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🔥 Heat Transfer, Simply Explained — Conduction, Convection & Radiation
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Conduction: Direct Transfer of Heat Through a Solid
Imagine touching a hot pan handle — ouch! The heat transfers directly from the pan to your hand through conduction. Conduction occurs when heat energy moves through a solid material from molecule to molecule. Molecules in a hotter region vibrate rapidly, passing their energy to cooler neighboring molecules.
Materials vary greatly in their ability to conduct heat. Metals, like copper or aluminum, have high thermal conductivity, allowing heat to move swiftly. Conversely, materials like wood, plastic, or insulation have low thermal conductivity, hindering heat flow.
q = (k × A × ΔT) / d
q = Heat transfer rate (W)
k = Thermal conductivity (W/m·K)
A = Cross-sectional area (m²)
ΔT = Temperature difference (K or °C)
d = Thickness (m)HVAC example: wall insulation reduces heat transfer by increasing thermal resistance (low‑k materials), improving comfort and lowering energy bills.
Convection: Transfer of Heat by Fluid Motion
Ever observed how warm air rises from a hot cup of coffee? That’s convection! Convection transfers heat via fluid movement—either liquids or gases. This motion can occur naturally due to buoyancy (warm rises, cool sinks) or be driven by fans and pumps (forced convection).
Natural convection happens because warmer fluids are lighter and rise, while cooler, denser fluids sink, creating continuous circulation. Forced convection enhances heat transfer by actively moving fluid over surfaces.
q = h × A × (Tₛ − T_f)
q = Heat transfer rate (W)
h = Heat transfer coefficient (W/m²·K)
A = Surface area (m²)
Tₛ = Surface temperature (°C)
T_f = Fluid temperature (°C)HVAC example: Efficient duct design and smart diffuser placement leverage forced convection to distribute comfort and cut energy use.
Radiation: Heat Transfer Through Electromagnetic Waves
Ever felt the sun’s warmth even on a cool day? That’s radiant heat! Radiation transfers heat via electromagnetic waves and needs no medium. Unlike conduction and convection, radiation can occur across a vacuum — like outer space.
Dark, matte surfaces absorb and emit radiation effectively, whereas shiny, reflective surfaces reduce absorption and emission.
Stefan–Boltzmann (black body): q = σ × A × T⁴
For real surfaces: q = ε × σ × A × T⁴
σ = 5.67 × 10⁻⁸ W/m²·K⁴ · ε ∈ [0,1]HVAC example: solar gains through glazing and roofs can dominate cooling loads; reflective coatings and low‑e glass reduce radiative heat flow.
Why these modes matter for HVAC
- Conduction impacts insulation levels, thermal bridges, and envelope performance.
- Convection drives room air mixing, diffuser selection, and coil sizing.
- Radiation governs solar/long‑wave gains, glazing choices, and roof reflectivity.
Practical Applications and Examples
- Conduction: Select low‑k insulation for walls/roofs to reduce heat loss or gain.
- Convection: Optimize duct layout, vent placement, and fan control for uniform comfort.
- Radiation: Use reflective roofs, shading, and low‑e windows to cut cooling loads.
📚 Citations (optional)
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Conduction Estimator
Convection Estimator
Radiation (Grey Surface)
Tip
Try different ε values: matte black ~0.95, clean aluminum ~0.05–0.2. See how radiative heat skyrockets with temperature (T⁴)!
