Heat Exchangers - C HAPTER 11: H EAT E XCHANGERS Heat...

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Unformatted text preview: C HAPTER 11: H EAT E XCHANGERS Heat exchangers are one of the most common unit operations used in industry and are used to transfer heat between two fluids for the purpose of: Recovering thermal energy Transferring heat to a closed-loop process Maintaining reaction stability Example: Air Conditioner Two horizontal, continuous finned heat exchangers (evaporator and condenser) one sits inside your house while the other sits outside the house. T YPES OF H EAT E XCHANGERS Heat exchangers are classified by: 1) Type of Construction Cross-Flow (Single Pass): Fluid flows over a bank of tubes o Outside baffles or fins may be present to increase heat transfer and control mixing Concentric Tube (Double Pipe): One tube inside an outer tube (annular tubes) o Cheapest but least effective Fluid Flow in Tube Free-flowing Fluid Shell-and Tube (Multiple Pipe): Multiple tubes inside an outer tube (shell) or multiple outer tubes (multishell) o Baffles often installed to increase turbulence ( b increase h ) and physically support tubes in the shell to prevent vibrations o Multiple passes possible Shell side Tube side Shell Inlet Tube Outlet Tube Inlet Shell Outlet Baffle 2 Shell Passes 4 Tube Passes Shell Inlet Tube Outlet Tube Inlet Shell Outlet 1 Shell Pass 2 Tube Passes Compact: Dense arrays of finned tubes or plates o Typically used if one fluid is a gas (lower h b need high A s ) o Flow passages small ( D h <5mm) b pressure drops may be high o Flow typically laminar o Most expensive to fabricate o Typically fin-tube or plate-fin Flat tube Plate fin Circular tube Circular fin Corrugations (fins) Parallel Plates Single pass Multiple pass FIN- TUBE PLATE-FIN Tube flow - unmixed Tube flow - unmixed 2) Flow arrangement Direction: Co-current (hot/cold streams flow in same direction) vs. counter-current (hot/cold streams flow in opposite directions) Co-current Counter-current Mixing: Mixed (flow unconstrained) vs. unmixed (flow constrained in one direction pipe or fins) Cross-flow T=f(x,y)- unmixed Cross-flow T=f(x) - mixed x y Baffles prevent y- direction mixing y- direction fluid flow unrestricted Tube flow - unmixed Tube flow - unmixed A NALYZING H EAT E XCHANGERS The key parameter of interest for defining heat exchanger performance is the overall heat transfer coefficient: R total = sum of all resistances in thermal circuit R total incorporates resistances related to convection in both the heating and cooling liquid streams and conduction through the heat exchanger walls. b Clean, unfinned heat exchangers In reality, we must also consider the non-idealities in heat transfer introduced by fouling of the heat exchanger walls due to fluid impurities, rust, scaling, deposition......
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Heat Exchangers - C HAPTER 11: H EAT E XCHANGERS Heat...

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