The core function of any heat transfer fluid is to move thermal energy efficiently from one part of a system to another, either to heat or cool in an industrial process or application.
While water is still recognised as the most cost-effective and efficient heat transfer fluid, it cannot always cope with the extreme operating demands of modern industry. It possesses several limitations, both physical and chemical, that can have a negative impact on key areas of operations. These include mechanical system reliability, thermal performance and maintenance costs. As a result, today’s leading lubrication and industrial fluid manufacturers have formulated alternative fluids that can facilitate improve the effectiveness of heat transfer, but also enhance the basic attributes of water.
Among the alternative heat transfer fluids to water is propylene glycol. As well as being viscous and colourless, propylene glycol is almost completely odourless and possesses just a slight sweet taste. Like all glycols (also referred to as diols) it comprises two alcohol groups and is miscible with a wide range of different solvents, including water. Propylene glycol is non-irritating and non-toxic, and it also has exceptionally low volatility.
In this blog, we’ll examine both water and propylene glycol and answer the common question of which is better.
Water as a heat transfer fluid
Pure water supplies outstanding heat-transfer properties for multiple reasons. A relatively inexpensive fluid, it is less viscous than many other media, which can contribute to improved pumpability and therefore lower pumping costs.
However pure water will freeze at 0°C and this makes it inappropriate for use in systems that are exposed to freezing temperatures and equipment designed to run at below-freezing temperatures. An example of this is refrigeration systems. In such applications, a water and glycol solution is used to provide freeze-protection. Propylene glycol is miscible with water, making it an ideal option for formulating fluids as the correct concentration of glycol within water can deliver freeze protection under temperatures even as low as -51°C.
Freeze protection is vital and when neglected, nature takes its course. Damage like burst pipes is common in freeze-up and can be catastrophic to systems. However, when the system works at temperatures far above water’s freezing point and no risk of freezing exists, pure water is typically the preferred medium.
As a result, when answering whether propylene glycol is better than water as a heat transfer fluid, this is the first point to note. When temperatures of 0°C and below are present, propylene glycol is a superior solution. However, once temperatures drop below freezing, water is a more suitable solution, allowing operators to capitalise on its affordability and improved pumpability.
Propylene glycol in heat transfer fluids
As touched upon, propylene glycol is mixed with water to widen the temperature range at which it can operate, improving its performance as a heat transfer fluid in cooling systems like refrigeration applications. The freezing point of water is effectively lowered when it is mixed with propylene glycol. However, in some applications, propylene glycol is used undiluted.
A common use of a water-glycol mixture is the de-icing of aircrafts. A 50% heated and water-diluted solution is employed for removing icing accretions from commercial aircraft fuselages on the ground (known as the de-icing process). However, an entirely undiluted cold solution is then used on the aircraft’s wing and tail surfaces to mitigate ice from forming during a set period before take-off (known as the anti-icing process).
Mixtures of water and propylene glycol are often dyed pink to show that products are relatively nontoxic and are often sold as marine antifreeze. Propylene glycol is often selected as a substitute to ethylene glycol because of its low toxicity, making it an eco-friendly automotive antifreeze. Additionally, it is sometimes used to winterise plumbing systems in structures that are vacant.
However, operating a system using the proper solution of water and propylene glycol can eliminate any need for special winterisation measures. When properly tended to, the initial fill of a supply of high-quality industrially glycol-based fluid won’t need any significant attention for many years. With proper maintenance, inhibited glycol heat-transfer fluids have been known to last for two decades in some systems.
The main disadvantage of using a propylene glycol-based heat-transfer fluid is its higher viscosity levels compared to a pure water heat transfer fluid. The higher viscosity increases the energy use required to pump the heat transfer fluid throughout the system.
To sum up, the advantages of water over propylene glycol are relatively lower costs and increased pumpability, which leads to reduced energy requirements and operational expenditure. However, water becomes ineffective as a heat transfer fluid under freezing conditions. Propylene glycol can be mixed with water in a precise concentration, which can improve its performance under freezing conditions, making it the most effective heat transfer fluid solution when temperatures of 0°C or lower exist.