Cooling
Airflow CFM Reference
CFM formula, BTU-to-kW conversion, ASHRAE TC 9.9 operating envelopes, altitude derating, typical rack densities and cooling system rules of thumb. Use alongside the Airflow CFM Calculator.
Last updated: June 2026
Terms
- ASHRAEAmerican Society of Heating, Refrigerating and Air-Conditioning Engineers
- The US industry body that publishes the standard temperature and humidity ranges data-center equipment is designed to handle. When a chart says "ASHRAE recommended range", it means the safe zone most server makers design for.
- CRACComputer Room Air Conditioner
- The specialized air-conditioning unit that cools a server room.
- CRAHComputer Room Air Handler
- Similar job to a CRAC unit — it cools a server room — but instead of cooling the air itself, it uses chilled water piped in from elsewhere in the building.
The CFM Formula
CFM = Q(BTU/h) ÷ (1.085 × ΔT°F)
Where: Q = IT load in BTU/h = kW × 3412.14 | ΔT°F = supply-to-return temperature rise in °F = ΔT°C × 1.8
m³/h = CFM × 1.699
At altitude: CFMalt = CFMsea level ÷ air-density ratio
The constant 1.085 = (0.075 lb/ft³ × 0.24 BTU/lb·°F × 60 min/h) — assumes standard sea-level air at ~25 °C.
CFM per kW — Quick Reference
Values for ΔT = 15 °C (27 °F) at sea level. Divide by altitude factor for high-elevation sites.
| IT Load | BTU/h | CFM (ΔT 15 °C) | m³/h | Cooling tons (TR) |
|---|---|---|---|---|
| 1 kW | 3 412 | 117 | 199 | 0.29 |
| 5 kW | 17 060 | 585 | 994 | 1.42 |
| 8 kW | 27 297 | 936 | 1 591 | 2.27 |
| 10 kW | 34 121 | 1 170 | 1 988 | 2.84 |
| 15 kW | 51 182 | 1 755 | 2 982 | 4.27 |
| 20 kW | 68 243 | 2 340 | 3 976 | 5.69 |
| 30 kW | 102 364 | 3 510 | 5 964 | 8.53 |
| 50 kW | 170 607 | 5 850 | 9 940 | 14.22 |
| 100 kW | 341 214 | 11 700 | 19 880 | 28.43 |
| 500 kW | 1 706 070 | 58 500 | 99 401 | 142.17 |
Rule of thumb: ~120 CFM per kW IT load at ΔT = 15 °C. Increase margin to 150 CFM/kW for high bypass airflow environments.
ASHRAE TC 9.9 IT Equipment Classes
ASHRAE defines operating envelopes for IT equipment. Most data centre class equipment is rated A1 or A2. Exceeding the envelope risks equipment faults, throttling and reduced reliability.
| Class | Inlet temp range | Max dew point | Max relative humidity | Typical equipment |
|---|---|---|---|---|
| A1 | 15 – 32 °C | 17 °C | 80 % | Servers, storage — top-tier enterprise IT |
| A2 | 10 – 35 °C | 21 °C | 80 % | Most modern servers, enterprise storage |
| A3 | 5 – 40 °C | 24 °C | 85 % | Extended-range equipment, edge/ruggedised |
| A4 | 5 – 45 °C | 24 °C | 90 % | High-temperature-capable IT (rare) |
| B1 | 5 – 35 °C | 28 °C | 85 % | Personal computers, office IT |
| H1 | 5 – 25 °C | 21 °C | 80 % | High-density / mainframe, legacy |
Altitude Derating
Air density decreases with altitude. The same volume of air carries less heat, so more CFM is needed. Apply the inverse of the density ratio as a multiplier to sea-level CFM values.
| Altitude | Air density ratio | CFM multiplier | Example: 100 kW site at ΔT 15 °C |
|---|---|---|---|
| 0 – 300 m (sea level) | 1.000 | × 1.00 | 11 700 CFM |
| 600 – 900 m | 0.929 | × 1.08 | 12 636 CFM |
| 1200 – 1500 m | 0.864 | × 1.16 | 13 573 CFM |
| 1800 – 2100 m | 0.804 | × 1.24 | 14 571 CFM |
| 2400 – 3000 m | 0.747 | × 1.34 | 15 688 CFM |
Typical Rack Densities
| Rack density | Classification | Typical CFM per rack (ΔT 15 °C) | Cooling approach |
|---|---|---|---|
| ≤ 5 kW/rack | Low density | ≤ 600 CFM | Room-level CRAC/CRAH, standard perforated tiles |
| 5 – 10 kW/rack | Medium density | 600 – 1 200 CFM | Hot/cold aisle containment, raised floor plenum |
| 10 – 20 kW/rack | High density | 1 200 – 2 400 CFM | In-row cooling, rear-door heat exchangers |
| 20 – 40 kW/rack | Very high density | 2 400 – 4 800 CFM | In-row, direct liquid cooling (DLC) recommended |
| > 40 kW/rack | HPC / GPU | > 4 800 CFM | Direct liquid cooling, immersion cooling |
Rules of Thumb
| Parameter | Typical value | Notes |
|---|---|---|
| CFM per kW IT load | ~120 CFM/kW | At ΔT = 15 °C. Use 150 CFM/kW with 25 % bypass airflow |
| Cooling design margin | 15 – 25 % | ASHRAE TC 9.9 recommendation; include future load growth |
| N+1 redundancy threshold | > 250 kW total IT | Below this, single-unit failure is often acceptable; above, N+1 CRAC required |
| PUE benchmark (cooling) | 1.2 – 1.6 | Cooling typically 20–40 % of total facility power; target PUE ≤ 1.3 for efficiency |
| Supply air velocity (raised floor) | 400 – 700 ft/min | Higher velocity increases noise; perforated tiles ~25 % open area |
| Hot aisle temperature | ≤ 45 °C | Above 40 °C risks A1-class equipment warnings; above 50 °C, thermal trips likely |
Frequently Asked Questions
What is the difference between CFM and m³/h?
CFM (cubic feet per minute) is the unit used in ASHRAE standards and most North American HVAC equipment specifications. m³/h (cubic metres per hour) is the SI equivalent used in European and international standards. Conversion: 1 CFM = 1.699 m³/h. For design purposes they are interchangeable — just pick the unit that matches your equipment datasheets.
What ΔT should I design for?
ASHRAE TC 9.9 recommends a 15–20 °C temperature rise (ΔT) across IT equipment for air-cooled systems. A larger ΔT means less airflow volume required, but higher return temperatures, which reduces CRAC coil efficiency and risks A1-class equipment envelope violations. Start with ΔT = 15 °C and model alternatives before finalising the design.
How much of the total facility power should I budget for cooling?
For a traditional raised-floor data centre, cooling accounts for 30–50 % of total facility power, giving a PUE of 1.5–2.0. Modern efficient designs target PUE ≤ 1.4 with economisers, aisle containment and hot/cold separation, bringing cooling to 20–30 % of total power. ASHRAE defines a Cooling Efficiency Ratio (CER) for CRAC units — look for CER ≥ 2.0 in new equipment.
When should I consider liquid cooling instead of air?
Air cooling becomes impractical above roughly 20–30 kW per rack — the required CFM per rack exceeds what standard perforated tiles or in-row units can deliver without high noise and hot-spot risk. Direct liquid cooling (DLC) — rear-door heat exchangers or chassis-level manifolds — handles 20–80 kW per rack. Immersion cooling (single-phase or two-phase) is practical above 100 kW/rack and is standard in modern HPC and AI clusters.
Does containment improve cooling efficiency?
Yes — hot-aisle containment (HAC) or cold-aisle containment (CAC) eliminates recirculation between hot and cold air streams, reducing the effective bypass airflow from 30–60 % in an open room to under 5 % with full containment. This means smaller CRAC units, lower fan energy, and fewer hot spots. ASHRAE TC 9.9 strongly recommends containment for all medium and high-density deployments.