Water-based heat transfer fluids made from propylene glycol, ethylene glycol, or bio-based glycol are widely used across various applications, including HVAC systems, industrial chillers, process heating and cooling, and data centers. The glycol in these fluids provides critical benefits such as freeze protection at low temperatures, burst protection, an extended operating temperature range, high boiling point, and low vapor pressure. Customers often request inhibited glycol concentrates for their closed loop systems which can then be diluted on-site to achieve the desired glycol concentration. There are several advantages to performing on-site dilution or blending of glycols:
- Cost Savings: By shipping concentrates instead of pre-diluted fluids, customers save on shipping costs, as they are not paying to transport water weight.
- Custom Concentrations: Glycol concentrates can be diluted on-site to a desired concentration depending on the specific thermo-physical properties required, such as freeze point, burst point, and heat transfer efficiency.
- Storage Efficiency: Storing concentrated glycol takes up less space compared to pre-mixed solution, which can be beneficial for facilities with limited storage areas.
- System Adaptation: If system requirements change, the glycol concentration can be modified on-site using either an inhibited glycol concentrate (to increase the concentration) or water (to decrease the concentration).
Selection of the right type of water is crucial for maintaining system efficiency and longevity. In HVAC applications, ASTM D80391 outlines the quality of water recommended for predilution (by manufacturer) as well as for on-site dilution or blending (by end-user) of glycol-based heat transfer fluids used. In the emerging data center market, the majority of direct-to-chip cooling is performed by inhibited propylene glycol blends. Open Compute Project (OCP) has included specifications for water quality in the event that on-site dilution of the concentrate is performed2. Some important criteria for water used for dilution are summarized below in Table 1.
Table 1: Water quality guidelines for dilution of glycol heat transfer fluids.
Water Property | Prediluted (ASTM D8039) | On-site Dilution (ASTM D8039) | On-site dilution (OCP) |
Total Solids | – | 340 ppm | – |
Total Hardness | 20 ppm | 170 ppm | < 50 ppm |
Chloride | 25 ppm | 40 ppm | < 25 ppm |
Sulfate | 50 ppm | 100 ppm | < 25 ppm |
pH | 5.5 to 8.5 | 5.5 to 9.0 | – |
Iron | 1 ppm | – | – |
The water properties in Table 1 such as total hardness, total dissolved solids (TDS), and certain ion concentrations in the water can have a big impact on heat transfer systems.
As seen in Table 1, applications such as cold plate (direct-to-chip) cooling might have more stringent requirements (OCP) compared to general HVAC applications (ASTM).
Effects of poor water quality
Total hardness of water is the amount of dissolved calcium and magnesium present in the water and is expressed as ppm of calcium carbonate. Hard water (> 120 ppm of calcium carbonate) can increase chances of scaling, fouling, and negative interaction with certain corrosion inhibitors. A hardness scale thickness of 0.01 inch on a pipe can lead to a 10% drop in heat transfer efficiency3. A higher pumping-power is then needed to compensate for the loss of efficiency, thereby increasing energy usage. Fouling causes formation, transport, and deposition of particulates which significantly reduces the efficiency of various process equipment4. Use of filtration can help in this circumstance by removing particulates from the system.
Total Dissolved Solids (TDS) is another parameter that should be monitored in different types of water. TDS is directly proportional to the electrical conductivity of the water. A high TDS value means a higher concentration of dissolved ions, which can cause scaling, corrosion deposits and clogging of pipes, leading to excessive maintenance and even system shutdown. Depending on the coolant pH and system temperature, small quantities of dissolved anions such as chloride and sulfate can increase the chances of localized corrosion in copper, steel and aluminum even in the presence of corrosion inhibitors. Copper plumbing pipes frequently experience pitting corrosion due to the presence of chloride, leading to the formation of pinhole leaks in the piping5.
Types of Water
The following is an overview of various types of water: deionized water, distilled water, RO water, well water, and tap water, and their potential effect on closed-loop systems:
Deionized (DI) Water:
Deionized (DI) water is strongly recommended for performing any on-site dilution or blending with glycol, and is preferred by most heat transfer fluid manufacturers when preparing prediluted coolant. The deionization process passes water through mixed-bed ion-exchange resin, ensuring that the result has very low electrical conductivity (< 5 mS/cm) and is free of almost all types of dissolved ions. Using DI water in a heat transfer fluid blend will significantly reduce the chances of corrosion, scaling, and fouling. Use of DI water is essential for low electrical conductivity applications to prevent shorts and thermal runaways in case of accidental spills or leaks.
Distilled Water
Distilled water can be used for blending and dilution of glycol-based heat transfer applications. Distilled Water is produced by boiling water into steam and condensing that steam back into liquid water. The distillation process removes both organic and non-organic dissolved contaminants, resulting in a high-purity water with properties comparable to that of deionized water. However, distilled water is more expensive than DI water due to the energy intensive distillation process.
Reverse Osmosis (RO) Water
Reverse osmosis water is another type of processed water that can be used for on-site blending of glycol. RO water is produced by passing the water through a semi-permeable membrane (such as carbon filter) which allows only the water molecules to go through, blocking dissolved ions and other contaminants. One advantage of RO water is that it can be produced readily from various types of water sources including municipal water, sea water, wastewater, etc., but the purity of the final product is dependent on the type of feed water used. An RO system requires more initial capital and higher maintenance than the more common deionization system, leading to a higher overall cost.
Well Water/Ground Water
Well water or other ground-sourced water is generally not recommended for blending with glycol due to its abundant mineral content. Groundwater is typically rich in dissolved minerals (such as calcium, magnesium, and iron) and other impurities depending on the geology of the area, with a water hardness that is typically > 180 ppm calcium carbonate6 and TDS > 1,000 ppm7. A small percent (2.5%) of wells in urban areas exceeds the US EPA maximum limit of 250 ppm chloride in drinking water, due to various contaminants8. All these minerals and ions can cause corrosion and fouling in the HVAC systems, along with adverse reactions with the inhibitor package.
Due to its low price and ease of availability, especially in rural areas, customers may want to use ground water for dilution purposes. It is recommended to do a quality check on this water and consult with your heat transfer fluid manufacturer before using this water. Purification methods such as particulate filtration, deionization and/or water softening methods are strongly recommended before using ground water in a heat transfer system.
Tap Water/City Water
Tap water should not be the first choice for glycol blending and dilution. While it is treated to remove contaminants harmful to human health, it still contains dissolved ions such as chloride and calcium, which are responsible for pitting corrosion and pipe scaling. Along with natural sources, surface water is also affected by deicing salt runaways, leading to wide variation in the chloride concentrations (average concentrations 100 to 200 ppm chloride)8. Typically, the hardness of tap water is > 100 ppm8 and TDS between 200 to 400 ppm9. Like well water, the quality of tap water can vary by region, and it is necessary to do a water quality check to see if purification may be necessary before using tap water for heat transfer applications.
Summary
In summary, selection of the right type of water is critical for the optimal performance of a heat transfer system, as use of high-purity water reduces scaling, fouling, pitting, and other types of corrosion. Scaling and fouling adversely affect heat transfer efficiency and may lead to frequent, costly extra maintenance of the system. Deionized water is strongly recommended for blending inhibited glycol concentrates. Distilled and RO water can be good alternatives to DI water at an added expense. Since the quality of groundwater and tap water vary from region to region and these types of water sources often have high dissolved mineral and ion concentrations, please consult with your heat transfer manufacturer before using them for blending. DI water can be procured from the heat transfer manufacturer for carrying out on-site dilution.
References:
- ASTM Standard D8039, 2023, ” Standard Specification for Heat Transfer Fluids (HTF) for Heating and Air Conditioning (HVAC) Systems,” ASTM International, West Conshohocken, PA, 2003, DOI: 10.1520/D8039-16, astm.org.
- Open Compute Project. (Year). Guidelines for Using Water-Based Heat Transfer Fluids in Single-Phase Cold Plate-Based Liquid Cooled Racks.
- Association of Water Technologies. Science of scaling. https://www.awt.org/resources/seed-program/water-careers/science-of-scaling/
- Al-Haj, H. (2012). Fouling in Heat Exchangers. InTech. doi: 10.5772/46462
- Lytle, D. A., Williams, D., & White, C. P. (2012). A simple approach to assessing copper pitting corrosion tendencies and developing control strategies. Journal of Water Supply: Research and Technology – Aqua, 61(3), 164-175.
- S. Geological Survey, Hardness of water. https://www.usgs.gov/special-topics/water-science-school/science/hardness-water?form=MG0AV3&form=MG0AV3
- National Ground Water Association, Groundwater quality basics. https://wellowner.org/resources/water-quality/groundwater-quality-basics/
- World Health Organization, Chemical fact sheets: Hardness
- Fondriest Environmental, Inc. (2014, March 3). Conductivity, salinity and total dissolved solids. Fundamentals of Environmental Measurements. https://www.fondriest.com/environmental-measurements/parameters/water-quality/conductivity-salinity-tds.htm
