STEAM SYSTEMS
at one point (both for Low Temperature Hot Water and domestic hot water) is that it allows the size and potential of the CHP plant to be maximised, providing greater electrical output. From this central point, the boiler or CHP plant can then produce steam to be distributed around the site. Steam does not require distribution pumps, eliminating the associated electrical consumption and cost that would be present with water systems.
The distribution system and its impact
Let’s think about the distribution system, and the impact it has at the point of use, in the plantroom, and in the heat exchangers. Steam contains large amounts of energy in every kg, but how does this compare with the alternative? Water systems are often used with a flow and return difference of 20˚C, in which case, each kg (or litre) of water distributed will contain about 84 kJ of energy (4.186 kJ/kg˚C x 20˚C). In contrast, the same single kg of steam contains over 2700 kJ when delivered at 7barg. Even when one considers that hot condensate (with a residual energy content) will be returned back to the boilerhouse from the point of use, the useful energy in the steam – the bit that gets imparted to the hot water via heat exchange – typically has over 2000 kJ/kg; that is 23 times more energy per kg than the water example. So, what does this mean for the installation? The answer is smaller pipes, lighter pipes, and less space needed in the plantrooms and ducts – in other words, a compact, pump- free way of distributing large amounts of energy across site.
Effective domestic hot water production
Here we come to the crux of this article – effective DHW production, and the impact of good practice in the wider steam system. The importance of achieving dry steam must be considered when designing a new installation or managing an existing system. Distributing the steam at high pressure keeps pipe sizes small, which results in a low pipe surface area and minimises radiation losses; this of course must be combined with effective lagging, whatever medium is being used.
Any condensate formed in the distribution pipework should be removed by using steam traps at regular intervals and at key points in the system. Remember, the aim is to deliver dry steam. This condensate is returned to the boilerhouse for re-use. We want good quality, dry steam arriving at the heat exchanger in order to maximise efficiency and minimise steam consumption. Wet steam contains a proportion of water and,
Taking action in the boilerhouse and in the distribution system directly influences the quality of the steam, and hence the effectiveness of the heat exchangers in the plantrooms being used to generate Domestic Hot Water.
as seen earlier in the article, it is the steam that has the very high energy content when compared to water.
A barrier to heat transfer Non-condensable gases should be removed from the distribution system, as they create a significant barrier to heat transfer if they enter the heat exchanger with the steam. This is achieved with the use of air vents fitted at strategic points in the pipe system.
Taking a step back from the distribution, the impact of gases can be minimised with good practice in the boilerhouse. The feedwater delivered to the boiler should be de-oxygenated as much as possible, and this can be done by ensuring that oxygen-scavenging chemicals are correctly used. It is important that the feedwater is kept at a high temperature in the boiler feedtank to allow dissolved oxygen to be driven out of solution, even before it reaches the boiler. Typically it is good to see temperatures of at least 85˚C in an atmospheric hotwell, or higher if it can be achieved without feed pump cavitation. This is the case for atmospheric feedtanks, but there is a more effective alternative. Pressurised dearation is the best way to de-oxygenate the feedwater, as it allows the water to reach temperatures above 100˚C, driving non- condensable gases out of solution. This is why the bigger picture is so important. Taking action in the boilerhouse and in the distribution system directly influences the quality of the steam, and hence the effectiveness of the heat exchangers in the plantrooms which are being used to generate Domestic Hot Water. Good quality steam will also have a positive effect on the longevity of the components such as controls valves and steam traps, minimising maintenance and maximising uptime.
By avoiding reliance on storing hot water, and employing best practice across the site, healthcare estates teams can improve energy efficiency, reduce the risk of Legionella, and deliver reliable, consistent hot water to healthcare workers and patients alike.
Angelo Giambrone
Angelo Giambrone has built a wealth of experience in steam over the past 30 years. He joined Spirax Sarco as a Technical Sales engineer, working with steam users across the industry spectrum, and developing a varied application knowledge. As part of the Engineered Systems department, his focus moved towards identifying energy-saving schemes and delivering packaged solutions into the market, which included the introduction in 2000 of the EasiHeat plate heat exchange system now used widely within the hospital sector.
In recent years he has worked in Business Development, looking at how Spirax can best work with its customers. His ongoing involvement with hospitals led to him recently being appointed as Regional Business Development manager for the Healthcare sector in Northern Europe, which covers both hospital and pharmaceutical responsibilities.
October 2020 Health Estate Journal 83
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