# Recuperators and regenerators: Science and thermodynamics

Why the hottest furnaces save the most fuel

Source page: https://www.materialinsights.org/energy/e_recuperators/rc-sci/

What this tab covers The thermodynamics: the grade (temperature or pressure) of the energy being recovered, energy versus exergy efficiency, and how harsh the waste stream is on equipment.

Recuperators, regenerators and air preheaters take heat from furnace or boiler exhaust and hand it back to the incoming combustion air. Furnace exhaust is the hottest and most valuable waste heat in industry, so the physics is favourable; the difficulty is that the gas is dirty and hot enough to damage ordinary steel.

#### Grade of energy
Favourable

Exhaust from glass melting, steel arc furnaces and aluminium furnaces leaves at well over 1,000 C, which the US Department of Energy (DOE) classes as high grade (above about 650 C). Heat this hot can be returned straight to the burner as preheated air, with no need to find a separate user. A paired regenerative burner fed with 1,200 C exhaust can, according to its manufacturer, deliver combustion air at about 1,000 C. DOE guidance treats furnaces above about 870 C as good candidates and finds preheating hard to justify below about 540 C.^[1](https://www1.eere.energy.gov/manufacturing/intensiveprocesses/pdfs/waste_heat_recovery.pdf)^[4](https://www.daigasgroup.com/en/rd/topic/1768584_56103.html)^[2](https://www1.eere.energy.gov/manufacturing/tech_assistance/pdfs/et_preheated.pdf)
 Glass melting furnace exhaust 1,300 to 1,540 C^[1](https://www1.eere.energy.gov/manufacturing/intensiveprocesses/pdfs/waste_heat_recovery.pdf) Steel electric arc furnace exhaust 1,370 to 1,650 C^[1](https://www1.eere.energy.gov/manufacturing/intensiveprocesses/pdfs/waste_heat_recovery.pdf) Air preheat from regenerative burner about 1,000 C from 1,200 C exhaust^[4](https://www.daigasgroup.com/en/rd/topic/1768584_56103.html) Screening threshold good above about 870 C; hard to justify below about 540 C^[2](https://www1.eere.energy.gov/manufacturing/tech_assistance/pdfs/et_preheated.pdf)

#### Exergy versus energy efficiency
Favourable

Fuel saving rises with both furnace temperature and preheat temperature. For a natural gas furnace with 10% excess air, DOE tables show 22% fuel saving when a 870 C furnace gets 425 C air, rising to 51% when a 1,315 C furnace gets 870 C air. Regenerative burners save 35 to 50% against no recovery, on manufacturer figures. Rotary regenerative (Ljungstrom) air heaters on power boilers typically reach about 85% temperature effectiveness. Because the recovered heat replaces fuel at the flame, energy and exergy gains are close: heat at 1,300 C has a work potential (Carnot factor) of about 0.81 of its energy content, so little quality is lost. In practice ordinary recuperators are economic at 40 to 60% effectiveness, and add-on devices typically capture under half of the heat available.^[2](https://www1.eere.energy.gov/manufacturing/tech_assistance/pdfs/et_preheated.pdf)^[4](https://www.daigasgroup.com/en/rd/topic/1768584_56103.html)^[5](https://www.energy.gov/sites/default/files/2015/02/f19/QTR%20Ch8%20-%20Waste%20Heat%20Recovery%20TA%20Feb-13-2015.pdf)^[9](https://www.asme.org/wwwasmeorg/media/ResourceFiles/AboutASME/Who We Are/Engineering History/Landmarks/185-Ljungstrom-Air-Preheater.pdf)
 Fuel saving, 870 C furnace, 425 C air 22%^[2](https://www1.eere.energy.gov/manufacturing/tech_assistance/pdfs/et_preheated.pdf) Fuel saving, 1,315 C furnace, 870 C air 51%^[2](https://www1.eere.energy.gov/manufacturing/tech_assistance/pdfs/et_preheated.pdf) Regenerative burner saving (manufacturer figure) 35 to 50%^[4](https://www.daigasgroup.com/en/rd/topic/1768584_56103.html) Carnot factor of 1,300 C heat (25 C ambient) about 0.81 (calculated)^[1](https://www1.eere.energy.gov/manufacturing/intensiveprocesses/pdfs/waste_heat_recovery.pdf)

#### Media compatibility and corrosiveness
Mixed

Hot exhaust carries dust, alkali vapours and sulphates. In glass furnaces sulphates deposit below about 270 C, so the cold end must stay hot. Carbon steel starts to oxidise above about 425 C and stainless steel above about 650 C. The 2008 DOE report says metals are generally not used above about 870 C, though some metallic recuperators work up to about 1,090 C, depending on alloy and design margin. Above that, ceramic tubes (hot side up to about 1,540 C) or refractory brick regenerators are used; regenerators tolerate dirty gas best but are bulky and costly.^[1](https://www1.eere.energy.gov/manufacturing/intensiveprocesses/pdfs/waste_heat_recovery.pdf)
 Glass furnace sulphate deposition below about 270 C^[1](https://www1.eere.energy.gov/manufacturing/intensiveprocesses/pdfs/waste_heat_recovery.pdf) Metallic recuperator upper limit about 870 to 1,090 C, depending on alloy^[1](https://www1.eere.energy.gov/manufacturing/intensiveprocesses/pdfs/waste_heat_recovery.pdf) Ceramic recuperator hot side up to about 1,540 C^[1](https://www1.eere.energy.gov/manufacturing/intensiveprocesses/pdfs/waste_heat_recovery.pdf)

#### Variants compared^[1](https://www1.eere.energy.gov/manufacturing/intensiveprocesses/pdfs/waste_heat_recovery.pdf)^[5](https://www.energy.gov/sites/default/files/2015/02/f19/QTR%20Ch8%20-%20Waste%20Heat%20Recovery%20TA%20Feb-13-2015.pdf)^[4](https://www.daigasgroup.com/en/rd/topic/1768584_56103.html)^[9](https://www.asme.org/wwwasmeorg/media/ResourceFiles/AboutASME/Who We Are/Engineering History/Landmarks/185-Ljungstrom-Air-Preheater.pdf)^[10](https://www.powermag.com/power-101-improving-the-performance-of-boiler-auxiliaries-part-i)

| Variant | Where used | Temperature or size limit | Note |
|---|---|---|---|
| **Metallic recuperator** | Steady air preheat on furnaces and kilns | Exhaust up to about 870 to 1,090 C | Economic at 40 to 60% effectiveness |
| **Ceramic recuperator** | Very hot exhaust | Hot side up to about 1,540 C | Costly; used where metal fails |
| **Stationary regenerator (refractory checkers)** | Glass furnaces and other large dirty exhausts | Cost justified above about 1,090 C and about 15 MW (50 MMBtu/h) firing (draft DOE assessment) | Large and expensive but tolerant of dust |
| **Regenerative burner pair** | Reheating and forging furnaces | About 1,000 C air from 1,200 C exhaust | Saves 35 to 50%; switches every few tens of seconds |
| **Rotary air preheater (Ljungstrom-type wheel)** | Boiler and power-plant air heaters; about 20,000 supplied by 1994 | Low and medium temperatures because of thermal stress | About 85% effective but leaks 8 to 12% of air, more as seals wear |

#### Sources

1. [Waste Heat Recovery: Technology and Opportunities in U.S. Industry (BCS Inc. for US DOE Industrial Technologies Program)](https://www1.eere.energy.gov/manufacturing/intensiveprocesses/pdfs/waste_heat_recovery.pdf) US Department of Energy, 2008
2. [Preheated Combustion Air, Process Heating Tip Sheet #1 (DOE/GO-102007-2482)](https://www1.eere.energy.gov/manufacturing/tech_assistance/pdfs/et_preheated.pdf) US DOE Industrial Technologies Program, 2007
3. [Industrial furnaces, burners: regenerative burners (manufacturer R&D page; company claims, undated, about 2023)](https://www.daigasgroup.com/en/rd/topic/1768584_56103.html) Daigas Group (Osaka Gas), 2023
4. [Quadrennial Technology Review 2015, Chapter 8 technology assessment: Waste Heat Recovery (draft, pre-decisional)](https://www.energy.gov/sites/default/files/2015/02/f19/QTR%20Ch8%20-%20Waste%20Heat%20Recovery%20TA%20Feb-13-2015.pdf) US Department of Energy, 2015
5. [Ljungstrom Air Preheater, Stockholm 1920: International Historic Mechanical Engineering Landmark brochure](https://www.asme.org/wwwasmeorg/media/ResourceFiles/AboutASME/Who We Are/Engineering History/Landmarks/185-Ljungstrom-Air-Preheater.pdf) ASME, 1995
6. [Power 101: Improving the Performance of Boiler Auxiliaries, Part I (Nenad Sarunac)](https://www.powermag.com/power-101-improving-the-performance-of-boiler-auxiliaries-part-i) POWER magazine, 2010

## Other sections

- [Summary](https://www.materialinsights.org/energy/e_recuperators/index.md)
- [Overview](https://www.materialinsights.org/energy/e_recuperators/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_recuperators/correlation/index.md)
- [Engineering and integration](https://www.materialinsights.org/energy/e_recuperators/rc-eng/index.md)
- [Economics and contracts](https://www.materialinsights.org/energy/e_recuperators/rc-econ/index.md)
- [Impact and additionality](https://www.materialinsights.org/energy/e_recuperators/rc-impact/index.md)
- [What to track](https://www.materialinsights.org/energy/e_recuperators/tracking/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_recuperators/links/index.md)