When a syrup batch scales from 500 kg to 1000 kg, the syrup’s own viscosity does not change. Viscosity is set by composition, temperature, and shear rate, not by tank size. What changes is the shear. At the same impeller tip speed, a larger vessel puts less power into each kilogram of syrup, so the average shear rate falls and a non-Newtonian, shear-thinning syrup reads at a higher apparent viscosity in the low-shear regions away from the impeller. The thicker-appearing syrup reduces convective flow and increases resistance to mixing, which in turn lowers the heat transfer coefficient. As a result, the bulk heats and cools more slowly, extending the time required to reach target temperatures during cooking and cooling stages.
Rheological Shifts at Double Scale
The behaviors that shift at double scale include:
- Shear thinning behavior - A shear-thinning syrup thins under shear and thickens when it rests. In the larger vessel, with less power per kilogram, a greater share of the batch sits at low shear near the walls and away from the impeller, so that share reads at the thick end of the syrup’s viscosity curve and the batch feels stiffer.
- Yield stress - The syrup’s yield stress is a material property and does not change when the batch doubles. What shifts is the flow pattern: with less power per kilogram, a yield-stress syrup leaves stagnant regions near the walls, and more agitation force is needed to sweep those regions and restart flow after standing.
- Thermal conductivity - While not a direct rheological property, the higher apparent viscosity reduces the syrup’s ability to carry heat via convection. This changes the effective heat transfer from the jacket or coils.
Impact on Heat Transfer
Heat transfer in a 1000 kg system typically becomes less efficient compared to 500 kg for three reasons:
- The surface-area-to-volume ratio drops - a 1000 kg vessel has roughly 1.6× the surface area of a 500 kg vessel (doubling volume only increases tank diameter by ~26% if height scales proportionally), so less jacket area per kilogram of syrup.
- Higher viscosity reduces the Nusselt number (convective heat transfer coefficient) in laminar or transitional flow regimes, which are common in larger batches due to limited agitation speed.
- The thicker syrup acts as an insulating layer near the vessel walls, causing temperature gradients that slow bulk temperature equilibration.
Effect on Setting Time
Setting time - the period for the syrup to reach a solid or semi-solid state after cooking - is prolonged in the larger batch because:
- Cooling is slower - Reduced heat transfer means the syrup takes longer to reach its set point temperature, delaying crystallization or gelation.
- Nucleation may be retarded - Higher viscosity impedes molecular mobility, which can slow crystal formation for syrups that set via crystallization.
- Temperature uniformity - The larger mass holds more thermal energy, and non-uniform cooling can lead to uneven setting. The center may remain hot and thin while the edges cool and thicken, causing inconsistencies in final texture.
Power per kilogram vs. tip speed when scaling up
The loss of mixing and heat transfer at 1000 kg traces to one choice: running the larger vessel at the same impeller tip speed as the 500 kg vessel. Power per unit volume scales with N³D², where N is impeller speed and D is impeller diameter. At constant tip speed, N falls as 1/D, so power per kilogram falls as 1/D, and a vessel whose diameter grows about 26% when volume doubles delivers roughly a fifth less power per kilogram. Less power per kilogram means less shear and slower circulation. Holding power per kilogram constant instead keeps mixing intensity close to the small batch. That calls for a higher tip speed, which raises shear at the impeller and adds more heat to the syrup. Neither rule is automatically correct; the choice depends on whether the syrup is shear-sensitive and whether the target is blend time, heat-up time, or set time. For a shear-thinning syrup, the safer default is usually to hold power per unit volume constant and then verify heat-up and cool-down curves against the 500 kg baseline.
To mitigate these effects at double scale, process engineers often increase agitation speed (if vessel design allows) or extend cooking/holding times, but both must be managed to avoid scorching or over-concentration. Our experience at KorNutra has shown that careful adjustment of the heating profile and cooling parameters is essential to replicate the same set time and product quality from the smaller batch.