Before deciding on the design and dimensions of a heat exchanger to handle viscous products such as ketchup or yoghurt, you need to know the key characteristics of the product. Here are the questions you need to ask:
- Does the product acquire its viscosity from starch, such as pudding; enzymatic action, such as culture in Greek yoghurt; or from a high level of solids and/or fat, such as baby food?
- Does the product contain particles and, if so, what is the size and sensitivity to shear and pressure?
- What is the sensitivity to heat? For example, does the product have a lot of protein, where fouling would be an issue?
- Is the product corrosive in any way? The most common material for the construction of heat exchangers for food processing is stainless steel. Other materials such as Superaustenitic stainless steel or titanium are used when necessary, depending on corrosion concerns as well as mechanical strength requirements.
- Last but not least, what is the “apparent” viscosity of the product at various temperatures across varying shear rates?
Just measuring the viscosity of a liquid using a standard Brookfield viscometer is not enough. Shear rates affect viscous food products, which are usually either shear thinning or shear thickening.
For example, products containing starch must have two tests run. One when the product is heated and the starch blooms, and then a second test as the product cools and thickens. You can give a sample of your product to the heat exchanger designer so they can perform the necessary tests over a range of temperatures and shear rates.
Viscosity plays an important part in the overall heat transfer coefficient, and thus determines the amount of heat transfer surface the heat exchanger must have to perform the thermal duty. Not knowing the actual viscosity can result in an oversized or undersized heat exchanger.
You have to ask the right questions. Based on the answers to the questions above, a heat exchanger can be designed and dimensioned to deliver the necessary amount of heat transfer efficiently.
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