Heat Transfer

 Chapter 4 

HEAT TRANSFER

 INTRODUCTION 

• Heat is a form of energy. As we know matter is made-up of atoms and molecules, atoms in molecules are always in motion represented by translation, rotation and vibration. This motion leads to generation of heat energy. The more the motion of atoms in molecules more is the generation of heat. This heat can only be transferred from a region of higher temperature to a region of lower temperature. Heat is transferred by conduction, convection, and radiation. Conduction is the main method by which heat is transferred through solids. In solids no appreciable displacement of matter occurs. 

• The flow of heat in non-metals is due to transfer of vibrational energy from one molecule to another and, in the case of metals, by the movement of free electrons. In convection, energy is transferred from or to a region by the motion of fluids. The heat flow is caused by buoyancy forces (caused by difference in temperature) induced by variations in the density of the fluid. All substances above absolute zero radiate heat in the form of electromagnetic waves. The radiation may be transmitted, reflected, or absorbed by matter; the fraction absorbed being transformed into heat. Thus, it is known that energy is transferred between systems either as heat or as work. The energy transfer that results across a finite temperature difference is heat transfer and the rest of the energy transfer interactions can be brought under one or other type of work transfer. Heat transfer involves first and the second law of thermodynamics.

 OBJECTIVES

• We know that across a finite temperature difference present between a system and its surrounding, heat transfer depends upon the mode of heat transfer for example, conduction, convection or radiation. The amount of heat (q) that transfers depends on several parameters such as the system and surrounding temperatures, their shape, relative movement or flow and thermo-physical properties such as specific heat, thermal conductivity, viscosity, and emissivity. These parameters can be used in understanding the principle and mechanisms involved in heat transfer process which can be further exploited for rapid and economical manufacturing of pharmaceutical API, intermediates and finished dosage forms.

The objectives of heat transfer are given below:

• Formulate basic equation for heat transfer problems.

• Calculate heat transfer between objects with simple geometries. 

• Evaluate the impact of initial and boundary conditions on the solutions of a particular heat transfer issue. 

• Recognize basic heat transfer mechanism and apply appropriate methods for quantification.  

• Evaluate the relative contributions of different modes of heat transfer.

• Perform an energy balance to determine temperature and heat flux.

• Apply heat transfer principles to design and size to evaluate performance of heat exchangers.

APPLICATIONS OF HEAT TRANSFER 

• Evaporation: Heat is supplied in order to convert a liquid into a vapour. 

• Distillation: Heat is supplied to the liquid mixture for separation of individual vapour component. Drying: For drying the wet granules and other solids. 

• Crystallization: Saturated solution is heated to bring out super saturation, which promotes crystallization of drugs.

• Sterilization: Autoclaves are used with steam as a heating medium. 

• Food processing industries: The principles of heat transfer are widely used in food processing industries for pasteurization, refrigeration, chilling, and freezing and refrigerative evaporation.

• Chemical process industries: Heat transfer methods find a variety of applications in the chemical process industries. Heating and cooling of batch tanks will allow the user to calculate the time it takes to heat up and then cool a batch vessel or tank. 

• Manufacturing of bulk drugs and dosage forms: Principles of heat transfer is of significance in manufacturing of various bulk drugs, excipients and dosage form.

FOURIER’S LAW

• The law of heat conduction is also known as Fourier’s law. It states that “the time rate of heat transfer through a material is proportional to the negative gradient in the temperature and to the area.” conductivity is one of the transport properties. Other properties include viscosity associated with the transport of momentum and diffusion coefficient associated with the transport of mass. Thermal conductivity provides an indication of the rate at which heat energy is transferred through a medium by conduction process.

• The assumptions of Fourier equation include steady state heat conduction, one directional heat flow, isothermal bounding surfaces with constant and uniform temperatures at the two faces, isotropic and homogeneous material and thermal conductivity constant, constant temperature gradient and linear temperature profile and no internal heat generation.

• The unique features of Fourier equation are that this equation is valid for all matter solid, liquid or gas. The vector expression indicates that the heat flow rate is normal to an isotherm and is in the direction of decreasing temperature. It cannot be derived from first principle and helps to define the transport property ‘k’. 

MECHANISMS OF HEAT TRANSFER 

• Heat transfer mechanisms are simply ways by which thermal energy is transferred between objects. It is based on the basic principle that kinetic energy tries to be at equilibrium or at equal energy states. There are three different ways for heat transfer to occur namely conduction, convection, and radiant heat. There is one more related phenomenon that transfers latent heat called evapotranspiration.

Conduction 

• Conduction heat transfer is energy transport due to molecular motion and interaction. Conduction heat transfer through solids is due to molecular vibration. Fourier determined that Q/A, the heat transfer per unit area (W/m2 ) is proportional to the temperature gradient dT/dx . The constant of proportionality is called the material thermal conductivity k.

• In conduction the molecules simply give their energy to adjacent molecules until equilibrium is reached. Conduction models do not deal with the movement of particles within the material. The thermal conductivity k depends on the material, for example, the various materials used in engines have the thermal conductivities (W/m K) as given in Table 4.1. The thermal conductivity also depends somewhat on the temperature of the material.

Convection  

• Convection heat transfer is energy transport due to bulk fluid motion. This type of heat transfer through gases and liquids from a solid boundary results from the fluid motion along the surface. Newton determined that the heat transfer/area (Q/A), is proportional to the fluid solid temperature difference (Ts − Tf). The temperature difference usually occurs across a thin layer of fluid adjacent to the solid surface. This thin fluid layer is called a boundary layer. The constant of proportionality is called the heat transfer coefficient (h).

• The movement of the thermal energy in convection is due to movement of hot fluid. Usually this motion occurs as a result of differences in density. Warmer particles are less dense, so particles with higher temperature will move to regions where the temperature is cooler and the particles with lower temperature will move to areas of higher temperature. Thus, the fluid will remain in motion until equilibrium is reached. The heat transfer coefficient depends on the type of fluid and the fluid velocity. The heat flux depending on the area of interest is the local or area averaged. The various types of convective heat transfer are usually categorized into the following areas:

Radiation 

• Radiation heat transfer is energy transport due to emission of electromagnetic waves or photons from a surface or volume. All moving charged particles emit electromagnetic radiation. This emitted wave will travel until it hits another particle. The particle that receives this radiation will receive it as kinetic energy. Particles will receive and emit radiation even after everything is at the same temperature, but it is not noticed due to the fact that the material is at equilibrium at this point. The radiation does not require a heat transfer medium and can occur in a vacuum.

Evapotranspiration  

• Evapotranspiration is the energy carried by phase changes, like evaporation or sublimation. Water takes a fair amount of energy to change phase, so this process recognizes that water vapour has a fair amount of energy associated with it. This type of energy transfer mechanism is often not listed among the different types of transfer mechanism as it's harder to understand

HEAT EXCHANGERS 

•    A heat exchanger is a device that allows heat from a fluid (a liquid or a gas) to pass to a second fluid (another liquid or gas) with the two fluids at different temperatures and in thermal contact. In heat exchangers, there are usually no external heat and work interactions. Heat exchangers are classified according to transfer processes, number of fluids, and degree of surface compactness, construction features, flow arrangements, and heat transfer mechanisms.

Applications: 

• Heating or cooling of a fluid stream. 

• Evaporation or condensation of single- or multicomponent fluid streams. 

• To recover or reject heat. 

• Sterilize, pasteurize, fractionate, distill, concentrate, crystallize, or control process fluids. 

• Chemical and petrochemical plants. 

• Air conditioning systems. 

• Power production. 

• Waste heat recovery. 

• Automobile radiator.

• Central heating system.

• Electronic parts.

Classification of heat exchangers:

In general, based on the relative flow direction of the two fluid streams heat exchangers are classified into two general classes as follows.

• Cross flow exchangers: Both the fluid streams cross each other in space at right angle.

• Parallel flow exchangers: Both the fluid streams move in parallel direction in space. For example, shell and tube heat exchanger. If the fluid flows in the parallel direction, two situations may arise

1.  Fluids flow in same direction.
2. Fluids flow in opposites direction. 

Shell and Tube Heat Exchanger 

• The shell and tube heat exchangers are the most commonly used heat exchangers in the chemical process industries. This type of heat exchanger consists of a bundle of tubes properly secured at the ends in tube sheets. The metal sheets have holes into which the tubes are fixed up to have leak proof joints. The entire tube bundle is placed inside a closed shell in such a way that it forms two immiscible zones for hot and cold fluids. One fluid flows through the tubes whereas the other fluid flows around the outside of the tubes within the space between the tube sheets and is enclosed by the outer shell. The proper fluid distribution of the shell side fluid is achieved by placing baffles normal to the tube bundle. Baffle creates turbulence in the shell side fluid and enhances the transfer coefficients for the shell side flow.

• The tube heat exchanger, have one shell and one tube pass since both the shell and tube side fluid make a single traverse through the heat exchanger. Thus, this type of heat exchangers is designated as 1-1 exchanger. If the tube fluid passes twice, it is designated as 1-2 exchangers. Similarly, if heat exchanger has 2 shell pass and 4 tube passes, it is designated as 2-4 exchangers. The number of passes in tube side is done by the pass partition plate. A pass partition plate is shown. The shell side pass can be created by a flat plate as shown in 

• In reality, this type of shell and tube heat exchanger is used in the process industry and is quite complex and is improved in design for thermal expansion stresses, tube fouling due to contaminated fluids, ease of assembling and disassembling, size, weight etc. The area available for flow of the tube side fluid is inversely proportional to the number of passes. Thus, on increasing the number of pass the area reduces and as a result the velocity of fluid in the tube increases and henceforth the Reynolds number also increases. It results in increased heat transfer coefficient, but it is at the cost of high pressure drop. Generally, even numbers of tube passes are preferred for the multi-pass heat exchangers.

• The commonly used method of classifying heat exchangers is indirect contact type or direct contact type heat exchangers.

• Indirect contact type heat exchangers: In many heat exchangers, the fluids are separated by a heat transfer surface, and ideally, they do not mix or leak. Such exchangers are referred to as direct transfer type

1. Direct transfer type
2. Storage type 
3. Fluidized bed type 

•  Direct contact type: Exchangers in which there is intermittent heat exchange between the hot and cold fluids via thermal energy storage and release through the exchanger surface are referred to as indirect transfer type.

1. Immiscible fluids 
2. Gas-liquid 
3. Liquid-vapour 

Indirect-Contact Heat Exchangers  

• In an indirect-contact heat exchanger, the fluid streams remain separate and the heat transfers continuously through dividing wall or into and out of a wall. There is no direct contact between thermally interacting fluids. This type of heat exchanger is also called as surface heat exchanger. These type of heat exchangers can be classified into direct-transfer type, storage type, and fluidized-bed type heat exchangers as described below\

Direct-transfer type exchangers:

• In this type, heat transfers continuously from the hot fluid to the cold fluid through a dividing wall. Although a simultaneous flow of two fluids is required in the exchanger, there is no direct mixing of these fluids because each fluid flows in separate fluid passages. In general, there are no moving parts in most such heat exchangers. This type of exchanger is designated as a recuperative heat exchanger or simply as a recuperator. Recuperator is a form of heat exchanger in which heating air is waste gases.

• Some examples of direct transfer type heat exchangers are tubular, plate-type, and extended surface exchangers. The term recuperator is not commonly used in the process industry for shell-and-tube and plate heat exchangers, but they are considered as recuperators. Recuperators are further sub-classified as prime surface exchangers and extended-surface exchangers. Prime surface exchangers do not use fins or extended surfaces on any fluid side. Plain tubular exchangers, shell-and-tube exchangers with plain tubes, and plate exchangers are good examples of prime surface exchangers.

Storage type exchangers:  

•  In storage type exchanger, both the fluids flow alternatively through the same flow passages and thus the heat transfer is intermittent. The heat transfer surface is generally cellular in structure and is referred to as a matrix or it is a permeable solid material, referred to as a packed bed. When hot gas flows over the heat transfer surface the thermal energy from the hot gas is stored in the matrix wall, and thus the hot gas is cooled during the matrix heating period. As cold gas flows through the same passages later, the matrix wall gives up thermal energy, which is absorbed by the cold fluid. Thus, heat is not transferred continuously through the wall as in a direct-transfer type exchanger, but the corresponding thermal energy is alternately stored and released by the matrix wall. This storage type heat exchanger is also referred to as a regenerative heat exchanger, or simply as a regenerator.

• To operate continuously and within a desired temperature range, the gases, headers, or matrices are switched periodically so that the same passage is used for hot and cold gases. The actual time required for hot gas to flow through a cold regenerator matrix is called the hot period or hot blow. Whereas time required for cold gas to flow through the hot regenerator matrix is called the cold period or cold blow. It is not necessary to have hot- and cold-gas flow periods of equal duration. There is some unavoidable carryover of a small fraction of the fluid remained in the passage to the other fluid stream after switching of the fluids; this known as carryover leakage. If the hot and cold fluids are at different pressures, the leakage is from the high-pressure fluid to the low-pressure fluid past the radial, peripheral, and axial seals, or across the valves, referred as pressure leakage. These leakages being unavoidable, regenerators are used exclusively in gas-to-gas heat and mass transfer applications with sensible heat transfer. In some applications regenerators may transfer about 5% moisture from humid air to dry air

Fluidized-bed heat exchangers: 

• In a fluidized-bed heat exchanger, one side of a two-fluid exchanger is immersed in a bed of finely divided solid material, as shown in Fig. 4.10. If the upward fluid velocity on the bed side is low, the solid particles will remain fixed in position in the bed and the fluid will flow through the interstices of the bed. If the upward fluid velocity is high, the solid particles will be carried away with the fluid. At a ‘‘proper’’ value of the fluid velocity, the upward drag force is slightly higher than the weight of the bed particles. This causes the solid particles to float with dilation of bed behave as a liquid and is referred to as a fluidized condition. In this condition, the fluid pressure drop through the bed remains almost constant, independent of the flow rate, and a strong mixing of the solid particles occurs. This causes uniform temperature in the whole bed with an apparent thermal conductivity of the solid particles as infinity.

• Very high heat transfer coefficients are achieved on the fluidized side compared to particle-free or dilute-phase particle gas. Chemical reaction is common on the fluidized side in many process applications, and combustion takes place in coal combustion fluidized beds. The common applications of the fluidized-bed heat exchanger are drying, mixing, adsorption, reactor engineering, coal combustion, and waste heat recovery. Initial temperature difference between the inlet temperature of the hot fluid and the fluidized bed is reduced by to fluidization and thus the exchanger effectiveness is lowered.   

Direct-Contact Heat Exchangers 

• In a direct-contact exchanger, two fluid streams come into direct contact, exchange heat, and are then separated. Common applications of a direct-contact exchanger involve mass and heat transfer, such as in evaporative cooling and rectification. The enthalpy of phase change in such an exchanger represents a significant portion of the total energy transfer. The phase change generally enhances the heat transfer rate. Direct-contact heat exchangers have very high heat transfer rates, the exchanger construction is relatively inexpensive, and the fouling problem is generally non-existent due to the absence of a heat transfer surface between the two fluids. These exchangers are classified as follows. 

Immiscible Fluid Exchangers:  

• Liquid-liquid exchanger: In this type, two immiscible fluid streams are brought into direct contact. These fluids may be single-phase fluids, or they may involve condensation or vaporization. For example, condensation of organic vapors and oil vapors with water or air     

Gas–liquid exchangers: 

• In this type, one fluid is a gas (usually air) and the other a low-pressure liquid (commonly, water) and is readily separable after the energy exchange. In either cooling of liquid (water) or humidification of gas (air) applications, liquid partially evaporates, and the vapour is carried away with the gas. In these exchangers, more than 90% of the energy transfer is by virtue of mass transfer due to the evaporation of the liquid and convective heat transfer is a minor mechanism. For example, water cooling tower with forced- or natural-draft airflow, air-conditioning spray chamber, spray drier, spray tower, and spray pond.

Liquid–Vap our Exchangers:

•  In this type, steam is partially or fully condensed using cooling water, or water is heated with waste steam through direct contact in the exchanger. Non-condensable and residual steam and hot water are the outlet streams. For example, de-superheaters and open feed water heaters (de-aerates) in power plants.

HEAT INTERCHANGERS 

• Most of the chemical and pharmaceutical industries uses a variety of heat transfer equipment. 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 includes 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 interchangers 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 use interchangeably.

• 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 interchangers 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 use interchangeably.

Baffles

• Baffles are circular discs of metal with one side cut away. These discs are perforated through which tubes are fitted. In order 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 setscrews. 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 increases 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\

• The cut portion of the baffle is called as baffle window. It provides the area for flow of the shell fluid. The baffle window area ranges from 15% to 50%. At 20% cut 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 increase 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 Interchanger 

• The liquid-to-liquid heat interchanger is a 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 few modifications and its working remains approximately same.

Working: 

• 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: 

• The construction of a liquid-to-liquid heat interchanger. It consists of baffles, tube sheets, spacer rods and the tubes. The most important parts in any heat interchanger are the baffles. Appropriate size of tube sheets is used for the fabrication. Guide rods are fixed to the tube sheets and tighten by means of screws. As mentioned before baffles are placed at 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 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 interchanger. On both the sides of the tubes, distribution chambers are provided. At the top of left-side chamber an inlet for fluid to be heated is provided. The outlet for the heated fluid is provided near to the right-side distribution chamber. 

Working:  

• 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 right angles to the tube, which creates more turbulence. This help in reducing the resistance to heat transfer outside the tubes. Baffles lengthen the path and decrease the cross-section of path of the cold fluid. The baffles get heated and provide 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 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 righthand side distribution chamber.

Advantages:  

In a liquid-to-liquid interchanger, heat transfer is rapid as the liquid.

1. passes at high velocity outside the tubes 
2. flows more or less at right angles to the tubes.

Double Pipe Heat Interchangers 

• Double pipe heat interchangers are efficient equipment for the heat transfer as they have few pipes (tubes) per pass.

Construction:

• The double-pipe heat interchanger uses two pipes arranged as one inserted into the other. 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 its outer surface. Outer pipe size varies from 2 to 14 inch with inner tubes varying from 0.75 to 2 inch 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:  

• 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 bottom on right side. During 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 at 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: 

•  Double pipe heat interchanger is useful when not more than 0.9 to 1.4 m2 of surface is required. 

• It is best suited when the volume of liquid inside tubes is less and obtain 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 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 low interface temperature between the liquid and heat transfer surface for a given overall temperature driving force.

• These types of exchangers have a rotating element with spring loaded scraper blades, 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 actually touch the surface over which they pass but move in close proximity to 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 base of the fin, heat transfer surface area is increased. A wide variety of fins are used. Rectangular discs of metal may be pressed on to the tube at right angles to them. Spiral fins may be attached to the tubes. Transverse and longitudinal fins are other forms. 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 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 tube) are significantly spacesaving and is 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 vapours, and vice versa

Applications: 

• Finned tube heat exchangers are often used in power plants as an exhaust gas heat exchanger 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:   

• Robust construction of finned tube heat exchanger can withstand contrarious operating conditions over a long period. 

• They have maximum transmission quality and high condensation rate. 

• The have wide application and temperature spectrum (range) thus value for money. 

• They are ideal for gas-liquid or gas-vapor heat transfer. 

• They show 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 this exchanger 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. Plates are provided with grooves for rubber gaskets. A number of 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 one piece pressing of metal plates. Stainless steel is a commonly used metal for the plates due to its ability to withstand high temperatures, its strength, and its corrosion resistance.