Hot Water & Glycol
U-Tube Heat Exchangers

Hot Water & Glycol U-Tube Heat Exchangers

Closed Loop Heating and Freeze Protection

Hot water and glycol loops do three jobs: they isolate a process from a primary heat source, they carry freeze protection into equipment exposed to ambient, and they distribute heat around a plant from one central source rather than piping steam everywhere.

Glycol is not water with antifreeze in it. It is more viscous, less thermally conductive, and has a lower specific heat. All three work against you at once: the film coefficient falls, the pressure drop rises, and you need more mass flow to move the same heat. A loop converted from water to glycol without resizing the exchanger will not make its duty, and the shortfall is usually larger than people expect.

Size for the cold condition, not the operating point. Glycol viscosity climbs steeply as temperature falls. A loop that is comfortably turbulent at operating temperature can be laminar at startup on a cold morning, and once flow goes laminar the coefficient collapses and warm-up takes far longer than the calculation predicted. Check the design at the coldest realistic starting condition.

Concentration costs performance. Every percentage point of glycol above what the freeze protection genuinely requires costs heat transfer and pumping power for no benefit. Loops are routinely over-concentrated on the theory that more must be safer. Decide the protection temperature you actually need, set concentration to match it, and monitor it — concentration drifts as make-up water is added.

Propylene or ethylene. Propylene glycol wherever any possibility of food or potable contact exists, because it is far less toxic. Ethylene otherwise, since it performs better thermally and is less viscous at equal concentration. Many plants standardise on propylene simply to remove the question from the risk assessment.

Water quality decides exchanger life. A properly treated, genuinely closed loop should stay clean for years. Persistent fouling in a closed system is almost always oxygen ingress and an exhausted inhibitor package rather than an exchanger problem, and corrosion products from that ingress are what foul the surface.

Related pages: process cooling, thermal fluid heating, tube passes and velocity and 304L and 316L construction.