Heat Transfer to Boiling Liquids: Applications in Food Processing

When you heat a liquid to its boiling point, something remarkable happens – the rate of heat transfer shoots up dramatically. Bubbles form, the liquid churns vigorously, and energy moves from the heated surface into the fluid far more efficiently than in simple convection. This phenomenon, known as boiling heat transfer, is one of the most powerful mechanisms of thermal energy exchange. In food processing, it underpins operations like evaporation, pasteurization, blanching, and sterilization. Understanding how heat moves into boiling liquids – and how factors like temperature difference and surface conditions affect the process – is essential for designing efficient, safe food processing systems.

What happens when a liquid boils? 🔗

Boiling occurs when a liquid reaches its saturation temperature – the temperature at which its vapor pressure equals the surrounding atmospheric pressure. For water at sea level, this is 100°C. Once the heated surface in contact with the liquid exceeds this saturation temperature by even a few degrees, vapor bubbles begin forming at tiny imperfections on the surface called nucleation sites.

These bubbles grow, detach from the surface, and rise through the liquid. As they move upward, they create intense agitation and mixing in the surrounding fluid. This turbulent movement is what makes boiling such an effective heat transfer mechanism – far more effective than heating a liquid through simple natural convection alone. The continuous cycle of bubble formation, growth, departure, and liquid replacement at the heated surface keeps pulling fresh, cooler liquid toward the hot surface, maintaining a high rate of energy transfer.

The boiling curve: understanding different regimes 🔗

The relationship between the temperature difference (often called excess temperature, ΔT = surface temperature − saturation temperature) and the heat flux is represented graphically by a boiling curve. This curve, first established experimentally by Nukiyama in 1934, reveals that boiling is not a single uniform process – it occurs in distinct regimes, each with different heat transfer characteristics.

Natural convection boiling 🔗

When the surface temperature is only slightly above the saturation point (roughly 2-6°C for water), no bubbles form. Heat is transferred from the surface to the liquid by natural convection currents. The warmer, less dense liquid near the surface rises, and cooler liquid sinks to replace it. Heat transfer rates in this regime are relatively low.

Nucleate boiling 🔗

As the excess temperature increases beyond about 5-10°C, bubbles begin forming at nucleation sites on the heated surface. This is the nucleate boiling regime, and it is the most important zone for practical food processing applications. Two sub-regions exist within nucleate boiling:

Isolated bubble region: At lower excess temperatures (approximately 4-10°C above saturation for water), individual bubbles form at scattered nucleation sites. These bubbles detach and rise, creating significant mixing near the surface. Most of the heat transfer in this sub-region occurs through direct convection between the surface and the moving liquid rather than through the vapor itself.

Column and slug region: At higher excess temperatures (roughly 10-30°C above saturation), bubble production becomes so rapid that individual bubbles merge into continuous columns and slugs of vapor. The combined effect of liquid entrainment and rapid evaporation produces extremely high heat fluxes – up to around 10⁶ W/m² for water at atmospheric pressure.

The peak of the nucleate boiling regime is the critical heat flux (CHF), which represents the maximum heat flux achievable before the boiling process becomes unstable. This is a crucial design limit in food processing equipment – exceeding it leads to a sudden, sharp drop in heat transfer efficiency.

Transition boiling 🔗

Beyond the critical heat flux, increasing the surface temperature actually causes the heat flux to decrease. A layer of vapor begins blanketing portions of the heated surface. Since vapor conducts heat much less effectively than liquid, the overall heat transfer drops. This unstable region, where conditions oscillate between nucleate and film boiling, is generally avoided in industrial practice.

Film boiling 🔗

At very high excess temperatures (above approximately 120°C for water), a stable, continuous vapor film covers the entire heated surface. This is known as film boiling, and it is also referred to as the Leidenfrost effect. The vapor film acts as an insulator, keeping the liquid away from the surface. Heat transfer occurs through the vapor layer mainly by conduction and, at very high temperatures, by radiation. Heat transfer rates are significantly lower than in the nucleate boiling regime.

Heat transfer coefficient in boiling 🔗

The heat transfer coefficient (h) quantifies how effectively heat moves from a surface into the boiling liquid. In food processing, this value determines how quickly a product heats up, how uniformly it is treated, and how energy-efficient the process is. Typical values of the convective heat transfer coefficient vary widely depending on the boiling regime:

Free convection (no boiling): Approximately 200-1,000 W/(m²·°C). Nucleate boiling water: Can range from 3,000 to 100,000 W/(m²·°C). Condensing steam: Also in the range of 5,000 to 100,000 W/(m²·°C).

The dramatic jump in the heat transfer coefficient during nucleate boiling – often 10 to 100 times higher than in non-boiling convection – is what makes boiling-based processes so thermally efficient.

Factors affecting the boiling heat transfer coefficient 🔗

Several factors influence how efficiently heat transfers to a boiling liquid:

Temperature difference (excess temperature): The greater the difference between the heated surface temperature and the liquid’s saturation temperature, the more nucleation sites become active and the higher the heat flux – up to the critical heat flux limit.

Surface characteristics: Rougher surfaces have more nucleation sites, which promotes earlier and more vigorous bubble formation. Polished, smooth surfaces tend to delay the onset of nucleate boiling. Surface material also matters – different metal-liquid combinations yield different heat transfer coefficients, as described in the well-known Rohsenow correlation.

System pressure: Increasing pressure raises the saturation temperature of the liquid and changes the fluid’s thermophysical properties. Higher pressures generally increase the heat transfer coefficient by affecting bubble size and the rate of bubble formation.

Fluid properties: Thermal conductivity, viscosity, surface tension, and latent heat of vaporization all play a role. These properties collectively determine how easily bubbles form, grow, and detach from the surface.

The Rohsenow correlation 🔗

One of the most widely used methods to predict nucleate boiling heat transfer is the Rohsenow correlation, developed by Warren Rohsenow at MIT. This semi-empirical equation relates the heat flux to the excess temperature using fluid and surface properties. It accounts for the specific heat of the liquid, the latent heat of vaporization, surface tension, and a surface-fluid constant (C_sf) that varies depending on the particular combination of heated surface material and liquid.

For example, the C_sf value for water boiling on a mechanically polished stainless steel surface is around 0.0132, while for water on nickel it is about 0.006. These constants are determined experimentally and reflect how easily a given surface promotes bubble formation. The Rohsenow correlation remains a foundational tool in food processing engineering for designing equipment like evaporators, kettles, and heat exchangers.

Applications of boiling heat transfer in food processing 🔗

Boiling heat transfer is not just a theoretical concept – it is the driving force behind several critical food processing operations.

Evaporation and concentration 🔗

Evaporators in the food industry rely directly on boiling heat transfer to remove water from liquid food products. Fruit juices, milk, sugar syrups, and tomato paste are all concentrated using evaporators where the liquid food is heated to its boiling point. The water vaporizes, leaving behind a more concentrated product with enhanced flavour, colour, and longer shelf life.

In many cases, vacuum evaporation is used, where the pressure inside the evaporator is reduced below atmospheric. This lowers the boiling point of the liquid, allowing evaporation at lower temperatures. The benefit for food processing is significant – heat-sensitive nutrients, vitamins, and flavour compounds are better preserved. The rate of evaporation still depends on the boiling heat transfer coefficient, the available heat transfer area, and the temperature difference between the heating medium (usually steam) and the boiling liquid.

Pasteurization and sterilization 🔗

While pasteurization itself typically operates below boiling temperatures, the steam used as the heating medium in pasteurizers and sterilizers involves condensation – the reverse of boiling – which also has very high heat transfer coefficients. Understanding boiling and condensation together allows food engineers to design efficient plate heat exchangers and tubular heat exchangers that rapidly bring liquid foods like milk, juices, and sauces to the required temperatures for pathogen destruction.

In retort sterilization, where canned or packaged foods are heated above 100°C under pressure, the boiling behaviour of the liquid food inside the container directly affects how quickly the product reaches the target temperature throughout its volume.

Blanching 🔗

Blanching involves immersing fruits or vegetables in boiling water or exposing them to steam for a short duration. The vigorous agitation caused by boiling water ensures rapid and uniform heat transfer to the food surface, inactivating enzymes that would otherwise cause quality degradation during frozen storage. The high heat transfer coefficient of boiling water (compared to hot water below its boiling point) is precisely what makes blanching a quick and effective pre-processing step.

Cooking in jacketed kettles 🔗

Large-scale food production facilities use steam-jacketed kettles for cooking soups, sauces, jams, and confectionery. Steam condenses on the outer surface of the inner vessel, transferring its latent heat. Inside the vessel, the food product may itself reach boiling, creating the characteristic vigorous agitation that helps with uniform cooking and prevents scorching or burning on the heated walls.

Boiling under different conditions: practical examples 🔗

The conditions under which boiling occurs can change the heat transfer behaviour significantly. Here are a few practical scenarios relevant to food processing:

Boiling water at atmospheric pressure: At 100°C, water boils freely. The heat transfer coefficient in the nucleate boiling regime can be exceptionally high, making atmospheric boiling effective for processes like blanching and cooking.

Boiling under vacuum: Reducing the pressure lowers the boiling point. At half atmospheric pressure, water boils at roughly 82°C. While this protects heat-sensitive food components, the lower temperature difference available between the steam and the boiling product can reduce the heat transfer rate, requiring larger heat transfer surfaces or multi-effect evaporation systems to compensate.

Boiling of sugar solutions: As dissolved sugars increase in concentration, the boiling point of the solution rises (a phenomenon called boiling point elevation). This means that at the same steam temperature, the available temperature difference for heat transfer decreases as the product concentrates, progressively slowing the evaporation rate.

Boiling with agitation: Mechanical stirring or pumping the liquid enhances heat transfer by preventing vapour from accumulating on the heated surface and by continuously bringing fresh liquid into contact with it. Scraped-surface heat exchangers, for instance, use rotating blades to scrape the heated wall, maintaining high heat transfer rates even with viscous food products.

Why it matters for food engineers 🔗

Designing efficient food processing equipment requires a solid understanding of boiling heat transfer. An engineer selecting an evaporator for concentrating fruit juice needs to know the expected heat transfer coefficients under the operating conditions – including the type of juice, its viscosity, sugar content, and the pressure inside the evaporator. Underestimating the heat transfer area leads to undersized equipment that cannot meet production demands. Overestimating leads to unnecessary capital costs.

Similarly, understanding the boiling curve helps avoid dangerous conditions. Operating too close to the critical heat flux in a heat exchanger can cause localized overheating (known as burnout), damaging both the equipment and the food product. Maintaining operations within the nucleate boiling regime ensures the highest heat transfer rates with safe, predictable performance.

The thermal properties of the food product – density, viscosity, thermal conductivity, and specific heat – all change during processing as temperature rises and concentration increases. A food engineer must account for these dynamic changes when designing and optimizing thermal processing systems.

What do you think? How might the choice between atmospheric and vacuum boiling affect the quality and cost of a concentrated fruit juice product? And in what situations might a food processor deliberately avoid the nucleate boiling regime in favour of gentler heating methods?

References

1. https://www.engineersedge.com/heat_transfer/water_boiling_graph_curve_13825.htm
2. https://en.wikipedia.org/wiki/Nucleate_boiling
3. https://cdn.prexams.com/7278/chapter%204.pdf
4. https://www.sciencedirect.com/topics/food-science/vacuum-evaporation
5. https://www.wattco.com/2021/12/heat-transfer-food-processing/
6. https://auctoresonline.org/article/thermal-processing-in-food-preservation-a-comprehensive-review-of-pasteurization-sterilization-and-blanching
7. https://thermtest.com/heat-exchangers-and-thermal-fluids-in-the-food-processing-industry