Upper Gas Layer Temperature (MQH)Steady-state hot upper-layer temperature for a vented compartment

What this calculates

This calculator uses the MQH correlation to estimate the steady-state temperature of the upper hot gas layer in a vented compartment fire. The compartment is assumed to be small enough and the fire size large enough that the smoke layer is roughly uniform.

The same geometry inputs are used as the flashover calculator — compartment dimensions, vent dimensions, interior lining material and thickness — together with the heat release rate of the fire. Compare the predicted layer temperature against the flashover criterion (≈ 600 °C / 873 K) as one screening test.

How to use it

Enter the compartment dimensions and vent dimensions in the yellow cells. Choose the wall lining material and enter its thickness; the thermal conductivity is looked up automatically. Enter the heat release rate of the fire and the ambient temperature. The upper-layer temperature is reported in K and °C.

Variables

T_g
Upper-layer temperature [K]
T_a
Ambient temperature [K]
Q
Heat release rate [kW]
A_o
Vent area = W_v · H_v [m²]
H_o
Vent height [m]
A_T
Total area of compartment surfaces (excluding vent) [m²]
h_k
Effective heat transfer coefficient = k / δ [kW/m²·K]

Equations

$$ T_{g} = T_{a} + 6.85 \, \left( \dfrac{ Q^{2} }{ A_{o} \, \sqrt{H_{o}} \, h_{k} \, A_{T} } \right)^{\!1/3} $$McCaffrey, Quintiere & Harkleroad upper-layer temperature

Discussion

This correlation assumes a vented compartment in a steady state — the heat release rate is constant, the layer has had time to equilibrate, and there is a single ventilation opening. For very small compartments, very large fires, or transient conditions, the prediction degrades.

A common engineering rule is that a layer temperature near 600 °C (≈ 873 K) is associated with flashover; pair this calculator with the Compartment Flashover page for a cross-check.

Worked example

Compartment 14.5′ × 11.5′ × 8′ with a single 14.5′ × 8′ opening, ½″ gypsum board lining, fire of 50 kW, ambient temperature 72 °F.

Expected result: upper-layer temperature ≈ 334.6 K (61.5 °C / 142.7 °F).

References

  1. McCaffrey, B. J., Quintiere, J. G., and Harkleroad, M. F. — upper-layer temperature correlation as published in the SFPE Handbook of Fire Protection Engineering, 3rd ed. (2002), Section 3.
  2. Thermal conductivities of wall lining materials taken from NUREG 1805, Chapter 13.