LESSON 1. HEAT EXCHANGERS

Transcription

1 LESSON 1. HEAT EXCHANGERS 1 Contents (I) Definition. Classification. Regenerators. Mixers or direct contact heat exchangers. Packed bed heat exchangers (Intercambiadores de lecho compacto). Direct flame heat exchangers (Intercambiadores de llama directa). Recuperators: Classification. Double-pipe heat exchangers. Cross-flow heat exchangers. 2

2 Contents (II) Recuperators: Shell-and-tube heat exchangers. Compact heat exchangers. Plate or plate and frame heat exchangers. Fouling factors (Factores de impureza o suciedad). Heat transfer coefficients (Coeficientes de convección). Overall heat transfer coefficient (Coeficiente global de transferencia de calor). Selecting the circulation side. Bibliography. 3 Definition Device used to implement the process of heat exchange between two fluids that are at different temperatures and separated by a solid wall. Many engineering applications: Heating, Air Conditioning and Ventilation (H.A.C.V.) of buildings, power generation, heat recuperation, chemical processes... 4

3 Classification According to the operation mode: Recuperators: both fluids pass simultaneously over the two surfaces of the separating wall. Regenerators: both fluids pass alternatively over the same surface of the separating wall. Mixers or direct contact: there is no separating wall. Packed bed: it is a regenerator with a porous medium as fixed matrix. Direct flame: unique fluid in contact with a flame through a wall. 5 Regenerators (I) Both fluids pass alternatively over the same surface of the separating wall. Heat exchange in two stages: 1. The hot fluid heats the matrix. 2. The matrix heats the cold fluid. Matrix = separating wall. The matrix can be fixed or rotary. 6

4 Regenerators (II) Matrix materials characteristics: High c p. High k in ^ direction to the flow and low in // direction. High thermal diffusivity (a). Low thermal dilatation coefficient. 7 Regenerators (III) Fixed matrix regenerator: 8

5 Regenerators (IV) Rotary matrix regenerator: Both heat transfer process occurs simultaneously. 9 Regenerators (V) Rotary matrix regenerator: 10

6 Regenerators (VI) Rotary matrix regenerator: 11 Regenerators (VII) Rotary matrix regenerator: Applications: 12

7 Mixers (I) The heat is exchanged between the two fluids without separating wall between them (direct contact). There is heat and mass transfer (water evaporation). Two examples: Cooling or refrigeration towers: used to dissipate heat from an industrial process to the atmosphere instead of to a river or sea. Evaporative cooler (enfriador evaporativo): used in air conditioning installations. Air is cooled by means of water evaporation. 13 Mixers (II) Cooling or refrigeration towers: 14

8 Mixers (III) Cooling or refrigeration tower diagram: 15 Mixers (IV) Cooling or refrigeration tower diagram: 16

9 Mixers (V) Evaporative cooler diagram: 17 Packed bed heat exchanger It is a regenerator where the matrix is a porous medium. Figure: regenerator of double fixed matrix for continuous operation. 18

10 Direct flame heat exchanger Heat transmission by means of radiation mode (flame) and convection mode (exhaust or flue gases). Diagram of an industrial boiler or steam generator: 19 Recuperators (I) Both fluids pass simultaneously over the two surfaces of the separating wall. Classification: according to the flow arrangement and according to the construction type. Double-pipe heat exchangers (Intercambiadores de tubos concentricos o doble tubo): Parallel flow (flujo paralelo). Counter-flow (contraflujo). 20

11 Recuperators (II) Double-pipe heat exchangers: 21 Recuperators (III) Cross-flow heat exchangers (Intercambiadores de flujo cruzado): Tubes finned (both fluids unmixed). Tubes unfinned (one fluid mixed and the other unmixed). 22

12 Recuperators (IV) Cross-flow heat exchangers: Used to cool a liquid with air instead of water (shell-and-tube heat exchangers). Advantage: Elimination of water dependence and high cost of its treatment. Disadvantage: Require more space and produce more noise. 23 Recuperators (V) Cross-flow heat exchangers: Two types: Forced flow fan: The fan is below the tubes. Advantage: high turbulence and heat transfer. Disadvantage: low outlet speeds recirculation problems. Induced flow fan: The fan is over the tubes. Advantages: needs less power and no recirculation problems. Disadvantages: higher cost and more noise. 24

13 Recuperators (VI) Shell-and-tube heat exchangers (Intercambiadores de carcasa y tubos o de pasos múltiples): 25 Recuperators (VII) Shell-and-tube heat exchangers: 26

14 Recuperators (VIII) Shell-and-tube heat exchangers: Advantage: Great number of tubes increase of heat transfer area. Disadvantage: Increase of heat transfer area increase of cross-sectional area decrease of flow speed decrease of heat transfer coefficient. Less efficient than the counter-flow heat exchnagers. 27 Recuperators (IX) Shell-and-tube heat exchangers: Components. 28

15 Recuperators (X) Shell-and-tube heat exchangers: Components. 29 Recuperators (XI) Shell-and-tube heat exchangers: Subdivided according to the number of passes of each fluid through the tubes and the shell. 30

16 Recuperators (XII) Shell-and-tube heat exchangers: Baffles to increase the turbulence and to create a component of cross flow. 31 Recuperators (XIII) Types of shell-and-tube heat exchangers: 1. With fixed tubular plates: Types: welded, riveted, with expansion joint, double-plate. Longitudinal movement due to dilatation not permitted. Operation limited to shell and tube temperature difference under 50 ºC. Welded type: no possibility of dismantling chemical cleaning and fluid not corrosive. 32

17 Recuperators (XIV) Types of shell-and-tube heat exchangers: 2. With U tubes: Inlet and outlet of the tubes at the same side of the exchanger. Tubes curved in U shape at the other side. Bigger shell than the previous. Lower manufacturing cost than the previous (saving in connections). Chemical cleaning of the tubes due to the curves shape. 33 Recuperators (XV) Types of shell-and-tube heat exchangers: 3. With floating head: Longitudinal movement due to dilatation permitted. Higher manufacturing cost and construction difficulty. 34

18 Recuperators (XVI) Types of shell-and-tube heat exchangers: Comparison: U tubes Fixed plates Floating head Relative cost» 0.75» Dilatations absorbed Yes No Yes Shell change allowed Yes No Yes Tubes change Yes Yes Yes allowed Tubes Chemical or cleaning Chemical Chemical mechanical method Shell cleaning method Chemical or mechanical Chemical Chemical or mechanical 35 Recuperators (XVII) Compact heat exchangers (Intercambiadores de calor compactos): The objective is to obtain a high heat transfer area per volume unit (up to 700 m 2 /m 3 ). Formed by complex arrangements of tubes (plane or circular) with fins (rectangular or annular): Plate fin, tube fin or parallel plate heat exchangers. Used when one of the fluids is a gas. Small cross-sections laminar flow. 36

19 Recuperators (XVIII) Compact heat exchangers: 37 Energy Technology A. Y Recuperators (XIX) Compact heat exchangers: 38 Energy Technology A. Y

20 Recuperators (XX) Plate or plate and frame heat exchangers (Intercambiadores de placas): Formed by several thin, corrugated plates joined together by means of gaskets (juntas) or welded, depending of the liquids inside. The whole is compressed by a rigid frame, formed by a fixed plate in one side, a mobile plate on the other, a guide bar and a carrying bar. An arrangement of parallel flow channels is created. One fluid travels in the odd numbered channels and the other in the even. 39 Recuperators (XXI) 40

21 Recuperators (XXII) Plate heat exchangers: 41 Recuperators (XXIII) Plate heat exchangers: 42

22 Recuperators (XXIV) Plate heat exchangers: Corrugated plates provide: Greatest surface area for heat transfer. Maximum turbulence of the flow. Optimum fluid distribution. Minimal fouling. 43 Recuperators (XXV) Plate heat exchangers: Operation limitations: Welded plates: 350 ºC and 40 bar. With gaskets: 260 ºC and 20 bar. Disadvantage: higher pressure drops than shell-and-tubes. Advantage: easy cleaning (dismantle) and counter-flow arrangement. 44

23 Recuperators (XXVI) Design and calculation of plate heat exchangers: 4A P 4 a e 2 ( a + e) c Re = f (D h ) with Dh = =» 2 e if a >> e. mojado a = width of the plate; b = height of the plate; e = separation between plates. There are convection correlations specific for plate heat exchangers consult bibliography. Same calculation methods used for recuperators but with specific formulas consult bibliography. 45 Recuperators (XXVII) Design and calculation of plate heat exchangers: Definition of Plate Number, PN: 2 n U Su 2 U S PN =» n + 1 ( m& c ) ( m& c ) p min n = number of plates; S u = unit surface of one plate (projected for corrugated plates); m& u = unit mass flow per channel. n + 1 S T = n S u and m& = m& u so PN and NTU are related: 2 If NTU = PN, the heat exchanger is well designed. If NTU < PN, the heat exchanger is oversized, the power exchanged is higher than the prescribed one. If NTU > PN, the heat exchanger is undersized and it transfers less power then the required. u u p u min 46

24 Recuperators (XXVIII) Types of plate heat exchangers: a) One pass per channel of each fluid, pure counterflow. b) Multi-pass U arrangement. c) Multi-pass Z arrangement. d) Complex arrangements. 47 Fouling factors Fluid R f (m 2 K/W) Sea water (T < 50 ºC) Sea water (T > 50 ºC) River water Treated boiler feed water Liquid gasoline and oil Refrigerant liquids Methanol, ethanol Natural gas Steam (non-oil bearing) Steam (oil bearing) Compressed air

25 Heat transfer coefficients Process Fluid h (W/m 2 K) Natural or free convection Gas 2-25 Natural or free convection Liquid 50 1,000 Forced convection Gas Forced convection Liquid 50 20,000 Phase change convection Boiling 2,500 25,000 Phase change convection Condensation 5, , Overall heat transfer coefficient Fluid combination (hot cold) U (W/m 2 K) Water - Water 1,000 2,500 Water - Oil Ammonia - Water 1,000 2,500 Gases - Water Steam condensing- Water (in tubes) 1,000 6,000 Steam condensing - Gases Ammonia condensing Water (in tubes) 800 1,400 Alcohol condensing Water (in tubes) Finned-tube heat exchanger (Water in tubes, Air in cross flow)

26 Selecting the circulation side Thermal point of view: hot fluid in the shell side if the objective is to cool it. Corrosion: more corrosive fluid inside the tubes. Pressure: Highest pressure fluid inside the tubes. Dilatations: Hot fluid inside the tubes. Fouling: dirtiest fluid through the side with easiest cleaning. Viscosity: highest viscosity fluid through the more turbulent side (shell). Pressure drop: fluid with lower flow resistance through the tubes. 51 Bibliography (I) F. P. Incropera and D. P. De Witt, Fundamentals of Heat and Mass Transfer, 5 th ed., John Wiley & Sons, Inc., New York, F. Kreith y M. S. Bohn, Principios de Transferencia de Calor, 6ª ed., Thomson, Madrid, A. Bejan, Heat Transfer, 1 st ed., John Wiley & Sons, Inc., New York,

27 Bibliography (II) E. Torrella, J. M. Pinazo, R. Cabello, Transmisión de Calor, 1ª ed., Servicio de Publicaciones, Universidad Politécnica de Valencia, Valencia, J. Irigaray, Tecnología Energética II, Unicopia, Servicio de Publicaciones de TECNUN, San Sebastián,