Choosing heat exchangers for viscous food products

By Antonio Guillermo Jara Ponce, Systems Sales Manager, HRS Heat Exchangers

Heat exchangers are key components in many processes in the food industry, from heating and cooling to pasteurisation, sterilisation and evaporation. They come in many forms, from the simplest plate heat exchangers to corrugated tube-in-tube and scraped surface designs. With so many different food and drink products in production across the globe, choosing the right heat exchanger for the right product and the right process is vital, not only to ensure effective processing and optimum product quality, but also to maximize operational and energy efficiency.

This is particularly true when dealing with viscous foodstuffs (such as honey, mayonnaise, syrups, purees and nut butters). They require more energy to pump and move and often have a higher potential to foul pipework and heat exchangers, reducing heat transfer and operational efficiency. However, as their texture and viscosity are also important quality characteristics, it is important that processing equipment, including heat exchangers, does not change or disrupt them during production.

What is meant by viscosity?

When considering different sorts of viscous food and drink materials, it is useful to clarify what we mean by viscosity. It is normally defined as a measure of a substance’s resistance to motion under an applied force, based on the amount of force required to remove one layer in relation to another (shear stress) and the change in speed of the layers relative to each other (the shear rate). It is measured in units called centipoise (cP) with one cP being equal to 1 mPa sec-1 (millipascal per second). For example, depending on temperature, milk may have a typical viscosity between one and 20 cP, while molasses may be 5,000 – 10,000, ketchup (which thins when sheared) 10,000 – 18,000 cP, and peanut butter as much as 250,000 cP.

However, the viscosity of different products can change as they are subjected to different levels of sheer stress and temperature. Because of this, most fluids are classified as being either Newtonian, or non-Newtonian. Newtonian fluids have the same viscosity irrespective of changes in temperature or shear stress – the most common example of this is water. Non-Newtonian fluids (which can then be sub-divided into five different categories) have viscosities which fluctuate depending on the shear rate applied.

In practice, this means that when dealing with non-Newtonian products (such as cheese, cream, batter or custard, and certain fat-free products), elements of the processing operation – including pumping, heating, cooling and passing through pipework – all have the potential to affect a product’s viscosity and end quality if not handled correctly.

The HRS AS Series is ideal for products such as thick sauces, honey and syrups

Corrugated tube heat exchangers: design & selection

Choosing the correct type of heat exchanger, together with careful system design, helps avoid such problems. Corrugated tube heat exchangers, such as those designed and produced by HRS, ensure that delicate products such as cream can be processed efficiently without damage. This is because the corrugated tube design helps minimize fouling, increasing thermal efficiency during operation, and extending operational periods between cleanings. In addition, corrugated tube heat exchangers have a lower pumping requirement than smooth tubular heat exchangers due to their compact nature, which results in a lower pressure drop. This helps to increase operational life while reducing maintenance costs compared with other types of heat exchanger.

Products with low or medium viscosities, such as milk, thin sauces, soups and creams, fruit and vegetable juices and purees, can usually be efficiently processed in multi-tube corrugated heat exchangers, like the HRS MI Series. These feature multiple tubes that carry the product within a large vessel which contains the service fluid. Where products contain pieces or particulates, such as fruit and vegetable dices, then a double-tube heat exchanger, like the HRS DTA Series, is recommended. In a double-tube heat exchanger, one large tube carries the product, so there is less chance of blockage and fouling by the particles contained in the food product.

For products such as thick sauces, honey and syrups, an annular-space heat exchanger like the HRS AS Series is preferable. These consist of three or four concentric tubes, with the product flowing through the annular space between the tubes and the service fluid flowing through the inner and outer tubes to ensure even heating and cooling. This arrangement increases both heat transfer and energy efficiency and minimizes potential fouling.

The HRS RHD Series has been developed for products such as nut butters

Scraped surface heat exchangers for the toughest challenges

For the most viscous products, the use of corrugated tubes will not be sufficient to prevent fouling or maintain movement through the exchanger. In these cases, scraped surface heat exchangers (SSHEs) are ideal, particularly for evaporation. At HRS we produce two types of SSHE: the rotating HRS R Series, which includes a unique spiral scraper bar system and baffle placement; and the HRS Unicus Series, which uses a reciprocating movement to mix the fluid whilst cleaning the heat exchange surface.

For extremely viscous products and demanding applications, such as nut butters, a heavy-duty version of the R Series, the HRS RHD Series has been developed. This comprises all the features and benefits of the standard R Series but with increased motor size and scraping rods, together with extra mounting supports for the scrapers and motor.

The separate hydraulic action of the Unicus Series means that the speed of the scrapers (which are available in a number of different designs) is highly controllable and can be optimized for the product being processed. Materials which are susceptible to shear stress or pressure damage can therefore be handled gently to prevent such damage while still providing high levels of heat transfer.

Between our range of corrugated tube heat exchangers, and the three models of SSHE, HRS has a heat transfer solution for any viscous food product, including condensed milk, tomato paste, purees, juice concentrates, honey, butter, margarine, nut butters, chocolate, sauces, minced meats, and many more.