Heat Transfer By Gavhane Free Download Rar
Theodora Glime
We shall consider steady one-dimensional heat conduction. By steady we mean that temperatures are constant with time; as the result, the heat flow is also constant with time. By one dimensional we mean that temperature is a function of a single dimension or spatial coordinate.
A rising film or vertical long tube evaporator is a type of evaporator that is essentially a vertical shell and tube heat exchanger. The liquid being evaporated is fed from the bottom into long tubes and heated with steam condensing on the outside of the tube from the shell side. This is to produce steam and vapour within the tube bringing the liquid inside to a boil. The vapour produced then presses the liquid against the walls of the tubes and causes the ascending force of this liquid. As more vapour is formed, the centre of the tube will have a higher velocity which forces the remaining liquid against the tube wall forming a thin film which moves upwards. This phenomenon of the rising film gives the evaporator its name.
Applications:There is a wide range of applications for rising tube evaporators, including effluent treatment, production of polymers, food production, thermal desalination, pharmaceuticals, and solvent recovery. Aschner, F.S. & Schaal, M. & Hasson, D. (1971). Large Long-Tube Evaporators for Seawater Distillation. In terms of applications within these industries, rising tube evaporators are mainly used as reboilers for distillation columns, or as pre-concentrators or flash evaporators or pre-heaters designed to remove volatile components prior to stripping.
Thermal desalination.A specific application of rising tube evaporators is the thermal desalination of sea water. Sea water is pumped into the long tubes of the evaporator while the heating media (usually steam) heats it up. As vapour forms inside the tubes it flows upwards. This evaporation occurs under vacuum conditions that allow for the use of lower temperatures.
The main advantage of the rising film evaporator is the low residence time of the liquid feed in the evaporator compared to other evaporator designs like plate-type evaporators. This is crucial because it allows the usage of the evaporator in higher operating temperatures and gives assurance of high product quality despite the product being heat sensitive. Additional advantage is the availability of the option to operate the evaporator as a continuous process which is overall more energy and time efficient than a batch process operation.[1]
High heat transfer coefficients:-Another significant advantage is the relatively high heat transfer coefficient of this evaporator type. This is essential as it reduces the overall heat transfer area requirement which in turn will lower the initial capital cost of the evaporator. This is accentuated by the fact that the components, which consist of a shell and tubes, are easily obtainable with customized designs making them cost effective for construction and ideal for simple evaporation requirements. Moreover, this type of evaporator also can easily contain those widely available vapour separators for foaming products.
While the rising film evaporators are relatively efficient and have several advantages, some literature suggests that they are not as efficient as the vertical or the horizontal tube falling film evaporator. As such, in recent times falling film evaporators are usually chosen in place of rising film evaporators because they have similar advantages as rising film evaporators and have the additional benefits of better efficiency. Moreover, rising film evaporators requires a driving force to move the film against gravity and this causes a limitation because there is a requirement for a sufficient temperature difference between the heating surfaces to provide the driving force.[1]
Considering that rising film evaporators use the same heat transfer principle as a general shell and tube heat exchanger. Therefore, the overall heat transfer rate is crucial in determining the performance of the evaporator. This factor will determine the capacity of the rising film evaporator. The fundamental general formula which gives the overall heat transfer rate is,[3]
This is the contact area of heat transfer which involves the outer surface area of the long vertical tubes that are parallel and in direct contact with the heating media housed within the shell of the evaporator.
The hot fluid in the case of the rising film evaporator would be the steam in the shell side and the cold fluid would be the liquid inside the long tubes. In relation to the overall heat transfer rate, there are several key parameters that affect this characteristic specifically in terms of a rising film evaporator.
For a rising film evaporator, the main paths of heat transfer are conduction and convection. These occur as the steam in the shell side heats up the tube and as the tube heats up the liquid and vapour within. The design of the long vertical tubes in rising tube evaporators promote the formation of the long, thin and continual film of liquid formed by the pressure exerted by the vapour which occupies the centre part of the tube and rises up. This ascending motion of film and vapour in the centre promotes great turbulence which then allows for higher heat transfer coefficients leading to a better efficiency heat transfer. Another significant factor that affects the value of the heat transfer coefficients is the design of the evaporator.
According to the laws of heat transfer, initially the heat transfer rate increases as the temperature difference increases to approach the boiling point of the materials in the case of a constant feed flow rate. Hence, it is generally good to have a large temperature difference for this process. However, as the vapour bubbles gradually fill the entire centre of the tube, the steam pressure reaches a peak value. Beyond this point, the temperature difference, which acts as the main driving force for the heat transfer and also the rising film, will start reducing if there is any increase in steam pressure. In addition to that, there are other constraints in terms of the product quality and consistency when considering increasing the temperature difference. The temperature difference for driving force is also defined heavily by the properties of steam and the boiling liquid.[3]
This is the total contact area between the heating media and the liquid requiring the heating or any intermediate surfaces in between them. In the case of the rising film evaporator this is the specific region between the film and the surface of long tubes and also the surrounding heating media within the shell of the evaporator. As shown in the equation, the heat transfer area is one of the chief factors that affect the rate of conductive heat transfer. Therefore, it would be favourable to maximise the amount of area available for heat transfer. The limitations for these can be in terms of the cost per unit area as increasing the area would mean increasing the length of the tubes and also the shell of the evaporator which can significantly increase the cost of construction and maintenance,.[4][5]
The rising film which is formed due to the pressure of the vapor within the vertical tubes greatly influences the efficiency of the heat transfer. This is because the thickness of the film will affect the heat transfer coefficients as well as the contact area for heat transfer. In relation to this, a thin and long film is favored as it reduces the distance between the two heat transfer surfaces. This would give higher heat transfer coefficients and overall heat transfer rate.
This can be solved in various ways such as having a pre-heating system prior to the main evaporator with the function of heating the liquid feed until the temperature almost approaches the boiling point. This will lower the load capacity of evaporator and reduce the residence time.
In relation to that, the Artisan Rising Film Evaporator is designed to eliminate the majority of the volatile components before stripping and is usually used as a flash evaporator or pre-heater.[citation needed] This design allows the operator to control the feed rate or steam rate in order to remove residues, to adjust to the product behaviour, and to maximise steam economy. This evaporator is appropriate for high temperature application and materials with high viscosity that has a propensity to foul the transfer surfaces.[6]
Semi Kestner, also known as Semi-Rising Film Evaporator, is widely used in sugar industries. This equipment provides Polly-baffle catcher to avoid juice entrapment and there would be more effective juice distribution by means of juice coil and Flushing of juice. This design has high brix of syrup and high vapour pressure is supplied as less steam quantity is required.[8] With short retention time of liquor and good heat transfer design features, the juice flows back without any discharge by going through the heating surface only once.[9]
The temperature difference which is log mean temperature difference between the heating media and the boiling liquid must be high enough to generate sufficient ascending force of the steam vapour in the tube side to force the liquid film to flow upwards. In general, the greater the temperature difference, the better the driving force of the steam. Additionally, having a high temperature difference will also increase the flow rate of the liquid and vapour within the tube. This increase in flow rate causes higher turbulence which results in an increase of the heat transfer coefficient. However, the overall temperature difference has to be within the range of boiling points of the two components as it could affect the quality and the purity of the products.[10]
The determination of the size of a rising film evaporator is generally a sensitive task because it involves obtaining a good understanding of the process requirements and behaviour of the materials involved.[10] In terms of cost efficiency, it is generally accepted that long and thin tubes are relatively cheaper because the thicker shell sizes for shell and tube heat exchangers are usually more expensive. Nonetheless, while weighing in the cost of the construction against the requirement, the size can be always adjusted and customized depending on the required application in producing the desired products. Generally the size of the length ranges between 4 and 8 m and 25 mm to 50 mm in diameter.[11]
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