Assessing product properties for optimal heat exchanger performance

14 May, 2020

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 feed back the most relevant information to the manufacturer and ensure the correct exchanger is supplied.

Why undertake product assessment?

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; particularly important in the case of food or drink products.

The key aspects of product analysis

The key aspects of product analysis are studying viscosity and flow behaviours, 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:

Viscosity

Density

Shear behaviour

Thermal behaviour (e.g. specific heat, latent heat and thermal conductivity)

To ensure the correct heat exchanger is specified, it isrecommended 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.

How are these parameters assessed?

Like most heat exchange engineers, HRS uses specialist laboratories often attached to universities to perform a range of tests, the exact nature of which depends on the product being tested 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 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 sensitiverheometer which can detect very small changes and differences in zeroshear 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 1degC over temperature can result in scrambled, rather than liquid, eggs.