Question: Do you know that glycol provides both a freeze point and a burst point? Do you know the difference between them? Do you know which one your system should be protected to? If you answered no to any of those questions, you might want to read on.
We recently recommissioned a project where the owner had a closed-cell fluid cooler sitting outside. The fluid cooler is not needed for heat rejection from the condenser water system during cold weather and a 3-way bypass valve is normally positioned to prevent flow through the fluid cooler during the winter. The building boilers are connected to the same condenser water loop to add heat when needed to maintain the loop temperature within the normal setpoint range.
The sequence of operation in the BAS was partially opening the fluid cooler bypass valve, allowing some flow through the fluid cooler, when the outside air temperature dropped below an adjustable setpoint that we found set to +10°F. They were doing this to prevent bursting the coil in the fluid cooler. After looking through the test reports, we found the glycol concentration in the loop was 30% propylene glycol. So was that sequence needed to protect the equipment?
Solution: Most of you probably know that both ethylene and propylene glycol solutions are commonly used in HVAC hydronic heating and cooling systems to protect systems from damage. They are the “anti-freeze” of the HVAC industry. How much you need (i.e., the concentration of glycol in your glycol/water mixture) depends primarily on three things:
- The type of glycol being used. Ethylene glycol and propylene glycol have different freeze and burst points at the same concentration. Charts are usually readily available from your supplier.
- The coldest temperature that your system may be exposed to. For example, if you have a fluid cooler sitting outside that will remain full of fluid in the winter, the all-time record low outdoor air temp for your climate is what is important. But if you only have coils in air handling units that never see 100% outside air, you may be able to use something higher than that record low outside air temperature based on the worst case you can expect to see entering the coil in the event of damper failures, etc.
- The coldest temperature the fluid will be when you have to pump it.
Let’s first talk about the difference between Freeze and Burst points of glycol solutions. As your glycol solution cools below the Freeze point, ice crystals, or slush starts to form in the piping and it becomes more viscous. This makes it difficult, if not impossible to pump, as the fluid temperature drops further and further below the Freeze point. However, as long as the fluid has not dropped below the Burst point, you do not run the risk of bursting piping, tubes in coils, etc. Burst point is where the fluid expands enough that it may damage piping or coils that are exposed to temperatures below that value.
So, if you test a sample, or have a sample of your system tested by an outside firm or water treatment company, make sure the report clearly identifies if they are providing a Freeze point or a Burst point. If you aren’t sure, ask.
So let’s look at our real case study of 30% (by volume) propylene glycol solution mixed with water. The Freeze point is about +10°F and the Burst point is about -20°F. The design outdoor temperature is about -15°F for this site, but the all-time record low temperature is -31°F. The pumps and bypass valve are located inside the building. They need to pump the fluid within the building all year round, but the outdoor piping and fluid cooler do not need flow in them in the winter. If the bypass valve is closed, the outdoor loop is basically isolated from the indoor loop. So letting the glycol solution in the fluid cooler and outdoor piping drop below the Freeze point is not an issue in this case. Even though slush will form in the outdoor piping, it will not expand to the point of bursting until the temperature drops below the Burst point. The current fluid concentration of 30% protects them down to a -20°F Burst point, so there is some risk to the fluid cooler or outdoor piping if they have extreme cold weather that approaches their all-time record low.
So the recommended solution (no pun intended) for our client is to boost their glycol concentration from the current 30% to 35%, which provides burst protection throughout the range of -40°F to -60°F. If they maintain at least 35%, that provides a good safety factor to the all-time record low of -31°F. Once that is done they can eliminate that logic from the BAS that modulates the bypass valve open based upon outside temperature.
This will reduce unnecessary heat loss to the outside and result in a little more capacity in a system that struggles to keep up with the peak heating demand. For years they have been wasting energy simply because they didn’t understand the difference between Freeze and Burst points and how to apply it to their system. Now they know!
- Always check with your particular glycol manufacturer’s charts and be sure that you are mixing the solutions to the right percentage based upon weight or volume to achieve the desired protection levels for your system.
- If you have a system containing glycol that is connected to an automatic fill system providing plain makeup water, it should be disconnected whenever possible. All makeup to glycol based systems should be a pre-mixed glycol solution of the same concentration as the system itself to maintain the proper freeze and burst protection.