Delta-T 101: Server Exhaust to CRAC/CRAH Delta-T
Welcome to Keep Your Cool—a series tackling simple cooling optimization strategies for busy data center operators.
We’re in Week 3 of our Delta-T 101 series. We started with the four different Delta-Ts of data center cooling, and then broke down the cooling unit delta-T. This week we’re exploring the return path delta-T.
Introduction
Continuing our discussion on the Four Delta-Ts of the Data Center, today we focus on the Return Air Delta-T—the temperature difference between server exhaust and the return air entering the CRAC/CRAH units. This Delta-T is a key area where improvements in efficiency can be made. Without proper airflow management, hot and cold air mix unnecessarily, reducing the effectiveness of cooling systems and increasing operational costs. In this article, we’ll break down how return airflow works, what a good and bad Delta-T looks like, and best practices to improve efficiency.
How Return Airflow Works in a Data Center
Data centers rely on an organized airflow cycle to maintain optimal cooling efficiency. Servers take in cool supply air, process workloads, and expel hot exhaust air into the hot aisle. Ideally, this hot exhaust air should travel directly back to the CRAC/CRAH units without mixing with cool supply air. However, in many data centers, the absence of hot aisle or cold aisle containment leads to unwanted air mixing, reducing cooling efficiency.
As the return air temperature drops due to cold air mixing, the CRAC/CRAH units must work harder to remove heat. This increases Delta-T and energy consumption, lowering overall cooling efficiency.
What is Return Air Delta-T?
Return Air Delta-T is the temperature difference between the hot air leaving the servers and the air entering the CRAC/CRAH units. The larger the Delta-T, the more efficient the system is at removing heat. However, if the return air temperature is lower than expected due to mixing, it reduces efficiency and drives up cooling costs.
Common causes of poor Return Air Delta-T include:
Lack of Containment: Without hot or cold aisle containment, hot air rises and mixes with supply air, reducing return temperature.
Missing Blanking Plates: Open gaps in server racks allow cool air to pass through unused spaces instead of cooling IT equipment effectively.
Unsealed Floor Openings: Holes in the raised floor to accommodate cabling can allow cool air to escape into the hot aisle, reducing airflow efficiency.
Short Cycling: Poor CRAC/CRAH placement can result in cold air bypassing equipment entirely and returning directly to the cooling units, leading to inefficiencies.
What is a Good vs. Bad Delta-T?
A good Return Air Delta-T occurs when minimal air mixing allows the CRAC/CRAH units to receive hot return air at a high enough temperature to maximize heat removal efficiency. In an ideal scenario, the return air should be as hot as possible before entering the cooling unit, ensuring effective cooling.
A bad Return Air Delta-T happens when cold supply air mixes with hot return air, lowering return temperatures and reducing system efficiency. If the cooling system is receiving return air at significantly lower temperatures than expected, it suggests that containment and airflow management need improvement.
How to Improve Return Air Delta-T
To optimize Return Air Delta-T and improve cooling efficiency, consider implementing the following best practices:
Monitor and Measure Delta-T: Regularly measure Return Air Delta-T to identify inefficiencies and adjust airflow management strategies accordingly.
Deploy Containment Systems: Use hot aisle or cold aisle containment solutions to prevent air mixing.
Seal Gaps in Racks: Install blanking plates to block gaps in server racks and prevent cool air from bypassing IT equipment.
Close Floor Openings: Seal cable openings with brush grommets to maintain plenum pressurization and prevent cool air leakage.
Optimize CRAC/CRAH Placement: Ensure cooling units are positioned strategically to prevent short cycling and improve airflow patterns.
The Delta-T as Your Efficiency Gauge
Think of Delta-T like the fuel efficiency gauge in your car. If your vehicle suddenly drops from 25 miles per gallon to 20, you wouldn’t ignore it—you’d get it checked to avoid unnecessary fuel costs. The same applies to your data center: a lower-than-expected Delta-T means you’re paying more in energy costs. Monitoring and optimizing Delta-T can help cut cooling expenses and enhance operational efficiency.
Conclusion
Understanding and optimizing Return Air Delta-T is essential for efficient cooling in data centers. By addressing common airflow issues and implementing containment strategies, data center operators can improve cooling performance and reduce operational expenses.
About the Author
Gregg Haley is a data center and telecommunications executive with more than 30 years of leadership experience. Most recently served as the Senior Director of Data Center Operations - Global for Limelight Networks. Gregg provides data center assessment and optimization reviews showing businesses how to reduce operating expenses by identifying energy conservation opportunities. Through infrastructure optimization energy expenses can be reduced by 10% to 30%.
In addition to Gregg's data center efforts, he has a certification from the Disaster Recovery Institute International (DRII) as Business Continuity Planner. In November of 2005, Gregg was a founding member and Treasurer of the Association of Contingency Planners - Greater Boston Chapter, a non-profit industry association dedicated to the promotion and education of Business Continuity Planning. Gregg had served on the chapter's Board of Directors for the first four years. Gregg is also a past member of the American Society of Industrial Security (ASIS). Gregg currently serves as the Principal Consultant for Purkay Labs.
You can follow Gregg’s adventures on LinkedIn.
About Purkay Labs
At Purkay Labs, we specialize in cooling measurement solutions for data centers. Our AUDIT-BUDDY system allows operators to measure Return Air Delta-T with precision, helping identify inefficiencies in airflow management. Our software provides real-time analytics to optimize containment strategies, improve cooling efficiency, and reduce energy costs. If your interested in learning more about how to measure the return delta-T, click here.