Choosing between ethylene glycol (Dowcal 100 class) and propylene glycol (Dowcal 200 class) for secondary cooling circuits, how concentration sets freeze protection and penalises pumping, why inhibited fluids and not automotive antifreeze belong in process circuits, and commissioning checks.
Ethylene glycol (EG) — the Dowcal 100 class, stocked as 28 kg concentrate — gives the better thermophysical properties: lower viscosity at low temperature, higher heat capacity per unit flow, smaller pumps. Its drawback is toxicity: it is excluded wherever leakage could reach food, potable water or occupied spaces that drain uncontrolled. Propylene glycol (PG) — the Dowcal 200 class, stocked as 25 kg and 220 kg drum, plus a red-dyed MPG 210 kg — is the food-safe-adjacent choice used in breweries, dairies, fish processing and HVAC circuits in occupied buildings. The decision rule is simply: PG wherever people or product could contact the fluid; EG where the better hydraulics pay and the circuit is contained (industrial and marine machinery circuits).
Freeze protection comes from concentration, and every percentage point costs viscosity and heat capacity. Typical inhibited-glycol planning values (volume %, verify against the fluid maker's tables for the batch in use):
| Vol % | EG freeze point ≈ | PG freeze point ≈ |
|---|---|---|
| 25 % | −12 °C | −10 °C |
| 30 % | −16 °C | −13 °C |
| 40 % | −25 °C | −21 °C |
| 50 % | −37 °C | −33 °C |
Design to hold the fluid ~5 K below the coldest surface temperature in the evaporator (not the air setpoint). Below about −25 °C duty, PG viscosity grows steep enough that EG or a different secondary fluid class is usually the engineering answer. Above 60 % concentration both chemistries lose — freeze point gains reverse and viscosity soars; 50 % is the practical ceiling for most plants.
Process fluids like the Dowcal series carry industrial corrosion-inhibitor packages matched to mixed-metal circuits (steel, copper, brass, solder) and are silicate-free; automotive antifreeze carries silicate/OAT packages designed for engine alloys and service lives, and its additives foul plate heat exchangers and drop out in low-flow process circuits. Never mix inhibitor families: a circuit being converted should be flushed, cleaned and refilled, not topped with a different brand. Make-up water matters equally — use demineralised or at least soft water within the fluid maker's chloride and hardness limits; chlorides drive pitting in stainless plate exchangers.
Secondary circuits pair naturally with the primary-side hardware elsewhere in the catalogue — plate-exchanger condensing units (Tecumseh WLX water-cooled series), water-regulating valves (Penn V46) and the temperature controls in the controls guide.