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Heat Transfer Operations

Introduction to Heat TransferConduction, Convection, Radiation

Dr. Muhammad Rizwan Assistant Professor COMSATS Course Outline Introduction to fundamental concepts and modes of heat transfer. General heat conduction equation, one dimension steady state conduction in rectangular coordinatesConvection heat transfer and related laws, Free convection and forced convectionHeat transfer with phase change (Condensation, boiling, and evaporation)Heat exchangers Some aspects of process design principles of double pipe and shell and tube heat exchangers. Related calculationsRadiation heat transfer basic concepts, black and gray body radiationProblems related to conduction, convection and radiation

Reference Text Coulson & Richardsons. Chemical Engineering Vol.1 6 Ed., Butterworth Heinmenn Ltd., 1996.Cengel Y. A. Heat Transfer A practical approach, 2nd Ed, McGraw Hill, 1988.Kern D. Q. Process Heat Transfer Tata McGraw Hill Edition, 1997.Incropera F. P.; DeWitt D. P. Fundamentals of Heat and Mass transfer John Wiley & Sons. 2007. McCabe W. L.; Smith J. C. and Harriett P. Unit Operations of Chemical Engineering, 6th edition, McGraw Hill, 2001. Importance of heat transfer for chemical engineer The students should be able to make right assumptions and approximations for tackling practical situations involving heat transfer. Almost all industrial processes are involved with heating or cooling, which requires heat transfer.One of the most important objectives of this module is to apply fundamentals of heat transfer to understand the design of various heat transfer equipments and to be able to specify the type and size of heat exchanger to satisfy the needs of a particular chemical engineering process application.How to find the optimum thickness of insulation materialThermodynamics and Heat transfer: Heat is a form of energy, that can be transferred from one system to another as a result of temperature difference.The science that deals with the determination of the rates of such energy transfer is called HEAT TRASFER.Thermodynamics analysis simply tells how much heat must be transferred to realize a specified change of state to satisfy the conservation of energy principle.Determining the rates of heat transfer and the variation of temperature is the subject of heat transfer.Thermodynamics deals with the equilibrium states and changes from one equilibrium state to another, whereas heat transfer deals with systems that lack thermal equilibrium

Application areas of Heat transfer: Heat transfer is commonly encountered in engineering systems and other aspects of life.

The optimal insulation thickness in the walls and roofs of the houses, on hot water or steam pipes is again determined on the basis of a heat transfer analysis with economic consideration.

Engineering Heat Transfer: Heat transfer equipments which are designed primarily on the basis of heat transfer analysis include:Heat exchangersBoilersCondensersRadiatorsFurnacesRefrigeratorsSolar collectors

Heat transfer problems encountered in practice can be considered in two groups:Rating problemsSizing problems

Specific heats of gases, liquids and solids: Specific heat is defined as the energy required to raise the temperature of a unit mass of a substance.Specific heat at constant volume, CvSpecific heat at constant pressure, CpCp always greater than Cv because at constant pressure the system is allowed to expand and the energy for this expansion work must also be supplied to the system.The specific heat of a substance depend on two variables, temperature and pressure.The constant volume and constant-pressure specific heats are identical for incompressible substances.The specific heats of incompressible substances depend on temperature only.In that case, the change in the internal energy of solids and liquids can be as

Energy Transfer: Heat transfer (Q) and work (W)An energy transfer is heat transfer if its driving force is a temperature difference.The amount of heat transfer per unit time is called heat transfer rate

When the rate of heat transfer is available, then the total amount of heat transfer during a time interval can be determined from:

The rate of heat transfer per unit area normal to the direction of heat transfer is called HEAT FLUX

Energy Transfer:

Basic Modes of Heat Transfer ConductionConvectionRadiationAll modes require the existence of a temperature differenceConduction: Can be in solids, liquids or gases due to a temperature difference which is a driving force. Distinct characteristic of conduction is, the molecules or particles of the medium are not displaced from their original positions.Possible means of heat energy transfer can be:Molecular vibrations (General solids and metals)Drift of free electrons (Metals)Collisions (Gases)The rate of heat conduction through a medium depends on:Geometry of the mediumThicknessThe material of the mediumTemperature difference across the mediumFouriers Law of Heat Conduction Hence, the rate of heat conduction is proportional to the temperature difference across the layer and the heat transfer area, but is inversely proportional to the thickness of the layer.Where k is called the thermal conductivity of material and it tells about the capability of material to transfer heat.

Called Fouriers law of heat conduction after J. Fourier, 1822

Fouriers Law of Heat Conduction The heat transfer surface area is always normal to the direction of heat transfer.The thickness of the wall has no effect on the area

Fouriers Law of Heat Conduction: Estimate the heat loss per square metre of surface through a brick wall 0.5 m thick when the inner surface is at 400 K and the outside surface is at 300 K. The thermal conductivity o f the brick may be taken as 0.7 W/mK.Example 9.2 (Coulson volume 1, 6th Ed):