Enclosure temperature rise calculator

How it works

In steady state, the heat dissipated inside a sealed enclosure leaves through its walls:

ΔT = P / (h · A_eff)

• P is the power dissipated inside, h the overall heat transfer coefficient from the inside air through the wall to the room, and A_eff the effective cooling surface.
• A_eff = Σ A_o · b: each surface’s area times its surface factor b from IEC TR 60890: 1.4 for an exposed top, 0.7 for a covered top, 0.9 for exposed sides (front, rear and side walls), 0.5 for covered sides (such as the back of a wall-mounted enclosure) and for the sides of an enclosure between two others. The floor is not counted.
• h = 5.5 W/(m²·K) is the value enclosure makers give for sheet steel. Change it for other materials.

For a single free-standing enclosure this gives A_eff = 1.8·H·(W + D) + 1.4·W·D.

Fan airflow

If the internal temperature would exceed the maximum, the walls still shed h·A_eff·(T_max − T_a) and a fan must carry the rest:

Q = (P − h · A_eff · ΔT_allowed) / (ρ · c_p · ΔT_allowed)

Here ΔT_allowed = T_max − T_a is the rise from the incoming air to the outgoing air, c_p = 1006 J/(kg·K), and ρ is the density of the incoming air at the ambient temperature and the standard-atmosphere pressure at your altitude, so the flow is the volume at the fan inlet. “Ignoring the walls” gives the flow if the fan carries all of P, an upper bound. If the ambient is at or above the maximum, no amount of ambient air can hold it, and the enclosure needs a cooling unit.

Sources

• IEC TR 60890, A method of temperature-rise verification of low-voltage switchgear and controlgear assemblies by calculation: the effective cooling surface and its surface factors (Table 3 in the 2014 edition).
• The single-coefficient model ΔT = P / (h·A), with h = 5.5 W/(m²·K) for sheet steel, as in enclosure makers’ climate control catalogs, for example Rittal’s.

Limits

• A first-order estimate for a sealed enclosure with the heat spread evenly inside. The result is an average; air near the top runs hotter, and parts that concentrate heat run hotter still.
• It does not implement IEC TR 60890’s full calculation, which gives the air temperature at mid-height and at the top from enclosure constants and correction factors.
• Steady state only; solar gain, ventilation openings and internal partitions are not modeled.
• The fan flow is the flow delivered against the enclosure’s filters and grilles, not the fan’s free-air rating. Check it on the fan’s pressure–flow curve.

Worked examples

1. A 600 × 1200 × 400 mm free-standing steel enclosure with 150 W inside, 25 °C ambient. A_eff = 1.8 · 1.2 · 1.0 + 1.4 · 0.6 · 0.4 = 2.50 m², so ΔT = 150 / (5.5 · 2.496) = 10.9 °C and the inside settles at 35.9 °C, within a 40 °C maximum.

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2. The same enclosure with 500 W. It would reach 61.4 °C. At 40 °C the walls shed 5.5 · 2.496 · 15 = 206 W, leaving 294 W for a fan: 59.3 m³/h (34.9 CFM) of 25 °C air at sea level.

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3. A 400 × 500 × 200 mm wall-mounted enclosure with 60 W, 35 °C ambient, 50 °C maximum, at 1500 m. A_eff = 0.572 m² and the inside would reach 54.1 °C. The walls carry 47.2 W at 50 °C, so the fan needs only 3.20 m³/h (1.88 CFM); carrying all 60 W would take 15.0 m³/h.

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