|Gauges in a Boiler House |
Where fossil fuels are used, efficiency gains made in steam systems ultimately translate into reductions in the amount of boiler fuel used, which has economic and environmental impacts. The optimisation of steam systems can also result in reduced water consumption, reduced effluent generation rates and also positive impacts on effluent quality.
In future posts I will examine individual aspects of steam systems and how efficiency gains can be realised, but it what I’d like to do in this article is highlight what the components of a steam system are, and why it is necessary to evaluate steam systems in an integrated way. In simple terms, steam is produced by heating treated water in a boiler, changing the phase of the water from liquid to gas. This gas (which may be saturated steam or superheated steam) is then distributed to point of use. The specific purpose for which the steam has been produced determines what happens next. The steam may be used directly, in which case it may be incorporated into products or process streams, it may be used to transfer heat to a process stream using a heat exchanger, or it could be used to drive a turbine. In the case of heating applications, the condensate may be recovered and returned to the boiler for reuse, reducing energy requirements and saving treatment chemicals. In the case of turbines, these may be of the back-pressure variety (in which the exhaust steam is at a lower pressure, but may still be used for various purposes, with or without condensate recovery) or the condensing variety, in which case the condensate may be recovered for reuse.
The major components of a steam system are:
- The boiler, which is used to generate the steam – these may be water-tube or fire-tube boilers, depending on required steam characteristics and quantities. An individual site may employ a number of boilers, and these may use different fuels.
- The distribution system, which is used to distribute the steam to points of use and comprises items such as steam headers, letdown valves (or pressure reducing stations), isolation valves, and traps.
- The individual steam users, the nature of which depends on the industry concerned – some of these users may use the steam consumptively, in which case condensate cannot be returned.
- The condensate recovery system used to capture and return condensate to the boiler – this comprises steam traps and piping along with vessels used to store the condensate before it is pumped.
|General Overview of a Typical Steam System|
This above is a general steam system architecture, but I need to stress that individual sites can have very different steam systems, and each site has to be evaluated on its own merits. The cost inputs to each site’s system also mean that there are no general rules in terms of the feasibility of individual steam system efficiency options. It is therefore vital that a comprehensive system model is constructed that can be used to test various assumptions regarding individual options as well as various option permutations. This latter point is very important, and here I’ll address why a systems approach is so necessary when considering resource efficiency for steam.
Systems and systems thinking are everywhere in the realm of industrial sustainability. To consider steam systems as defined in this post as self-contained would be wrong, since steam systems interact with broader process systems on industrial sites in ways that go beyond thermodynamics and heat transfer. For example, in the dairy or beverage industry, too low a steam pressure in a sterilisation unit can lead to microbial contamination of food products, with dire economic, environmental and possibly social consequences. Always bear the broader system within which steam systems operate in mind when dealing with steam. The construct of a steam system as described here (and note that this is not my concept, this is a generally accepted model, used globally by organisations like UNIDO and the US DOE) is however powerfully relevant from a resource efficiency perspective, and in the series of articles that will follow this one, the focus will be on resource efficiency.
From this perspective, it is easy to see why a systems approach is needed, given the impacts that projects carried out in various parts of the steam system can have on the system as a whole. Here are a few simple examples:
- Condensate recovery is increased. This increases energy recovery to the boiler and reduces fuel consumption. The amount of make-up water required is reduced, as is the quantity of treatment chemicals used.
- High-pressure condensate which was previously discarded is flashed into a lower pressure steam header, reducing the amount of low-pressure steam that has to be produced by the boiler. This reduction in generation rate reduces fuel requirements. Water and chemical consumption are also reduced.
- High-pressure condensate flash from a condensate recovery tank is used to pre-heat boiler feedwater in a heat exchanger, and the condensate produced is recovered for reuse. While still positive from a system perspective, the gains are less than for the “flash-to-header” option, since the efficiency of the boiler now becomes a consideration when converting the recovered condensate into steam. Additional energy is also needed for the boiler feedwater pumps with this option.
There are many more approaches than these that are available in terms of increasing steam system efficiency. Where a basket of options is implemented, expect that they will interact with each other. For example, reducing boiler blowdown rate could hurt the viability of previous projects aimed at recovering blowdown flash, or aimed at blowdown heat recovery (or both, since these could be implemented in tandem). Radical reductions in steam consumption can lead to a boiler operating a far below its rated capacity, negatively impacting on its efficiency. Without the integrated consideration of impacts such as these, you run the risk of defeating the object of your resource efficiency projects. In future posts I will explore individual steam system optimisation opportunities and how they fit together within the context of a steam system. I will also compile a glossary of steam terms which I will make available through my company website, and I will link to that from this blog as soon as it is available. In the interim, start thinking about the individual elements of your steam system and how they relate to each other. Steam systems typically represent a sizable economic opportunity, and are worthy of detailed investigation at every industrial organisation interested in reducing emissions and energy intensity.