BIZOMAT MAKEOS · Engineering
Enclosure Cooling Airflow (Heat Load, Wall Losses, Fan Airflow Needed)
For an electrical or control enclosure with equipment giving off heat, how much heat escapes through the walls, and what fan airflow keeps the inside within its temperature limit?
Ventilation of a closed cabinet: the heat lost through the enclosure surface at the temperature difference from a wall heat transfer coefficient, the heat the fan must remove, the mass and volume of air per hour to carry it at the allowed temperature rise (as the heating energy tool, applied to the cooling air), and the fan airflow against the fan you have.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Heat given off inside | W | Total heat given off by drives, power supplies and other equipment inside. |
| Highest outside air temperature | °C | Highest temperature of the air drawn in. |
| Highest allowed inside temperature | °C | Highest temperature the equipment inside may see. |
| Effective enclosure surface area | m² | Surface area of the enclosure that can lose heat (walls against other walls or the floor count less). |
| Wall heat transfer coefficient | W/(m²·K) | Heat transfer coefficient through the walls to still air (about 5.5 for painted steel). |
| Air density | kg/m³ | Density of the air at the inlet. |
| Air specific heat | kJ/(kg·K) | Specific heat of air. |
| Fan airflow (at its working pressure) | m³/h | Airflow of the fan at the pressure drop of its filters and grilles. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Heat given off inside
- What it is
- The total heat released inside the enclosure by drives, power supplies, transformers and other equipment.
- Units and format
- Enter a number in W: watts. The form accepts 0 W to 10000000 W. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 500 W as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Airflow needed rises with the heat left after wall losses.
- Get it right
- Add the losses from each maker’s data sheet, not the rated power the equipment controls.
What this means: Highest outside air temperature
- What it is
- The highest temperature of the air around the enclosure that the fan draws in.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -50 °C to 80 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 35 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the hottest air near the enclosure, including sun or nearby machines, not the average room temperature.
What this means: Highest allowed inside temperature
- What it is
- The highest air temperature allowed inside the enclosure for the equipment to run reliably.
- Units and format
- Enter a number in °C: degrees Celsius. The form accepts -40 °C to 100 °C. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 45 °C as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger difference between inside and outside cuts the airflow needed.
- Get it right
- Take it from the most temperature-sensitive device’s data sheet; it must be above the outside air.
What this means: Effective enclosure surface area
- What it is
- The enclosure surface that can lose heat to the surrounding air, counting walls against other walls or the floor as less effective.
- Units and format
- Enter a number in m²: square metres. The form accepts 0 m² to 10000 m². Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 m² as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More effective area removes more heat through the walls.
- Get it right
- Follow the enclosure maker’s rules for counting surfaces against walls or other enclosures.
What this means: Wall heat transfer coefficient
- What it is
- The heat transfer coefficient through the enclosure walls to still air: watts per square metre per kelvin of temperature difference.
- Units and format
- Enter a number in W/(m²·K): watts per square metre per kelvin. The form accepts 0 W/(m²·K) to 100 W/(m²·K). Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5.5 W/(m²·K) as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Wall heat loss rises in direct proportion.
- Get it right
- Take it from the enclosure maker for the material and finish.
What this means: Air density
- What it is
- The density of the air entering the enclosure, used to turn the mass of air needed into a volume flow.
- Units and format
- Enter a number in kg/m³: kilograms per cubic metre. The form accepts 0.000001 kg/m³ to 2 kg/m³. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.16 kg/m³ as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the density at the inlet temperature and site altitude from air property tables.
What this means: Air specific heat
- What it is
- The specific heat of air: the energy needed to warm one kilogram by one kelvin.
- Units and format
- Enter a number in kJ/(kg·K): kilojoules per kilogram per kelvin. The form accepts 0.000001 kJ/(kg·K) to 5 kJ/(kg·K). Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.005 kJ/(kg·K) as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter kJ/(kg·K), not J/(kg·K).
What this means: Fan airflow (at its working pressure)
- What it is
- The airflow the fan actually moves at the pressure drop of its filters and grilles, compared with the airflow needed.
- Units and format
- Enter a number in m³/h: cubic metres per hour. The form accepts 0 m³/h to 10000000 m³/h. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 150 m³/h as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Read it from the fan curve with filters fitted, not the free-blowing catalogue figure.
Method and formulas
- Wall loss = k A (T_inside − T_outside); fan heat = heat load − wall loss.
- Air mass per hour to carry the fan heat with a rise of T_inside − T_outside: m = Q ÷ (c_p ΔT) (as the heating energy tool, Q = m c ΔT).
- Airflow = mass ÷ density.
- Example: the walls lose 110 W; the fan must move 120.43 m³/h.
Limitations
- Air drawn in at the outside temperature and leaving at the inside limit; poor air paths leave hot spots.
- Solar gain on outdoor cabinets is not included.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size a filter fan for a control cabinet
- Check whether a panel needs a fan at all
- See the effect of a hotter plant room
Who it is for
- Electrical engineers
- Panel builders
- Maintenance engineers
What you get
- Heat lost through the walls
- Heat for the fan to remove
- Airflow needed against the fan
Interpreting the result
- When the inside limit is close to the outside temperature, the airflow needed rises steeply; an air conditioner may then be the better answer.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 371–374, 2683. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Explains the concept · BIZOMAT original book
The cabinet heat load
In Thermoelectric Cooling by the Numbers
What it covers: Internal heat, heat through the walls, sol-air temperature for sun on the cabinet, what insulation gains and costs, why fans cannot cool below ambient, and a load estimate that left out the walls.
Method explained here · BIZOMAT original book
Seal, vent, ventilate or heat: choosing the strategy
In Condensation Control by the Numbers
What it covers: What an IP rating does and does not say about humidity, and how sealing, venting, fan ventilation and heating each change the dew point and the surfaces inside an enclosure.
Method explained here · BIZOMAT original book
Heat, cooling and the hot spot
In Distribution Transformers by the Numbers
What it covers: How losses become oil and winding temperatures, how to estimate top-oil and hot-spot temperatures at any load, and how to size a substation room's ventilation.
Method explained here · BIZOMAT original book
Inverter heat, reliability and spares
In Rooftop Solar for Factories by the Numbers
What it covers: How to ventilate an inverter room so the inverters do not derate, read inverter failure records with confidence bounds, and decide whether a spare inverter is worth holding.
Method explained here · BIZOMAT original book
Junction temperature and cabinet cooling
In Variable-Speed Drives by the Numbers
What it covers: How to estimate the junction temperature of a drive's switches through its thermal resistances, size the cabinet ventilation for a panel of drives, and keep dusty air from defeating it.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Enclosure Cooling Airflow (Heat Load, Wall Losses, Fan Airflow Needed) calculator