Heat Exchanger

Heat Transfer Equipments

ObjectivesRecognize what is heat exchangerDifferentiate numerous types of heat exchanger, their classification and their applicationsKnow the heat transfer equipment terminologiesKnow the primary consideration in the selection of heat exchangersWhat is a heat transfer equipment?An equipment that permits efficient transfer of heat from a hot fluid to a cold fluid without any or with direct contact of fluidsSuch an equipment is calledHeat Exchanger

What is a Heat Exchanger?Technically speaking

A heat exchanger is a device that is used totransfer thermal energy (enthalpy) betweentwo or more fluids, between a solid surface anda fluid,or between solid particulates and a fluid,at different temperaturesand in thermal contact.Heat ExchangerHeat exchangers, can be seen in daily life, like ?? as well as in industries, like??

What are these?

Heat Exchanger Equipement in daily life such as household refrigerator or air conditioner7Aim and Application of HETypical applications involve heating or cooling of a fluid stream of concern and evaporation or condensation of single- or multicomponent fluid streams. In other applications, the objective may be to recover or reject heat, or sterilize, pasteurize, fractionate, distill, concentrate, crystallize, or control a process fluid.Common examples of heat exchangers are shell-and tube exchangers, automobile radiators, condensers, evaporators, air preheaters, and cooling towers.Aim and Application of HEThere could be internal thermal energy sources in the exchangers, such as in electric heaters and nuclear fuel elements.Combustion and chemical reaction may take place within the exchanger, such as in boilers, fired heaters, and fluidized-bed exchangers. Mechanical devices may be used in some exchangers such as in scraped surface exchangers, agitated vessels, and stirred tank reactors.Aim and Application of HEHeat exchanger found applications in almost all Chemical and petrochemical plantsAir Conditioning SystemsPower productionWaste Heat recovery Automobile RadiatorCentral Heating System Electronic Parts

Different Terminologies of Heat Transfer EquipmentHeat exchanger: both sides single-phase and process streamsCooler: one stream a process fluid and the other cooling water or air.Heater: one stream a process fluid and the other a hot utility, such as steam or hot oil.Condenser: one stream a condensing vapor and the other cooling water or air.Chiller: one stream a process fluid being condensed at sub-atmospheric temperatures and the other a boiling refrigerant or process stream.Reboiler: one stream a bottoms stream from a distillation column and the other a hot utility (steam or hot oil) or a process stream.Different Terminologies of Heat Transfer EquipmentDiscuss and elaborate the examples in daily life and/or industrial process of each of the above mentioned equipmentsHeat Exchanger ClassificationHeat exchangers are classified according toTransfer processNumber of fluidsDegree of surface compactnessConstructionFlow arrangementsHeat transfer mechanisms

Shah, 1981Heat Exchanger ClassificationHeat exchangers are classified according toTransfer processClassification by Transfer Processes

Classification by Transfer ProcessesIndirect contact type The fluid streams remain separate and the heat transfers continuously through a dividing wall into and out of the wall in a transient manner.

Classification by Transfer ProcessesIndirect contact type Direct transfer type heat exchangerStorage type heat exchangerFluidized bed heat exchanger

Classification by Transfer ProcessesDirect Transfer Type Heat ExchangerIn this, type heat transfers continuously from the hot fluid to the cold fluid through a dividing wall. There is no direct mixing of the fluids because each fluid flows in separate fluid passages. It is also known as recuperator. Examples, tubular exchangers, plate and frame heat exchangers and extended surface exchangers.

Tubular ExchangerPlate and Frame ExchangerClassification by Transfer ProcessesStorage Type Heat Exchanger (Regenerative Heat Exchanger)In a storage type exchanger, both fluids flow alternatively through the same flow passages, and hence heat transfer is intermittent.The heat transfer surface (or flow passages) is generally cellular in structure and is referred to as a matrix, or it is a permeable (porous) solid material, referred to as a packed bed.When hot gas flows over the heat transfer surface (through flow passages), the thermal energy from the hot gas is stored in the matrix wall, and thus the hot gas is being cooled during the matrix heating period. As cold gas flows through the same passages later (i.e., during the matrix cooling period), 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 (recuperator), but the corresponding thermal energy is alternately stored and released by the matrix wall.Classification by Transfer ProcessesStorage Type Heat Exchanger (Regenerative Heat Exchanger)

Fixed Bed RegeneratorContinuous-passage matrices for a rotary regenerator: (a) notched plate; (b) triangular passage.

Classification by Transfer ProcessesStorage Type Heat Exchanger (Regenerative Heat Exchanger)Regenerative heating was one of the most important technologies developed during the Industrial Revolution when it was used in the hot blast process on blast furnaces.It was later used in glass and steel making, to increase the efficiency of open hearth furnaces, and in high pressure boilers and chemical and other applications, where it continues to be important today.Classification by Transfer ProcessesFluidized bed heat exchangerIn a fluidized-bed heat exchanger, one side of a two-fluid exchanger is immersed in a bed of finely divided solid material, such as a tube bundle immersed in a bed of sand or coal particles.The common applications of the fluidized-bed heat exchanger are drying, mixing, adsorption, reactor engineering, coal combustion, and waste heat recovery

Classification by Transfer ProcessesDirect-Contact Heat ExchangerIn 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 transfer in addition to heat transfer, such as in evaporative cooling and rectification.However, the applications are limited to those cases where a direct contact of two fluid streams is permissible.

Classification by Transfer ProcessesDirect-Contact Heat ExchangerImmiscible Fluid ExchangersGasLiquid ExchangersLiquidVapor Exchangers

Classification by Transfer ProcessesImmiscible Fluid ExchangersIn 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. Condensation of organic vapors and oil vapors with water or air are typical examples.

Classification by Transfer ProcessesGasLiquid ExchangersIn this type, one fluid is a gas (more commonly, air) and the other a low-pressure liquid (more commonly, water) and are readily separable after the energy exchange. In either cooling of liquid (water) or humidification of gas (air) applications, liquid partially evaporates and the vapor 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. A wet (water) cooling tower with forced- or natural-draft airflow is the most common application. Other applications are the air-conditioning spray chamber, spray drier, spray tower, and spray pond.

Classification by Transfer ProcessesLiquidVapor ExchangersIn this type, typically steam is partially or fully condensed using cooling water, or water is heated with waste steam through direct contact in the exchanger. Noncondensables and residual steam and hot water are the outlet streams. Common examples are desuperheaters and open feedwater heaters (also known as deaerators) in power plants.Classification by Transfer ProcessesDirect-Contact Heat ExchangerCompared to indirect contact recuperators and regenerators, in direct-contact heat exchangers, very high heat transfer rates are achievable, the exchanger construction is relatively inexpensive, and the fouling problem is generally nonexistent, due to the absence of a heat transfer surface (wall) between the two fluids. Heat Exchanger ClassificationHeat exchangers are classified according toNumber of fluidsClassification by Number of FluidMost processes of heating, cooling, heat recovery, and heat rejection involve transfer of heat between two fluids. Hence, two-fluid heat exchangers are the most common. Three fluid heat exchangers are widely used in cryogenics and some chemical processes (e.g., air separation systems, a heliumair separation unit, purification and liquefaction of hydrogen, ammonia gas synthesis). Heat exchangers with as many as 12 fluid streams have been used in some chemical process applications.Heat Exchanger ClassificationHeat exchangers are classified according toDegree of surface compactnessClassification by Surface Compactness Heat exchangers are characterized by a large heat transfer surface area per unit volume of the exchanger, resulting in reduced space, reduce weight, reduce support structure and footprint, energy requirements and cost, as well as improved process design and plant layout and processing conditions, together with low fluid inventory.Classification by Surface Compactness The ratio of the heat transfer surface area of a heat exchanger to its volume is called the area density or surface compactness .

A heat exchanger with = 700 m2/m3 (or 200 ft2/ft3) is classified as being compact. Examples of compact heat exchangers are car radiators ( 1000 m2/m3) and the human lung ( 20,000 m2/m3).

Heat Exchanger ClassificationHeat exchangers are classified according toHeat Transfer MechanismsClassification by Heat Transfer MechanismsThe basic heat transfer mechanisms employed for transfer of thermal energy from the fluid on one side of the exchanger to the wall (separating the fluid on the other side) are single-phase convection (forced or free), two-phase convection (condensation or evaporation, by forced or free convection), and combined convection and radiation heat transfer. Any of these mechanisms individually or in combination could be active on each fluid side of the exchanger.Some examples of each classification type are automotive radiators, passenger space heaters, regenerators, intercoolers, economizers and so on.36Heat Exchanger ClassificationHeat exchangers are classified according toFlow arrangementsClassification by Flow Arrangement

38Classification by Flow ArrangementThe choice of a particular flow arrangement is dependent on the required exchanger effectiveness, available pressure drops, minimum and maximum velocities allowed, fluid flow paths, packaging envelope, allowable thermal stresses, temperature levels, piping and plumbing considerations, and other design criteria.39Classification by Flow ArrangementSingle Pass flow arrangementA fluid is considered to have made one pass if it flows through a section of the heat exchanger through its full length. Counterflow exchangerIn a counterflow or countercurrent exchanger, the two fluids flow parallel to each other but in opposite directions within the core.The counterflow arrangement is thermodynamically superior to any other flow arrangement. It is the most efficient flow arrangement, producing the highest temperature change in each fluid compared to any other two-fluid flow arrangements for a given overall thermal conductance (UA), fluid flow rates and fluid inlet temperatures. The maximum temperature difference across the exchanger produces minimum thermal stresses in the wall for an equivalent performance compared to any other flow arrangements.

Classification by Flow ArrangementSingle Pass flow arrangementParallelflow exchangerIn a parallelflow (also referred to as cocurrent or cocurrent parallel stream) exchanger, the fluid streams enter together at one end, flow parallel to each other in the same direction, and leave together at the other end.This arrangement has the lowest exchanger effectiveness among single-pass exchangers for given overall thermal conductance and fluid flow rates and fluid inlet temperatures.In a parallelflow exchanger, a large temperature difference between inlet temperatures of hot and cold fluids exists at the inlet side, which may induce high thermal stresses in the exchanger wall at the inlet.

Classification by Flow ArrangementSingle Pass flow arrangementCrossflow ExchangerIn this type of exchanger, the two fluids flow in directions normal to each other.Thermodynamically, the effectiveness for the crossflow exchanger falls in between that for the counterflow and parallel flow arrangements. The largest structural temperature difference exists at the corner of the entering hot and cold fluids.This is one of the most common flow arrangements used for extended surface heat exchangers, because it greatly simplifies the header design at the entrance and exit of each fluid.Classification by Flow ArrangementSingle Pass flow arrangementSplitflow ExchangerIn this exchanger, the shell fluid stream enters at the center of the exchanger and divides into two streams.These streams flow in longitudinal directions along the exchanger length over a longitudinal baffle, make a 180 turn at each end, flow longitudinally to the center of the exchanger under the longitudinal baffle, unite at the center, and leave from the central nozzle. The other fluid stream flows straight in the tubes.

Classification by Flow ArrangementMultipass flow arrangementAfter flowing through one full length, if the flow direction is reversed and fluid flows through an equal- or different-sized section, it is considered to have made a second pass (or multipass) of equal or different size.44Take Home AssignmentWhat are the types, examples and applications of the Multipass Flow Exchangers?Heat Exchanger ClassificationHeat exchangers are classified according toConstructionClassification by constructionTubularDouble pipeShell and tubeSpiral tubePipe coilsPlate typePlate and frame SpiralPlate coilPrinted circuitExtended surfacePlate-finTube-finRegenerativeRotaryFixed-matrixRotating hoods

Shell and Tube HE

Double Pipe HE

Plate and Frame HE

Plate-Fin HE

Could you guess which type of heat exchanger is this?Types of Heat ExchangersWe will study industrially important heat exchanger in more detail in upcoming lecturesApproach to Heat-Exchanger Design The proper use of basic heat-transfer knowledge in the design of practical heat-transfer equipment is an Art Designers must be constantly aware of the differences between the idealized conditions for, and under which the basic knowledge was obtained and the real conditions of the mechanical expression of their design and its environment. Approach to Heat-Exchanger DesignThe H.E design must satisfy process and operational requirements (such as availability, flexibility, and maintainability) and do so economically. An important part of any design process is to consider and offset the consequences of error in the basic knowledge of heat transfer, in its subsequent integration into a design method, in the translation of design into equipment, in the operation of the equipment and the process. Heat-exchanger design is not a highly accurate art under the best of conditions.Selection criterion for heat exchangersMaterial of constructionOperating temperatures and pressures conditionsFlow ratesFlow arrangementsPerformance parameters such as thermal effectiveness and pressure dropsFouling tendenciesTypes and phases of fluidMaintenance, inspection, cleaning ,extension and repair possibilitiesOverall economyFabrication techniques

1. Material of constructionFor reliable and continuous use, the material of construction of heat exchangers should have well defined corrosion rate in service environment. the material should exhibit strength to with stand with operating and temperature and pressure

2. Operating temperature and pressure conditionsPressureThe design pressure is important to determine the thickness of pressure retaining components. The higher the pressure, the greater will be the required thickness of pressure retaining equipment.TemperatureDesign temperature: This parameter is important as it indicate whether a material at design temperature can withstand the operating pressure and various load imposed on component.Shell and tube heat exchanger units can be designed for almost all condition of temperature and pressure. In extreme cases, high pressure may impose a limitation by fabrication problems associate with material thickness.Compact Heat exchanger: Compact Heat exchanger are constructed from thinner material by mechanical bonding like welding. Therefore they are limited in operating pressure and temperatureGasketed plate heat exchanger and spiral exchanger: these exchanger are limited in pressure and temperature. Wherein the limitation are imposed by the capability of gaskets

3. Flow rateFlow rate determine the flow area: the higher the flow rate the higher will be cross flow area

4. Flow arrangementThe choice of typical flow arrangement (cocurrent or countercurrent) is dependent of required exchanger effectiveness, exchanger construction types.

5. Performance ParameterThermal effectivenessHeat exchanger effectiveness is defined as the ratio of the actual amount of heat transferred to the maximum possible amount of heat that could be transferred with an infinite area.

Pressure dropPressure drop is an important parameter in heat exchanger design. The heat exchanger should be design in such a way that unproductive pressure drop should be avoided to maximum extent in area like inlet and outlet bends ,nozzles and manifolds6. Fouling Tendencies

Fouling is defined as formation on heat exchanger surface of undesirable deposit that decrease the heat transfer and increase the resistance to fluid flow, resulting in high pressure drop. The growth of those deposit decrease the performance of exchanger with time.

7. Type and Phases of fluidThe phase of fluid within the unit is an important consideration in selection of heat exchanger type. Various combination of fluid dealt in exchanger are Liquid-Liquid, Liquid-Gas and Gas-Gas

8. Maintenance, inspection, cleaning, extension and repair possibilitiesThe suitability of various heat exchanger depend upon it maintenance cleaning and repairing maintenanceRepairing and maintenance of shell and tube exchanger is relatively easy but repairing of expansion joint is somehow difficult. Repairing and maintenance of compact heat exchanger of tube/plate fin type heat exchanger is very difficult except by plugging of tube.

9. Overall EconomyThere are two major cost to consider in designing of heat exchanger, the manufacturing cost and operating cost, including maintenance costIn general the less heat transfer area the less is the complexity of design, the lower in manufacturing cost. The operating cost is pumping cost due to pumping device such as pumps, fans and blowers. The maintenance cost include cost of spares that require frequent renewal due to fouling and corrosion

10. Fabrication techniqueFabrication technique is also determining factor for heat exchanger design. For example shell and tube exchanger mostly fabricated by welding, plate fin heat exchanger and automobile aluminum radiator by brazing. Most of circular tube fin exchanger fabricate by mechanical assembling.Study Reference MaterialsFundamentals of Heat Exchanger Design. Ramesh K. Shah and Duan P. Sekulic, John Wiley & Sons, Inc.Heat Exchanger Design Handbook. Kuppan Thulukkanam, CRC Press.