Glycols Heat Transfer Fluids: A Look at Thermal Conductivity and Thermal Stability
Thermal conductivity of the three glycols in a typical mixture of 50 percent deionized water and 50 percent glycol is presented in Table 1. The thermal conductivity of ethylene glycol is the best, followed by propylene glycol and the bio-based glycol, which have very similar numbers.
Ethylene glycol is well documented to have better thermal conductivity than propylene glycol. However, it is a toxic material and, for most applications that are involved in a residential or commercial HVAC use, a nontoxic solution is preferred.
The viscosity data for the glycols in table 2 shows that ethylene glycol has the lowest viscosity profile over the temperature range. The bio-based glycol has a lower viscosity than propylene glycol at the lower temperatures and a similar viscosity at the higher temperatures. In lower temperature applications, the bio-based glycol will be more efficiently pumped and have better overall heat transfer coefficient than propylene glycol. For instance, this lower viscosity would be well suited for use in a geothermal system with deep wells, where lower power will be needed to pump the fluid at low temperatures through a number of pipes underground.
|Table 2 shows viscosity data for the glycols.|
Thermal stability of the glycol fluids is an indicator of how durable the heat transfer fluid is. Thermal stability comes into play at higher temperatures - usually near the upper temperature limits of glycols, around 220°F (104°C). As glycols degrade at these higher temperature, or they can form organic acids. These acids, in turn, can cause damage to systems by corroding and fouling heat exchangers and other components in the fluid loop. The fluids also experience diminished heat transfer properties.
Looking at the stability of the glycol chemistries, ethylene glycol degrades the quickest under constant temperature and constant pressure, meaning it might not be the best choice for constant high temperature applications in boiler systems or solar thermal hot water systems. The bio-based glycol has the slowest thermal degradation at about five times less than ethylene glycol and half of propylene glycol. This stability makes it a good choice for higher temperature applications and for extending the life of typical systems.