CRAC vs. CRAH Cooling Units: What’s the Difference?

December 10, 2021

Balancing the temperature in data centers is crucial. When data centers were first established, keeping them cool was an easy task. Managers would add air conditioners to the server rooms to manage the temperature. Adding more servers meant adding more air conditioners. At a certain point, this method becomes ineffective. Today we have larger data centers with higher server densities, and cooling computer rooms via standard air conditioners is not enough. More robust equipment is needed to optimize air circulation for better data center cooling. Two of the top cooling choices are CRAC and CRAH cooling units.

What Are Computer Room Air Conditioner (CRAC) Units?

A CRAC unit is similar to traditional air conditioning. It is designed to maintain the temperature, air distribution and humidity control in a data center’s computer rooms. CRAC units use a direct expansion refrigeration cycle. Air is cooled by blowing over a cooling coil. The cooling coil is filled with refrigerant, which itself is kept cool by compression. The extra heat is ejected using a glycol mix, water or ambient air.

The primary components of a CRAC unit include:

  • Mechanical items: Mechanical items depend on the type of CRAC unit being used and its application. Sometimes the CRAC unit includes a belt used to adjust tension. Over tensioning or under tensioning can reduce the performance of your cooling unit if not properly monitored. Maintaining the mechanical systems helps the unit work better and last longer.
  • Refrigerant: Refrigerant is a chemical compound commonly used in cooling systems and air conditioners. Check a CRAC unit’s refrigerant levels at least annually for optimal unit performance.
  • Air filters: Air filters are vital to a CRAC cooling unit’s performance. The accumulation of particles on the air filters can increase the internal temperature. Due to this, it is crucial to clean dirty filters as they could lead to overworked motors and reduced cooling capabilities.
  • Compressors: Compressors are best utilized when adequately oiled. The incorrect amount of oil can reduce a unit’s service life.
  • Evaporator coils: Coils need to be kept clean to ensure a well-functioning CRAC unit.

CRAC cooling units can be used to pressurize the spaces below the floors. They vent out cool air through perforated tiles into the server intakes. After moving through the server, the cool air is pushed out as hot exhaust and returned to the CRAC unit for cooling. Newer models of CRAC units can vary the airflow with the assistance of multistage compressors. However, most older models only have on/off capability. This method of cooling works best for small and low-density data centers.

What Are Computer Room Air Handler (CRAH) Units?

CRAH cooling units have a similar function to chilled water air handling units in buildings. Like CRAC systems, these units use fans to blow air over cooling coils to remove excess heat. However, these cooling coils are filled with chilled water rather than refrigerant. The chilled water typically comes from a separate chiller or chilled water plant. The unit draws in warm air from the computer room, and the air flows over the chilled water coils. Heat transfers from the air to the water, which then returns to the chiller. CRAH units can regulate fan speed, ensuring humidity and temperature levels stay stable while also allowing variability.

This type of cooling is typically used in mid-to-large-sized data centers. The air used is classified as “free” air, which can mean lower cooling costs based on the climate. Because CRAH units don’t use compressors, they use less energy, are more efficient and require less maintenance, all of which can also mean lower costs.


What Are the Differences Between CRAC and CRAH Cooling Units?

The main difference between CRAC and CRAH cooling units is that CRAC units use refrigerants and compressors, whereas CRAH units use chilled water and control valves.

CRAC cooling units have a simpler functionality as there is typically only one mode once turned on. Due to this, it is much harder to adjust cooling according to the computer room’s needs. Though CRAC units usually have simpler functionality, the system contains more parts, which means more maintenance and more components that can fail. Regular maintenance is critical to keep CRAC cooling units functioning at optimal performance. However, when operating correctly, CRAC cooling systems have the highest level of reliability. CRAC units are ideal for data centers with electrical loads of less than 200 kilowatts (kW) and lower availability requirements.

Because CRAH cooling units use a more efficient cooling cycle, they have greater heat removal capabilities than CRAC units while still having the same energy footprint. These systems are often more cost-effective long-term but can be expensive when installed in centers with less than 100 kW of electrical IT loads. CRAH systems are generally designed for data centers with electrical loads of 200 kW or more with moderate-to-high availability requirements. The efficiency of a computer system’s air handling unit improves with increased data capacity, making them the best choice for large data centers.

DataSpan Has the Cooling Solutions for You

At DataSpan, we have 50 years of data storage solution experience. We can provide your company with a customized and efficient data center cooling solution that provides maximum performance and reliability. Our vast range of cooling solutions includes down-flow, in-row, rack-mounted, ceiling mounted and portable designs so we can meet your needs, whatever your capacity requirements or room size. Our solutions are proven to reduce downtime and energy costs with the potential for utility rebates. Contact us now to learn more about our extensive data center cooling solutions and how you can keep your data center cool.