PG25 vs Non-Polar Coolant Filter Requirements | Filson

You may find that, in today’s AI liquid cooling systems, two typical liquid cooling methods are widely used: cold plate cooling and immersion cooling. The main difference between them lies in whether the coolant comes into direct contact with the electronic components (CPU/GPU).

PG25 and non-polar coolants are the two types of coolants used in these distinct liquid cooling systems, and they differ significantly in composition, performance, and application methods—which, in turn, places different demands on the filters integrated into each cooling system. Let’s explore this further in this blog post.

What is PG25 Coolant?

PG25 is a type of polar coolants that typically consists of a mixture of 25% propylene glycol and water, with the addition of corrosion inhibitors, pH buffers, and dispersants. Due to its conductive properties, this water-based coolant is suitable only for non-contact liquid cooling systems, e.g. cold plate cooling or direct-to-chip (D2C) cooling.

Other common water-based cooling liquids also involve EG25 (25% ethylene glycol) coolants, PG50/EG50 coolants, but why PG25 is a top-priority option for cold plate cooling? It is essentially a focus on environmental safety and low cost.

* PG50/EG50: 50% concentration of the diol, the freezing point can be lowered to around -35 °C, particularly suitable for extremely cold regions with high frost protection requirements.

What is Non-Polar Coolant?

Non-polar coolants are usually dielectric fluids, including mineral oil, synthetic hydrocarbons, or fluorinated fluids. The most important characteristic of them is excellent electrical insulation properties, which is also the key distinction compared to PG25 and other water-based coolants. This allows them to be used directly in immersion cooling—that is, by submerging servers entirely into the coolant.

PG25 Coolant Vs. Non-Polar Coolant

Even though non-polar coolants clearly outperform PG-25 in terms of heat dissipation efficiency and temperature control, why does PG-25 still dominate the liquid cooling market?

Actually it’s a battle between cold plate cooling and immersion cooling.

▶High Thermal Conductivity

Water typically has a thermal conductivity 5 to 10 times greater than that of non-polar coolants, that means easier to cool the GPUs with high heat flux density, lower flow rate requirements, and lower pump power. That’s why you can see almost cold plate systems from NVIDIA, AMD, or Intel adopt water-based architecture.

▶Low Viscosity

PG25 coolant features much lower viscosity than that of the non-polar coolants, giving it better flow properties. As a result, it flows more easily through the microchannels of cold plates, resulting in lower pressure drop and reduced demands on the pumps in the coolant distribution unit (CDU), as well as lower power consumption.

▶Low Cost

The unit price of PG25 coolant may be only 1/10 to 1/30 that of non-polar coolants. For large AI data centers, which often require tens of tons of coolant, so changing to immersion cooling with non-polar coolants represents a significant cost increase.

▶Anti-Freeze & Anti-Corrosion

After adding 25% PG(propylene glycol), the freeze resistance, corrosion inhibition, lubricity, and microbial inhibition of coolant are incredibly improved. Therefore, it is more ideal for various outdoor dry cooler, chilled water system, and cold climates in Europe and North America.

Let’s check their differences more clearly via the chart below:

(Grade: ***** > **** > *** > ** >*)

Cold Plate Cooling Vs. Immersion Cooling: Different Requirements in Filtration Strategies

Filters used to purify circulating coolants are typically integrated into coolant distribution units (CDUs), located on the section side of the CDU pumps or discharge side. As a result, they are commonly referred to as CDU filters.

So, what are the different requirements for the filters integrated into cold plate cooling coolant distribution units (CDUs) and those integrated into immersion cooling coolant distribution units (CDUs)?

▶Material Compatibility

▶For Filter Housings / Elements

CDU filter housings are typically made of stainless steel, but components in PG25 coolant—such as chloride ions and sulfates—can cause severe corrosion to stainless steel. So higher-grade stainless steels, such as 316L which offers superior corrosion resistance, you might find that, are the most ideal and preferred materials for CDU filtration system.

316L stainless steel filter housing is also the optimal option for immersion cooling. Not only offers excellent chemical compatibility with a wide range of non-polar coolants, but it also present a very low risk of contaminating the coolant due to metal ion leaching.

If you’re considering glass fiber filter elements, keep in mind that they can only be used in cold plate cooling systems. Glass fiber is never suitable for immersion cooling, as oils or fluorinated liquids will damage the surface fibers, allowing contaminants to enter the liquid cooling tank and hurt motherboards.

Furthermore, don’t forget the galvanic corrosion between dissimilar metals. Metal combinations with similar electrode potentials, such as stainless steel and copper, or aluminum and its alloys, should be your top priority. If, under certain special circumstances, it is necessary to use metals with different potentials, be sure to isolate them using insulating washers or special coatings.

▶For Sealing Materials

Filters in P25 cooling system primarily use rubber compounds for sealing materials, e.g. EPDM (ethylene propylene diene monomer) or NBR. They are tried and true within cooling liquids containing water and propylene glycol.

Conversely, NEVER use NBR/EPDM seal rings for the filters handling non-polar coolants, since they’re susceptible to corrosion or swelling by oils.

FFKM is a preferred sealing material choice, however requires higher initial costs. Across the entire temperature range, it is virtually impervious to fluorinated liquids, strong acids, and strong alkalis, exhibits virtually no swelling.

Certainly, some FKM/FPM (viton) seal rings can serve as cost-effective alternatives, provided that their compatibility is verified through immersion testing.

The chemical compatibility of all wetted parts must be validated by your coolant supplier.

▶Micron Rating

In cold plate cooling systems that contain PG25 coolant, filters are mainly in charge of intercepting impurity particles including weld slugs, metal fillings, pipeline spalling, and biofilm fragments resulting from microbial growth.

So, the micron rating is usually designed to 50-100 μm or can be down to 25um to maintain smooth flow and optimum thermal performance. Finer designs (< 20 μm) are not in the recommendation since it may lead to raised pressure drop and frequent maintenance.

Non-polar liquids (particularly electronic fluorinated liquids) that come into direct contact with chips or PCBs are extremely sensitive to particulate matter, as minute conductive particles can cause localized electric field breakdown.

Consequently, the filter system for non-polar coolants is typically designed with finer precision, often using micron rating ranging from 10 μm to 25 μm, with the strictest requirements reaching the 1-micron level. For high-precision chip immersion applications, a multistage filtration system may even be implemented.

▶Installation & Layout

Filters in PG25 involved CDUs are commonly positioned upstream of the pump (to protect the pump), at the return inlet of the secondary circuit inside the CDU, or on the piping connecting the CDU to the equipment.

We recommend using a bypass configuration to replace filter elements without shutting down the system, while preventing clogging—which can cause a sharp increase in flow resistance and compromise thermal management performance.

Immersion cooling systems typically employ full-flow filtration. Since nonpolar fluids tend to generate fine suspended impurities during circulation, filters are often located downstream of the pump or configured as a separate bypass purification loop (offline filtration).

Plus, non-polar coolants are highly permeable, so a high level of sealing is required during installation. The entire filter replacement process should be performed in a sealed environment. And you should pay more attention to anti-static, anti-bubble, and leak-proof designs, with provisions for venting and cleaning/maintenance.

Conclusion

In immersion liquid cooling systems that use non-polar coolants, the requirements for filter material, sealing performance, and filtration rating are significantly higher than those for water-based PG25 liquid cooling systems. These systems are also highly sensitive to system sealing and chemical compatibility, so you’re recommended to follow the selection criteria from your coolant supplier.

At 2026, many AI training clusters still rely primarily on cold-plate liquid cooling, but interest in exploring immersion liquid cooling is also growing in many ultra-high-density computing scenarios. Regardless of which liquid cooling method your project chooses, contact FILSON  team via sales@filsonfilters.com or +(86) 157-3695-8886 to discuss any challenges you face in coolant management, particularly regarding filtration.

FAQs

▶Why does immersion liquid cooling place higher demands on O-rings?

In immersion liquid cooling, the coolant is in constant contact with all materials over extended periods. Many dielectric fluids—particularly PAO, synthetic hydrocarbons, and mineral oil—cause rubber to swell. This manifests as softening, expansion, dimensional changes, and permanent compression set in O-rings, ultimately leading to seal failure and leakage.

Immersion systems are constantly exposed to high temperatures ranging from 50 to 70 degrees Celsius, and the combination of heat and chemical fluids accelerates swelling, aging, and the leaching of additives (plasticizers, additives, organic contaminants), which ultimately contaminates the dielectric fluid.

▶Why do more and more AI servers require a filtration rating of 5um or even 1um?

This is an inevitable trend resulting from the evolution of AI GPU cold plates. GPU thermal flux density is skyrocketing; for example, the thermal load of a single NVIDIA GB200 cabinet could reach 120 kW or more, and may even reach 200 kW or 300 kW in the future.

To improve cooling efficiency, the internal channels of cold plates are becoming increasingly narrow, with many now approaching “semiconductor-grade channel dimensions,” where channel sizes may be as small as 100–150 μm. This means that even 20 μm particles could pose a risk.

In that case, traditional CDU filters designed with a 25 μm rating are no longer sufficient; we must develop filtration systems with higher filtration precision to keep pace with the evolution of GPU cold plates. However, this does not simply mean upgrading the filter cartridge’s precision; we must also balance the system’s flow rate and pressure drop. Filson is currently conducting R&D and testing on filters in this direction.