Heat Exchangers Heat Exchangers

Heat Exchangers 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

. Flow in a double-pipe heat exchanger.

Heat Exchangers Chee 318

•Heat Exchanger (HEX) Rating Checkingthe 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

steady-state, steady flow• no heat generation in the HEX 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

Heat Exchanger Analysis LMTD Method Expression for convection heat transfer for flow of a fluid inside a tube: For case 3 involving constant surrounding fluid temperature: Heat Exchangers

Concentric Tube Construction • • - : : Heat Exchangers

Heat Exchangers

Heat Exchangers

Heat Exchangers

Heat Exchangers

Heat Exchangers

Heat Exchangers

Overall Heat Transfer Coefficient 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. (11.3a) Note that: where inner tube surface outer tube surface Heat Exchangers

Log Mean Temperature Difference (LMTD) is best suited when the temperature difference is known. The Effectiveness-NTU is best used when the surface area is known.

Assumptions for HEX analysis Steady-state, steady flow No heat generation in the HEX Negligible ΔPE, ΔKE Adiabatic processes No phase change Constant specific heats and other physical properties.

Under these assumptions, the first law of thermodynamics requires that the rate of heat transfer from the hot fluid equals the rate of heat transfer to the cold fluid.

Notes

Example