What is an Exhaust Gas Heat Exchanger?

An engine exhaust produces waste heat - and lots of it! Find out how an exhaust gas heat exchanger converts this heat into a valuable 'free' energy source.

An exhaust gas heat exchanger recovers heat energy from the exhaust gases of a reciprocating diesel, gas or bio gas fuelled engine and utilises this energy to heat water, which can then be used for space or process heating, cooling, via a chiller system or generating more electricity, via an ORC system. When added to an engine powered generating set, it converts the engine from ‘Power Only’ to ‘Combined Heat and Power’ (CHP), increasing its overall efficiency significantly.

Operating principle

A reciprocating engine converts fuel into power. A by-product of this process is the creation of heat, which is generated in every part of the engine. However, the largest single area of heat generation is from the exhaust system. Up to 50% of all fuel energy used to power the engine is turned into heat and of this heat 55% comes from the engine’s exhaust system. Without some form of heat recovery this valuable energy source would simply be lost into the earth’s atmosphere. However, by installing an exhaust gas heat exchanger into the engine’s exhaust stream, it can be recovered and used for a variety of heating or cooling requirements, at no additional cost, in terms of fuel used.

How does it work?

Exhaust gases exit the engine at temperatures in excess of 500 °C, passing through the heat exchanger’s central ‘tube core’. Simultaneously, water is circulated around the outer ‘shell’ of the heat exchanger, travelling over and around the tube core, cooling the exhaust gases and transferring much of the heat in the gases to the water circuit.

What can the heat be used for?

The recovered heat energy can be used for numerous domestic, commercial or industrial applications, including commercial space heating, residential district heating and hot water provision, process heating, plus heating thermal oil. Used in conjunction with Stirling engine or ORC system, it can also be used to generate more electricity or run a chiller for cooling.

Benefits to the end user

In addition to capturing this valuable ‘free energy’ source, the exhaust gas heat exchanger also improves the overall efficiency of a gen-set, increasing it from around 30% (power only) to around 60% (CHP). This can be improved to around 80% efficiency, when waste heat is recovered from other areas of the engine, such as the cooling, lubrication and air induction systems.

Things to consider

Consideration must be given to the amount of heat recovered from the exhaust stream, as the exhaust temperature should not normally be allowed to drop below 120 °C (or 180 °C on diesel powered engines), in order to avoid fouling or condensation forming inside the heat exchanger, which could lead to premature failure.

Automatic engine shutdown equipment must also be installed with temperature probes located in both the exhaust gas heat exchanger and the engine. Should the gas circuit shut down, it is important that the water circuit continues to operate for a period of time to enable residual heat to disperse from the heat exchanger.

In summary

Exhaust gas heat exchangers are an extremely efficient way of recovering a valuable energy resource that would otherwise be lost. Once recovered, the energy can be utilised for a range of uses, which contribute to reducing energy costs. As the UK’s leading manufacturer of exhaust gas heat exchangers, Bowman offer a wide range of high-quality units for applications up to 1 MW using biogas, diesel or natural gas, which are proven in some of the most challenging installations on earth.

For further information on Bowman exhaust gas heat exchangers, download our product brochure or contact our technical sales team on +44 (0)121 359 5401.

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