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Viscosity Control

The operational principal of the capillary viscometer is that a fluid flowing within a tube under laminar flow conditions generates a pressure drop along the tube that is proportional to the length of the tube, the volumetric flow rate within the tube, and the fluid’s viscosity.

The governing relationship that represents the above situation, is Poiseuille’s equation.

To remove the influence of changes in static head along the tube as the fluid’s density changes, the capillary tube is mounted horizontally.

Figure 2.1 below, represent a schematic of the Capillary Viscometer.

The length L, is measured between the centers of the 2 sanitary isolating diaphragms of the differential pressure installation.

The equation that is used to calculate the viscosity of the fluid in the capillary tube is:

Why the capillary viscometer is the “preferred” instrument to use on concentrate feed systems to spray dryers.

  1. To maximize the output of a spray dryer for any set of operating conditions requires that the Total Solids to the dryer is optimized. The limit on the maximum Total Solids is set by the concentrate viscosity, which is mainly determined by the protein content of the concentrate and also by the preheating conditions to which the concentrate has been subjected too.
  2. Having too low a Total Solids and hence viscosity reduces dryer output and produces a finer atomized droplet, whilst too high a Total Solids and hence viscosity produces a larger spray droplet which is difficult to dry and may cause fouling issues within the plant.
  3. Having a reliable measure of the concentrate viscosity to the nozzles of a spray dryer allows the Total solids of the concentrate to be optimized, thus allowing the plant to run at its maximum potential for the prevailing conditions.
  4. The majority of commercially available inline viscometers that are available, use “vibration” to measure viscosity. Unfortunately they are not suitable because of the way the operate. The shear induced by the vibration of the probe, fork or tube varies and results in  a shear rate which is varying. For Newtonian fluids this does not cause an issue, but for milk concentrates which are non-Newtonian this becomes a problem in that the varying shear rate affects the viscosity of the concentrate.  To make matters worse, the shear rate also varies with the concentrate flowrate past the probe. To achieve consistent repeatable results, the concentrate viscosity should be measured under well-defined  shear rate conditions.
  5. The most suitable device to measure concentrate viscosity is a “Capillary Viscometer”. This device provides a “viscosity” output which is measured under constant shear rate conditions. The principle of operation is that a fluid flowing in a tube generates a pressure drop along the tube that is proportional to the length of tube, the volumetric flow rate and the fluid viscosity.

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