The Best Heat Transfer Fluids for Liquid Cooling

Once you’ve determined liquid cooling is the solution, do you know what heat transfer fluid to use? One of the most important factors when choosing a liquid cooling technology for your application is the compatibility of the heat transfer fluid with the wetted surfaces of the cooling components (like liquid cold plates or heat exchangers) or system and your application. Heat transfer fluid compatibility is critical in ensuring long-term system reliability. Some other requirements for a heat transfer fluid may include high thermal conductivity and specific heat, low viscosity, low freezing point, high flash point, low corrosivity, low toxicity, and thermal stability1. Based on these criteria, the most commonly used coolants for liquid cooling applications today are:

• Water
• Deionized Water
• Inhibited Glycol and Water Solutions
• Dielectric Fluids

By selecting a compatible pairing of heat transfer fluid and wetted materials you’ll minimize the risk of corrosion as well as optimize thermal performance. Copper is compatible with water and glycol/water solutions and aluminum is compatible with glycol/water solutions, dielectric fluids, and oils. When using deionized water or other corrosive fluids, however, stainless steel is generally recommended since it is more corrosion resistant than other metals. (See Table 1.) Most cooling systems are compatible with water or glycol/water solutions but require special plumbing for compatibility with deionized water or a dielectric fluid such as polyalphaolefin (PAO).

Table 1

Water is one of the best choices for liquid cooling applications due to its high heat capacity and thermal conductivity. It is also compatible with copper, which is one of the best heat transfer materials to use for your fluid path. Water for liquid cooling comes from different sources. Tap water, for example, comes from a publicly owned water treatment facility or a well. The benefit to using facility or tap water is that it is readily available and inexpensive. What is important to note about facility water or tap water is that it is likely untreated and is therefore likely to contain impurities. These impurities could cause corrosion in the liquid cooling loop and/or clog fluid channels. Therefore, using good quality water is recommended in order to minimize corrosion and optimize thermal performance.

Water’s ability to corrode metal can vary considerably depending on its chemical composition. Chloride, for example, is commonly found in tap water and can be corrosive. Facility or tap water should not be used in liquid cooling loops if it contains more than 25 PPM of chloride. The levels of calcium and magnesium in the water also need to be considered, since calcium and magnesium can form scale on metal surfaces and reduce the thermal performance of the components. (See Table 2.)

Table 2

If you determine that your facility water or tap water contains a large percent of minerals, salts, or other impurities, you can either filter the water or opt to purchase filtered or deionized water. If your facility or tap water is relatively pure and meets recommended limits it is still generally recommended that you add a corrosion inhibitor for additional protection. Phosphate is an effective corrosion inhibitor for stainless steel and most aluminum components. It is also effective for pH control. One disadvantage of phosphate, however, is that it precipitates with calcium in hard water. For copper and brass, tolyltriazole is a common and highly effective corrosion inhibitor. For aluminum, organic acids such as 2-ethyl hexanoic or sebacic acid offer protection.

Deionized water is water that has had its ions removed, including sodium, calcium, iron, copper, chloride, and bromide. The deionization process removes harmful minerals, salts, and other impurities that can cause corrosion or scale formation. Compared to tap water and most fluids deionized water has a high resistivity. Deionized water is an excellent insulator which is why it is used in the manufacturing of electrical components where parts must be electrically isolated. However, as water’s resistivity increases, its corrosivity increases as well. Deionized water will pH at approximately 7.0 but will quickly become acidic when exposed to air. The carbon dioxide in air will dissolve in the water, introducing ions and giving an acidic pH of around 5.0. It is necessary to use a corrosion inhibitor when using water that is virtually pure. When using deionized water in a recirculating chiller special high purity plumbing is needed. The fittings should be nickel-plated and the evaporators should be nickel-brazed. When using deionized water in cold plates or heat exchangers, stainless steel tubing is recommended.

The two types of glycol most commonly used for liquid cooling applications are ethylene glycol and water (EGW) and propylene glycol and water (PGW) solutions. Ethylene glycol has desirable thermal properties including a high boiling point, low freezing point, stability over a wide range of temperatures, and high specific heat and thermal conductivity. It also has a low viscosity and, therefore, reduced pumping requirements. Although EGW has more desirable physical properties than PGW, PGW is used in applications where toxicity might be a concern. PGW is generally recognized as safe for use in food or food processing applications and can also be used in enclosed spaces³.

Even though EGW’s thermal conductivity is not as high as water it provides freeze protection that can be beneficial during use or shipping. In fact, ethylene glycol is the chemical used in automotive antifreeze. However, automotive glycol should not be used in a cooling system or heat exchanger because it contains silicate-based rust inhibitors. These inhibitors can gel and foul, coating heat exchanger surfaces and reducing their efficiency. Silicates have also been shown to significantly reduce the lifespan of pump seals. While the wrong inhibitors can cause significant problems, the right inhibitors can prevent corrosion and significantly prolong the life of a liquid cooling loop. Inhibited glycols can be purchased from companies such as Dynalene, Houghton Chemical, or the Dow Chemical Company and are highly recommended over non-inhibited glycols.

As the concentration of glycol in the solution increases the thermal performance of the heat transfer fluid decreases. It is best to use the lowest possible concentration of inhibited glycol necessary to meet your corrosion and freeze protection needs. Dow Chemical recommends a minimum concentration of 25-30% EGW⁴. At this minimum concentration the ethylene glycol also serves as a bactericide and fungicide. With recirculating chillers a solution of 30% ethylene glycol will result in only about a 3% drop in thermal performance over using water alone but will provide corrosion protection as well as freeze protection down to -15°C (5°F).

The quality of the water used in the glycol solution is also important. The water should meet or exceed the limits specified in Table 2, even if you’re using an inhibited glycol. Ions in the water can cause the inhibitor to fall out of solution, resulting in fouling and corrosion.

While the food industry might be more likely to select PGW over EGW for heat transfer, the power electronics, laser, and semiconductor industries might be more likely to choose dielectric fluids over water. A dielectric fluid is non-conductive and therefore preferred over water when working with sensitive electronics. Perfluorinated carbons, such as 3M’s dielectric fluid Fluorinert™, are non-flammable, non-explosive, and thermally stable over a wide range of operating temperatures. Although deionized water is also non-conductive, Fluorinert™ is less corrosive than deionized water and therefore may be a better choice for some applications. However, water has a thermal conductivity of approximately 0.59 W/m°C (0.341 BTU/hr ft °F), while Fluorinert™ FC-77 has a thermal conductivity of only about 0.063 W/m°C (0.036 BTU/hr ft °F).⁵ Fluorinert™ is also much more expensive than deionized water.

PAO is a synthetic hydrocarbon used frequently in military and aerospace applications for its dielectric properties and wide range of operating temperatures. For example, the fire control radars on today’s jet fighters are liquid cooled using PAO. For testing cold plates and heat exchangers that will use PAO as the heat transfer fluid, PAO compatible recirculating chillers are also available. PAO has a thermal conductivity of 0.14 W/m°C (0.081 BTU/hr ft °F). So although dielectric fluids provide low risk liquid cooling for electronics, they generally have a much lower thermal conductivity than water and most water-based solutions.

Water, deionized water, glycol/water solutions, and dielectric fluids such as fluorocarbons and PAO are the heat transfer fluids most commonly used in high performance liquid cooling applications. It’s important to select a heat transfer fluid that is compatible with your fluid path, offers corrosion protection or minimal risk of corrosion, and meets your application’s specific requirements. With the right chemistry, your heat transfer fluid can provide very effective cooling for your liquid cooling loop. For more information on liquid cooling technologies and the proper working fluid to use in your system, contact Aavid, Thermal Division of Boyd Corporation.

¹ Mohapatra, Satish C., “An Overview of Liquid Coolants for Electronics Cooling,” ElectronicsCooling, May 2006, p. 22.

² The Dow Chemical Company, “The Importance of Using Good-Quality Water in Heat Transfer Fluid Solutions,” www.Dow.com , Form No.180-01396-1099QRP, October 1999.

³ The Dow Chemical Company, “How to Choose the Right Heat Transfer Fluid”, Process Heating, January 2008, Troy, MI, p. 52.

⁴ The Dow Chemical Company, “Engineering and Operating Guide for DOWTHERM SR-1 AND DOWTHERM 4000 Inhibited Ethylene Glycol-based Heat Transfer Fluids”, www.Dow.com , Form No. 180-1190-0901 AMS, September 2001, p. 6.

⁵ 3M, “3M Fluorinert™ Electronic Liquid FC-77”, www.3M.com , 98-0212-2309-8 (HB), May 2000, p. 1.