Slide 1
Transfer heat across a barrier
A heat exchanger moves thermal energy between fluids while keeping their main flows separated. A conducting wall provides the thermal connection.
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Separation is useful when fluids differ in cleanliness, pressure or chemistry. A leak can compromise that boundary even if heat transfer remains strong, so integrity and thermal performance are separate requirements.
Slide 2
Area and temperature difference matter
More transfer area and a larger temperature difference can increase heat flow. Resistance on both fluid sides and through the wall limits the rate.
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Fins add area where a fluid transfers heat poorly, such as air. Their benefit depends on conducting heat into the fin, so adding arbitrarily long thin fins does not guarantee proportionate improvement.
Slide 3
Flow arrangement changes the opportunity
Parallel and opposing fluid directions create different temperature profiles. Counterflow can maintain a useful temperature difference along more of the exchanger.
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In counterflow, the cold fluid can leave at a higher temperature than the hot fluid leaves. Heat still flows from hot to cold because the hot stream is warmer at each adjacent location.
Slide 4
Fouling adds a barrier
Deposits on heat-transfer surfaces increase thermal resistance and may obstruct flow. Performance can decline even though the metal wall remains intact.
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Cleaning access and water chemistry can therefore influence lifetime cost. A compact design with inaccessible passages may be difficult to restore after fouling, despite excellent initial performance.
Slide 5
Pressure loss is part of the tradeoff
Making flow more turbulent can improve heat transfer but also increase pumping demand. Thermal effectiveness is not the only measure of a good exchanger.
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An efficient system balances heat recovered against the work needed to move both fluids. The preferred design may change with operating hours, fluid properties and available pressure.
Sources
- Decarbonizing Building Thermal Systems
US Department of Energy
- District Energy Fact Sheet
US Department of Energy
- Heat Exchangers
Massachusetts Institute of Technology