Choosing Heat Exchangers for Viscous Food Products

Heat exchangers are key components in many processes in the food industry, from heating and cooling to pasteurisation, sterilisation and evaporation, coming in many forms, from 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 maximise 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 viscosity?

Viscosity 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).

Milk may have a typical viscosity between one and 20 centipose (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.

The viscosity of different products can change as they are subjected to different levels of sheer stress and temperature. Newtonian fluids have the same viscosity irrespective of changes in temperature or shear stress – the most common example of this is water, while non-Newtonian fluids have viscosities which fluctuate depending on the shear rate applied.

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.

 

Corrugated tube heat exchangers: design & selection

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 minimise fouling, increasing thermal efficiency during operation, and extending operational periods between cleanings. Corrugated tube heat exchangers also have a lower pumping requirement than smooth tubular heat exchangers due to their compact nature, which results in a lower pressure drop.

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 minimises potential fouling.

 

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. 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.

 

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.