Dairy-Process-Engineering-1.0.pdf

Dairy Process Engineering J.B. Upadhyay Sunil M. Patel  Dairy Process Engineering Author J.B. Upadhyay & Sunil M. Patel Department of Dairy Engineering AAU, Anand  Course Outline...

Dairy Process Engineering J.B. Upadhyay Sunil M. Patel  Dairy Process Engineering Author J.B. Upadhyay & Sunil M. Patel Department of Dairy Engineering AAU, Anand  Course Outline Lesson Page No Module 1. Evaporation Lesson 1.Basic Principles of Evaporators 5-8 Lesson 2.Construction and Operation of Evaporators 9-13 Lesson 3.Different Types of Evaporators Used In Dairy Industry 14-22 Lesson 4.Calculation of Heat Transfer Area and Water 23-34 Requirement of Condensers Lesson 5.Basic Concepts of Multiple Effect Evaporators 35-42 Lesson 6.Operations and various feeding systems 43-46 Lesson 7.Economy of operation, Thermo processor and MVR 47-51 system Lesson 8.Care and maintenance of evaporators 52-55 Module 2. Drying Lesson 9.Introduction to Principle of Drying 56-59 Lesson 10.Equilibrium Moisture Content, Bound and Unbound 60-62 Moisture etc. Lesson 11. Rate of drying constant and falling rate, Effect of 63-70 shrinkage Lesson 12.Classification of Dryers, Spray Dryers, Drum Dryers 71-87 and problems on Dryers Lesson 13.Air Heating Systems, Atomization & Feeding System 88-94 Lesson 14.Factors Affecting Bulk Density of Powder, Spray Drying 95-107 Controls Lesson 15.Theory of Solid Gas Sepearation, Cyclone Separators, 108-117 Bag Filters etc. Lesson 16.Care and Maintenance of Spray and Drum Dryers 118-120 Module 3. Fluidization Lesson 17.Mechanisms of Fluidization, Characteristics of Gas 121-124 Fluidization Systems, Minimum Porosity, Bed Weight, Pressure Drop in Fluidized Bed. Lesson 18.Application of Fluidized Bed in Drying 125-128 Lesson 19.Batch Fluidization, Fluidized bed Dryer 129-133  Module 4. Mechanization in manufacture of indigenous dairy products Lesson 20.Butter making machines 134-141 Lesson 21.Ghee making machines 142-143 Lesson- 22.Ice-cream making equipment 144-151 Lesson- 23.Cheese making equipment 152-156 Module 5. Packaging Machines for Milk & Milk Products Lesson 24.Packaging Machines for Milk 157-168 Lesson 25.Packaging Machines for Milk Products 169-174 Module 6. Membrane Processing Lesson 26.Ultra Filtration 175-182 Lesson 27.Reverse Osmosis 183-186 Lesson 28.Electro dialysis 187-192 Lesson 29.Materials for Membrane Construction 193-197 Lesson 30.Ultra Filtration of Milk 198-201 Lesson 31.Effect of Milk Constituents on Operation of Membrane 202-205 Process Lesson 32.Membrane for Electro Dialysis 206-209 Dairy Process Engineering Module 1. Evaporation Lesson-1 Basic Principles of Evaporators 1.1.INTRODUCTION Evaporation and vapouration are two processes in which simultaneous heat and mass transfer process occurs resulting into separation of vapour from a solution. Evaporation and vapourization occur where molecules obtain enough energy to escape as vapour from a solution. The rate of escape of the surface molecules depends primarily upon the temperature of the liquid, the temperature of the surroundings, the pressure above the liquid, surface area and rate of heat propagation to product. 1.2. VAPOURIZATION AND EVAPORATION Evaporation and vaporization are quite different from each other. The differences are shown in Table 1.1. Table-1.1: Differences between evaporation and vaporization Vapourization Evaporation (i) Vapourization occurs when entire (i) It is only a surface phenomenon with only mass of liquid is raised to the boiling surface molecules escaping at a rate point. depending upon area of open surface. (ii) It is a much faster process for the (ii) It is a relatively slow process and depends production of vapours. mainly on temperature difference and on difference of vapour pressure between air and liquid. (iii) Boiling point is related to the pressure (iii) Evaporation occurs at normal room above the liquid surface and the temperature and application of heat is not amount of solute. necessary under normal evaporation 5  www.AgriMoon.Com process. (iv) Vapourization of liquid is visual in the (iv) Evaporation is not usually visual and form of vapour bubbles rising and hence not detected easily. escaping at the surface. (v) Vapourization can be controlled by (v) Liquid will evaporate until the pressure of variation in pressure. its vapour is equal to the equilibrium value. It is the vapour pressure of the liquid for the given temperature of the liquid, for closed system. In case of open system, evaporation will continue till there is no more liquid in the vessel. Evaporation and vapourization occur where molecules obtain enough energy to escape as vapour from a solution. The rate of escape of the surface molecules depends primarily upon the temperature of the liquid, the temperature of the surroundings, the pressure above the liquid, surface area and rate of heat propagation to product. In a closed container with air space above the liquid, evaporation will continue until the air is saturated with water molecules. Removal of water from a liquid product by evaporation is enhanced by adding heat and by removing the saturated air from above the liquid. This is done by removal of vapour from the space above the liquid surface and there by creating vacuum. The boiling point of solution due to dissolved solutes is higher than that of pure water and depends on the molecular weight of the solute. Vacuum is utilized to remove water from liquid/solids at lower temperatures to reduce damage to heat sensitive products which might decompose at higher temperatures. In the dairy industry evaporation means the concentration of liquid milk products containing dissolved, emulsified or suspended constituents. During this process water is removed by boiling. This process is used in the dairy industry for manufacture of evaporated milk, condensed milk and traditional Indian Dairy products i.e. Kheer, Basundi, Khoa etc. In milk condensing plant, milk is condensed by evaporating a part of its water content by using saturated steam. The milk is boiled under vacuum. As the milk boils, water vapour is formed. This vapour is utilized for heating the milk further in the next stage which is at a higher vacuum. 6 Dairy Process Engineering Modern dairy plants use evaporators to remove part of water from milk by boiling it under low pressure. The process of evaporation takes place at a maximum temperature of about 70 0C corresponding to an absolute pressure of 230 mm (9.0 inch) of mercury (Hg). Evaporation of milk under low pressure or vacuum is carried out in a specially designed plant. The plant design depends much on the characteristics of liquid milk during boiling at low pressure than any other factor. Some of the important properties of evaporating milk are as under. i Concentration of solids (initial and final) ii Foaming under vacuum iii Heat sensitivity iv Viscosity change The engineering design of plant requires certain other factors which provide a suitable milk contact surface, cleaning without frequent dismantling, faster heat transfer and economy of steam/power used for operating the plant. Following factors are important for evoparation process. (i) Concentration: The initial and final concentration of solute in the solution should be considered. As the concentration increases, the boiling point rises. (ii) Foaming: Few products have tendency to foam, which reduce heat transfer and there is difficulty in controlling level of liquid which ultimately increases product (entrainment) losses. (iii) Heat sensitivity: Milk, like many other food products, is sensitive to high temperatures. If time of exposure is more, there will be severe damage to milk proteins. (iv) Scale formation / Fouling: It is a common phenomenon of deposition of solids on the heat exchanger surface. However, the scale forming tendency can be very much reduced by maintaining reasonably low temperature difference and relatively clean and smooth heat transfer surface. The flow velocity of product has also significant effect. If scale formation starts, rate of heat transfer decreases and cleaning becoming more difficult. 7  www.AgriMoon.Com (v) Materials of construction: Stainless steel is the most common metal for evaporators in the dairy and food industry. Other metals may be used in chemical evaporators. The factors like strength, toughness, weldability, non-toxicity, surface finish, cost etc. are important in the selection of material of construction. (vi) Specific heat: It changes with concentration of solution. More heat is required to be supplied at high specific heat values. (vii) Gas liberation: Few products liberate gases when heated under boiling pressures. (viii) Toxicity: The gases liberated in few cases may be toxic and should be handle carefully. (ix) Viscosity: There is increase in viscosity of solution during evaporation which increases time of contact and hence chances of burning or damage the product. (x) Capacity: It is expressed as the amount of water evaporated per hour. It depends on the surface area of heat transfer, temperature difference and the overall heat transfer co-efficient. (xi) Economy: It is based on the amount of water evaporated per kg of steam used. It increases with number of effects. ***** ☺***** 8 Dairy Process Engineering Lesson-2 Construction and Operation of Evaporators 2.1. INTRODUCTION A number of evaporators of different design have been developed. The need of large scale operation and of improvement in quality has replaced the batch type evaporators. We shall discuss here the most important types of evaporators which are of interest to the dairy industry. 2.2. EVAPORATOR SYSTEMS Basically an evaporator system consists of a heat exchanger, supplying the sensible heat to raise the product to its boiling point and provide the latent heat of vaporization, a separator for the separation of vapour and concentrate, a condenser to remove the resultant vapour as condensate and a vacuum system as the process is carried out under reduced pressure. The heating medium is steam. For vacuum, barometric leg condenser, vacuum pump and steam ejector are generally used. The evaporators may employ natural or forced circulation of the product. In natural circulation units, circulation of the liquid is brought about by convective currents arising from the heating surface. In forced circulation evaporators, the increased velocity of the liquid over the heat transfer surface will bring about a marked increase in the liquid heat transfer coefficients. The circulation is achieved with the aid of an external circulating pump generally a centrifugal pump. The simplest evaporator as shown in following figure, consists of an open pan and kettle in which liquid is boiled. The heat is supplied by condensation of steam in one side of a metal surface and the liquid material to be evaporated on the other side. Sometimes heating coils immersed in the liquid. In some cases the kettle is indirect fired. These evaporators are inexpensive and simple to operate, but the heat economy is poor. Paddles and/or scrappers are used to improve the economy and quality of the product. 9  www.AgriMoon.Com Fig-2.1: Evaporator System Based on the nature of the heat transfer surface, evaporators can be classified as: (1) Tubular surface with natural or forced circulation. a. Horizontal tube evaporator b. Vertical short tube evaporator c. Falling film evaporator d. Rising /Ascending / climbing film evaporator (2) Flat heating surface: plate evaporator (3) Stationary cylindrical surfaces with scraped surface evaporator. 2.3. TYPES OF EVAPORATORS Evaporators used in food industry may also be classified in different ways as under. 1. Operating pressure – Vacuum and Atmospheric 2. Number of effects – Single effect and multiple effect 3. Type of convection – Natural convection and forced convection 4. Continuity of operation – Batch and continuous 5. Type of heat exchanger – Tube type, plate type, scraped surface etc. (i) Horizontal Tube Evaporator A simple unit, not used to a great extent on new installations, is the horizontal tube evaporator. Horizontal tubes from 2 to 3 cm diameter extended across the bottom of a cylindrical chamber from one to three meters in diameter and 2.5 to 4.5 meter high. Steam 10 Dairy Process Engineering enters a chest on the end of the tubes, moves through the tubes and the condensate is removed from the chest at the opposite end. The vapour is removed from the top of the chamber and the product circulation is by natural circulation over the heating coil. (ii)Vertical Tube (short tube calandria) Evaporator: Tubes carrying the steam internally are placed vertically in the bottom of the cylindrical evaporator chamber. It is easier to clean the tubes in a vertical unit than in a horizontal tube evaporator, here also the product circulation is by natural convection. This type of unit is known as the Roberts evaporator in Europe and is the calandria evaporator in the United States. (iii) Basket Type Evaporator: In a basket type evaporator the tubes may be placed in the shape of a ring or tubes. This unit provides an open space on the periphery so that the liquid may circulate more freely through the coils, with the liquid moving up through the coils as it is heated and the colder product moving down through the annulus around the basket. Components of evaporator system other than heat exchanger are as under : Vapour and Entrainment separators These are used in most evaporators to obtain vapours without product particles. Entrainment results wherever a vapour is generated from a liquid boiling vigorously. When this occurs the vapour carries with it varying quantities and sizes of liquid droplets. Separators provide a means for separating the vapour from the liquid with minimum liquid carry-over. Various mechanisms such as inverted U-tube, spiral, baffle, centrifugal type are adopted for such separation. 11  www.AgriMoon.Com Fig 2.5 (c) Evaporator with vapour separator and Entrainment separator Vacuum producing device In an evaporator, evaporation of milk occurs at temperature ranging from 45-75 0C and operated under vacuum. To create vacuum, it is necessary to remove vapour and other gases and. The vapour constitutes by far the greatest part of the total volume of the gases. As the vapour is condensed, vacuum is maintained because the volume occupied by the liquid is considerably less than the original vapour. To change vapour to liquid requires that the latent heat must be removed from the vapour. The heat is extracted through an indirect or a direct heat exchanger. The vapour is removed after last calandria by condensation either by condensing on water cooled surfaces or by condensing through direct contact with cooling 12 Dairy Process Engineering water. The condenser is therefore the main vacuum pump and additional vacuum pumps such as steam jet air suction pumps, liquid seal pumps and others have only the purpose of removing non-condensable gases and thus maintaining the vacuum. ***** ☺***** 13  www.AgriMoon.Com Lesson-3 Different Types of Evaporators Used In Dairy Industry 3.1. INTRODUCTION The major types of evaporators used in dairy industry are a. Vertical tube circulation evaporator b. Batch vacuum pan evaporator c. Long tube vertical (rising and falling film type). d. Plate evaporators Film evaporators with mechanically moved parts (SSHE) Expanding flow evaporator  3.2. DIFFERENT TYPES OF EVAPORATORS Evaporators are of many different shapes, sizes and types of heating units. The major objective is to transfer heat from heat source to the product to evaporate water or other volatile liquids from the product. The general classification for evaporator bodies may be made based on (i) Source of heat, (ii) Position of tubes for heating (iii) Method of circulation of product (iv) Length of tube 14 Dairy Process Engineering (v) Direction of flow of film of product (vi) Number of passes (vii) Shape of tube assembly for heat exchanger (viii) Location of steam (ix) Location of tubes The most important and widely used evaporator is the long tube vertical (calandria) type evaporator with climbing or falling film principle. The type is of the forced circulation type with steam condensing in the jacket surrounding a most of small diameter tubes. This type of evaporator has higher rate of heat transfer, less contact time with hot surface, flexibility of operation, economy of evaporation and easy-in-place cleaning. It can be operated in stages reusing vapours by Thermo-Vapour Recompression (TVR) and Mechanical Vapour Recompression (MVR), for steam economy. 3.2.1. Long Tube Vertical (Rising and Falling film type) Evaporator In natural convection evaporators, the velocity of the fluid is usually less than one to 1.25 m/s. It is difficult to heat viscous materials with a natural circulation unit. Therefore the use of forced circulation to obtain a velocity of liquid up to 5 m/s, at the entrance of the tubes is desired for more rapid heat transfer. The liquid head above the heat exchanger is usually great enough to prevent boiling in the tubes. A centrifugal pump is normally used for circulation of milk products, but a positive pump is used for highly viscous fluids. Tubes of 3 to 5 cm diameter and 300 to 500 cm long are used to move the liquid on the inside. These are placed in a steam chest. So that steam heats from the outside of the tube. The Long Tube Vertical (LTV) evaporator is used normally with the heating element separate from the liquid-vapour separator. The product enters at the bottom of the evaporator body and as it is heated by steam condensing on the opposite side of the tube, the product moves rapidly to the top of the tube and then into a separation chamber. The evaporator is thus a continuous one in operation. Within the tubes there are three distinct regions. At the bottom, under the static head of liquid, no boiling takes place, only simple heating occurs. In the center region the temperature rises sufficiently for boiling and vapour is produced, heat transfer rates are still low. In the upper region the volume of vapour increase and the 15  www.AgriMoon.Com remaining liquid is being wiped into a film on the tube surfaces resulting in good heat transfer conditions. The disadvantages of this type are the relatively large hold up of liquid in the lower regions of the tube giving long contact times (15-30 min.) Also evaporation ratio in a single pass is usually not sufficient to reach the required concentration, so that recycling is necessary, extending residence time. In the central portion of the tubes formation of scale, protein deposits and other fouling is often found to be most severe. Vapour is removed by the separation chamber and the concentrated product removed or recirculated through the evaporation chamber again, depending on the concentration desired. The falling film evaporator is used to reduce the amount of heat treatment and exposure of heat to the product. The tubes are from 4 to 5 cm diameter and up to 600 cm long in the falling film evaporator. The product is sprayed or other wise distributed over the inside of the tubes which are heated with steam. Unless the tubes are fairly heavily loaded there is a risk that some of the tubes may not get their fair share of feed and will overheat or over concentrate the liquid flowing down. The distributor is provided for uniform distribution of feed to each and every tubes of calandria to form thin film over the inner surface of tubes. The smaller the tubes for a given output the easier it is to get even distribution, also small tubes result in a larger pressure drop across their length. The ideal plant might well have conical tubes which would maintain a good initial velocity, would prevent overloading with vapour at the bottom of the tubes and might make the distribution of the feed easier. Moisture removed moves downward along with the concentrated product and finally separated in the vapour separator. The product may be recirculated for further concentration or removed from the system. The Reynolds number of the falling film should exceed 2000 for good heat transfer. The great advantage of the falling film is the short time the product remains inside the tube. This gives better quality product with minimum changes or damage to the product. The other advantages are as stated below. 1. The overall heat transfer coefficient is much larger than vacuum pan or other types of evaporator. The U-value of vacuum pan is 500-700 W/m2 K, while for multi effect evaporators it is 1500-2200 W/m2 K. 16 Dairy Process Engineering 2. More than one effect can be used in series with great saving in steam per kg of vapour. 3. There is no static head and hence no change in the boiling point due to hydrostatic head. 4. It can be used for concentrating most of the heat sensitive products including milk and fruit juices, due to lower temperature gradient. 5. Evaporation is carried out at lower temperature due to higher vacuum and temperature difference required is relatively low. Disadvantages are as under:- 1. Chocking of tubes due to scale formation and difficulty in cleaning of tubes. 2. Operation is highly sensitive to fluctuation in steam pressure to plant. 3. Sudden failure of vacuum causes heavy entrainment losses and fouling of tubes. 4. Great care is needed in keeping all joints leak proof to maintain desired vacuum. Fig.3.2 Single Effect Falling Film Evaporator (courtesy Kessler H.G.) 17  www.AgriMoon.Com Table-3.1: Difference between rising and falling film evaporators Rising Film Evaporators Falling film Evaporators More residence time Less residence time More temperature difference is required Less temperature difference is required between heating medium and feed between heating medium and feed Less overall heat transfer coefficient More overall heat transfer coefficient There is a static head and hence change in There is no static head and hence no change the boiling point due to hydrostatic head in in the boiling point due to hydrostatic head the tube in the tube Higher vacuum is not possible Higher vacuum is possible It is not used for heat sensitive products Used for heat sensitive products as gentle heating More fouling problem Less fouling problem Materials of construction Evaporator bodies and tubes are fabricated from the materials mostly of stainless steel (AISI- SS-316) is used when corrosive action is to be prevented. Design consideration Evaporator drums invariably operate under vacuum. These are designed for an external pressure of 0.1 N/mm2 (100 kPa). The bottom head may be conical in many cases and may be designed for similar pressure rating. The top head may be flanged for flared and dished shape or conical. The calandria which has the tubular heating surface is designed as a shell and tube heat exchanger. Since steam under pressure in usually accepted as the heating medium, the design is based on the pressure of steam. The entire evaporator body must be rigid. The conical head, the calandria and the vapour drum are connected by flanged joints or directly welded. The vapour drum may be made up to separate cylindrical pieces and joined by flanges. Large openings like manholes, sight glasses must be reinforced with compensating rings. Supports may be placed below the brackets welded to the vapour drum or to the calandria. External calandria is also designed as a shell and tube heat exchanger. 3.2.2. Plate evaporator The plate evaporator is characterized by a large heat exchanger surface occupying a relatively small space which need not be very high. Like the plate heat exchanger, it is constructed from profile plates, with the condensing steam used as heating medium and the evaporating product passing between alternate pairs. 18 Dairy Process Engineering High heat transfer coefficients are obtained and viscous materials are handled at relatively high temperature but for shorter contact times. The plate arrangement may be such that it offers a combination of rising and falling film principle or falling film principle alone. By varying the plate gap, width of the plates and the relative dimensions of the various channels, the vapour velocity is controlled for efficient heat transfer. As the diagram shows, larger cross-sectional areas are provided for the inlet of the steam used for heating than for the discharge of condensate. Similarly, the cross-sectional areas for discharge of vapour and of concentrate are also enlarged. The advantages of plate evaporator are its, flexibility, low head space, sanitary construction and shorter residence time which makes evaporation of heat sensitive products possible. It also offers possibility of multiple effects. However, rubber gaskets for sealing are costly; Liquid having suspended matter cannot be easily processed. For even distribution and to ensure good wetting of the surface, orifice pieces are to be inserted at header ports. Sometimes recirculation is necessary to ensure proper wetting. 3.2.3. Film evaporators with mechanically moved parts (SSHE) When highly viscous products (viscosity more than 1 Pa s) or fluids containing suspended matter are to be evaporated, it may happen that the forces which normally move the liquid along with gravity and propelling power of the vapour, are not sufficient to move the product satisfactorily. This intensifies the problem of maintaining high rates of heat transfer and proper distribution. The Figure 3.4 depicts the cross-section of an evaporator with a rotating inner section. A shaft fitted with wiper blades, scrapers, vanes or other device rotates within a vertical tube of relatively large diameter. This tube is surrounded by a heating jacket. The rotor may have a fixed clearance of 0.2 – 2.0 mm or fixed blades with adjustable clearance, or blades which actually wipe the heat exchanger surface. The purpose of the blades etc. is to produce thorough mixing of the film, to distribute it evenly and to transport the product through the evaporator. The film thickness differs from one liquid to another depending on its physical properties. 19  www.AgriMoon.Com The advantages of this evaporator are: 1. It can handle highly viscous, pulpy and foaming materials. 2. Evaporation rates are high. 3. Fouling problem non-existent. The disadvantages are: 1. Requires precise alignment because of small blade clearance. 2. Difficult to clean. 3. High capital and operating cost. 4. High headspace required for demounting rotor for inspection and cleaning. 3.2.4. Expanding Flow Evaporator It is compact and its heating element and expansion vessel are a single unit. In put milk acts as coolant in condenser. Steam condensate is used in milk pre heater. CIP is possible. Flexible in its capacity. One can get concentration in one pass. It has shorter residence time of < 1 min. Hence it is giving the advantage of gentle heating. Also because of low holding the plant has the characteristic of quick start up. It is made up of number of inverted, S.S. cones. Gaskets maintain narrow passages between cones. The alternative passages for feed and steam is provided. Entrainment separators The entrainment separators are basically depend on principles of impingment theory, where liquid droplets get returned due to spiral tubes and baffles installed in the path of the vapour. The other principle is change in direction as well as velocity. For industrial applications centrifugal type of entrainment separators are in use.Entrainment results whenever a vapour is generated from a liquid. So the vapour carries these liquid droplets. Separators provide a separation of liquid from vapour. 20 Dairy Process Engineering Vapour Release Chamber A large chamber is used to reduce the velocity of vapour stream. This enables the droplets to settle out by gravity. The vapour release drum may either be placed just above the bundle shell or it is a separate unit placed adjoining to tube bundle shell, being connected to it by a large pipe. It may not be economically practical to make the vapour head large enough to accomplish the entire decontamination of the vapour. Further, increasing vapour space decreases entrainment of larger drops, but has not effect on small drops. The vapour disengagement rate from a boiling liquid surface should not normally be more than 30 cm per second for normal solutions at atmospheric pressure and may be about 3 cm per second with crude solutions. Even allowing for sufficient vapour disengagement space it is common practice to provide spray traps. These traps are merely a series of baffles giving rapid changes of direction to the vapour stream. Wire Pad Pads of finely woven wire set in the vapour release chamber at right angles to the vapour flow are used for entrainment. As the vapour and its entrained liquid pass through the pad, the liquid particles agglomerate, eventually falling back into the vapour release chamber. A highly purified dry vapour leaves the top of the pad. Application of such pads may be difficult for vapours with suspended solids, fibers or scale forming materials, which will block the wire mesh. In such cases washing facilities at proper intervals may be provided. Wire pads are not generally used in the food industry for the unhygienic condition it creates. Vapour Release Drum Size The size of drum provided above the tube bundle in most of evaporators, is decided by three important considerations. They are: (a)the foaming of the liquid in the evaporator, (b)the vapour velocity, and (c)entrainment separation. Foaming takes place above the liquid level and occupies a certain space of the vapour drum. The vapour velocity sets the minimum drum diameter. A thumb rule commonly employed in evaporator design of this kind is that the height of the vapour space above the calandria should not be less than one vessel diameter and the bottom space below calandria should be one-fourth of vessel diameter. In cases where the 21  www.AgriMoon.Com entrainment separator forms an independent unit, the main drum can have a shorter disengaging height. Centrifugal Separator This is a separate drum in which the vapours are admitted tangentially and are made to flow in a helical path by use of baffles. The vapours leave either from the top of the drum or through a central pipe. A centrifugal type baffling system as shown in figure is fitted at the top of the drum. The vapours enter from the central passage and are diverted by the baffles separating the liquid in the process. ***** ☺***** 22 Dairy Process Engineering Lesson-4 Calculation of Heat Transfer Area and Water Requirement of Condensers 4.1. INTRODUCTION In the evaporation system, steam is used as a heating medium as well as vapour produced from the previous effect is used as a heating medium in next effect. While the vapour from the final effect enters in the vapour condenser where cooling water is circulated to condense the vapour and thereby maintains the vacuum. The proper calculation of heat transfer area and the water requirement will decide the energy requirement and the capacity of each system components. 4.2. HEAT TRANSFER IN BOILING LIQUIDS Heat transfer in boiling liquids and condensing vapour is accompanied by a change in the phase of liquid or vapour, the saturation temperature of the forming vapour (ts) is determined by the ambient pressure P. This temperature ts remains constant through out the process of boiling of any liquid at constant pressure. Experience however shows that the boiling liquid is usually overheated. (feed temperature, tf is greater than ts). Experience shows that a sharp increase in the temperature is observed only in a layer 2- 5mm thick over the heated surface. Hence the temperature of the liquid on the heating surface is higher than the saturation temperature by ∆t=tw-ts. The value of ∆t rises with an increase in the rate of heat transfer q (W/m2.K.). The temperature profile of liquid near heating surface is shown in following curve. Fig-4.1: Temperature profile in heating tube 23  www.AgriMoon.Com It has been established by visual observation that vapour bubbles form only on the heated surface, where liquid superheat is maximum, and only at individual points of that surface called starting boiling. The surface where the effect of adhesion is minimum may become evaporation starting points. Adhesion is defined as the effort required detaching the liquid from unit area of the hard surface. Further, experience shows that the number of starting points of bubble (Z) depends on the degree of superheat at the heating surface, i.e. on the temperature difference, ∆t = tw - ts. A rise in ∆t entails an increase in Z and boiling intensifies. Essentially, this dependence is traced to the phenomenon of surface tension appearing at the liquid vapour interface. Surface tension is defined as the stress causing the free surface of the fluid to contact; this stress is tangential to the surface. The pressure inside a bubble p 1 is greater than the surrounding liquid pressure p owing to surface tension. According to the Laplace equation, for a bubble in equilibrium the difference between the two pressures is determined by the equation ∆ p = p1-p = 2s/r, Where s = surface tension kg/m, r = curvature radius of bubble, m. After their formation, the bubble grows rapidly and detach from the surface on attaining a certain size. The size of bubble at the moment of its separation from the surface is determined mainly by the interaction of the weight of gravity and surface tension. Besides, the generation of bubbles and their separation from the surface depend to a great measure on whether or not the liquid wets the surface. The wetting capacity of a liquid is characterized by the contact angle , formed between the wall and the free surface of the liquids, the larger the angle , the poorer the wetting capacity of the liquid. Following figures illustrate the same. 24 Dairy Process Engineering Since the temperature of the boiling liquid tf, is higher than the saturation temperature ts, intensive heat transfer between the liquid and bubble takes place, and that causes the bubble to continue to grow after its separation from the surface. The bubble increases in volume dozens of times, this depending on the rise time and degree of liquid superheat. The multiple increase in the volume of separated bubbles evidences that from the heating surface that is transferred mainly to the liquid; it is then transported into the volume by convection and is further utilized to evaporate the liquid into bubbles. Direct transfer of heat from the heating surface to the vapour is possible only during the growth of bubbles prior to their detachment from the surface. But, owing to the small bubble-to-surface contact area and the low thermal conductivity of vapour, only a relatively small amount of heat can be transferred to the bubbles in that period. Boiling is classified in three types: 1) Interface evaporation 2) Nucleate boiling 3) Film boiling. In interface evaporation, regime first, liquid contacts heated surface to produce vapour which rises due to convection forming convection currents to circulate the liquid, the phenomenon is free or natural convection. With further rise in temp, ∆t=(t w-ts) bubbles are formed adjacent at the surface due to high energy in liquid particles; they rise above the water surface but condense before reaching liquid surface. The phenomenon, nucleate boiling commences in the next regime in figure. With further rise in temperature, ∆t, liquid gets heated up, bubbles do not condense, they help evaporation, and this phase is known as nucleate boiling as shown in the next regin in the diagram. There is limit to this phenomenon as shown by crest of the curve in the figure. The peaking point is a critical point and heat flux at this critical state is called critical heat flux. Beyond the crest of the curve, bubbles cover metal surface providing an insulating effect, thereby decreasing heat flux. The film of bubble is unstable during the next regime, it reforms and collapses. On further increase of ∆t the stable film forms and the heat flow is low. Any further increase of ∆t involves heat transfer by radiation also. 25  www.AgriMoon.Com 4.3. HEAT TRANSFER DURING CONDENSATION The condensation phenomenon is very important in the evaporators for efficient heat transfer. The condensation of steam in the calandria is of two types i.e. (i) Film condensation (ii) Drop wise condensation. Film condensation gives steam side film co-efficient in the range of 10,000 to 12,000 W/m2K, where as drop wise condensation gives steam side film co- efficient in the range of 25,000 to 30,000 W/m2K. a) Factors affecting the boiling point: (1) The pressure or vacuum respectively in the evaporating space. Vapour and liquid are in equilibrium with each other and their temperature at any time is a function of the saturated vapour pressure. A lowering of the pressure lowers the boiling point. (2) The concentration of the solution (as osmotic pressure) Boiling point is influenced by the amount of dissolved matter. Dissolved substances exert osmotic forces which lower the vapour pressure of a solution at constant temperature or raise the boiling point if the pressure is constant. (3) The hydrostatic pressure of a column of liquid. Due to hydrostatic pressure boiling point increases. A column of liquid of height H produces an increase in pressure of P=H.ρ.g b) Factors affecting the size of heating surface: It depends upon the amount of heat to be exchanged. If milk enters at boiling point: Then mw.L = A.U(tH-tB), tB=Boiling point, tH=Temp. of steam If cold milk enters: mwL + mM CM (tB - to), to=Initial temp., tB milk at boiling point. For superheated milk: mw.L - mM CM (tV-tB), tV=High temp. than boiling point. Surface area is also dependent on the over all heat transfer coefficient and the temperature difference. In practice, following table gives the normal range of values of overall heat transfer coefficients (U): 26 Dairy Process Engineering Table-4.1: Normal range of values of overall heat transfer coefficients Number of Effect Skim milk Whole milk (W/m2K) (W/m2K) 1st effect 2300-2600 2000-2200 2nd effect 1900-2200 1700-1900 3rd effect 1000-1200 900-1100 Note: 1 Watt=0.86 kcal/h, 1kcal/h =1.163 Watt During continuous operation ‗U‘ drops as deposition of precipitated protein or calcium phosphate on the calendria tubes. Deposits may also form on steam side if complete separation of product from vapour is not carried out in vapour separator and this vapour taken in steam chest of the next effect. So plant must be cleaned at regular interval with sequence of water-caustic soda solution-water-nitric acid-water. c) Factors affecting the heat transfer coefficients: (i) Steam side film coefficient: (a) It depends on the temperature drop. (b) Condensing temperature of steam. (a) and (b) are fixed by condition of operation. (c) Amount of non condensable gas present. It depends on evaporator construction. (ii) Boiling liquid side surface coefficient: (a)Velocity of flow of milk (b) Viscosity of fluid (c) Cleanness of the heating surface. 4.4. HEAT AND MATERIAL BALANCE OF SINGLE EFFECT EVAPORATOR: Heat Balance F.hf + S.Hs = L.hL+ V.H + Shc (neglecting heat losses) Therefore , F.hF + S(Hs-Hc) = LhL + VH …………………….(1) Where, F= Feed rate, kg/h 27  www.AgriMoon.Com hf= heat content of feed, kJ/kg S=steam supply rate, kg/h Hs= Enthalpy of steam, kJ/kg L= output rate of concentrate, kg/h hL= heat content of concentrate, kJ/kg V= rate of vapour production, kg/h H= Heat content of vapour, kJ/kg hc=heat content of condensate, kJ/kg Material Balance: F= L+V………….(2) Based on the total solid, we have FxF = LxL + Vy ……………(3) Where, xF = Feed concentration, % TS xL= Concentration of condensed milk, % TS y= solids in vapour (taken as zero) Capacity: From equation (3) FxF = LxL as Vy=0 Therefore , F= L.(xL / xf) and L= F.(xf/ xL) Therefore , (F-V) = F. (xf/ xL), ( as L= F-V) Therefore , V = F[ 1- (xf/ xL)] …………………………..(4) 4.5. MATERIAL AND HEAT BALANCE OF DOUBLE EFFECT EVAPORATOR Material Balance: 1st effect, F = L1 + V1 …………………………..(1) 2nd effect, L1 = L2 + V2………………………….(2) Substituting for L1 in equation (1) from equation (2) F = L2+ V2+V1 Therefore , F = L2+ E …………………………..(3) where, V1 + V2 = E Based on solid, 1st effect, F.xF = L1 xL1 …………………………(4) 2nd effect, L1xL1 = L2xL2 ………………………...(5) Therefore, F.xf = L2 xL2 …………………..…....(6) Therefore, L2 = F (xF / xL2) Substituting the value of L2 into equation (3) we get, F= (FxF / xL2 ) + E or F= (FxF / xL2 ) + V1 + V2 ………………..(7) 28 Dairy Process Engineering In terms of total evaporation, E kg/h E = F - (FxF / xL2 ) Therefore , E = F [1- (xF / xL2 )] ………………..(8) Heat balance: F.hF + SHs = L1 hL1 + V1H1 + Shc1 Therefore, FhF + S(Hs – hc1 ) = L1 HL1 + V1 H1 ……………………..…..…(i) Therefore, FhF + S(Hs – hc1 ) = (F- V1) hL1 + V1H1 ( as L1 = F- V1) Therefore, FhF + S(Hs – hc1 ) = F hL1 - V1hL1 + V1 H1 Therefore, F (hF – hL1 ) + S (Hs – hc1 ) = V1 (H1 – hL1) …………………......(ii) Similarly taking heat balance on 2 nd effect, L1 hL1 + V1 H1 = L2 hL2 + V2 H2 + V1 hc2 Therefore , L1 hL1 + V1 (H1 - hc2) = L2 hL2 + V2 H2 …………………………(iii) Therefore , L1 hL1 + V1 (H1 - hc2) = (L1 -V2 ) hL2 + V2H2 ( as L2 = L1 -V2 ) Therefore , L1 hL1 + V1 (H1 - hc2) = L1 hL2 - V2 hL2 + V2H2 Therefore , L1 (hL1 - hL2 ) + V1 (H1 – hc2) = V2 (H2 – hL2) ……………………(iv) 4.6. HEAT AND MATERIAL BALANCE OF TRIPLE EFFECT EVAPORATOR 1st effect: FhF + SHs = V1 H1 + S hc1 + L1 h1 Therefore , FhF + S(Hs - hc1) = V1 H1 + L1 h1 Therefore , FhF + S(Hs - hc1) = V1 H1 + (F - V1 )h1 Therefore , F(hF - h1) + S(Hs - hc1) = V1 (H1 - h1) ……………………..(i) 2nd effect: L1h1 + V1 (H1 – hc2) = V2H2 + L2h2 = V2H2 + (F – V1 – V2)h2 = V2H2 + (L1 - V2)h2 = V2H2 + L1 h2 - V2h2 Therefore , L1( h1- h2) + V1 (H1 – hc1) = V2H2 - V2h2 = V2( H2 - h2) ………….……..(ii) 3rd effect : V2H2 + L2h2 = V3 H3 + V2hc2 + L3h3 Therefore , V2( H2-hc2) + L2h2 = V3 H3 + (F – V1 – V2 – V3) h3 = V3 H3 + (L2 – V3 ) h3 V2( H2 - hc2) + L2( h2 – h3 ) = V3 (H3 – h3) …………………..…………(iii) 29  www.AgriMoon.Com Material balance: F = L1 + V1 , L1 = L2+ V2 , L2 = L3+ V3 Therefore , L1 = L3+ V3 + V2 Therefore , F = L3+ V1 + V2 + V3 ……………………………………….(i) Based on solids, Fxf = L1x1, L1x1 = L2x2, L2x2 = L3x3 Therefore , Fxf = L3x3 , Therefore , F = L3 (x3/xf) Therefore , F = (F - V1 - V2 - V3 ) (x3/xf) Therefore , E= F – F(xf/x3) = F [1- (xf/x3)] Combined: FhF + SHs = S hc1 + V1hc2 + V2hc3 + V3 H3 + L3 h3 Different Vacuum producing devices: Production and maintenance of vacuum in the evaporator is very important for the smooth and efficient evaporation operation. The various vacuum producing devices used in dairy industry are as follows 1) Indirect (Shell and Tube) type condenser 2) Direct type condenser 1) Indirect type: The surface condenser is an indirect type heat exchanger in which cold water on one side causes vapour coming from the product to condense on the other side. A common indirect heat exchanger used as a surface condenser is the tubular unit. 2) Direct type: A jet condenser is a heat exchanger in which cooling water is sprayed into the unit where vapour is to be condensed. Jet type condenser can be further classified as parallel flow or counter flow type. The parallel flow condenser is normally operated as a wet condenser and the counter current flow condenser as a dry condenser. In parallel flow condenser, air and water are removed at the same temperature whereas in counter current condenser, the non- condensable are removed at the temperature of incoming water. In case of dry condenser, the cooling water is removed by one pump and the non- condensable including the air removed by another pump. In a wet condenser, condensed vapour and non condensable are all removed together. 30 Dairy Process Engineering A jet condenser will use cooling water amounting 20 to 50 times the weight of vapour. Thus, the vapour being removed from the product and the cooling water is mixed. A jet condenser is normally used in milk evaporating operation in preference to a surface condenser, the surface condenser being more expensive The quantity of water required is less for a counter flow type of condenser. Another advantage is that the air and vapour need not enter at the top of the unit as is done with parallel flow. The quantity of air removed from the evaporator system is about 15-25% of the volume of cooling water. Leaks in the system can cause the quantity of air to be considerably higher and result in expensive operation. Table 4.1 Pressure, volume and boiling point relationship Boiling Point oC mm of Hg absolute pressure Specific Volume of Vapour (m3/kg) 75 434 2.8 70 233 4.8 60 149 7.7 50 92 12.0 40 55 19.6 Barometric Leg Condenser: It is placed high enough so that water and condensate from the condenser escapes from it by a barometric leg. In order to remove water and condensate from the plant without losing vacuum it is necessary that a leg of liquid be maintained with a hydrostatic head Hρ, equal to the difference between vacuum and atmospheric pressure; where H is the height and ρ is the density. In this manner the upper surface of the liquid in the tail pipe is at a pressure corresponding to the vacuum and the liquid at the bottom of the tail pipe is at atmospheric pressure due to the weight of the hydrostatic head. Thus liquid under vacuum continuously enters the tail pipe and liquid at atmospheric pressure continuously leaves from the bottom of the tail pipe by way of the hot well at the bottom. Atmospheric pressure corresponds to a hydrostatic head of 10.35 meters of water and complete vacuum corresponds to zero hydrostatic head. To maintain a process at substantially complete vacuum requires a leg of 10.35 meter of water be maintained between the barometric condenser and the hot well. If a pump is used to remove the tail pipe liquid instead of a total barometric height, whatever head is supplied by the pump can be deducted from the total barometric height and the assembly is known as a low-level condenser. 31  www.AgriMoon.Com P=Hρ, where P=1.013x105 Pa =1.013 bar i.e. Atmospheric pressure Specific steam consumption and water requirement: The following table gives the idea about steam required to vapourize one kg of water and cooling water requirement. Range Average Water required in kg. per kg vapor to condense (Steam in kg.) (Steam in kg.) Single effect 1.83-1.00 1.17 20.00 Double effect 0.63-0.50 0.57 9.00 Triple effect 0.40-0.34 0.37 6.00 Quadruple effect 0.30-0.26 0.28 5.00 Quintuple effect 0.24-0.22 0.23 4.00 Water requirement in condenser: In a condenser the circulating water extracts heat from the vapour to be condensed and the temperature of the circulating water is raised. In direct condenser the cooling water and vapour come in direct contact, hence the temperature of condensate is same as that of outlet temperature of circulating water. Heat received by circulating water from one kg. of vapour = m × S × (t 2 – t1) kJ (i) Heat given out by one kg. of vapour to circulating water = H – h1 kJ (ii) From equation (i) and (ii), we get m × S × (t2 – t1) = H – h1 Therefore m = (H – h1) / [S × (t2 – t1)] kg. Where, m = quantity of circulating water required to condense one kg. vapour S = specific heat of water kJ/kg K t1 and t2 = Inlet and outlet temperature of circulating water H = h + x L = Heat content of one kg vapour at corresponding vacuum h1 = S × t2 = Heat of one kg condensate of vapour 32 Dairy Process Engineering Example: Determine the amount of cooling water required in jet condenser to condense one kg vapour, if cooling water inlet and outlet temperature is 20 0C and 30 0C respectively. Take heat of vapour (H) as 2556 kJ/kg at 30 0C and specific heat of cooling water as 4.187 kJ/kg. Solution: m = (2556 – 125.6) / 41.87 = 58 kg Working of steam jet ejector: Positive pump of reciprocating type and steam jet ejectors are commonly used to produce a vacuum. The pump is normally used for producing 24‖ Hg vacuum or less. The single stage steam jet ejector may be used for 25‖ Hg vacuum. The two stage steam jet ejector may be used for 28.8‖ Hg vacuum or three stages to produce 29.8‖Hg vacuum using steam at 7.0 bar. High pressure is admitted to a nozzle A that sends a jet of very high velocity into throat of a venturi.-shaped tube. The non condensable gases to be removed enter through suction chamber as shown in figure. By proper proportioning of a throat of the venturi, volume and velocity of steam used, the steam can be made to entrain (suck) non-condensable gases from the space under vacuum. For a very high vacuum, the steam air mixture from these jets goes to a condenser B, where the water vapour is condensed by a jet of cold water and the residual air passes to a second nozzle c. The discharge from second nozzle can usually be made to reach atmospheric pressure and is discharged at D to the air. Another important advantage of steam jet over reciprocating vacuum pump is that it has no moving parts and repairs are reduced to a minimum. Fig. 4.10 steam jet ejector Different type of pumps may be used for evacuation such as mechanically operating pump (ring pump) or steam ejectors which operates on similar principles to steam jet vapour compressors. Oil lubricated vacuum pumps The diagram shows a cross-section of a typical rotary vacuum pump. It consists of a horizontal cylindrical casing, with a rotor mounted eccentrically so that it is virtually in contact with the casing at one point of the circumference. The space between the rotor body 33  www.AgriMoon.Com and the casing is thus crescent shaped, and communicates through the elongated inlet port with the vacuum pipeline, and through the elongated outlet port with the exhaust pipe. The rotor has longitudinal slots, usually four, which house vanes free to slide radially as the rotor turns. The vanes, which are usually made of asbestos fiber composition, are kept in contact with the casing by centrifugal force. In some designs the vanes slide tangentially, the purpose being to reduce frictional losses. As the rotor turns, pockets of air are enclosed between the vanes and transferred from the inlet to the outlet. ***** ☺***** 34 Dairy Process Engineering Lesson-5 Basic Concepts of Multiple Effect Evaporators 5.1. INTRODUCTION Milk condensing in vacuum pan uses high amount of steam to evaporate water. Multiple effect evaporator is used for steam ecomomy. 5.2. MULTIPLE-EFFECT EVAPORATION In any evaporation operation, the major process cost is the steam consumed. Therefore, methods of reducing steam consumption (or of increasing economy, defined as mass of vapour produced per unit mass of steam consumed) are very important. The most common of the available methods is to use the vapour generated in the first evaporator as the heating medium for a second evaporator. Ideally, this method should produce almost 2 kg of vapour for every kg. of steam consumed. The method is feasible if the second evaporator is operated at a lower pressure than the first, so that a positive value of ∆t is obtained across the steam- chest surface of the second evaporator. Several evaporators can be connected in series. In this way the amount of vapour (kg) produced per kg of steam consumed equal to the number of evaporator bodies. The increase in latent heat with decreasing pressure and additional radiation losses affect , the economy as the number of evaporators used is increased. This method of evaporation in series is called multiple-effect evaporation, and each stage is called an effect. The amount of steam consumption in multiple effect evaporators is already mentioned in Lesson 4. 5.2.1. Different level of vacuum in each effect of Multiple effect Evaporator Here we assume that initially in all the calandria the level and temperature of feed is same. Now during starting of the plant steam is introduced in first calandria. So in that calandria milk initially will be heated and then raised to corresponding boiling point and vapour will be released. This vapour is going in the next calandria‘s heating jacket where the milk is cold and nowstart heating thereby the temperature difference between heating vapour and milk will decrease. It gives less condensation of vapour in second calandria. This gives rise of back pressure in first calandria tube and thereby the boiling point in first calandria will rise. The 35  www.AgriMoon.Com same principle will work for subsequent calandrias and last calandria will correspond to condenser vacuum. Once this established than all the calandrias will operate at different vacuum levels. By controlling the opening of orifice plates present in the airline of heating jackets is also used to control different vacuum levels in all the calandrias. 5.2.2. Multiple effect Evaporator capacity and steam economy In addition to the economy increase in multiple-effect evaporation, a capacity variation would be expected. Note, however, that the temperature difference from initial steam to the final condenser which was available for a single-effect evaporator will be unchanged by inserting any additional effects between the steam supply and the condenser. For the simplest case, where each effect has area and coefficient equal to that of every other effect and where there are no boiling point rises qt = q1 + q2+ q3 + …………..where qt is the total heat-transfer rate in all effects and q1, q2, q3 are the heat transfer rates in each of the individual effects. qt = U1 A1 ∆t1 + U2 A2 ∆ t2 + U3 A 3 ∆ t3 + ……… Since the areas and heat transfer coefficients are equal, qt = U1 A1 (∆ t1 + ∆ t2 + ∆ t 3 …….) = U1 A1 ( ∆ t) total This rate of heat transfer is the same as that obtained with a single effect operating between the same ultimate temperature levels. Thus, multiple-effect evaporation using n effects increases the steam economy but decreases the heat flux per effect by a factor of about 1/n relative to single-effect operation under the same terminal conditions. Therefore, no increase in capacity is obtained and in fact, the additional complexity of equipment usually results in increased heat losses to the surroundings and a reduction in capacity. The increased steam economy must then, be balanced against the increased equipment cost. The result is that the evaporation using more than five or seven effects is rarely economical. When the solution being evaporated has a significant boiling-point rise, the capacity obtained is very much reduced, for the boiling-point rise reduces the ∆t in each effect. 36 Dairy Process Engineering 5.2.3. Calculations for Multiple- Effect Evaporators For a multiple – effect evaporator system calculations, the values required to be obtained are (i) The area of the heating surface in each effect, (ii) The kg of steam per hour to be supplied, and (iii) the amount of vapour leaving each effect, particularly in the last one. The given values are usually as follows (1) Steam pressure to the first effect, (2) Final pressure in the vapour space of the last effect, (3) Feed conditions and flow to the first effect, (4) Final concentration in the liquid leaving the last effect, (5) Physical properties such as enthalpies and / or heat capacities of the liquid and vapours, and (6) Overall heat – transfer coefficients in each effect. Usually, the areas of each effect are assumed equal. The calculations are done using material balances, heat balances, and the capacity equations q = UA∆T for each effect. A convenient way to solve these equations is by trial and error. The basic steps to follow are given as follows for a triple – effect evaporators. 5.2.4. Triple – Effect Evaporators’ Calculation Method 1. Determine the boiling point in the last effect from the known outlet concentration and pressure in the last effect, 2. Determine the total amount of vapor evaporated by performing an overall material balance, 3. Estimate the temperature drops ∆T1, ∆T2 and ∆T3 in the three effects. Then calculate the boiling point in each effect, 37  www.AgriMoon.Com 4. Calculate the amount vaporized and the flows of liquid in each effect using heat and material balance in each effect, 5. Calculate the value of heat transferred in each effect. Using the rate equation q=UA∆T for each effect, calculate the areas, A1, A2 and A3. If these areas are reasonably close to each other, the calculations are complete and a second trail is not needed. Otherwise a second trial should be performed. Example: Evaporation of Milk in a Triple – Effect Evaporator A triple effect forward – feed evaporator is being used to evaporate a milk containing 10 % solids to a condensed milk of 50% T.S. The boiling point rise of the milk (independent of pressure) can be estimated from BPR°C = 1.78x + 6.22x 2, where x is weight fraction of T.S. in milk (K1). Saturated steam at 205.5 kPa (121.1°C saturation temperature) is being used. The pressure in the vapor space of the third effect is 13.4 kPa. The feed rate is 22680 kg / h at 26.7 oC. The heat capacity of the milk is (K1) C p = 4.19 – 2.35x kJ/kg.K. The heat of milk is considered to be negligible. The coefficients of heat transfer have been estimated as U1 = 3123, U2 = 1987, and U3 = 1136 W / m2. K. If each effect has the same surface area, calculate the area, the steam rate used, and the steam economy. Solution : Following the above steps outlined, the calculations are as follows: Step 1. For 13.4 kPa, the saturation temperature is 51.67 oC from the steam tables. Using the equation for BPR for evaporator number 3 with x = 0.5, BPR3 = 1.78x+6.22x2 = 1.78(0.5)+6.22(0.5)2 = 2.45 oC, so T3 = 51.67+2.45 = 54.12 oC Step 2. Making an overall and a solids balance to calculate the total amount vaporized (V1+V2+V3) and L3, F = 22680= L3 + (V1+V2+V3) FxF = 22680(0.1) = L3 (0.5) + (V1+V2+V3) (0) L3 = 4536 Kg/h total vapour = (V1+V2+V3) = 18 144 kg/h Assuming equal amount vaporized in each effect, V1 = V2 = V3 =6048 kg/h. Making a total material balance on effects 1, 2, and 3 and solving. (1) F = 22 680 = V1 + L1 = 6048 + L1, L1 = 16 632 kg/h (2) L1= 16 632 = V2 + L2 = 6048 + L2, L2 = 10 584 kg/h (3) L2= 10 584 = V3 + L3 = 6048 + L3, L3 = 4536 kg/h 38  Dairy Process Engineering Making a solids balance on effects 1, 2 and 3 and solving for x, (1) 22 680(0.1) = L1x1 = 16 632 (x1), x1 = 0.136 (2) 16 632(0.136) = L2x2 = 10 584 (x2), x2 = 0.214 (3) 10 584(0.214) = L3x3 = 4536 (x3), x3 = 0.50 Step 3. The BPR in each effect is calculated as follows: (1) BPR1 = 1.78 x1 + 6.22(x1)2 = 6.22(0.136)2 = 0.36oC (2) BPR2 = 1.78(0.214) + 6.22(0.214)2 = 0.65oC (3) BPR3 = 1.78(0.5) + 6.22(0.5)2 = 2.45oC Σ ΔT available = Ts1 – T3 (saturation) – (BPR1 + BPR 2 + BPR 3) = 121.1 - 51.67 – (0.36 + 0.65 + 2.45) = 65.97oC Now ΔT1 = 12.40 oC similarly ΔT2 = 19.50oC and ΔT3 = 34.07oC However, since a cold feed enters at effect number 1, this effect requires more heat. Increasing ΔT1 and lowering ΔT2 and ΔT3 proportionately as a first estimate, ΔT1 = 15.56oC , ΔT2 = 18.34oC , ΔT3 = 32.07oC To calculate the actual boiling point of the milk in each effect, (1) T1 = Ts1 - ΔT1 = 121.1 – 15.56 = 105.54oC Ts1= 121.1oC (Condensing temperature of saturated steam to effect 1) 39  www.AgriMoon.Com (2) T2 = T1 –BPR1 - ΔT2 = 105.54 – 0.36 – 18.34 = 86.84oC Ts2= T1 –BPR1 = 105.54 – 0.36 = 105.18oC (Condensing temperature of steam to effect2) (3) T3 = T2 –BPR2 - ΔT3 = 86.84 – 0.65 – 32.07 = 54.12oC Ts3= T2 –BPR2 = 86.84 – 0.65 = 86.19oC (Condensing temperature of steam to effect3) 40 Dairy Process Engineering 41  www.AgriMoon.Com ***** ☺***** 42 Dairy Process Engineering Lesson- 6 Operations and various feeding systems 6.1 Introduction: The most commonly used evaporation plant in the dairy is falling film evaporating plant consists of the following components which are assembled together in the required manner. a) Heat transfer surface or calandria b) Liquid/vapour separation system c) Vapour removal system and vacuum control system d) Ancillary equipment such as pumps for extracting and conveying milk, cooling water pumps, valves, gauges, thermometers etc. a) Heat transfer Surface: Efficient heat transfer from steam to liquid product is vital; both to the process and to the quality of the final product. Chemical, physical and biological changes in the product depend partly on time as well as temperatures. In general, high temperature and short time heat treatment produce less chemical and more biological effect than lower temperature sustained for long times. For economy it is important to keep the temperature as high, heating time as short as possible. Heat transfer rates and flow of product through the plant creates specific conditions within each type of equipment. During heat transfer to milk, protein denaturation may take place on milk side if temperature of milk is high and when steam is condensing at high temperature. The build up of scale, which is hard and difficult to remove, reduces the rate of heat transfer at a much faster rate. Thus there is a need to wash the calandria after a period of operation to remove scale and restore the evaporation rate. 43  www.AgriMoon.Com The use of two or more evaporators in series with gradually decreasing temperatures may be used to reduce the heat shock, especially when the more viscous product approaches final density. Thus, a triple effect plant operating at 70 0C, 57 0C and 44 0C in the final stage will give desirable product quality, concentration level and plant economy. This arrangement permits use of vapour from one effect to be reused for the next effect and so on, thus achieving greater economy as well as more gentle heat treatment. The liquid product moves along the heating surface by convection assisted by vapour propulsion. The liquid is fed from the top of the tubes by means of weir or other device so as to form a thin film on the tube surface. Vapour from the evaporating liquid occupies the center of the tubes. Thus, only a thin quickly moving film is in contact with the heating surface. The falling film evaporator tends to provide a gentle heat treatment if properly operated. b) Liquid/Vapour Separation System: The separation of vapour from boiling liquid is possible by giving centrifugal or rotary motion at the entry to vapour separator. The adaptation of cyclone principle has removed the need for deflecting plants, baskets and entrainment separators in the vapour space. The tendency is to reduce the vapour system in size, because of the higher efficiency of separation. c) Vapour Removal and Vacuum Control System: The heart of the evaporator is the vacuum system, on which depends the successful operation, ease of operation and final product quality. It is important to remove vapour evaporated from the product as well as the noncondensable gases, which enter through leaks or are entrained in the product d) Ancillary Equipment: The main economy in evaporation is obtained by the continuous re-use of vapour. This vapour may be recompressed by live steam (TVR ). The use of preheaters, interstage heaters and raw product heaters in the final condensate stage, all provide for economical operation. The use of heaters in this way also requires pumps to transfer the liquid product. These pumps are often operating against vacuum, and require water seals to maintain the vacuum. 44 Dairy Process Engineering 6.2 Different feed flow arrangements in Multiple-effect evaporators. In a forward feed system, the flow of process fluids and of steam are parallel. Forward feed has the advantage that no pumps are needed to move the solution from effect to effect (not applicable to modern calandria type evaporator). It has the disadvantage that all the heating of cool feed is done in the first effect, so that less vapour is generated here for each kg of steam resulting in lower economy. It has the further disadvantage that the most concentrated solution is subjected to the coolest temperatures. Low temperatures may be helpful in preventing decomposition of organics, but the high viscosity that may be found sharply reduces the heat transfer coefficient in this last effect. In a backward feed system, the feed flows counter to the steam flow. Pumps are required between the effects. The feed solution is heated as it enters each effect, which usually results in better economy than that obtained with forward feed. The viscosity spread is reduced since the concentrated product evaporates at the highest temperature but heat sensitive materials may be affected. Forward feed system is generally used for heat sensitive product , while the backward feed is used for highly viscous product. For best overall performance, evaporators may be operated with flow sequences that combine these two (i.e. mixed feed), or they may be fed in parallel with fresh feed evaporating to final concentration in each effect. 6.3 Capacity of Multiple – effect evaporators: Although the use of the multiple-effect principle increases the steam economy, it must not be thought that there are no compensating disadvantages coordinate in importance with the economy of an evaporator system is the question of its capacity. By capacity is meant the total evaporation per hour obtained since latent heats are nearly constant over the ranges of pressure ordinarily involved, capacity is also measured by the total heat transferred in all effects. The heat transferred in these effects can be represented by the following equations q = q1 + q2 + q3 = U1A1 ∆ t1 + U2A2 ∆t2 + U3A3 ∆ t3 Assume, now that all effects have equal areas & that an average coefficient U av can be applied to the system. Then equation can be written as q = Uav A ( ∆ t1+ ∆ t2 + ∆ t3) 45  www.AgriMoon.Com However, the sum of the individual temperature drops equals the total over-all temperature drop between the temperature of the steam and the temperature in the condenser & therefore q = Uav A ∆ t. Suppose now that a single-effect evaporator of area A be operated with the same over-all temperature difference viz. with steam at 110o C and a vapour temp of 52oC Assume also that the over-all coefficient of the single-effect is equal to the Uav of the triple effect. The capacity of the single effect will be q = Uav A ∆t. This is exactly the same equation as that for the triple effect. No matter how many effects one use, provided the average over-all coefficients are the same exactly the same equation will be obtained for calculating the capacity of any evaporator. It follows from this that if the number of effects of an evaporation system is varied and if the total temperature difference is kept constant, the total capacity of the system remains substantially unchanged. ***** ☺***** 46 Dairy Process Engineering Lesson- 7 Economy of operation, Thermo processor and MVR system 7.1 Introduction The economy of the evaporation system increases with number of effects, simultaneously the capital investment is also increase with number of effects. Hence overall economy of the operation is based on the cost benefit ratio. Further the improvement in economy Is possible by the use of vapour recompression system. 7.2 Cost factors in evaporator selection If the cost of 1 m2 of heating surface is constant, regardless of the number of effects, the investment required for an N-effect evaporator will be N times that of a single effect evaporator of the same capacity. The choice of the proper number of effects will be dictated by an economic balance between the saving in steam obtained by multiple effect operation and the added investment costs brought about by the added effect. Fig.7.1 Cost factors in evaporator selection 47  www.AgriMoon.Com The relations are shown in the graph. The annual fixed charges may be taken as a percentage of the first cost of the evaporator. Since the cost per sq.mt. of heating surface increases somewhat in small sizes, the curve for the first cost is not a straight line except in the upper part of its range. The cost of steam and water fall off rapidly at first but soon show the effect of the law of diminishing returns. Labour costs may be considered constant, since only one operator is needed except with a very large number of effects. The total cost of operating the evaporator is the sum of all these curves and usually shows a marked minimum for the optimum number of effects. 7.3 Vapour Recompression in Milk Condensing Plant:. Vapour recompression is a process by which the low pressure vapour produced from the boiling milk in the calandria is recompressed to a higher pressure. This recompressed vapour is used for heating the milk again in either same effect or in previous effect. Because of this, the steam consumption per kilogram of evaporated water is reduced considerably, lowering the processing cost for the condensed milk. There are two ways for recompressing the low pressure steam i.e. Thermal Vapour Recompression (TVR) by steam-jet vapour compressor or thermo compressor and Mechanical Vapour Recompression (MVR). At present in India TVR is common, while MVR is getting importance recently due to its extremely favourable characteristics for conservation of energy. 7.3.1 Thermal Vapour recompression (TVR) In thermo compressor, the kinetic energy of a jet of steam is used to compress the vapour. It consists of a steam nozzle, suction chamber with inlet for sucking in the vapour, mixing chamber and recompression chamber as shown in Figure. The process of thermo compression on enthalpy-entropy (h-s) diagram is depicted in Figure. Here, live steam at pressure P1(state-1) is almost isentropically expanded in the nozzle to suction pressure P2 (state-2). Steam pressure usually employed in the condensing plant is about 8-12 bar and suction pressure about 0.2-0.3 bar depending on the effect from which the vapour is drawn. The expanded steam emerges from the nozzle as a jet of steam. The velocity of the steam is about 1000 m/sec. 48 Dairy Process Engineering In the mixing chamber the sucked-in vapour is entrained and carried away by the expanded steam. The vapour is accelerated as the steam transfers its kinetic energy to it. The mixing occurs at constant pressure, the enthalpy is increased and state point-3 is reached. From this point onwards, the cross-section of the thermo compressor increases, and so the kinetic energy of mixture is converted into potential energy. The pressure of the mixture is increased almost isentropically from state-3 to state-4. In this way the low pressure steam taken from a lower effect is compressed to a higher pressure corresponding to the inlet pressure of previous stage operating at higher pressure and temperature. Fig.7.3 Enthalpy - Entropy diagram of Thermal Vapour recompression (TVR) The amount of vapour, MV and amount of live steam, Ms are related as follows (Kessler, 1981). MV/Ms= [0.8 (h1-h2) / (h4-h3)] – 1 Where h1, h2, h3, h4 is enthalpies of steam at various state points. It can be seen from the above equation, that if the vapour to live steam ratio is higher, the factor h4-h3 decreases. This means that rise in temperature of compressed vapour is smaller. A thermo compressor with a vapour to live steam ratio of 50 : 50, gives a temperature rise in 49  www.AgriMoon.Com the compressed vapour of about 15 oC. But, if the proportion is 60 : 40, the temperature rise is only 11oC. Generally, the vapour drawn from the first effect is recompressed and used for the same effect again. However, more recently, the trend is to draw vapour from the second or third effect and use the recompressed vapour in the first effect. This is due to the fact that the evaporating capacity of earlier effects is increased. With thermo compressor drawing vapour from the second effect, one must choose the right thermo compressor to achieve a temperature which lies at least 5 oC above the boiling temperature of the first effect. The performance of thermo compressor is influenced by the heat transfer rate in calandria, suction pressure, discharge pressure and the motive steam pressure. 7.3.2 Mechanical Vapour Recompression (MVR) Here, the low pressure vapour is compressed mechanically i.e. employing single or multiple stage radial flow compressors or by axial flow compressors. These compressors may be driven by electric motors, I.C. engines or steam turbines. Layout for single effect evaporation and the process of mechanical compression is shown on enthalpy-entropy diagram. The quantity of vapour MV drawn from the evaporator is at saturation condition with pressure P 1, temperature t1 and enthalpy h1. This condition is shown on h-s diagram as state-1. The mechanical compressor compresses the vapour almost isentropically to a pressure P2, temperature t2 and enthalpy h2. This is superheated steam, which is not suitable for heating the milk as such, because of its bad heat transfer properties. It is cooled down to saturated state-3 i.e. temperature t3 and enthalpy h3 at constant pressure P2. This is done by diverting a portion of condensate at temperature t 4 and injecting it in the superheated steam. The condensate evaporates by consuming superheat from the compressed vapour. The mixture thus achieves final state-3 with temperature t3 enthalpy h3 and pressure P2. The amount of steam available is thus increased by the amount of condensate mixed. 50 Dairy Process Engineering Fig.7.5 Enthalpy - Entropy diagram of Mechanical Vapour recompression (MVR) At this stage is should be remembered that the energy required to drive the MVR may be costlier than steam. Thus actual saving will be somewhat less depending on the prices of steam and other forms of energy employed to run MVR. With increasing energy costs, evaporators with MVR become increasingly competitive with multi effect evaporators with TVR. Apart from being extremely economic MVR has other advantages. 1. The maximum evaporating temperature of first effect can be reduced to such as extent that burning on of product is minimized. 2. The lowest effect of evaporating temperature i.e. of last effect can be high enough which results in lower viscosity of the concentrate facilitating easy handling of concentrate. The pre-heating of concentrate before drying may be avoided or may be reduced to a less drastic treatment. 3. The higher temperature in the final effect results in reduced choking of calandria. Thus the plant can be run for a longer period before cleaning. 4. The need for cooling water is considerably reduced or totally eliminated. A major disadvantage of MVR is the greater expenditure on equipment, maintenance cost and noise problem, but most of the studies indicate that the payback period for MVR is about 2-2.5 years. 51  www.AgriMoon.Com Lesson- 8 Care and maintenance of evaporators 8.1 Introduction: The dairy plants mostly use tubular falling film evaporators for concentrating milk to the desired level of total solids. Most of the equipment, fittings, gauges, valves and density measuring devices are selected by individual plant manufacturer, but special care is needed in their selection and operation to get satisfactory results. The flow rates and flow pattern and the temperature gradients are of vital importance to efficient operation. Thus, loose jets, spindles and seats in steam valves can produce a fluctuating vacuum. 8.2 Operation of the Plant: After going through general discussion on various components of the plant, we can now look into the details of operating parameters which affect the performance of the complete plant. The performance of the evaporating plant is generally based on economy in the use of steam and the output capacity of the given plant. The economy is improved by either increasing the number of effects or by utilizing the outgoing vapour separated from milk. The increase in number of effect involves additional cost of calandria, pumps, piping system and also the operating cost. The other limitation is due to smaller temperature difference between heating medium and product as the numbers of effect are increased. Hence three to five effect plants are common in our country. On the other side, the thermo-compressor system is less costly and can give economy of steam equivalent to one additional effect, if steam pressure and other operating conditions remain steady as per the design of thermo- compressor operation. The thermo-compressor is most suited where high pressure steam is available and the evaporator can be operated with low pressure steam. Space limitation would favour a thermo-compressor installation. The following table gives steam and cooling water consumption of evaporator. 52 Dairy Process Engineering SN No. of effect Kg of steam to Evaporate Kg of water to condense one Kg of water one Kg of vapour 1 Single effect 1.17 20.00 2 Single effect using 0.57 9.00 recompression of vapour 3 Double effect 0.57 9.00 4 Double effect using 0.37 7.00 recompression of vapour The term capacity of evaporating plant gives the output in terms of water evaporated per hour. It depends on the surface area of heat transfer, temperature difference and the overall heat transfer coefficient. The surface area may be used on the external or inner diameter of the tube. The number of tubes and the length of tube will decide the area of heat transfer with fixed diameter of tubes. Thus the capacity of plant increases in direct proportion to the number of tubes of same diameter and length. With larger number of effects, the total surface area may be distributed among different effects keeping in mind the product flow rate. Thus the first effect has always larger number of tubes while the subsequent effects will have reduced number of effect. The other important parameter is the temperature difference which is the difference in temperature of steam condensing in the first effect jacket and the temperature of vapour condensing after the last effect. With first effect temperature of 70 0C, the steam temperature may be assumed to be around 80 0C and the last effect vapour may be assumed to condensing at 45 0C. Thus, the available temperature difference (80-45=35) is about 35 0C. With the increase in number of effects the available temperature difference will decrease and hence the capacity will remain constant or may even decrease because of increased loss of heat from surface of calandria. The last and one of the important parameter is the overall heat transfer coefficient expressed in W/m2.K. The U-value or the overall heat transfer coefficient is affected by number of factors, such as flow velocity, thermal conductivity of metal and the scale and the turbulence of the liquid product flowing down the tube. The change in specific gravity with change in concentration also affects the U-value. Thus the U-value is much higher for the first effect but goes on decreasing with increased concentration of liquid flowing in subsequent effects. The surface tension and viscosity indirectly affects the flowing velocity and thickness of film causing reduced heat transfer rate. The fouling of milk contact surface 53  www.AgriMoon.Com with hard scale is the main cause of reduction in U-value with time. The scale formation is basically due to inverse solubility of calcium and phosphorus salts present in milk at the tube wall temperature and protein denaturation at temperature above 70 0C occurring at heat transfer wall. In addition to this low flow rate or inadequate wetting of tube wall causes burning of the thin film at the tube wall even at slightly low temperature. Therefore, care is required in the operation of the plant to avoid conditions for hard and tough scale formation in the evaporator calandria. Over hard scale deposit, softer deposits are also formed causing reduction in heat transfer rate. The other factors which affect the plant operation are live steam pressure, flow rate of product, even distribution of feed to all tubes, maintaining constant vacuum, preventing leakage of air and operation of condenser for rapid condensing and removal of condensate. If thermo-compressor is used, the working of plant is affected by variation of steam pressure at inlet to thermo compressor. The provision of separate steam main from boiler with regulating valve will solve the problem. The flow rate can be easily controlled by feed pump of adequate size and type and needle valves for flow regulation. Automatic flow controller and flow measuring device may be used. The uniform distribution of product to all the tubes is another major problem in the design and operation of falling film evaporator. The term wetting rate indicates the relationship between product feed rate and heating surface. Every tube requires a certain minimum quantity of product to cover the entire surface. Therefore in a single pass evaporator, the number of tubes per calandria will decrease from effect to effect, because the milk volume is constantly reduced as the concentration is increased. The first effect is always the largest, and in large evaporators, the calandria will be divided in two sections in order to overcome the problem of distribution of the product over a very large number of tubes, and to obtain the correct wetting rate. When the first effect is in two sections, the milk flow will be in series, but the steam flow will be parallel. 8.3 Care and maintenance: 1. Follow the preventive maintenance norms from the manufacturer guidelines. 2. Check the air leaks which may develop around valves, joints, cover and observation posts, as it fluctuates the operating vacuum and temperature. 3. All gaskets must be changed periodically. 4. Avoid the use of high pressure steam. 5. Economy of cooling water to the water cooled condenser should be checked. 6. Keep about 30C temperature difference between condenser discharge water and cooling water for better economy. 7. Ensure heating surfaces are clean and free from deposits. 8. Scaling of heat transfer surfaces constitutes a problem and it should be cleaned after an optimum operation time. Descale the plant at least once a year with a suitable acid solution. 9. Check water vacuum system (working of vacuum pumps). 10. Periodical detection of steam coil leakage by hydraulic pressure test. 11. Condensate must be removed properly from the heating surface. 12. Release vacuum immediately in case of sudden power failure. Also steam valves and cooling water supply must be shut off at once. 54 Dairy Process Engineering ***** ☺***** 55  www.AgriMoon.Com Module 2. Drying Lesson 9 Introduction to Principle of Drying 9.1 Introduction: The purpose of drying food products is to allow longer periods of storage with minimized packaging requirements and reduced shipping weights. The quality of the product and its cost are greatly influenced by the drying operation. The quality of a food product is judged by the amount of physical and biochemical degradation occurring during the dehydration process. The drying time, temperature, and water activity influence the final product quality. Low temperatures generally have a positive influence on the quality but require longer processing times. Low water activity retards or eliminates the growth of microorganisms, but results in higher lipid oxidation rates. Maillard (nonenzymatic) browning reactions peak at intermediate water activities (0.6 to 0.7), indicating the need for a rapid transition from medium to high water activities. Many dried foods are rehydrated before consumption. The structure, density and particle size of the food plays an important role in reconstitution. Ease of rehydration is increased with decreasing particle size, and the addition of emulsifiers such as lecithin or surfactants. Processing factors which affect structure, density, and rehydration include puffing, vacuum, foaming, surface temperature, low temperature processing, agglomeration, and surface coating. Storage stability of a food product increases as the water activity decreases, and the products that have been dried at lower temperatures exhibit good storage stability. Since lipid-containing foods are susceptible to lipid oxidation at low water activities, these foods must be stored in oxygen impermeable packages. Poor color retention has been a problem in the freeze-drying of coffee because the number of light-reflecting surfaces is decreased during rapid drying. This problem has been improved by slow freezing, partial melting, and refreezing to insure large ice crystal formation. Other food materials have different drying problems and specific solutions must be developed. Drying should fulfill the following goals: (i) Minimal chemical and biochemical degradation reactions (ii) Selective removal of water over other salts and volatile flavor and aroma substances (iii) Maintenance of product structure (for a structured food) (iv) Control of density (v) Rapid and simple rehydration or redispersion (vi) Storage stability: less refrigeration and packaging requirements 56 Dairy Process Engineering (vii) Desired color (viii) Lack of contamination or adulteration (ix) Minimal product loss (x) Rapid rate of water removal (high capacity per unit amount of drying equipment) (xi) Inexpensive energy source (if phase change is involved) (xii) Inexpensive regeneration of mass separating agents (xiii) Minimal solids handling problems (xiv) Facility of continuous operation (xv) Noncomplex apparatus (reliable and minimal labor requirement) (xvi) Minimal environmental impact 9.2 Drying Fundamentals: Drying is defined as a process of moisture removal due to simultaneous heat and mass transfer. Heat transfer from the surrounding environment evaporates the surface moisture. The moisture can be either transported to the surface of the product and then evaporated or evaporated internally at a liquid vapor interface and then transported as vapor to the surface. The mechanisms of water transfer in the product during the drying process can be summarized as follows-: water movement due to capillary forces, diffusion of liquid due to concentration gradients, surface diffusion, water vapor diffusion in pores filled wi