A cheese plant in Argentina has outgrown its old double-effect evaporator and must concentrate far more whey before drying. Three proposals arrive: a larger multiple-effect plant heated by steam, the same plant with a thermocompressor, and a falling-film MVR evaporator driven by an electric fan. One quotes tonnes of steam per hour, another kilowatts of electricity, and the areas run to hundreds of square metres. To compare them fairly, the engineer needs a solids balance, the idea of steam economy and an honest view of boiling point elevation.
In short
- An evaporator concentrates liquid foods by boiling off water, usually under vacuum at roughly 45 to 70 °C.
- A single-effect evaporator uses about 1 kg of steam per kg of water removed: a steam economy of roughly 0.9 to 1.
- Multiple effects reuse each effect’s vapour to heat the next; each added effect raises the economy by roughly one, at the cost of more area.
- Thermal vapour recompression (TVR) adds roughly one effect’s worth of economy with a steam jet. Mechanical vapour recompression (MVR) compresses all the vapour with an electric fan, using roughly 10 to 20 kWh per tonne of water.
- MVR suits low-viscosity products with small boiling point elevation, such as milk and whey, more than high-Brix juices.
How does a food evaporator work?
An evaporator is a heat exchanger that boils water out of a liquid food and separates the vapour from the concentrate. Steam or compressed vapour condenses on one side of the heating surface, called the calandria, and the product boils on the other. The vapour passes to a separator, then to the next effect or a condenser; each heating body with its own separator is one effect. A vacuum pump removes air and other non-condensable gases, which would otherwise blanket the heating surface.
Food evaporators run under vacuum, where water boils at about 65 °C at 25 kPa absolute and about 46 °C at 10 kPa, to protect heat-sensitive products. Every design starts with a solids balance:
F × x_F = P × x_P and V = F − P
F, P and V are the feed, product and vapour flows (kg/h) and x is the solids mass fraction.
What is steam economy in a single-effect evaporator?
Steam economy is the mass of water evaporated per mass of steam supplied, in kg/kg, the standard yardstick of evaporator energy use. In a single effect, each kilogram of condensing steam releases roughly the latent heat needed to evaporate a kilogram of water, so the economy is close to 1, and lower when the feed enters below its boiling point.
Worked example
A dairy concentrates 15,000 kg/h of skimmed milk from 9% to 45% total solids in one effect, boiling at 65 °C and heated by steam condensing at 80 °C. Steam-table latent heats: 2,345 kJ/kg at 65 °C and 2,308 kJ/kg at 80 °C. Feed cp = 3.9 kJ/(kg·K). Boiling point elevation and heat losses are ignored.
Solids balance: P = 15,000 × 0.09 ÷ 0.45 = 3,000 kg/h; V = 15,000 − 3,000 = 12,000 kg/h.
Feed preheated to 65 °C: steam S = 12,000 × 2,345 ÷ 2,308 = 12,190 kg/h. Economy = 12,000 ÷ 12,190 = 0.98.
Feed at 10 °C: S = [12,000 × 2,345 + 15,000 × 3.9 × (65 − 10)] ÷ 2,308 = 13,586 kg/h. Economy = 0.88.
Heat duty with preheated feed: 12,190 × 2,308 ÷ 3,600 = 7,815 kW, about 7.8 MW.
A single effect therefore uses roughly a tonne of steam per tonne of water removed.
How do multiple-effect evaporators save steam?
A multiple-effect evaporator uses the vapour from one effect as the heating steam for the next, which runs at a lower pressure and temperature. Only the first effect takes fresh steam, and only the last effect’s vapour goes to the condenser. With n effects the economy approaches n, falling a little short because of feed heating, heat losses and boiling point elevation.
For the skimmed milk duty, a full balance of three forward-feed effects between steam at 80 °C and a last effect at 45 °C, with feed at 50 °C, small boiling point elevations and U falling from 2.2 to 1.2 kW/(m²·K) as the milk thickens, gives about 4,440 kg/h of steam: an economy of about 2.7. One effect with the same feed temperature needs about 12,570 kg/h, so the three effects save about 65% of the steam.
The saving is paid for in surface area. The same overall temperature difference is now shared between three heating surfaces, so each works with a smaller driving force. The window is capped at the top by product damage and fouling, with dairy first effects often run at about 70 °C or below, and at the bottom by condenser cooling water, which limits the last effect to roughly 40 to 45 °C in many climates. Forward feed, where product moves from the hottest to the coolest effect, keeps the most concentrated product at the lowest temperature and suits heat-sensitive foods.
What does thermal vapour recompression (TVR) add?
Thermal vapour recompression uses a steam jet, called a thermocompressor, to raise part of the vapour from an effect back to a useful heating pressure. High-pressure motive steam expands through a nozzle, draws in low-pressure vapour and discharges the mixture to a calandria. There are no moving parts, and it is cheap to add.
A well-matched thermocompressor typically adds roughly one effect’s worth of steam economy, so a single effect with TVR performs roughly like a double effect. It needs high-pressure steam, works best near its design point and turns down poorly.
How does an MVR evaporator work, and how much energy does it use?
An MVR evaporator compresses all the vapour leaving the separator with an electrically driven fan or compressor, raising its saturation temperature by a few kelvin, and returns it to the calandria of the same effect as the heating medium. Mechanical vapour recompression therefore reuses the latent heat instead of dumping it in a condenser, and needs only a small steam make-up once running.
The fan supplies only the lift, the small temperature rise that drives heat transfer, so the energy per kilogram of water is small.
Worked example
Vapour leaves the boiling skimmed milk at 65 °C (saturation pressure 25.0 kPa). The fan raises it to the saturation pressure at 70 °C (31.2 kPa), a pressure ratio of 1.25, giving a 5 K temperature difference. Take an isentropic efficiency (ideal work divided by actual work) of 0.78 (illustrative).
Isentropic enthalpy rise, from steam tables: about 35 kJ/kg.
Shaft work = 35 ÷ 0.78 ≈ 45 kJ per kg of water evaporated, or about 12.5 kWh per tonne.
For 12,000 kg/h: 12,000 × 45 ÷ 3,600 ≈ 150 kW of shaft power, plus motor and drive losses, against about 7.8 MW of steam heat for a single effect.
Area at 5 K with U = 2.0 kW/(m²·K): 7,815 ÷ (2.0 × 5) ≈ 780 m², about three times the 260 m² a single effect needs at 15 K.
Compressor work rises roughly in proportion to the lift: about 10 kWh per tonne for a 4 K lift and about 15 kWh per tonne for 6 K at the same efficiency. So MVR plants run small temperature differences across large surfaces. The discharge vapour is superheated, about 88 °C here, and is usually desuperheated with a little condensate.
Counted as heat, 12.5 kWh (45 MJ) equals the latent heat of roughly 20 kg of steam, so an MVR evaporator has a very high equivalent steam economy. Electricity, though, is a higher-grade energy, usually dearer per kilowatt-hour and often generated at well below 100% efficiency, so compare options on local energy prices and carbon intensity, not kilowatt-hours alone. The Intermediate Food Process Engineering course solves these balances, from single effects to MVR, step by step.
Why do falling film and boiling point elevation matter?
A falling-film evaporator feeds product to the top of long vertical tubes so it runs down the inside wall as a thin film, heated by condensing steam or vapour outside. It is standard for milk, whey and many juices because it offers:
- Short residence time, often seconds to a minute per pass, protecting heat-sensitive products.
- No hydrostatic head, so product at the bottom of a tube does not boil hotter.
- High heat transfer at small temperature differences, which MVR needs.
The catch is wetting: if too little liquid flows per metre of tube perimeter, the film breaks up, dry patches overheat and product burns on. Suppliers set a minimum wetting rate for each product.
Boiling point elevation (BPE) is the rise in a solution’s boiling temperature above that of pure water at the same pressure, caused by dissolved solutes. The vapour condenses at the saturation temperature of pure water, so the BPE is lost from the driving force. Milk and whey show little BPE at the concentrations where MVR is used, but sugar-rich juice concentrates at high Brix can show several kelvin. With a 5 K lift and 2 K of BPE, only 3 K remains for heat transfer, so the fan must lift further or the area must grow. For design, use measured data or Dühring’s rule: at fixed concentration, a solution’s boiling point varies almost linearly with that of water. Juice evaporators also strip volatile aroma, so they normally include aroma recovery, a quality topic also covered in Food Technology for Industry Professionals.
Which evaporator should you choose?
| Feature | Single effect | Multiple effect | Multiple effect with TVR | MVR |
|---|---|---|---|---|
| Energy input | Live steam | Live steam to the first effect | High-pressure live steam as motive steam | Electricity, small steam make-up |
| Approximate energy per tonne of water | About 1.0 to 1.1 t of steam | A little over 1/n t of steam for n effects | Roughly as for a plant with one more effect | About 10 to 20 kWh of electricity |
| Temperature difference per effect | Large | Shared between effects | Moderate | A few kelvin |
| Turndown | Set mainly by film wetting in falling-film designs | Set mainly by film wetting in falling-film designs | Poor; the ejector prefers its design point | Good with a variable-speed fan, within wetting limits |
| Best fit | Small or intermittent duties | Wide range, including high-BPE products | Retrofits and plants with plentiful steam | Large, continuous, low-BPE duties such as milk and whey |
- Choose MVR for large, continuous, low-BPE duties where electricity is affordable or low-carbon.
- Choose multiple effects, usually with TVR, where steam is plentiful or BPE is significant.
- Combine them where it pays: a common layout pre-concentrates with MVR and finishes in steam-heated effects.
- Consider membranes first. Reverse osmosis removes the first part of the water with no phase change, leaving a smaller evaporator load.
Frequently asked questions
What is a good steam economy for an evaporator?
A single effect achieves about 0.9 to 1 kg of water per kg of steam, less when the feed is cold. Each added effect raises the economy by roughly one, a little less in practice, so three effects give about 2.5 to 3. A thermocompressor adds roughly one more effect’s worth. MVR evaporators are compared in kWh of electricity per tonne of water instead.
What is the difference between TVR and MVR?
Both raise the pressure of evaporator vapour so it can heat the product again. TVR uses a steam jet with no moving parts: high-pressure motive steam entrains part of the vapour, adding roughly one effect’s worth of economy. MVR uses an electric fan or compressor to recompress all the vapour, so it needs very little steam and works with small temperature differences across large surfaces.
Why does an MVR evaporator need so much heat transfer area?
Compressor power rises roughly in proportion to the temperature lift, so MVR plants are designed for small temperature differences, often around 5 K, to keep electricity use low. Heat transfer area is inversely proportional to temperature difference, so a 5 K design needs about three times the area of a 15 K single effect for the same duty and overall coefficient. Falling-film tubes keep that area affordable and effective.
Can MVR be used for fruit juice concentrates?
Yes, but with care at the concentrated end. Sugar-rich juices develop several kelvin of boiling point elevation at high Brix, which eats into the small MVR lift, and rising viscosity lowers the heat transfer coefficient. A common approach is to use MVR or membranes for the dilute stages and steam-heated effects to finish, with aroma recovery to capture volatile flavour compounds.
Why are food evaporators operated under vacuum?
Reducing the pressure lowers the boiling point: water boils at about 65 °C at 25 kPa absolute and about 46 °C at 10 kPa. Lower temperatures protect proteins, vitamins, colour and flavour, slow browning and fouling, and allow low-pressure steam or recompressed vapour to act as the heating medium. The penalty is a condenser, a vacuum system and vessels built for vacuum.
Next step. The Intermediate Food Process Engineering course covers evaporator balances, multiple effects, TVR and MVR, falling-film design and membrane concentration. It ends with a proctored final assessment and an ASC certificate. You can also see all eleven food science and technology courses.
Sources. R. Paul Singh, Dennis R. Heldman and Ferruh Erdogdu, Introduction to Food Engineering, 6th edn (Academic Press, 2024). P. J. Fellows, Food Processing Technology: Principles and Practice, 5th edn (Woodhead Publishing, 2022). Zeki Berk, Food Process Engineering and Technology, 3rd edn (Academic Press, 2018). Romeo T. Toledo, Rakesh K. Singh and Fanbin Kong, Fundamentals of Food Process Engineering, 4th edn (Springer, 2018). ISO 50001:2018, Energy management systems, from ISO.
This article is general guidance on evaporator selection and calculation and is not a substitute for the applicable standard, your national legislation, supplier data or the advice of a qualified engineer.