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Engineers know different. Most of our electricity is generated by turbines spun by steam at high pressure. The energy that boils the water comes from a variety of sources but effectively we can still be said to be in the age of steam.
So it’s vital to ensure that as little as possible of the energy that generates that steam is wasted. In particular, if a fuel is burned to generate steam by boiling water, then the gases given off from burning the fuel will be hot, and that heat can be used to boil more water. The equipment used to do this is called a heat recovery steam generator (HRSG), and these are standard and vital components, not only in power stations but in most industrial plants that have boilers, as the steam they generate can also be used as process steam (for example to heat chemical reactions), or as part of a heating system.
Heat exchangers
HRSGs are a specialised branch of engineering. They share commonalities with process engineering, especially where it overlaps with power engineering, as they are effectively a subcategory of heat exchanger, where hot gases flow past tubes containing a cooler circulating fluid, arranged so that the hottest flue gases are brought against the coolest circulating fluid.
There are several different types of HRSG. One classification derives from the direction of flow of the flue gases that enter the equipment. This can be horizontal, in which case the water that the gases are heating flows in vertical tubes; or vertical, in which case the water flows in horizontal tubes. Apart from the orientation, the main difference between the two is that, in a horizontal HRSG, fluid flow is powered by the higher density of cold fluids than a hot mixture of water and steam; these are therefore also sometimes known as natural circulation systems. This can’t be the case in a vertical HRSG, so their horizontal fluid flow is assisted by pumps: these are sometimes known as forced circulation systems. Vertical HRSGs tend to occupy less space than horizontal systems and are faster to start up.
Another classification for HRSGs derives from how the steam raised by the system is handled. In one type, steam is stored in a drum and sent for further heat exchange with hot flue gases in a section called a superheater, while liquid water is recirculated; the pressure in the drum builds up over time, so it must be made from thick metal to withstand its maximum pressure level. Another type is the 'once-through' HRSG – as the name implies, water is not recirculated and is only heated by the hot flue gases once. Other classifications refer to whether the steam generated is at a single pressure or at multiple pressure levels; power plants tend to use multiple pressure level systems, with several steam drums at different pressures.
Heat recovery steam generator user group 2024
Enjoy peer-reviewed technical presentations and learning opportunities at this year's HRSG User Group meeting on 6-7 November at the Holiday Inn, Coventry.
Attend to understand how to keep your technology up to date, enhance its operational lifespan and remain competitive in the market.
Hear successful strategies and new technologies to enhance the efficiency and operational lifespan of your heat recovery steam generator assets. Share expertise and guidance with manufacturers, owners, operators, engineering thought leaders and maintenance contractors.
Find out more at imeche.org/events.
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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.