Recover lost heat to optimise thermal efficiency

Economisers are a type of heat exchanger used to recover heat from the flue gases of boilers (or other gaseous heat sources). The heat recovered is used to pre-heat the feedwater, incoming combustion air or other streams. Improvements in the overall thermal efficiency of the process can be very significant.

How an industrial economiser works

The exhaust gases of a boiler have extremely high temperatures, meaning the system loses a lot of energy if it does not use heat recovery methods. A boiler economiser fits the boiler’s flue, where the waste gases are expelled. It transfers the heat to the incoming process fluid, such as boiler feedwater. An oil economiser does the same job for a boiler that runs on thermal oil.

Design considerations

Here are some key aspects that we consider when designing an economiser heat exchanger:

  • Exhaust temperature
  • Flue gas properties
  • Solids content
  • Condensing temperature

For example, the characteristics of the flue gases can vary significantly depending on the fuel used. We must factor this into the design. Similarly, the temperature of the process fluid inlet varies depending on ambient conditions and operational requirements. We also account for this.Photo of a flue gas economiser designed and manufactured by Sterling Thermal Technology

When it comes to the flow of the streams, it is usually most efficient when the process feeds the heated medium counterflow to the flue gas stream. However, sometimes we have found it is advantageous to use co-current flow for to control the fin and tube temperatures.

Another thing to consider is the configuration of the fins. We determine the best option based on the calculated fin root and tip temperatures. We then work on secondary and tertiary phases of cooling, ensuring that we remain within the design envelope of the selected fin form in each case.

Last but not least, equipment can’t be effective if it isn’t easy to operate. We work with our customers to review a range of operating points and ensure that the economiser characteristics permit easy control of the equipment as a whole.

Though the configurations of economisers and other heat exchangers can be complex, our experts make the design process painless. Whether you’re an end-user or an original equipment manufacturer, we will provide the solution that suits you.

Why partner with Sterling Thermal Technology for your economisers?

Customised design

We have a an extensive range of materials and design options available to us to find a solution optimised for your needs.


Our designs are cost and energy-efficient. We tailor the finned surfaces to the design temperature profile. We use more economical materials and extended surfaces as the exhaust gas temperature reduces.

Compact arrangement

We ensure the most compact arrangement while minimising the pressure drop required. We achieve this by using a range of finned surfaces, from welded fins to plate fins.


We design economisers to lower the gas temperature to a level marginally above the level at which condensation would occur. When there is a risk of corrosion under start-up and shut down conditions, we will advise on appropriate materials and the best operational parameters to minimise the potential for corrosion.

Exhaust gas economiser on truck ready to leave Sterling Thermal Technology's factoryAs a recognised economiser manufacturer, we can help you get the best equipment for your application. Sterling Thermal Technology is a leader in our field with over 100 years of experience in heat exchangers. Find out what we can do for you: contact us today.

If you feel you need another type of heat exchanger, visit our products and services page or contact us.

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