Types and Applications of Heat Exchangers in Food Processing

Every time you drink pasteurised milk, open a can of fruit juice, or enjoy a scoop of ice cream, a heat exchanger has played a critical role behind the scenes. Heat exchangers are devices that transfer thermal energy between two or more fluids without allowing them to mix. In food processing, they are essential for operations like pasteurisation, sterilisation, cooling, and concentration – all of which directly affect product safety, shelf life, and quality. Understanding how these devices work, the types available, and the principles governing their performance is fundamental for anyone involved in food engineering.

What is a heat exchanger? 🔗

At its core, a heat exchanger allows heat to flow from a hotter fluid to a cooler one through a separating barrier – typically a thin metal wall. The two fluids never come into direct contact, which is critical in food applications where contamination must be avoided at all costs. The hotter fluid loses thermal energy while the cooler one gains it, and this exchange continues until the desired temperatures are reached.

In practice, food processors use heat exchangers to raise the temperature of milk for pasteurisation, cool freshly extracted juices before packaging, sterilise canned products, and even crystallise ice cream. According to Tetra Pak, there are over 27,000 heat exchangers operating at their customer facilities worldwide, covering virtually every type of food and beverage processing. The sheer diversity of food products – from thin juices to thick sauces – means that no single heat exchanger design suits all applications.

Principles of heat transfer in heat exchangers 🔗

Three key factors govern how efficiently a heat exchanger performs: temperature difference, heat transfer surface area, and the overall heat transfer coefficient. Together, these determine the rate at which thermal energy moves between fluids.

Temperature difference (ΔT) 🔗

Heat naturally flows from a region of higher temperature to a region of lower temperature. The greater the temperature gap between the two fluids, the faster the heat transfer. This is the driving force behind every heat exchange process. In a food plant, for example, steam at 150°C heating milk at 4°C creates a large temperature difference, enabling rapid heat transfer. As the milk warms up and the steam condenses, this difference shrinks and the rate of transfer slows.

Engineers often use counter-current flow arrangements – where the hot and cold fluids move in opposite directions – to maintain a higher average temperature difference throughout the length of the exchanger. This configuration is more efficient than parallel flow, where both fluids travel in the same direction.

Heat transfer surface area (A) 🔗

The larger the area of contact between the separating wall and the fluids, the more heat can be transferred per unit time. This is why plate heat exchangers are so popular – their stacked corrugated plates create an enormous surface area within a compact footprint. Tubular heat exchangers increase surface area by using multiple tubes bundled inside a shell. Designers can adjust the number, length, and diameter of tubes or plates to match the thermal load of a specific application.

Overall heat transfer coefficient (U) 🔗

The overall heat transfer coefficient combines all the resistances that heat must overcome as it passes from one fluid to the other. These resistances include convection on the hot fluid side, conduction through the metal wall, and convection on the cold fluid side. If fouling deposits build up on the wall surface – a common problem in food processing – an additional resistance layer is introduced, reducing the U value and lowering performance.

The fundamental equation governing heat exchanger performance is: Q = U × A × ΔT, where Q is the rate of heat transfer. This simple relationship shows that to increase the heat transfer rate, an engineer can raise the temperature difference, increase the surface area, or improve the overall heat transfer coefficient by choosing better materials, increasing turbulence, or reducing fouling.

Types of heat exchangers used in food processing 🔗

Different food products have different viscosities, particle sizes, and thermal sensitivities. This is why several heat exchanger designs exist, each suited to particular processing conditions.

Tubular heat exchangers 🔗

Tubular (or shell-and-tube) heat exchangers are among the most widely used types in food plants. They consist of one or more inner tubes enclosed within an outer shell. One fluid flows through the inner tubes while the other flows through the surrounding shell space. According to Processing Magazine, multitube designs are the simplest configuration and are well-suited for products with low to medium viscosity.

Tubular exchangers handle higher pressures better than plate designs and can also accommodate fluids with suspended particles, depending on tube diameter. They are commonly found in evaporation systems, UHT processing lines, and juice cooling operations. A variation called the double-tube or triple-tube exchanger places one tube concentrically inside another, allowing the product to be heated or cooled from both sides simultaneously – improving efficiency for moderately viscous products like cottage cheese wash water or ice cream mix.

Plate heat exchangers 🔗

Plate heat exchangers (PHEs) are the most popular type in the food industry, and for good reason. They consist of a series of thin, corrugated metal plates stacked together and sealed with gaskets. The corrugations create channels through which the two fluids flow alternately – hot fluid in one channel, cold fluid in the next. The result is a very large heat transfer surface area packed into a small volume.

PHEs offer several advantages for food processors:

High thermal efficiency – corrugated plates induce turbulence in the fluid, boosting the heat transfer coefficient significantly. Some PHEs achieve heat transfer efficiencies of up to 95%.
Compact design – they occupy far less floor space than an equivalent tubular exchanger.
Easy maintenance – plates can be separated for cleaning, inspection, and gasket replacement.
Flexible capacity – adding or removing plates adjusts the thermal capacity without replacing the entire unit.

These exchangers are ideal for low- to medium-viscosity products such as milk, fruit juice, beer, and liquid egg. They are widely used in pasteurisation lines where precise temperature control is essential. However, PHEs are generally not suitable for very viscous products or those containing large particles, as these can block the narrow channels between plates.

Scraped surface heat exchangers 🔗

Some food products are too viscous, too sticky, or too prone to fouling for conventional tubular or plate designs. Scraped surface heat exchangers (SSHEs) are purpose-built for these challenging applications. They feature a cylindrical barrel with a heated or cooled outer wall and a rotating shaft fitted with scraper blades inside. As the shaft turns, the blades continuously scrape product off the inner wall surface, preventing buildup and ensuring uniform heat distribution.

SSHEs are essential in ice cream manufacturing, where ice crystals would otherwise accumulate on cold surfaces, reducing heat transfer and ruining product texture. They are also used for processing peanut butter, chocolate, fruit jams, honey, and other high-viscosity products. ICARUS Heat Exchangers notes that the scraping action also introduces turbulence, which further enhances heat transfer rates in thick products.

Two main SSHE designs exist. Rotating scrapers spin continuously, providing thorough mixing and high shear – ideal for very thick products. Reciprocating scrapers move back and forth gently, making them suitable for delicate products like whole fruit pieces that must retain their structure during processing.

Vat (jacketed vessel) heat exchangers 🔗

Vat or jacketed vessel heat exchangers are the simplest and oldest design used in food processing. They consist of a tank with a double wall – the annular space between the walls is filled with a heating or cooling medium (usually steam or chilled water). The food product sits inside the inner vessel, and an agitator may be used to keep the product moving over the heat transfer surface for more even temperature distribution.

Vat exchangers are most commonly associated with batch pasteurisation of milk and other dairy products. They are also used in small-scale operations for heating soups, sauces, and brewing wort. While they are inexpensive and easy to operate, vat exchangers are less thermally efficient than plate or tubular types because of their lower surface-area-to-volume ratio. They are best suited for batch processing rather than continuous operations.

Flow configurations: counter-current vs. parallel flow 🔗

The direction in which the two fluids travel relative to each other significantly affects heat exchanger performance. In parallel flow, both fluids enter the exchanger at the same end and move in the same direction. The temperature difference is highest at the inlet and decreases along the length. In counter-current flow, the fluids enter at opposite ends and move in opposite directions. This arrangement maintains a more uniform temperature difference along the entire length, resulting in greater overall heat transfer.

Counter-current flow is the preferred configuration in most food processing applications. It allows the cold fluid to exit at a temperature closer to the hot fluid’s inlet temperature, which is not possible with parallel flow. This is why plate heat exchangers – which naturally operate in counter-current mode – are so thermally efficient.

Energy recovery in heat exchangers 🔗

Modern food plants place strong emphasis on energy efficiency. Regenerative heat exchange is a technique where the heat from a product that needs to be cooled is used to pre-heat incoming cold product. In a milk pasteurisation line, for example, hot pasteurised milk passes through one side of a plate heat exchanger while cold raw milk flows through the other side. The hot milk is cooled down while the raw milk is warmed up – reducing the energy needed for both heating and cooling.

This approach can recover a significant portion of the thermal energy used in pasteurisation, substantially lowering utility costs and reducing the plant’s environmental footprint. Research indicates that heat exchangers can recover up to 50% of wasted heat depending on the application. In well-designed dairy pasteurisation systems, regeneration rates can reach 90-95%.

Factors to consider when selecting a heat exchanger 🔗

Choosing the right heat exchanger for a specific food processing application involves evaluating several product and process characteristics:

Product viscosity – thin liquids like water and juice work well in plate exchangers, while thick products like honey or chocolate require scraped surface designs.
Particle content – products with large particles (fruit pieces, vegetable chunks) need heat exchangers with wider flow channels, such as tubular or scraped surface types.
Fouling potential – products that tend to deposit on hot surfaces (dairy proteins, sugars) require designs that minimise buildup or incorporate self-cleaning mechanisms.
Hygienic requirements – food-grade heat exchangers must be built from materials like 316L stainless steel, which resists corrosion and is easy to sanitise. Clean-in-place (CIP) compatibility is also essential.
Temperature sensitivity – heat-sensitive products (vitamins, flavour compounds) benefit from high-efficiency exchangers that minimise exposure time at elevated temperatures.
Pressure requirements – high-pressure applications like sterilisation favour tubular designs over plate types.

Common food processing applications 🔗

Heat exchangers are involved in nearly every thermal process in a food plant. Pasteurisation of milk and juice uses plate or tubular exchangers to rapidly heat product to a target temperature (e.g., 72°C for 15 seconds for HTST milk pasteurisation) and then cool it quickly. UHT processing raises temperatures above 135°C for a few seconds to achieve shelf-stable products without refrigeration. Sterilisation of canned goods relies on retort or tubular systems. Concentration of fruit juice involves evaporators coupled with heat exchangers. Cooling and chilling of cooked products before packaging uses plate or tubular coolers to rapidly bring temperatures down and limit bacterial growth.

In confectionery, scraped surface heat exchangers are used for tempering chocolate – carefully controlling the crystallisation process to achieve the desired snap and gloss. In the brewing industry, plate heat exchangers cool hot wort rapidly after boiling, and in the dairy sector, they handle everything from cream pasteurisation to yoghurt incubation temperature control.

The role of heat transfer fluids 🔗

The fluid used on the service side of a heat exchanger matters just as much as the design itself. In food processing, steam is the most common heating medium because it delivers a large amount of energy through condensation and is easy to control. Hot water is used for gentler heating applications, while chilled water, glycol solutions, and brine are used for cooling and freezing.

When heat transfer fluids could potentially contact food – for instance, in the event of a gasket leak – they must be classified as food-safe under FDA and NSF standards. These are designated as HT1 fluids and are non-toxic, odourless, and safe for incidental food contact. Choosing the right heat transfer fluid ensures both processing efficiency and consumer safety.

What do you think? How might the choice of heat exchanger type affect the final quality and texture of a processed food product? If you were designing a processing line for a thick, particle-laden product like a chunky vegetable soup, which type of heat exchanger would you prioritise and why?

References

1. https://thermtest.com/heat-exchangers-and-thermal-fluids-in-the-food-processing-industry
2. https://www.tetrapak.com/en-us/solutions/integrated-solutions-equipment/processing-equipment/heat-transfer
3. https://www.sciencedirect.com/topics/engineering/plate-heat-exchanger
4. https://www.tandfonline.com/doi/full/10.1081/JFP-120005796
5. https://www.processingmagazine.com/process-control-automation/heating-cooling/article/53079262/how-to-choose-the-right-heat-exchanger-for-food-processing-applications
6. https://www.alfalaval.my/products/heat-transfer/plate-heat-exchangers/gasketed-plate-and-frame-heat-exchangers/heat-exchanger/how-plate-heat-exchanger-work/
7. https://www.icarus-hex-group.com/applications/heat-exchanger-food-processing/
8. https://www.sanitaryexchangers.com/FoodHeatExchangers.html