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Glycol & Low-Temperature Guide

Choosing a Glycol Percentage

Freeze protection against heat-transfer performance

Overview

More glycol isn't automatically better - it's a tradeoff between freeze protection and heat-transfer performance in an Advantage chiller loop.

Glycol lowers the freeze point of the fluid circulating through an Advantage chiller loop, which is what makes sub-freezing process temperatures and outdoor winter operation possible in the first place. As a rough guide, around 25% glycol protects to roughly 15°F, around 40% protects down to roughly -5°F, and 50% or more gets into the -25 to -35°F range depending on the specific glycol type - these are general, rounded figures for planning, not a substitute for confirming the right fluid and percentage with us for your process.

The tradeoff is that glycol doesn't carry heat as efficiently as plain water, and the higher the concentration, the more pronounced that effect becomes - heat transfer drops and the pump needs more horsepower to move the same flow rate through the loop. Running a higher percentage than the process requires costs performance and energy for no benefit.

The right approach is to size around the minimum glycol percentage that protects against the actual lowest expected temperature or process requirement - not the highest percentage available or a round number picked out of habit. If the process fluid itself needs to run below 32°F, that required temperature (not the ambient outdoor low) is usually the number that sets the percentage; if the concern is purely outdoor freeze protection during winter shutdown, the ambient design low is the relevant number instead.

Propylene glycol and ethylene glycol behave differently at the same concentration and have different suitability depending on the process, so confirm which one applies before finalizing a percentage. Propylene is the standard choice anywhere food, beverage or pharmaceutical contact risk is a factor, since it's non-toxic; ethylene glycol is more common on purely industrial loops with no contact risk, since it costs less and transfers heat slightly more efficiently at a given concentration. Mixing the two fluids in one system, or on one site across separate loops that could ever cross-connect, is a mistake worth designing against from the start. See Glycol Chillers for the Advantage indoor and outdoor packages built to run these solutions.

Freeze pointnoun

The temperature below which the circulating fluid freezes. Glycol lowers it, which is what makes sub-freezing process temperatures and outdoor winter operation possible - at a cost in heat transfer and pump horsepower as the concentration rises.

Concentration and Protection

Rounded planning figures; confirm the fluid and percentage with us for the actual process.

Approx. Glycol Concentration Approx. Freeze Protection Notes
~25% ~15°F Light freeze protection, minimal effect on heat transfer
~40% ~-5°F Common for moderate sub-freezing process or winter outdoor duty
~50%+ ~-25 to -35°F Range depends on glycol type (propylene vs. ethylene); pump sizing impact is significant

PROPYLENE VS ETHYLENE

Which Glycol Type Applies

The type question comes before the percentage question, since the two fluids don't perform identically at the same concentration and aren't interchangeable once a system is filled. Propylene glycol is the standard for any loop with food, beverage or pharmaceutical contact risk, or anywhere a leak could reach a process, product or people - it's non-toxic, which is the deciding factor irregardless of cost. Ethylene glycol is more common on purely industrial loops with no contact risk, where its slightly lower cost and marginally better heat-transfer performance at a given concentration make it the more economical default.

Once type is settled, percentage follows from the lowest temperature the loop has to survive - whichever is colder between the process fluid's required setpoint and the site's outdoor freeze exposure, if the unit is outdoors. Send us both numbers and your glycol type preference (or contact-risk requirement) and we'll confirm the exact mix for your Advantage chiller.

Send Us

  • Required leaving fluid temperature, if below 32°F
  • Outdoor ambient design low, if the unit is outdoors
  • Any food, beverage or pharmaceutical contact risk
  • Existing glycol type in the loop, if this is a refill or top-off

Running glycol: margins, mixing math and fluid choice

About 9 percent more at a typical mix, before viscosity is counted. The 500 in the usual water formula (500 x GPM x deltaT) assumes plain water; for about 35% propylene glycol near 25°F the factor is closer to 460, because the mix holds less heat per pound. A 10-ton load at a 6°F rise is 40 GPM of water but about 43 to 44 GPM of glycol. The thicker fluid also raises friction through every foot of pipe, so the pump delivers more flow against more head. On an Advantage BG chiller the pump is matched to glycol from the start; on a converted loop, check it.

Roughly 25°F under the coldest setpoint, which is the margin Advantage builds its glycol chillers around. The gap is wide because refrigerant in the evaporator boils well below the leaving fluid temperature, so a mix that freezes near the setpoint can form ice on the plate surface while the outlet reading still looks fine. For the standard 25°F duty of an Advantage BG chiller that puts the freeze point near 0°F, which is why that duty calls for 30-35% inhibited propylene glycol and the 25% line in the table falls well short. An outdoor unit is mixed for the site's winter low when that is colder still.

No, and the difference can save glycol on idle outdoor loops. Below its freeze point a glycol mix turns to slush first; it takes a lower temperature still before it expands enough to split pipe or a heat exchanger. A loop that only has to survive a winter shutdown without damage can be protected against bursting at a lower concentration than one that has to keep flowing. A loop that must start and pump at the cold extreme needs true freeze protection. Say which case applies, because it changes the percentage and, with it, the pump and capacity penalty.

With a refractometer that has a scale for your glycol type - propylene and ethylene read differently, and a hydrometer is unreliable on propylene because its density is so close to water. Check after every top-off, since makeup water dilutes a loop quietly. A 200-gallon loop at 35% that loses 30 gallons to a leak and is refilled with plain water drops to about 30%, and its freeze point rises with it. Keep premixed fluid at the right strength for top-offs rather than adding water or concentrate by eye.

No. Automotive coolant carries silicates and additives meant for engine blocks, which can drop out as gel or deposits on brazed-plate evaporators and pump seals in a process loop. Use an inhibited industrial glycol, propylene or ethylene, sold for heat-transfer duty. Uninhibited glycol is no better, since it ages into acids that attack metal. The inhibitor package is what protects the loop, so the label matters as much as the percentage.

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