With liquid cooling, one of the most important decisions is what heat transfer fluid to use in your application. Ethylene glycol (EG) and water solutions and propylene glycol (PG) and water solutions are often used as the coolant in cold plates, cooling systems, chillers, and heat exchangers. Although straight water provides better thermal performance than EG and PG solutions, EG and PG solutions provide corrosion protection and freeze point suppression needed in many applications. In addition to EG and PG, there is now another glycol option for heat transfer applications that many people might not yet be aware of – a 1,3 propanediol (PDO) product derived from corn sugar.

PDO, also known as trimethylene glycol, actually has the same chemical formula as propylene glycol but is different in molecular structure. PDO can be manufactured from petroleum by conventional chemical processing, or can be produced from corn sugar by a bio-processing route that involves fermentation.[i] Since 2006, DuPont Tate & Lyle has been producing Bio-PDO™ for heat transfer applications under the brand name Susterra™. According to the company’s website, “From ‘cradle-to-gate,’ the production of Bio-PDO consumes up to 40% less energy and reduces greenhouse gas emissions by more than 40% versus petroleum-based 1,3-PDO and PG.”[ii] Bio-PDO is also non-flammable, non-toxic, and biodegradable.

There are tradeoffs between EG and PG, which is one reason that some engineers and equipment operators may want to consider Bio-PDO based glycols. EG has desirable chemical properties, including a high boiling point, low freezing point, stability over a wide range of temperatures, and relatively high specific heat and thermal conductivity. If toxicity is a concern, however, engineers and equipment operators may opt for PG instead of EG. PG is generally considered safe for the environment and for food processing applications. PG has a lower thermal conductivity than EG though, resulting in a reduction in thermal performance. In addition, PG has a higher viscosity than EG[iii], which translates to higher power consumption for recirculation pumps and a higher minimum operating temperature if you use PG instead of EG. Bio-PDO solutions offer comparable or better performance than PG solutions while also being non-toxic and renewable.

Dynalene is one of the companies that is using Susterra to manufacture ready-to-use heat transfer fluids. The company’s BioGlycol® product includes Susterra and a proprietary corrosion inhibitor package called PE-1. Dynalene recommends a concentration of 20% or greater of BioGlycol in water solutions for inhibiting corrosion and suppressing the freezing point. According to Dynalene[iv], BioGlycol offers greater thermal stability at high temperatures while possessing similar or better physical properties compared to EG and PG fluids. BioGlycol’s thermal conductivity, for example, is approximately 10% better than PG’s. (See Table 1: Glycol Thermal Performance Comparison.)

Table 1: Glycol Thermal Performance Comparison in 50% Water Solution[v]

Heat Transfer Fluid	Thermal Conductivity (50%)	Specific Heat (50%)
Ethylene Glycol	0.24 BTU/ft-hr-°F	0.77BTU/lb-°F
BioGlycol™	0.21 BTU/ft-hr-°F	0.85 BTU/lb-°F
Propylene Glycol	0.19 BTU/ft-hr-°F	0.86 BTU/lb-°F

In addition, BioGlycol provides 30% lower viscosity at low temperatures as compared to traditional petroleum derived propylene glycol. (See Table 2: Viscosity Comparison for Glycols in 50% Water Solution.)

Table 2: Viscosity (μ) Comparison for Glycols in 50% Water Solution[vi]

Temperature	BioGlycol	PG	EG
32°F / 0°C	12.27	15.94	10.85

There are many coolant options available for your liquid cooling loop. Whichever coolant you choose, it’s critical that your coolant be chemically compatible with all of the materials within your loop. Check with your coolant supplier for information on compatibility or test your coolant in a non-mission critical version of your application. In addition to chemical compatibility, you should review the thermal conductivity, temperature stability, freeze point, viscosity, toxicity, environmental impact, cost, availability, and other chemical properties of your heat transfer fluid.  

For more information on BioGlycol or Dynalene’s heat transfer fluids, please visit http://www.dynalene.com/products/bioglycol.asp or call +1-610-262-9686. For more information on cold plates, chillers, cooling systems, or heat exchangers for liquid cooling or air cooling, please contact Lytron at +1-781-933-7300.

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[i] Mingzhang Wang and Satish C. Mohapatra, Dynalene, “Corrosion Performance of a Bio-Based Glycol for Process Cooling Applications”, NACE International, Paper No. 08378, Houston, Texas, 2008.

[ii] http://www.duponttateandlyle.com/life_cycle.html.

[iii] According to http://physics.info/viscosity/, EGW’s viscosity is approximately 16 η (μPa s) versus PGW’s viscosity, which is approximately 40 η (μPa s) at 25°C.

[iv] http://www.dynalene.com/products/bioglycol.asp.

[v] and [vi] Data provide by Patrick McMullen, Dynalene, http://www.dynalene.com/.