Common Coolant Types and Their Applications in Liquid Cooling Systems

As the integration level of electronic systems continues to increase, leading to a sustained rise in power density and thermal load, and as specific analytical testing processes become increasingly demanding in terms of temperature stability—aiming to enhance test accuracy and result reliability—more and more equipment designers, R&D laboratories, and testing institutions are beginning to adopt liquid cooling solutions to optimize thermal management performance.

High-quality coolant can enhance the accuracy of experimental results, improve equipment operational efficiency, reduce system downtime, lower maintenance costs, ensure operational safety, and support environmental compliance. However, selecting the right coolant requires a comprehensive consideration of multiple factors, including temperature requirements, heat transfer efficiency, anti-corrosion properties, material compatibility, environmental impact indicators, and safety regulations.

Pure water or deionized water, as the most common and efficient basic cooling medium in circulating cooling systems, has become the benchmark against which the performance of other coolants is measured. Other commonly used coolant types include ethylene glycol, propylene glycol, mineral oil, and dielectric fluids.

The properties of these coolants differ significantly and can even directly influence the overall design architecture of the cooling system. This article will systematically review common types of coolants and explain how to select the optimal coolant based on specific application scenarios.

Coolant and Its Applications

Purified water

In liquid cooling applications, water is suitable for use within a temperature range of 0°C to 100°C (32°F to 212°F). Although purified water has been stripped of chemicals and contaminants, it may still contain trace minerals. Distilled water, as a type of purified water, has all contaminants and minerals completely removed. It’s important to note that while certain impurities can cause corrosion problems, highly pure water—such as highly distilled water—possesses strong ion-adsorption properties that can strip electrons from the metal components of the cooling system, leading to corrosive damage. Therefore, highly distilled water is not recommended for use in recirculating cooling systems.

The water quality of municipal tap water depends on its storage conditions, transmission pipelines, and source type (groundwater or surface water). It may contain corrosive impurities such as chlorides, alkaline carbonates, or suspended particles. If the system contains dissimilar metals, impure water can also create an electrochemical bridging effect, leading to galvanic corrosion.

Circulating water cooling systems are also prone to biological contamination. Depending on the extent to which the system is exposed to light and heat, as well as the nutrient content of wet-zone components, algae, bacteria, or fungi may proliferate. The resulting slime or biofilm can impede heat transfer between the fluid and the surfaces in the wet zone, and may also cause leakage from the mechanical seals of pumps. It is essential to add sufficient additives—such as ethylene glycol, commonly used as a bacteriostat. If the concentration falls below 20%, the bacteriostatic effect will be limited; indeed, when the concentration drops below 1%, propylene glycol and ethylene glycol can actually serve as nutrient media for bacterial growth.

➤ Purified water

➤ Deionized water

Deionized water (DI) removes ionic impurities such as sodium, calcium, iron, copper, chloride, and bromide through a resin-bed filter. This high-purity coolant is suitable for medium- and high-pressure systems and excels particularly in applications where ion-induced corrosion must be prevented. It is widely used in fields such as cooling electronic devices, industrial processes, and laboratory equipment.

Deionized water is an excellent electrical insulator, with an extremely low content of mineral ions that results in very low electrical conductivity. However, even if the wet surfaces in the cooling circuit are passivated, water will continue to adsorb ions from the contact interfaces—because deionized water lacks ion content, it exhibits a strong ion-adsorption effect.

To maintain the dielectric properties of water, it must be continuously circulated through the resin bed. Over time, the resin bed will gradually lose its effectiveness and will need to be periodically regenerated. Mixed-bed regeneration requires a sophisticated system, as cation and anion resins must be treated with different regenerating agents. Contaminants such as oil, sludge, or metal particles can also shorten the lifespan of the resin bed.

Deionized water

➤Ethylene glycol-water mixture

When selecting an ethylene glycol-water mixture, it is essential to take into account a variety of interrelated factors. The most commonly used options are ethylene glycol (EG—widely employed for internal combustion engine cooling) and propylene glycol (PG). These organic compounds can lower the freezing point of water and raise its boiling point, making them suitable for temperature ranges from approximately -50°C to 150°C (-58°F to 302°F). Ethylene glycol is more widely used but exhibits biological toxicity, whereas propylene glycol has lower toxicity and is therefore more appropriate for safety-sensitive applications such as food and pharmaceutical industries.
Propylene glycol has a higher specific heat capacity than ethylene glycol, but it has lower thermal conductivity and higher viscosity. Therefore, ethylene glycol generally offers superior overall performance. Typically, low-concentration mixtures of diols and water are used, because water itself has better basic properties than pure diol solutions—when achieving the same reductions in freezing point, increases in boiling point, and improvements in freeze-thaw resistance, ethylene glycol requires a lower concentration than propylene glycol.

Copper and copper-nickel alloys possess natural anti-corrosion and antibacterial properties. However, similar to aluminum, they require the addition of corrosion inhibitors to prevent acidic corrosion. The concentration of glycol depends on the balance between antifreeze requirements and heat-transfer performance: a higher concentration enhances antifreeze effectiveness but reduces heat-transfer efficiency. It is important to note that the handling and disposal of glycol solutions must comply with environmental safety regulations.

Ethylene glycol-water solution

Propylene glycol-water solution

➤Synthetic mineral oil

The oil-cooled heat exchanger is specifically designed for circulating transformer oil and is ideal for applications where the heat source temperature exceeds the limits of water-based coolants or where insulation properties are required. The mineral oil immersion cooling technology provides uniform heat dissipation to immersed components. Its non-conductive and non-corrosive characteristics are optimized specifically for cooling applications, with an operating temperature range from -40°C to 290°C (-40°F to 554°F). This thermally stable synthetic oil features low reactivity and a slow evaporation rate; it is odorless, non-toxic, and offers significantly better noise control compared to other liquid- or air-cooled systems.

However, mineral oil cooling systems are complex to build and prone to contamination: custom-sealed housings are challenging to design, and the oil can become contaminated with dust, requiring regular cleaning and maintenance. It’s important to note that copper materials and certain elastomers are not suitable for immersion cooling.

Mineral oil

➤Dielectric fluid

As insulating media for equipment such as transformers, capacitors, and high-voltage cables, dielectric fluids include formulations like synthetic oils and fluorinated liquids, with an applicable temperature range from -40°C to 105°C (-40°F to 221°F) or even higher. Selected based on dielectric strength, thermal conductivity, and chemical stability, these engineered fluids—such as Shell Diala S4, XG Galden, or Fluorinert—can enable full-immersion cooling of electronic devices while providing insulation, suppressing corona discharge, and preventing electric arcs.

These non-conductive fluids, which exhibit high chemical stability, generally have viscosities higher than those of water. When selecting a pump, it is essential to verify the flow and pressure characteristics based on the viscosity parameters. Although dielectric fluids offer significant advantages in terms of insulation and heat dissipation, they also present challenges such as system complexity, difficult maintenance, high costs, and potential environmental risks. Therefore, when using these fluids, it is crucial to comprehensively evaluate the specific requirements of the application scenario.

Electrohydrodynamics

Fluid Performance

To evaluate the thermal performance of a fluid, it is necessary to consider parameters such as thermal conductivity, specific heat capacity, density, and viscosity. These properties directly affect the heat transfer efficiency at the fluid–heat-exchange interface. The thermal conductivity coefficient must be determined using correlation equations applicable to specific geometric conditions:
Thermal conductivity: A measure of a fluid's ability to conduct heat (unit: W/mK); higher values indicate more efficient heat transfer.
Specific heat capacity: the amount of energy required to raise the temperature of 1 gram of a substance by 1°C (unit: J/g·K). Water’s specific heat capacity, at 4.186 J/g·K, is the primary reason it is preferred as a coolant.
Fluid density: mass per unit volume (kg/m³). High-density fluids generally have better heat-storage capacity.
Viscosity: A measure of a fluid's resistance to flow (unit: cP), affecting pumping performance and heat transfer efficiency.
Heat transfer coefficient: A comprehensive measure of the heat transfer rate (unit: W/m²·°C); a typical value for water is approximately 1000 W/m²·°C.

Fluid Performance Parameter Table

Material compatibility

The 300-series stainless steels, thanks to their chromium(III) oxide passivation layer, are compatible with the vast majority of heat-transfer fluids. When using deionized water, stainless steel and nickel alloys are ideal materials for wetted areas; however, their thermal conductivity is significantly lower than that of aluminum/copper.

Aluminum alloys have a thermal conductivity ranging from 160 to 210 W/mK, but they are susceptible to corrosion by impurities in non-pure water. Even when using a mixture of diethylene glycol and distilled water, EG and PG can oxidize on the surface of aluminum, forming acidic compounds that necessitate the addition of corrosion inhibitors or anodic oxidation treatment. Copper and copper-nickel alloys possess inherent anti-corrosion and antibacterial properties, yet they still require corrosion inhibitors to protect against acidic corrosion.
The wet-end components of the pump (including seals) must be compatible with both the fluid characteristics and the operating conditions. Contact between dissimilar metals can lead to galvanic corrosion, resulting in seal failure and leakage of toxic fluids.

Cost considerations

Tap water is the lowest-cost option, while the cost of purified water increases as the required purity level rises. Maintenance costs deserve particular attention, covering aspects such as filtration, ion-exchange resin regeneration, cathodic protection, and replenishment for evaporation or leaks. Disposal is also an important factor—ordinary water can be discharged directly, but solutions containing alcohols, organic substances, or any organic fluids must undergo special treatment. For certain coolants that need to be replaced periodically, the disposal costs at the end of their service life may even exceed the initial purchase price.

For systems that are not completely sealed (where there are seams or sealing leaks), the liquid level will decrease over time. When replenishing the coolant, it is crucial to strictly ensure that the concentration matches the existing system. At the same time, it is necessary to monitor organic acids generated by the degradation of diols—by measuring the pH value and indicators of solid/biological contamination, you can determine whether the coolant needs to be replaced.

Conclusion

Selecting a coolant requires a comprehensive understanding of the fluid’s characteristic parameters and thermophysical properties, including operational efficiency, material compatibility, and maintenance considerations. An ideal coolant should combine low cost, non-toxicity, excellent thermophysical characteristics, and long service life. Each coolant has a unique combination of performance attributes—such as thermal conductivity, specific heat capacity, and thermal stability—and the final selection decision ultimately depends on system reliability requirements and economic evaluations.