Design of a low pressure superheated steam drying unit

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(1)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.

(2) 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.

(3) 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.. iii.

(4) 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. iv.

(5) 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. v.

(6) 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. vi.

(7) 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. vii.

(8) 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. viii.

(9) 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.. ix.

(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. 10.

(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.. 11.

(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.. 12.

(13) CHAPTER ONE DRYING T HEORY 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. 13.

(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. 14.

(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 DR YING 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.. 15.

(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].. 16.

(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. 17.

(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.. 18.

(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.. 19.

(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 TEMPER ATURE 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.. 20.

(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.. 21.

(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 1 resistances or by radiation from the walls of the cabinet. Fig. 2 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. 22.

(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 2. handled. An illustrative representation of a tunnel dryer is shown in Fig. 3 . If the y tra ys 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/conveyordryer.gi f. 23.

(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 3 stationary. Fig. 4 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. 24.

(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 con vection. 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 4 large when compare to simpler dryers as the tray and tunnel dryer. In Fig. 5 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. 25.

(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 5 Fig. 6 .. Figure 6: Schematic representation of a fluidized bed dryer 5. Taken from: http://www.nzifst.org.nz/unitoperations/drying7.htm#tray. 26.

(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].. 27.

(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:. 28.

(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). 29.

(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.. 30.

(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.. 31.

(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. 32.

(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].. 33.

(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 laboratoryscale 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. 34.

(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. 35.

(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.. 36.

(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 37.

(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].. 38.

(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].. 39.

(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].. 40.

(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. 41.

(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. 42.

(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. 43.

(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. 44.

(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, 45.

(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 JouleThompson 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. Water. 2x102. 102. 105°C. P [kPa]. 90°C. 75°C. 60°C. 101. 4x100 2438. 2500. 2563. 2625. 2688. 2750. h [kJ/kg] 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 46.

(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) Capacity (gal/hr) Inlet connection Material. 30 3,5 ¼” NPT 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. 47.

(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. 48.

(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 49.

(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) Radius (cm) Thickness (cm) Total weight (gr) Maximum capacity (gr) Material. 4 15 0,5 430 200 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. 50.

(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. 51.

(52) Characteristic Length (cm) Diameter (cm) Thickness Material Insulation thickness (cm) Insulation material. 90 60 3/8” Structural steel 5 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.. 52.

(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.. 53.

(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.. 54.

(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 55.

(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.. 56.

(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 57.

(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. 58.

(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.. 59.

(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 onoff 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 60.

(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. 61.

(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 Tanks. Ancillary. Individual cost. Quantity. Cost. Drying chamber. 4.900.000,00. 1. 4.900.000,00. 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. Electric fan. 60.000,00. 1. 60.000,00. Thermostat. 85.000,00. 1. 85.000,00. RTD probe. 147.000,00. 1. 147.000,00. Controller. 0,00. 1. 0,00. Vacuum pump. 4.661.460,00. 1. 4.661.460,00. Load cell Measurement. Auxiliary. Total Table 5: List of costs. 62. 11.910.636,00.

(63) Constituent/device Tanks Atomizing nozzle Steam traps Air venting Heating resistances Electric fan Load cell Thermostat RTD probe Vacuum pump. Supplier Calderas Continental Spraying Systems Steam Control Steam Control Resistencias Luengas Contactores y Breaker LCM Systems (United Kingdom) Electronic Control Omega GyG Sucesores. Table 6: List of suppliers. 63.

(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.. 64.

(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.. 65.

(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. 66.

(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.. 67.