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36 — Calculators

Calculate control cabinet cooling

A control cabinet gives off part of its heat through the enclosure surface, how much depends on the effective area, and that depends on how the cabinet is installed. Only what exceeds this has to be removed actively.

Input

Appears on the printout. Does not affect the calculation.

Heat dissipation inside the enclosureW

Total— W

The actual figures are in the device data sheet or in a maintained parts database. The preset is an example of a drive enclosure, not a basis for design.

Determines the effective surface to IEC 60890.

Covered when a platform, cable floor or another enclosure sits on top.

The upper limit permitted inside the enclosure. It follows from the most sensitive component.

The lower limit that must not be undercut at standstill. The component with the highest minimum operating temperature governs.

The design case, the cooling unit is selected for this.

The normal case. It determines how much the unit actually runs.

At night and at standstill. Determines whether a heater is needed.

Moisture content of the ambient air in the design case.

Result

Required cooling capacityIn the design case
—W
Cooling capacity in the normal case
—W
Air flow of a filter fanFree-blowing, in the design case
—m³/h
Effective surface areaTo IEC 60890
—m²
Self-dissipation of the enclosure
—W
Cooling assessment
—
Dew point of the ambient airAt maximum temperature and assumed humidity
—°C
Heating power against condensationTo hold the internal temperature at the dew point at standstill
—W
Governing criterion for the heating power
—
Condensation assessment
—

A guide, not a design to standard. We give no warranty for the correctness of the results, the applicable standards and case-by-case verification govern.

Worked example

The default case the calculator starts with, worked through once. Change the values above and it recalculates immediately.

Inputs

Enclosure designation
0
2 × Variable-frequency drive 7.5 kW
390 W
2 × Variable-frequency drive 4 kW
220 W
1 × Power supply 24 V / 20 A
55 W
1 × Controller with modules
45 W
5 × Contactors and motor protection
40 W
Heat dissipation inside the enclosure
750 W
Width
1,200 mm
Height
2,000 mm
Depth
600 mm
Installation
Free-standing, all sides free
Top surface
Free
Maximum internal temperature
35 °C
Minimum internal temperature
5 °C
Maximum ambient temperature
30 °C
Expected ambient temperature
25 °C
Minimum ambient temperature
5 °C
Assumed relative humidity
60 %

Result

Required cooling capacity
544 W
Cooling capacity in the normal case
338 W
Air flow of a filter fan
337 m³/h
Effective surface area
7.49 m²
Self-dissipation of the enclosure
206 W
Cooling assessment
The requirement is in the range of a cooling unit. Filter ventilation is no longer sufficient once ambient temperature approaches the permissible internal temperature.
Dew point of the ambient air
21.4 °C
Heating power against condensation
675 W
Governing criterion for the heating power
The dew point governs the heating power. More heating is needed against condensation than the minimum internal temperature demands.
Condensation assessment
Uncritical in operation, but not at standstill: if the enclosure cools to the minimum ambient temperature it drops below the dew point. An enclosure heater with a hygrostat prevents the condensation.

Formula

  • Heat output: Q = k · A · ΔT with k = 5.5 W/(m²·K) for painted sheet steel
  • ΔT = internal temperature − ambient temperature
  • Cooling requirement = heat dissipation − heat output
  • Free-standing: A = 1.8 · H · (W + D) + 1.4 · W · D
  • Covered on top: the top area 1.4 · W · D drops out
  • Dew point to Magnus: α = ln(φ/100) + 17.62 · t / (243.12 + t), τ = 243.12 · α / (17.62 − α)
  • Heating power against condensation: P = k · A · (τ − minimum ambient temperature)

Assumptions and standards

  • Effective surface to IEC 60890 depending on the installation type.
  • k = 5.5 W/(m²·K) applies to painted sheet steel. Stainless steel is lower, aluminium higher.
  • If the ambient is warmer than the permissible internal temperature, filter fans are ineffective, only a cooling unit will do.
  • The air flow follows V = 3.1 · Q / ΔT. The factor 3.1 m³·K/(W·h) is the reciprocal of air density times specific heat capacity, converted to the hour, and applies at sea level. Density falls with altitude, so the factor rises.
  • The figure is the free-blowing air flow. Filter mat, outlet grille and internal components create back pressure — a fan delivers less than its free-blowing rating once installed. Select the unit by its air flow under actual back pressure.
  • The design case governs, that is the warmest ambient assumed. At the expected ambient temperature the difference is larger and the required air flow considerably smaller.
  • The covered top surface is simplified here: the top area drops out entirely. IEC 60890 lists separate rows with their own factors for covered surfaces. The simplification errs on the safe side. It reduces the area and therefore increases the calculated cooling requirement.
  • The heating power is the value needed to hold the enclosure at the dew point at standstill. An upper bound. In operation a hygrostat switches, and the heat from the installed components contributes.
  • Solar gain, installation in recesses and dirty filters worsen the result in operation.

Frequently asked

Where do I get the heat dissipation figure?

From the data sheets of the installed components. Drives, power supplies and transformers dominate; contactors and terminals barely matter. As a rough figure, drives dissipate about 3 % of rated power.

Is a filter fan enough?

Only if the ambient temperature is below the permissible internal temperature. A fan cannot cool below ambient. Otherwise a cooling unit or an air-to-water heat exchanger is needed.

Why does the required air flow rise so steeply?

Because the temperature difference sits in the denominator. At 10 K it takes half the air of 5 K; at 2 K it takes two and a half times as much. As the ambient approaches the permissible internal temperature the air flow tends to infinity — and that is exactly where filter ventilation stops being an option.

Why does the installation matter so much?

A side placed against a wall or built into a row gives off hardly any heat. Between free-standing and enclosed on both sides there is easily a 40 % difference in effective area. The same applies to a covered top. A platform or cable floor resting on it takes the top area out of the calculation.

Why three ambient temperatures?

Because three different questions sit behind them. The cooling unit is selected for the maximum temperature, otherwise it falls short on the hottest day. The expected temperature says how much the unit actually runs. And the minimum temperature decides whether a heater is needed, the case most often overlooked.

What is the humidity for?

Temperature and relative humidity give the dew point. If the enclosure cools below that value at standstill, water condenses on the components. This is why corrosion appears in an unheated plant over winter even though cooling was adequate in summer.

Calculating is the easy part.

A formula gives you a number. Designing a plant also demands installation method, grouping, discrimination, the standards in force and a look at the installed base. That is what we take on.

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