Heat Transfer in Chemical Engineering: Managing Thermal Energy

Temperature is one of the most important variables in chemical processing. Reactions proceed at rates determined by temperature. Separation processes depend on phase changes driven by heat addition or removal. Equipment must be protected from temperatures that exceed material limits. Heat transfer is the discipline that manages thermal energy in all its forms, enabling chemical processes to operate at their required conditions.

Modes of Heat Transfer

Heat moves by three mechanisms: conduction, convection, and radiation. Each operates according to different physical principles.

Conduction: Heat Through Solids

Conduction transfers heat through stationary material by molecular vibration and electron transport. Fourier’s law states that heat flux is proportional to temperature gradient: q = -k dT/dx. The thermal conductivity k varies widely: copper conducts about 400 W/m·K, carbon steel about 50, glass about 1, and polymer foams about 0.03.

For steady-state conduction through a flat wall, the heat transfer rate equals the temperature difference divided by the thermal resistance. For cylindrical geometry—as in pipes and tubes—the resistance increases logarithmically with radius. Insulation on pipes takes advantage of this principle: adding insulation increases the outer radius and the resistance.

Convection: Heat Transfer to Moving Fluids

Convection transfers heat between a solid surface and a moving fluid. The convective heat transfer coefficient h depends on the fluid properties, flow velocity, and geometry. Typical values range from 10 W/m²K for natural convection in gases to 10,000 W/m²K for boiling water.

Forced convection occurs when fluid motion is caused by external means—pumps or fans. Natural convection occurs when buoyancy forces from density differences drive fluid motion. Forced convection coefficients are generally much higher than natural convection coefficients. Understanding fluid behavior in these regimes draws on fluid-flow-chemical principles.

Radiation: Heat Through Electromagnetic Waves

Thermal radiation transfers heat through electromagnetic waves without requiring a medium. All objects emit radiation according to their temperature and surface properties. The Stefan-Boltzmann law states that the radiant power is proportional to the fourth power of absolute temperature.

Radiation becomes significant at high temperatures. In furnaces, radiation is the dominant heat transfer mechanism, with refractory walls radiating to process tubes. At typical chemical process temperatures below 300°C, radiation is usually negligible compared to conduction and convection.

Heat Exchanger Design

Heat exchangers transfer heat between two or more fluids at different temperatures. They are among the most common and most important equipment in chemical plants.

Shell-and-Tube Heat Exchangers

The shell-and-tube exchanger is the most widely used type. Tubes carry one fluid while the shell carries the other. Baffles on the shell side direct flow across the tubes, enhancing heat transfer.

The design process involves calculating the required heat transfer area based on the heat duty, the overall heat transfer coefficient, and the log mean temperature difference. The LMTD accounts for the varying temperature difference along the exchanger length.

Fouling—the accumulation of deposits on heat transfer surfaces—reduces the heat transfer coefficient over time. Designers include a fouling factor that represents the expected thermal resistance from deposits. Periodic cleaning restores performance.

Compact Heat Exchangers

Plate heat exchangers use corrugated plates to create flow channels between hot and cold fluids. They achieve higher heat transfer coefficients than shell-and-tube designs in a smaller volume. Gasketed plate exchangers can be opened for cleaning, making them suitable for fouling services.

Spiral heat exchangers use a spiral-wound channel arrangement that provides high turbulence and self-cleaning characteristics. They are used for slurries and highly viscous fluids.

Air-cooled heat exchangers use fans to blow air over finned tubes. They eliminate cooling water requirements, making them attractive in water-scarce locations. The trade-off is higher capital cost and dependence on ambient air temperature.

Heat Exchanger Network Design

Individual heat exchangers do not exist in isolation. They are part of a network that must be designed for overall energy efficiency. Pinch analysis provides a systematic method for designing heat exchanger networks that minimize hot and cold utility consumption.

The pinch point divides the process into two regions. Above the pinch, the process requires external heating. Below the pinch, it requires external cooling. Designing heat exchangers that transfer heat across the pinch reduces utility consumption.

Heat Transfer in Chemical Reactors

Temperature control is often the most critical aspect of reactor design and operation.

Exothermic Reactor Cooling

Exothermic reactions release heat. If this heat is not removed adequately, the temperature rises, accelerating the reaction rate, which generates more heat—a potential runaway condition. Reactor cooling systems must remove heat at least as fast as it is generated.

Cooling strategies include jackets surrounding the reactor vessel, internal cooling coils, external heat exchangers with pump-around loops, and evaporative cooling where a volatile component boils and the vapor is condensed and returned. The choice depends on the heat release rate, the required temperature, and the reactor size.

For highly exothermic reactions such as oxidation and hydrogenation, cooling is the primary design constraint. The reactor size may be determined not by the reaction kinetics but by the heat transfer area required for cooling.

Endothermic Reactor Heating

Endothermic reactions require continuous heat input to proceed. Steam reforming of methane, for example, operates at 800 to 1000°C and requires heat input of about 206 kJ per mole of methane converted.

Fired heaters typically provide the heat for high-temperature endothermic reactions. The reactor tubes are located in the radiant section of the heater, where they receive heat by radiation from the burner flames and refractory walls. The design must ensure uniform heat flux to all tubes to avoid hot spots that could damage the catalyst or tube material.

Evaporation and Condensation

Phase-change heat transfer involves much higher coefficients than single-phase heat transfer.

Boiling Heat Transfer

Boiling occurs when a surface temperature exceeds the saturation temperature of the liquid. In nucleate boiling, vapor bubbles form at nucleation sites on the surface and rise into the liquid. This is the most efficient boiling regime because the bubble formation and departure create intense local mixing.

At higher surface temperatures, a vapor film forms between the surface and the liquid—film boiling. The vapor film acts as insulation, drastically reducing the heat transfer coefficient. The minimum heat flux between nucleate and film boiling is the Leidenfrost point.

Condensation Heat Transfer

Condensation occurs when vapor contacts a surface below its saturation temperature. Dropwise condensation produces droplets that grow and roll off the surface, leaving bare surface for further condensation. Film condensation produces a liquid film that covers the entire surface.

Dropwise condensation has heat transfer coefficients 5 to 10 times higher than film condensation but is difficult to maintain. Surface coatings that promote dropwise condensation degrade over time, and most industrial condensers are designed for film condensation.

Thermal Design of Process Equipment

Beyond heat exchangers, many types of equipment involve heat transfer as a central function.

Fired Heaters

Fired heaters provide high-temperature heat for distillation column reboilers, reactor feeds, and heat transfer fluids. The heater consists of a radiant section where most heat transfer occurs by radiation, and a convection section where flue gas heats process fluid.

Thermal efficiency depends on the flue gas exit temperature. Modern heaters achieve efficiencies above 90 percent by recovering heat from flue gas through combustion air preheaters. The limiting factor is the acid dew point—the temperature at which sulfuric acid condenses from combustion products, causing corrosion.

Cooling Towers

Cooling towers reject process heat to the atmosphere by evaporating a portion of the cooling water. Hot water from the process enters the top of the tower and falls through packing while air flows upward. Evaporation cools the remaining water, which collects in the basin for recirculation.

The approach temperature—the difference between the cold water temperature and the ambient wet-bulb temperature—determines the tower size. A smaller approach requires a larger tower. Typical approaches are 5 to 10°C.

Heat Transfer Fluids and Systems

The choice of heat transfer fluid affects system design, operating cost, and safety.

Water and Steam

Steam is the most common heating medium in chemical plants. Saturated steam condenses at constant temperature, providing uniform heating. The latent heat of vaporization—about 2200 kJ/kg for steam at atmospheric pressure—provides high heat capacity per unit mass.

Steam pressure determines temperature. Low-pressure steam at 5 barg provides heat at about 160°C. High-pressure steam at 40 barg provides heat at about 250°C. For temperatures above those achievable with steam, other fluids must be used.

Thermal Fluids

Thermal fluids such as Dowtherm and Therminol provide heat at temperatures up to 400°C without the high pressures required for steam at these temperatures. The fluids operate as liquids at atmospheric pressure, simplifying system design.

Thermal fluid systems require expansion tanks, pumps, and heaters. The fluid degrades over time at high temperatures, requiring periodic replacement. Fire safety is a concern because thermal fluids are combustible.

Molten Salts and Liquid Metals

For temperatures above 400°C, molten salts provide heat transfer and thermal storage. Solar thermal power plants use molten salt mixtures—typically 60 percent sodium nitrate and 40 percent potassium nitrate—to store heat at 550°C.

Liquid sodium is used in fast nuclear reactors for heat transfer at temperatures up to 800°C. Its excellent thermal conductivity and high heat capacity make it an exceptional heat transfer fluid, though its chemical reactivity presents safety challenges.

Energy Recovery and Sustainability

Heat transfer is central to industrial energy efficiency and sustainability.

Waste Heat Recovery

Process streams leaving at elevated temperatures contain energy that can be recovered. Waste heat boilers generate steam from hot process gases. Recuperators preheat combustion air using hot flue gas. Heat pumps upgrade low-temperature heat to useful temperatures.

The economic value of waste heat depends on the temperature level and the available recovery technology. High-temperature waste heat is valuable because it can generate steam or power. Low-temperature waste heat is harder to recover economically.

Thermal Energy Storage

Thermal energy storage enables processes to operate flexibly, decoupling energy supply from demand. Sensible heat storage uses the heat capacity of a material like water or rock. Latent heat storage uses the phase change of materials like paraffin wax or salt hydrates.

TES is critical for concentrating solar power plants, which store heat in molten salt for electricity generation after sunset. In chemical plants, TES can capture waste heat for use in batch operations or to handle variations in steam demand.

Conclusion: The Energy That Makes Processes Work

Heat transfer is not a peripheral activity in chemical engineering—it is central to virtually every process. Without effective heat transfer, reactions would overheat, separations would fail, and products would not meet specifications. The discipline connects thermodynamics (how much heat must be added or removed), transport phenomena (how fast heat can be transferred), and equipment design (what hardware accomplishes the transfer).

As energy costs rise and environmental regulations tighten, heat transfer expertise becomes increasingly valuable. Engineers who can design efficient heat exchanger networks, recover waste heat, and optimize thermal processes contribute directly to plant profitability and sustainability.

Frequently Asked Questions

What is the overall heat transfer coefficient?

The overall heat transfer coefficient U combines all resistances to heat transfer between two fluids: the convective resistances on each fluid side plus the conductive resistance of the tube wall and fouling layers. It is a key parameter in heat exchanger design, typically ranging from 100 to 1000 W/m²K for liquid-liquid exchangers.

How do engineers prevent heat exchanger fouling?

Fouling prevention strategies include maintaining sufficient velocity to prevent deposition, selecting materials that resist corrosion and scaling, filtering feed streams to remove particulates, and designing for easy cleaning. Chemical additives can inhibit biological growth and scale formation.

What is the difference between log mean temperature difference and number of transfer units methods?

LMTD is used when the heat exchanger outlet temperatures are specified and the required area must be calculated. NTU is used when the area is known and the outlet temperatures must be determined. Both methods give the same results when applied correctly.

Why are fins used on heat exchanger tubes?

Fins increase the surface area on one side of a heat exchanger, compensating for a low heat transfer coefficient. They are typically applied to the gas side of gas-liquid exchangers, where gas-side coefficients are much lower than liquid-side coefficients. Fins can increase the effective area by a factor of 10 to 20.