Heat Release Rate Calculation

Use 1.0 for ideal calculation; typically 0.7 – 0.9 for real fires.

Calculation Results:

Heat Release Rate: 0 kW

Equivalent to: 0 MW

function calculateHRR() { var m = parseFloat(document.getElementById('massLossRate').value); var h = parseFloat(document.getElementById('heatOfCombustion').value); var chi = parseFloat(document.getElementById('efficiency').value); if (isNaN(m) || isNaN(h) || isNaN(chi) || m < 0 || h 1 || chi < 0) { alert("Efficiency must be between 0 and 1."); return; } // Formula: HRR (kW) = m (g/s) * H (MJ/kg) * chi // Since 1 MJ = 1000 kJ and 1 kg = 1000 g, the units (g/s * MJ/kg) directly cancel to (kJ/s) which is kW. var hrr = m * h * chi; var hrrMW = hrr / 1000; document.getElementById('hrrValue').innerText = hrr.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2}); document.getElementById('hrrValueMW').innerText = hrrMW.toLocaleString(undefined, {minimumFractionDigits: 4, maximumFractionDigits: 4}); document.getElementById('hrrResultBox').style.display = 'block'; }

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Understanding Heat Release Rate (HRR)

In fire protection engineering, the Heat Release Rate (HRR) is considered the single most important variable in defining a fire hazard. It measures the amount of energy released by a fuel source per unit of time, typically expressed in kilowatts (kW) or megawatts (MW).

A higher HRR indicates a more intense fire that spreads more quickly, produces more smoke, and reaches flashover conditions faster. This calculator helps engineers and safety professionals quantify that energy based on the mass of the fuel being consumed.

The HRR Formula

The calculation used in this tool is based on the mass loss rate method:

HRR = ṁ × ΔH_c,eff × χ

• ṁ (Mass Loss Rate): The rate at which the fuel is losing mass as it turns into gas/vapors (measured in g/s).
• ΔH_c,eff (Heat of Combustion): The amount of energy released per unit mass of fuel (measured in MJ/kg).
• χ (Efficiency): A factor representing incomplete combustion. Most real-world fires have an efficiency between 0.7 and 0.9.

Typical Values for Common Materials

Material	Heat of Combustion (MJ/kg)
Wood (Cellulose)	16 – 20 MJ/kg
Polyurethane Foam	23 – 28 MJ/kg
Gasoline / Heptane	44 – 45 MJ/kg
Polystyrene	39 – 40 MJ/kg

Practical Example

Imagine a small wastepaper basket fire. If the paper and plastic liner are burning at a rate of 5 grams per second, and the weighted average heat of combustion for the materials is 18 MJ/kg, with a combustion efficiency of 0.8:

HRR = 5 g/s × 18 MJ/kg × 0.8 = 72 kW

In comparison, a fully upholstered sofa fire can reach peak heat release rates of 2,000 kW (2 MW) to 3,000 kW (3 MW), which is enough to cause flashover in a standard-sized living room within minutes.

Why Is HRR Important?

Fire safety professionals use HRR data to:

• Design sprinkler systems and determine required water flow.
• Estimate the time available for building occupants to evacuate.
• Calculate the temperature of the "smoke layer" near the ceiling.
• Assess the risk of structural failure due to heat exposure.