Review Of Literature On Scraped Surface Heat Exchangers PDF

REVIEW OF LITERATURE 2. REVIEW OF LITERATURE Extensive literature survey was under taken for better understanding of scraped surface heat exchanger (SSHE) and to develop vapour ejector systems. The review of literature chapter has been divided into following sections: 2.1...

REVIEW OF LITERATURE 2. REVIEW OF LITERATURE Extensive literature survey was under taken for better understanding of scraped surface heat exchanger (SSHE) and to develop vapour ejector systems. The review of literature chapter has been divided into following sections: 2.1 Scraped surface heat exchanger 2.2 Design aspects of SSHE 2.3 Factors affecting performance of the SSHE 2.4 Heat transfer studies in SSHE 2.5 Power consumption in SSHE 2.6 Residence time distribution in SSHE 2.7 Applications of SSHE 2.8 Fan and blower systems 2.1 Scraped surface heat exchanger Scraped surface heat exchangers are essentially double pipe construction with the process fluid in the inner pipe and the cooling or heating medium (steam) in the annulus. A rotating element is contained within the tube and is equipped with spring-loaded blades scraping the inside tube wall. In operation, the rotating shaft scraper blades continuously scrape product film from the heat transfer tube wall, thereby enhancing heat transfer and agitating the product to produce a homogenous mixture. For most applications, the shaft is mounted in the center of the heat transfer tube. An off-centered shaft mount or eccentric design is recommended for viscous and sticky products (Thulukkanam, 2013). During operation, the product is brought in contact with a heat transfer surface that is rapidly and continuously scraped, thereby exposing the surface to the passage of untreated product. In addition to maintaining high and uniform heat exchange, the scraper blades also provide simultaneous mixing and agitation. High heat transfer coefficients are achieved because the boundary layer is continuously replaced by fresh material. Moreover, the product is in 3  REVIEW OF LITERATURE contact with the heating surface for only a few seconds and high temperature gradients can be used without the danger of causing undesirable reactions. SSHEs are versatile in the use of heat transfer medium and the various unit operations that can be carried out simultaneously (Rao and Hartel, 2006). The heating or cooling media for scraped surface heat exchangers can be brine, water, steam, freon and in some cases, oil, which can then achieve temperatures of up to 315ºC (Richardson, 2001). Generic design of SSHE is shown in fig 2.1. 1. Feed tank 2. Feed pump 7 6 9 8 5 3. Flow regulating valve 4. Flow meter 4 11 5. Steam jacketed cylinder 3 6. Steam regulating valve 10 7. Pressure gauge 8. Scraper blade assembly 9. Vapor duct 2 1 10. Concentrate outlet 11. Rotor drive motor Fig. 2.1 Scraped Surface Heat Exchanger 2.2 Design aspects of SSHE In its simplest form, the scraped surface heat exchanger consists of a jacketed cylinder, within which a rotor carrying scraper blade rotates. It may be of batch or continuous type heat exchanger. Batch type systems are generally of small capacity. In continuous type, product is admitted through an inlet at one end of the cylinder and is discharged through an outlet of the other end. There are different types of SSHE used in the industry (Fig. 2.2). There are two major types of SSHEs: Thin film SSHE and Liquid full SSHE. 4  REVIEW OF LITERATURE Fixed Horizontal clearance Liquid Full variable Vertical clearance Spring loaded SSHE blades Cylindrical Rotating disc Sraight sided Swinging type blades Forward Thin film Horizontal Tapered Reverse Vertical tapered Fig. 2.2. Classification of scraped surface heat exchangers A. Thin film and liquid full scraped surface heat exchanger i. Thin Film Scraped Surface Heat Exchanger Thin film SSHEs have been in use in food industry for many years because of their unique characteristics of handling difficult process fluids. The problems like viscosity variation, heat sensitivity and foaming, encountered during processing of food products can easily be handled in thin film SSHEs. It has spring loaded scraper blades to scrap the inside surface. Generally a double pipe construction is used, the scraping mechanism is in the inner pipe, where the process fluid flows, and the cooling or heating medium is in the outer pipe (Mutzenburg, 1965). ii. Liquid Full Scraped Surface Heat Exchanger In the liquid full vertical SSHEs the product falls down along the inner side of the vertical heat transfer tube by gravity. The heat transfer is promoted by rotation of blades that scraps the product off the surface of the heat transfer tube. Centrifugal force throws the product back on to the tube. And vapour rises in the free space close to the rotor (Harrod et al., 1986). 5  REVIEW OF LITERATURE B. Position of the heat exchanger i. Vertical and horizontal SSHE Scraped surface heat exchangers can be installed in either the vertical plane or the horizontal plane. The vertical format can save floor space depending on plant design and so may be more advantageous in plants where there is little available space. Vertical designs now incorporate hydraulic units for the removal of the central shafts for servicing operations which makes for rapid inspection and maintenance. The vertical design also has a further advantage in that as the product enters the heating chamber from the base and travels upwards to the exit, this allows for effective purging of any air that may be present in the chamber so ensuring efficient heat transfer through the system. Horizontal scraped surface heat exchangers benefit from an equal loading on both bearings within the system instead of all of the loading on the lower bearing as in the vertical designs (Richardson, 2001). ii. Inclined SSHE Punjrath et al. (1990) designed and developed inclined SSHE. Angle of inclination can be varied from 0o to 30o.The heat exchanger comprises of inner cylinder, outer steam jacket, rotor and drive (Fig. 2.3). The inner shell is provided with steam jacket which has three separate compartments to maintain different steam pressures. Partitioning of the steam jacket enables better control over heating and evaporation. 4 3 1 2 1. Scraped surface heat exchanger 2. Steam jacket 5 3. Scraper 4. Milk inlet 5. Milk tank 6 6. Feed pump 7 7. Drive mechanism Figure 2.3 Inclined scraped surface heat exchanger 6  REVIEW OF LITERATURE C. Scraper blade geometry and arrangement Different types of scraper assembly are being used in the industry like scraper blades, wiper blades, scraping cum conveying blade etc. D. Shaft mounting SSHE can be classified on the basis of shaft mounting as (Mgnewell, 2006): Concentric: For most applications the shaft is mounted in the center of the heat transfer tube, or Concentric Eccentric: An off centered shaft mount or Eccentric design is recommended for viscous and sticky products. This shaft arrangement increases product mixing, eliminates mass rotation, and reduces the mechanical heat load. Oval tubes: Oval tubes are used to process extremely viscous products. This design eliminates product channeling within the tube, it reduces mechanical heat by a double cam action of the scraper blades, and it balances the internal forces to prevent shaft deflection. E. Number of stages Scrapes surface heat exchanger may be classified on the basis of stages (single, double, multiple). F. Based on the pass/circulation Scraped surface heat exchanger may be operated on single, double or multiple pass. 7  REVIEW OF LITERATURE 2.3. Factors affecting performance of the SSHE The rate of heat transfer is affected by several factors like effective are of heat transfer, temperature difference, free or forced convention, type of flow, type of product, viscosity of product and fouling of heat transfer surfaces (Kessler, 1981, Chapman, 1984, Heldman, 1992): a. Effective area of heat transfer The rate of heat transfer is directly related to the effective area of heat transfer. b. Temperature difference The rate of heat transfer is directly related to the temperature difference between product and heating/cooling medium. As this temperature difference increases, the rate of heat transfer will increase and vice-versa. c. Type of flow In the indirect heat exchanger, heat transfer depends on the flow pattern of the product. With continuous flow and with rotating scrapers the flow through the exchanger is both radial and axial. Type of flow depends on geometrical shape of heat exchanger, flow velocity rpm of agitator/ scraper and physical properties of the liquid. In this type of exchanger, the flow is the result of the superposition of a Poiseuille flow in an annular space and a Couette flow, on which perturbations created by the blades are added (Trommelen and Beek, 1971a; Naimi, 1989; Burmester, Winch & Russel, 1996). Depending on the rotational speed of the rotor, two different flow regimes can be shown: laminar or vortex flow (Dumont et al., 2000).Trommelen and Beek (1971a) have studied the flow patterns and residence time. At low speeds and flows the flow streams are concentric circles (couette flow) except at the blades where the streamlines are displaced in passing the blade. At higher speeds torodial (taylor) vortices for which have a width equal to the annular clearance. The transition from couette to Taylor flow patterns seems to occur at the same conditions as for a simple rotating inner cylinder i.e. the presence of blades 8  REVIEW OF LITERATURE had no detectable effect. This transition occurs at the Reynolds number (Shah et al., 1988). d. Type of Product The physical properties and flow condition of the product affects the rate of heat transfer. The product is homogenous or heterogeneous, viscosity, density and thermal properties of the product are of importance. If plant is designed only for milk then calculation of heat transfer is not a real problem, but for viscous product calculation and design is complicated as viscosity results into decreased heat transfer rate. The viscous product requires larger heating surface and residence time. The other aspect is that in a liquid, heat is mainly transferred by convection, while in a solid material as conduction. Conduction heat transfer is slower, hence heat transfer rate will be slower. To avoid such problem, the product shall be agitated thoroughly to have uniform distribution of heat energy. e. Viscosity of Product An important factor influencing the overall heat transfer coefficient is the viscosity of the product which is related to the compositional properties. Viscous material requires larger heating time, as the heat transfer will be slower due to lower heat transfer coefficient. Some viscous materials may not be suitable for indirect systems as it will result into severe burning and fouling of the heat transfer surface. In addition to the technological difficulties, high viscosity products may cause problems of less uniform flow. In such cases scraped surface heat exchangers are used, which are suitable for processing liquid foods exhibiting non-Newtonian rheological properties, high viscosity, containing suspended solids or which tends to form deposits during thermal processing. Scraped surface heat exchangers are relatively expensive equipment, and they can be justified only for very viscous fluids, eg, viscosities higher than 10 Pa.s and particulate suspensions (Maroulis and Saravacos, 2003). 9  REVIEW OF LITERATURE f. Fouling on Heat Transfer Surfaces The formation of deposits of milk on heating surface as a result of superheating and partial dehydration of milk is called fouling of heat transfer surface. If the plant will run continuously without intermediate cleaning of the heating surfaces then fouling will increase. The fouling of heat transfer surface cause problems like:  reduced processing rate  product quality is often impaired  removal of deposited material is often a difficult part of cleaning operation  shutdown of plant for cleaning after some time  heat transfer capacity of heating surface is decreased  requires higher temperature difference, hence increased steam consumption  appearance of burnt particles in the final product Deposition of scale on heat transfer surfaces during evaporation lowers the overall heat transfer coefficient. The deposited scale will offer some resistance to the heat flow, which will reduce the overall heat transfer coefficient. In almost heat transfer situations, even a very thin layer of almost any material on the heat transfer surface seriously retards the rate of heat transfer. The layers can be removed very effectively by the use of blades or scrapers and this the principle on which scraped surface heat exchangers are designed. Scraped surface heat exchangers are finding increased use in thermal processing applications, especially for those liquid foods that have suspended solids, high viscosity or display non-Newtonian rheology. g. Annulus –clearance gap between scraper and inner cylinder It is found that as the blade tip clearance is reduced, the overall U heat transfer coefficient based on the total evaporator surface, increases. When the blade tip clearance is reduced to a certain value, however, higher liquid loading does, not cause any change in heat transfer ·co-efficient once the whole surface becomes wetted. Mechanical difficulties arise in reducing the 10  REVIEW OF LITERATURE clearance between the rotating blades and heating surface. Therefore self adjusting blades are ideal (Relay et al.,1963). The gap between the edge of the blade and the wall surface has to be considered. This gap varies with the rotational speed, the shape of the blades and their angle with the exchange surface. The sensitivity of the product to mechanical stress and rheological properties has also to be considered to define optimal operating conditions. These parameters have to be considered to realize a thermal treatment in a SSHE without damaging the product by mechanical treatment (Mabit et al.,2003). h. Overall heat transfer co-efficient (U) The thermal design of SSHE is based on empirical (experimental) overall heat transfer coefficient (U), with typical values in the range of 500 to 1000 W/m 2K, which are relatively low due to the high resistance of thick walls of the heat transfer pipe. In order to obtain a high heat flux, temperature differences Δ T between the heating medium and the product off about 25°C are used, which are much higher than the low Δ T (about 5°C) used in plate heat exchangers (Maroulis and Saravacos, 2003). U value is a function of scraper blade assembly, scraper speed, product properties and temperature gradient. Overall heat transfer coefficient increases with increase in speed of rotor and mass flow rates. Heating can be improved in the transition regions using alternate blade in SSHE, leading to a significant increase of the overall heat transfer coefficient. Right selection of blade geometry can result in higher thermal efficiencies (D‟Addio et al. 2013). i. Speed of the scraper assembly Heat transfer co-efficient increases with the scraping action of the blade. Design of scraper blade assembly also plays an important role in heat transfer (Kohli and Sharma, 1983).Scraper speed may be low or high and will depend on the process requirements(Shah et al., 1988): Low speed scraping- Low speed scraping, 2 to 15 m/min is done for the purpose of removing solids accumulating on the surfaces such as in a 11  REVIEW OF LITERATURE dewaxing or crystallization process. Mere a solid and liquid separation is desired. High speed scraping-The speed of scraping 1 to 15 m/s is often determined by mixing or process requirements rather than high heat transfer rates. In these units the annular clearance is relatively small (shaft dia/shell dia. = 0.75 to 0.65) and blades of several types (wiping, scraping, or fixed close clearance) are used. In some Instances to improve mixing the shaft may be eccentric to the shell or the shell made slightly oval. j. Vapour removal rate It involves either vapor removal through a porous heated surface in nucleate or film boiling, or fluid withdrawal through a porous heated surface in single- phase flow (Rajput et.al, 2014). 2.4. Heat transfer studies in SSHE SSHEs are most often used for products that are highly viscous in nature. For evaporation, thin film operation is used where process liquid moves along the heat transfer surface as a thin film. The greatest advantage of SSHEs for either heating or cooling is the constant removal of the stagnant film near the wall and subsequent increase in the heat transfer coefficients and reduction in fouling at the wall. Literature survey on SSHE show that studies on heat transfer & residence time distribution is far more extensive than on flow behavior phenomena or power consumption. The heat transfer is one of the most discussed phenomena in case of Scraped surface heat exchangers (Skelland et al., 1962; Trommelen 1967; De Goede and De Jong, 1993; Hillmann and Walzel, 2014). Bolanowski and Lineberry (1952) measured overall heat transfer co-efficients for various food products using a votator-type SSHE. They presented overall 12  REVIEW OF LITERATURE heat transfer coefficients for a variety of food products (margarine, starch, eggs, peas, applesauce, milk, etc.) with a variety of heat transfer media. Investigations were also carried out to find the type of heat transfer mechanisms occurring in SSHEs during evaporation. Two types of mechanisms were suggested. Kramers et al. (1955) suggested the first mechanism. According to this, heat transfer takes place by conduction across the vapor films combined with evaporation of the more volatile component at the surface of the film. It takes place at low values of specific heat flux. The various forces acting on the film include internal friction, force of gravity and peripheral forces created by rotation. Second mechanism is based on penetration theory. The penetration theory to a two-bladed rotator type SSHE and arrived at an empirical relation for Nusselt number with limitation of heat transfer on viscosity and velocity of liquid through the exchanger. (Lateinen et.al, 1958) Nu = 1.6 (Re)0.5 (Pr)0.5 Krischbaum and Dieter (1958) conducted evaporation studies and showed that scraped heat transfer coefficient decreased with increasing ∆T at constant rotor speed, low flow rate and higher value of ∆T.The following correlation was observed. hs = 437 (N)0.33k /  Bjorklund and Kays (1959) and Simmers and Coney (1979) have reported heat transfer between the outer surface and the fluid in an annulus with use of Reynolds‟s analogy and come up with an equation. Cuevas et al. (1982) studied the performance of SSHE under ultra high temperature conditions. The U-values is a function of blade rotational speed and mass flow rate. Trials on water has indicated that U-value increase as a result of scraping up to certain point and further increase in blade speed did not induce any significant improvement in the transfer of heat but lower the U-value at very high value of blade speed. Use of agitation nozzle in the jacket improved the U -value of the vessel by three fold. 13  REVIEW OF LITERATURE Qin et.al (2006) examined heat transfer coefficient of a laboratory scraped- surface heat exchanger (SSHE) for freezing operation. The heat transfer coefficient with phase change (ice formation) was about three to five times greater than sensible heat transfer. A mathematical model of an ice cream freezer (SSHE) was developed by considering the freezer barrel as a series of well mixed stages and employing heat and mass transfer equations by Bongers (2006). The developed model can be used to predict local temperature and shear conditions within ice cream freezer for scale up design. Landfeld (2006) used scraped surface homogenizer for heating and cooling of granular stuffs and found relationship of Nusselt number and Peclet number for determining surface heat transfer co-efficient. Dodeja et.al (1990) studied heat transfer during evaporation of milk to high solids in thin film scraped surface heat exchanger. Variation in mass flow rate, steam condensing, temperature and their effect on scraped film co-efficient was discussed. Appropriate dimensionless groups were formulated and fitted in model and come up with correlation. In scraped heat exchanger crystallizer, influence of flow field on wall temperature and local heat transfer distribution has been studied by direct measurements of heat exchanger surface temperature and fluid velocity field inside the crystallizer (Rodriguez et.al, 2008). Bocaardi et.al (2010) developed heat transfer correlation with dimensional analysis. Thermal behavior of a new kind of scraped surface heat exchanger operated in laminar regime to treat hazelnut paste assessed. This unit, A- SSHE with an alternate arrangement of the scraping blades improves doubling the exchanger thermal efficiency with back mixing of fluids. Further, estimation of overall heat transfer coefficient with respect to process parameters: blades rotational speed, flow rate and wall temperature was carried out using 3D-CFD simulation (D‟Addio., 2013). 14  REVIEW OF LITERATURE 2.5 Power consumption in SSHE Several studies have especially evaluated the power consumption in the scraped surface heat exchanger (Benezech & Maingonnat, 1988; Cox, Gerrard, & Wix, 1993; Trommelen & Beek, 1971b). Power requirements for very low Reynolds numbers calculated analytically for hinged blades by Toh and Murakami (1982a) and for floating blades by Toh and Murakami (1982b) considering shape of the blades, number of blades in a set, scraping angle of the blades and slit in the blades. On the other side, Weisser (1972) and Dinglinger (1964) found increased power requirements due to friction during freezing with formation of crystals and such phenomena was also reported by Qin et.al (2006) using laboratory type scraped surface heat exchanger. 2.6 Residence time distribution in SSHE The residence time distribution (RTD) of a heat exchanger or reactor is a probability distribution function that describes the amount of time that a fluid element could spend inside the equipment. The time the particles or molecules have spent in the thermal treatment unit is called the residence time and the distribution of the various particles coming out the unit with respect to time is called the residence time distribution. The residence time distribution in heat exchangers (e.g., heaters, coolers, evaporators, dryers) has an important effect on the quality of the food product. The product should remain in the heat transfer equipment only for the minimum time required to accomplish the desired effect, such as sterilization or concentration. Any additional residence time is undesirable, because the quality and nutritive value of the product may suffer heat damage. Three types of distribution are possible in continuous flow heat exchangers: plug flow, where the product flows through and exits the equipment without mixing; complete mixing, where the product is mixed thoroughly in the equipment and exits gradually; and intermediate mixing, where the product is partially mixed and exits gradually from the equipment. The mean residence time (t m, s) in a continuous flow equipment is related to the holdup volume (V, m 3), the 15  REVIEW OF LITERATURE product flow rate (m, kg/s), and the density of the product (ρ, kg/m 3) as follows (Saravacos and Kostaropoulos, 2002): Tm = V ρ / m Ramaswamy et al. (1995) reported that residence time distribution (RTD) of particles is key factors that play a role in aseptic processing. In a continuous flow system, some of the particles remain in the system longer than others, residence time of particles must be as narrow as possible in order to reduce the degree of over heating/ cooling during processing. Extensive studies which involve research of residence time distribution of liquid and with particulates within it. Various types of methods used in determination of RTD includes optical methods, use of magnetic particles, magnetic resonance imaging, digital image analysis, chemical tracers (dye or components), fractional collection, metal detectors, radioactive tracers, and ultrasonic Doppler velocimetry methods. Some of techniques based on computerized simulation which gives the flow behavior of system with in short time. Determining RTD of liquids introducing a tracer solution has been widely adopted. Bateson (1971) examined distribution of fluid particle in heat exchanger using saline solution and Milton and Zahradnik (1973) performed research for a heat-cold-cool system using tracer in glycol solution as carrier fluid. The concentration was measured by means of polarimeter to analyse RTD. Sancho and Rao (1992) used a pulse input of sodium chloride solution to examine RTD of various solutions flowing in a straight holding tube. Fluid residence time distributions within the SSHEs were measured using a tracer-response technique reported by Levenspiel (1972). Abichandani and Sarma (1988) analyzed a similar analysis to determine the RTD of water in a scraped surface heat exchanger (SSHE). Saturated NaCl solution was used as the disturbance at the inlet and outlet as response was measured at periodic intervals of 5 by a conductivity bridge. This relates the studies previously carried out by Lee et al. (1996) using conductivity change to determine the RTD of fluids. 16  REVIEW OF LITERATURE Earlier researchers de Ruyter and Brunet (1973), Manson and Cullen (1974), and Dail (1985) observed RTD of particles within the SSHE to be critical. They suggested fastest moving particle must be used for process calculations. Taeymans et al. (1985) were first to perform experimental RTD work on water and calcium alginate beads mixtures and effects of physical and fluid properties on RTD. They determined RTD through Reynolds and centrifugal Archimedes number. In contrast, Alcairo and Zuritz (1990) showed that increased particle diameter, mutator speed, and flow rate narrowed the residence time distribution, while carrier fluid viscosity has no effect on the RTD curve. Alcairo and Zuritz (1990) was investigated Residence distribution time of scraped-surface heat exchanger using spherical particles suspended in non- Newtonian fluid. Lee and Singh (1991) studied particle residence time distribution of particles in a horizontal scraped surface heat exchanger. Further, Lee and Singh (1990) determined mathematical model based RTD of vertical and horizontal SSHE using potato particles. Alhamdan and Sastry (1997) determined RTD of transparent SSHE and holding tube by videotaping in order to detect tracer particles flowing through system. A mirror was used to inspect the particles that were blocked by the shaft of the SSHE. Residence times of the particles were obtained through the built-in digital stopwatch. Relatively simple technique based on introducing particles as pulse was suggested by Abdelrahim et al. (1993) for evaluating the RTD of a scraped surface heat exchanger and holding tube. Over a period of time, Wang et.al (2002) studied influence of processing conditions on particle damage as results of residence time distribution using potato cubes in carboxymethyl cellulose solution as carrier fluid in SSHE. Mabit et al. (2008) developed a tracer in order to follow the thermal treatment of Newtonian or Non-Newtonian fluids in scraped surface heat exchanger in conditions similar to pasteurization. The time-temperature integrator based on degradation of Betanin was carried out experimentally and compared with a dispersion RTD model. Belhamri et al. (2009) studied the product flow rate and in rotational speed led to a narrowing of the RTD in SSHE used for cooling. Recently, Arellano et al. (2013) investigated influence of the operating conditions on the 17  REVIEW OF LITERATURE RTD and the axial temperature profile of the product in a SSHE to characterize the product flow behavior. RTD experiments were carried out in a continuous laboratory pilot-scale SSHE by means of a colorimetric method. 2.6 Application of scraped surface heat exchanger (SSHE) 2.6.1 Freezing The scraped surface of the SSHEs is kept clean and has an exceptionally high heat transfer rate that remains constant as long as the cooling medium continues at design. The scraping action also minimizes product pressure drop. Because of the rapid rate of cooling, SSHEs provide an excellent product. Ice cream plants use this type of freezer to harden ice cream to 20°F (- 7°C) or so before it is packaged and directed to the hardening process where it is frozen to -20°F ( - 29°C). The citrus industry uses SSHEs to freeze citrus concentrates to about 20°F (- 7°C) before canning. The cans are then directed to a belt freezing system where the product is frozen. Other products that are cooled or partially frozen by SSHEs include: margarine, butter, process cheese, chili, baked beans, pet foods, and marshmallow topping (Briley et al., 2004). Freezing of ice cream is performed in a scraped surface heat exchanger, where rotating scraper blades continually remove frozen product from the cooled surface and thus maintain a high heat transfer rate. It is within the freezer barrel that much of the product structuring occurs. These product structuring mechanisms include ice crystallization, aeration and fat de- emulsification. The quality of the final product depends to a large degree on how these structuring processes have been carried out. In order to optimize the freezing process for a given product formulation or to maintain a desired product quality on scale-up, it is necessary to know the local conditions inside the heat exchanger and how these change with operating conditions. Since direct measurement of temperature and shear conditions in a freezer barrel is difficult to achieve, a mathematical modeling approach has been applied in this work to predict these quantities (Bongers et al., 2006). 18  REVIEW OF LITERATURE 2.6.2. Crystallization Chong et al. (2001) studied a scraped-surface heat exchanger for an ice- slurry application. They accounted for the thermodynamic properties of ice slurry in the development of their model, which was formulated in terms of both the axial and rotational ReynoIds numbers. The experimental results showed that the influence of rotational speed on the heat transfer coefficient was not directly proportional. Moreover, the influence of axial flow rates in 1aminar region was study on the ice production, where the heat transfer coefficient was not significantly affected. Scraped surface heat exchangers (SSHEs) are widely used in the food industry for crystallization applications in several food processes, such as the crystallization of margarine (Shahidi, 2005), the tempering of chocolate (Dhonsi and Stapley, 2006), the freeze concentration of milk and the freezing of sorbet or ice cream (Cook and Hartel, 2010). Scraped surface heat exchangers (SSHEs) are widely used for crystallization applications in several food processes (i.e. crystallization of margarine, tempering of chocolate, freezing of ice cream and sorbet). The final quality of these food products is highly related to crystal size distribution and apparent viscosity, both of which are determined by the operating conditions. 2.6.3 Heating-Cooling Scraped surface heat exchangers (SSHEs) are one of the most versatile pieces of processing equipment. SSHEs handle products that are viscous, that contain particles, and that tend to deposit and form films on the heat transfer surface. Operational versatility makes SSHEs especially attractive for food processors (Darlington, 1972). SSHEs are mechanically aided, turbulent film heat exchangers. The rotating scraper blades continually remove any deposition on the heat exchanger surface, thereby maintaining the heat transfer rate and enabling extended runs without fouling (McDonald, 1974). SSHEs are used for sterilization of particulate foods (de Ruyter and Brunet, 1973). For the sterilization process, particulate foods are pumped through the SSHE under constant scraper action. The critical factors in determining the efficacy of SSHEs in heat sterilization are the residence time distribution of 19  REVIEW OF LITERATURE the particulate matter, the overall heat transfer coefficients and the potential for damage to the particulates (Alhamdan and Sastry, 1998). Whey proteins (WP) were heated in a scraped-surface heat exchanger to produce products with different levels of denaturation. Model processed cheese spreads containing the WP were prepared. Rheological measurements showed that higher levels of denaturation in the WP produced softer (lower elastic modulus) cheeses. Temperature scans indicated that the cheeses prepared with high levels of native whey proteins did not melt (the elastic modulus was higher than the viscous modulus at all temperatures), whereas those prepared with high levels of denatured whey proteins melted on increasing the temperature. Micro structural examination of selected cheeses indicated that, when denatured whey proteins were used in processed cheeses, the whey proteins were incorporated as large aggregates dispersed within the cheese matrix. These aggregates did not contribute to, and may disrupt the structure of the cheese. When native whey proteins were used, they were incorporated into the cheese matrix, producing a denser network structure (Lee et al., 2013). SSHEs are also used for the chilling of low fat emulsion spreads and whole milk powder (Aceto etal., 1996). 2.6.4 Product manufacturing SSHEs in thin-film operation are extensively used in ultra-high temperature sterilization of milk, manufacture of lactose from whey, and continuous production of ethnic products like ghee and khoa (Dodeja et al., 1989). Equipment like Multi-Purpose three Cylinder Thin Film Scraped Surface Heat Exchanger (closed/ open circuit with specially designed scrapers to have thin film and continuous/ batch operation) for indigenous milk products designed. This machine can be used to manufacture valued TIDP like basundi, kulfi mix, kheer, khoa, peda, thabdi, burfi, gajar halwa, and dhudhi halwa with better hygienic rheological qualities and improved shelf life at lower cost of processing (Patel et al., 2006). Three stage SSHE which was designed and fabricated for continuous manufacture of khoa is used for establishing the efficacy of the equipment for 20  REVIEW OF LITERATURE manufacture of basundi in continuous mode. Comparison of the quality of product in terms of sensory and physico-chemical characteristics was made using white crystalline and caramelized sugar. Optimization of process parameters in first stage and second stage is also made for acceptable product quality. It is established that excellent product quality is made using rotor speed as 125 rpm in first and second stage SSHE using caramelized sugar (Singh and Dodeja, 2012). Development of an industrial scale khoa making plant has been a challenge to dairy scientists since several years. Many efforts have been made in mechanization of khoa making process to improve heat transfer and also to overcome the drawbacks of traditional methods as well as to commercialize the process for industrial requirements. Extensive studies were also carried out on the hydrodynamics and heat transfer of thin film scraped surface heat exchanger (TFSSHE) for processing of liquid, concentrated liquids, and particulate viscous products. 2.7 Fan and blower systems Fans and blowers are mainly used for ventilation and number of industrial process requirements. Fans generate a pressure to move air against a resistance caused by ducts, dampers, or other components in a fan system. The fan rotor receives energy from a rotating shaft and transmits it to the air (Balkrishna et al., 2014). Fan and blower selection depends on the volume flow rate, pressure, type of material handled, space limitations, and efficiency (Ain and Gupta, 2014). Fan efficiencies differ from design to design and also by types. In centrifugal flow, airflow changes direction twice - once when entering and second when leaving (Forward curved, backward curved or inclined, radial). In axial flow (propeller, tube axial, vane axial), air enters and leaves the fan with no change in direction (BEE, 2005). There are two primary types of fans centrifugal and axial fans. These types are characterized by the path of the airflow through the fan. 21  REVIEW OF LITERATURE 2.7.1 Centrifugal fans Centrifugal fans use a rotating impeller to move air first radially outwards by centrifugal action, and then tangentially away from the blade tips. Incoming air moves parallel to the impeller hub and it turns radially outwards towards the perimeter of the impeller and blade tips. As the air moves from the impeller hub to the blade tips, it gains kinetic energy. This kinetic energy is converted to a static pressure and increase the pressure of the air or gas stream that in turn moves them against the resistance caused by ducts, dampers and other components (Kumar and Rubinson, 2014). Centrifugal fans are capable of generating relatively high pressures. They are frequently used in „„dirty‟‟ airstreams (high moisture and particulate content), in material handling applications and in systems operated at higher temperatures (Vijay and Chaturvedi, 2010). Centrifugal blower consists of an impeller with small blades on the circumference, a shroud to direct and control the air flow into the center of the impeller and out at the periphery. The blades move the air by centrifugal force and throwing it out, thus creating suction inside the impeller and suction duct. The pressure rise and flow rate in centrifugal blowers depend on the peripheral speed of impeller and blade angles (Pathak et al., 2012). 2.7.2 Axial fans Axial fans move an air stream along the axis of the fan. The air is pressurized by the aerodynamic lift generated by the fan blades. Axial fans are commonly used in “clean air,” low-pressure, high-volume applications (Vijay and Chaturvedi, 2010). Axial fans have less rotating mass and are more compact than centrifugal fans of comparable capacity. Additionally, axial fans tend to require higher rotational speeds and are somewhat noisier than inline centrifugal fans of similar capacity (EERG, 2008). 22  REVIEW OF LITERATURE Radial blades Radial Modified radial outlet blades housing open radial blades Centrifugal fans Forward curved Scroll Fans housing Airfoil Backward curved Propeller Axial fans Backward Tube axial inclined Vane axial Fig. 2.4 Classification of fan 2.7.3 Selection criteria The selection criteria for blower/fan is as follows (LCC, 1999) Air volume required (CFM) System resistance Air density (Altitude and Temperature) Type of service Environment type Materials/vapors to be exhausted Operation temperature Space limitations Fan/blower type Drive type (Direct or Belt) Noise criteria Number of fans/impellers Discharge Rotation Motor position Expected life in years 23  REVIEW OF LITERATURE Table 2.1 Classification of centrifugal fans Type Characteristics Applications Radial High pressure, medium flow, Various industrial efficiency close to tube axial, application, suitable for power increases continuously. dust, laden, moist air/gases Forward Medium pressure, high flow Low pressure, HVAC, curved blades rate, din in pressure curve, packaged units for clean efficiency higher than radial and dust laden air/gases. fans, power rises continuously. Backward High pressure, high flow rate, HVAC, various industrial curved blades high efficiency, power reduces applications forced draft as flow increases beyond point fans, etc. of highest efficiency. Airfoil type Same as backward curved type, Same as backward curved, higher efficiency. but for clean air applications. Source: BEE, 2005 Table 2.2 Classification of axial flow fans Type Characteristics Applications Propeller Low pressure, high flow rate, Air-circulation, ventilation, low efficiency, peak efficiency exhaust close to point of free air delivery (zero static pressure) Tube-axial Medium pressure, high flow, HVAC, drying ,ovens, higher efficiency than propeller exhaust systems type, dip in pressure flow curve before peak pressure point Vane-axial High pressure, medium flow High pressure application rate, dip in pressure-flow curve, including HVAC systems use of guide vanes improves efficiency exhausts Source: BEE, 2005 Shrinath and Reddy (2013) described operating principle is a combination of two effects: Centrifugal force which produces more static pressure and again deflection of the air flow by the blades, but here the deflection is from a radial 24  REVIEW OF LITERATURE outward direction in to a spiral flow pattern. In case of forward curved blades the air deflections have a strong influence on the flow pattern on the performance. Wagh and Panchagade (2014) termed fans and blowers are turbo machines which deliver air at a desired high velocity (and accordingly at a high mass flow rate) but at a relatively low static pressure. The pressure rise across a fan is extremely low and is of the order of a few millimeters of water gauge. The upper limit of pressure rise is of the order of 250mm of water gauge. The rise in static pressure across a blower is relatively higher and is more than 1000 mm of water gauge that is required to overcome the pressure losses of the gas during its flow through various passages. A blower may be constructed in multi stages for still higher discharge pressure. A large number of fans and blowers for relatively high pressure applications are of centrifugal type. A blower consists of an impeller which has blades fixed between the inner and outer diameters. The impeller can be mounted either directly on the shaft extension of the prime mover or separately on a shaft supported between two additional bearings. Air or gas enters the impeller axially through the inlet nozzle which provides slight acceleration to the air before its entry to the impeller. The action of the impeller swings the gas from a smaller to a larger radius and delivers the gas at a high pressure and velocity to the casing. The flow from the impeller blades is collected by a spiral-shaped casing known as volute casing or spiral casing. The casing can further increase the static pressure of the air and it finally delivers the air to the exit of the blower. A centrifugal fan impeller may have backward swept blades, radial tipped blades or forward swept blades. The backward swept blades are employed for lower pressure and lower flow rates. The radial tipped blades are employed for handling dust-laden air or gas because they are less prone to blockage, Dust erosion and failure. According to Mark Stevens (2007) fan and air system market is mature. There is a wide variety of fans to fill market niches that have specific performance requirements. For example, centrifugal fans with narrow blades operating at high speeds are suited for systems requiring low volume flow rate and high 25  REVIEW OF LITERATURE pressure. Propeller fans are generally used to move air against low pressures from one open space to another. Systems handling material often have radial bladed fans. Fans in residential applications are smaller and have single phase motors. This case addresses a simple ventilation system in which the general fan type is already known. This means that with rudimentary knowledge in the hypothetical fan selection process, the range of available fan sizes and models can be significantly narrowed to a much smaller range that best fits the system‟s requirements. A system designer can select a fan, specifically suited for a job, which achieves a balance of maximizing efficiency and minimizing cost. However, the designer must understand how system design affects performance. 26