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Companies and government agencies alike are looking to reduce the greenhouse gas (GHG) emissions associated with their activities. Methods include increasing the use of renewable and low carbon energy sources as well as improving the energy efficiency of processes.

Energy efficiency represents more than 40 percent of the emissions abatement needed by 2040, according to the International Energy Agency (IEA) Sustainable Development Scenario.^[1] “Energy efficiency is the ‘first fuel’,” says the IEA report. “Reining in the scale of this unprecedented challenge, supporting net zero energy goals at lower costs, and delivering a wide array of benefits for society.” The report also notes: “According to the IEA Efficient World Scenario, currently existing cost-effective technologies are sufficient to double global energy efficiency by 2040.”

Heat exchangers are just such an existing, cost-effective technology, technologically proven for more than a century. Developments in materials and design mean that many types of heat exchangers are now more energy efficient than before. Among them are those designs utilizing corrugated tubes and energy recovery.

The biggest efficiency benefits of heat exchangers come about when they facilitate the reuse of as much of the thermal energy generated or used during a process — that is, the heating, cooling, pasteurization, evaporation and the like — as possible. Distributing heat more efficiently throughout production facilities has been recognized as a key factor in improving efficiency and reducing greenhouse gas emissions in industries such as chemical refining, water treatment and manufacturing.

Many processes require heat but not all of them utilize all of it. For example, a process using steam at 212°F (100°C) or more may result in a hot water stream with a temperature of 176 to 194°F (80 to 90°C). In some cases, this will be reheated in a continuous cycle. In the least efficient situations, however, it may simply be dumped and can even require cooling before it can be discharged. Yet, water within this temperature profile has a range of potential uses, including pasteurization and low temperature evaporation. Rather than continually heat and dump process hot water, it makes more sense to reuse it where possible by transporting it to where else it is needed in the facility.

Heat recovery can be applied to gases as well as liquids. | Photo credit: HRS Heat Exchangers

Heat Recovery with Heat Exchangers

A few examples can illustrate how heat exchangers can be used for heat recovery in process applications.

The first example is food production. Imagine a product that needs to be pasteurized: The product must be heated to the necessary temperature to achieve pasteurization and then rapidly cooled to maintain shelf life and quality. This can be achieved through the use of two heat exchangers. The first uses hot water to raise the temperature while the second uses chilled water to cool the product. In the second process, the temperature of the cooling water is increased significantly. There are three options for dealing with this heated water

• Discard or discharge it elsewhere.
• Cool it again for reuse.
• Cool it again for reuse but use some of the heat it contains toward the heat required for the pasteurization phase.

Hot water left over from other processes can be utilized for applications such as pasteurization. | Photo credit: HRS Heat Exchangers

The third option utilizes heat recovery or heat regeneration, reducing the amount of new energy required for the subsequent first heating phases.

A second example shows how excess heat from one process can be recaptured to be used elsewhere. Many anaerobic digestion plants use heat exchangers to pasteurize the digestate produced during the anaerobic digestion process so that it can be sold as an agricultural fertilizer. The “surplus” heat that is generated after the system has been running for two hours is used to preheat the digestate. This reduces total heat load and improves overall plant efficiency by increasing the amount of generated energy that is available for export or other uses.

Combining multiple heat exchanges often can provide the greatest energy benefits. The third example looks at a multi-effect evaporation system. This approach uses heat exchangers and evaporation to reduce the volume and increase the concentration of sludges and digestate. In the process, the first evaporation stage heats liquid digestate and uses a cyclone separator. The steam produced from this first cycle is used as the heating media for the second effect, whereby the process is repeated. The subsequent then steam is 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. After the final stage, the steam is condensed back to water, and this heat can be used to preheat the incoming product before the first stage of evaporation.

Heat recovery is not limited to systems dealing with liquids. For instance, a gas-to-gas heat exchanger can be used to recapture heat from high temperature exhaust gases leaving a large chemical reactor. This recovered heat then can be used to help preheat the chemicals entering the reactor.

In conclusion, these examples show that where the situation allows, heat exchangers have the potential to reduce the energy consumption — and, therefore, greenhouse gas emissions — of thermal processes in a wide range of industries. The capital costs of including energy recovery in a heat exchanger system are likely to be higher than similar systems without heat recovery, but these can be recovered over the working life of the unit.