Heat Transfer Rate
Sensible Heat & Energy Balance
Sensible Heat (Q = ṁ × Cp × ΔT)
Heat Transfer Rate
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Heat Exchanger (Q = U × A × ΔT_lm)
Heat Duty
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Heat Transfer Equations
Sensible Heat Transfer
Q = ṁ × Cp × ΔT
Q = Heat transfer rate (W)
ṁ = Mass flow rate (kg/s)
Cp = Specific heat capacity (J/kg·K)
ΔT = Temperature difference (K or °C)
ṁ = Mass flow rate (kg/s)
Cp = Specific heat capacity (J/kg·K)
ΔT = Temperature difference (K or °C)
Heat Exchanger Duty
Q = U × A × ΔT_lm
U = Overall heat transfer coefficient (W/m²·K)
A = Heat transfer surface area (m²)
ΔT_lm = Log mean temperature difference (K)
For counter-flow or parallel flow:
ΔT_lm = (ΔT₁ - ΔT₂) / ln(ΔT₁/ΔT₂)
A = Heat transfer surface area (m²)
ΔT_lm = Log mean temperature difference (K)
For counter-flow or parallel flow:
ΔT_lm = (ΔT₁ - ΔT₂) / ln(ΔT₁/ΔT₂)
Energy Balance on Streams
Q_hot = Q_cold
For ideal heat exchanger (no losses):
ṁ_hot × Cp_hot × ΔT_hot = ṁ_cold × Cp_cold × ΔT_cold
ṁ_hot × Cp_hot × ΔT_hot = ṁ_cold × Cp_cold × ΔT_cold
Specific Heat Capacity of Common Fluids
| Fluid | State | Cp (kJ/kg·K) | Temp Range |
|---|---|---|---|
| Water | Liquid | 4.18 | 0-100°C |
| Steam | Vapor | 1.87 | 100-200°C |
| Air | Gas | 1.005 | 25°C |
| Oil (mineral) | Liquid | 2.0-2.2 | 40-100°C |
| Ethylene Glycol | Liquid | 3.50 | 20°C |
| Refrigerant (R-134a) | Liquid | 1.42 | 25°C |
Unit Conversions for Heat Transfer
| From | To | Multiply by |
|---|---|---|
| kJ/kg·K | BTU/lb·°F | 0.2389 |
| BTU/lb·°F | kJ/kg·K | 4.187 |
| W/m²·K | BTU/h·ft²·°F | 0.1761 |
| BTU/h·ft²·°F | W/m²·K | 5.678 |
| W (watts) | BTU/h | 3.412 |
| kW | kcal/h | 860.4 |