Glossary term
Heat Exchanger
A device that transfers heat between two or more fluids without necessarily mixing them.
Definition
deviceA heat exchanger is a device that transfers thermal energy between fluid streams or between a fluid and a solid surface.
Heat exchangers are used to heat, cool, condense, evaporate, recover energy, reject waste heat, and control process temperatures. Common forms include shell-and-tube, plate, finned-tube, double-pipe, air-cooled, spiral, compact, and regenerative exchangers. Their performance depends on heat-transfer area, fluid properties, flow arrangement, heat-transfer coefficients, fouling, pressure drop, phase change, material compatibility, and operating margins.
A heat exchanger transfers heat from one medium to another. In many designs the fluids remain separated by a wall, as in a shell-and-tube exchanger or plate heat exchanger. In other systems, heat may be transferred between a fluid and a solid matrix, as in a regenerative heat exchanger. The engineering goal is to move a required heat rate while controlling pressure drop, temperature approach, fouling, leakage risk, material compatibility, size, cost, and maintainability.
The basic steady heat-transfer expression is:
where \dot{Q} is heat-transfer rate, U is overall heat-transfer coefficient, A is heat-transfer area, and \Delta T_{lm} is the log mean temperature difference. The overall coefficient includes convection on both sides, conduction through the wall, and fouling resistance. For many design tasks, effectiveness-NTU methods are used instead of LMTD when outlet temperatures are not known in advance.
Flow arrangements
Parallel-flow exchangers send both fluids in the same direction. Counterflow exchangers send them in opposite directions and usually achieve a closer temperature approach for a given area. Crossflow exchangers move streams roughly perpendicular to each other and are common in air coolers, radiators, and HVAC coils. Multi-pass shell-and-tube exchangers use baffles and tube passes to improve heat transfer while managing pressure drop and mechanical constraints.
Phase change changes the design problem. Condensers and evaporators can transfer large heat rates at nearly constant temperature, but they require attention to drainage, vapor distribution, dryout, flooding, pressure drop, and fouling.
Design considerations
Thermal sizing must be paired with hydraulic and mechanical checks. Increasing velocity can improve heat-transfer coefficients but also raises pressure drop, erosion risk, vibration risk, and pumping power. Larger area can reduce temperature approach but increases cost, footprint, fluid inventory, and cleaning burden. Material selection must consider corrosion, galvanic coupling, temperature, pressure, fatigue, thermal stress, and compatibility with cleaning chemicals.
Fouling is often the dominant lifecycle issue. Deposits, scale, biological growth, polymerization, corrosion products, or particulate buildup reduce heat transfer and increase pressure drop. A design that meets duty when clean may fail after months of operation if fouling resistance is underestimated.
Common mistakes
A common mistake is treating catalogue heat duty as a universal capacity. Actual performance depends on flow rate, inlet temperatures, fluid properties, fouling condition, and installation. Another mistake is optimizing for heat transfer alone while ignoring pressure drop and controllability. Good heat-exchanger specifications state duty, allowable pressure drop, design pressure, design temperature, fouling allowance, fluid properties, materials, flow arrangement, cleaning method, and acceptance test conditions.