Question 1

What is Fourier's Law of heat conduction?

Accepted Answer

Fourier's Law: Q = k × A × ΔT / L, where Q = heat flow rate (W), k = thermal conductivity (W/m·K), A = cross-sectional area (m²), ΔT = temperature difference (K or °C), L = thickness (m). Example: A 10mm thick carbon steel plate (k = 50 W/m·K), 1 m² area, with 100°C temperature difference: Q = 50 × 1 × 100 / 0.01 = 500,000 W = 500 kW. Used in boiler design, furnace walls, pipe insulation calculations, and process vessel heat-through calculations.

Question 2

What is Newton's Law of cooling for convection?

Accepted Answer

Newton's Law of Cooling: Q = h × A × ΔT, where h = convective heat transfer coefficient (W/m²·K), A = surface area (m²), ΔT = temperature difference between surface and fluid (K). Typical h values: Natural air convection: 5–25 W/m²·K. Forced air convection: 25–250 W/m²·K. Forced water: 500–10,000 W/m²·K. Boiling water: 2,500–100,000 W/m²·K. Condensing steam: 5,000–100,000 W/m²·K. U-values for overall HEX design combine conduction and convection resistances in series.

Question 3

What is the Log Mean Temperature Difference (LMTD) and how is it used?

Accepted Answer

LMTD is the effective mean temperature driving force for heat exchanger design. LMTD = (ΔT₁ - ΔT₂) / ln(ΔT₁/ΔT₂). For counter-flow HEX: ΔT₁ = T_hot,in − T_cold,out, ΔT₂ = T_hot,out − T_cold,in. Required area: A = Q / (U × F × LMTD), where F = correction factor (0.7–1.0 for multi-pass/cross-flow). Counter-flow gives higher LMTD → smaller area required. Shell-and-tube HEX design per TEMA standards used widely in South African petrochemical and mining industries.

Question 4

What is heat exchanger effectiveness (NTU method)?

Accepted Answer

Effectiveness ε = Actual heat transfer / Maximum possible heat transfer = Q_actual / Q_max. Q_max = C_min × (T_hot,in − T_cold,in), where C_min is the smaller of ṁ_h×Cp_h or ṁ_c×Cp_c. NTU = U × A / C_min. Effectiveness depends on NTU and the capacity ratio C_min/C_max. The NTU method is preferred when outlet temperatures are unknown. Used for rating existing HEX versus LMTD method used for sizing new HEX.

Question 5

How do I estimate pipe heat loss and insulation thickness?

Accepted Answer

Pipe heat loss: Q = 2πkL(T_inner - T_outer) / ln(r_outer/r_inner) for cylindrical geometry. For insulated pipe: resistances add in series. Insulation selection: target heat loss to meet safety requirements (touch temperature < 55°C for personnel protection per SANS 10082) and energy cost targets. Economic thickness of insulation: balance insulation cost vs. energy savings. For steam pipes, ASHRAE/SANS guidelines recommend insulation for all lines above 60°C. Common insulation k values: Rockwool 0.04 W/m·K, Calcium silicate 0.09 W/m·K, Polyurethane 0.026 W/m·K.

Question 6

What is the Stefan-Boltzmann Law for radiation heat transfer?

Accepted Answer

Q_rad = ε × σ × A × (T₁⁴ − T₂⁴), where σ = Stefan-Boltzmann constant = 5.67 × 10⁻⁸ W/(m²·K⁴), ε = emissivity (0 = perfect reflector, 1 = blackbody), T in Kelvin. Emissivity values: Polished metal: 0.02–0.10. Oxidised steel: 0.70–0.90. Painted surfaces: 0.85–0.95. Firebrick: 0.75. Radiation becomes dominant above ~800°C. Used in kiln and furnace design, and for heat shield and surface coating selection in high-temperature process equipment.

Question 7

What types of heat exchangers are used in South African industry?

Accepted Answer

Common heat exchanger types in SA industry: Shell-and-tube (most common): Used in petrochemical, power generation, mining. Designed per TEMA standards. Plate HEX: HVAC, food processing, brewery. High thermal efficiency per volume. Air coolers (fin-fan): Process cooling where water is scarce, especially Mpumalanga and Northern Cape plants. Double-pipe: Simple, maintenance-friendly. Spiral: High-fouling fluids. Brazed plate: Refrigeration. SA-specific challenges: water scarcity makes air coolers preferred; high-solids mine water requires frequent cleaning or special materials (titanium, duplex SS).

Question 8

How do I choose the correct thermal conductivity value for my material?

Accepted Answer

Key factors: Material composition matters — stainless 304 (16 W/m·K) vs carbon steel (50 W/m·K). Temperature affects k — verify at operating temperature. For composites: use effective conductivity or thermal resistance calculations. For detailed design: use manufacturer datasheets or ASME/ASTM material standards. Rules of thumb: metals have high k (good conductors), insulation materials have low k (poor conductors). A ceramic fibre blanket at 0.1 W/m·K provides 500× more insulation per mm than carbon steel.

Material	k
Copper	386
Aluminium (6061)	167
Carbon Steel	50
Stainless 316	16
Cast Iron	52
Mild Steel	45
Concrete	0.8–2.0
Brick	0.6–1.0
Insulation (rockwool)	0.04
Water (20°C)	0.598
Air (20°C)	0.026

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