Energy Balance Calculator

Calculate steady-state energy balance for a process stream. Includes sensible heat (Q = mCpΔT), latent heat of phase change, and reaction heat contributions.

About this calculator

Calculates a steady-state energy balance for a single process stream passing through a heater, cooler, or reactor, breaking total duty into three additive pieces: sensible heat (Q = ṁ·Cp·ΔT, the energy needed just to change temperature), latent heat (mass flow times the heat of vaporization/condensation times the fraction of the stream that actually changes phase), and reaction heat (heat generated or consumed by a chemical reaction per unit mass, entered as negative for exothermic reactions by this tool's convention). All three are computed on a per-second basis internally (mass flow is converted from kg/hr to kg/s) and summed to a total duty in kW, then also reported in BTU/hr, MW, and GJ/hr for different audiences. A positive total means net heating is required; negative means net cooling.

From that duty, the calculator provides two practical utility-sizing estimates: the saturated steam flow needed at a typical 10 barg condensing enthalpy (~2015 kJ/kg) if heating, or the cooling water flow needed assuming a fixed 10°C temperature rise if cooling — both are rough utility-sizing checks, not substitutes for a real utility balance that accounts for actual steam pressure or cooling water supply/return temperatures on site. A key limitation: the phase-change fraction is a single lumped value rather than a true vapor-liquid equilibrium calculation, so mixtures with wide boiling ranges (rather than a pure component boiling at a fixed temperature) will need a more rigorous flash calculation for accurate latent heat.

Inputs

Mass flow rate (kg/hr)

Specific heat Cp (kJ/kg·K)

Inlet temperature (°C)

Outlet temperature (°C)

Latent heat (kJ/kg)

Phase change fraction (0–1)

Heat of reaction (kJ/kg)

Tab to move between fields

Results

Total duty (kW)

319.76

Sensible heat (kW)319.76

Latent heat (kW)0

Reaction heat (kW)0

Total duty (BTU/hr)1,091,079

Total duty (GJ/hr)1.15

Steam estimate (kg/hr)571.3

Cooling water estimate (kg/hr)0

Total Duty MW0.32

How to Use This Calculator

Enter Mass flow rate (kg/hr), Specific heat Cp (kJ/kg·K), and Inlet temperature (°C).
Set Outlet temperature (°C), Latent heat (kJ/kg), and Phase change fraction (0–1).
Adjust Heat of reaction (kJ/kg) as needed.
Review the Total duty (kW) result.
Use Sensible heat (kW) and Latent heat (kW) to inform your decision.

How the result changes with Outlet temperature (°C)

Outlet temperature (°C)	Total duty (kW)
40	87.21
60	203.49
120	552.32
200	1,017.43

What each input means

Mass flow rate (kg/hr): Total mass flow rate through the process unit.
Specific heat Cp (kJ/kg·K): Specific heat capacity. Water = 4.186, oil ≈ 2.0, air ≈ 1.005 kJ/(kg·K).
Inlet temperature (°C): Temperature of the stream entering the process unit.
Outlet temperature (°C): Temperature of the stream leaving the process unit.
Latent heat (kJ/kg): Heat of vaporization/condensation (water ≈ 2257 kJ/kg). Set 0 if no phase change.
Phase change fraction (0–1): Fraction of the stream that undergoes phase change (0 = none, 1 = complete).
Heat of reaction (kJ/kg): Exothermic = negative, endothermic = positive. Set 0 if no reaction.

What each result means

Total duty (kW): Net heat duty. Positive = heating needed, negative = cooling needed.
Sensible heat (kW): Heat required to change temperature: mCpΔT.
Latent heat (kW): Heat for phase change.
Reaction heat (kW): Heat generated or consumed by chemical reaction.
Total duty (BTU/hr): Duty in imperial units.
Total duty (GJ/hr): Duty in gigajoules per hour.
Steam estimate (kg/hr): Estimated 10 barg saturated steam consumption (for heating duties only).
Cooling water estimate (kg/hr): Estimated cooling water flow at 10°C rise (for cooling duties only).

How this is calculated

Worked example, using the default values

Identify Input Parameters
4 parameters
Mass flow rate (kg/hr) = 5000, Specific heat Cp (kJ/kg·K) = 4.186, Inlet temperature (°C) = 25, Outlet temperature (°C) = 80 = 7 input(s) provided
Calculate Total duty
Total duty = sensibleKW + latentKW + reactionKW
319.76 = 319.76
Calculate Sensible heat
Sensible heat = massFlowKgS * cpKJ * deltaT
319.76 = 319.76
Calculate Latent heat
Latent heat = massFlowKgS * latentHeat * phaseChangeFraction
0 = 0

Engine last updated September 1, 2026. Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Why is heat of reaction entered as negative for exothermic reactions?

This calculator's convention treats reaction heat as a contribution to the total duty the process needs to supply from outside, so an exothermic reaction (one that releases heat) is entered as negative because it reduces the external heating duty required — or drives the total duty negative, meaning cooling is needed instead. An endothermic reaction consumes heat, so it's entered as positive and adds to the duty.

What do the steam and cooling water estimates actually assume?

The steam estimate assumes saturated steam condensing at roughly 10 barg with a condensing enthalpy of about 2015 kJ/kg, and only appears when total duty is positive (heating). The cooling water estimate assumes a fixed 10°C temperature rise through the cooler and only appears when total duty is negative (cooling) — both are quick utility-sizing checks, not a substitute for a real utility balance using your site's actual steam pressure or cooling water supply/return temperatures.

How does the phase-change fraction interact with latent heat?

Latent heat duty is calculated as mass flow times the entered latent heat times the phase-change fraction, so a fraction of 1 means the entire stream fully vaporizes or condenses, while 0.5 means only half the stream undergoes the phase change. This is a single lumped fraction rather than a true vapor-liquid equilibrium flash, so it works best for a pure component at a fixed boiling point rather than a wide-boiling-range mixture.

Why are there three separate heat terms instead of just one duty number?

Sensible heat (temperature change), latent heat (phase change), and reaction heat (chemical reaction) are physically distinct mechanisms that can all occur in the same process unit simultaneously — for example, a reactor that heats reactants, drives an exothermic reaction, and boils off a solvent all at once. Breaking them out separately, in addition to the summed total duty, makes it possible to see which mechanism dominates the overall energy requirement.

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