Plant & Works Engineering June 2022

36 | Plant & Works Engineering www.pwemag.co.uk June 2022 Energy & Environmental Management Focus on: HVAC Heat regeneration should be key consideration Many industrial processes require energy, but only a portion of that energy input is used for each operation such as pasteurisation or evaporation. Unused energy is wasted, often passing to the environment as hot gas or liquid. However, by using heat exchangers, it is possible to recapture most of this untapped energy through waste heat regeneration. Matt Hale* explains further. H eat regeneration (also known as heat recovery) is the process whereby heat from a process which would otherwise be lost or wasted (for example by being vented to the atmosphere) is recaptured and used for useful heating purposes. For the purposes of this article, heat regeneration should not be confused with ‘regenerative heat exchangers,’ which are a specific type of heat exchanger in which the product and service fluids flow alternately and the heat is stored in the structure of the heat exchanger. When we talk about heat regeneration, we mean the recovery of as much surplus heat (or cooling capacity) as possible after the primary function of the heat exchanger has been performed. This can then be reused to either improve the efficiency of heat exchange process or used elsewhere. ‘Recovery and re-use of industrial waste heat is an attractive concept that could simultaneously reduce energy costs and CO 2 emissions.’ 1 Given the importance of energy efficiency in reducing the use of fossil fuels and greenhouse gas (GHG) emissions, it can be argued that it is imperative to employ heat regeneration and recovery at every opportunity. As at least one paper has pointed out, ‘The use of excess heat could also be important to improve the economic and climate footprint feasibility of new processes… by avoiding the addition of new heat production capacity.’2 Benefits of heat regeneration Heat recovery improves the energy efficiency of heat exchange processes, and so the greatest and most obvious benefit of heat regeneration (recovery) is that less energy is required for a particular heating or cooling operation. This obviously provides financial benefits but is also better for the environment compared to systems without heat recovery. Repurposing recovered heat can also reduce the amount of heat required for certain processes. For example, if a material is pre-heated with recovered heat, then it may be possible to complete the necessary heating (for example for pasteurisation) using hot water from another source or part of the factory, instead of requiring a dedicated boiler to provide the necessary temperature rise. By increasing the energy efficiency of the heat transfer process, heat recovery can also make it possible to reduce the size of the heat exchange equipment required or reduce the necessary processing time. Heat recovery in action Heat recovery is often used in dedicated systems. For example, the use of heat recovery in building heating, ventilation systems and air conditioning (HEVAC), where heat is captured from air before it is expelled and used to warm incoming air, is becoming more widespread. In industrial situations, gas-to-water heat exchangers can be installed to capture heat from boiler flues and exhausts and used to pre-heat water in the system, raising the inlet temperature at the boiler and reducing the energy required to produce hot water or steam. When it comes to other heat exchanger applications, one example can be found in food pasteurisation processes, where products such as cream need to be heated to the necessary temperature to achieve pasteurisation, then rapidly cooled to maintain shelf life and quality. Such systems involve the use of two heat exchangers: one uses hot water to raise the temperature, while the second uses chilled water to cool the cream down again. The cooling process produces warm water which can be discarded, cooled for re-use, or cooled with some of the heat contained being used to pre- heat the cream before the pasteurisation process. This last option utilises heat recovery or heat regeneration, reducing the amount of new energy required for the subsequent first heating phases. As another example, many biogas plants use heat exchangers to pasteurise the digestate produced during the anaerobic digestion (AD) process. The ‘surplus’ heat which is generated after the system has been running can also be used to pre-heat the digestate, reducing total heat load and improving overall efficiency. Heat recovery can also be used in gaseous applications. Whether it is using the heat from the flue gas of a biogas combined heat and power (CHP) engine to pre-heat digestate, or a large gas-to-gas heat exchanger to capture waste heat from chemical processing, there is no reason to Heat regen can reduce overall energy requirements by up to 40% in some systems