DESIGN OF A LOW PRESSURE SUPERHEATED STEAM DRYING UNIT
Transcript of DESIGN OF A LOW PRESSURE SUPERHEATED STEAM DRYING UNIT
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
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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.
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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
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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
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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
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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
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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
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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.
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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
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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.
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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.
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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
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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
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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.
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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].
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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
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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.
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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.
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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.
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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.
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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
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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
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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
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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
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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
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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].
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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:
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- 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)
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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.
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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.
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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
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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].
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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.
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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
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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