What is a Heat Transfer Calculator?
This calculator computes the heat required for sensible and latent heat processes, and calculates the log mean temperature difference (LMTD) used in heat exchanger design.
How it's calculated
Sensible heat Q = m × c × ΔT (when temperature changes)
Latent heat Q = m × L (when phase changes)
LMTD = (ΔT₁ − ΔT₂) ÷ ln(ΔT₁/ΔT₂)
Variable definitions
| Variable | Meaning | Unit |
|---|---|---|
| Q | Heat energy | kJ, J |
| m | Mass | kg |
| c | Specific heat | kJ/kg·K |
| L | Latent heat (fusion/vaporization) | kJ/kg |
Assumptions
This assumes specific heat and latent heat stay constant across the temperature range involved. In reality specific heat shifts slightly with temperature, but the effect is negligible for typical engineering calculations.
Worked example
Heating 2kg of water from 20°C to 100°C, then converting it all to steam
- Sensible heat: 2 × 4.18 × (100-20) = 668.8 kJ
- Latent heat: 2 × 2,260 = 4,520 kJ
- Total heat required: 5,188.8 kJ (latent heat is about 6.8x the sensible heat)
Why does boiling water stay at 100°C?
Sensible heat raises temperature; latent heat changes state without changing temperature. Once water reaches 100°C, all additional heat goes into converting liquid to gas, so the temperature stops rising.
Heating 1kg of water from 0°C to 100°C takes about 420 kJ, but turning that same 100°C water into steam takes 2,260 kJ — more than 5 times as much energy.
Good to know
- This is exactly how air conditioners and refrigerators work — refrigerant absorbs latent heat as it evaporates, cooling the space, then releases that heat outside as it condenses.
- Sweat cools you through latent heat too, absorbing heat from your skin as it evaporates — which is why humid heat feels so much worse than dry heat, since evaporation slows down.
- Water's specific heat is unusually high (4.18 kJ/kg·K), meaning it resists both heating and cooling — the reason coastal climates are milder and inland areas see bigger daily temperature swings, and why water is used as engine coolant.
- LMTD uses a logarithmic rather than simple average because temperature difference changes exponentially along the length of a heat exchanger.
- Counter-flow heat exchangers outperform parallel-flow because they maintain a more uniform temperature difference along the entire exchanger length.
Frequently asked questions
Q. How do I tell sensible heat from latent heat?
If temperature changes, it is sensible heat. If temperature stays constant while the state (solid/liquid/gas) changes, it is latent heat.
Q. Why does LMTD use a logarithm?
Because the temperature difference inside a heat exchanger changes exponentially along its length, a simple average would introduce significant error.
Q. Is counter-flow or parallel-flow better?
Counter-flow is generally more efficient, since it maintains a more even temperature difference across the entire exchanger.