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Views: 28 Author: Site Editor Publish Time: 2025-04-30 Origin: Site
The digital world runs on data centers, and as our reliance on data grows exponentially, so does the power consumed—and heat generated—by these critical facilities. The rise of Artificial Intelligence (AI), High-Performance Computing (HPC), and increasingly dense server hardware has pushed traditional air-cooling methods to their breaking point. Heat densities within server racks are soaring, making efficient thermal management paramount for operational stability, hardware longevity, and environmental sustainability.
Enter immersion cooling, a transformative approach that moves beyond air and brings liquid directly into contact with heat-generating components. By submerging IT hardware in specialized dielectric (non-conductive) fluids, immersion cooling offers vastly superior heat transfer capabilities. However, "immersion cooling" isn't monolithic; it primarily encompasses two distinct technologies: single-phase immersion cooling (1-PIC) and two-phase immersion cooling (2-PIC). Understanding the fundamental differences, advantages, and trade-offs between these two approaches is crucial for making informed decisions about next-generation data center cooling. This article provides a comprehensive comparison to guide that choice.
In a single-phase immersion cooling system, IT equipment (servers, GPUs, ASICs) is vertically or horizontally submerged in a tank filled with a dielectric coolant. Heat generated by the components transfers directly to the surrounding fluid primarily through convection. Crucially, the coolant remains in its liquid state throughout this primary cooling process – hence, "single-phase."
To remove the absorbed heat, the warmed dielectric fluid is actively circulated by pumps out of the immersion tank. It flows to an external heat rejection unit, typically a Coolant Distribution Unit (CDU), which contains a heat exchanger (often liquid-to-liquid). Here, the heat is transferred from the dielectric coolant to a secondary loop, usually facility water. The now-cooled dielectric fluid is then pumped back into the immersion tank to continue the cooling cycle.
Fluids: Typically utilize hydrocarbon-based engineered fluids (like mineral oils or synthetic oils) characterized by relatively high boiling points, low volatility (reducing evaporative loss), good thermal stability, and often lower cost compared to two-phase fluids.
Hardware: Requires tanks (which can be designed as open baths or sealed units), robust pumps capable of handling the viscosity and flow rates of the dielectric fluid, and external CDUs or heat exchangers to interface with the facility's heat rejection system.
Two-phase immersion cooling also involves submerging hardware in a dielectric fluid, but with a key difference: the fluid is specifically engineered to have a very low boiling point (often around 50-60°C or 120-140°F at operating pressure).
As components heat up, they cause the surrounding fluid to boil directly on their surfaces. This liquid-to-vapor phase change absorbs a large amount of energy (latent heat of vaporization) very efficiently and at a nearly constant temperature. The generated vapor, being less dense, naturally rises to the top of the tank. There, it encounters a condenser (typically coils cooled by facility water or another secondary loop) integrated into the tank lid or upper section. The vapor releases its latent heat to the condenser, changes back into liquid, and then drips back down into the main fluid bath via gravity. This internal boiling/condensation cycle provides the primary cooling mechanism, largely eliminating the need for pumps to circulate the primary dielectric fluid within the tank itself.
Fluids: Employs specialized engineered fluids, predominantly fluorochemicals (historically PFCs, now more commonly HFEs, FKs, or newer proprietary low-GWP formulations like 3M™ Novec™/Fluorinert™ or Chemours™ Opteon™). These have low boiling points, high latent heat values, excellent dielectric strength, but are typically much more expensive and volatile than 1-PIC fluids. Environmental impact (GWP) and potential regulatory scrutiny (PFAS) are significant considerations for certain fluid types.
Hardware: Requires carefully designed tanks that are usually sealed or semi-sealed to manage the vapor pressure and minimize the loss of expensive, volatile fluids. Integrated condensers are critical components. While primary fluid pumping within the tank is often eliminated, pumps are still needed for the external loop that cools the condenser.
Understanding the nuances between these technologies requires a direct comparison across several key areas:
1-PIC: Relies on sensible heat transfer via convection. Efficiency depends on fluid properties (thermal conductivity, specific heat, viscosity) and flow rate generated by pumps. Effective, but less efficient at the point of heat absorption than boiling.
2-PIC: Relies primarily on latent heat transfer via nucleate boiling. This mechanism has inherently higher heat transfer coefficients, allowing for more efficient heat removal directly from component surfaces.
1-PIC: Capable of handling high rack densities, often cited up to 100 kW or even approaching 200 kW per rack, depending on the specific implementation and fluid used.
2-PIC: Generally considered capable of handling higher maximum power densities, often exceeding 200-250 kW per rack and potentially much higher, due to the superior efficiency of boiling heat transfer. This makes it attractive for the most extreme compute-intensive applications.
1-PIC: Can achieve good temperature control, but there will inherently be a temperature rise in the fluid as it flows past components and a temperature gradient across the system.
2-PIC: The boiling process occurs at a near-constant saturation temperature (dependent on pressure). This typically results in excellent temperature uniformity across the surfaces of the immersed components, reducing thermal stress and mitigating hotspots more effectively.
1-PIC: Uses higher boiling point oils/synthetics. Generally lower cost, much lower volatility (minimal evaporative loss), simpler handling, and often fewer environmental concerns (depending on the specific fluid).
2-PIC: Uses low boiling point fluorochemicals. Significantly higher cost, high volatility (requires sealed systems to prevent costly losses), potentially higher GWP for some fluids (though newer options are improving), and regulatory considerations around PFAS chemicals. Excellent dielectric properties are a must.
1-PIC: Simpler concept – requires robust primary loop pumps and external CDUs. Tank design can be less complex (open baths are common, simplifying access).
2-PIC: Eliminates primary loop pumps but requires efficient integrated condensers and sealed/semi-sealed tanks, adding complexity and potentially cost to the tank design itself. Still requires an external loop to cool the condenser.
Both 1-PIC and 2-PIC offer dramatic PUE improvements over traditional air cooling, often achieving figures very close to ideal (1.0).
2-PIC often claims a slight edge (e.g., PUE potentially as low as 1.01-1.02) due to the elimination of primary pumping energy and highly efficient heat transfer.
1-PIC PUE is also excellent (e.g., 1.02-1.03 or better), with the main cooling energy overhead coming from the primary circulation pumps and the external heat rejection system (common to both). Real-world PUE depends heavily on the specific design, scale, and integration with facility cooling.
1-PIC: Often perceived as easier to service. Open bath designs allow direct access to hardware (though fluid dripping needs management). Fluids are less volatile and generally easier to handle.
2-PIC: Maintenance can be more complex. Sealed tanks may need to be opened carefully to minimize vapor loss. Handling volatile fluids requires specific procedures. Some sources claim simpler maintenance as fluid draining isn't required for component swaps, but vapor management remains a key consideration.
Pros:
Simpler system mechanics and potentially lower hardware complexity.
Lower cost and much lower volatility of working fluids.
Easier fluid handling and potentially easier hardware access (open bath).
Mature and widely deployed technology.
Cons:
Lower maximum heat density capability compared to 2-PIC.
Requires significant energy for primary fluid pumping.
Less inherent temperature uniformity compared to boiling.
Pros:
Highest heat transfer efficiency and capability for extreme power densities.
Excellent temperature uniformity due to isothermal boiling.
Eliminates primary fluid pumping energy (though condenser loop still needs cooling).
Potentially the lowest achievable PUE.
Cons:
Higher system complexity (sealed tanks, condensers, vapor management).
Significantly higher fluid cost and high volatility (requires sealing, risk of loss).
Environmental/regulatory concerns associated with some fluorinated fluids (GWP, PFAS).
Potentially more complex maintenance procedures.
The choice between 1-PIC and 2-PIC is not always clear-cut and depends heavily on specific requirements:
1-PIC is often favored for applications requiring significant density improvements over air cooling (up to the ~100-200 kW/rack range) where simplicity, lower initial cost, and ease of operation are high priorities. It's a robust and proven solution for many HPC, AI, and enterprise data centers.
2-PIC is typically considered for applications pushing the absolute boundaries of power density (>200 kW/rack), such as cutting-edge supercomputers, extreme AI/ML training clusters, or specialized high-flux electronics where maximum thermal performance and the lowest possible PUE are paramount, and the higher complexity/cost can be justified.
Factors like budget constraints, operational expertise, tolerance for fluid management complexity, local environmental regulations, and long-term TCO goals all play a critical role in the decision.
The development of immersion cooling fluids is ongoing. For 2-PIC, significant research focuses on creating new fluids with lower GWP, improved material compatibility, and lower costs, while retaining excellent thermal and dielectric properties. Regulations surrounding PFAS chemicals may influence future fluid choices. For 1-PIC, advancements continue in optimizing fluid properties for better thermal conductivity and lower viscosity. We may also see hybrid approaches emerge, attempting to combine benefits from both technologies.
Both single-phase and two-phase immersion cooling represent groundbreaking advances in data center thermal management, offering substantial improvements in efficiency, density, and sustainability compared to traditional air cooling. 1-PIC provides a simpler, often more cost-effective path to high-density liquid cooling using less volatile fluids. 2-PIC offers the pinnacle of heat transfer performance, enabling extreme power densities but comes with greater complexity and considerations around fluid cost, volatility, and environmental impact.
The optimal choice is not universal. It demands a thorough assessment of current and future power density needs, capital and operational budgets, technical expertise, risk tolerance, and sustainability objectives. Careful evaluation and often consultation with cooling experts are essential to select the immersion cooling strategy that best aligns with a facility's specific goals.
As the demand for sophisticated cooling solutions intensifies across data centers and other high-power electronic applications, deep thermal engineering expertise becomes indispensable. At Winshare Thermal, founded in 2009, we are dedicated to designing, simulating, and manufacturing high-performance thermal management solutions capable of meeting these evolving challenges.
Our extensive background in advanced liquid cooling systems provides us with a strong foundation in the core principles critical to deploying effective immersion cooling, whether single-phase or two-phase. This includes expertise in:
Designing high-efficiency heat exchangers, vital components in CDUs (for 1-PIC) and condensers (for 2-PIC).
Optimizing fluid dynamics and heat transfer within complex thermal assemblies.
Conducting detailed thermal simulations (CFD) to predict and validate system performance.
Precision manufacturing of intricate thermal components and assemblies.
While our core focus may be on component and system-level solutions like heat pipe modules, vapor chambers, and custom cold plates, our understanding of heat transfer physics, material science, and quality manufacturing processes (certified to ISO9001, ISO14001, IATF 16949) allows us to contribute effectively to the development and implementation of cutting-edge liquid cooling strategies. We partner with innovators across the new energy, ICT, power electronics, and data center industries to deliver reliable, optimized thermal performance.
Contact Winshare Thermal to discuss how our thermal engineering knowledge and manufacturing capabilities can support your next advanced cooling project.
