Glycol Heat Transfer Fluids – Technical Information

Ethylene and Propylene Glycols

General

Water is probably the most efficient heat-transfer fluid known. If it did not freeze, water would be the ideal heat-transfer fluid for cooling applications. When freeze conditions exist (<32 F), ethylene and propylene glycols can be added to water to provide freeze and burst protection.

Both glycols have lower heat-transfer efficiencies than water and are denser, resulting in higher volumetric flow rates or heat-exchange areas required to maintain the same temperature levels

(see Tables 1 and 2). Higher flow rates lead to higher pressure drops, energy consumption, and equipment wear. As a result, it is important to accurately determine the minimal concentration of glycol needed to do the job in order to maintain system efficiency.

Between the two, ethylene glycol (C₂H₆O₂) is a better heat transfer fluid than propylene glycol (C₃H₈O₂). Propylene glycol is much less toxic and should be used when toxicity is a concern.

Table 1 – Ethylene Glycol versus Propylene Glycol Thermal Conductivities

Table 2 – Flow Increases to Achieve Same Heat Transfer as Pure Water

Makeup Water Quality

High quality water will help maintain system efficiency and prolong glycol fluid life.

• Less than 50 ppm calcium (as CaCO₃),
• Less than 50 ppm magnesium (as CaCO₃),
• Less than 100 ppm total hardness (as CaCO₃),
• Less than 10 ppm chloride (as Cl)
• Less than 25 ppm sulfate (as SO₄).

In the real world it is highly unlikely that such makeup water can be obtained without pretreatment.

Cation exchange to remove the total hardness is highly recommended whenever possible. To reduce chloride and sulfate to such low values, use of reverse osmosis permeate or mixed bed deionization will be required.

Most facilities with water-glycol closed loops just use city water as makeup without any pretreatment. In this case, a corrosion & scale inhibitor product must be added to the water-glycol solution, either as a component of the glycol or as a separate additive to the water-glycol solution.

All glycols produce acids in the presence of air (oxidants). These acids reduce pH which results in corrosion. When the system pH drops below 7, rust will form on any ferrous metal, and

nonferrous metals start to corrode. Due to these reactions, all glycol-water solutions need to have a corrosion & scale inhibitor added. The corrosion & scale inhibitor will also typically be buffered to prevent acidic pH levels from developing.

Bio Control

A minimum of 20% by weight of glycol should be present in ay water-glycol system to prevent biodegradation of the glycol by bacteria which feed on it. If this level cannot be maintained, an effective biocide must be dosed into the system. Typical biocides would be polyquat 10% (PCT 3001) at 250 to 500 mg/l, dibromo nitrlo propionamide 20% (PCT 3014) at 100 to 200 mg/l, isothiazolin 1.5% – copper free (PCT 3020) at 300 to 1,800 mg/l, or glutaraldehyde 15%  (PCT 3021) at 650 to 1,300 mg/l. Do not use any oxidizers or halogens on a closed water-glycol system. Open water-glycol systems can be treated with hydrogen peroxide 32% (PCT 3009) at 20 to 1,000 mg/l.

Concentration of Glycol

The easiest means to determine the % by weight of glycol in a water glycol mix is by determining its specific gravity and using the following Table 3.

Table 3 – % by Weight to Specific Gravity of Water Glycol Solutions

Freeze Protection Versus Burst Protection

Water volume expands by 9% when frozen. Glycols depress water’s freezing point providing protection to temperatures as low as -60 F. Freeze protection prevents ice crystal formation at the lowest temperature expected in the coolant circuit. This type of protection is necessary for year-round pumping. Continuous pumping will also prevent freezing. but is costly and risky because of possible power failures.

Burst protection requires less glycol and allows some freezing to turn the coolant into a slush that is not easily pumped, but will not cause the pipe to burst. This method is used in closed circuits that are not operated in cold weather.

Table 4 – % Wt to Freeze Point

System Monitoring

Glycols can typically be expected to last 12 years or longer, providing corrosion & scale inhibitor levels are maintained and a side steam filter is in use. Inhibitor levels can be easily field checked with typical levels of nitrite based products (PCT 6101) maintained at 400 to 800 mg/l as NO₂ and molybdate based products (PCT 6103) at 100 to 200 mg/l as Mo.

Many “inhibited” glycols marketed have only phosphate compounds, 7,500 to 9,500 mg/l as PO₄,  present as a combination pH buffer and corrosion & scale inhibitor. Putting hard makeup water into systems with such products will often result in formation of calcium phosphate scale. Systems where such products are in use require addition of formulated products to provide adequate corrosion & scale control with most makeup waters.

Glycol fluid pH can be a good barometer for the condition of the water-glycol solution. Although the pH is primarily a function of the corrosion & scale inhibitor and, therefore, will vary from product to product, a few rules of thumb are helpful in determining what constitutes proper pH.

Most concentrated inhibited glycols have a pH in the range of 8.5 to 10.5. A pH reading below 8.5 indicates that a significant portion of the inhibitor has been depleted and that more inhibitor needs to be added.

Caution is required in any system with zinc (galvanize) or aluminum components. Water-glycol solutions should be kept in the pH range of 7.0 to 8.5 and an inhibitor designed for such metallurgy like PCT 6103 or PCT 6108 used.

When the pH falls below 7.0, most manufacturers recommend replacing the fluid. A pH value of less than 7.0 indicates that oxidation of the glycol has occurred. The system should then be drained and flushed before severe damage occurs. Should the system require cleansing after removing old or damaged antifreeze, flush the system with a solution of PCT 6009, 21 to 42 lb/1000 gallons system volume for two to four hours then drain and rinse thoroughly.

Mixing Glycols

Do not mix ethylene and propylene glycols in the same system. Our associated laboratory, ASI, can determine what glycols are present in a system using GC-MS analysis.

Other Considerations

Automatic makeup water systems can easily cause undetected dilution or loss of glycol.

A makeup water proportional add system for the corrosion & scale inhibitor is strongly recommended to avoid problems when an automatic makeup water system is used.

Sidestream filtration is strongly recommended for all water-glycol systems for removal of suspended solids and products of corrosion. Cartridge type units with micron ratings varying from 50 to 1 micron can be used dependent upon the condition of the system when filtration is started.

For very large systems, a multimedia type unit can be used.

ProChemTech International, Inc. “Innovation in Water Management”
Apache Junction, AZ, and Brockway, PA
480-983-5385              www.prochemtech.com          814-265-0959 prochem@prochemtech.com