FEATURE HEAT TRANSFER PROPERTIES AND PERFORMANCE
Matt Hale, International sales & marketing Director, HRS Heat Exchangers, says understanding the key properties of a food product will ensure the correct type of heat exchanger is specified
I
n 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 feed back relevant information to the manufacturer and ensure the correct exchanger is supplied. Understanding the physical properties of
a food product will help to determine the best type of heat exchanger for the project. For example, high fouling products such as syrups and thick sauces may require a scraped-surface unit. It will also ensure that the heat exchange process does not alter the characteristics of the material. The key aspects of product analysis are
studying viscosity and flow behaviours, 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 include: • Viscosity • Density • Shear behaviour • Thermal behaviour (e.g. specific heat,
latent heat and thermal conductivity) To ensure the correct heat exchanger is
specified, it is recommended that the following measurements of different parameters are taken to model the product’s behaviour 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 (the stress which must be
applied to initiate flow of the product) The way in which a product shears is
also important and can determine the best type of equipment to prevent (or encourage) shearing during processing. The basic type of material will also be a key consideration – for example, whether the product is a gel, liquid, emulsion, suspension or other. Like most heat exchange engineers, HRS uses specialist laboratories often attached
6 DECEMBER 2019/JANUARY 2020 | PROCESS & CONTROL
to universities to perform a range of tests. One of the key items of equipment is a
rheometer; a laboratory device which 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: The
temperature at which proteins in the product are denatured can also be tested and can be useful in processes like liquid egg pasteurisation, where being 1˚C over 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: This is a chemical
reaction which results in food browning, often providing a distinctive flavour. 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. For some food products, additional
organoleptic testing, including the sensory judgement 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.
HRS 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 client. For example, corrugated tubes will deliver heat transfer benefits in products which have a Reynolds number above 2,000 and which display transition or turbulent flow characteristics
Another measurement which relates to
the potential effects of processing is how (or if) the viscosity and structure of the product recovers after processing; a property known as thixotropy. Specific methods for assessing this have been developed. Once key parameters such as the
viscosity and Non-Newtonian shear thinning factors are known, they can be used to select the best type of heat exchanger. The measurements also allow designers
to use heat exchanger software to calculate additional information which is required for the design, but which cannot be directly measured in the laboratory. These values include: • Heat transfer coefficient • Flow type • Nusselt number (Nu): The ratio of
convective heat transfer to heat transfer by conduction in the fluid. • Prandtl number (Pr): The ratio of
Some of the key measurements of food products relate to viscosity and flow behaviours, and include density, shear behaviour and thermal behaviour
momentum diffusivity to the thermal diffusivity, representing the ratio of heat transfer to fluid motion. • Reynolds number (Re): The ratio
between the fluid’s dynamic forces and viscous drag forces. The various properties of the product are
also entered into 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. 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.
HRS Heat Exchangers
www.hrs-heatexchangers.com
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