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Heat Transfer

Heat Duty Explained

Heat duty is the rate of heat transfer needed to change a fluid temperature. Learn the sensible heat formula, required inputs, and common mistakes.

TypeEngineering guide — concept explainer

Definition

Heat duty (Q̇) is the rate of heat transfer required to change a fluid's temperature by a specified amount. For sensible heat (no phase change), it equals the product of mass flow rate, specific heat capacity, and temperature difference: Q̇ = ṁ × Cp × ΔT. Heat duty is expressed in watts (W), kilowatts (kW), or BTU per hour (BTU/h).

Why it matters

Heat duty is the starting point for sizing heaters, coolers, and heat exchangers. Before selecting equipment, an engineer must know how much heat needs to be added or removed. Errors in heat duty propagate through the entire design — an undersized heater fails to reach the target temperature, and an oversized cooler wastes capital. Getting the units right (especially Cp units and flow rate time basis) is critical.

Formula

Sensible heat duty
Q̇ = ṁ × Cp × ΔT
Temperature difference
ΔT = T₂ − T₁

Units involved

  • •Q̇ — heat duty in W, kW, or BTU/h
  • •ṁ — mass flow rate in kg/s, kg/h, or lb/h
  • •Cp — specific heat capacity in kJ/(kg·K), J/(kg·K), or BTU/(lb·°F)
  • •ΔT — temperature change in K, °C, or °F

Concept diagram

heaterQ̇ = ṁ Cp ΔTT₁ (cold)ṁ, CpT₂ (hot)Q̇heat input (kW)ΔT = T₂ − T₁

Worked example

A water stream at 2 kg/s must be heated from 20 °C to 30 °C. Cp of water is 4.184 kJ/(kg·K). What is the heat duty?

  1. 01ṁ = 2 kg/s
  2. 02Cp = 4.184 kJ/(kg·K)
  3. 03ΔT = 30 − 20 = 10 K
  4. 04Q̇ = 2 × 4.184 × 10
  5. 05Q̇ = 83.68 kW
Result

Heat duty = 83.68 kW

Common mistakes

  • •Forgetting to match the time basis — if ṁ is in kg/h and Cp is in kJ/(kg·K), the result is in kJ/h, not kW. Divide by 3600 to get kW.
  • •Using Cp at the wrong temperature — Cp varies with temperature. Use the average Cp over the temperature interval, or at least the value at the midpoint.
  • •Applying this formula across a phase change — latent heat (boiling, condensation, freezing) requires a separate enthalpy-based calculation. This formula is for sensible heat only.
  • •Confusing ΔT with absolute temperature — the formula uses the temperature difference, not the absolute temperature.
  • •Using volumetric flow instead of mass flow — heat duty requires mass flow. Convert volumetric flow using ṁ = ρQ first.

When to use the calculator

Use the Heat Duty calculator when you know the mass flow rate, specific heat capacity, and temperature change, and need the heat duty in your preferred units. The calculator handles unit conversions between kW, BTU/h, and other power units.

Heat Duty Calculator →

FAQ

What is the difference between sensible heat and latent heat?
Sensible heat changes a fluid's temperature without changing its phase (Q̇ = ṁCpΔT). Latent heat changes the phase (boiling, condensation) at constant temperature and uses the enthalpy of vaporisation. This guide covers sensible heat only.
Where do I find Cp values?
Cp values for common fluids are published in engineering references such as Perry's Chemical Engineers' Handbook or NIST webbook. Water at 25 °C has Cp ≈ 4.184 kJ/(kg·K). ProcessConvert does not include a Cp lookup — you must supply the value.
Can I use this formula for gases?
Yes, for sensible heating or cooling of gases at moderate pressures. However, gas Cp varies more strongly with temperature than liquid Cp, and for high-pressure or near-critical conditions, more detailed enthalpy calculations may be needed.
How do I convert heat duty from kW to BTU/h?
Multiply kW by 3412.14 to get BTU/h. For example, 83.68 kW × 3412.14 = 285,464 BTU/h. You can also use the power/energy conversions on ProcessConvert.