Heat Interchangers: Most of the chemical and pharmaceutical industries use a variety of heat transfer equipments. The materials to be heated may be liquids, gases, or solids. The heating media is a hot fluid or condensed steam. In pharmacy, operations involved in heat transfer include preparation of starch paste for granulation, crystallization, evaporation, distillation, etc.
In industrial processes, heat energy is transferred by various methods. The heat exchangers are the devices used for transferring heat from one fluid (hot gas or steam) to another fluid (liquid) through a metal wall. Heat interchanges are the devices used for transferring heat from one liquid to another or from one gas to another gas through a metal wall. This classification is vague and may time used interchangeably.
In a heat interchanger, when heat is transferred the film coefficients on both sides of the tube are of the same order of magnitude. To increase the overall coefficient, along with an increase in the coefficient on one side the fluid velocity on both sides is also increased to enhance both the coefficients. The fluid velocity and heat transfer coefficient could be achieved by placing baffles outside the tubes. The baffle arrangement increases the path length and decreases the cross-section of the path of the second fluid. Thus, the provision of baffles increases the velocity of the liquid outside the tubes and also makes it flow at right angles to the tubes. It leads to additional turbulence which reduces the resistance for heat transfer outside the tubes.
Baffles are circular discs of metal with one side cutaway. These discs are perforated through which tubes are fitted. To avoid or at least minimize the leakage, the clearance is kept small between the baffles, shell, and tubes. The baffles are supported on metal rods and are fastened between the tube sheets by set-screws. Baffles are used to create turbulence in the shell side fluid by changing the flow pattern parallel or cross-flow to the tube bundles and thus increase the shell side heat transfer coefficient. It also has a function to support the tube all along its length otherwise the tube may bend. Moreover, these baffles may have horizontal or vertical cuts (segmental baffle) as shown in Fig.1.
The cut portion of the baffle is called a baffle window. It provides the area for the flow of the shell fluid. The baffle window area ranges from 15% to 50%. At 20% cut, the segmental baffle means that the area of the cut-out portion is 20% of the area of the baffle. The spacing between the baffles has significance in terms of pressure drop and heat transfer coefficient. A larger spacing reduces the shell side pressure drop, decreases turbulence and heat transfer coefficient. A smaller spacing increases the turbulence and heat transfer coefficient but the pressure drop may increase significantly, thus the advantage attained due to the higher heat transfer coefficient may be nullified. Thus, baffle spacing is selected based on the allowable shell side pressure drop and the desired heat transfer coefficient. Generally, the minimum spacing of segmental baffles is 1/5th of the shell diameter.
Liquid-to-Liquid Heat Interchangers
The liquid-to-liquid heat interchanger is a piece of single-pass equipment wherein the fluid to be heated is passed only once through the tubes before it gets discharged. Thus, the heat transfer, in this case, is not efficient. The basic construction includes a few modifications and its working remains approximately the same.
Baffles are placed outside the tubes. The presence of baffles increases the velocity of liquid outside the tubes. Baffles make the liquid flow more or less right angles to the tubes, which creates more turbulence. This helps in reducing the resistance to heat transfer outside the tubes. The construction of a liquid-to-liquid heat interchanger illustrates the principle of introducing the baffles into the equipment.
Construction of Liquid-to-Liquid Interchanger:
The construction of a liquid-to-liquid heat interchanger is shown in Fig.2. It consists of baffles, tube sheets, spacer rods, and tubes. The most important parts of any heat interchanger are the baffles. The appropriate size of tube sheets is used for the fabrication. Guide rods are fixed to the tube sheets and tightened using screws. As mentioned before baffles are placed at the right places with the support of guide rods. The baffles are separated with proper spacing using short sections of the same tubing. Tubes are inserted through the perforations in baffles whereas the ends of tubes are expanded into the tube sheets. This whole assembly is enclosed in a shell for introducing the heating fluid. The outlet for the heating fluid is at top of one end of the interchanger. On both sides of the tubes, distribution chambers are provided. At the top of the left-side chamber, an inlet for fluid to be heated is provided. The outlet for the heated fluid is provided near the right-side distribution chamber.
Working of Liquid-to-Liquid Interchanger:
The hot fluid (heating medium) is pumped from the left-side top of the shell. The fluid flows outside the tubes and moves down directly to the bottom. Then, it changes the direction and rises again. This process is continued till it leaves the heater. Baffles increase the velocity of the liquid outside the tubes. Baffles also allow the fluid to flow more or less at right angles to the tube, which creates more turbulence. This helps in reducing the resistance to heat transfer outside the tubes. Baffles lengthen the path and decrease the cross-section of the path of the cold fluid. The baffles get heated and provide a greater surface area for heat transfer. Simultaneously, during the flow, the tubes also get heated. As a result, the film coefficient inside the tube also increases. The liquid to be heated is pumped through the inlet provided on the left-side distribution chamber. The liquid passes through the tubes and gets heated. The flow of liquid is single-pass. The heated liquid is collected from the right-hand side distribution chamber.
Advantages of Liquid-to-Liquid Interchanger:
In a liquid-to-liquid interchanger, heat transfer is rapid as the liquid.
- passes at high velocity outside the tubes
- flow more or less at right angles to the tubes.
Double Pipe Heat Interchangers
Double pipe heat interchangers are efficient equipments for heat transfer as they have few pipes (tubes) per pass.
Construction of Double Pipe Heat Interchangers:
The double-pipe heat interchanger uses two pipes arranged as one inserted into the other, Fig.3. The inner pipe is used for the pumping of cold fluid to be heated whereas the outer acts as a jacket for the circulation of the hot fluid. The components of this interchanger are inter-connected within the shell. As mentioned earlier the number of pipe sections is limited and in addition, the length of the pipe is also less. The glass tube, standard iron pipe, and graphite materials are used for construction. The metal pipes are assembled with return bends. Few pipes are connected in parallel and stacked vertically and may have longitudinal fins on their outer surface. Outer pipe size varies from 2 to 14 inch with inner tubes varying from 0.75 to 2 inches in size. Some have longitudinal fins on the outside of the inner tube. Counter-current flow in these interchangers is advantageous when very close temperature approaches are required.
Working of Double Pipe Heat Interchangers:
The heating medium (hot liquid) is pumped into the outer jacket and is circulated through the annular spaces between them and carried from one part to the next part and finally, it leaves the jacket at the bottom on the right side. During the movement of fluid, the pipes get heated and thus hot fluid loses its temperature. The fluid to be heated is pumped into the inner tube through the inlet provided on the right side. The liquid gets heated up and flows through the bent tubes into the part of the pipe. The liquid further gets heated during flow and finally discharges through the exits point on the left side.
Uses of Double Pipe Heat Interchangers:
- A double pipe heat interchanger is useful when not more than 0.9 to 1.4 m2 of the surface is required.
- It is best suited when the volume of liquid inside tubes is less and obtains desired velocity and the size of the tube.
- Since one liquid flows through the inside of the pipe and the second liquid flows through the annular space between the pipes, these are primarily used for low flow rates, high-temperature drops, and high-pressure applications.
Scraped Surface Exchangers
In some of the heat interchangers the drag forces due to the flow of viscous liquid a quite thick viscous sub-layer or due to turbulent conditions in the core, liquid exhibits no pressure loss with excessive pumping costs. This problem is solved by physically removing the layers of fluid at the heat transfer surfaces and mixing them with the bulk fluid in the heat exchanger. In this way, if the fluid is being heated, heat is conveyed directly from the wall to the bulk liquid. The technique is particularly attractive for heat-sensitive liquids used in pharmaceutical products because it has a low interface temperature between the liquid and heat transfer surface for a given overall temperature driving force.
These types of exchanges have a rotating element with spring-loaded scraper blades, Fig.4, to scrape the inner heating surface to effectively remove liquid from it. The blades move against the heat transfer surface under the influence of the rotational forces. Simultaneously, as liquid layers are removed any fouling substance deposited on the surface is also removed. This ensures no contamination of the processed liquid with no change in product qualities. The number of scraper blades may vary but as the number of blades is increased the capital cost rises. A large number of blades are not necessary, since the time interval between successive scrapes is relatively short. The choice of the number of blades is an empirically based compromise between capital cost, acceptable speed of rotation, and liquid viscosity. Rotating parts in these exchangers makes the maintenance costlier.
Some exchangers have blades that do not touch the surface over which they pass but move near it. Such designs may be termed as “wiped surface” heat exchangers and may be preferred, where the wear of components cannot be tolerated from a viewpoint of mechanical or contamination effects. Scraped surface heat exchangers can either run full of liquid or the liquid may enter the exchanger as a peripheral stream. The design of this heat exchanger is complex and is made usually based on empirically determined parameters derived from experience. Scraped surface heat exchangers are, in general, used only for special applications.
Finned Tube Exchanger
In a heat exchanger, while heating the air outside the tubes with steam inside the tubes, the steam side coefficient will be very high and the air-side coefficient will be extremely low. While heat transfer, as the overall coefficient approximates that of the lower side (the air-side coefficient), the only remedy to increase q is to increase the area term on the air-side without putting more tubes in the heater. As we know, metals generally have high thermal conductivity, the temperature of the metal surface rapidly approximates that of steam. If metal fins are attached on the outside of the tubes such that there is good contact between the surface of the tube and the base of the fin, heat transfer surface area is increased. A wide variety of fins are used as shown in Fig.5. Rectangular discs of metal may be pressed onto the tube at right angles to them. Spiral fins may be attached to the tubes. Transverse and longitudinal fins are other forms. The use of finned tubes greatly reduces the size of the apparatus. In cases where the heat transfer coefficients on the two sides are close, the question of where fins are to be attached is entirely one of economics in design. There are certain cases where fins are used on the inner as well as on the outside of the tubes.
Finned tube heat exchangers are used for heat transfer between air, gas, and liquids or steam. Heat exchangers with finned heating surfaces (finned tubes) are significantly space-saving and are more efficient than exchangers with straight tubes. These heat exchangers are designed to transfer heat from clean air and gases with high efficiency on liquids or vapors, and vice versa.
Applications of Finned Tube Exchanger:
Finned tube heat exchangers are often used in power plants as exhaust gas heat exchangers to increase the efficiency factor. Further applications in power plants are the preheating of combustion air as well as the condensation of exhaust steam from steam or turbines. In industrial dryers, these heat exchangers are used for heating air by hot water, steam, or thermal oil in large quantities. In many industrial production processes, such as for the air conditioning of buildings, these heat exchangers are used as an air cooler for cooling down or re-cooling of liquids.
Advantages of Finned Tube Exchanger:
- The robust construction of a finned tube heat exchanger can withstand contrarious operating conditions over a long period.
- They have maximum transmission quality and a high condensation rate.
- They have a wide application and temperature spectrum (range) thus value for money.
- They are ideal for gas-liquid or gas-vapor heat transfer.
- They show the highest reliability of operation.
Plate Type Exchangers
The plate heat exchanger is a specialized design well suited for transferring heat between medium- and low-pressure fluids. Welded, semi-welded, and brazed heat exchangers are used for heat exchange between high-pressure fluids or where a more compact product is required.
In these exchanges, the media can be heated, cooled or condensed, in a closed space. Plate-type heat exchangers can be used for different applications and in a variety of designs. Several forms of plate heat exchangers are available. These essentially consist of standard plates which serve as heat transfer surfaces, Fig.6. Plates are provided with grooves for rubber gaskets. Several such plates are supported on a frame and assembled in such a way that the plates can be separated individually for cleaning or replacing. A corrugated plate design is also used to impart rigidity to the plate. In place of a pipe passing through a chamber, there are instead two alternating chambers, usually thin in-depth, separated at their largest surface by a corrugated metal plate. The plates used in a plate and frame heat exchanger are obtained by the one-piece pressing of metal plates. Stainless steel is a commonly used metal for plates due to its ability to withstand high temperatures, its strength, and its corrosion resistance.
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