Ventilating a Hot Factory: A Heat Removal Calculation, Step by Step

By the NextAir Systems engineering team. Last reviewed October 2026.

The usual way to size factory ventilation is to pick an air change rate from a table. Eight changes an hour, ten, twelve. It is quick, and it ignores the thing the ventilation is actually for: carrying away heat. When the heat load is known, the airflow follows from a short energy balance, and it can be very different from the rule of thumb.

Here is the calculation, worked for a 100 × 60 × 10 m hall, with a table showing what each choice of allowed temperature rise does to airflow, fan count and make-up air area.

The short version

• Airflow = heat load ÷ (air density × specific heat × allowed temperature rise over outdoor air).
• For a 400 kW heat load, a 5 K rise over outdoor air needs about 70.4 m³/s (253,347 m³/h), which is 4.2 air changes an hour in a 60,000 m³ hall.
• Halving the allowed rise doubles the airflow, and fan power rises faster than airflow, so the temperature target is the most expensive number in the specification.
• A fan is only as good as its installation, and system effect can take a large share of the airflow, as shown in the system effect article.

The energy balance

Heat generated inside a hall is carried out by the air that passes through it. In steady state the heat removed equals the heat generated, and the heat removed is the mass flow of air times its specific heat times the temperature it gains on the way through. Rearranged for volume flow:

• Q (m³/s) = heat load (kW) ÷ (ρ × cp × ΔT)
• With ρ about 1.13 kg/m³ for air at 40 °C and cp about 1.006 kJ/kg·K, ΔT is the allowed rise of the exhaust air over the outdoor air.

The heat load is the sum of everything that warms the building: machines and process equipment, lighting, people, solar gain through the roof and walls, and any hot materials. Machine power is the easiest to estimate, since most of the electrical power ends up as heat in the space. Solar gain on a large metal roof in a hot climate is often the surprise, and it can be a substantial fraction of the total.

The choice that matters: allowed temperature rise

Ventilation cannot cool the hall below the outdoor temperature. The question is how far above it you will tolerate. We take an outdoor design temperature of 38 °C and a heat load of 400 kW, and work out the airflow for four allowed rises. The fan count assumes 50,000 m³/h per fan, which is an assumption chosen for the example. Fan power uses 100 Pa and 55% efficiency. The last column is the make-up air opening area at 2 m/s.

Allowed rise over outdoor (K)	Indoor temperature (°C)	Airflow (m³/s)	Airflow (m³/h)	Air changes per hour (60,000 m³)	Fans of 50,000 m³/h	Fan power at 100 Pa (kW)	Make-up air opening at 2 m/s (m²)
3	41	117.3	422,245	7.0	9	21.3	58.6
5	43	70.4	253,347	4.2	6	12.8	35.2
8	46	44.0	158,342	2.6	4	8.0	22.0
12	50	29.3	105,561	1.8	3	5.3	14.7

Worked example. Allowing a 5 K rise puts the hall at 43 °C when it is 38 °C outside, and needs 70.4 m³/s, or 4.2 air changes an hour, with 6 fans using 12.8 kW. Tightening to 3 K raises that to 117.3 m³/s, 7.0 air changes and 21.3 kW. Loosening to 8 K cuts it to 44.0 m³/s, 2.6 air changes and 8.0 kW. The rule of thumb of 10 air changes an hour would have given 167 m³/s, about 3.8 times what the 8 K case needs.

In practice the ventilation takes place alongside roof stack effect and wind, and the temperature near workers differs from the temperature at the roof. The calculation gives the right order of magnitude, and a proper design adds detail. It is far more informative than starting from an air change rate.

Reduce the load first

• Insulate or reflect the roof. Solar gain on metal sheeting can be a large part of the heat load in hot countries.
• Capture heat at the source. A hood over a furnace or oven removes heat before it enters the hall, and a small exhaust does what a large general fan would otherwise do.
• Separate hot and cool zones. Don’t ventilate the whole hall for one hot corner.
• Move the heat out of the building. Processes that reject heat to cooling water or to flue gas leave less in the space.

Provide the air a way in

A fan can only extract what can enter. Make-up air openings sized at about 2 m/s, as in the last column of the table, keep the pressure drop on the inlet side low. Starved openings raise the inlet pressure drop, and the fans deliver less than the datasheet, which is system effect again. Place the openings low and on the side that brings the coolest air, and keep them away from sources of dust, fumes and the exhaust itself.

Selecting the fans

With the airflow and the system resistance, select fans using the method in how to size an axial fan. Many factories use several smaller fans on the roof or in the walls, and the trade-offs of that choice are in one large fan or several small ones.

Where this fits with NextAir

NextAir manufactures axial fans up to 1,600 mm and 185,000 m³/h, in MS, SS304 and SS316, with direct and belt drive. Send the hall dimensions, heat load and site temperature to our axial fan manufacturer team and we will propose a layout. See our axial fan guide, and the foundries and metal casting and steel plants pages for heavier heat loads.

Sources

1. Plant Engineering (US DOE Motor System BestPractices material). Building energy efficiency into fan systems.
2. AMCA / ANSI Webstore. AMCA 201-23 Fans and Systems.

Figures and tables on this page are calculated or compiled by NextAir Systems and you are welcome to reuse them with a link back to this article.

Frequently Asked Questions