Heat transfer fluids come in many different chemistries, from water-based to organics, depending on the application. The most important characteristics of any industrial heat transfer fluid are:
- It removes or supplies heat quickly and efficiently.
- It does not corrode the materials of construction.
- It does not pose any safety hazards during operation.
Most commonly, water-based solutions are used between the temperatures of -40 to 250°F (-40 to 121°C) for food and beverage, pharmaceutical and HVAC. Mineral oils and synthetic oils generally are used at higher temperatures — in the range of 300 to 750°F (149 to 399°C) — where the system pressure from using superheated water would be too high. At subzero temperatures, hydrocarbons also can be used, but there are more effective options. Glycols and brines are better suited to that range due to their low viscosities and good heat transfer properties. In fact, glycols are most commonly used as an antifreeze additive to water and found in applications such as the coolant in engine blocks and HVAC systems.
Propylene glycol (PG) and ethylene glycol (EG) are the most widely used glycols. But, there is another member of the glycol family that fits the needs of liquid process cooling and heating systems.
Comparing Glycol Fluids for Industrial Heat Transfer
Triethylene glycol (TEG) may be best known for its service as a dehumidifying agent in natural gas applications, but it also can be used in many high temperature heat transfer processes. The most important thermal properties of heat transfer liquids are viscosity, thermal conductivity and specific heat. While heat transfer oils may possess thermal durability that well exceeds the temperature limitations of glycols, glycol-water mixtures offer better heat transfer properties than oils.
Water has the highest thermal conductivity and specific heat of any liquid. Because glycols can leverage water’s advantage, the properties of glycols are much better than the properties of oils. The high heat transfer coefficient of the triethylene glycol-water fluid in comparison to a mineral or synthetic oil results in good system and heater efficiency, fast process temperature responses and reduced pumping power.
For applications running between 200 and 400°F (93 and 204°C), TEG fulfills a suitable niche in the transition temperature range from aqueous solutions to heat transfer oils. Table 1 compares the properties of TEG, PG and EG.
Systems in colder environments also may need freeze protection during plant shutdown. Many heat transfer oils used at higher temperatures will freeze below 32°F (0°C) or have high viscosities that may require immense pumping power during cold starts. A 60 percent by volume TEG solution will not freeze until it reaches -31°F (-35°C). Also, it has lower viscosity than hydrocarbons at colder temperatures, making the cold-start process significantly quicker.
The Science Behind the TEG Molecule
TEG is rarely seen in consumer applications and other typical process cooling because it is more viscous than ethylene glycol and propylene glycol. TEG is a molecule composed of three linked ethylene glycol molecules. Because of the bigger molecule, it is more viscous. This larger molecule provides an advantage as well: it is less prone to thermal degradation in comparison to its monomer.
When a glycol breaks down due to heat, it begins to form acids (glycolic, lactic, oxalic, acetic and formic acids). The glycol slowly evolves into a pale yellow and eventually a dark orange color, depending on the temperature and length of use. Due to the acid formation, the pH will drop. If the glycol fluid is not reinhibited and buffered, there is the potential for a runaway reaction to occur, where the glycol is so acidic it will begin to corrode metals.
The longer chain length of the TEG molecule allows the vibrational energy that is generated from heat to be dissipated and diluted throughout the TEG backbone. This reduces the rate of degradation and acid formation. Glycols running continuously above 200°F (93°C) must be closely monitored and sampled for quality control periodically. Also, they must be checked regularly for inhibitor content because it will be consumed over time. This is the same for TEG, but it will degrade slower than the other glycols. TEG needs less adjustment and fewer replacement over the lifetime of a system.
Thermal Stability Testing of TEG
Ethylene and propylene glycol are limited to about 300°F (149°C) with special inhibitor packaging. When operating near or above this temperature, EG and PG will degrade and need to be replaced.
The engineers at one heat transfer fluid manufacturer tested TEG to see how it compares to EG and PG. The TEG fluid was tested in the 350 to 400°F (149 to 204°C) temperature range to determine if it could be feasibly used at these high temperatures.
A two-loop, single heat exchanger system was constructed (figures 1 and 2). The hot loop used the fluid manufacturer’s synthetic oil and the test loop used 60 percent by volume TEG inhibited with the heat transfer fluid manufacturer’s inhibitor package). Each loop used a centrifugal pump that provided a fluid velocity of 5.57 ft/sec (1.7 m/sec) through 1” carbon steel piping. The materials of construction were entirely AISI 1020 carbon steel with the exception of components such as the pump housing and measurement devices, which were 316 stainless steel.
During testing, the silicone fluid was heated electrically with a recirculation heater and fed into the jacket side of the heat exchanger. The TEG fed through the tube side. Carbon steel corrosion coupons were placed in the TEG side, and fluid samples were extracted every week. Nitrogen was pressurized in the head space to remove oxygen.
Testing on the TEG started around 356°F (180°C) and was held for two weeks. After samples were taken, the temperature was incrementally increased to 392°F (200°C). After another two weeks, a sample was taken, and the temperature was increased to 415°F for two weeks. The final increment was at 425°F (213°C) for two weeks. Throughout the testing, due to the vapor pressure of TEG, the maximum pressure the system experienced was 275 psia at 425°F (213°C).
Several fluid properties were monitored: density, pH, iron content, reserve alkalinity, solids content and color. The carbon steel coupons were checked for weight loss, surface topography and surface corrosion and discoloration.
Once the test was completed, the thermal stability results of the TEG were compiled (figure 3). The AISI 1020 carbon steel coupon results with the TEG fluid are shown in figure 4, where weight gain indicates that a scale or layer was building on the coupon surface.
Testing showed that thermal breakdown of the TEG fluid started to occur quickly when the test was turned up to 415°F (213°C). After holding it at 415°F (213°C), the pH drop and reserve alkalinity drop were both evident as more acids began to form. The fluid started to turn to a dark orange color. At 392°F (200°C), the properties of the TEG fluid essentially remained constant with a slight dip in reserve alkalinity; however, this stabilized while testing continuously at 392°F (200°C). There was no corrosion observed in the carbon steel test coupons because the inhibitor package was effective at preventing corrosion throughout the high temperature test.
Testing demonstrates that TEG is an effective heat transfer fluid capable of withstanding temperatures up to 390°F (199°C). If using TEG continuously or intermittently near 390°F (199°C), fluid samples must be taken every few months to check for fluid quality and potential degradation. While at high temperatures, TEG is a more effective heat transfer fluid than propylene glycol and ethylene glycol, it may need to be periodically re-inhibited to maintain stable pH levels and prevent corrosion.
In conclusion, TEG can serve as an effective heat transfer replacement for hydrocarbon-based heat transfer fluids for temperatures up to around 390°F (199°C). Due to its thermophysical properties, TEG can provide exceptional heat transfer ability and freeze protection down to -31°F (-35°C) for very cold environments if needed.