Thermodynamics Basics for Industry
What Is Heat?
In a steel mill, the furnace operates at 1500°C while cooling water circulates at 30°C. Heat is not a substance — it is energy transferred due to a temperature difference. Temperature measures the average kinetic energy of molecules. Faster molecules mean higher temperature. Heat always flows from hot to cold until equilibrium is reached.
First Law: Energy Is Conserved
The First Law of Thermodynamics states that energy cannot be created or destroyed — only converted.
ΔU = Q - W
Where ΔU is the change in internal energy, Q is heat added, and W is work done by the system.
Factory example: In a diesel generator, fuel combustion releases heat (Q). Part becomes mechanical work (W) driving the generator. The rest raises engine temperature (ΔU) and exits with exhaust. Every joule is accounted for.
Second Law: Entropy
The Second Law states that heat flows spontaneously from hot to cold — never the reverse without external work. This directionality is measured by entropy, a quantity that always increases in real processes.
No engine can be 100% efficient. A steam boiler at 400°C feeding a turbine exhausting at 100°C has a maximum Carnot efficiency:
η = 1 - T_cold / T_hot = 1 - 373/673 ≈ 44.6%
The remaining 55.4% must be rejected as heat — this is a law of nature, not a design choice.
Heat Transfer
Conduction
Conduction transfers heat through a solid without material movement. Fourier's Law describes it:
q = -k × A × (dT/dx)
Where k is thermal conductivity. Copper (k ≈ 400 W/m·K) is excellent for heat exchangers. Insulation (k ≈ 0.04) wraps steam pipes to reduce losses.
Convection
Convection transfers heat via fluid motion. Natural convection occurs when heated fluid rises due to reduced density — like hot air above a smelting furnace. Forced convection uses pumps or fans — like cooling water in injection molds. Forced convection delivers far higher heat transfer rates, which is why nearly every industrial cooling system uses pumps.
Radiation
Thermal radiation travels as electromagnetic waves — no medium required. The Stefan-Boltzmann Law governs it:
q = ε × σ × A × T⁴
Where ε is emissivity. Energy scales with T⁴ — doubling temperature increases radiation 16-fold. At 1500°C in smelting furnaces, radiation dominates all other heat transfer modes.
Heat Exchangers
A heat exchanger transfers heat between two fluids without mixing them.
Shell-and-tube: One fluid flows through tubes inside a shell carrying the other fluid. Handles high pressures (up to 100 bar) and temperatures. Common in oil refineries and power plants.
Plate heat exchangers: Stacked corrugated metal plates. Large surface area in a compact volume with higher transfer efficiency, but suited to lower pressures. Used in food processing and HVAC systems.
Thermal Efficiency
Thermal efficiency is the ratio of useful work to heat input:
η = W_useful / Q_input × 100%
Typical industrial values:
- Diesel engine: 35-45%
- Modern steam boiler: 85-92%
- Gas turbine: 30-40%
- Combined cycle (gas + steam): 55-62%
Each percentage point improvement saves significant fuel. A plant consuming 10 MW of thermal energy gains 200 kW from a 2% efficiency improvement — thousands of dollars annually. Key techniques include waste heat recovery from exhaust gases, pipe insulation, and cleaning heat exchanger fouling.
Summary
Thermodynamics governs every industrial process involving heat — from smelting furnaces to cooling systems. The first law accounts for every joule. The second law sets an upper bound on efficiency. Understanding heat transfer mechanisms and selecting the right heat exchanger means designing systems that are more efficient and less energy-intensive.