DOWFROST HD, Heat Transfer Fluid, propylene glycol, Dow, Dowfrost, single fluid process heating and cooling, closed-loop, water-based HVAC applications.
DOWFROST™ HD heat transfer fluid is a formulation of propylene glycol and a specially designed package of industrial corrosion inhibitors. The fluid is dyed fluorescent yellow to aid in leak detection. Suitable applications include: single fluid process heating and cooling, closed-loop, water-based HVAC applications where propylene glycol solutions are preferred or required. Not approved by the USDA or FDA for food use, and is not suitable for food or beverage application.
Suitable applications: single fluid process heating and cooling, closed-loop, water based HVAC applications where propylene glycol solutions are preferred or required. For health and safety information for this product, contact your Hood Chemical sales representative.
The Ground Source Heat Pump (GSHP) industry recognizes the need for an antifreeze or freeze depressant in most installations, however, industry standards for fluid performance in ground source heat pump applications do not currently exist. Historically, fluid decisions have been based on three performance considerations: antifreeze/freeze point depressant capability, heat transfer efficiency, and initial cost to fill the system.
Over the many years Dow has been supplying heat transfer fluids to the HVAC industry, it has become increasingly clearer that a more sophisticated evaluation is needed when selecting fluids for use in ground source heat pumps if system performance is to be optimized and long-term costs minimized. A thorough evaluation requires careful consideration to several key selection criteria.
First and foremost, because this application calls for underground installation of fluids, utmost consideration must be placed on environmental and handling implications. Additionally, while heat transfer efficiency is crucial, fluid selections should also consider longer term issues, such as protecting equipment from loss and/or failure due to corrosion.
An “ideal” fluid for use in GSHP would exhibit the following performance characteristics:
To date, no single fluid is known to exhibit optimum performance in each of the key selection criterion. The paragraphs below compare performance characteristics of DOWFROST™ HD and methanol (selected as the basis for comparison) versus key performance criteria.
Regulatory Considerations – This criterion is high priority because potential leaks could lead to contamination (of both the soil and water supplies). The fluid must be low in toxicity to wildlife, readily biodegradable, and not listed or characterized as hazardous material subject to RCRA regulation (e.g., the flash point must be greater than 140°F). DOWFROST HD fluid satisfies all these requirements. However, methanol is highly toxic with a flash point of 54°F and subject to RCRA regulation even when diluted to 20% by volume concentration. This low flash point causes methanol to be characterized as a hazardous waste upon disposal.
Toxicity and Flammability – In the interest of fire safety, the fluid selected should not have a low flash point (<140°F), and should be low in acute oral toxicity to humans. DOWFROST™ HD fluid is very low in acute oral toxicity, and its primary ingredient (>94% propylene glycol) is an FDA approved food additive. By comparison, methanol is characterized as relatively high in both acute oral and inhalation toxicity. And, due to its low flash point, extra safety precautions should be taken during installation and system operations to guard against fire and protect both the health and safety of those involved.
Efficient Heat Transfer – At the concentration required for typical (25°F) freeze protection, the fluid is not likely to be the limiting factor in heat transfer efficiency for the overall system. Though the heat transfer coefficient of methanol is somewhat better than that of DOWFROST™ HD fluid, in the great majority of GSHP applications, the limiting factor is the heat transfer of the earth/piping or the refrigerant in the evaporator. Therefore, slight differences in fluid heat transfer efficiency do not impact the overall system’s heat transfer. It is possible (even likely) that changing from methanol to DOWFROST™ HD fluid will have no measurable impact on overall system heat transfer efficiency, even though the individual fluid coefficients differ.
Corrosion Protection – In order to ensure the long-term success of the system and its equipment, the fluid must pass ASTM D1384 corrosion evaluation, and should provide 20-year corrosion protection for heat pump systems without any required maintenance during that period. DOWFROST™ HD fluid provides corrosion protection well within the recommended guidelines for typical metals, and can provide 20+ years of worry-free protection. However, when methanol is used, the fluid is often installed without, or with improper, corrosion inhibitors. This could lead to serious corrosion problems and equipment failure.
Proven Utility – The product selected should have demonstrated utility for all above criteria for similar HVAC freeze protection situations. The DOWFROST™ HD formulation has demonstrated its effectiveness for corrosion protection, heat transfer, human safety, and environmental protection in typical HVAC applications over decades of use in the industry.
Total Cost – Cost evaluation should consider the long-term system maintenance and replacement costs, total pumping costs, and initial installation costs. Comparisons which simply look at initial purchase price of the fluid may skew the long-term system economics by ignoring other factors such as maintenance and replacement costs of fluid and equipment.
DOWFROST™ HD glycol-based coolants contain specially formulated industrial inhibitor packages that are effective in preventing corrosion of metals commonly used in HVAC, food processing, and process heat transfer equipment. These inhibitors prevent corrosion of metals in two ways.
First, they passivate the surface of metals, reacting with the surface to prevent acids from attacking it. Unlike inhibitors used in some other fluids, Dow inhibitors perform this passivation process without fouling heat transfer surfaces. The inhibitors in automotive antifreeze, on the other hand, contain silicates that coat heat transfer surfaces with a thick silicate gel that reduces heat transfer.
Second, the inhibitors in DOWFROST™ HDfluids buffer any acids formed as a result of glycol oxidation. All glycols produce organic acids as degradation products. This degradation is accelerated in the presence of oxygen and/or heat. Left in solution, such acids lower pH and contribute to corrosion. Properly formulated inhibitors such as those in DOWFROST™ HD fluids neutralize these acids.
The standard ASTM D1384 corrosion test is a screening test that measures the relative corrosion protection provided by different solutions on standard metals under standard test conditions. The data in Table 7 show relative corrosion rates for DOWFROST™ HD fluids compared to uninhibited propylene glycol and plain water. The data indicate that solutions of DOWFROST™ HD fluids fall well within the generally accepted corrosion limits considered adequate under this test. Rates in excess of 0.5 mpy (2.5 mpy for aluminum) are generally evidence of inadequate corrosion protection. (Since it is only a screening test, ASTM D1384 may not be indicative of performance in an actual system.)
The presence of excessive amounts (>25 ppm) of contaminants such as chlorides, sulfates, and/or ammonia could contribute to system corrosion not evident in these tests. For example, excessive concentrations of chloride ions will result in the formation of iron chloride. With any available oxygen, iron chloride will react to form iron oxide, which is insoluble. The resulting deposition of precipitant forms an area where under-deposit corrosion can occur. This corrosion will be further accelerated by the presence of chlorides and cannot be eliminated through the use of a non-chromate-based inhibitor. This underscores the importance of dilution water quality.