Evaporator Steam Economy Calculator

Watson Dairy Training Academy
Develop practical understanding of evaporator operation and steam economy.

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.

Practical ways to reduce energy and product losses · Use the calculator

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

Multiple effects reuse product vapour at successively lower pressures and temperatures. The idealised steam demand falls as vapour is reused, but preheating, boiling-point elevation, losses and the actual arrangement determine the operating economy. University of Guelph: Evaporation and Dehydration.

Additional effects reuse vapour at successively lower pressures. Available temperature difference, fouling, viscosity, preheating and plant arrangement limit the actual benefit. The calculator's plain multi-effect economies (0.88, 1.75, 2.55, 3.20, 4.00, 4.40 and 5.00 for one to seven effects) are illustrative screening assumptions, not a verified range for every plant. Replace them with measured or supplier-guaranteed performance.

2. Thermal vapour recompression (TVR)

A thermocompressor uses motive steam to recover part of the process vapour. Its benefit depends on pressure levels and integration; a fixed allowance of one additional effect is unsuitable for comparing all TVR systems. The calculator uses the following fixed screening cases, excluding heat treatment. They are model inputs for comparison; establish the actual economy from the duty and pressure levels.

These examples are not a performance guarantee. No seven-effect TVR default is supplied; enter a known economy for that arrangement.

3. Mechanical vapour recompression (MVR)

MVR uses an electrically driven compressor to raise process-vapour pressure so that the vapour can supply useful heat again. The calculator starts at 12 kWh per tonne of water removed, as an illustrative model input, not a site-performance claim. This covers the stated evaporation duty, not every energy requirement of the installation. Start-up, heat treatment and other duties may still require steam or another heat source. Use the actual compressor duty and the full plant heat balance for a project comparison.

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 for vapour recompression; heat-treatment, start-up and auxiliary duties are excluded

For a five-effect TVR screening case at 0.09 kg steam/kg water, the same duty requires 1,350 kg/h of steam. Compare these limited utility estimates using consistent operating boundaries and add the excluded plant duties before making a decision.

Related page: Spray Dryer Fines: Causes & Recovery — concentrate solids and viscosity affect atomisation and drying behaviour; this page discusses fines, recovery and emissions.

Technical References and Model Basis

• University of Guelph: Evaporation and Dehydration. Academic background on evaporation, recompression and drying.
• US Department of Energy: Steam System Survey Guide. Steam distribution, drainage and non-condensable gases.
• US Department of Energy: Return Condensate to the Boiler. Recovery of suitable hot condensate.

Fixed calculator values are screening assumptions. Establish actual economy, compressor electricity and purchased-water requirements from measured duty or an agreed design basis. The worked mass balances and operating-review sequence are original illustrations.