARY DRIER

12