Glycol in heat exchanger coils: propylene vs ethylene glycol

Propylene glycol vs ethylene glycol in a coil loop

Both are used in heating and cooling coil loops. The safety requirement usually decides the fluid; the coil is then calculated for it.

Key facts at a glance

Published freezing points of glycol–water mixtures. Inhibited commercial fluids differ slightly from pure glycol, so use the supplier's data for the product you buy.

• Ethylene glycol: better heat transfer and lower viscosity; moderately toxic if swallowed
• Propylene glycol: low oral toxicity, FDA-affirmed GRAS for food use; more viscous when cold
• 40–50 % mixtures carry roughly 10–17 % less heat per kilogram per degree than water
• Keep inhibited glycol at 25 % or more so the corrosion inhibitors stay effective
• For a loop that must keep running, choose a freezing point at least 3 °C (5 °F) below the lowest temperature

Freeze protection or burst protection

Glycol suppliers distinguish two goals. Freeze protection keeps the fluid liquid and pumpable at the lowest expected temperature. Burst protection accepts that ice crystals and slush form, but leaves enough liquid that expansion does not split pipes or coils. Burst protection needs less glycol, but the loop cannot be relied on to circulate normally while it is in that state.

Guidance published for Dow's DOWFROST propylene glycol fluids recommends a freezing point at least 3 °C (5 °F) below the lowest expected temperature when the loop must keep running. A coil that only has to survive a winter shutdown can be filled for burst protection instead, provided the system has room for expansion.

• Freeze protection: the loop must keep running at the lowest temperature
• Burst protection: the loop may stop, but must not split when cold
• Decide which one applies before choosing the concentration

Propylene glycol or ethylene glycol

Ethylene glycol transfers heat better and is less viscous at low temperature, so it gives the smaller performance penalty. It is rated as moderately toxic if swallowed. Propylene glycol has low oral toxicity and is affirmed by the US FDA as generally recognized as safe for direct food use (21 CFR 184.1666), which is why it is the usual choice where incidental contact with food, beverage or drinking water is possible. Check the documentation of the specific inhibited product, because the inhibitor package matters as well as the glycol.

The trade-off shows up in the coil. In one supplier comparison for an air-cooled heat exchanger at 50 % concentration, ethylene glycol reduced performance by 4.7 % and propylene glycol by 11.7 %, mainly because of propylene glycol's higher viscosity. The figure depends on the exchanger and its operating temperatures, so it has to be recalculated for each coil rather than applied as a rule.

What glycol does to coil sizing

Adding glycol lowers the specific heat of the fluid. Published property tables put the specific heat of 40–50 % mixtures roughly 10–17 % below that of water, so the same flow rate carries less heat for the same temperature change. Higher viscosity also lowers the heat-transfer coefficient inside the tube and raises the pressure drop, most noticeably at low temperature and at start-up.

A coil selected on water therefore does not deliver the same duty on glycol at the same flow and temperatures. The thermal calculation has to use the actual fluid, concentration and temperature range. The result may call for more tube surface, a different circuit arrangement or a higher flow rate.

• Less heat carried per litre for the same temperature change
• Lower heat-transfer coefficient inside the tube, especially when cold
• Higher pressure drop through long or small-bore coil circuits

Use enough glycol, but not more

Each step up in concentration lowers the freezing point but costs heat transfer and pumping power, so the concentration should match the protection actually needed. Closed-loop water-treatment guidance recommends at least 25 % inhibited ethylene or propylene glycol so that the inhibitor package stays effective, and advises against simply diluting below that level.

Concentration tables are published either by volume or by mass, and for propylene glycol the two do not always match. Always state which basis you are using.

Inhibitors, materials and maintenance

Use an inhibited heat-transfer glycol rather than raw glycol. Uninhibited glycol degrades over time into organic acids such as glycolic and formic acid, which lower the pH and attack metals in the loop. Galvanized piping is not recommended with inhibited glycol above about 49 °C (120 °F).

Confirm with the fluid supplier that its inhibitor package covers every metal in the loop, including the coil tube (304 stainless steel or copper), fittings and any aluminium or galvanized parts. Test glycol concentration, pH and inhibitor level regularly; a refractometer gives a quick field check of concentration.

What to send with a coil enquiry for a glycol loop

Uniinox reviews coils for glycol service from the same drawing-based inputs as any other coil, plus the fluid data below. Without them, a quotation can confirm physical fit but not performance.

• Glycol type and concentration, and whether it is by volume or by mass
• Product name of the inhibited fluid, if already chosen
• Lowest ambient or process temperature, and whether freeze or burst protection is needed
• Inlet and outlet temperatures and flow rate on the glycol side
• Allowable pressure drop through the coil
• Liquid, product or air outside the coil, and any cleaning chemicals

Freeze and burst protection concentrations for propylene glycol

Guidance for Dow's inhibited DOWFROST propylene glycol fluid lists recommended concentrations for both goals. Burst protection needs far less glycol at very low temperatures, because the fluid only has to stay as a movable slush rather than a pumpable liquid.

Related: Immersion coil heat exchangers, HVAC coils and finned tube heat exchanger cores