Data Centers: How to Dispose of Secondary Circuit Heat

The growth of artificial intelligence, cloud computing, and high-performance applications is rapidly increasing the thermal density of data centers . More computing power means more heat to be continuously removed , with increasingly stringent requirements for efficiency, reliability, and operational continuity.

In this scenario, attention cannot focus solely on server cooling . Once the heat has been transferred from IT equipment to a heat transfer fluid, it must be effectively disposed of to the external environment. This is where dry coolers, evaporative cooling towers, adiabatic systems, heat exchangers, and chillers come into play—fundamental components in the thermal architecture of modern data centers.

From the chip to the outside: heat must pass through multiple levels

In liquid systems, the heat generated by the CPU and GPU is transferred to a circuit that carries it to the building's plant engineering. This transfer can occur through cold plates, CDUs, and heat exchangers, all the way to the building's circuit responsible for transporting the thermal energy to the final dissipation system.

The key point is that cooling doesn't end once the server temperature is kept under control. The absorbed heat must still be released into the environment , and the efficiency with which this final step occurs directly impacts the data center's overall power consumption .

With increasing power densities, liquid cooling is becoming increasingly important because liquids can transport heat more efficiently than air. This allows cooling to be brought directly to the most critical heat sources and reduces some of the energy required to move the air.

The most common configurations may include:

• direct-to-chip with cold plate;
• immersion systems;
• water or water-glycol circuits;
• CDU to separate IT circuit and facility circuit;
• air-liquid hybrid systems.

Whatever solution is adopted near the servers, it remains necessary to correctly design the final heat rejection phase.

Dry coolers, evaporative towers or chillers?

The choice of heat rejection technology depends on loop temperatures, climate, water availability, space, redundancy requirements, and energy goals.

• Dry coolers dissipate heat directly into the outside air through finned coils and fans. The circuit remains closed and does not normally require water consumption for evaporation. This solution can be particularly attractive when fluid temperatures are high enough to allow good exchange with the outside air. New architectures for high-density data centers are increasing circuit operating temperatures, making it possible, in some cases, to operate without chillers for very long periods of the year.
• Evaporative cooling towers, on the other hand, exploit water evaporation to achieve lower cooling temperatures than those normally achieved with purely air-cooled systems. They can ensure high performance even with heavy loads, but they introduce water consumption and additional requirements for water treatment and maintenance.
• A middle ground solution is represented by adiabatic coolers , in which the air is pre-cooled through evaporation only when external conditions require it. The goal is to combine some of the benefits of dry cooling with improved performance on hot days.
• The chiller , however, remains necessary when the circuit must be maintained at temperatures lower than those obtainable directly from the ambient air. It offers greater control over operating conditions but introduces the energy consumption of the refrigeration cycle.

The best solution is therefore not universal: the design must find a balance between PUE, water consumption, reliability and local climate conditions.

Higher temperatures may reduce the need for mechanical refrigeration

One of the most interesting trends concerns the increase in temperatures of circuits dedicated to liquid cooling .

When heat is harvested directly from the CPU and GPU, the fluid can exit the racks at much higher temperatures than those typical of traditional chilled water systems. ASHRAE notes that some new architectures can operate with incoming water temperatures up to approximately 45°C and return temperatures up to 65°C.

A higher thermal level creates a greater temperature difference compared to the outside air, facilitating disposal through dry coolers and reducing or eliminating the need for mechanical refrigeration in certain climatic conditions.

This can produce several advantages :

• reduced use of compressors;
• reduction of electricity consumption for cooling;
• possibility of using free cooling more frequently;
• lower water consumption than evaporative systems;
• greater potential for heat recovery.

Recovery represents another important opportunity . The higher the temperature of the available heat, the greater the potential for using it for district heating networks, nearby buildings, or other thermal processes. The U.S. Department of Energy specifically identifies the reuse of waste heat as one of the strategies to consider before final disposal.

Cooling becomes a strategic part of the IT infrastructure

With increasingly powerful data centers, cooling cannot be considered a simple auxiliary system.

The design must take into account IT load, circuit temperatures, water availability, climate conditions, redundancy, and energy consumption. A technology that is highly efficient in a cool climate may be less advantageous in a high-temperature area; similarly, a cooling tower may offer excellent thermal performance but be unsuitable for a water-stressed area.

The growth of liquid cooling also strengthens the relationship between IT and industrial refrigeration. Heat exchangers, pumps, chillers, dry coolers, towers, and control systems become part of an infrastructure that must operate 24/7 with extremely high levels of reliability.

The challenge, therefore, is not just removing heat from the servers, but designing the entire thermal path to the external environment. It is precisely in the management of the circuit and the choice of waste disposal system that a key part of the efficiency of next-generation data centers will be played out.