What is an energy balance?

An energy balance is the application of the first law of thermodynamics to a control volume. For a steady-state open system it states Q - W_s = H_out - H_in, where Q is heat added, W_s is shaft work done by the system, and H is total stream enthalpy (mass flow × specific enthalpy).

What does a positive Q mean?

A positive Q means heat is added to the system (endothermic process, e.g., a fired heater or steam boiler). A negative Q means heat is removed (exothermic process, e.g., a condenser or cooler).

What units does this calculator use?

In SI mode: mass flow in kg/s, specific enthalpy in kJ/kg, and heat duty in kW. In Imperial mode: mass flow in lb/h, specific enthalpy in BTU/lb, and heat duty in BTU/h.

How do I handle shaft work?

Enter shaft work (W_s) as positive if work is done BY the fluid on a turbine, and negative if work is done ON the fluid (pump or compressor). The heat duty is then Q = (H_out - H_in) + W_s.

Can I model multiple inlet and outlet streams?

Yes. Click "Add Inlet Stream" or "Add Outlet Stream" to include mixing or splitting points. Each stream's enthalpy contribution (ṁ × h) is summed independently.

What is thermal efficiency in this context?

When shaft work is provided, thermal efficiency is the ratio of useful energy output (net stream enthalpy gain) to total energy input (heat supplied + shaft work input). It is only shown when at least one energy input is positive.

Energy Balance Calculator

Name: Energy Balance Calculator
Availability: InStock
Author: Nham Vu

Enter inlet and outlet enthalpy streams to compute net energy balance (Q = H_out - H_in), heat duty, and system efficiency.

Unit System

SI: kg/s · kJ/kg → kW | Imperial: lb/h · BTU/lb → BTU/h

Inlet Streams (H_in)

Outlet Streams (H_out)

Shaft Work W_s (optional)

kW (positive = turbine output)

Positive: work done by fluid (turbine). Negative: work done on fluid (pump/compressor).

Add streams and click Calculate to see the energy balance.

Total H_in

—

Total H_out

—

Net Heat Duty Q

—

Q = H_out − H_in + W_s

Stream Breakdown

Stream	ṁ	h	H = ṁ·h

Summary

Enter inlet and outlet enthalpy streams to compute net energy balance (Q = H_out - H_in), heat duty, and system efficiency.

How it works

Select the unit system (SI or Imperial).
Add one or more inlet streams — enter mass flow rate and specific enthalpy for each.
Add one or more outlet streams — enter mass flow rate and specific enthalpy for each.
Optionally enter shaft work (W_s) if the system involves a pump, compressor, or turbine.
Click "Calculate" to compute total H_in, H_out, and net heat duty Q.
Review the breakdown table showing each stream contribution.

Use cases

Heat exchanger duty calculations for process design.
Boiler and furnace heat input estimation.
Chemical reactor heat balance for exothermic or endothermic reactions.
HVAC system thermal load analysis.
Distillation column condenser and reboiler duties.
Turbine and compressor energy analysis.
Quick sanity checks on process simulation results.
Coursework and exam practice for chemical and mechanical engineering.

Frequently Asked Questions

Last updated: 2026-06-10 · Reviewed by Nham Vu