Data Center Cooling Fans: Selection Guide for CRAC, CRAH, Fan Walls and Dry Coolers
A data centre cooling fan is not one product. It is a family of duties — CRAH coil airflow, EC fan walls, condenser and dry-cooler exhaust, hot-aisle extract, and liquid-cooling dry coolers — and each one wants a different impeller. This guide sets out which fan goes where, how to size it from airflow and static pressure, why part-load efficiency decides the energy bill, and what to put in a specification so the fans you buy still hold up when the IT load changes.
Which fan goes where
| Duty | Typical fan | Static pressure | What decides the choice |
|---|---|---|---|
| CRAH / CRAC coil airflow | Backward-curved centrifugal plug fan, or EC fan wall | 300 – 800 Pa | Part-load efficiency, stable airflow against a loading coil and filter bank |
| EC fan wall (in-row and per-rack) | EC plug fans in an array, 6 – 12 per unit | 200 – 600 Pa | Redundancy (N+1), individual fan speed control, shallow depth |
| Condenser / dry cooler | Axial, 630 – 1250 mm | 80 – 200 Pa | Lowest kW per m³/s, fan count, acoustic limit at the boundary |
| Hot-aisle extract | Axial or mixed-flow | 50 – 150 Pa | Redundancy, easy replacement, low noise into the hall |
| Liquid-cooling dry cooler (CDU) | Large axial, up to 1600 mm | 60 – 150 Pa | Turndown range, wind tolerance, low ambient starting |
| Make-up and pressurisation | Small axial | 30 – 100 Pa | Filter loading, controllability |
Axial or centrifugal: the deciding number is static pressure
An axial fan moves a lot of air at low pressure. A centrifugal holds its flow as the pressure climbs. The crossover sits around 250 – 400 Pa. Below it, an axial fan almost always uses fewer watts per cubic metre. Above it, an axial fan starts to stall and the centrifugal wins — which is why CRAH units with dense coil faces use plug fans rather than propellers.
Two practical checks before you choose:
- What is the system resistance at the real filter condition? A clean filter is not the design case. Size for the loaded filter and check the airflow at the dirty end, not just at day one.
- How far does the fan turndown? IT load varies far more than the nameplate suggests. A fan that is efficient at 100 % and poor at 40 % will cost more over a decade than one that is good across the range.
Why EC fans change the calculation
An EC (electronically commutated) motor carries its own speed control, so the fan follows the load instead of running at fixed speed and dumping energy through a damper or inlet guide vane. In most cooling systems resistance falls as airflow falls, so shaft power drops roughly with the cube of speed. Two things follow:
- Part-load efficiency is where the money is. A data centre spends most of its hours well below design airflow. The number to compare is not the best-point efficiency on the catalogue cover, but the efficiency at 50 % and 70 % of design flow.
- Control becomes part of the fan, not the panel. An EC fan accepts a 0–10 V or PWM speed signal and reports back over Modbus RTU or similar. That makes per-fan speed control and N+1 arrays practical, and it removes the belt, the pulley and the separate variable speed drive.
The trade-off is honest: the EC motor and its electronics cost more than a belt-driven AC fan of the same duty, and the electronics have their own temperature limit, so the fan has to be applied where the airstream temperature is within the motor's rating — a hot-aisle extract at 40 – 45 °C is a different duty from a dry cooler at ambient.
How to size a cooling fan
Four numbers define the duty, and every fan supplier should be asked for all four:
- Volumetric flow in m³/s or m³/h (or cfm) — the air the coil or the room actually needs, not the fan's free-air rating.
- Total pressure in Pa, at that flow — the resistance of coil, filter, duct, louvre, silencer and discharge, added up at the dirty-filter condition.
- Air density at the fan inlet — altitude and temperature both change it, and power follows density directly. A fan selected at sea level will be short of flow at 2000 m unless the density is corrected.
- Sound limit in dB(A) at the relevant distance or at the boundary — this often decides the tip speed, which then decides the fan size.
From these, the required shaft power follows from the air power divided by efficiency: P = Q × Pt / (1000 × η) with Q in m³/s, Pt in Pa and P in kW. Note that total pressure is the comparable figure, not static pressure alone: Pt = Ps + Pd, and Pd = ½ρv² at the fan outlet. Comparing a fan quoted on static pressure with one quoted on total pressure is a common way to buy the wrong machine.
What to put in the specification
Most data centre fan problems appear two years after handover, and most of them are specification gaps rather than manufacturing faults. Six lines worth writing into the enquiry:
- Caliber. State whether the quoted pressure is total or static, and to which standard the performance is stated — AMCA 210 or ISO 5801. Ask for the test standard in writing.
- Part-load points. Require efficiency at 100 %, 75 % and 50 % of design flow, not just one point. This is the only way to compare offers that look similar on the cover page.
- Turndown and minimum speed. Confirm the fan is stable and coolable down to the lowest speed the control loop will command, not just at design.
- Redundancy. For fan walls and CRAH arrays, state the N+1 logic and whether the remaining fans must hold the load with one off. That sets the fan size, not the average duty.
- Control interface. Name the signal (0–10 V, PWM, Modbus RTU/TCP, BACnet gateway) and who supplies the controller. Ambiguity here is a commissioning delay.
- Balance grade and bearing life. ISO 1940 G6.3 or better for the impeller, and a stated L10 bearing life at the actual speed and temperature.
One data center fan or a whole fan wall: where the airflow actually goes
Most selection errors start before the fan is chosen, in the way the air path is drawn. Each data center fan in a unit does not see the room average; it sees the pressure at its own inlet and outlet, and in a hot aisle containment arrangement the return path is what sets that pressure.
- Supply and return are two different duties. A CRAH supply fan works against the raised-floor or ducted supply path; the hot aisle extract sees the containment, the ceiling plenum and often a longer return route. They are sized separately, even when the two fans are identical.
- Containment changes the resistance, not the flow. Closing the hot aisle removes bypass mixing, so the same rack load is met with less supply air but a higher return-air temperature and density correction. On a fixed fan that reads as lower power at the same flow.
- Redundancy is a fan count, not a percentage. N+1 on a fan wall means the remaining fans must carry the full load at a higher speed and usually at a worse efficiency. Size the fan wall so that the N condition is acceptable on the curve, not only the design condition.
- Low PUE is reached at part load. A unit spends most hours in the 40-70% band, so the fan that wins is the one with a flat part load power curve and a control range wide enough to stay out of on/off cycling.
The order that avoids rework: fix the air path and the containment first, then the duty point per fan, then the redundancy state, and only then the fan itself. A data center fan chosen against the room average will be wrong in every one of the four cases above.
Common mistakes
- Sizing on clean-filter resistance. The fan then runs at the dirty end of its curve for most of its life, at lower flow and often at a worse operating point.
- Comparing static pressure with total pressure. A static-pressure figure looks lower for the same machine, and a total-pressure figure looks higher — the comparison has to be made on one basis, and then the system resistance has to be counted the same way.
- Ignoring density. Data centres at altitude, or fans handling hot extract air, need the density correction applied to both flow and power.
- Buying on best-point efficiency. A fan with a very high peak efficiency and a narrow curve can lose to a slightly lower peak with a broad, flat efficiency island once the load starts moving.
- Leaving the control interface open. The fan arrives, the BMS speaks a different protocol, and the unit runs at fixed speed for a year.
Where SINDFAN fits
We manufacture axial fans and impellers from Ø250 to Ø3150 mm, and backward-curved centrifugal impellers, at our own works in Tianjin. For data centre duty that means:
- Dry cooler and condenser axial fans — standard sizes from Ø630 to Ø1250 mm, with the volume flow, total pressure and sound power stated at the duty point.
- CRAH plug fan impellers — backward-curved centrifugal impellers supplied as impeller-only for OEM units, or as fan assemblies.
- Replacement impellers — for existing CRAC/CRAH and dry cooler units where the motor and housing stay and only the impeller is renewed.
Performance is stated to AMCA 210 / ISO 5801, impellers are balanced to ISO 1940 G6.3 or better, and test reports are supplied with each contract. We are a manufacturer, not a trading house: impellers are pressed, welded, heat treated and balanced in our own plant, which is why a non-standard diameter or blade angle is a normal order for us rather than a special project.
Ask us for the actual numbers
If you have a duty point — airflow, total pressure, density, sound limit and the number of fans — send it and we will come back with the fan size, the shaft power at 100 %, 75 % and 50 % flow, and the sound power. If you are replacing an existing impeller, send the diameter, the hub diameter, the blade count and a photo of the old impeller and we will match it.