Your questions answered: Can you specify CRAC systems to deliver free cooling in a data center?

In this case, yes you can. Read more questions and answers from the team that designed energy-efficient cooling in a California data center

By Eric Jensen and Dan McInnis, PE April 3, 2020

Questions left unanswered during the March 26, 2020, webcast “Can you specify CRAC systems to deliver free cooling in a data center?” are answered here.


  • Eric Jensen, vice president and general manager, Data Aire Inc.
  • Dan McInnis, PE, manager, applications engineering, Data Aire Inc.

Question: Can you tell us the inlet server air temperature requirement for the Los Angeles project? It will help provide a reference. This metric is typically the measurement used by the information technology professionals. Also, are you using electronically commutated motor fans?   

Answer: The design server inlet temperature is 72°F, with a return temperature of 95°F to the computer room air conditioning units. Yes, we used ECM fans.

Question: I would like more information about ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings and ASHRAE 90.4: Data Center Standard.     

Answer: ASHRAE 90.1 includes the minimum efficiency requirements for CRAC units (upflow/downflow) in sensible coefficient of performance.

ASHRAE 90.4 is a newer standard created as a spinoff from ASHRAE 90.1, and is specific to data centers.

Question: How do you achieve the same pressure drop for the two modes of cooling?    

Answer: We needed to analyze each operation mode independently and determine the system pressure drop through the related components. This included the piping, valves and coils, to name a few.

Question: Was the condenser water system existing, or was that provided as part of this retrofit?

Answer: It was installed around the same time as the equipment we provided, but it is being used by the entire building.

Question: What about rooftop applications in data centers? Instead of using chiller and close control unit, rooftop system can be more efficient by using air side economizer. Did you also make comparison?     

Answer: Existing space is always a consideration, however roof space was a limitation on this project so rooftop units were not considered.

Question: Is the 1.2 power usage effectiveness year-round or just using economizers?   

Answer: That is an annualized PUE estimate, so yes, it includes the entire year.

Question: What was the difference between the cooling tower and dry cooler?   

Answer: The cooling tower used was a closed-circuit evaporative cooling tower. The difference is that using evaporative cooling by spraying water on the tubes allows you to take advantage of the wet-bulb temperature when the water evaporates (typically only used in summer), while a dry cooler is using air only and cooling is purely based on the dry-bulb temperature.

Question: What is your recommendation for sizing the equipment specifically compressor cooling? Do we use the worst case even though compressor cooling is only partially used?   

Answer: I think that this falls into a good control strategy for the environment being in put place. An understanding of your desired temp/humidity conditions and the acceptance of allowable ranges of the equipment is important here. Referencing ASHRAE Environmental Classes, your target may be in the recommended zone, which you are able to reach through economization procedures in place. But when economization is not as available, there may be a shift into a higher zones within A1, A2 or even A3, which have higher temperatures. Much of the equipment in place may be able to handle this condition while others may not. The performance impact at these conditions may be acceptable for the amount of exposure they have.

To understanding the impact, it is important to see the operation from a whole. Designing to the worst case may at times be a good option, and in some cases have minimal impact on the overall facility, but consideration needs to be given to line power sizing, heat rejection to the atmosphere, usable space and future capacity expansion with regard to cooling loads.

Author Bio: Eric Jensen, vice president and general manager, Data Aire Inc.; Dan McInnis, PE, manager, applications engineering, Data Aire Inc.