Bulletin 1034 : Using Glycol In Closed Loops


Jul 21, 2015

Using Glycol In Closed Loops

How does the glycol protect the loop from freezing?

Pure water freezes at 32° F, but a 60% solution of ethylene glycol pushes the freeze point down to -60° F. While the freeze point of pure glycol is only -39° F, the synergy between glycol and water results in a much lower freezing point. This is very important for closed loop systems that may be exposed to freezing conditions.

What is the difference between freeze protection and burst protection? As the temperature of the water-glycol solution falls, the water will begin to freeze and “precipitate” out of solution causing the fluid to become slushy. Most systems can handle short periods of slush if the viscosity remains low. The “slush” period is termed burst protection. As the temperature continues to decrease, the glycol begins to freeze. Unlike water which expands when it freezes, glycol contracts when it freezes. Therefore, when the glycol freezes, the volume in the system actually decreases. With freeze protection, there will not be any freezing of water or glycol in the mixture.

What is the difference between ethylene and propylene glycol?

Both types of glycol will provide adequate freeze protection in most systems. Ethylene glycol is the industry standard for closed loop freeze protection and is the product we use in most applications. Where food or potable water contact may be a concern, specify propylene glycol. Propylene glycol is less toxic, breaks down more rapidly and is more environmentally friendly. Unfortunately, the freeze point of propylene glycol is not as low and its volume remains stable as it freezes and therefore does not offer the burst protection of ethylene glycol. The visual differences may be as follows:

Types of Glycol

  1. Dowtherm SR-1 (ethylene glycol)
  2. Dowtherm 4000 (ethylene glycol)
  3. DowFrost (propylene glycol)
  4. DowFrost HD (propylene glycol for food applications low toxicity
  5. Generic Inhibited Glycol (typically automotive antifreeze)

Color of Glycol

  1. Fluorescent Pink
  2. Fluorescent Orange
  3. Water White (not much color at all)
  4. Bright Yellow
  5. Light Green

Can I use automotive antifreeze instead of Glycol?

While automotive antifreeze does contain glycol, most brands are formulated to protect the aluminum components found in modern cars. The primary corrosion inhibitor in most automotive antifreeze blends is silicate. Silicates tend to form thick, visible passivation films that can adversely effect heat transfer. Silicates also tend to be gritty and can shorten the life of pump seals. Any type of corrosion inhibitor will require periodic replacement or supplementation. Silicate-based corrosion inhibitors have shorter replacement intervals than traditional blends such as SN-7 or SN-10. Most automotive blends also contain fluorescent dyes that make many water chemistry test results impossible to see.

Is there a problem using too much Glycol?

Since glycol has a lower specific heat than water, higher concentrations of glycol in your closed loop water will reduce the heat carrying capacity of the system. Too much glycol will therefore increase energy costs as the system works harder to accomplish the desired heating or cooling.

Does the Glycol protect the metal in my systems?

The glycol in a system does not protect any metal from corrosion. It is only the corrosion inhibitors that are added to the glycol that protect the metal. Glycol will not affect plastic but will affect aluminum (above about 150°F) and galvanized steel. The zinc in the galvanizing will react with most inhibitors and cause loss of the zinc coating leading to localized corrosion.

How often do I need to replace my Glycol?

The replacement cycle for glycol depends on the environment in which it is used. If the glycol is exposed to high temperatures (>250°F) or has significant amount of contamination, the cycle will be far more frequent. It’s likely that a truly closed loop with no make up can go for years without changing glycol. However, glycol will break down to glycolic acid over time and will require changing. Even the glycol in your automobile requires periodic changing. Testing for iron, copper and ammonia levels will determine when to change the glycol. Additionally, the pH of the mixture may become acidic which is another strong indication of glycol breakdown.

Can I add SN-7 (or other SN products) to loops already containing inhibited Glycol?

Yes, however if the loop contains one of the special Dow brand glycols, additional inhibitors are not required. Many times the “green” colored glycol will benefit from addition of inhibitors.

How can I test for Glycol?

Using an instrument called a refractometer. It will display the freeze point of the glycol solution and then you can refer to the chart below to determine the percentage of glycol in the sample. Glycol makes closed loop water feel very slippery and it becomes sticky as the water dries. We recommend using ethylene or propylene glycol at 50% by volume. This will afford a freeze protection to -25° F (below zero). The table below will help you calculate how much glycol your system will require.