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How to Choose and Buy a CRAC Unit for Your Organization

By Andrew Erickson

April 2, 2023


Computer rooms and data centers play a critical role in businesses and government agencies. These rooms require a controlled environment with specific temperature and humidity levels. This ensures the smooth operation of servers, network equipment, and other sensitive electronics.

Computer room air conditioning (CRAC) units are designed to provide this environment by regulating temperature, humidity, and air quality. Let's take a few minutes to discuss the components of a CRAC unit. We'll also review how CRAC units work. Last, we'll cover factors to consider when selecting and purchasing a CRAC unit for your company or government agency.

Components of a CRAC Unit

A CRAC unit is a specialized air conditioning system designed to cool computer rooms and data centers. The unit comprises several critical components. These include a computer room air handling unit (CRAHU), a cooling coil, an air filter, and an air distribution system.

The CRAHU is the central component of a CRAC unit. It receives hot air from the server room and cools it using a cooling coil. The cooled air is then distributed back into the server room through an air distribution system.

The cooling coil is where the heat exchange occurs. The hot air from the server room blows over the cooling coils. These contain a refrigerant.

The heat is transferred from the air to the refrigerant. The cooled air is then blown back into the server room.

The air filter removes dust and other particulate matter from the air before it enters the CRAHU. This is important because dust and dirt can damage electronics and interfere with the operation of the CRAC unit.

The air distribution system ensures that the cooled air is distributed evenly throughout the server room. This is typically achieved by using raised flooring to create a plenum. This allows the cool air to flow under the floor and into the server racks.

How CRAC Units Work

CRAC units work by removing the hot air generated by the electronic equipment in the server room. This air is replaced it with cool, clean air. The hot air is drawn into the CRAHU, where it is cooled by blowing air over the cooling coils filled with chilled water. The cooled air is then distributed back into the server room through the air distribution system.

CRAC systems are typically designed to maintain a specific temperature and humidity level within the server room. The temperature is controlled by adjusting the amount of chilled water flowing through the cooling coils. Meanwhile, the humidity is controlled by using a humidifier or dehumidifier.

Factors to Consider When Selecting and Purchasing a CRAC Unit

When selecting and purchasing a CRAC unit for your company or government agency, there are several factors to consider:

  1. Determine the size of the unit required to cool your server room. This will depend on the size of the room, the amount of electronic equipment, and the heat generated by that equipment. A qualified HVAC technician can help you determine the correct size for your needs.
  2. Check the efficiency of the CRAC unit. Energy-efficient units can save you money on your energy bills and reduce your carbon footprint. Look for units that have high SEER (Seasonal Energy Efficiency Ratio) ratings.
  3. Consider the type of cooling system used by the CRAC unit. Some units use chilled water, while others use refrigerant-based systems. Chilled water systems are typically more energy-efficient but require a separate chiller to cool the water. Refrigerant-based systems are simpler and more cost-effective but are less energy-efficient.
  4. Think about the type of air distribution system required for your server room. Hot aisles and cold aisles are commonly used in server rooms to optimize airflow and cooling. Hot aisles contain the hot air generated by the servers, while cold aisles contain the cool air delivered by the CRAC unit. The hot air is then exhausted out of the server room through a return air plenum, while the cool air is drawn back into the CRAC unit through the supply air plenum.
  5. Review the level of air filtration required for your server room. The air filters in a CRAC unit can remove dust, dirt, and other particulate matter from the air, which can help prevent damage to sensitive electronics. Look for units with high-efficiency particulate air (HEPA) filters, which can remove up to 99.97% of airborne particles.
  6. Consider the level of redundancy required for your CRAC unit. Redundancy refers to the ability of the CRAC unit to continue functioning in the event of a component failure. For mission-critical applications, it is recommended to have at least "N+1 redundancy", which means having one extra CRAC unit to take over in the event of a failure. The majority of remote sites we deal with at DPS have a pair of CRAC/HVAC units. They primarily operate in a "lead-lag" configuration to minimize short-cycling.

What about HVAC vs. CRAC? What's the difference?

HVAC (Heating, Ventilation, and Air Conditioning) and CRAC (Computer Room Air Conditioning) are both systems used for cooling and regulating the temperature in indoor spaces. However, there are some key differences between the two systems.

One major difference is their application

HVAC systems are commonly used to cool and heat large commercial and residential buildings. CRAC units are specifically designed to cool and regulate the temperature and humidity in server rooms and data centers where sensitive electronic equipment is housed. HVAC systems are more general-purpose, while CRAC units are specialized and designed to meet the specific needs of IT infrastructure.

Another difference is the level of control and precision provided by the two systems

CRAC units typically have more advanced controls and monitoring systems than HVAC systems. This is because the performance of IT equipment in a data center is heavily dependent on the temperature and humidity of the air in the room.

CRAC units are designed to maintain a tightly controlled temperature and humidity level in the server room. This is often within a range of only a few degrees and a few percentage points of humidity. HVAC systems, on the other hand, typically have a wider range of temperature and humidity settings.

The way the two systems deliver cool air is also different

HVAC systems typically use a centralized air handling unit to circulate air throughout a building, with individual rooms controlled by dampers and vents.

In contrast, CRAC units are localized systems that deliver cool air directly to the server room or data center. This means that CRAC units can provide more precise control over the cooling and air quality of the server room. This occurs without affecting the rest of the building.

The components and features of the two systems differ

CRAC units are specifically designed to handle the unique needs of a server room. This includes features like high-efficiency air filters, redundant cooling systems, and sophisticated monitoring and control systems.

HVAC systems, on the other hand, are designed to handle a wider range of indoor environments. They have features like variable speed fans, humidity control, and air purification.

Although there are some differences, plenty of HVAC systems are used for cooling telecom assets

While both HVAC and CRAC systems provide cooling and temperature regulation, they differ in their application, precision, air delivery, and components.

HVAC systems are more general-purpose, while CRAC units are specialized for data centers and server rooms. CRAC units provide more precise temperature and humidity control, deliver cool air directly to the server room, and have advanced monitoring and control systems.

HVAC systems are more versatile, with a wider range of features for different indoor environments.

With all of that said, the remote telecom huts and power substation environments that I work with at DPS are primarily cooled using HVAC systems. These are projects where our HVAC Controller is commonly purchased and installed. All of the concepts I've described related to CRAC selection and purchase apply equally to HVAC systems.

What we're dealing with here is primarily a question of vocabulary. There are also differences in some of the finer points of the respective feature sets.

What is a lead-lag controller?

A lead-lag controller is a device that manages the operation of multiple HVAC or CRAC units. It helps ensure that the units work together efficiently to regulate the temperature and humidity in a building or server room.

A lead-lag controller determines which unit should operate as the "lead" unit and which should operate as the "lag" unit. The lead unit is responsible for cooling the space, while the lag unit is on standby, ready to take over if the lead unit fails or needs maintenance.

The controller regularly switches the roles of the units to prevent any one unit from being overworked and to ensure even wear and tear.

The lifespan of a cooling system can be effectively extended by reducing "short cycling" in favor of longer run cycles.

The basic premise here is much like what you know from operating a car. You get more miles out of less wear and tear on the highway. It's much easier on the system to get up to "cruising speed" than to constantly start and stop as with city driving. The same is true for HVAC systems.

Short cycling occurs when a cooling system turns on and off frequently, such as when it runs for a few minutes and then shuts off before running again shortly thereafter. Short cycling causes the system to work harder and wear out faster, leading to costly repairs and a shorter lifespan.

On the other hand, longer run cycles, where the cooling system runs for longer periods before turning off, are beneficial for the system's health. Longer run cycles allow the system to cool more effectively and efficiently, reducing the overall workload and wear and tear on the system. This can extend the lifespan of the system, reduce maintenance costs, and improve energy efficiency.

Compared to just always activating both HVAC units at the same time to cool a site, you're cutting your number of run cycles by more than half. It's much better for one HVAC unit to run for twice as long. That's one cycle vs. two that would be required for a simultaneous activation.

If the heat load is so high that the lead unit can't keep up, the lead-lag controller adds the lag unit to deliver the benefits of simultaneous operation. In this way, you get the power (and cost for improved ROI) of simultaneous operation only when it's actually needed for improved ROI.

By using a lead-lag controller, cooling systems can be managed more effectively, reducing the likelihood of short cycling and extending the life of the equipment. With longer run cycles and proper maintenance, HVAC and CRAC units can function effectively for many years,

Your CRAC purchase decision will have an impact (good or bad) for years

Selecting and purchasing a CRAC unit for your company or government agency is a critical decision. It requires careful consideration of several factors. Considerations such as the size of the unit, efficiency, cooling system, air distribution, air filtration, and redundancy can all impact the performance and reliability of your CRAC unit.

By taking the time to understand and evaluate these factors, you can select and purchase a CRAC unit that meets your specific needs and provides a controlled environment for your critical electronic equipment.

Make sure that you purchase something that will serve your organization well for years to come. Investing in a quality CRAC unit will pay off in the long run. You'll see improved energy savings, increased system reliability, and enhanced equipment performance.

Speak with a DPS engineer now about your CRAC requirements

At DPS, we design and build remote monitoring systems. One of the most common uses of our devices is for monitoring temperatures in critical areas like server rooms and data centers. That gives us a background in computer room air conditioning technology.

When you call us, we'll guide you in the right direction. We can point you in the right direction for CRAC units and also give you preliminary information about the remote monitoring tech. Good remote monitoring will extend the life of your cooling system. It will also preserve the mission-critical equipment that requires cooling in the first place.

To speak with a DPS engineer about what you're trying to accomplish, simply call 1-800-693-0351 or email sales@dpstele.com

Andrew Erickson

Andrew Erickson

Andrew Erickson is an Application Engineer at DPS Telecom, a manufacturer of semi-custom remote alarm monitoring systems based in Fresno, California. Andrew brings more than 17 years of experience building site monitoring solutions, developing intuitive user interfaces and documentation, and opt...