Evaporator Steam Economy Calculator

What Steam Economy Means

Steam economy is the simplest measure of how efficiently an evaporator uses energy. It is defined as the mass of water evaporated for each unit mass of live steam consumed:

A single-effect evaporator has a steam economy slightly below 1.0 — it takes roughly 1 kg of steam to evaporate 1 kg of water, because some energy is lost heating the feed to boiling point and to ambient losses. The whole art of evaporator design is reusing the latent heat held in the vapour boiled off the product, instead of throwing it away in the condenser. There are three established ways to do that, and they are the three options in the calculator below.

The Calculator

Enter your feed rate, the total solids of the feed and the concentrated product, then choose the evaporator configuration. The calculator returns the water evaporation duty (an exact mass balance), the indicative steam or electricity consumption, and — using your own energy rates — the running cost. A side-by-side comparison of all three technologies on the same duty is shown beneath.

The Three Ways to Reuse Vapour Energy

1. Multiple effects

In a multiple-effect evaporator, the vapour boiled off in the first effect is used as the heating medium for the second effect, whose vapour heats the third, and so on. Each effect operates at a progressively lower pressure and temperature so that the vapour from the effect before is hot enough to drive boiling. The practical result, as the Tetra Pak Dairy Processing Handbook puts it, is that the live steam required is approximately the total water evaporated divided by the number of effects.^[1]

Steam economy therefore improves with each effect added, but with diminishing returns and rising capital cost. Published typical figures:^[2][3]

2. Thermal vapour recompression (TVR)

A TVR system adds a steam-jet ejector (a thermocompressor) that uses high-pressure motive steam to entrain and recompress part of the low-pressure vapour from an effect, raising its pressure and temperature so it can be reused as heating steam. It has no moving parts and is robust and inexpensive. A thermocompressor typically improves steam economy by roughly the equivalent of adding one effect, and in highly integrated modern dairy systems with full preheat trains the gain can be considerably greater — Tetra Pak cites a five-effect evaporator with thermocompressor needing only about 0.09 kg of steam per kg of water evaporated.^[1] Because the achievable gain depends heavily on the specific design, the calculator treats TVR conservatively as approximately one extra effect; enter your known economy for a precise figure.

3. Mechanical vapour recompression (MVR)

MVR replaces live steam almost entirely. A high-speed electric blower (or compressor) takes the vapour boiled off the product, compresses it through a small temperature lift — typically around 5–10 °C — and returns it as the heating medium for the same evaporator. The cycle is an open-cycle heat pump, and at the small temperature lifts used in dairy it achieves a very high coefficient of performance, often close to 50.^[4] Live steam is used only for start-up and balancing.

Because MVR runs on electricity rather than steam, its energy is expressed in kWh per tonne of water evaporated rather than as a steam economy. Dairy MVR evaporators typically use in the order of 8–15 kWh per tonne of water at low temperature lift.^[4] MVR has become the standard choice for new greenfield milk powder plants: New Zealand alone has over 100 MVR blowers on milk evaporators, and one early installation showed 5.8 GWh of electricity displacing 49 GWh of coal for the same duty.^[4] Whether MVR or steam works out cheaper at a given site depends entirely on the local ratio between electricity and steam prices — which is exactly why the calculator above uses your own rates.

Why Boiling-Point Elevation Matters

One reason real economy never quite reaches the simple "water evaporated divided by number of effects" ideal is boiling-point elevation (BPE). Dissolved solids raise the temperature at which a solution boils above that of pure water at the same pressure. As milk is concentrated, its BPE rises, eating into the temperature difference available to drive heat transfer in each effect. The effect is small in the early effects on dilute feed but becomes significant in the final effects on high-solids concentrate — which is one reason finishing duties are often handled differently from the bulk of the evaporation, and why running an effect too close to saturation can cause instability. Accurate design accounts for BPE effect by effect; this calculator does not, which is one of several reasons it is a screening tool rather than a design.

Worked Example

Take 20,000 kg/h of whole milk at 12% total solids, concentrated to 48% before the dryer:

• Water to evaporate = 20,000 × (1 − 12/48) = 15,000 kg/h
• Concentrated product = 20,000 − 15,000 = 5,000 kg/h at 48% solids
• Concentration ratio = 48 / 12 = 4.0×
• 6-effect at economy ~4.4: steam ≈ 15,000 / 4.4 ≈ 3,400 kg/h
• MVR at 12 kWh/tonne: ≈ 15 × 12 ≈ 180 kW of electrical power, no live steam

Whether the 3,400 kg/h of steam or the 180 kW of electricity costs less is entirely a function of your local energy prices — the comparison the calculator makes for you.

Related page: Spray Dryer Fines: Causes & Recovery — concentrating feed to higher solids in the evaporator also reduces fines in the dryer; this page explains the link, recovery and emissions.

References

1. Tetra Pak. Dairy Processing Handbook — Evaporators. States that the live steam required is approximately the total water evaporated divided by the number of effects, and gives the five-effect-with-thermocompressor figure of about 0.09 kg steam per kg water. dairyprocessinghandbook.tetrapak.com.
2. American Institute of Chemical Engineers (AIChE) (2018). The Essentials of Continuous Evaporation, Chemical Engineering Progress. Gives single-effect steam economy of 0.75–0.95 kg water/kg steam and the multiple-effect relationship. aiche.org.
3. University of Guelph. Dairy Science and Technology eBook — Evaporation and Dehydration. Covers falling-film multiple-effect operation and economy. books.lib.uoguelph.ca.
4. EECA (Energy Efficiency and Conservation Authority, New Zealand) (2019). MVR (Mechanical Vapour Recompression) Systems for Evaporation, Distillation and Drying. Documents dairy MVR coefficient of performance near 50 at low temperature lift, the New Zealand dairy MVR fleet, and the Anchor Products case displacing 49 GWh of coal with 5.8 GWh of electricity. genless.govt.nz.