In every situation where a heat exchanger is required, the combination of products and service fluids, application, temperature and other variables will be different. Understanding these properties, and how and why they affect heat exchanger performance, will enable project engineers to provide the most relevant information to the manufacturer and ensure a well-suited exchanger is supplied.
Understanding the physical properties of a food product is especially important because it will help to determine the type of heat exchanger for the project. For example, high fouling products such as syrups and thick sauces may require a scraped-surface unit.
Also, understanding the food’s physical properties will ensure that the heat exchange process does not alter the characteristics of the material, which is particularly important in the case of food or drink products.
Key Aspects of Product Analysis
The key aspects of product analysis are studying viscosity and flow behaviors, the study of which is known as rheology. This forms the basis of most of the tests required regarding the products handled by heat exchangers, particularly in terms of consistency. Some of the key measurements that should be taken include:
- Shear behavior.
- Thermal behavior (e.g. specific heat, latent heat and thermal conductivity).
To ensure that an appropriate heat exchanger is specified, it is recommended that the following measurements of different parameters be taken to model the product’s behavior and calculate key parameters:
- Apparent viscosity (the viscosity at a quoted shear rate).
- Heat transfer coefficient (the rate of heat transfer per unit area and unit temperature difference).
- Flow type at different conditions (i.e., whether the product displays smooth laminar flow or turbulent flow).
- Yield stress (a measure of the stress that must be applied to initiate flow of the product).
Using corrugated tubes is thermally beneficial for materials that display transition or turbulent flow.
The way in which a product shears also is important. It can determine the types of equipment to prevent — or encourage — shearing during processing.
The basic type of material also will be a key consideration. For example, is the product is a gel, liquid, emulsion, suspension or something else?
How Are These Parameters Assessed?
Most heat exchange engineers use specialist laboratories — often attached to universities — to perform a range of tests. The exact nature of these tests depends on the product being examined and the potential forces and stresses that it will be subjected to during normal processing.
One of the key items of equipment is a rheometer, a specialist laboratory device that measures the way in which a liquid, suspension or slurry flows in response to applied forces. In order to adequately profile the viscosity and shear rate of different products, particularly where there may be subtle changes, it is important to use a suitably sensitive rheometer, which can detect very small changes and differences in zero-shear viscosity (the point at which viscosity stops increasing with reducing shear rate).
It is also important to determine key thermal limits for many food products. These include:
- Protein denature temperature.
- Starch activation temperature.
- Maillard reaction.
Some heat exchanger manufacturers employ design and modeling software. It uses computational fluid dynamics (CFDs) to predict and study the flow of product through the heat exchanger as well as the thermal changes which occur.
Protein Denature Temperature. The temperature at which proteins in the product are denatured can be tested. Knowing this value can be useful in processes like liquid egg pasteurization, where being one degree over the required temperature can result in scrambled rather than liquid eggs.
Starch Activation Temperature. Once the product has reached this critical temperature, its viscosity increases rapidly.
Maillard Reaction. A chemical reaction that results in food browning, the Maillard reaction is used to provide a distinctive flavor in some products. In other products such as juices and smoothies, however, a fresh non-processed taste is equally important. Almost all food is pasteurized, but understanding the temperature at which the Maillard reaction occurs means you can ensure that an apple-based smoothie tastes like fresh apples rather than toffee apples, for example. Conversely, where browning is required to give the product its taste, it is important to ensure that suitable temperatures are achieved during processing.
For some food products, additional organoleptic testing, including the sensory judgment of how a food feels in the mouth (known as psychorheology), may be required to ensure that processing the product has had the desired effect without any unwanted outcomes on quality. Another measurement that relates to the potential effects of processing is how — or if — the viscosity and structure of the product recovers after processing. This is a property known as thixotropy. Specific methods for assessing this — the viscometric three-step thixotropy test is one example — have been developed.
How Are Product Characteristics Data Used?
Once key parameters such as the viscosity and Non-Newtonian shear thinning factors are known, they can be used to select the type of heat exchanger. For example, corrugated tubes will deliver heat transfer benefits in products that have a Reynolds number above 2,000 and that display transition or turbulent flow characteristics.
The measurements also allow designers to use heat exchanger software to calculate additional information required for the design but that cannot be directly measured in the laboratory. Some of the values that are calculated when designing a heat exchanger include:
- Heat transfer coefficient.
- Flow type.
- Nusselt number (Nu), which is the ratio of convective heat transfer to heat transfer by conduction in the fluid. Higher Nusselt numbers represent effective heat transfer.
- Prandtl number (Pr), which is the ratio of momentum diffusivity to the thermal diffusivity, representing the ratio of heat transfer to fluid motion.
- Reynolds number (Re), which is the ratio between the fluid’s dynamic forces and viscous drag forces. The value indicates the flow regime, i.e., whether the flow can be described as laminar, transitional or turbulent.
The Reynolds Number is one of many parameters that can be measured or calculated by material analysis and is required to design effective heat exchangers.
The various properties of the product also are entered into design and modeling software, which uses computational fluid dynamics (CFD) to predict and study the flow of the product through the heat exchanger and the thermal changes which occur. In order to do this accurately, the design temperature, pressure and maximum allowable pressure drop must be defined for the product and service fluids.
In short, the more information the manufacturer has on the physical properties of the product involved, the more accurate the design of the heat exchanger will be. Engineers and designers can adjust the design of the heat exchanger until the optimal combination of efficiency, productivity and cost is achieved before making any recommendation to the processor.