Enter design inputs
Plotly graph
The chart shows predicted ceiling temperature across radial distance for the current fire size and ceiling height.
Example data table
| Case | Q (kW) | H (m) | r (m) | Ambient (°C) | Region | ΔT (°C) | Tjet (°C) | Velocity (m/s) |
|---|---|---|---|---|---|---|---|---|
| Case A | 500 | 4.00 | 0.50 | 20 | Near-axis | 105.63 | 125.63 | 4.73 |
| Case B | 750 | 4.50 | 2.00 | 22 | Radial | 62.17 | 84.17 | 2.13 |
| Case C | 1,000 | 6.00 | 4.00 | 25 | Radial | 35.58 | 60.58 | 1.52 |
| Case D | 2,500 | 5.00 | 4.00 | 20 | Radial | 78.66 | 98.66 | 1.88 |
Formula used
This calculator applies the Alpert ceiling jet temperature relation for a smooth, unobstructed ceiling and reports supporting ceiling jet velocity.
Near-axis temperature region: when r/H ≤ 0.18
ΔT = 16.9 × Q^(2/3) / H^(5/3)
Radial temperature region: when r/H > 0.18
ΔT = 5.38 × (Q / r)^(2/3) / H
Total ceiling jet temperature:
Tjet = Tambient + ΔT
Supporting velocity relation:
For r/H ≤ 0.15, u = 0.947 × (Q/H)^(1/3)
For r/H > 0.15, u = 0.197 × Q^(1/3) × H^(1/2) / r^(5/6)
Units: Q in kW, H and r in m, temperature in °C, and velocity in m/s.
How to use this calculator
- Enter the fire heat release rate in kilowatts.
- Enter the ceiling height above the fuel source.
- Enter the radial distance from the fire plume axis.
- Set the ambient temperature and a critical threshold temperature.
- Choose the ceiling and space condition to document applicability.
- Set the graph radius to view how temperature decays outward.
- Press the calculate button to show the result above the form.
- Use the CSV or PDF buttons to export the current report.
Frequently asked questions
1. What does ceiling jet temperature mean?
It is the hot gas temperature flowing beneath the ceiling after plume gases strike the ceiling and spread outward from the fire axis.
2. Which correlation does this calculator use?
It uses the Alpert smooth-ceiling correlation for ceiling jet temperature rise and the companion velocity relation for engineering screening.
3. When is the near-axis equation used?
The near-axis temperature equation applies when the radial ratio r/H is 0.18 or less. It represents points close to the plume centerline.
4. Why does temperature fall as distance increases?
The ceiling jet spreads and entrains cooler air as it moves away from the plume axis, reducing the temperature rise at larger radial distances.
5. Can I use this for sprinkler or detector activation?
Yes, as a first screening step. Activation timing also needs the device response model, such as RTI, activation temperature, and transient fire growth.
6. Does it work well in every room?
No. Confined compartments with hot gas layer buildup can run hotter than this estimate, so use caution and apply detailed fire modeling when needed.
7. What units should I enter?
Use kilowatts for heat release rate, meters for ceiling height and radial distance, and degrees Celsius for ambient and critical temperatures.
8. What does margin to critical temperature show?
It shows how far the predicted ceiling jet temperature is below or above your chosen threshold. A negative margin means the threshold is exceeded.