What is CRAH vs CRAC vs in-row cooling: the 3 ways to cool your data center and when each applies

Operating ranges by cooling technology

An enterprise data center needs to solve the same problem every time it grows: how to move the heat generated by the servers from the rack to the external condenser. The three dominant technology families in precision air conditioning respond to that question with different architectures, and the right choice depends on the density per rack, the redundancy required, the energy efficiency target, and the physical constraints of the data center. There is no single answer that fits all cases: CRAC, CRAH, and in-row cooling are complementary, not substitutes.

The decision is not just about cooling capacity: it is about the total cost of ownership over the 10-15 year life of the data center, the operational flexibility to support changes in IT load, and the alignment with the energy efficiency strategy. A choice that is optimal for a 2 kW per rack legacy data center is suboptimal for a 15 kW per rack HPC cluster. The decision matrix evolves as the data center grows.

What the ASHRAE TC 9.9 framework says

ASHRAE Technical Committee 9.9 (TC 9.9) is the industry body that publishes the recommended thermal guidelines for data centers. The current version (ASHRAE TC 9.9 2021 Thermal Guidelines) defines several equipment classes (A1-A4) with different operating temperature and humidity ranges, and the cooling technology must be able to maintain the conditions within the class of the equipment installed.

The relevant classes for a typical data center are A1 and A2, which allow operating temperatures of 18-27°C (A1) and 18-32°C (A2) at the rack inlet, with humidity between 40% and 55% relative. Equipment class A3 and A4 (with higher temperatures) are used in specific applications, but most data center equipment is class A1 or A2.

ASHRAE also defines the concept of “allowable” ranges (the manufacturer guarantees the equipment operates within specifications) and “recommended” ranges (where the equipment operates with maximum reliability and life). Operating outside the recommended range but within the allowable range is possible but reduces equipment life. The cooling technology must be able to maintain the conditions within the recommended range, not just the allowable range, for the data center to have the expected reliability.

ASHRAE TC 9.9 also publishes the heat density classifications and the corresponding cooling strategies. Up to 10 kW per rack, air cooling (CRAC, CRAH, in-row) is sufficient. Between 10 kW and 30 kW per rack, rear-door heat exchangers or in-row with high airflow are needed. Above 30 kW per rack, liquid cooling (DLC or immersion) is required.

Computer Room Air Conditioner (CRAC)

A CRAC (Computer Room Air Conditioner) is a self-contained precision air conditioning unit that cools, dehumidifies, and conditions the air in a data center. It uses a direct expansion (DX) refrigeration cycle, with the compressor, condenser, and evaporator integrated in a single unit. The CRAC discharges cold air into the raised floor (or into a duct) and returns hot air from the top of the racks.

Operating range of a typical CRAC: 5-30 kW per unit, with 3-5 kW per rack density. The CRAC is most efficient in small data centers (below 200 kW of total IT load) with uniform density per rack. The efficiency (kW of cooling per kW of electrical consumption) of a CRAC is typically 2.5-3.5 (EER), which is lower than a CRAH because the CRAC consumes electrical power in the compressor.

Advantages of the CRAC: lower initial cost, simpler installation, independent of external chilled water. Disadvantages: lower efficiency, limited capacity per unit, more difficult to scale (each additional CRAC is a separate unit with its own condenser).

Computer Room Air Handler (CRAH)

A CRAH (Computer Room Air Handler) is a precision air conditioning unit that conditions the air in a data center using chilled water from an external source (chiller, district cooling, or chilled water from a cooling tower). The CRAH does not have a compressor or condenser: it has a coil through which chilled water circulates, and a fan that moves the air through the coil. The heat is rejected in the external chiller, not in the CRAH itself.

Operating range of a typical CRAH: 30-300 kW per unit, with 5-15 kW per rack density. The CRAH is more efficient than the CRAC (the chiller is more efficient than the DX compressor for large capacities) and scales better (additional CRAH units can be added as the data center grows, sharing the same external chiller).

Advantages of the CRAH: higher efficiency (kW of cooling per kW of electrical consumption, with EER of 4-6 including the chiller), scalability, integration with free cooling. Disadvantages: requires external chiller infrastructure, higher initial cost, more complex installation.

In-row cooling

In-row cooling is a precision air conditioning unit that is installed between the racks in a row, in line with the cold aisle. The in-row unit takes hot air from the hot aisle (or from the rear of the rack) and discharges cold air directly into the cold aisle in front of the rack. The proximity to the heat source allows the in-row unit to operate with a smaller temperature difference and higher efficiency.

Operating range of a typical in-row unit: 20-80 kW per unit, with 10-30 kW per rack density. The in-row cooling is the most efficient air cooling option for medium and high density (10-30 kW per rack) because it shortens the air path and reduces the fan energy required.

Advantages of the in-row cooling: high efficiency, ability to handle medium and high density, modular scalability (one in-row unit per row or per group of racks). Disadvantages: requires more floor space between racks, requires raised floor or overhead ducting, more expensive than CRAC/CRAH per kW of capacity.

The decision matrix

The decision between CRAC, CRAH, and in-row cooling depends on the data center characteristics:

• Total IT load below 200 kW, density below 5 kW per rack: CRAC is the most economic option. The simplicity of the installation and the lower initial cost compensate for the lower efficiency.
• Total IT load 200 kW to 2 MW, density 5-15 kW per rack: CRAH is the most efficient option. The higher initial cost is compensated by the lower operating cost over the 10-15 year life of the data center.
• Density 10-30 kW per rack (HPC, AI clusters): in-row cooling is the most efficient air cooling option. The proximity to the heat source and the high airflow allow the in-row to handle the density that the CRAC and CRAH cannot.
• Density above 30 kW per rack: liquid cooling (DLC or immersion) is required. Air cooling is not viable at these densities.

Many data centers combine several technologies: CRAH for the general rooms, in-row for the high-density rows, and rear-door heat exchangers for the transition zones. The cooling architecture is not exclusive, but adaptive to the load profile of each area of the data center.

What changes with liquid cooling

Liquid cooling (DLC direct-to-chip, or single-phase/two-phase immersion) is a paradigm shift in data center cooling because the heat transport medium is water or dielectric fluid, not air. Water has a heat capacity approximately 3,500 times greater than air per unit volume, which means that liquid cooling can handle much higher densities with much smaller flow rates.

Direct-to-chip (DLC) cooling uses cold plates mounted on the processors (CPU, GPU) that circulate a water-glycol mixture at 25-35°C, removing the heat directly from the chip. DLC allows densities of 30-100 kW per rack, which is the typical density of HPC and AI clusters. The data center still needs air cooling for the rest of the equipment in the rack (memory, storage, power supplies), but the heat load on the air cooling system is significantly reduced.

Single-phase immersion cooling submerges the entire server in a dielectric fluid that circulates by natural convection or by pumping. The heat is removed by a heat exchanger in the immersion tank. Immersion allows densities of 50-200 kW per rack, but requires a specialized data center design (no air conditioning in the immersion room, dielectric fluid handling, specialized maintenance procedures).

Liquid cooling is not a substitute for air cooling: most data centers combine DLC or immersion for the high-density IT load with CRAH or in-row for the rest of the data center. The decision to adopt liquid cooling is driven by the density per rack and the energy efficiency target.

What your next RFP should ask

If the data center needs precision air conditioning, the questions that must appear in the RFP:

• What is the design density per rack? Is the cooling sized for the current density or the projected density in 5 years?
• What is the technology (CRAC, CRAH, in-row, hybrid) recommended for the data center profile? What is the justification?
• What is the efficiency of the cooling system at the design load? At 50% load? At 25% load? (Efficiency varies with load.)
• What is the N+1 or N+N redundancy of the cooling system? Is the redundancy in the units, in the chiller, or in both?
• What is the integration with the data center’s BMS? What communication protocols does it support? Modbus, SNMP, BACnet?
• What is the procedure for the seasonal commissioning of the cooling system? What tests are run?
• What is the O&M cost of the cooling system? Are there preventive maintenance kits? What is their frequency?
• What is the integration with free cooling or economizer? What is the energy savings in the climate of the specific location?
• What is the expected useful life of the equipment? What is the warranty?
• Does the manufacturer have local technical support in Mexico? What is the response time in case of failure?

The answers to these questions determine whether the cooling system is properly sized for the data center profile, and whether it will provide the efficiency and reliability the operation requires over the 10-15 year life of the data center.

Sources

1. ASHRAE (2021). 2021 Thermal Guidelines for Data Processing Environments — Expanded Data Center Classes and Usage Guidance. https://www.ashrae.org/technical-resources/bookstore/thermal-guidelines-for-data-processing-environments
2. ASHRAE (2015). Datacom Equipment Power Trends and Cooling Applications (3rd Edition). https://www.ashrae.org/file%20library/technical%20resources/bookstore/datacom-3rd-edition.pdf
3. Vertiv (2024). SmartAire CRAC Unit — Product Data Sheet. https://www.vertiv.com/en-us/products-catalog/thermal-management/precision-cooling/smartaire-crac/
4. Stulz (2024). CyberAir 3 CRAH Unit — Data Center Precision Cooling. https://www.stulz.com/en/products/cyberair
5. Climico HVAC (2025). Designing Resilient HVAC for Data Centers: CRAC, CRAH and Beyond. https://climicohvac.com/blog/designing-resilient-hvac-for-data-centers-crac-crah-beyond
6. Anvilfield (2025). In-row and close-coupled cooling commissioning field guide. https://anvilfield.com/field-guides/datacenter/in-row-close-coupled-cooling-commissioning
7. Heather Technologies (2025). Modular Data Center Cooling: Scaling CRAH and In-Row Units with Demand. https://heathertechnologies.com/pages/modular-data-center-cooling-scaling-crah-and-in-row-units-with-d
8. E3S Web of Conferences (2025). Thermal Performance Evaluation of Cooling Solutions with Three ITE Power Densities. https://e3s-conferences.org/articles/e3sconf/pdf/2025/72/e3sconf_roomvent2025_06002.pdf