DESIGN OF A LOW PRESSURE SUPERHEATED STEAM DRYING UNIT

ANDRES FELIPE TAFUR AGUDELO

UNIVERSIDAD DE LOS ANDES FACULTY OF ENGINEERING

MECHANICAL ENGINEERING DEPARTMENT BOGOTA

2007

DESIGN OF LOW PRESSURE SUPERHEATED STEAM DRYING UNIT

ANDRES FELIPE TAFUR AGUDELO

Thesis project presented to obtain the bachelor of science in Mechanical Engineering

Advisor Gregorio Orlando Porras

Mech. Eng. Msc. PhD.

UNIVERSIDAD DE LOS ANDES FACULTY OF ENGINEERING

MECHANICAL ENGINEERING DEPARTMENT BOGOTA

2007

iii

ACKNOWLEDGEMENTS

First of all I would like to thank my parents Jose Tafur and Adriana Agudelo, to

whom, I owe everything I am and have. Because of them I have managed to get

this far. I would also like to thank other members of my family who have been there

when needed: my sister Maria Fernanda and my aunties Sandra Agudelo and

Imelda Tafur who have make me feel they are proud of me.

I also want to thank Professor Orlando Porras, advisor of this work, who has made

possible this project.

Finally I would like to thank my friends: Ana Plata and Julian Quiñonez for

supporting me for so many years, Jairo Herazo for being such a good friend, and

Nadia Chavez, Catalina Rojas, Luisa Gomez and Virginia Covo for being there

when they were needed.

iv

CONTENTS

1. List of tables vi

2. List of figures vii

3. List of appendixes viii

4. Summary ix

5. Introduction 10

6. Chapter One: Drying theory 13

6.1. Introduction 13

6.2. Air drying 15

6.3. Superheated steam drying 17

6.4. The inversion temperature 20

6.5. Drying equipment 21

7. Chapter Two: Drying Model 28

7.1. Introduction 28

7.2. Heat transfer 29

7.3. Mass transfer 31

8. Chapter Three: Design problem Formulation 34

8.1. Introduction 34

8.2. Purposes and applications 34

8.3. Variables to be measured and controlled and other requirements 35

8.4. Study and evaluation of alternatives 38

9. Chapter Four: Dryer Design 43

9.1. Introduction 43

9.2. Steam and vacuum generation 44

9.3. Storing chamber 45

9.4. Drying chamber 49

9.5. Steam traps and air venting 55

v

9.6. Weighing system 58

9.7. Temperature and pressure control 60

9.8. Costs 62

10. Chapter Five: Procedures 64

10.1. Introduction 64

10.2. Selection of operational conditions 64

10.3. Start-up and shut-down procedures 66

11. Nomenclature 68

12. Bibliography 70

13. Appendixes 72

vi

LIST OF TABLES

Table 1: Atomizing nozzle characteristics

Table 2: Storing chamber characteristics

Table 3: Holder plate characteristics

Table 4: Drying chamber characteristics

Table 5: List of costs

Table 6: List of suppliers

vii

LIST OF FIGURES

Figure 1: Drying process seen in a psychometric chart

Figure 2: Schematic representation of a tray dryer

Figure 3: Schematic representation of a tunnel dryer

Figure 4: Schematic representation of a rotary dryer

Figure 5: Schematic representation of a spray dryer

Figure 6: Schematic representation of a fluidized bed dryer

Figure 7: Schematic representation of the experimental set-up used in research

works in universities of Thailand and Singapore

Figure 8: Schematic representation of the experimental set-up used in research

works in universities in Japan

Figure 9: Schematic representation of the experimental set-up used in research

works in a research institute in India

Figure 10: Schematic representation of the changes on steam prior to drying

Figure 11: 3-D representation of the storing chamber

Figure 12: 3-D representation of the drying chamber and distribution of internal

ancillaries

Figure 13: Representative failure curve for vessels under external pressure

Figure 14: Schematic representation of a ball float steam trap

Figure 15: Position of air venting

Figure 16: Image of the UF1 load cell

viii

LIST OF APPENDIXES

Appendix 1: Storing chamber drawing planes

Appendix 2: Drying chamber drawing planes

Appendix 3: Connection and disposition of tanks

Appendix 4: EES program

Appendix 5: Steam trap specifications

Appendix 6: Air venting specifications

Appendix 7: Load cell specifications

Appendix 8: Temperature controller specifications

Appendix 9: RTD probe specifications

Appendix 10: Pressure controller specifications

Appendix 11: Atomizing nozzle specifications

ix

SUMMARY

The purpose of this work is to carry out the design of a low pressure superheated

steam drying (LPSSD) unit. To reach this goal a preliminary study of designs

already working around the world was undertaken. Different possibilities were

considered with the purpose of designing a multifunctional unit capable of drying

foodstuffs primarily, but with the possibility of being implemented in other drying

applications.

Once a preliminary design was developed, a factability and economical study was

carried out in order to have some ideas. Definition and search of several devices

necessary for the correct performance of the drying unit are included as well.

10

INTRODUCTION

Drying is the process by which moisture is removed from some product in order to

fulfill different purposes, mainly the improvement of properties and quality of the

product being dried. This process has several industrial applications, including the

food industry (drying of fruits, vegetables, pulp), the textile industry (drying of paper

and tissues), and some other industries (drying of wood, coal, sludges, etc).

In the process of drying two transport phenomena occur: heat transfer in order to

supply the heat necessary to evaporate the moisture, and mass transfer to take the

evaporated moisture away from the dried product. The heat transfer can be

accomplished by any of the mechanisms of heat transfer (conduction, convection

and radiation) or by a combination of them. Nevertheless the mass transfer can

only be accomplished if a flowing (convective) medium is present in the process.

For a long time the convective medium used to supply the latent heat and carry

away the moisture has been air, which can provide large rates of drying.

Nevertheless, air drying encounters some disadvantages in certain applications:

when drying combustible materials, a great deal of safety considerations must be

taken into account in order to avoid combustion which may lead to fire and

explosion hazards, while in the food industry and due to the presence of oxygen in

the air, it is inevitable the oxidation of the final products.

In the last two decades [1] the concept of using superheated steam in place of air

has gained considerable interest, and several industries are already using this new

technology. In principle, any direct or direct and indirect (convective and

conduction) air dryer can be operated as a superheated steam dryer (SSD),

although the conversion is not simple. With the use of SSD the disadvantages of

11

air drying are overcomed; because there is no oxygen, oxidation reactions are

avoided, and in this way foodstuff quality is highly improved. Moreover, the lack of

oxygen also minimizes the danger of combustion reaction that may lead to danger

hazards.

But the advantages do not stop yet; the SSD also works in a phytosanitary way

destroying all kinds of microorganisms. If the products to be dried are temperature

sensitive, the process can be carried out at near vacuum pressures, allowing the

superheated steam to be at lower temperatures; this version of SSD is commonly

referred as low pressure superheated steam drying (LPSSD). Another important

advantage is that higher drying rates can be achieved if the process is carried out

above the inversion temperature, which is defined as the temperature at which the

SSD overcomes the air drying. This temperature depends on the product being

dried. Finally, the SSD can be implemented along with pasteurization and

sterilization processes [1].

Another important aspect of SSD is that higher efficiencies can be attained [2]. This

is due to the fact that the latent heat given away by the steam in the drying process

can be recovered in the exhaust gas by mechanical and thermal means,

implementing a vapor recompression cycle.

All these advantages come with an increase in complexity as well as inversion

costs. The dryer unit is not just one unit precisely; the whole dryer can be seen as

consisting of three main vessels and some special devices to guarantee the correct

performance of the dryer. The vessels have their own function: the first one, which

can be a boiler, is in charge of producing the steam, the second one, is used to

store the steam, and the last one, is the proper drying vessel, where the steam is

bring into contact with the product to be dried.

12

The special devices to be supplied include steam traps to extract condensates

from the vessels, air vents to guarantee that no air is present in the process,

heating coils to control the temperature of the steam during the process, an electric

fan to maintain the flow and finally a vacuum pump to reach the desired sub

atmospheric pressures. Moreover, instrumentation equipment must be supplied to

record and study how the moisture contents change during the drying process and

temperature and pressure measurements are needed in order to control these two

variables.

13

CHAPTER ONE

DRYING THEORY

INTRODUCTION

Drying simply means the removal of small quantities of moisture contents from a

solid or liquid to an acceptable level. The moisture is usually water, but it may also

be any other volatile liquid. According to this definition, care must be taken in order

to not include mechanical processes such as filtration and pressing as drying

processes [3].

A drying process uses thermal evaporation to vaporize and remove the moisture

from the final products. There are several ways in which this thermal energy can be

transferred to the solid and they are directly related to the mechanisms of heat

transfer [4]. Dryers that use convective heat transfer using a flowing hot fluid in

contact with the product are usually called adiabatic or direct dryers, while those

using external sources of heat such as condensing steam or electrical heaters,

usually through a metal surface (transfer by conduction) or radiant or microwave

energy from an emitting surface (transfer by radiation), are known as nonadiabatic

or indirect dryers. Combinations of these two kinds of dryers are also applied; this

usually leads to a reduction in the size of the dryer.

The product to be dried is usually handled as particulate solids or coarse elements;

prior drying there is usually a milling operation. Nonetheless, large individual

pieces can also be dried as is the case of paper drying. The way in which the

product is handled in industrial dryers depends on whether the unit is adiabatic or

nonadiabatic [4]. In the later case, the product remains stationary in horizontal

surfaces which are heated by external sources; the horizontal surface may be

14

moving, or it may be a cylindrical surface in which agitation or conveyor transport is

applied.

In the case of adiabatic dryers the possibilities are more: the solid may be

stationary in a plate holder and the gas may be blown across the surface of the

product (cross-circulation drying) while in other cases the holder is a screen and

the gas is blown through the product (through-circulation drying); in these two

cases the product is considered to be a fixed bed. Another option is to fluidize the

product, passing the gas at a velocity large enough to suspend the product; if the

gas velocity is increased, the product is entrained in the gas and pneumatically

conveyed while drying occurs. Finally if the product is a liquid, drops of it may be

suspended in the gas stream as in spray dryers.

Drying is present in several manufacturing processes in industry. It may be an

intermediate part of the whole process, but in most cases, it is located at the end

as part of the quality enhancement of the final products. The main objectives of

drying are summarized as follows:

- Storage life: dried products are less susceptible to damage caused by

microorganism’s activity and oxidative reactions. In this way the life of the

products is extended.

- Handling, packing and transportation: these activities are cheaper and

easier to apply in dried products, because the volume and weight is reduced

after the removal of moisture. Besides, dried products flow easier than wet

ones, improving operation of loading and unloading.

- Quality enhancement: many of the properties of the products are improved

(this is the main reason drying is carried out). In the food industry, color and

15

flavor are changed according to the market needs, while palatability and

digestibility are improved as well. In the coal industry, the calorific value of

coal is increased when moisture content is reduced, leading to improved

combustion efficiency.

- Further processing: some industrial operations consume less energy and

are more easily applied to dried products; in milling operations, the

consumption of energy is reduced if the product is in a dry form. Because

wet products are sticky, it is difficult to carry out mixing operations, so a

drying process is required before mixing. Another example occurs in the

treatment of sludges: before incineration, a drying operation is required to

reduce the calorific value of the sludges and recover energy sources that

may be used elsewhere in the process.

In the drying of solids two drying rate periods can be distinguished: a constant

drying rate period, where essentially superficial moisture is evaporated, and a

falling drying rate, in which the internal moisture contents are transport from the

inside of the solid to the surface.

AIR DRYING

In air drying, the latent heat of vaporization necessary to evaporate the moisture

contents in wet products is supplied by a hot stream of air. In mixtures of air and

vapor, each one exerts a pressure on each other; this pressure is known as the

partial pressure of each of the components in the gaseous mixture, and the sum of

them equals the total pressure of the mixtures according to Dalton s law.

16

The difference between the partial pressure of the vapor in the air-vapor mixture

and the pressure exerted by the condensed form such vapor or moisture in the

product is the driving force for drying [2]. In a more rigid way, the actual driving

force for the drying process can be seen as two driving forces instead of one: a

thermal driving force for the transfer of heat as a result of a difference in the

temperatures of the hot air and the product, and a mass transfer driving force

which results from the difference in chemical potential of the moisture in both of its

forms (condensate and vapor).

It is actually this last driving force the one that limits the drying process. When this

force vanishes, an equilibrium is reached between the gaseous phase and the

product being dried; once the moisture concentration in equilibrium (which is

determined by the moisture s chemical potential) has been reached, further drying

is not possible.

When the moisture to be eliminated is water, the equilibrium relations that limit the

process can be seen in a psychometric chart, which shows the interrelationships

between air and water vapor at a fixed pressure. Equilibrium is attained when the

relative humidity is 1.0, that is, when the vapor pressure equals the saturation

vapor pressure; once this state is reached, the humid air is no longer capable of

receiving more water vapor, and any additional quantity of vapor added condenses

immediately. A schematic diagram of a drying process using humid air can be seen

in Fig. 1 [2].

17

Figure 1: Drying process seen in a psychometric chart

In the figure, the line connecting point 1 and 2 represents a heating process prior to

the drying process in order to increase the temperature of the air, and in this way

increased the capacity of air to carry off water vapor. The line connecting points 2

and 3 is the drying process itself; it can be seen how the relative humidity of the

mixture increases as the drying process proceeds, and at the same time, how the

air is cooled as a consequence of the energy it must supply (reduction in specific

enthalpy) to the product.

SUPERHEATED STEAM DRYING

Although the concept was first conceived at the beginning of the XIX century, and

used in Germany after the Second World War [1], it was not until the two last

decades that superheated steam drying (SSD) started to gain credibility, becoming

in an innovative alternative in the drying processes. Basically, SSD uses

superheated steam in place of hot air, as the fluid in charge of providing the latent

18

heat of vaporization and carrying off the evaporated moisture. The operational

principles are the same, and according to studies around the world it is possible to

operate any air drying equipment as an SSD unit [1].

One of the main advantages of using SSD is that the exhaust gas is also steam,

although its specific enthalpy is lower than that of the inlet steam. Because of this,

the latent heat supply to the product in the evaporation of moisture can be

recovered in a cyclic process, or the exhaust steam can be used somewhere else

in order to minimize the use of extra energy sources needed in another unit (an

evaporator heated by electricity).

The main advantages of using SSD can be summarized as follows:

- Because of the lack of oxygen, no oxidation or combustion reactions are

developed during the drying process. This is of great importance when

drying combustible materials, which require a safe handling so as to avoid

fire or explosion hazards. In the industry of foodstuffs, the elimination of

oxidation reactions allows the products to count with an extended life.

- A better quality product is possible when using SSD instead of air drying. An

example of this is found in the food industry, where the use of SSD yields

higher porosity dried products as a result of an evolution of steam within the

product being dried [5].

- Higher drying rates are possible in both drying rate periods. In the constant

rate period, this is possible as long as the temperature of the steam is above

the inversion temperature. For the decreasing rate period the drying rates

are faster due to the lack of a diffusional resistance in the vapor phase, to

the movement of evaporated moisture toward the superheated steam.

19

Furthermore, it has been proved that the formation of case-hardening skins

(layers which introduce a new resistance to mass transfer) is completely

avoided [1].

- SSD can be implemented along with other processes such as

pasteurization, sterilization and deodorization of food products [1].

Although the many advantages of SSD over air drying, there are some limitations

that must be taken into account before selecting an SSD as the choice of drying:

- Complex systems need to be developed. Leakage must be totally avoided,

as well as the infiltration of air. Means must be provided to guarantee that

there is no air present in the drying chamber. Start-ups and shutdowns of

the equipment are more complex than with air drying [1].

- Because the saturation temperature at ambient pressure is high, some

temperature sensitive materials may melt, go through a glass transition

phase change, or be damaged [5]. To overcome this problem the use of low

pressure superheated steam drying (LPSSD) may be implemented; in this

way the saturation temperatures are decreased, and the drying process may

be carried out.

- Condensation of steam is inevitable. Although condensation can be avoided

as the drying process proceeds by means of temperature control, it is

impossible to avoid the condensation during the start-up of the equipment

due to the contact of the steam with the walls of the system, which are at

ambient temperature. Nevertheless, this inconvenient can be minimized by

implementing a pre-warming of the unit walls. Additional ancillaries must be

provided to evacuate the condensed water.

20

According to these limitations, it is clear that the equipment cost (use of vacuum

pumps and other devices to guarantee the correct performance of the unit) is

higher when comparing it with the cost of a simple air dryer. The same is not true

for operational costs if the superheated steam in the exhaust gas of the dryer is

used properly.

THE INVERSION TEMPERATURE

The inversion temperature is defined as the point where both rates of drying, using

superheated steam drying and air drying, are equal; that is, at this temperature

none of the two drying techniques has advantages over the other. Below this

temperature, air drying has a better drying rate, but if the drying temperature is

greater than the inversion temperature, then SSD will reach shorter times of drying.

Before continuing it is important to set that this definition only holds for the constant

drying rate period.

Despite the inferior thermal properties of air compared to those of steam (for given

conditions of temperature and pressure, it can be proved that the convective heat

transfer coefficient is greater in SSD, although the difference is not too big) air

drying reaches greater drying rates below the inversion temperature. The reason

for this phenomenon lies in the temperature gradients created in both types of

drying; as the laws of transport phenomena establishes, the rate of dying (which

depends of heat transfer) equals the product of the convective heat transfer

coefficient and the temperature gradient. So, it is possible for air drying to have

greater rates as long as its temperature gradient is overcomes the disadvantage in

heat transfer coefficient.

21

Such gradient is developed between the drying medium (air or steam) temperature

and the surface product temperature. In the case of air the surface temperature is

quite close to the wet bulb temperature, while for SSD it is the boiling point

temperature at the prevailing pressure [9]. As the drying temperature is increased,

the wet bulb temperature does so, and the temperature gradient does not change

appreciably in air drying. On the other hand, the boiling point temperature is fixed

by the operating pressure, so in the case of SSD the temperature gradient changes

significantly, and this is the reason SSD improves the air drying rates when the

temperature is increased.

In recent studies [9] it has been proved that the operating pressure has a strong

effect on the inversion temperature. As the pressure is reduced, the boiling point of

water is reduced as well; this causes greater temperature gradients for SSD, which

in turn reduce the value at which the inversion temperature occurs.

Nevertheless, a further reduction in pressure can cause a considerable reduction in

the heat transfer coefficient as a result of a reduction in steam flow, so care must

be taken when choosing the operational pressure. This last point will be further

analyzed in Chapter 4.

DRYING EQUIPMENT

As was mentioned previously in this chapter, any air dryer can in principle be

operated as an SSD. Therefore, the following list of equipments applies for both,

air drying and SSD.

22

1. Tray dryers

It is one of the most elemental types of dryers. In these dryers the product is

spread over trays positioned on a cabinet chamber. It is easy to handle and

control, it is operated in a batch form, with the convective fluid flowing across the

surface of the product (cross-flow drying). It is suitable for the dehydration of fruits,

vegetables and meat [1].

Heating may also be accomplished by conduction through the trays using heating

resistances or by radiation from the walls of the cabinet. Fig. 21 shows a schematic

diagram of a tray dryer.

Figure 2: Schematic representation of a tray dryer

1 Taken from: http://www.fao.org/inpho/content/documents/vlibrary/ac306e/img/ac306e11.gi f

23

2. Tunnel dryers

These dryers may be seen as an evolution of the tray dryer. In these units, the

trays carrying the product are moved along a tunnel. The circulation of the gaseous

phase may be parallel with the movement of the product or countercurrent. This

unit is versatile and easy to control and products of all kinds of shape can be

handled. An illustrative representation of a tunnel dryer is shown in Fig. 32.

If they trays are screened, through flow drying can be implemented. There is also

the possibility for heating by conduction and radiation as well.

Figure 3: Schematic representation of a tunnel dryer

3. Conveyor band dryers

This dryer works in a similar way as the tunnel dryer except that the flow of the fluid

is strictly through the solid and not across (through flow drying). In this unit the

2 Taken form: http://www.spiraxsarco.com/us/images/applications/industries/packaged-food/conveyor-dryer.gi f

24

product is spread over a screen band, which moves the product through the dryer.

The direction of flow of the fluid may be upward or downward.

4. Rotary dryers

This dryer makes uses of an inclined cylindrical chamber for the drying process.

The product is moved through the cylinder and heat transfer may be due to a

convective flow and/or conduction through the walls of the chamber. In some cases

the cylinder rotates around its axis, but other applications make use of paddles and

screws within the cylinder to move the material, while the cylinder remains

stationary. Fig. 43 shows an illustration of what a rotary dryer may look like.

Figure 4: Schematic representation of a rotary dryer

3 Taken from: http://www.fao.org/docrep/ fi eld/003/AC059E/AC058E04.gif

25

4. Spray dryers

In this class of dryers, liquids or very fine solid materials are sprayed through the

drying medium, which may move parallel or countercurrent with the product while

the transfer of heat is primarily by convection. The dryer is complex and makes use

of several important devices: an atomization device (when drying liquids), a

dispersion ancillary to introduce and distribute the product, a heating and blowing

system to move the drying medium and finally a device to separate (in the case of

parallel flow) and collect the product.

Despite its complexity, this dryer has found great applicability in the dehydration of

liquids and drying of particles in slurries. Commercial dryers of this type can very

large when compare to simpler dryers as the tray and tunnel dryer. In Fig. 54 it is

shown a spray dryer in which fine solid particles are dried, while a cyclone is used

to separate and recover the product.

Figure 5: Schematic representation of a spray dryer.

4 Taken from: http://www.dtu.dk/upload/institutter/kt/chec/particl e_tech/spray_drying.jpg

26

5. Fluidized bed dryers

In this type of dryer, the product in form of particles is suspended in the flowing

drying medium while the drying process occurs. The direction of flow of the drying

fluid is upward and the transfer of heat is mostly by convection. The velocity of the

fluid must be large enough to overcome the minimum fluidization velocity of the

particle which increases with size particle and the difference between product s

density and fluid density.

Care must be taken to maintain the fluid velocity below the terminal velocity of the

particle, which is the velocity at which the particles are dragged and carried away

with the fluid. For particle-fluid systems in which the difference between the two

velocities is small, cyclones must be used to recover the solids from the leaving

exhaust gas. A schematic representation of this type of dryer can be observed in

Fig. 65.

Figure 6: Schematic representation of a fluidized bed dryer

5 Taken from: http://www.nzifst.org.nz/unitoperations/drying7.htm#tray

27

6. Impinging jets dryers

The drying process is accomplished by hitting the product to be dried with hot, high

velocity, localized jets of the drying fluid. In this way the heat transfer coefficient is

increased, although the control and complexity of these systems is increased as

well. This technique is appropriate for special drying applications such as drying of

tissue papers and textiles.

The dryers just described can work with either air or superheated steam as the

drying medium. More detailed information of these equipments can be found in the

literature [2].

28

CHAPTER TWO

DRYING MODEL

INTRODUCTION

Several models have been proposed [6, 7, 8] around the world to describe the

physical phenomena developed during drying processes using superheated steam.

The differences among them depend on the assumptions made in each model,

which may simplify the solution despite of introducing an error.

A model that has proved to predict experimental results with great accuracy is the

one proposed by Suvarnakuta et al [6]. In this model is mainly based on the

assumption that mass transfer within the solids being dried is controlled only by

diffusion, and that no evaporation is developed in the interior of the solid, but in the

surface, once the moisture has reached it by mass diffusion. This idealization

avoids the need for the estimation of a convective mass transfer coefficient within

the solid as is the case of the model proposed by Tatemoto et al [7].

Another important assumption in this drying model is that the sensitive heat

necessary to raise the temperature of the solid from the initial point to the boiling

temperature of the moisture at the specified pressure (moment at which

evaporation begins) is negligible in comparison with the latent heat that must be

supplied to evaporate the moisture contents of the solid. As evaporation is

considered to be developed only on the surface of the solids, the temperature there

is then expected to be the boiling temperature already mentioned.

Other important assumptions made by Suvarnakuta are:

29

- Isotropic and homogenous properties. This can be accomplished if particles

of similar shape and origin are provided to the dryer. If pieces from a big

sample are to be dried, such pieces should be cut from a common area.

- Initial condensation is neglected. To count with this assumption, pre-warning

of the dryer should be carried out.

- Physical and thermal properties such as density, viscosity, thermal

conductivity, and specific heats are considered as functions of the moisture

contents. The diffusion coefficient should be expressed as a function of

moisture content and temperature.

HEAT TRANSFER

Heat is transferred within the solid by conduction as a consequence of the

development of temperature gradients in all directions. Such temperature gradients

cannot be expected to be equal due to shrinkage effects in the solid. As the

stresses developed in different regions of the solid differ from one another

depending on the internal structure, it is expected that shrinkage is not uniform

throughout the body.

The starting point to obtain an expression for the heat transfer is the diffusion of

heat equation. For the case of rectangular coordinates, this equation takes the

form:

(1)

30

The last term in the left hand side can be neglected since no internal evaporation is

considered to take place within the body of the solid. Also, by assuming steady

state, the density and heat capacity can be considered to not change with time.

Moreover, the thermal conductivity is equal in all directions since isotropy was

considered to be true. With these modifications, the heat transfer equation reduces

to:

(2)

Similar expressions can be obtained if cylindrical or spherical coordinates are

used. It is important to establish that this equation is applicable in the range from

the initial temperature to the boiling temperature.

The boundary conditions necessary to solve this equation include an initial

temperature of the product just before drying begins, and a convective boundary

condition that establishes that energy enters the solid by convection transport from

the superheated steam. These two conditions have the form:

(3)

(4)

In the last expression, the first term in the right hand side represents the heat

transferred by convection from the superheated steam to the product´s surface.

Part of this heat is used to vaporize the moisture, while the rest in transported to

the interior by conduction.

31

The transfer heat coefficient can be obtained in two ways: it can either be

measured, or it can be calculated using correlations developed and found in the

literature. In the first case, a method of measuring proposed in some works [5, 6]

calculates the coefficient from information on the drying rates of evaporation during

the constant rate period:

(5)

When using this method, measures should be made at different working pressures

in order to obtain a correlation between the two variables. Alternatively a

correlation proposed and used by Tatemoto [7] can be used to estimate the

Nusselt number and:

(6)

MASS TRANSFER

As was already mentioned, the mass transfer is considered to occur by mass

diffusion within the products. Moreover, since no evaporation is considered in the

interior, no convective mass transfer is developed there. The gradient force for

mass transfer in then the difference between the moisture content in the interior

and the moisture content in the surface which is considered to be in its equilibrium

state at the boiling temperature and pressure of the drying medium.

32

By applying Fick´s law of mass diffusion in rectangular coordinates, an expression

that describes the mass transfer within the product is obtained:

(7)

Once again similar expressions can be obtained for cylindrical and spherical

coordinates. The dependent quantity in the last equation is the free moisture

content, which is the difference between the instantaneous moisture content, and

the moisture at equilibrium:

(8)

The boundary conditions for this differential equation include an initial moisture

content prior to the beginning of the drying process and a boundary condition at the

surface of the product:

(9)

(10)

This last expression establishes that the moisture content at the surface of the

solid equals the equilibrium moisture contents as was already stated. This is a

consequence of the lack of a mass transfer resistance between the surface and the

superheated steam; it is clear that there is no resistance there because both the

33

solute and solvent are water, and water does not have a self resistance to the

movement of its molecules.

Modifications to these equations can be implemented to model internal evaporation

and convective mass transfer in the interior of solids. For further information you

may consult in the literature [7].

34

CHAPTER THREE

DESIGN PROBLEM FORMULATION

INTRODUCTION

Before starting with the design stage, it is necessary to establish what

characteristics the equipment should have, and what it is made for. The main

purpose of the unit and some side usage possibilities must be defined, so as to

have a starting point for the design.

Next, the variables to be measured must be specified. The variables chosen must

be selected according to their influence in the process and drying model. In this

way experiments to determine the best conditions to dry different products can be

carried out.

Finally, a control strategy must be implemented to guarantee the correct

performance of the unit. Variables and parameters that have a strong effect on the

equipment performance must be identified; then a control technique must be

developed to maintain such variables at the desired level. As this is a laboratory-

scale dryer, and not an industrial one, research on works done at such level

around the world can be a very important source of information and ideas to

accomplish the dryer´s design.

PURPOSES AND APPLICATIONS

At this moment a very important question arises: what is this drying unit for? There

must be a main objective to answer this query; as this is a laboratory unit, the main

35

objective should be testing. This dryer is then meant to be used as a research tool

to study the phenomena of drying on different products (foodstuffs, wood, etc)

when using low pressure superheated steam drying.

According to this, some purposes of this drying unit are:

- Testing of different operational conditions to find the best performance of the

drying process.

- Determination of drying kinetics to design industrial drying units.

- Testing and comparison of superheated steam drying with conventional air

dying.

- Determination of convective heat transfer coefficients.

- Scaling and estimation of energetic costs for industrial dryers.

It is important to clarify that this unit is designed to dry foodstuffs and heat sensitive

materials primarily (this is where LPSSD is being used the most, around the world),

but testing in other products is also possible. Moreover, there will be restrictions

and limitations on operational conditions depending on the capacity of the ancillary

inside the drying chamber to withstand high temperatures and low pressures.

VARIABLES TO BE MEASURED AND CONTROLLED AND OTHER

REQUIREMENTS

In drying testing the most important variable is perhaps the moisture content of the

product under drying. As the main purpose of drying is to eliminate or reduce the

36

moisture content of products, a measured of how much moisture remains after

drying can indicate how well the drying operation is functioning. To study this

variable, a load cell is used to measure the change of weight of the product sample

as moisture evaporates; such device must be able to work under conditions of high

temperature and humidity, and have a great accuracy and the capacity of

measuring changes of the order of milligrams. These measurements must be

recorded as time passes by in order to have data on drying kinetics.

There are another two important variables in this operation: the steam temperature

and the vacuum pressure inside the drying chamber. Nevertheless these two

variables are considered to be operational parameters as they are selected prior to

the start of the process. Therefore, these variables must be controlled to keep

them at their set point in order to guarantee the correct performance of the unit and

obtain trustable results.

The temperature of the steam changes over time as it supplies the energy

necessary to evaporate the moisture. Therefore, that energy must be supplied

back to the steam, or the temperature will drop toward the boiling point at the

corresponding operational pressure, which may lead to condensation of the steam,

spoiling the whole process.

To control the temperature and avoid the condensation problem, a heating

resistance will be used to supply the energy the steam gives away in evaporating

moisture. Such resistance will be operated by a temperature controller in charge of

determining how much energy must be supplied by the resistance. This controller

must be connected to a temperature sensor located inside the steam containing

chamber to measure the temperature of the steam, and in this way provide the

necessary information for the controller to work properly.

37

On the other hand, the operational pressure is also subjected to changes; as the

moisture content is evaporated, it is transferred to the steam, therefore the mass of

steam in the chamber increases, which in turn produces a reduction in its specific

volume and according to the ideal gas law an increase in pressure if temperature is

held constant. These variations in the operational pressure may lead to incoherent

and unreliable results, because the heat transfer coefficient depends on such

pressure.

Pressure must be then kept at its set point by controlling the vacuum pump

connected to the drying chamber. This will be accomplished by using a pressure

controller, which measures the pressure in the chamber in order to manipulate the

vacuum pump.

There are another two important variables that must be controlled: the presence of

condensable phases as well as the presence of air inside the drying chamber.

More than controlling them, they must be reduced as much as possible. As was

already stated it is inevitable to avoid the condensation of steam at the start-up of

the equipment, so a means to extract this condensed phase must be supplied. This

is accomplished by using a steam trap; this device opens in the presence of a

condensed phase, allowing its passage through, and closes once such phase has

been evacuated, maintaining the vapor phase inside.

Now, to withdraw the air from the drying chamber, an air venting will be used. This

ancillary works in a similar fashion as the steam trap: it allows the passage of air

through it, while maintaining the steam inside the drying chamber.

Another important requirement is that the drying equipment must be rigidly sealed.

Nor steam runaways, neither air entrance can happen. The sealing is also

necessary to maintain operational conditions inside the unit. This requirement

38

introduces a cumbersome design; it will be necessary to maintain auxiliary

ancillaries such as sensors, heating resistances and the fan inside the sealed

equipment. To accomplish this, only wires will be allowed to cross the equipment

boundaries.

Finally, to minimize energy losses to the surroundings, it is necessary to provide

the drying chamber with insulation. For this requirement, an insulator such as glass

fiber can be used.

STUDY AND EVALUATION OF ALTERNATIVES

The different kinds of industrial dryers have been already studied in the first

chapter. An additional research on the laboratory scale dryers can provide some

more ideas for the design. The bibliographic research consulted is based on dryer

units developed in eastern Asia, where this technology is growing rapidly. Most of

the research work on this field has been carried out in King Mongkut´s University of

Technology in Thailand and the National University of Singapore. Such works are

based on the experimental set-up shown in Fig. 7 [5].

39

Figure 7: Schematic representation of the experimental set-up used in research works in

universities of Thailand and Singapore

This drying unit consists of three main chambers: a boiler (1) to generate the

steam, a steam reservoir (3) to storage the steam prior to drying, and a rectangular

drying chamber (7) where the steam and product samples are bring into contact; it

is in this chamber where the drying process takes place. Additional ancillary

include: steam valves (2) and (6), a steam trap (5), a steam distributor (8), an

electric fan (9), the sample holder (10), a heating resistance (11), a temperature

sensor (12), a vacuum break-up valve (13), the insulator (14), a vacuum pump (15)

the weight measuring system (16) and a PC to record information (17).

Another important source of information on the field of SSD [7 and 10] comes from

works done in the universities of Shizuoka and Nagoya, both located in Japan.

Their research is based on a drying unit similar to the schematic representation

shown in Fig. 8 [7].

40

Figure 8: Schematic representation of the experimental set-up used in universities of Japan

This unit consists of a cylindrical chamber (1) where drying takes place, an electric

fan (2), a heater (3) the sample holder (4), a damper (5), the vacuum pump (6) and

a balance (7).

A third source of articles on the subject of SSD is located in the Indian Institute of

Technology in New Delhi, India. There, experiments are carried out in a drying unit

such as the one represented schematically in Fig. 9 [11].

41

Figure 9: Schematic representation of the experimental set-up used in a research institute in India

Several similarities and differences can be found in the dryers used. All of them

share similar devices, but the first one is the most specific because it shows how

the steam is generated and transported to the drying chamber; besides, it also

shows several ancillaries necessary for the correct performance of the equipment.

The drying chamber can be of box shape or cylindrical, and the drying process can

be carried out in batch or continuous form. All of the units are supplied with fans to

make circulate the steam inside the drying chamber, as well as temperature

controllers and heating systems to maintain the temperature inside. All this agrees

with the requirements presented in the previous section.

The cylindrical shape may be better for the drying chamber because the flowing

patterns can be controlled more easily, avoiding excessive turbulence when the

42

steam contacts the walls of the chamber. It is not totally clear what is the location

of the fan and heater. The later should be positioned in front of sample holder as to

create a longitudinal flow, while the former should be located in the suction zone of

the fan as to heat the steam just before it makes contact with the product. The

temperature sensor can be placed just after the fan discharge; in this way the

temperature being controlled is that of the steam making contact with the sample.

Now, the way in which weight measurements are made differs in the three cases.

In the two last dryers a balance supporting the sample holder is used, although one

of the dryers locate the balance under the drying chamber, while the other does so

above the chamber. The main problem with using these balances is that they must

be located outside the chamber, so it is necessary to extract from the chamber

mechanical structures to connect the sample holder and the balance. These

structures may be subjected to friction due to the contact with the walls of the

chamber, just in the zone where such structures come out of the chamber. This

friction may lead to misleading measurements causing a lack of reliability on the

results obtained.

The solution to this problem was somehow already proposed in the preceding

section: the use of load cells, as the first dryer does. The weighing system is

positioned inside the chamber, and only the wires that connect the load cells with

the recorder system are allowed to cross the walls of the equipment. In this way,

the measurement is made inside the chamber, while the signal is transmitted

through the wire and recorded outside the drying chamber.

All the dryers are of the industrial cabinet type: the product is held still while the

steam is circulated through the drying chamber. This is probably the best choice,

as is the less complicated and cheaper; it is important to remember that this design

has testing purposes at a laboratory level, so there is no need to complicate it.

43

CHAPTER FOUR

DRYER DESIGN

INTRODUCTION

According to the problem formulation the dryer must consist of two main chambers:

one for storing and preparing the drying operational conditions, and a second one

for the drying process itself. As the department of Mechanical Engineering recently

bought a boiler, the dryer can count with the generation of superheated steam.

There is also need for a vacuum pump to produce the necessary conditions.

The dimensions and shape of the recipients for storing and drying are chosen

based on the experimental set-ups found in the literature. Proper thicknesses must

be chosen for both recipients to avoid mechanical failure. Both recipients must be

insulated to minimize the energy losses to the surroundings.

The operational conditions are also based on the works developed in the literature.

The dryer will be designed to work at absolute pressures of 10 kPa minimum and

temperatures as high as 100 °C.

To guarantee the correct performance of the equipment, steam traps must be

installed to extract condensed water in the start-up of the process. Moreover, to

secure the development of SSD, air vents are necessary to remove the air initially

present in the two chambers.

To study how the moisture content changes with time a weighing measuring

system using load cells will be implemented. This system must be positioned inside

44

the drying chamber. Temperature and pressure systems must also be designed as

these variables tend to change as was already discussed in the previous chapter.

Finally a cost study is carried out including manufacturing costs for both recipients

and prices of auxiliary ancillaries coming from the national and international

market.

STEAM AND VACUUM GENERATION

To generate the superheated steam, the dryer is designed to be connected to the

boiler unit of the Energy Transformation Laboratory. According to this boiler´s

specifications, the steam can be produced as a saturated steam at any given

pressure above 101,3 kPa. This means, that if it is chosen to use steam at this

pressure, its temperature would be 100 °C (saturation temperature at the specified

pressure), being this the lowest temperature that can be obtain from the boiler.

As the dryer is supposed to work at temperatures as low as 70°C, a cooling

process is required on the steam. Prior to such process it is necessary to reduce its

pressure in order to provide some degree of superheating, otherwise any cooling

will cause steam to condense. This whole process will be carried out in the storage

chamber; an extensive description of this process follows in the next section.

The saturated steam is transported to the storage recipient by means of pressure

drop. This can be achieved by applying vacuum to the storage chamber. After, the

steam has been treated to produce superheated steam at the desired drying

conditions, it is moved to the drying chamber; this movement is also a result of a

pressure drop, so this last recipient must be at lower absolute pressure than the

first one.

45

To produce the vacuum conditions necessary (as low as 10 kPa.) a centrifugal

vacuum pump can be used. Unfortunately, the department lacks of a vacuum

pump capable of producing such reduced pressures. It is then necessary to get a

vacuum pump. From the research done in costs (at the end of this chapter) it is

proposed to get a Fisher Scientific single stage pump capable of reaching vacuum

pressures of 75 torr. The pump makes use of a 5/8 in O.D. hose for the suction

connection, so it would be necessary to place connection ports in both chambers.

In principle, the pump would be used to produce the vacuum conditions in both

recipients, connecting it to them in decreasing order of absolute pressure.

STORING CHAMBER

This chamber was initially meant for storing purposes only, as seems to be the

case in one of the laboratory dryers of the previous chapter. But because of the

conditions at which the steam is produced, this tank has also the responsibility of

adequating the steam for the drying conditions.

Basically in this chamber, the saturated steam is subjected to: a reduction in

pressure at constant volume to generate superheated steam, and a cooling

process to reduce the temperature to the desired drying temperature.

To illustrate what happens in this recipient, let us take a look at an example:

suppose it is desired to dry at a pressure of 10 kPa and a temperature of 75 °C.

The changes that suffer the steam in this first chamber are represented in the P-h

diagram shown in Fig. 10, obtained by using the program EES. The initial point

represents the state of saturated steam obtained from the boiler (it is located on

the saturated vapor line of the diagram); from there, the vertical line represents the

reduction in pressure to a pressure just above the operating pressure (in this way,

46

the steam will be able to flow to the next chamber by pressure drop). It can be

seen that the steam has been superheated; in addition it is observed that the

temperature drop is not considerable, which agrees with the value of the Joule-

Thompson coefficient for water at the prevailing conditions in the chamber (0,067

K/kPa). Next the horizontal line represents the cooling process to obtain a

temperature close to 75 °C. At this point, the superheated steam is almost ready

for the desired drying conditions. The remaining two lines represent changes in the

drying chamber and will be discussed in the next section.

2438 2500 2563 2625 2688 27504x100

101

102

2x102

h [kJ/kg]

P [k

Pa]

105°C

90°C

75°C

60°C

Water

Figure 10: Schematic representation of the changes on steam prior to drying

The cooling process is attained by adiabatic mixing of the superheated steam with

a little quantity of liquid water at ambient conditions. The necessary amount of

water to reach a certain temperature can be estimated by material and energy

47

balances remembering that the system is closed. A detailed procedure to calculate

the amount of water can be found in the next chapter.

To introduce the liquid water to this chamber it is necessary to use an atomizing

nozzle as to spray the water as very fine drops, otherwise, there will not be enough

time for mass transfer, and the liquid may go directly to the bottom of the recipient

where it will be evacuated by the steam trap.

Differing atomizing spraying nozzles can be found among the products of Spraying

Systems de Colombia. The atomizing nozzle is selected according to its capacity

and pressure drop across it [16]. The 1/4LN-SS4 nozzle is chosen as a first

alternative; its characteristics are shown in Table 1. Further information can be

found in appendix 11.

Characteristic Pressure drop (psi) 30 Capacity (gal/hr) 3,5 Inlet connection ¼” NPT Material 303 Stainless steel

Table 1: Atomizing nozzle characteristics

The storing chamber must also be equipped with a steam trap to extract

condensed water and an air venting to remove air. Information on these two

ancillaries is provided ahead in this chapter.

The chamber is mechanically designed against failure according to the norm from

the ASME to design recipients under pressure [12]. This topic will be treated

together with the mechanical design of the drying chamber in the next section. The

dimensions of the recipient are chosen arbitrarily to be smaller than the drying

chamber. The main characteristics of the storing chamber are summarized in Table

2.

48

Characteristic Height (cm) 25 Diameter (cm) 20 Thickness 3/8” Material Structural steel

Table 2: Storing chamber characteristics

A 3-D scheme of the storing chamber obtained from SolidEdge can be observed in

Fig. 11; detailed drawing planes are shown in appendix 1.

Figure 11: 3-D representation of the storing chamber

49

DRYING CHAMBER

The drying chamber is where the drying process takes place by bringing into

contact the superheated steam and the product sample. Referring back to Fig. 10,

the superheated steam is brought from the storing chamber (second vertical line)

by pressure drop. At this point, some heating may be required to reach the drying

operating temperature (second horizontal line)

As was already stated in preceding chapters, the energy given up by the steam to

evaporate the sample´s moisture content must be supplied back to it by a heating

resistance. This heating system has also the responsibility of providing the energy

just mentioned in the preceding paragraph for the steam to be at the operating

temperature during the start-up. It is important to clarify, that although the steam

reaches the drying temperature in the storing chamber, a temperature drop (due to

the pressure drop) is developed during the transport from this tank to the drying

recipient.

If the start-up procedure is chosen properly (to be discussed in the next chapter) it

is expected than an increment of no more than 5 °C will be enough. This energy, to

be supplied by the heating system, must be transferred very fast because once the

steam enters this tank, the drying process begins. Based on calculations using

EES a supplied power of 363,3 W (see appendix 4) is necessary. To facilitate the

temperature control strategy, this power will be supplied by two resistances, each

of 181,6 W; one of them will be on the start-up period, turning off once the

operating temperature is reached, while the other will remain working maintaining

the steam temperature.

These resistances can be manufactured in the national market by specifying the

power they must supply and the space they are supposed to occupy. Only the

50

wires for electrical connection are allowed to cross the walls of the tank. The

resistances will be positioned inside the chamber in cantilever form. Detail

information on this can be checked in appendix 2.

Inside the tank, the superheated steam is circulated across the surface of the

product by means of an electrical fan positioned just between the heating system

and the stationary sample holder in such a way that the fan suctions hot steam and

discharges it directly to the sample.

The electric fan as well as its driving motor is to be totally located inside the sealed

chamber; once again only the wires to connect the motor to a source of electrical

energy are allowed to cross the walls of the tank. To avoid damage of the motor by

the presence of steam, it must be isolated in an internal smaller chamber. It is

proposed in this design that a commercial electrical fan with sealed motor

purchased in the national market be used.

The sample holder will be hanging from the measuring system device. This holder

is a simple circular plate where no more than 200 gr of product sample can be

placed. The main characteristics of this plate are summarized in Table 3.

Characteristic Height (cm) 4 Radius (cm) 15 Thickness (cm) 0,5 Total weight (gr) 430 Maximum capacity (gr) 200 Material Structural steel

Table 3: Holder plate characteristics

The maximum amount of sample to be placed in the plate is limited by the capacity

of the load cell; this topic will be treated further, later in this chapter. The

51

dimensions chosen of the plate, as well as the tank dimensions, are inspired on the

laboratory scale designs of the previous chapter.

The drying chamber is of cylindrical form with rounding ends on both sides. A list of

characteristics can be found in Table 4, while an isometric diagram showing the

inside distribution of the heating resistances, electric fan and measuring system is

shown in Fig. 12. Detailed planes and dimensions of the drying chamber are

attached in appendix 2.

Figure 12: 3-D representation of the drying chamber and distribution of internal ancillaries

52

Characteristic Length (cm) 90 Diameter (cm) 60 Thickness 3/8” Material Structural steelInsulation thickness (cm) 5 Insulation material Glass fiber

Table 4: Drying chamber characteristics

The insulation thickness is chosen by approximated calculations of the heat losses

to the surroundings. By supposing resistances due to internal convection, drying

chamber wall conduction, insulation material conduction and external convection,

the total resistance to the flow of heat can be approximately estimated as:

(11)

Information on the convective heat transfer coefficients is obtained from the

literature [15] by approximating the internal coefficient as air moving at 7 m/s

(8,353 W/m2*K) and the external coefficient as still air (3,4 W/m2*K). The heat

losses can then be calculated as:

(12)

The area of transfer is the sum of all the wall´s areas of the chamber. These

approximations are used only to have an idea of how much heat is lost to the

surroundings depending on the insulation thickness. When this variable equals 5

cm (a common value in industry) the heat losses are of the order of 0,016 W, less

than 0,1% of the heat supplied by the heating resistances.

53

The drying chamber thickness, which happens to be the storing chamber thickness

as well, is chosen by applying the design procedures of the ASME for vessels

subjected to external pressure [12]. The failure in this type of application may be

due not only to plastic yielding at stresses above the yield limit, but also by bucking

at much lower stresses.

The collapse by buckling occurs at a critical stress in the elastic region of the

material. Such stress depends on the geometrical dimensions of the vessel and

can range from a minimum critical value (for long cylinders) to the yield limit. In this

way [13] vessel subjected to external pressure can be classified as:

- Long cylinders: Fail by buckling; the critical pressure is independent of

length, so that all cylinders in this group fail at the same pressure.

- Short cylinders: Fail by plastic yield. This group is characterized by very

thick cylinders.

- Intermediate length cylinders: Collapse by buckling. Nevertheless, in this

group the critical pressure depends on the length, so there are different

values of the critical pressure.

The failure curve [13] shown in Fig. 13 can help in understanding the classification

above. It also shows expression to calculate the critical pressure for failure for the

different kind of cylinders.

54

Figure 13: Representative failure curve for vessels under external pressure

The point at which a cylinder becomes large can be estimated for materials of

µ=0.3 as:

(13)

For the dimensions established above, we obtain a critical length of 747.5 cm, so

we can expect our design to fail as either an intermediate cylinder or a short one,

although this last alternative is quite unsure, as our design is of thin wall and not

thick.

55

To include all kind of cylinders and materials, ASME has developed two charts to

estimate the allowable pressure a vessel can support without collapsing. Such

charts have been prepared using a safety factor of 4. The first chart relates

geometrical properties to mechanical properties for different d/th ratios. The

mechanical properties are grouped in what is called FACTOR A, which is:

(14)

For the geometrical dimensions chosen in this design a FACTOR A of 0.001 is

obtained. With this value and using the second chart, a FACTOR B can be

estimated. This second chart is a stress-strain curve for all materials at different

temperatures. Once this last factor is determined, the permissible pressure can be

calculated from:

(15)

For the maximum temperature of work expected, a value of 12000 for FACTOR B

is obtained. This leads to a permissible external pressure of 640 kPa, which is far

above the atmospheric pressure at which our tanks will be subjected. Therefore,

there is good confidence no failure will develop.

STEAM TRAPS AND AIR VENTING

In previous sections it has been outlined the need of steam traps to extract

condensed water, and air venting to remove the air, in both tanks. The steam traps

56

can be classified in three groups according to its way of operation [14]:

thermostatic traps, which are activated by changes in fluid temperature below the

condensation temperature, mechanical traps, which operate when sensing a

difference of density, and thermodynamic traps operated by changes in fluid

dynamics such as the formation of flash steam.

For the operational conditions of both tanks, the mechanical steam traps are highly

recommended. From this group, the ball float trap is chosen; a schematic diagram

of such a trap is shown in Fig. 14 [14].

Figure 14: Schematic representation of a ball float steam trap

This trap senses the difference of density between the steam and the condensate

water. As long as there is condensate in the trap the ball will float whilst leaving

open a discharge orifice. Once the condensed water is drained, the ball falls back

to its seat closing the valve and avoiding the lost of steam.

57

On the other hand, air venting allows the extraction or air initially present in any

recipient. It remains open as long as there is air in the chamber, and closes to

avoid the runaway of steam. The location of the air venting is very important. For

small recipients the steam entering acts as a piston pushing the air in his way in.

For this reason, the air venting should be located opposite to the steam inlet.

According to this, there are two possibilities: to incorporate the air venting to the

steam trap with the steam inlet at the top, or to locate it alone in the top of the

recipient with the steam inlet in an inferior zone. This last option is depicted in Fig.

15 [14], and chosen for this design.

Figure 15: Position of air venting

The steam trap and air venting are chosen according to their discharge capacity

and pressure drop across them. A national supplier of these devices,

SteamControl, provides graphs for the selection of the appropriate steam traps and

air venting (see appendixes 5 and 6). The steam trap selected offers a discharge

58

rate of 0,05 Kg/s, approximately a 5% of the total amount that can be stored in the

storing tank at the lowest pressure; the condensation expected is not as high, so

this trap is expected to satisfy the conditions required.

On the other hand, the air venting chosen has a discharge capacity of 5670 cm3/s

which means, which means that it could evacuate the air expected in the storing

chamber in less than two seconds, and that present in the drying chamber in about

a minute. Further information and drawing diagrams of these two devices can be

found in appendixes 5 and 6.

WEIGHING SYSTEM

In the preceding chapter it was stated the requirements that the weighing system

should fulfill. It was stated that the whole system needs to be placed inside the

chamber; in this way more accurate data can be obtained. It was also proposed the

use of load cells as the measuring device. After a wide search, a load cell

satisfying the required conditions was found. The supplier of this proposed

alternative is LCM Systems from the United Kingdom and the load cell is

referenced as UF1 (Low range isometric force sensor). An image of the load cell

can be appreciated in Fig. 166.

6 Taken from: http://www.lcmsystems.com/iqs/sid.07635940011711042605731/tension_and_compression_load_cells.html

59

Figure 16: Image of the UF1 load cell

As it can be seen, the cell is equipped with four terminals for sensor signal

connections, an actuator rod from where the sample holder can be hung and four

mounting holes to attach the cell to a structure in vertical position. Moreover, the

load cell is supplied with the instrumentation necessary to monitor and record the

measurements (if desired).

According to the design of the holder and maximum allowed weight for the sample,

a total of 630 gr must be supported by this cell. Because of this, a 1000 gr range

cell is chosen. Further information and schematic diagrams of this cell can be

found in appendix 7.

60

TEMPERATURE AND PRESSURE CONTROL

As was already stated, two important heating process must be controlled in the

drying chamber: an initial heating to supply the heat the steam looses by pressure

drop while being transported from the storing chamber, and a constant heating

during the drying process to provide the energy the steam gives up for moisture

evaporation.

The first heating process is designed to work with the two heating resistances. For

a bad case scenario (5 °C heating as a consequence of a bad selection procedure)

the power required would be 360 W approximately. For the second heating

process an idea of how much power is required can be inferred by calculating the

heat transferred from the steam to the product sample in a typical set of conditions.

Let us suppose the drying takes place at 10 kPa and 75 °C; an experimental

convective heat transfer coefficient is 8,353 W/m2*K [6]. The area and sample´s

surface temperature (boiling temperature) are known, so an estimated value of

13,6 W is expected to be supplied by the resistance during drying. This value is

much smaller than the heating power required initially, so it will not be necessary to

use both resistances while the drying process takes place.

This design proposes two different control techniques for each resistance: an on-

off control on one of the resistances for the starting heating and a proportional

control for the other resistance working during the whole process.

The first control will be carried out by a simple thermostat available in the national

market. This device will provide power to one of the resistances only in the starting

heating process; its set point must be chosen so that the resistance is turned off

once the temperature is sufficiently close for the other resistance to reach the

operating temperature. For example, if the drying temperature desired is 75 °C, the

61

thermostat must be set at 73 °C. In this way, the resistance controlled here will be

off during the rest of the process. It can be seen that this first control is quite

simple, and is designed only to work only during the start-up of the equipment.

The second control is of much more importance because it is in charge of

maintaining the temperature during the whole process. For this reason, a

proportional control is selected. In this application, a DIGIMEC FH-1 controller will

be used (the department counts with this equipment already). This controller is

capable of supplying 3 A at a voltage of 250 V, which means that the power it can

supply is 750 W, which is quite sufficient for our application. More information on

this controller can be found in appendix 8.

This controller can be activated by different kinds of sensors, including types J, K

and Pt100. This last one is chosen for this application. From the stock of products

of Omega, the RTD probe PR-11 was selected by recommendation of the same

company. This device consists of a variable length straight sheath insulated by a

jacketed cable with stripped lead terminal ends. The probe will be immersed in the

drying tank, and only its cables will be allowed to leave the tank. Additional

information on this probe is annexed in appendix 9.

To maintain the pressure at the specified drying condition, a pressure controller

must be used. This type of device is equipped with a pressure sensor and an on-off

actuator to control the performance of the vacuum pump. This type of device can

be operated by fixing the desired pressure with the appropriate set scale value, or

in differential form taking as reference the surrounding atmospheric pressure.

From the national market, a Danfoss pressure control, type RT1 is chosen.

Besides, a standard switch SPDT must be added for the pressure controller to act

over the pump. Further information can be consulted in appendix 10.

62

COSTS

The manufacture of both tanks was consulted with the national company “Calderas

Continental”, who happen to be the suppliers of the boiler recently bought by the

department. The price provided includes the manufacture of both tanks under the

specifications of this design, including connection ports for the steam traps, air

venting, atomizing nozzle and required tubing, and internal structures to attach the

weighing system, electric fan and heating resistances. The ancillaries required

were consulted in the national and international market. Some of the prices are

special offers, so they are susceptible to change with time. The suppliers chosen

are just an alternative, they are optional, and so there is no obligation in buying

them, in case a cheaper option arises. Tables 5 and 6 show the lists of costs and

suppliers consulted respectively.

Constituent/device  Individual cost  Quantity  Cost Drying chamber  4.900.000,00  1  4.900.000,00 

Tanks Storing chamber  400.000,00  1  400.000,00 

Atomizing nozzle  197.056,00  1  197.056,00 

Steam trap  453.150,00  2  906.300,00 

Air venting  247.410,00  2  494.820,00 

Heating resistance  29.500,00  2  59.000,00 

Ancillary 

Electric fan  60.000,00  1  60.000,00 

Load cell          

Thermostat  85.000,00  1  85.000,00 

RTD probe  147.000,00  1  147.000,00 Measurement 

Controller  0,00  1  0,00 

Auxiliary  Vacuum pump  4.661.460,00  1  4.661.460,00 

Total  11.910.636,00 

Table 5: List of costs

63

Constituent/device Supplier Tanks Calderas Continental Atomizing nozzle Spraying Systems Steam traps Steam Control Air venting Steam Control Heating resistances Resistencias Luengas Electric fan Contactores y Breaker Load cell LCM Systems (United Kingdom) Thermostat Electronic Control RTD probe Omega Vacuum pump GyG Sucesores

Table 6: List of suppliers

64

CHAPTER FIVE

PROCEDURES

INTRODUCTION

This final chapter has the purpose of explaining two important procedures to follow

for the proper performance of the equipment. First, it is important to choose the

operational variables to be imposed in both tanks in order to guarantee they will

function properly; these conditions are chosen according to some restrictions set

by the design, and proper understanding of such conditions is necessary prior to

using the equipment.

Once the operational conditions are chosen, a set of instructions must be followed

during the start-up in order to prepare the equipment for the drying process. These

instructions include a pre-warning stage for reducing condensation of steams and

directives to operate the valves and sample holder.

SELECTION OF OPERATIONAL CONDITIONS

Special care must be taken when selecting the operational conditions of both tanks

in order to assure that the drying conditions are attained at the drying tank. The

pressure in the storing tank must always be greater than that in the second one in

order to provide the movement of steam by pressure drop; it also must be less than

the pressure in the boiler as to afford a superheated steam, otherwise

condensation may occur.

65

Once the pressure of the first tank has been chosen, the necessary quantity of

water for the adiabatic cooling must be calculated. This water must be fed to the

atomizing nozzle feeding system in the top of the chamber. A procedure to select

the operational conditions is as follows:

- Select a pressure for the storing tank. The temperature at this tank is

determined by an isenthalpic balance:

(16)

The left side of this equation represents the saturated steam generated in

the boiler. The only unknown in this equation is T2, and then the equation

may be solved to find this variable.

- Estimate the quantity of water necessary for the adiabatic cooling to reach a

determined temperature. This can be accomplished by solving the mass and

energy balances of a closed system:

(17)

(18)

There are four unknowns in these two equations: the three amounts of mass

and the final pressure after mixing. The amount of steam and at the

beginning and the final pressure can be determined by applying the ideal

gas law to the initial and final states respectively.

66

(19)

(20)

The volume remains constant during the process; it can be calculated from

the geometry of the tank. With this additional information, the above

equations can be solved to find the necessary quantity of liquid water to

reach the drying temperature desired.

- Select a pressure for the drying tank. The temperature at this tank can be

found by a second isenthalpic balance.

(21)

The drop in temperature (due to the pressure drop) will be compensated by

the on-off temperature control.

A program for EES to solve the above equations can be found in appendix 4. This

same program was used to obtain the information in Fig. 10.

START-UP AND SHUT-DOWN PROCEDURES

What comes next is a recommended procedure for the manipulation of the

equipment. More details should be studied in the case this dryer gets to be

manufactured. According to the literature, a pre-warning stage should be carried

67

out before starting the drying process in order to minimize steam condensation.

This can be accomplished by making use of the air initially inside the drying

chamber; by setting the thermostat control at a value between 40 and 50 °C. The

fan must also be on to circulate the air so as to heat all the internal surfaces in the

chamber. This procedure must be carried out before introducing the sample to be

dried; otherwise some air drying may be carried out.

Once the preheating stage is complete, the chamber s door can be opened and

the sample introduced. Use of gloves and grippers is necessary to avoid burns

when both introducing and taking off the sample. Once the drying process is

complete, air must be allowed to enter the chamber from the outside in order to

break the vacuum. The sample cannot stay too long inside the chamber because

condensation may occur on its surface.

68

NOMENCLATURE

A = surface area (m2)

c = heat capacity (J/Kg*K)

d = diameter (m)

D = Diffusivity coefficient (m2/s)

E = Modulus of elasticity (Pa)

FACTOR A = Factor A for use in ASME procedures

FACTOR B = Factor B for use in ASME procedures

h = heat transfer coefficient (W/m2*K)

H = enthalpy (KJ/Kg)

k = thermal conductivity (W/m*K)

l = length (m)

MW = molecular weight (Kg/Kmol)

n = unit vector

N = evaporation rate (Kg/s)

P = pressure (kPa)

q´ = heat generation per unit of volume (W/m3)

r = heat transfer resistance (m2*K/W)

R = ideal gas constant (kPa*m3/Kmol*K)

t = time (s)

th = thickness (m)

T = temperature (K)

U = internal energy (KJ/Kg)

x = quality

X = free moisture content (Kg moisture/Kg dry solid)

69

GREEK SYMBOLS

ρ = density (Kg/m3)

µ = viscosity (Pa*s)

λ = latent heat of vaporization (J/Kg)

σ = stress (Pa)

SUBSCRIPTS

0 = initial point

1 = steam at boiler

2 = steam at storing chamber prior to adiabatic cooling

3 = steam at storing chamber after adiabatic cooling

4 = steam at the drying chamber

a = ambient condition

c = critic point

i = instantaneous point

in = inside condition

ins = insulation

eq = equilibrium point

out = outside condition

p = permissible

ss = superheated steam

w = wall

wt = water

x = rectangular coordinate x direction

y = rectangular coordinate y direction

z = rectangular coordinate z direction

70

BIBLIOGRAPHIC REFERENCES

[1] Mujumdar, Arun. Superheated Steam Drying. Handbook of industrial drying.

Second edition. New York. 1995.

[2] Sokhansanj, Shahab; Jayas, Digvir. Drying of Foodstuffs. Handbook of

industrial drying. First edition. New York.

[3] Treybal, Robert. Mass Transfer Operations. McGraw Hill. New York. 1997.

[4] McCabe, Warren; Smith, Julian; Harriot, Peter. Unit Operations of Chemical

Engineering. McGraw Hill. New York. 2001.

[5] Devahastin, S.; Suvarnakuta, P.; Soponronnarit, S.; Mujumdar, A. A

Comparative Study of Low Pressure Superheated Steam and Vacuum Drying of a

Heat Sensitive Material. Drying Technology, Vol 22 No. 8. 2004.

[6] Suvarnakuta, P.; Devahastin, S.; Mujumdar, A. A Mathematical Model for Low

Pressure Superheated Steam Drying of a Biomaterial. Chemical Engineering and

Processing 46, 675-683 . 2007.

[7] Tatemoto, Y.; Bando, Y.; Oyama, K.; Yasuda, K.; Nakamura, M.; Sugimura, Y.;

Shibata, M. Effects of Operational Conditions on Drying Characteristics in Closed

Superheated Steam Drying. Drying Technology 19(7), 1287-1303. 2001.

[8] Erriguible, A.; Bernada, P.; Couture, F.; Roques, M. Simulation of Superheated

Steam Drying from Coupling Models. Drying Technology 24, 941-951. 2006.

71

[9] Suvarnakuta, P.; Devahastin, S.; Soponronnarit, S.; Mujumdar, A. Drying

Kinetics and Inversion Temperature in a Low Pressure Superheated Steam Drying

System. Ind. Eng. Chem. Res. 44, 1934-1941. 2005.

[10] Sano, A.; Senda, Y.; Oyama, K.; Tanigawara, R.; Bando, Y.; Nakamura, M.;

Sugimura, Y.; Shibata, M. Drying Characteristics in Superheated Steam Drying at

Reduced Pressure. Drying Technology 23, 2437-2447. 2005.

[11] Rahman, N.; Kumar, S. Evaluation of Heat Transfer Coefficient during Drying

of Shrinking Bodies. Energy Conversion and Management 47, 2591-2601. 2006.

[12] American Society of Mechanical Engineers. ASME Boiler and Pressure Vessel

Code: rules for the construction of pressure vessels. ASME. New York. 1998.

[13] Harvey, J. Theory and Design of Pressure Vessels. Van Nostrand Reinhold.

New York. 1991.

[14] The Steam and Condensate Loop: an engineer´s best practice guide for

saving energy. Spirax-Sarco Limited. London. 2007.

[15] Beltran, R. Principios de Aire Acondicionado. Facultad de Ingenieria,

Universidad de los Andes. Bogota. 2002

[16] Spraying Systems Selection Catalogue consulted on the website

http://www.spray.com/cat70/e/E1.html.

72

APPENDIX ONE

STORING CHAMBER DRAWING PLANES

Because of digitalization requirements, the drawing planes that follow may not be

seen clearly when trying to visualize the little details. For a better visualization you

may want to check the attached CD using the program SolidEdge.

73

APPENDIX TWO

DRYING CHAMBER DRAWING PLANES

Because of digitalization requirements, the drawing planes that follow may not be

seen clearly when trying to visualize the little details. For a better visualization you

may want to check the attached CD using the program SolidEdge.

74

APPENDIX THREE

CONNECTION AND DISPOSITION OF TANKS

Because of digitalization requirements, the drawing planes that follow may not be

seen clearly when trying to visualize the little details. For a better visualization you

may want to check the attached CD using the program SolidEdge.

75

APPENDIX FOUR

EES PROGRAM

T[1]=100[°C] X[1]=1 P[1]=pressure(Steam;T=T[1];X=X[1]) P[2]=20[kPa] h[1]=enthalpy(Steam;P=P[1];X=X[1]) h[2]=h[1] T[2]=temperature(Steam;P=P[2];h=h[2]) m[1]=0,92[Kg] m[2]=m[1] m_1=0,014[Kg] T_1=20[°C] P_1=75[kPa] u_1=intenergy(steam;T=T_1;P=P_1) u[2]=intenergy(steam;T=T[2];P=P[2]) P[3]=20[kPa] m[2]+m_1=m[3] m[2]*u[2]+m_1*u_1=m[3]*u[3] T[3]=temperature(steam;P=P[3];u=u[3]) h[3]=enthalpy(steam;T=T[3];P=P[3]) P[4]=10[kPa] h[4]=h[3] T[4]=temperature(steam;P=P[4];h=h[4]) T[5]=75[°C] P[5]=P[4] h[5]=enthalpy(steam;T=T[5];P=P[5]) Q=m[3]*(h[5]-h[4])

76

APPENDIX FIVE

STEAM TRAP SPECIFICATIONS

The steam trap found in the national market is a TLV trap, J3X model, similar to the

one shown in the next figure7:

The trap is chosen according to its capacity and pressure drop across it using a

graph which shows this relation for a different number of orifices within the trap.

Such figure is shown next8:

7 Taken from the catalogue of products of SteamControl. 8 Taken from the catalogue of products of SteamControl

77

For a differential pressure of 0,065 MPa, a 5 orifices trap is chosen; the capacity of

such trap represents 10% of the total amount of steam to be handled by the tanks

under extreme conditions of operation. Further information on the trap follows

next9:

9 Taken from the catalogue of products of SteamControl

78

79

APPENDIX SIX

AIR VENTING SPECIFICATIONS

The same steam trap supplier provides the air vents for steam applications. A

LA13L model was recommended by the supplier; a picture of it can be observed

next10:

The capacity of this ancillary can be determined by using the next graph supplied

by TLV11:

10 Taken from the catalogue of products of SteamControl 11 Taken from the catalogue of products of SteamControl

80

For the same differential pressure of 0,065 MPa, a capacity of 250 lt/min is

obtained. This value is slightly less than the total volume of the tank, meaning, that

air can be extracted is a minute and a couple of seconds. Further information on

this equipment is presented next12:

12 Taken from the catalogue of products of SteamControl

81

82

APPENDIX SEVEN

LOAD CELL SPECIFICATIONS

A picture of the load cell chosen was already shown in chapter 4. Next there are

dimensions (in millimeters) and further specifications13 of such cell:

13 Taken from the catalogue of products of LCM Systems at: http://www.lcmsystems.com/c2/uploads/UF1.pdf

83

84

APPENDIX EIGHT

TEMPERATURE CONTROLLER SPECIFICATIONS

As was already stated the temperature controller chosen is the RTD FH1 from

Digimec. In the next figures it can be appreciated a picture of the device (the

department has some stored in the laboratory), a connection diagram for

installation, dimensions of the unit (in millimeters) and further specifications14

(information is written in Portuguese):

14 Taken from the catalogue of products of Digimec at: http://www.digimec.com.br/PDF/50-51_79_termor.pdf

85

86

APPENDIX NINE

RTD PROBE SPECIFICATIONS

The probe PR-11 from Omega has been chosen to fulfill the RTD measuring

requirements. A picture of such probe and specifications15 are presented next:

15 Taken from the RTD selection guide of Omega at: http://www.omega.com/toc_asp/frameset.html?book=Temperature&file=SEL_GUIDE_RTD

87

88

APPENDIX TEN

PRESSURE CONTROLLER SPECIFICATIONS

The pressure controller RT 1 from Danfoss has been selected. Next there is

information on such controller and the two types of switches16 that can be used

with it:

16 Taken from the catalogue of products of Danfoss at: http://rc.danfoss.com/TechnicalInfo/literature/manuals/01/DKRCCPDCB0A102.pdf

89

APPENDIX ELEVEN

ATOMIZING NOZZLE SPECIFICATIONS

The selection of the appropriate atomizing unit is carried out based on the

performance data of the different nozzles provided by Spraying Systems shown in

the next table17:

For the capacity expected, the capacity size 4 nozzle is chosen. A picture of it and

some dimensions18 follow next:

17 Taken from the catalogue of products of Spraying Systems (Colombian store) 18 Taken from the catalogue of products of Spraying Systems (Colombian store)

90