Exhaust Gas Heat Exchangers – The Great Pressure Drop Myth!

Adding a heat exchanger to a gen-set's engine exhaust stream not only recovers valuable heat energy, it can also help to reduce pressure drop!

There’s a commonly held theory that adding an exhaust gas heat exchanger (EGHE) to an engine powered generating set is detrimental to its performance.

The theory is based on the fact that pressure drop of between 8-10 kPa is often experienced in an exhaust system, taking into account the length of pipework (straight sections, plus elbows and bellows), plus the silencer itself and that by adding an EGHE, the pressure drop will increase still further.

So, for installations, where recovering waste heat would be a benefit, end users often lose out on this valuable ‘free’ energy source, simply due to a misunderstanding of the way an EGHE operates!

Adding any component into an exhaust system creates the potential for greater pressure drop, so how can an EGHE be the exception to this rule?

The fact is an EGHE reduces pressure drop due to the exhaust gases being cooled by the unit!

Cooler exhaust gas has a higher density, which results in a lower velocity for the same mass flowrate.

As pressure drop is directly proportional to the square of the velocity, a reduction in velocity will result in a lower pressure drop through the system.

To illustrate this, Bowman has performed fluid dynamics calculations based around their 6-40 exhaust gas heat exchanger, installed into a 6” diameter exhaust system, complete with a silencer geared for a noise sensitive installation.

In this programme, the exhaust temperature is reduced from 600 °C to 170 °C and the calculation shows a pressure drop improvement of 5% on the overall system (not including pipework), which typically has a maximum limit of 8-10 kPa.

Bowman exhaust gas heat exchangers are designed to ensure pressure drop remains under 2 kPa on engines rated from 16 kW to 950 kW.

For example, on a 140 kW engine, recovering around 100 kW of heat energy, the pressure drop is around 1.9 kPa, virtually the same as for a 950 kW engine recovering over 650 kW of exhaust heat energy!

In a world where there is growing pressure to improve energy efficiency, recovering waste heat is a relatively straightforward way of increasing an engine powered gen-set’s efficiency – typically from around 30% (power only) to around 80% (combined heat & power).

That heat energy can be used for a wide range of commercial or industrial uses, including district heating and hot water, process heating, or even generating more power!

Bowman have a range of exhaust gas heat exchangers, suitable for engines rated from 16 kW to 950 kW, offering heat recovery from 9.5 kW to 673 kW. All are built to high quality standards to offer high performance and durability in a compact design, easily integrated into most gen-sets.

Additionally, the company also has a computer based selection programme, so can recommend the most appropriate heat exchanger for the application, or operating conditions.

For more information on Bowman exhaust gas heat exchangers, call the technical sales team on +44 (0)121 359 5401, or email [email protected].