Heat Exchangers Heat Exchangers 1 How is the heat transfer? Mechanism of Convection Applications. Mean fluid Velocity and Boundary and their effect on the rate of heat transfer. Fundamental equation of heat transfer Logarithmic-mean temperature difference. Heat transfer Coefficients. Heat flux and Nusselt correlation 1
Heat can transfer between the surface of a solid conductor and the surrounding medium whenever temperature gradient exists. Conduction Convection Natural convection Forced Convection Natural and forced Convection Natural convection occurs whenever heat flows between a solid and fluid, or between fluid layers. As a result of heat exchange Change in density of effective fluid layers taken place, which causes upward flow of heated fluid. If this motion is associated with heat transfer mechanism only, then it is called Natural Convection 2
Forced Convection If this motion is associated by mechanical means such as pumps, gravity or fans, the movement of the fluid is enforced. And in this case, we then speak of Forced convection. A device whose primary purpose is the transfer of energy between two fluids is named a Heat Exchanger. 3
A heat exchanger is used to exchange heat between two fluids of different temperatures, which are separated by a solid wall. Heat exchangers are used to carry out energy conversion and utilization. They utlize a wide range of flow configurations. Applications in heating and air conditioning, power production, waste heat recovery, chemical processing, food processing, sterilization in bio-processes. Heat exchangers are classified according to flow arrangement and type of construction. Heat Exchangers prevent car engine overheating and increase efficiency Heat exchangers are used in Industry for heat transfer Heat exchangers are used in AC and furnaces 4
Heat Exchangers 9 The closed-type exchanger is the most popular one. One example of this type is the Double pipe exchanger. In this type, the hot and cold fluid streams do not come into direct contact with each other. They are separated by a tube wall or flat plate. 5
Chee 318 Heat Exchangers 11 The baffle heat exchanger design (Phillips Petroleum Co.) 6
Tube Bundles Tube Pitch 7
Baffles are used to establish a cross-flow and to induce turbulent mixing of the shell-side fluid, both of which enhance convection. The number of tube and shell passes may be varied One Shell Pass and One Tube Pass One Shell Pass, Two Tube Passes Two Shell Passes, Four Tube Passes Heat Exchangers 16 8
Chee 318 Heat Exchangers 17 Heat Exchangers 18 9
TEMA AES Exchanger TEMA Designations 10
Chee 318 Heat Exchangers 21 Chee 318 Heat Exchangers 22 11
Chee 318 Heat Exchangers 23 Finned - Both Fluids Unmixed Unfinned - One Fluid Mixed the Other Unmixed Heat Exchangers 24 12
Widely used to achieve large heat rates per unit volume, particularly when one or both fluids is a gas. Characterized by large heat transfer surface areas per unit volume (>700 m 2 /m 3 ), small flow passages, and laminar flow. Heat Exchangers 25 Chee 318 Heat Exchangers 26 13
Chee 318 Heat Exchangers 27 Baffles How do baffles help? Where are they installed and which fluid is directly affected? Common practice is to cut away a segment having a height equal to one-fourth the inside diameter of the shell. Such baffles are called 25 percent baffles. 14
Baffle Arrangement Tubes Standard tube lengths are 8, 12, 16 and 20 ft. Tubes are drawn to definite wall thickness in terms of BWG and true outside diameter (OD), and they are available in all common metals. 15
Heat Exchanger (HEX) Rating Checking the existing design for compatibility with the user requirements (outlet temperature, heat load etc.) given: flow rates, inlet temperatures, allowable pressure drop; thus HT area and passage dimensions find: heat transfer rate, fluid outlet temperatures, actual pressure drop HEX Sizing Thermal and pressure drop considerations, maintenance scheduling with fouling consideration. given: inlet and outlet temperatures, flow rates, pressure drop find: dimensions -type and size of HEX Heat Exchangers 31 Assumptions for Basic Design Equations for Sizing steady-state, steady flow no heat generation in the HEX negligible ΔPE, ΔKE adiabatic processes no phase change (later) constant specific heats and other physical properties. Heat Exchangers 32 16
LMTD Method Expression for convection heat transfer for flow of a fluid inside a tube: q mc T T ) conv p( m, o m, i For case 3 involving constant surrounding fluid temperature: To Ti q UA s T lm Tlm ln( T / T ) o i Heat Exchangers 33 In a two-fluid heat exchanger, consider the hot and cold fluids separately: qh m hcp, h( Th, i Th, o) (11.1) and q UA T lm (11.2) q m c ( T T ) c c p, c c, o c, i Need to define U and T lm Heat Exchangers 34 17
Parallel Parallel Flow Flow Counterflow Counterflow - : Heat Exchangers 35 Heat Exchangers 36 18
Heat Exchangers 37 Heat Exchangers 38 19
Heat Exchangers 39 Heat Exchangers 40 20
Heat Exchangers 41 Condenser: Hot fluid is condensing vapor (eg. steam) Evaporator/boiler: Cold fluid is evaporating liquid Heat Exchangers 42 21
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Parallel Flow For tubular heat exchangers we must take into account the conduction resistance in the wall and convection resistances of the fluids at the inner and outer tube surfaces. Note that: 1 1 UA U A i i 1 1 ln( Do / Di ) 1 UA h A 2kL h A 1 U A o o i i where inner tube surface o outer tube surface o Counterflow A D L A i o i D (11.3a) L Heat Exchangers 48 o 24
Heat Exchangers Heat exchanger surfaces are subject to fouling by fluid impurities, rust formation, or other reactions between the fluid and the wall material. The subsequent deposition of a film or scale on the surface can greatly increase the resistance to heat transfer between the fluids. An additional thermal resistance, can be introduced: The Fouling factor, R. f Depends on operating temperature, fluid velocity and length of service of heat exchanger. It is variable during heat exchanger operation. Typical values see Heat Transfer for Kern. The overall heat transfer coefficient can be written: i i " f, i 1 1 R UA h A A i " f, o ln( D R o / Di ) 2kL A o 1 h A o o (11.3b) 50 25
Heat Exchangers Heat Exchangers 26
Fins reduce the resistance to convection heat transfer, by increasing surface area. Expression for overall heat transfer coefficient includes overall surface efficiency, or temperature effectiveness, h o, of the finned surface, which depends on the type of fin. 1 1 UA U A ( o c 1 ha) c c 1 U A R ( " f, c o h A) h c R conduction R ( " f, h o A) h ( o 1 ha) h (11.3c) where c is for cold and h for hot fluids respectively Heat Exchangers 53 Example 1 Heat Exchangers 27
Heat Exchangers Heat Exchangers 28
Example 2 Heat Exchangers Heat Exchangers 29
Heat Exchangers 30