Heat Exchanger Notes

Introduction

• Temperature: Measure of hotness or coldness.

  • Factors affecting boiling point: impurities, vapor pressure.

• Energy: Cannot be created or destroyed, only changes form.

  • Stored energy: internal, potential, kinetic, chemical, electrical, nuclear.

  • Transitional energy: energy transferred between system and surroundings (e.g., heat, work).

• Heat: Form of thermal energy.

  • Adding heat raises temperature and changes size/state.

  • Chemical reactions give off heat.

  • Types: Sensible and Latent.

Types of Heat

• Sensible Heat: Changes temperature but not state.

• Latent Heat: Changes the state of the material but not temperature; 'hidden heat'.

Sensible Heat Formula

• $Q = m * Cp * \Delta T$

  • Q = heat transfer (J or kcal)

  • m = mass (kg)

  • Cp = specific heat capacity (J/kg K or J/kg °C)

  • $\Delta T$ = temperature difference (°C or K)

• Specific Heat Capacity (Cp): Heat to raise 1kg by 1K or 1°C.

Unit of Measurement (Heat and Power)

• Mass: kilogram (kg), 1 kg = 1000 g = 0.001 metric ton

• Heat: joule (J), kJ = 1000 J, MJ = 10^6 J, 1 Btu = 1055 J

• Power: watt (W), kW = 1000 W = 10^3 W, 1 W = 1 J/s

Heat Transfer

• Heat flows from high to low temperature.

• Types: Conduction, Convection, Radiation.

Conduction

• Molecular vibration spreads through solid.

• In heat exchangers: occurs in tube walls.

• Rate depends on: Surface Area, Temperature Difference, Thickness.

Convection

• Heat transfer within fluid.

• In heat exchangers: occurs within tubes and shell.

• Rate depends on: Viscosity, Density.

Radiation

• Heat transfer via wave motion, no medium needed.

• Rate depends on: Distance, Size, Temperature Difference, Surface Type.

Heat Exchanger Equation

• $Q = U * A * LMTD$

  • Q = Heat transfer rate (W)

  • U = Overall heat transfer coefficient (W/m².K)

  • A = Heat transfer area (m²)

  • LMTD = Log mean temperature difference (K or °C)

• Factors affecting U: Thermal conductivity, Fouling.

Operating Principles of Heat Exchanger

• Exchanges heat between fluids at different temperatures.

• Functions: Heat transfer from hot to cold streams, cool processed liquid.

Operating Principles of Cooling Tower

• Specialized heat exchanger: warm water droplets expose to air.

• Heat transferred via evaporation.

• Rate of evaporation depends on air humidity.

Pre-Startup of Heat Exchanger

• Air freeing: purge trapped gases.

• Thermal shock: avoid sudden temperature changes.

• Start with colder fluid first.

Startup - Shell Side

• Crack open recirculation line.

• Open shell vent valve until fluid overflows, then close.

• Fully open process fluid inlet/outlet valves; establish flow rate.

Startup - Tube Side

• Open vent valve.

• Crack open inlet valve to remove air.

• When full, close vent valve.

• Fully open inlet/outlet valves; start pump, establish flow rate.

Shutdown - Shell Side

• Lower pump flow rate.

• Switch off pump.

• Close all isolation valves.

Safety Shutdown

• Shut down hotter liquid side first to prevent overheating.

• Drain and clean after use.

Shell and Tube Heat Exchanger Construction

• Baffles: cause turbulent flow for better heat transfer.

• Tube Sheet: supports tubes.

• Channel (Head): inlet/outlet nozzles, partition plates; Removable Channel (dirty liquid), Bonnet (clean liquid).

Classification of Heat Exchanger by Application

• Condenser: removes heat from vapor.

• Cooler: cools hydrocarbon stream.

• Reboiler: supplies heat to column bottom.

Classification of Heat Exchanger by Structure

• Fixed Head: simple, for small temperature differences.

• Floating Head: for large temperature differences, allows thermal expansion.

• U-tube: for very high pressure, difficult to clean.

• Hairpin: for very high pressure, clean fluid only, true counter-current flow.

• Plate & Frame: compact, corrosive processes, clean conditions.

Types of Flow Path

• Co-current: fluids flow parallel; temperature difference decreases, limited heat transfer.

• Counter-current: fluids flow opposite; constant temperature difference, more efficient.

• Cross current: fluids flow perpendicular; tube bundle or finned-tube bundle.

Routing of Fluid

• Shell Side: viscous, corrosive, lower flow rate, fouling fluids.

• Tube Side: cleaner, cooling water, toxic, high-pressure fluids.

Performance of Heat Exchanger

• Multiple-pass exchangers improve efficiency.

• Log Mean Temperature Difference (LMTD): measures efficiency.

  • Co-current: $LMTD = \frac{(T1 - t1) - (T2 - t2)}{ln(\frac{T1 - t1}{T2 - t2})}$

  • Counter-current: $LMTD = \frac{(T1 - t2) - (T2 - t1)}{ln(\frac{T1 - t2}{T2 - t1})}$

  • (T1, T2 = tube side temps; t1, t2 = shell side temps)

  • Correction factor applied for multiple pass exchangers.

Factors Affecting Heat Transfer

• Fouling: deposits on tube surface reduce heat transfer; prevent by adding chlorine, back flushing.

• Tube leaks: caused by erosion/corrosion; prevent by using deflector plates; sacrificial anodes (Mg for freshwater, Al for saltwater).

• Stagnant film: fluid layer on pipe wall, reduces heat transfer; reduce by increasing flow velocity or turbulence.

Auxiliary Equipment

• Blowdown drum: removes dissolved solids from boiler.

• Steam trap: removes condensate from steam line.

  • Float trap: uses density difference.

  • Inverted-bucket trap: uses density difference, bucket moves valve.

  • Bimetallic trap: uses thermal expansion differences.