Effluent heat recovery | Informa-TECH by Énergir

A heat recovery boiler produces steam and/or hot water without additional combustion, thereby reducing primary energy consumption.

The system typically uses a tube heat exchanger to capture heat contained in high-temperature waste streams—such as exhaust gases from turbines, engines or industrial dryers, flue gases or liquid effluents—to produce saturated steam (3 bar to 16 bar) and/or hot water (80°C to 120°C). When the temperature of the thermal discharge is moderate, the boiler can be coupled to a heat pump to optimize heat recovery.

Equipped with an economizer and proper insulation, a heat recovery boiler can achieve efficiencies of 85% or more.².

Thanks to its versatility, a heat recovery boiler can easily be integrated into an existing thermal network and is particularly well suited to industrial facilities where waste heat is available at temperatures above 120°C, such as in food processing plants, pulp and paper mills, chemical plants, refineries and cement plants.

In industrial processes, low-pressure steam flows (flash steam) are typically routed to lower-pressure processes, to a condenser or vented to the atmosphere, resulting in a net energy loss.

A thermal vapour recompression (TVR) system recovers this steam using a device called a thermocompressor, which has three connections: two inlets (high pressure and low pressure) and one outlet (medium pressure). To capture the energy contained in low-pressure steam, high-pressure motive steam is injected into the thermocompressor and accelerated as it exits the nozzle. This creates a Venturi effect that draws the residual steam into the mixing chamber, where the two flows combine to produce an intermediate-pressure steam stream ready to be reused in an industrial process.

For optimal performance, a thermocompressor must be designed according to steam flow rates, suction pressure, motive steam pressure and system type. With few moving parts and no wear components, thermocompressors are reliable and durable.

A conventional thermocompressor offers very attractive efficiency compared with more complex systems such as multi-nozzle configurations.

TVR is particularly well suited to applications such as evaporation, distillation, drying and desalination, especially in the agri-food, chemical, pharmaceutical, and pulp and paper industries.

TVR applied to paper drying

The dryers in a paper machine are supplied with steam, which condenses in the rotating dryers and must be discharged to a tank where steam and condensate are separated. A siphon removes the condensate by creating a pressure differential and carrying it along with the steam. To maintain sufficiently low pressure in the siphon and allow the mixture to flow, a thermocompressor is used to draw in the entrainment steam and the portion of condensate that has re-evaporated at the lower pressure.

Mechanical vapour recompression (MVR) is an advanced evaporation technology that recycles the latent heat of vapour by using a mechanical compressor to compress the vapour produced during liquid evaporation to a higher pressure and temperature than the inlet vapour, allowing it to be reused as a heating medium.

Among low-pressure steam recovery technologies, MVR offers the highest energy efficiency and the greatest steam savings (more than 100 kg of water evaporated per kg of steam). By comparison, thermal vapour recompression (TVR) typically achieves between 1 and 10 kg of water per kg of steam. MVR can also be combined with a multi-effect system featuring several evaporators in series to further increase thermal efficiency.

MVR is particularly well suited to high-concentration, energy-intensive evaporation stages where other methods become less efficient due to reduced heat transfer and higher energy cost per kilogram of water removed. It supplies higher-temperature steam for heating, operates with low temperature differentials to prevent thermal degradation and minimizes the use of live steam, which is generally required only for preheating. Although it is by far the most efficient technology, it is also the most costly and complex because it requires a compressor.

MVR is therefore the highest-performing low-pressure steam recovery technology, but also the most costly and complex.

MVR is particularly attractive for processes such as evaporation concentration and crystallization in sectors including food processing, pulp and paper, water treatment, chemicals and pharmaceuticals.

MVR for sugar drying

MVR systems minimize live steam consumption during sugar evaporation by mechanically recompressing flash vapour and reusing it as the primary heat source. Compared with single-effect, multi-effect or TVR-only (steam ejector) systems, MVR significantly reduces specific steam demand while maintaining a stable temperature differential in the evaporators. Compared with a triple-effect evaporator, for example, an MVR system can reduce steam consumption by 80 to 95% Residual steam is mainly used for start-up and to compensate for losses.

For greater precision, hybrid TVR–MVR configurations are often used. TVR provides bulk recompression at low capital cost and debottlenecks the initial stage of sugar drying, while MVR reduces load and stabilizes operation at the end of the process, optimizing the electricity-to-steam cost balance.