Heat Exchanger Design Basis

Presentation on Heat Exchangers
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  TP - heat exchanger design.ppt 1 Transport Processes Overall heat transfer coefficient  From previous studies (CPP module): Q’  = U A Δ T LM ooiioiii  h1dd2k )ddln(d h1U1  Some typical U values (all in W/m 2 K): steam/water: 6000 to 18000 water/water: 850 to 1700 steam condenser (water in tubes) 1000 to 6000 ammonia condenser (water in tubes) 800 to 1400 alcohol condenser (water in tubes) 250 to 700 finned tube (air outside, water inside) 25 to 50 deduce others from charts  TP - heat exchanger design.ppt 2 Transport Processes Overall heat transfer coefficient ã Contains many combinations ã May need to transpose top and bottom fluids ã Gives rather conservative estimates  TP - heat exchanger design.ppt 3 Transport Processes Choosing right shell-and-tube type Decision as to TEMA code used depends on fluids used Shell& tube exchangers Severe thermal exapansion stresses? Are bellows allowed? Is chemical cleaning  possible? High shellside fouling > 0.00035 m 2 K/W? High tubeside fouling > 0.00035 m 2 K/W? Is chemical cleaning  possible? Removable  bundle design Fixed tubesheet Is interstream leakage allowed? Are T & P within range for lantern ring? Is F correction factor < 0.75? Are there more than 2 passes? Are F or multi shells allowed? Frequency of bundle removal AEL AEM BEM  No No Yes Yes No AEU AFU AEU AFU  No No Yes Yes No AEP BEP  No No Yes Yes No AEW BEW  No No Yes Yes No AET BET  No No Yes Yes No AES BES  No No Yes Yes No Is tubeside fouling > 0.00035 m 2 K/W? Do we require tube access without disturbing connections? Yes No Yes Yes Yes  No  No Yes Yes  No Yes  No Yes  No Yes  No Yes  No Low High Yes  TP - heat exchanger design.ppt 4 Transport Processes Log Mean Temperature Difference e.g. find  T LM  for both co-current & counter-current flow Fluid A T in = 120 T out = 90°C Fluid B t in = 20 t out = 80°C temperature  T 1  T 2 hot fluid T in T out cold fluid t out t in 2121LM TlnTln TTT 
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