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Thermal conductivity (k) describes how readily a fluid conducts heat. It is measured in W / (m · K) [SI] or BTU / (hr · ft · °F) [Imperial]. Fourier's Law of heat conduction states:
q = −k × A × (ΔT / d)
where q is heat flow rate (W), A is cross-sectional area, and d is the thickness of the medium.
Propylene glycol solutions have significantly lower thermal conductivity than water — roughly 20–30% lower at typical operating concentrations. Lower k reduces the overall heat transfer coefficient (U-value) of a heat exchanger, requiring larger surface areas or higher flow velocities to compensate. Engineers use k alongside viscosity and Cp to compute the Prandtl number and Nusselt number for heat exchanger design.
Note: Thermal conductivity decreases as propylene glycol concentration increases and generally increases with temperature.