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Calculating cooling inefficiency

You can gauge how inefficient you are using a simple formula. By Jim Fink, CTO, Upsite Technologies

Advances in compressorized system- capacity control, electronically commutated motors, variable speed drives and improved controls have undoubtedly made a favorable impact on the minimum achievable PUE (Power Usage Effectiveness) of modern data centers.

Many existing data centers, however, are operating at subpar efficiencies because of problems with airflow management. Bypass airflow is generally at the root of their problems.

CALCULATING YOUR EFFICIENCY ENTITLEMENT As evidence of this inefficiency, perform a simple experiment. Add up the total airflow rate in CFM (Cubic Feet per Minute) for all of the CRAC (Computer Room Air Conditioning) or CRAH (Computer Room Air Handler) units in your data center.

If you

don’t know the CFM specification on your cooling units, use a rough estimate: 550 CFM per ton of cooling, for example.

Then, estimate your air-cooled IT equipment A Estimate

Cooling Unit CFM

550 CFM per Ton Caption Needed B

Estimate Cooled IT

Equipment CFM IT load (kW) * 130 (CFM per kW)

If A > B, Then

Surplus Airflow

5-10% is excellent, 50% is bad

Cooling equipment Overhead & loss Computing equipment

Data Center Power Usage

CFM with the equation: IT CFM = IT load (kW) * 130 (CFM per kW). Now, compare the two airflow rate numbers. The amount by which the aggregate cooling unit CFM exceeds the IT CFM is representative of your efficiency entitlement.

From an efficiency standpoint, 5-10% surplus is excellent; 50% is bad. This is extra airflow that theoretically is not needed but is costing you money. Unfortunately, the remedy is not as simple as turning off a few cooling units until the two numbers match.

Because of imperfectly coupled cooling, hot spots often develop as the surplus cooling capacity is reduced.

In other words, just

matching the cooling system airflow rate to the IT airflow rate does not mean that bypass airflow will cease to exist.

Bypass airflow still occurs, and hot spots consequently develop, when cool supply air returns to an air conditioner without having passed through any IT equipment, and hot exhaust air returns to the IT equipment without being re-cooled.

With proper airflow-management solutions in place, energy-saving actions can be taken more aggressively, with less of a chance of hot spots. These actions may include reducing fan speed, turning off cooling units, modifying redundancy failure schemes and raising temperature set-points.

These are all relatively simple actions that do not require a lot of work and equipment, so the associated expense is minimal and payback periods are usually attractive. n


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