How do glycols prevent freezing in water treatment?

As outside temperatures drop to below 0oC in the winter months, there is a risk that water recirculating in chilled systems will freeze. In order to prevent the water from freezing, it is often necessary to add an a glycol-based antifreeze. 

There are two main types of glycol that are used as standard within the water treatment industry:

Monoethylene glycol (MEG) - This is the more cost-effective solution, but is highly toxic. About 125ml MEG is a lethal dose for typical sized adult. For this reason, mono propylene glycol (USP grade) should be chosen for any application where there is a possibility of contact with food, pharmaceuticals or humans.

Monopropylene glycol (MPG) – This is a pharmaceutical grade type of glycol with a specified purity of greater than 99.8%.

It is important to note that glycols, in particular their breakdown products, are corrosive to most metals so they should always be used with an appropriate corrosion inhibitor.

The level of either of the above glycol products within a system will affect:

1. The freezing point of the solution within a system.

2. The biostability of a system, i.e. susceptibility to microbial growth.

3. The thermal conductivity/heat transfer capacity of a system. Antifreeze decreases thermal conductivity & specific heat capacity of a system, leading to reduced heat transfer efficiency.

4. Viscosity of the solution to be pumped around the system. Antifreeze increases viscosity which affects pump horsepower and fluid flow which may result in increased power consumption.

Here, we take a look at each of these factors in turn.

1. The freezing point of the solution

Most standard specifications which include MEG will state a concentration of MEG at 25% w/w concentration within the system. This will bring the freeze point down to -12oC which is generally suitable for most UK based situations.

freezing-point-of-monoethylene-glycol

The freeze point depression of MPG is slightly less than that achieved by the addition of MEG. A 30% w/w solution of MPG is required to depress the freeze point of the system down to -12oC.

freezing-point-of-monopropylene-glycol

2. The biostability of a system

When closed circuits contain a minimum of 20% but preferably 25% MEG or MPG, the glycol has a biostatic effect, preventing the growth of bacteria. As concentration falls substantially below 25%, glycol can act as a food source for bacteria. As bacteria consume the glycol they produce acidic bi-products namely glycolic, glyoxylic, formic, carbonic and oxalic acids, thus lowering the pH of the system. This can, in itself, lead to corrosion.

effect-of-glycol-on-bacterial-activity-within-heat-transfer-systems

3. The thermal conductivity/heat transfer capacity of a system

Glycols can be used to decrease the thermal conductivity of the resultant solution. 20% MEG will decrease the thermal conductivity by approximately 10%, while 20% MPG will decrease the thermal conductivity by approximately 17%.

4. Viscosity of the solution to be pumped around the system

Glycols can also be used to increase the viscosity of the resultant solution. 20% MEG will increase the viscosity by 50% while 20% MPG will increase it by approximately 115%.

In order to properly evaluate the required levels of glycol within a system it is important to understand the above impacts that the levels of glycol are likely to have on the system. It is generally the case that a trade-off has to be made between system performance, biostability and freeze protection as previously outlined.

Summary

When all the above parameters are considered it is generally the case that most specifications will include the requirement for a 25% w/w addition of glycol to a system. In order to ensure biodegradation of glycol does not occur, glycol concentrations should not be allowed to fall below 20% in closed circuit systems.

It is essential to install and maintain the correct glycol concentration within a system. Too little glycol can lead to excessive microbial growth, biofilm formation and corrosion. If too much glycol is present it can result in poor heat transfer, increased viscosity and increased energy bills, often together with damage to pumps and valves within the system.

The correct inhibitor must be added to the system with the glycol to prevent corrosion. Inhibited MEG and inhibited MPG are available. In these formulations, the glycol and corrosion inhibitor are combined so that at 25% dose rate, the correct level of inhibitor is also present in the system.

If MPG (USP grade) is required for a specific application, only certain approved inhibitors can be used. The same is the case for biocides. For advice on selecting inhibitors for use in these applications, please contact our technical department.

Product Component Purpose
MEG Monoethylene glycol General purpose antifreeze
MPG Monoproptlene glycol Food grade antifreeze
Inhibited MEG MEG + molybdate-based inhibitor

General purpose inhibited antifreeze

First choice inhibited antifreeze (except for use in food factories)

Inhibited MPG MPG + inhibitor

Food grade inhibited antifreeze

First choice inhibited antifreeze for food factories

Jill Cooper

Jill Cooper

Jill Cooper is Own Label Manager at B&V Chemicals, a leading UK and Export distributor of Water Treatment chemicals for Cooling Water Systems chemicals, Closed Circuit chemicals, Boiler chemicals, Hot and Cold Water System Chemicals, Effluent and Wastewater chemicals. A Microbiologist and Chemist, she has 20+ years production, toll blending and regulatory knowledge.

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