Ch 1 Introduction

Chapter 1 Introduction Heat transfer is the transfer of thermal energy from a system at a high temperature to one at lower temperature. It plays an important part in many aspects of engineering. In some cases the engineer requires a high rate of heat transfer through a small area with a low temperature difference, for example in the temperature control of electronic components; alternatively it may be desirable to maintain a temperature difference while maintaining a low rate of heat transfer, for example in the fabric of a building or where pipework carries hot or cold fluid. We will examine many examples of these applications and pay particular attention to the design and selection of heat exchangers – devices whose purpose is to facilitate the transfer of thermal energy from one fluid stream to another. Most practical heat transfer problems require the engineer to make various assumptions and approximations. For hand calculations empirical correlations are widely used. These are relationships which may have some theoretical or conceptual basis but are underpinned by experimental results. Heat transfer occurs by one of three mechanisms, or a combination of these mechanisms: •

Conduction

•

Convection

•

Radiation

Conduction is the transfer of energy though a material without bulk motion. It is the only mechanism of heat transfer in most solids, and occurs in fluids at rest and in the layer of a fluid immediately adjacent to a solid surface. Convection occurs in liquids and gases. Fluid at a high temperature physically moves from one region to another, while cooler fluid replaces it. Energy is thus transferred from one region to another. We are primarily concerned with convection from a solid surface to the bulk of the fluid, or from a fluid to a solid wall. The study of convection is subdivided: if the movement of the fluid is induced by a blower, fan or pump then it is known as forced convection. Natural, or free,

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convection occurs when the fluid movement is induced by temperature differences within the fluid. Boiling and condensation are special cases of convection. Radiation: all surfaces above absolute zero emit electromagnetic radiation and absorb radiation from other surfaces or the surroundings. In this way energy is transmitted from a body to another. Radiation does not rely on a medium and can occur in a vacuum, it is the only form of heat transfer which can occur in a vacuum. All three modes of heat transfer may occur in one problem – either in series, as heat is transferred from a fluid to a solid and through the solid, or in parallel as heat is transferred from a hot body by both radiation and convection. Heat Exchangers are thermal devices in which heat1 is exchanged from one fluid stream (or exceptionally a solid) to one or more other fluid streams. The term heat exchanger encompasses a range of devices which permit heat exchange to take place in one of four ways: Heat transfer plays an important role in many engineering applications including: i. Recuperative heat exchangers - hot and cold streams flow in close proximity but are physically separated by a solid wall ii. Regenerative heat exchangers - hot and cold streams flow alternately through a matrix which is heated by the passage of the hot fluid and releases heat to the cold fluid. iii. Direct contact - two fluids are allowed to come into contact with each other and are then at least partially separated. If the temperatures of the fluids differ then energy will be transferred while the fluids are in contact. The most common application of this type of heat exchanger is the direct contact cooling tower. iv. From a solid to a fluid -thermal energy may be liberated in a solid due to the passage of an electric current or a chemical or nuclear reaction. This energy may be dissipated to a passing fluid.

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In the study of heat transfer we tend to be less rigorous in our terminology than thermodynamicists - strictly heat is an interaction describing the energy transfer from one system to another due to a temperature difference, it is energy that is transferred. In heat transfer the terms heat and thermal energy are used interchangeably.

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v. Through insulation: All materials will permit some heat transfer, however there are many applications in which it is undesirable, the engineer needs to be able to quantify the heat loss which will occur. In this module we will be dealing principally with the theory which allows us to tackle problems in I, iv and v above. Some comments about temperature and other units: Temperature may be given according to one of several scales. Two will be used in this module: The Celsius scale and the Absolute, or Kelvin, scale. The Celsius scale is based on two reference points; the freezing and boiling points of ware at normal atmospheric pressure are set at 0oC and 100oC, respectively. This is a convenient scale – many everyday processes occur within or close to this range. The Celsius scale is essentially arbitrary as far as thermodynamics is concerned, a fundamental measure of temperature based on absolute zero – the temperature at which atoms would have no kinetic energy and a perfect gas would have zero volume. Absolute zero, or 0 Kelvin may be expressed as -273.15oC. The unit of absolute temperature is the Kelvin, K, 1K has the same magnitude as 1oC. Therefore: (Temperature K) = (Temperature oC + 273.15) T K = t oC + 273.15 Upper case T is traditionally used for absolute temperature and lower case t is used for temperatures expressed in Celsius. (Some texts use other symbols for temperature e.g. θ) Temperature measurements and many tables of properties are expressed as Celsius. However, where temperature ratios are to be used or a temperature is to be raised to a power, then it is essential that the absolute temperature is used. Either temperature scale is acceptable when dealing with temperature differences and will give the same numerical result. Illustrative examples: (a)A building is maintained at an internal environment of 21oC while the external temperature is 4oC. What is the temperature difference across the building wall: t i − t o = 21 − 4 = 17 o C

Ti − To = (21 + 273.15) − (4 + 273.15) = 17 K

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(b)The pressure and temperature of a perfect gas are related by the expression pv = RT . A sealed pressure vessel has a pressure of 1.5 bar at 12oC it is then

heated to 24oC. Calculate the new pressure in the vessel.

p 2 T2 = since R and v are constant. p1 T1

T1 = 12 + 273.15 = 285.15K T2 = 24 + 273.15 = 297.15 K p1 = 1.5bar

p 2 = p1

T2 297.15 = 1.5 × = 1.563bar T1 285.15

Clearly a different, and wrong!! answer would be obtained if Celsius was used. Properties and heat transfer may be expressed in a range of units. We will use the SI system. It is important that all calculations are carried out with consistent units. In many heat transfer calculations dimensionless groups are used, in which case whether you work with kW, KW/m2 etc or W, W/m2 etc is largely a matter of personal preference. However you must be consistent!!. If a dimensional parameter is to be raised to a power, (e.g α= C(Q/A)0.67 ) then it is important that the unit used for Q is consistent with that used in the derivation of C in the correlation.

A little thermodynamics Energy is conserved. For a solid transferring heat at a rate Q& W (or kW) to a fluid flowing at a rate m& kg/s the temperature change of the fluid is given by:

Q& = m& (hout − hin ) where h is the specific enthalpy of the fluid J/kg or kJ/kg – a measure of the energy content of the fluid. For a single phase fluid undergoing a moderate temperature change:

Q& = m& (hout − hin ) = m& c p (Tin − Tout ) Where cp is the specific heat capacity of the fluid, J/kgK or kJ/kgK

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If we make some simplifying assumptions we can say that for a two-fluid heat exchanger: Rate of Energy gained by cold fluid =Rate of Energy lost by hot fluid

m& c (hc ,out − hc ,in ) = m& h (hh ,in − hh ,out ) = Q&

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m& c ∆hc = −m& h ∆hh If both fluids are single phase and have constant specific heat capacities: m& c c p ,c (Tc ,out − Tc ,in ) = m& h c p ,h (Th ,in − Th ,out ) m& c c p ,c ∆Tc = − m& h c p ,h ∆Th

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Recommended Text There are numerous texts covering heat transfer and heat exchanger design available. Some are written as text books, while others are comprehensive sources of reference material.

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Textbooks which include an introduction to heat exchangers Rogers G.F.C. and Mayhew Y.R., Engineering thermodynamics, work and heat transfer, Longman, 4th Edition,1992 Eastop T.D. and McConkey A., Applied thermodynamics for engineering technologists, Longman, 5th Edition, 1993. Kreith F. and Bohn M.S. Principles of heat transfer, PWS, 5th Edition Specialist Texts and Reference works Saunders E.A.D., Heat exchangers: selection, design and construction, Longman, 1988 Fraas A.P. Heat Exchanger Design, Wiley, 2nd Edition, 1988 Kays W.M.and London A.L.,Compact heat exchangers, McGraw-Hill, 2nd Edition, 1964 Hesselgreaves J.E., Compact heat exchangers, selection design and operation, Pergamon, 2001 Hewitt G.F. (Ed), Handbook of Heat Exchanger Design, Begell House, 1992 Yokell S. A working guide to shell-and-tube heat exchangers, McGraw-Hill,1990

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