Technical-economic assessment of ventilation systems incorporating energy recovery units

Outline of reasons and objectives
Collecting pollutants at source is very often the preferred recommended technique for reducing occupational exposure to hazardous substances. To ensure that the collection devices are effective and to secure thermal comfort for the employees, the extracted air must be made up for by an equivalent addition of air, that must be heated air in winter and cooled in summer. Unfortunately considerations regarding the energy costs associated with heating or cooling the new air often lead to solutions being adopted that are degraded from the occupational safety and health point of view. One way of reconciling the ventilation flow rates to be implemented with keeping running costs under control consists in adopting equipment that makes it possible to recover a fraction of the energy contained in the air removed by the ventilation systems.
The objective of this study was to make technico-economic assessments of existing facilities so as to show under what conditions it is possible to reconcile occupational risk prevention with keeping costs under control.
Approach
Three main recuperator systems making it possible to recover energy are used in industrial ventilation: plate heat exchangers; battery and water-circulation heat exchangers; and rotary heat exchangers. Four existing ventilation facilities equipped with such recuperator systems were monitored continuously over a minimum period of 50 days during the winter period. Each facility was equipped with sensors connected to acquisition units so that it was possible to monitor change over time in the ventilation flow rates extracted from and injected into the premises, change in the temperatures at various points of the facility, and change in electrical power consumptions. These measurements made it possible to assess the energy budget of the facility and to assess the cost-effectiveness of the recuperator system on the basis of the investment costs and of the operating costs.
Main results
During the measurements, the outside temperatures varied from -15°C to 23°C. The ventilation flow rates involved covered quite a wide range and varied, depending on the facilities, from 3,000 cubic metres per hour (m3/h) to 30,000 m3/h. Most of the time, the extracted air was polluted (with vapours, aerosols, dust, or chippings) and filtered before it went through the heat exchanger. In all of the observed situations, the energy recuperators were capable of preheating the new air with a maximum temperature increase close to or greater than 10°C. The efficiency of the devices varied on average in the range 50% to 90% depending on the climatic conditions of the facility. In spite of these results, only 10% to 55% of the energy needs for conditioning the new air were provided by the recuperators. However, for two installations, the energy recuperator system was advantageous, with the system paying for itself in a return-on-investment time lying in the range three to six years. That time was greater than ten years for the third facility, but would have been reduced to four years if the ventilation flow rates had been complied with. Finally, for the last case studied, the recuperator did not constitute a judicious investment because of its low rate of use, and because of the derisory cost of the fuel used (wood waste).
Discussion
The systems studied in this study are and remain high-performance in terms of thermal efficiency or effectiveness, even for processes that are highly emissive in pollutants.
Such performance levels are not sufficient to guarantee the cost-effectiveness of the investment made: two essential parameters need to be added: the rate of use of the energy recuperator system and the price of the energy/power.