Should I use glycol as a heat transfer fluid?

Although water alone is better at retaining and conducting heat from the associated process, glycol has antifreeze properties, and is more suited to chillers that are expected to function in low-temperature environments, where water alone would freeze and cause obstructions within the chiller.

There are two types of glycol heat transfer fluid, ethylene glycol/water combinations and propylene glycol/water combinations.

Ethylene Glycol

Ethylene glycol is the chemical used in antifreeze. Although similar, it is important to never use automotive glycol as a heat transfer fluid in chillers due to the presence of inhibitors specific to automotive processes that will cause fouling within the chiller system, reducing the lifespan of pump seals and overall efficiency of the chiller. Use of the correct inhibitors within chiller-specific heat transfer fluids will prevent corrosion and prolong the life of the chiller.

Ethylene glycol and water combinations as a heat transfer fluid has several desirable thermal qualities:

• High boiling point
• Low freezing point
• Stability over a wide temperature range
• Low viscosity, reducing pump requirements

When preparing or selecting an ethylene glycol/water combination, it is important to use the lowest concentration of glycol possible to meet the needs of the process. The higher the glycol concentration, the lower the performance of the heat transfer fluid. At a minimum combination of 25-30% ethylene glycol to water, the ethylene glycol will also serve as a bactericide and fungicide, protecting the chiller from microbial contamination.

A recirculating chiller using a 30% ethylene glycol/water combination will result in approximately a 3% reduction in performance. Although the thermal conductivity of ethylene glycol/water combinations is not as great as water alone, the added freeze protection, down to -15°C/5°F, can provide benefit both during use and shipping.

The quality of water used when preparing an ethylene glycol/water combination is important, as the presence of ions within the water may cause the inhibitor to fall out of the solution, leaving the system vulnerable to fouling and corrosion. The minimum requirements for good quality water can be seen below:

Applied Thermal Control stock CoolFlow, an industrial grade refrigerant antifreeze based on ethylene glycol. CoolFlow is sold by ATC in three variations:

• CoolFlow EG 
• (CoolFlow EG Pre-mix)
• CoolFlow 1
• CoolFlow B

All CoolFlow products are classed as harmful if swallowed and may cause irritation to skin and eyes. It is important to wear appropriate personal protective equipment when handling chemicals, including goggles and rubber gloves.

Propylene Glycol

Propylene glycol/water combinations offer many of the same benefits as ethylene glycol/water combinations but are often selected in process that require a food grade heat transfer fluid.

Applied Thermal Control are an exclusive supplier of Hexid, a propylene glycol/water combination. Hexid heat transfer fluid is optimised for temperatures from -45°C to 90°C. This is beneficial for chillers situated outside or in non-heated rooms.

Hexid is fully compatible with system components, protecting even copper and aluminium systems, preventing any leeching of ions into the circuit. Hexid is also able to provide complete protection from freezing and algal growth. A trace biocide is included within the fluid to prevent microbial contamination within the system.

To ensure that the system is running at optimum and the inhibitors within Hexid remain effective, it should be replaced annually. Hexid is safe and easy to dispose of as it is:

• Non-toxic
• Non-flammable
• Environmentally safe
• VOC-free (ozone benign)

Hexid is a cost-effective heat transfer to protect the investments made in chillers at low costs per bottle. It is also easy to store and remains stable for at least two years when stored at ambient temperatures, in closed containers, away from direct sunlight and other sources of UV light.