Before deciding on the design and size of a heat exchanger for storing viscous products like ketchup or yogurt, it’s important to understand the product’s basic properties. Here are some questions to ask yourself:
- Is the product’s viscosity due to starch, as in pudding? Enzyme activity , as in Greek yogurt? Or a high solids and/ or fat content, as in baby food?
- Does the product contain particles? If so, how large are they and how sensitive are they to shear and pressure?
- What is heat sensitivity? For example, does the product contain a high protein content that can lead to sedimentation?
- Is this product corrosive? The most common material for food-grade heat exchangers is stainless steel. Depending on corrosion concerns and mechanical strength requirements, other materials such as super-austenitic stainless steel or titanium can also be used.
- And last but not least: What is the apparent viscosity of the product at different temperatures and shear rates?
Measuring the viscosity of a liquid with a standard Brookfield viscometer is not sufficient.
Shear rate can affect viscous foods and typically causes them to become thinner or thicker.
For example, a starch-containing product needs to be tested twice: once during heating to solidification and once during cooling and thickening. You can provide samples of the product to the heat exchanger designer so they can test it at different temperatures and shear rates.
Viscosity plays an important role in the overall heat transfer coefficient and thus determines the heat transfer area required by the heat exchanger for its proper functioning. If the actual viscosity is unknown, this can lead to the selection of a heat exchanger with over- or under-estimated properties.
You need to ask yourself the right questions. Your answers will help you design and size a heat exchanger that delivers the required heat transfer efficiency.