One Air to Cool them All
Just like in “The Lord of the Rings”, there is an engineering that fascinates us all: Cooling.
And the fact is that there are (many) different Precision Cooling Formats: backpack, in-row, perimeter… which one to choose?
The growth of data centers has brought with it a technical challenge: keeping them cold, always, without failures. Cooling is no longer a simple task, and even less so when we talk about thermal loads that are increasingly concentrated per square meter.
The good news is that there is no single solution. There are multiple formats of precision cooling systems, and each has its advantages, limitations, and ideal scenarios.
🌡 What does ASHRAE say?
Before entering into the formats, it is useful to understand the regulatory framework. ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) is the global reference in technical air conditioning.
ASHRAE publishes the standard Thermal Guidelines for Data Processing Environments (TC 9.9), which defines the thermal classes (A1, A2, A3, A4). These classes indicate the recommended temperature and humidity ranges for IT equipment, depending on their thermal tolerance.
For example:
- Class A1: 15-32 °C, 20-80% RH
- Class A2: up to 35 °C
- Class A4: up to 45 °C (only on compatible equipment)
Most current equipment tolerates higher temperatures, but precise air control remains essential to avoid failures, condensation, or hot spots.
🏗 Design Standards: TIA-942 and Uptime Institute
The TIA-942 standard (Telecommunications Infrastructure Standard for Data Centers) defines the physical design of a data center, including the use of hot and cold aisles, the type of ductwork, and the classification of raised floor as an air distribution zone.
The Uptime Institute, for its part, establishes reliability levels (Tier I to IV). At high levels (Tier III or IV), N+1 or 2N redundancy in cooling is required, which also influences the type of systems selected.
🔧 Types of Precision Cooling Formats
Below, I explain the most commonly used formats, when each is suitable, and which brands manufacture them.
🟦 1. Perimeter (traditional CRAC or CRAH)
What are they?
Units that are placed around the room (perimeter) and blow cold air into the cold aisle, generally from the raised floor.
Types:
- CRAC: with compressor and direct expansion (DX)
- CRAH: with chilled water, they use coils connected to chillers
Advantages:
- Reliable and robust
- Easy maintenance
- Compatible with ducts or raised floor
Disadvantages:
- Lower efficiency in high-density environments
- Dependence on room design
Brands:
Vertiv (Liebert PDX), Stulz CyberAir, Schneider Uniflair
Relevant standards:
TIA-942 recommends raised floors of at least 30 cm if air is distributed from below.
🟩 2. In-Row (InRow)
What are they?
They are placed directly between the racks. They take hot air from the hot aisle and return cold air to the cold aisle.
Operation:
They can be DX, chilled water, or mixed systems.
Advantages:
- Very efficient in high density (>10 kW per rack)
- Localized cooling
- Easy modular expansion
Disadvantages:
- Lower capacity per unit
- Requires careful design of airflows
Brands:
- Schneider/APC InRow RC, RD
- Vertiv CRV
- Stulz CyberRow
Relevant standards:
ASHRAE TC 9.9 recommends this type in high-density clusters.
🟨 3. Backpack (Rear Door Heat Exchangers or in-rack cooling)
What are they?
Units installed on the rear door of the racks. They function as a heat exchanger that cools the hot air as it exits.
Types:
- Passive: they do not have fans
- Active: they incorporate fans to improve thermal transfer
Advantages:
- High efficiency
- They eliminate the mixing of cold/hot air
- Compact and discreet
Disadvantages:
- Requires hydraulic planning (water or refrigerant)
- Not all racks are compatible
Brands:
Rittal LCP, Vertiv DCL, Huawei iCooling
Standards:
Highly aligned with aisle containment strategies, promoted by ASHRAE and the Uptime Institute.
🟧 4. Overhead (mounted on ceiling or false ceiling)
What are they?
Systems that are installed above the IT area, and distribute air through ducts or in free fall towards the cold aisles.
Advantages:
- Frees up floor space
- Useful where there is no raised floor
- Easy integration with existing ductwork
Disadvantages:
- More difficult to maintain
- Require precise distribution calculation
Brands:
Vertiv CW overhead, Stulz TopCool, Schneider
📐 How to choose the correct format?
There is no single answer. Here are some keys:
| Criterion | Recommendation |
|---|---|
| High density per rack | InRow or Backpack |
| Small center or without raised floor | Overhead or InRow |
| Traditional centers with space | Perimeter |
| Need for energy efficiency | Rear Door or In-Row |
| Tier III or IV redundancy | Combination with CRAH and Chillers |
ASHRAE and Uptime recommend CFD thermal assessments (airflow modeling) to validate any design before implementing it.
🔚 Conclusion
Precision cooling is as critical as the power supply. Choosing the appropriate format not only protects your servers, it also saves energy, extends equipment life, and improves availability.
Remember: it is not about cooling more, but cooling better. Following ASHRAE guidelines and TIA and Uptime best practices, you can design an efficient, scalable, and reliable system.
