Evaporators · · 5 min read
Evaporator Selection Guide: Falling Film, Forced Circ., MVR
Choose between falling film, forced circulation, multiple-effect, MVR and vacuum batch evaporators for milk, juice and paste, with steam economy compared.
By the process engineering team at Shanghai Mars Industrial Co., Ltd.
Key takeaways
- Falling film evaporators suit heat-sensitive, low-viscosity liquids such as milk and clear juice, with residence times of seconds rather than minutes.
- Forced circulation evaporators suit viscous, fouling or crystallizing products such as tomato paste and are often used as the final high-concentration stage.
- Adding effects cuts steam use: a two-effect plant without thermocompression uses about 0.55 kg steam per kg water, a five-effect plant about 0.2 kg.
- MVR evaporators run mainly on electricity, evaporating roughly 80–100 kg of water per kWh, and pay back where power is cheap relative to steam.
Which evaporator type should you choose?
Use a falling film evaporator for heat-sensitive, low-viscosity liquids such as milk and clear juice; a forced circulation evaporator for viscous, fouling or crystallizing products such as tomato paste; multiple effects or MVR to cut energy at larger capacities; and a vacuum batch pan for small volumes, jams and frequent recipe changes.
Evaporator selection depends on four things: product viscosity at the final concentration, heat sensitivity, fouling tendency and the cost of steam versus electricity at your site. This guide compares the main designs and explains steam economy in plain numbers.
How do the main evaporator types work?
All food evaporators boil water off under vacuum to keep product temperatures low. They differ in how the liquid moves over the heating surface: as a thin falling film, as a pumped high-velocity stream, or as a stirred batch, and in how vapour energy is reused.
Falling film evaporators
Product is distributed at the top of long vertical tubes and flows down the inside wall as a thin film while steam condenses outside. The University of Guelph dairy science text gives tube lengths of 8–12 m, diameters of 30–50 mm and residence times of 20–30 seconds, compared with 3–4 minutes in older rising-film designs. Falling film units are the most common type in dairy. Their weak point is distribution: the heating surface must stay fully wetted, or dry spots foul and burn.
Forced circulation evaporators
A circulation pump drives product through a shell-and-tube heater at a pressure above its boiling point, so it does not boil on the tubes. When it enters the separator, the pressure drops and part of the water flashes off. Because boiling happens away from the heating surface, scaling is reduced. GEA recommends this design for highly viscous liquids, products with a high fouling tendency and the high-concentration stages of multi-effect plants, often after a falling film pre-concentrator.
Plate and scraped-surface evaporators
Plate evaporators spread the film over corrugated plates, giving short residence time and compact footprint; they are used for fruit juices with low solids. Scraped-surface (wiped film) units mechanically spread very viscous or heat-sensitive product in a single pass and are used for niche high-value concentrates.
Vacuum batch evaporators
A jacketed, stirred vessel under vacuum boils a fixed batch down to target concentration. Batch pans are simple and flexible, suiting jams, sauces, sweetened condensed products and pilot plants, but they give long heat exposure and no vapour reuse.
How do evaporator types compare?
Falling film offers the gentlest treatment and best energy efficiency for thin liquids, forced circulation tolerates viscosity and fouling, and batch pans offer flexibility at small scale. Most large plants combine types, for example falling film effects followed by a forced circulation finisher.
| Type | Viscosity handled | Residence time | Fouling tolerance | Typical products |
|---|---|---|---|---|
| Falling film (tubular) | Low to moderate | Short (tens of seconds per pass) | Low to moderate | Milk, whey, clear juice, plant-based milk pre-concentration |
| Plate falling / rising film | Low | Short | Low | Clear and low-solids fruit juices |
| Forced circulation | High | Longer (recirculating) | High | Tomato paste, fruit pulp concentrates, crystallizing liquids |
| Scraped surface | Very high | Very short, single pass | High | Heat-sensitive, sticky concentrates |
| Vacuum batch pan | High | Long (whole batch) | Moderate | Jam, sauces, condensed milk, small batches |
What is steam economy and how do effects and MVR change it?
Steam economy is the kilograms of water evaporated per kilogram of steam. A single effect evaporates slightly less than 1 kg per kg of steam; each additional effect reuses the vapour, so steam use per kg of water falls roughly in proportion to the number of effects. Thermocompression and MVR improve it further.
The Tetra Pak Dairy Processing Handbook gives about 0.55 kg of steam per kg of water for a two-effect evaporator without a thermocompressor, and about 0.2 kg for a five-effect plant. A thermal vapour recompressor (TVR) uses high-pressure motive steam, typically 600–1,000 kPa, to compress part of the vapour and reuse it, reducing consumption further.
| Configuration | Steam use | Electricity | Notes |
|---|---|---|---|
| Single effect | About 1 kg/kg or slightly more | Pumps only | Simple; suits small capacities |
| Two effects, no TVR | About 0.55 kg/kg | Pumps only | Handbook figure |
| Five effects, no TVR | About 0.2 kg/kg | Pumps only | Handbook figure |
| Multiple effects + TVR | Lower still | Pumps only | Needs 600–1,000 kPa motive steam |
| MVR | Start-up and make-up only | About 0.010–0.012 kWh/kg | Roughly 80–100 kg water per kWh |
In mechanical vapour recompression (MVR), a fan or compressor raises the vapour temperature by about 3–7 °C so it can reheat the same effect. The handbook states that MVR can halve operating costs compared with a six-effect thermocompression evaporator, but it raises capital cost and electrical demand. MVR makes most sense for long, continuous campaigns where electricity is competitively priced.
Which evaporator is best for milk, juice and tomato paste?
Milk: multi-effect falling film, often with TVR or MVR, operating at 40–70 °C. Clear juice: falling film or plate evaporators with aroma recovery. Tomato paste and pulpy concentrates: multi-effect forced circulation, or falling film pre-concentration followed by a forced circulation finisher.
Milk and whey
The handbook gives vacuum boiling at about 55–70 °C (160–320 hPa) with a maximum of about 70 °C in the first effect and about 40 °C in the last. Skim and whole milk are concentrated to about 48–50% total solids for condensed products, or to 32–40% as a pre-concentrate before spray drying.
Fruit juices
Juices are heat-sensitive and lose aroma during evaporation, so plants use short-residence falling film or plate evaporators and recover the volatile aroma fraction from the first vapours to add back later. Pulpy juices and purées need forced circulation as viscosity rises.
Tomato and fruit paste
Viscosity rises steeply as tomato juice is concentrated from about 5 °Brix towards 28–38 °Brix, so forced circulation is used for the high-concentration effects. Specialist suppliers offer dedicated forced-circulation tomato evaporators with tube-nest heat exchangers for this duty.
Plant-based milks and other dairy-like bases
Soy, oat and other plant-based bases behave much like milk in an evaporator: they are protein-rich, heat-sensitive and prone to fouling. Where concentration is needed, for example before drying into a powder, falling film effects at low boiling temperatures with short residence time are the usual choice, followed by thorough CIP to remove protein deposits from the tube walls.
How do you specify an evaporator?
- Define feed rate, feed and target solids, and calculate water to evaporate per hour.
- Measure viscosity and fouling behaviour at the final concentration.
- Set maximum product temperature and allowable residence time.
- Compare steam and electricity prices to choose effects, TVR or MVR.
- Check cooling water temperature, which limits vacuum and the last-effect temperature.
- Plan CIP: falling film tubes need complete wetting during cleaning as well as production.
When we engineer an evaporator, we start from the water-removal duty and the product's viscosity curve, then choose the number of effects from the client's steam and power costs. Share your feed rate and target concentration and we will propose a configuration and energy estimate.
Frequently asked questions
What is the difference between falling film and forced circulation evaporators?
In a falling film evaporator, liquid flows down heated tubes as a thin film and boils on the tube wall, which is gentle and efficient for thin liquids. In a forced circulation evaporator, a pump keeps product under pressure in the heater so it boils in the separator, which suits viscous and fouling products.
How much steam does a multiple-effect evaporator save?
Each added effect reuses the vapour from the previous one. According to the Tetra Pak Dairy Processing Handbook, a two-effect evaporator without thermocompression uses about 0.55 kg steam per kg water, and a five-effect plant about 0.2 kg, compared with roughly 1 kg for a single effect.
Is MVR better than a steam-driven evaporator?
MVR evaporates roughly 80–100 kg of water per kWh and can halve operating cost compared with a multi-effect steam plant, but it costs more to buy and needs substantial electrical capacity. It suits large, continuous operations where electricity is cheaper than steam.
Why are evaporators operated under vacuum?
Vacuum lowers the boiling point, so water can be removed at 40–70 °C instead of 100 °C. This protects heat-sensitive proteins, colour, flavour and vitamins, reduces fouling and allows vapour from one effect to heat the next effect at a lower pressure.
