Datacenter

Airflow / CRAC

Enter the heat load (kW) and the air ΔT (supply→return) and I compute the required airflow (CFM and m³/s) and the tons of refrigeration to size your CRAC/CRAH.

redzilla.cl — crac

Airflow

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Airflow (SI)

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cubic metres per second

Cooling

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tons of refrigeration

CFM —

m³/s —

Ton —

BTU/h —

Calculation breakdown

Quantity	Value	Formula

How it is calculated · airflow, m³/s and tons

1. Dry air at sea level: density ρ = 1.2 kg/m³, specific heat cp = 1005 J/(kg·K).

2. SI airflow: Q(m³/s) = P(W) ÷ (ρ · cp · ΔT), with ΔT in K (= °C).

3. To CFM: CFM = Q(m³/s) × 2118.88. Equivalent imperial formula: CFM = kW × 3412 ÷ (1.08 × ΔT_°F), with ΔT_°F = ΔT_°C × 1.8 (they agree within ±0.1 %).

4. Cooling: tons = kW ÷ 3.517 (1 TR = 3.517 kW) · BTU/h = kW × 3412.14.

5. This airflow is the theoretical minimum for the given ΔT: it excludes recirculation, leakage through poorly sealed containment and CRAC headroom. Size the unit with margin and validate the real ΔT with probes on supply and return.

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How it works

The calculator converts the heat load of a room or rack (in kW) into the airflow required to remove it, based on the ΔT between supply and return air. It reports the flow in CFM and m³/s, plus the load expressed in tons of refrigeration and BTU/h, which is how CRAC/CRAH units are specified.

The model is the thermal balance of air: Q(m³/s) = P(W) ÷ (ρ · cp · ΔT), with dry-air density ρ = 1.2 kg/m³ and specific heat cp = 1005 J/(kg·K) at sea level. The CFM conversion uses 1 m³/s = 2118.88 CFM (equivalent to the imperial formula CFM = kW × 3412 ÷ (1.08 × ΔT_°F)), and cooling is expressed with 1 ton = 3.517 kW. The result is the theoretical minimum: it does not include recirculation, containment leakage or unit margin.

Example: how much air does a 10 kW row need at a 12 °C ΔT?

SI flow: 10,000 W ÷ (1.2 × 1005 × 12) = 0.691 m³/s.
In CFM: 0.691 × 2118.88 ≈ 1,464 CFM.
Cooling: 10 ÷ 3.517 = 2.84 tons (about 34,121 BTU/h).
The CRAC must move at least 1,464 CFM at that ΔT; in practice you size with headroom.

Frequently asked questions

How many CFM per kW of IT load do I need?

With a typical ΔT of 10 to 14 °C, roughly 105 to 146 CFM per kW (146 CFM/kW at a 10 °C ΔT, 125 at 12 °C, 105 at 14 °C). The classic 160 CFM/kW figure corresponds to legacy rooms with a ΔT near 9 °C. The higher the ΔT, the less air you need to move for the same load.

What happens if the real ΔT is lower than the design ΔT?

You need more airflow for the same load: half the ΔT requires twice the air. A low ΔT usually points to bypass or recirculation (cold air returning to the CRAC without passing through the equipment), typical of poorly sealed containment or leaky raised floors. Before upsizing the CRAC, seal the leaks and measure the real ΔT with supply and return probes.

What is the difference between tons of refrigeration and kW?

They are two units for the same thermal power: one ton of refrigeration equals 3.517 kW (or 12,000 BTU/h) and comes from the heat needed to melt one short ton of ice in 24 hours. CRACs are usually rated in tons or BTU/h, while IT load is measured in kW; the calculator reports all three.

Is the heat load the same as the electrical consumption of the equipment?

In practice, yes: almost all the electrical power consumed by IT equipment ends up as heat inside the room. That is why you can use the PDU or UPS reading in kW as the thermal load, adding lighting and other sources if they are significant.

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