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evaporators and its types and its applications

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  1. 1. EVAPORATOR The name evaporator refers to the evaporation process occurring in the heat exchanger. In an evaporator, the refrigerant boils or evaporates and in doing so absorbs heat from the substance being refrigerated. An evaporator must be consist of a pump and condenser, heat exchanger, valves and a heated bath. Evaporator consists of a heat exchanger for boiling the solution and a means to separate the vapor from the boiling liquid. Different types are categorized by the length and alignment (horizontal or vertical) of the evaporator tubes. The evaporation tubes may be located inside or outside of the main vessel where the vapor is driven off. Figure 1 Evaporator There are three main measures of evaporator performance:  Capacity (kg vaporized / time)  Economy (kg vaporized / kg steam input)  Steam Consumption (kg / hour) INDUSTRIAL APPLICATIONS  Used in pharmaceuticals, foods, pulp and paper, chemicals and polymers, inorganic salts, acids, bases.  In pharmaceuticals industry it is used in the production of drugs, yeast extract, glycerin and sweet water.  In chemical industry it is used in the manufacture of Calcium Chloride, Caustic Soda, Pigments, Paints, Sodium Nitrate, and Magnesium Chloride. TYPES The most common types of evaporators are given as follow;  Falling Film Evaporators  Rising Film Evaporators  Thin Film Evaporators  Multi Effect Evaporators FALLING FILM EVAPORATOR In Falling Film Evaporator the liquid flows downwards in the form of a film along with the vapors at the central in a parallel flow. The mechanism of film forming and low residence time makes this evaporator suitable for heat sensitive, non-salting and non-crystallizing solutions. The liquid to be evaporated is initially pre-heated to the boiling temperature and enters the heating tubes through
  2. 2. distribution plates in the top bonnet of the evaporator. In falling film evaporator there is sufficient wetting of the heating surface and it is very important for the heat economy and trouble free operation. The wetting rates can be increased by extending or splitting the individual evaporator effects. Vapor liquid separation is performed at the bottom of the heater in a separated vapor body for obtaining very low entrainment. FEATURES OF FALLING FILM EVAPORATOR  Appropriate for low temperature applications (heat sensitive applications)  Steam economy is good  Multi-effect combination possible  Not very expensive DESIGN FEATURES  Well-designed distribution plate.  Optimum selection of variable parameters like temperature and pressure.  Accurate calculation of wetting surface area.  Proper design of vapor separator.  Hold up time is small.  Heat transfer co-efficient at all temperature is good. INDUSTRIAL APPLICATIONS  Dairy  Food  Pharmaceutical  Chemical RISING FILM EVAPORATOR In rising film evaporator the vapor bubbles produced during boiling causes the liquid and vapors flow upwards in a parallel flow. Simultaneously the vapor quantity also increase forming a "Climbing" film on the walls of a tube. Due to this there is a high turbulence in the liquid and these evaporators are beneficial for heat sensitive, highly viscous and moderately scaling liquids. The temperature difference in the rising film evaporators should be high between the heating chest and boiling chamber to ensure the circulation and formation of a climbing film. These evaporators are flexible enough to be converted to Forced Circulation Evaporators. FEATURES OF RISING FILM EVAPORATOR  Ideal for liquids which have high viscosity and have fouling tendency.  Suitable for high temperature application.  Steam economy is better as compared to falling film evaporator.  Not very expensive DESIGN FEATURES Easy removal of deposits which are achieved by increasing the feed rate or reducing steam rate.
  3. 3. INDUSTRIAL APPLICATIONS  Chemical Industry  Effluent Treatment Plants  Thin Film Evaporator  Thin film is categorized by external condensers that operate at high pressure (1-100 mbar) and can be used for effective separation of large quantities of components with high volatility. FEATURES THIN FILM EVAPORATOR  These are used for reducing the volume of water from the solids and to separate a mixture of liquids having different boiling points. These can be used to increase the concentration of liquids that are highly viscous or in paste and syrup forms.  Continuous processing  Heat sensitive, viscous, fouling and foaming products DESIGN FEATURES The Thin Film Evaporator consists of a jacketed or coated shell having a machined surface on the inner side. It also has a vapor nozzle that is typically arranged at the upper end of the evaporator in order to facilitate the vapors to the external condenser. The liquid flow can be of both bottom to top or top to bottom and this flow pattern depends on the fluid properties, chances of scale or crystal formation. Further, the rotating blades of thin film evaporators make sure that no material stays on the heat transfer surface for long duration thus preventing thermal degradation. INDUSTRIAL APPLICATIONS  Food Industry  Chemical Industry  Pharmaceutical Industry  Environmental Protection  Multi-Effect Evaporator The Multi Effect Evaporators are designed to economize the consumption of energy. These are designed in such a way that the steam from an outside source is condensed in the heating element of first effect. Further for second effect the boiling temperature of first effect is just too high that evaporated water can serve as the heating medium. The formed vapors than send to condenser, if it is the double effect evaporator But in the multi effect evaporators, the feed is generally transferred from the one effect to another, which leads to ultimate product concentration which further reaches to one effect of evaporator. There are two feed operations in Multi Effect Evaporator  Backward Feed  Forward Feed
  4. 4. BACKWARD FEED OPERATION In feed backward operation, the process of entering the raw feed into last effect takes place and its discharge works as a feed for the next to last effect. It is very commercial technique as the cold feed in case the feed is cold, as much less liquid must be heated to the higher temperature existing in the early effects. The same process is used to balance the viscosity of good and also to produce good heat transfer coefficients FORWARD FEED OPERATION In forward operation, the raw feed goes through parallel to steam flow to effect and the product comes through the last effect. This technique is useful when the feed is hot and is also useful for the high temperature deposit scale and concentrated product change FEATURES  Efficiently uses heat from steam to evaporate water  Uses distillation process  Used for concentration of liquids in the form of solutions INDUSTRIAL APPLICATIONS  Sugar beet factories  Pharmaceutical industries
  5. 5. FIRED TUBE HEATER Fired tube heater is also known as furnace. Furnace is a thermal enclosure used to heat the materials below or above their melting points. Furnaces and kilns are very similar in design, and are closely related to dryers. The major difference is that dryers only have an outer metal shell, whereas furnaces and kilns have refractory bricks for insulation. Several industries like iron and steel making, nonferrous metals production, glass making, manufacturing, ceramic processing, calcination in cement production etc. employ furnace. Figure 2 Fired Heater COMPONENTS OF A FIRED HEATER  Fire box (it consists of open flames and combustion gases. Temp is about 1800 °F)  Radiant Tubes (receive direct heat from the flames OR burners)  Convection Tubes Convection tubes (process fluid inside them, i.e. oil, etc.) are in the roof of the furnace so NOT in contact with the direct flames in the fire box.  Damper and Stack  Refractory Lining  Burners and Air Registers COMMON PROBLEMS OCCURRING IN FIRED HEATERS  Flameout  Process Fluid Feed Pump Failure  Fuel Composition Changes  Replace Refractory  Coke Formation TYPES OF FIRED HEATERS There are two types of furnaces  Oxy fuel furnace  Resistance furnace  Electric arc furnace OXY FUEL FURNACE  Uses a fuel gas such as propane, natural gas or acetylene  Oxygen or atmosphere is blown in and combined with fuel
  6. 6.  Heat from combustion melts the charge RESISTANCE FURNACE  A current passed through a material resulting in ohmic heating  Radiation heats charge material  Some convection and conduction occurs ELECTRIC ARC FURNACE  A current is passed from separate electrodes creating arcs of ionized gas  Heat is transferred from the arc into the charge material  Can be either DC or AC Figure 3 Different Classifications INDUSTRIAL APPLICATIONS  Metal injection molding  Annealing, quenching, tensile testing, brazing  Diffusion bonding  Sintering  Calcining  Metalizing  Thermal cycling