Glycol Loop Heat Transfer (Capacity Derate)
Also known as glycol derate · propylene glycol heat transfer
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Learning zone
Antifreeze buys you freeze protection and charges you capacity. Propylene glycol is denser than water but has a markedly lower specific heat — a 30 % mix at 40 °C runs about 1025 kg/m³ and 3850 J/(kg·K), so the product ρc falls from water's 4.18 MJ/(m³·K) to about 3.95, roughly a 6 % derate. Push to 50 % PG and you lose closer to 15 %, and the fluid gets thick enough that pump head rises too. That is why the same emitter that made design on water goes cold on the day the glycol truck leaves.
Field practice at HYDRONIC Water Treatment: check the mix with a refractometer, not with the invoice. We have opened plenty of "40 % systems" that measured 22 % because someone topped off a leaking loop with a garden hose for two winters — freeze protection gone, inhibitor package diluted below its threshold, and steel corroding underneath. Worked example: 25 gpm of 64 lb/ft³, 0.92 BTU/(lb·°F) glycol carrying 150,000 BTU/hr needs ΔT = 150,000 ÷ (500 × 0.92 × 0.96 × 25) ≈ 12.7 °F, where the same load on plain water would show 12 °F — a small number that tells you the pump has to move more fluid for the same job.
- = Heat transfer rate
- = Fluid density
- = Fluid specific heat
- = Flow rate
- = Supply-to-return ΔT
- Heat transfer rate — Hydronic Heat Transfer (Water), Newton's Law of Cooling (Q = hAΔT)
- Fluid density — Hydrostatic Pressure (P = ρgh), Pressure Head (h = P/ρg)
- Fluid specific heat — Stream Duty from Mass Flow (Q = ṁcΔT), Fourier Number
- Flow rate — Pump Water Horsepower, Pump Brake Horsepower
- Supply-to-return ΔT — Hydronic Heat Transfer (Water), Air Sensible Heat (1.08 Rule)