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Thermal Engineering: Heat Exchangers
Conduction, Convection, and Counterflow Design
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Heat exchangers are devices that transfer thermal energy from one fluid to another without necessarily mixing the fluids. They are essential in power plants, refrigeration systems, chemical processing, car radiators, and many other technologies. Good thermal design improves energy efficiency, lowers operating cost, and helps equipment stay safe and reliable. Engineers use heat exchangers to control temperatures, recover waste heat, and support stable industrial operation.
A shell-and-tube heat exchanger is one of the most common designs because it can handle high pressures and large heat loads. Hot fluid may flow through the tubes while cold fluid flows through the shell side, and heat moves through the tube walls by conduction while convection occurs on both fluid sides. Baffles inside the shell guide the flow, increase turbulence, and improve heat transfer. Engineers evaluate performance using energy balance, temperature difference, overall heat transfer coefficient, pressure drop, and exchanger effectiveness.
Key Facts
- Rate of heat transfer: Q = m_dot cp DeltaT
- Heat exchanger design equation: Q = U A DeltaT_lm
- Log mean temperature difference: DeltaT_lm = (DeltaT_1 - DeltaT_2) / ln(DeltaT_1 / DeltaT_2)
- Overall thermal resistance adds as 1 / U = 1 / h_hot + R_wall + 1 / h_cold + R_fouling
- Counterflow exchangers usually give a larger DeltaT_lm than parallel flow for the same inlet temperatures
- Increasing flow speed often increases heat transfer coefficient h, but it also increases pressure drop
Vocabulary
- Overall heat transfer coefficient
- The overall heat transfer coefficient, U, measures how easily heat passes through the combined fluid films, wall, and fouling layers.
- LMTD
- LMTD, or log mean temperature difference, is an average temperature driving force used in heat exchanger calculations.
- Fouling
- Fouling is the buildup of unwanted material on heat transfer surfaces that reduces performance.
- Baffle
- A baffle is a plate inside the shell that redirects fluid flow to improve mixing and heat transfer.
- Effectiveness
- Effectiveness is the ratio of actual heat transfer to the maximum possible heat transfer in a heat exchanger.
Common Mistakes to Avoid
- Using the simple temperature difference instead of DeltaT_lm, which is wrong because the temperature driving force changes along the exchanger length.
- Ignoring fouling resistance, which is wrong because deposits on surfaces can significantly lower U and reduce heat transfer over time.
- Assuming higher flow rate always improves design, which is wrong because pressure drop and pumping power can rise too much even if heat transfer improves.
- Mixing up parallel flow and counterflow temperature profiles, which is wrong because the arrangement changes DeltaT_lm, outlet temperatures, and overall performance.
Practice Questions
- 1 A hot water stream with m_dot = 2.0 kg/s and cp = 4180 J/kg K cools from 90 C to 70 C in a heat exchanger. Calculate the heat transfer rate Q.
- 2 A heat exchanger has U = 250 W/m^2 K, area A = 12 m^2, and DeltaT_lm = 18 K. Find the heat transfer rate Q.
- 3 Two exchangers have the same area and materials, but one uses parallel flow and the other uses counterflow. Explain which one usually transfers more heat and why.