Falling Film Evaporator: Design, Energy and Selection Guide

EVAPORATION & CONCENTRATION

Falling Film Evaporators: Concentrating Heat-Sensitive Liquids Without Cooking Them

Concentrating heat-sensitive liquids without cooking them - how falling film evaporation works, when it beats rising film or forced circulation, and the decisions that settle your product quality and your steam bill

Every concentration project runs into the same conflict: removing water requires heat, and the product you are trying to protect is exactly what that heat damages. A falling film evaporator resolves the conflict by doing the opposite of a boiling pan. Instead of holding a large volume at temperature for hours, it spreads the product into a film thin enough to be concentrated in a single pass, at a temperature the product tolerates, with residence time measured in seconds.

This article covers what a falling film evaporator is, where it fits against other evaporator types, how multi-effect, thermal and mechanical vapour recompression change the energy picture, and the questions worth settling before a system is built. The equipment shown throughout is from our own builds.

Filmed on a running unit: the instrumented vessel head, level and flow through the sight glass, and product discharging to the receiving tank.

Video file: https://sc02.alicdn.com/kf/H74328f887453403b9092073e11cffb4fj.mp4

What makes falling film evaporation different

Product is pumped to the top of a vertical tube bundle and distributed so that it runs down the inside wall of every tube as a thin, continuous film. The heating medium - steam, hot water or recompressed vapour - flows on the shell side. Because the product is a film rather than a flooded column, the vapour it generates can travel downward with it instead of forcing its way up through the liquid, and the entire mass does not have to reach boiling point before evaporation starts.

• Short residence time. The product spends seconds in the heating zone, not hours, so colour, aroma and heat-sensitive actives survive conditions that would degrade them in a kettle.
• Low temperature operation. Holding the effect under vacuum lowers the boiling point, so concentration happens at a temperature chosen for the product rather than for the atmosphere.
• A small temperature difference is enough. Falling film works with a modest driving temperature difference between heating medium and product - which is precisely what makes multi-effect and vapour recompression arrangements practical.
• Low liquid holdup. A small inventory means fast start-up and shutdown, less product stranded in the system at a changeover, and less time at temperature when the plant stops.
• Better behaviour on foaming and moderately viscous feeds than a flooded bundle, because the film does not churn.

Where falling film fits, and where it does not

Falling film is not the right answer for every duty. The differences that matter in practice are how the product moves through the heating zone, and what that does to residence time and to the risk of fouling.

The energy question: multi-effect, TVR or MVR

Evaporation is where most of the operating cost of a concentration plant sits, and the arrangement you choose is largely a utility question rather than a mechanical one.

Multiple effects

A multiple-effect evaporator chains vessels so that the vapour boiled off in one effect becomes the heating medium for the next. Each additional effect reuses vapour that would otherwise have to be condensed, so the steam needed per kilogram of water evaporated falls as effects are added. The trade-off is capital cost, footprint and control complexity - which is why the economic number of effects depends on energy prices and on how long you expect to run the plant.

Thermal vapour recompression (TVR)

A steam jet booster takes part of the vapour and recompresses it with motive steam, lifting its pressure and temperature high enough to be reused as heating medium. It is a compact way to cut steam consumption with no electrical load, and it is usually the option to consider when medium-pressure steam is already available on site.

Mechanical vapour recompression (MVR)

An MVR system compresses the vapour with a fan or compressor and returns it to the heating side as the primary heat source. The plant then runs largely on electricity instead of steam, which is attractive where steam is expensive, unavailable or regulated, and where the electricity tariff is favourable. MVR also removes most of the condenser duty and the cooling water demand of a conventional train - a decisive point for sites with limited cooling capacity or discharge restrictions.

There is no universally best answer. The right arrangement follows your utility prices, the stability of your feed and the turndown you actually need. A system specified around a utility that later becomes scarce or expensive is the most common mistake in evaporation projects - and it is decided on the first page of the specification, not during commissioning.

Inside the system: the modules that decide performance

Figure 1. A falling film evaporation train on its skid: vertical evaporation column, service and separation vessels, and the sanitary pipework that links the stages.

A falling film train is more than a heated column. These are the parts that determine whether it performs as designed once the feed changes.

• Calandria - tube bundle and shell. Tube diameter, length and surface condition set the heat transfer area and the pressure drop, while the shell side is arranged around the heating medium you selected.
• Feed distribution system. The single most important element of a falling film design: an uneven distributor starves some tubes, and a starved tube scales.
• Vapour-liquid separation vessel, sized so that entrainment does not carry product out with the vapour.
• Condenser, vacuum system and condensate handling, matched to the operating pressure of the last effect.
• Circulation and transfer pumps with sanitary connections, and the pipework that keeps the film stable between stages.
• Instrumentation: temperature and pressure at every effect, level control on vessels and separators, flow indication and sight glasses, and - on food and pharmaceutical duties - in-line concentration measurement.
• Cleanability: a drainable layout, hygienic welds, tri-clamp connections and a CIP route sized so that clean-down is short and verifiable rather than a guess.

Figure 2. Vessel head with sight glasses and pressure gauges on the separating vessels; the two-housing filtration module sits in the foreground.

Figure 3. The same train from the drive end: heat exchanger on the column head, product pumps and the collection vessel on the skid.

The control points that decide product quality

• Maximum product temperature - set by your product, not by the machine. Vacuum level is the lever that sets it, and it constrains every temperature difference available downstream.
• Residence time - a single-pass design keeps it short; every recirculation loop adds time at temperature.
• Concentration per pass - a high ratio saves energy but raises outlet viscosity and the risk of dry patches on the tube wall.
• Feed stability - evaporation is a steady-state process. A feed that swings in solids or flow swings your outlet concentration with it.
• Fouling margin - the surface area you buy should be sized to run between CIP cycles, not to look efficient on the first day.

Where falling film evaporators are used

• Fruit and vegetable juice concentration, where colour and aroma are the selling point
• Dairy and whey processing
• Sugar, glucose syrups and starch hydrolysates
• Glycerin and polyols
• Plant and herbal extracts
• Chemical intermediates and process streams
• Pharmaceutical and biological process liquids
• Food-industry wastewater and leachate concentration for zero-liquid-discharge duties

Selecting a system: the questions that settle the design

Frequently asked questions

Q: What is the difference between a falling film and a rising film evaporator?

In a rising film unit the liquid and vapour travel upward together, so the vapour has to push through the liquid column. In a falling film unit the product runs down the inside of the tube as a thin film with the vapour, which shortens residence time, lowers the temperature difference needed and tolerates more viscosity.

Q: Can a falling film evaporator handle a viscous product?

Up to a point. As viscosity rises, the film becomes harder to keep continuous and the risk of dry patches grows. Beyond that point the usual answer is a forced-circulation stage after the falling film, rather than accepting a badly wetting film.

Q: Why does capacity fall away over a production campaign?

Almost always fouling somewhere in the train, and often it starts with the feed distribution rather than the product itself. If the distributor is not feeding every tube evenly, the starved tubes foul first and the loss of surface area shows up as falling capacity.

Q: Is MVR always cheaper to run?

No. MVR shifts the cost from steam to electricity, so it wins where steam is expensive or unavailable and the electricity tariff is favourable. It also carries compressor capital cost and maintenance. The comparison belongs in the design stage, with your actual utility prices.

Q: What vacuum level do I need?

That follows from the maximum temperature your product tolerates, not from a standard figure. Once the temperature limit is known, the vacuum level is determined by the boiling point of your liquid at the concentration you are reaching.

Q: Can one unit concentrate several different products?

Yes, when it is designed for it: a drainable layout, effective CIP and recipe-based control are the enabling features. What you are buying in that case is changeover speed and repeatability rather than a single fixed duty.

Talk to our process engineers