Size a CRAC unit from the room’s design heat load and the cooling capacity the selected equipment can actually deliver at project conditions—not from nominal tonnage or a generic CFM-per-ton rule. Then check that air reaches rack inlets, the heat-rejection system can handle the load, and surviving equipment can meet demand during the failures or maintenance events the owner expects the design to withstand.
How to calculate the design heat load
For an operating room, start with measured IT or ICT electrical demand, using UPS or PDU data where available. Add heat from electrical equipment located inside the cooled space, including applicable UPS losses. For a new room, prepare a documented load schedule from the planned IT equipment and power design; installed electrical capacity is not automatically the same as the room’s current heat load. EPRI describes using measured kW for ICT load and adding UPS losses when the UPS is in the data-center space (EPRI, March 2015).
Convert units before comparing heat load with cooling capacity: EPRI gives 1 ton of cooling as 3.52 kW of cooling, and Btu/hr divided by 3,412 as cooling capacity in kW. Keep the load and capacity in consistent units, and distinguish sensible capacity from total capacity when the manufacturer reports both.
Document the owner’s expected growth scenario rather than adding an arbitrary universal percentage. Show how that scenario changes the required capacity, unit count, or planned modular expansion.
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How to check capacity at actual operating conditions
A unit’s nominal tonnage is not a guarantee that it will deliver that capacity in the room. Output depends on operating conditions, including return-air temperature. Obtain current manufacturer performance data for the selected model at the project’s entering or return-air temperature, supply-air target, outdoor or condenser conditions, or chilled-water conditions, as applicable. Confirm what capacity the quoted figure represents and account for the associated heat-rejection equipment as part of the complete system.
Dell’s June 2009 white paper illustrates why the rating label is not enough: in its historical example, a nominal 30-ton unit delivers less than 20 tons with return air in the 70s. That is an example from the paper, not a general performance curve or a claim about current products (Dell white paper, June 2009). The sources do not establish a universal current derating factor; use the selected manufacturer’s data rather than applying one.
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How to size and distribute airflow
Cooling capacity and airflow are separate checks. A system can have enough aggregate cooling capacity yet leave equipment hot if supply air bypasses racks, exhaust recirculates into inlets, supply air short-circuits to a return, or obstructions prevent air reaching the load. EPRI identifies leakage at cable cutouts and rack openings, bypass, recirculation, and short-circuiting as causes of unproductive airflow. Blanking panels can block airflow through unused spaces in partially loaded racks.
EPRI’s March 2015 report describes typical DX-CRAC configurations with airflow in the mid-400s to low-600s cfm per ton of cooling—about 125–175 cfm per kW of cooling at full capacity—and a coil temperature drop around 20°F at full capacity. These are descriptive ranges for configurations discussed in that report, not universal design targets. Required airflow depends on the selected unit, fan and static-pressure conditions, room distribution, rack airflow, and containment arrangement. Do not assume a generic 400 cfm/ton rule will suit every server room.
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- Check that the selected unit can deliver its required airflow against the project’s actual pressure conditions.
- Trace supply air to rack inlets and check for bypass, recirculation, short-circuiting, cable-opening leakage, and obstructions.
- Assess rack inlet conditions across the room; an adequate room-average temperature can conceal local hot spots.
- Investigate airflow distribution before treating added cooling capacity as the sole remedy for a hot spot.
EPRI reports a 32% reduction in cooling energy use per unit of ICT equipment energy use at 60% fan speed in its DX-CRAC field demonstrations. That finding applies to the tested units and sites; it is not a guaranteed saving for another facility. EPRI also states, “Reducing and managing airflow is one of the most cost-effective means of reducing power demand and energy use in data centers.” The practical implication for sizing is to design for useful airflow at the equipment, not simply more air volume.
How to define redundancy separately from sizing margin
Capacity margin and redundancy address different risks. A margin accommodates uncertainty or a documented future load; redundancy is the ability to continue serving the required load when equipment is unavailable because of failure or planned maintenance. Specify them separately and evaluate normal and failure cases independently.
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Define which failures the design must withstand: a cooling unit, power source, controls component, or shared heat-rejection element may have different consequences. Then calculate the capacity available from surviving equipment at the project’s design conditions and verify it can meet the required load. Extra unit count alone does not establish that air will be distributed effectively after a failure.
EPRI says N+1 or N+2 arrangements are commonly specified for continuously operating cooling systems. It also describes one spare unit for five to seven distributed units, or about 15–20% extra capacity, as common in the practice covered by its report. These are observations, not rules for every facility: the architecture, actual delivered capacity, airflow, and defined failure scenario determine whether the design meets its requirement.
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How to compare cooling design options
Compare complete designs at the same stated load and project conditions. EPRI describes DX as modular and often used in small-to-mid-sized data centers, while chilled-water systems are often selected at larger scale; it does not establish a universal size at which one should replace the other.
| Design question | What to compare |
|---|---|
| Delivered capacity | Manufacturer-rated capacity at the specified entering or return-air, supply-air, and heat-rejection conditions; compare sensible with sensible and total with total. |
| Air delivery | Airflow volume and the ability to deliver it to rack inlets under the project’s fan, pressure, room, and containment conditions. |
| Cooling architecture | DX versus chilled water, including whether the required plant and heat-rejection infrastructure are available for the design. |
| Resilience and maintenance | Normal and failure-case capacity, unit and shared-infrastructure failure assumptions, and planned service coverage. |
| Future operation | Modularity against the owner’s documented growth plan, plus controls, part-load operation, service requirements, and lifecycle energy implications. |
CRAC sizing dos and don’ts
- Do use measured IT load where available and include applicable heat from equipment inside the cooled room.
- Do confirm delivered capacity from current manufacturer performance data at the actual design conditions.
- Do check both cooling capacity and airflow to rack inlets, including the failure and maintenance case.
- Do make the owner’s growth assumption explicit and show how it affects the design.
- Don’t size from installed IT nameplate capacity alone when measured operating load is available.
- Don’t treat nominal tonnage, an office HVAC airflow rule, or a typical EPRI range as a project-specific guarantee.
- Don’t equate extra unit count with resilience unless surviving units and shared infrastructure can deliver the required cooling and airflow.
- Don’t assume added capacity alone will correct hot spots caused by bypass, recirculation, or poor distribution.
Final equipment selection requires the site’s load, room arrangement, design conditions, applicable environmental requirements, and current manufacturer ratings. A project-specific engineering calculation is necessary to turn these checks into a unit selection.
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