Plant & Works Engineering June 2022

June 2022 www.pwemag.co.uk Plant & Works Engineering | 37 Focus on: HVAC Energy & Environmental Management waste the heat present in gaseous products or waste streams. Perhaps the most common use of heat regeneration is demonstrated in multi-effect evaporation systems, where a number of heat exchangers are combined, for example in the HRS DCS Digestate Concentration System. The first evaporation stage heats liquid digestate and uses a cyclone separator; the steam produced from this first cycle (usually available at 70˚C) is then used as the heating media for the second effect, whereby the process is repeated. The subsequent steam (usually available at 60˚C) is then used as the heating media for the third cycle. The number of effects is determined by the level of dry solids required and the amount of surplus heat available, up to a maximum of four cycles. After the final stage, the steam is condensed back to water and this heat is used to pre-heat the incoming product before the first stage of evaporation. In all, the heat is regenerated up to four times in the process. Other considerations To determine the potential value of waste heat, and therefore determine what it can be used for, it is necessary to know a number a parameters about the process temperature, the product and heating (or cooling) medium being used, and the performance of the heat exchange process in terms of heat transfer area and flow rate, for example. It is therefore important to consider energy regeneration or recovery as early as possible. Heat recovery systems can be retrofitted to many processes, but their design is often a compromise and retrofitted solutions may involve excessive pipework and other connections. It is also important to note that not all sources of waste heat, particularly those at low temperatures, may not be suitable for heat recovery or waste heat utilisation. Online calculators are available or any reputable heat exchanger designer or manufacturer will be happy to provide advice on what is feasible. The amount of time that a process runs for will also have an effect on the viability of any project; processes which run for longer periods (such as 20 hours a day versus just a couple of hours), will be more attractive for heat regeneration. To maximise the benefits of heat regeneration it is important that waste heat is transferred to the storage media (e.g., water or thermal transfer fluid such as glycol) as soon as possible after its source. This is particularly true where the waste heat is in the form of a gas, as this has a much greater energy constant than liquid, meaning that the heat is lost much faster. By considering all of these factors, it will be possible to calculate both the additional capital costs associated with specifying heat regeneration in a project, together with the savings in running costs and energy, and from this determine the return on investment for the project. Although capital costs may be higher, the longer term financial and environmental benefits will make the use of heat regen in heat exchanger projects highly attractive. 1 The potential for recovering and using surplus heat from industry. Final report for DECC, 2014. https://assets.publishing.service.gov.uk/governm ent/uploads/system/uploads/attachment_data /file/294900/element_energy_et_al_potential _for_recovering_and_using_surplus_heat_from_i ndustry.pdf 2 https://www.frontiersin.org/articles/ 10.3389/fceng.2021.679454/full * Matt Hale is International Sales and Marketing Director, HRS Heat Exchangers Improve productivity with humidity control Steam Humidifiers Dehumidifiers Adiabatic Humidifiers ...take control of your environment. • Design, Installation & After Sales • Equipment to suit all applications • Planned Maintenance for consistent performance • Professional service & full technical backup 01372 571200 www.humiditysolutions.co.uk Service & Maintenance IF YOU’RE READING THIS, THEN SO ARE 30,000+ ENGINEERS IMAGINE THE IMPACT YOUR ADVERTISEMENT COULD HAVE Contact Damien Oxlee at PWE Tel: 01732 370342