Pros and Cons of Anti-Freeze in Water Coils
Operating water coils that are prone to air temperatures below freezing normally need the use of anti-freeze. The most common is glycol. This has been used for over 40 years, and there are two common types, ethylene and propylene. Anti-freeze provides a safeguard if there are mechanical or electrical breakdowns that may cause fluid to sit in a coil with below freezing air going across it. It doesn’t take long for standing water to freeze when very cold air is delivered at over 500 feet per minute velocity.
A normal mixture of either ethylene or propylene glycol would be 25% for climates at or above 15 degrees F, 35% for 5 degrees F, and 40% down to -5 degrees F. You can also mix up to 50% glycol, which provides protection down to -30 degrees F. Once you exceed a 55% mixture, then the freeze protection temperature goes up. Many cooling coils may be dormant during the winter season, and you can get by with a 20% or 25% mixture. A specific system set up is required to guarantee freeze proofing of dormant coils.
The question that is always asked is, “When should I use ethylene or propylene glycol?” This selection is more about state and local codes and specific industry requirements in that propylene glycol does not have the toxicity of ethylene glycol. Propylene glycol is used in 75% of the marketplace because of its toxicity difference. The selection of propylene does come with some negative aspects which includes a higher fluid volume, reduced heat transfer, and higher fluid side pressure drops.
Here is an example of the operating characteristics of a chilled water coil with 6 rows, 8 fins per inch, 5/8” OD tubes, 30” x 60” finned area, full circuit, 6,000 CFM, 80/67 entering air temperature, and 45 degree entering water temperature. All three selections below will use 41.1 GPM flow which is around a 10-degree fluid temperature difference.
|Leaving Air Temps
|Fluid Pressure Drop (feet)
|Leaving Fluid Temp
|40% Eth. Glycol
|40% Prop. Glycol
Analyzing the three selections above shows that there is a dramatic capacity difference between water and a 40% ethylene glycol mixture. There is also almost as much of a difference between the ethylene and propylene glycol mixtures.
Using ethylene glycol to achieve the same capacity as the water example above (208,491 Total BTUH), you would need to increase the GPM flow from 41.1 to 73 and then your fluid side pressure drop would increase from 3.93 feet up to 9.6 feet. Using propylene glycol to achieve the same capacity as the water example above, you would need to increase the GPM flow from 41.1 to 92 and then your fluid side pressure drop would increase to 14.7 feet.
It is, therefore, very important to have a professional coil engineer select these types of coils, due to the ramifications of increased flow, higher fluid pressure drops, and increased heat transfer surface. Improper selection may increase rows and fins per inch and may cause major problems with inadequate circuit selection.
The use of Sentry Guard Freeze Release Coils may allow for a decrease in the glycol percentage requirement and may also be the best alternative for freeze proofing dormant coils during the winter months.