¢º DESALINATION        ¢º PLATE TYPE HEAT EXCHANGER        ¢º FIN-TUBE TYPE HEAT EXCHANGER  
 




   The need for water is rapidly increasing, and current freshwater resources will not be able to meet all requirements.
A variety of desalting technologies has been developed over the years and, based on their commercial success, they can be classified into the major and minor desalting processes. The major process is thermal process, multi-stage flash distillation, multiple-effect distillation, vapor compression, with phase change and membrane process, electrodialysis and reverse osmosis, without phase change.
The minor process is freezing and solar humidification method. Two main directions survived the crucial evolution of desalination technology, namely evaporation and membrane technologies. The multi-stage flash(MSF) process is the most common technique for desalination. Among other evaporation techniques, the multi-effect distillation(MED) may be mentioned here, either with vertical or horizontal smooth tubes or doubly fluted tubes.
The vapor compression course is very popular for remote locations. Membrane processes, mainly reverse osmosis(RO), are currently the fastest-growing techniques in water desalination.

  The MSF process is the most common technique for desalination. It has operated commercially for more than 30 years.
In the MSF process, seawater is heated in a vessel called the brine heater. This is generally done by condensing steam on a bank of tubes that carry seawater which passed through the vessel.
This heated seawater then flows into another vessel, called a stage, where the ambient pressure is lower, using the water to immediately boil. The sudden introduction of the heated water into the chamber under reduced pressure condition causes it to boil rapidly, almost exploding or flashing into steam. Generally, only a small percentage of this water is converted to steam(water vapor), depending on the pressure maintained in this stage, since boiling will continue only until the water cools(furnishing the heat of vaporization) to the boiling point.

¢Æ Schematic diagram of a Multi-Stage Flash desalination plant.
   The MED process has been used for industrial distillation for a long time. Basically, the method can use low-temperature, low-pressure steam as the main energy source. MED takes place in a series of vessels(effects) and uses the principles of condensation and evaporation at reduced ambient pressure in the various effects. This permits the seawater feed to undergo boiling without the need to supply additional heat after the first effect. In general, an effect consists of a vessel, a heat exchanger, and devices for transporting the various fluids between the effects. Unlike the MSF technique where is produced mainly by turning sensible heat into latent heat of evaporation, the MED technique uses latent heat to produce secondary latent heat in each section. There are several methods of adding the feed water to the system. Adding feed water in equal portions to the various effects if the most common. The feed water is sprayed or otherwise distributed onto the surface of the evaporator surface in a thin film to promote rapid boiling and evaporation after it has been preheated to the boiling temperature on the upper section. MED process classify into ST(submerged tube design), VTE(vertical tube evaporator design), LTV(long tube vertical design) by configuration of evaporator.

¢Æ Schematic diagram of a mechanical vapor compression unit

The VCD operates mainly at a small scale, on small locations. The main mechanism is similar to MED except that it is based on compression of the vapor generated by evaporating water to a higher pressure, which allows reuse of the vapor for supplying heat for the evaporating process. The plants that use this process are also designed to take advantage of the principle of reducing the boiling point temperature by reducing the pressure. Steam ejectors and mechanical compressors are used in the compression cycle to run the process. The mechanical compressor is usually electrically or diesel driven, allowing the sole use of electrical or mechanical energy to produce water by distillation.


¢Æ Schematic diagram of a mechanical vapor compression unit
The vapor gains heat energy by being compressed by the vapor compressor. All steam is removed by a mechanical compressor from the last effect and introduced as heating steam into the first effect after compression where it condenses on the cold side of the heat transfer surface. Seawater if sprayed, or otherwise distributed on the other side of the heat transfer surface share it boils and partially evaporates, producing more vapor. The VCD operates mainly at a small scale, on small locations. The main mechanism is similar to MED except that it is based on compression of the vapor generated by evaporating water to a higher pressure, which allows reuse of the vapor for supplying heat for the evaporating process. The plants that use this process are also designed to take advantage of the principle of reducing the boiling point temperature by reducing the pressure. Steam ejectors and mechanical compressors are used in the compression cycle to run the process. The mechanical compressor is usually electrically or diesel driven, allowing the sole use of electrical or mechanical energy to produce water by distillation. The vapor gains heat energy by being compressed by the vapor compressor. All steam is removed by a mechanical compressor from the last effect and introduced as heating steam into the first effect after compression where it condenses on the cold side of the heat transfer surface. Seawater if prayed, or otherwise distributed on the other side of the heat transfer surface share it boils and partially evaporates, producing more vapor.

 
The RO membrane technique is considered the most promising for brackish and seawater desalination with successful commercialization occurring in the early 1970s. The RO is a membrane separation process in which the water from a pressurized saline solution is separated from the solutes(the dissolved material) by flowing through a membrane. No heating or phase change is necessary for this separation. The major energy required for desalting is for pressurizing the feed water. The RO uses dynamic pressure to overcome the osmotic pressure of the salt solution, hence causing water-selective permeation from the saline side of a membrane to the freshwater side. Salts are rejected from the membrane, and hence, the separation is accomplished. The RO system is made up of the pre-treatment, high-pressure pump, membrane assembly, and post-treatment. Pre-treatment is important in RO because the membrane surfaces must remain clean. Therefore, suspended solids must be removed and the water pretreated so that salt precipitation or microbial growth does not occur on the membranes. Post-treatment consists of stabilizing the water and preparing it for distribution. Two developments have helped to reduced the operating cost of RO plants during the past decade: the development of more efficient membranes and the use of energy recovery devices. It is common now to use energy recovery devices connected to the concentrate stream as it leaves the pressure vessel at about 1 to 4bar less than the applied pressure from the high-pressure pump. These can have a significant impact on the economics of operating large plants.

¢Æ Schematic diagram of reverse osmosis desalination plant with energy recovery unit
 

  

  A plate type heat exchanger consists of a stack of corrugated metal plates pressed together in a frame and sealed at their edges by a compressible gasket to form a series of interconnected narrow passages through which fluids are pumped.
  The hot and the cold fluids in alternate passages, and heat is transferred through the thin plates with relatively low thermal resistance. The corrugations are designed to provide enhancement of fluid heat transfer as well as mechanical support.

  1. Advantage of Plate heat exchangers

    (1) Flexibility
  The plate heat exchanger can be adapted to a wide range of fluids and conditions and can be modified to meet changing performance requirements.

    (2) Compactness
  A very large surface area can be formed in a small volume and the enhanced heat transfer associated with the narrow passages and corrugated surfaces produces a high overall heat transfer coefficient

    (3) Low Fabrication Costs
  Simple pressing combined with no welding means a low cost per unit surface area particularly with more expensive corrosion-resistant materials. For example, a stainless-steel plate exchanger may cost less than a carbon steel shell-and-tube unit designed for the same duty.

    (4) Ease of Cleaning
  Plate heat exchangers with gaskets can be dismantled for cleaning, and this may be a significant advantage.

  2. Plate Configuration
  The plates are of three types as far as the thermal duty is concerned. First, Low-NTU(Short duty) plates give a low heat transfer coefficient and a low pressure drop and they have high flow rates.  The angle of chevrons is low. Second, High-NTU(Long duty) plates give a high heat transfer coefficient and a high pressure drop and they have low flow rates and the angle of chevrons is high.
  An additional degree of flexibility in obtaining the required combination of heat transfer and pressure drop can be achieved with chevron plates by mixing high-NTU plates with low-NTU plates in correct proportion.


¢Æ Flow Pattern of the Plate Type Heat Exchanger

¢Æ Cross Section of the Herring Bone Plates



  Fin-tube heat exchangers form one of the main categories of compact heat exchangers designed to pack a high heat transfer capacity into small volume.
  These are units of tube bundles with a hot process liquid flowing inside the tubes and air flowing in cross-flow over the tubes to cool the process liquid.
  Finned tube heat exchangers are extensively employed in various industrial application. They are quite compact, light weight, and characterized by a relatively low cost fabrication.

   For practical application, the dominant resistance is usually on the airside; therefore, the use of enhanced fin surfaces is very common to effectively improve the overall heat transfer performance. In general, the most of air coolers are fin-tube type heat exchangers with various fin geometries for increasing heat transfer area on the air side because a air passing the fin side has low thermal conductivity.
  When the heat transfer coefficient on the outside of a tube is much lower than that on the inside, the use of externally enhanced surfaces are the following advantages.
  A decrease in the heat transfer surface, volume, and weight of an exchanger for a given heat duty, flow rate, and pressure drop. An increase in the heat transfer for a given size unit, flow rate, and pressure drop. This will be achieved when the plain tubes are replaced by the enhanced tubes.
  A decrease in the pressure drop for a given size unit and heat duty. A very large number of different surface geometries is available, and there is a large number of geometrical parameters associated with each type of tube-fin surface.