Options in Heat Transfer Fluids for Low Temperatures
Bio-based, sustainable heat transfer fluids can provide advanced performance.
Are you looking for the performance of an ethylene glycol-based system with the safety and toxicity performance of a propylene glycol-based system? Bio-based alternative fluids have technical properties and functionality compared to petroleum-based counterparts, providing alternatives to the historical debate over which heat transfer fluid to use.
Megatrends across the globe have created a demand for bio-based solutions. And, bio-alternative glycols have provided sustainable options to the heat transfer fluid market. At the same time, newer bio-based fluids based on 1,3 propanediol also offer sustainable fluids while showing improved pumping efficiencies across low temperature heat transfer applications.
For approximately the past decade, bio-based propanediol has been entering the heat transfer fluid market. This material, which is derived from natural corn starch and glucose, has the same chemical formula as propylene glycol, but it has a slightly different structure (figure 1). This modification to the structure gives it thermal stability for high heat applications and a lower viscosity profile.
Choosing a Fluid for a Low Temperature Applications
For most heat transfer applications, water is the most efficient and cost-effective choice for your system. The problem is that water freezes. It is only when the temperature drops below 33°F (0.5°C) that alternative materials such as glycols have to be considered. Plain water not only freezes but tends to be corrosive in chilling and freezing applications.
Heat transfer fluids are used in food processing, commercial refrigeration, geothermal and other low temperature heat transfer applications that typically operate in a temperature range from 0 to 42°F (-18 to 6°C). Most heat transfer fluids have lower heat transfer efficiencies than water and are denser, resulting in the need for more surface area for the heat exchanger or higher volumetric flow rates to maintain the same system temperature. The optimal heat transfer fluid would have a reduced viscosity at low temperatures and would not freeze as water does to maintain system efficiency.
In most low temperature heat transfer applications, ethylene-glycol-based fluids are your best choice because of their superior heat transfer efficiency due to the low viscosity profile. As the fluid is thinner at lower temperatures, the fluid’s performance also reduces the power consumption for recirculation pumps and enables the system to achieve an overall lower minimum operating temperature.
While ethylene glycol has advantages due to its low viscosity profile, the high acute toxicity of ethylene glycol has served to limit of its applications. Propylene glycol is nontoxic. Historically, propylene glycols are targeted for applications in which low acute oral toxicity is a requirement or for freeze-protection applications where incidental contact with food or beverage products is possible.
Propylene glycols do not have the same low viscosity profile as ethylene glycol, negatively impacting pump power consumption, flow rates and pump efficiency. This can be addressed in some cases with special equipment for circulation, by elevating operating temperatures or by lowering the glycol concentration below the manufacturer’s recommended concentration limit. These concessions can lead to lowered freeze protection, increased corrosion potential and microbial growth or contamination because propylene glycols can readily biodegrade at lower concentrations.
Bio-based propanediol also is nontoxic, approved for food contact and already approved as a food ingredient in some countries. It was developed through a joint venture to create sustainable solutions and move away from petroleum-based materials such as glycols. The viscosity profile is lower than propylene glycol but higher than ethylene glycol. Figure 2 compares ethylene glycol, propylene glycol and propanediol low temperature viscosities. Theoretically, based solely on viscosity, propanediol heat transfer fluids would offer slightly less system efficiency than ethylene glycol and enhanced system efficiency compared to propylene glycol.
Case Study: Low Toxicity Fluid Comparison
A case study was completed to compare the performance and operating costs for a food refrigeration system using propanediol or propylene glycol as the heat transfer fluid. The food refrigeration system operated at 32°F (0°C) and used a 5-hp centrifugal pump that circulated a 30 percent solution by volume of propylene glycol or propanediol at a flow (Q) of 175 gallons per minute with a maximum head of 50’.
Pump characteristics are usually based on water at around a temperature of 68°F (20°C), a density of pumped fluid of approximately 998 kg/m3 and a kinematic viscosity of 1 centistrokes (cSt). However, the viscosities of some liquids such as antifreeze solutions increase with lower temperature. For example, aqueous mixtures of 50 percent propylene glycol (50 percent water) experience a change in viscosity by a factor of 10 when the temperature is reduced from approximately room temperature 68 to -4°F (20 to -20°C). In this case, the viscosity of propanediol was 64 percent lower than propylene glycol under the same system operating conditions, which led to an 8.9 percent reduction in power use (figure 3).
Green and Sustainability
In most manufacturing environments, whether or not a heat transfer fluid is “green” is more a question of color for leak detection versus determining if it is bio-based. However, for the past 30 years, regulatory agencies around the world have been working to reduce greenhouse emissions. As commercially viable bio-based alternatives are becoming more readily available in the marketplace, companies are starting to evaluate their own manufacturing sustainability footprints. Considering a sustainable, bio-based heat transfer fluid would be a step in supporting the reduction of greenhouse emissions.
Propandiol is manufactured through a proprietary process that uses glucose from natural raw materials instead of petroleum-based feedstocks. The basic materials can be derived from renewable, farm-grown sources, including yellow dent corn. This makes the promise of carbon neutrality and independence from petroleum a real possibility. By one account, the production of bio-based 1,3 propanediol consumes 40 percent less energy and reduces greenhouse gas emissions by more than 40 percent versus petroleum-based 1,3-propanediol and propylene glycol.*
If you have a system where temperatures are operating less than 33°F (0.5°C), then consider using a heat transfer fluid versus water. Early in the fluid selection process, you should consider what local requirements might impact that choice. Local regulations or a specific application may require that you decide between the use of a toxic material such as ethylene glycol or nontoxic materials such as propanediol or propylene glycol.
Next, evaluate properties such as density, film coefficient, viscosity, pour point and thermal conductivity. Make sure the cooling capability charts offered by your distributor offer glycols and propanediols to select optimum system efficiency based on low viscosity profiles. Also, consider your company’s sustainability platform. Does a bio-based heat transfer fluid match the corporate direction of moving toward sustainable solutions? Throughout this process, remember that now there are bio-based options such as propanediol available for selection.