When scaling a syrup batch from 500 kg to 1000 kg, the primary rheological change is an increase in apparent viscosity due to the longer time under shear and higher hydrostatic pressure in the larger vessel. This thickening effect is especially pronounced in non-Newtonian syrups (e.g., honey, glucose, or sucrose solutions). The thicker syrup reduces convective flow and increases resistance to mixing, which in turn lowers the heat transfer coefficient. As a result, the bulk syrup heats and cools more slowly, extending the time required to reach target temperatures during cooking and cooling stages.
Rheological Shifts at Double Scale
Key rheological parameters that change include:
- Shear thinning behavior - At larger batch sizes, the longer residence time under low shear (e.g., near vessel walls) allows the syrup to build more structure, increasing viscosity at low shear rates. This can make the syrup appear “stiffer.”
- Yield stress - A larger batch may exhibit a higher static yield stress because the syrup has more time to form a network. This requires greater initial agitation force to restart flow after standing.
- Thermal conductivity - While not a direct rheological property, the increased 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 significantly less efficient compared to 500 kg. Why? Because:
- 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.
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.