Plant operators requiring relatively small amounts of process steam for variable or intermittent loads have long been turning to rapid steam-raising plant to meet their needs. It works in the opposite way to the decades-old technology of a firetube boiler. That latter type, typically used in large and diverse steam systems for instance, contains the water and the high pressure steam produced within the shell of the boiler. The hot gases from combustion are passed through tubes immersed in the water to produce steam. A rapid steam generator will most usually feature a continuous tubular coil that contains the water and steam and forms the pressure-containing part of the boiler. Water passes through this coil whilst combustion gases are channelled over the coil surface to produce the required quantity of steam. They are often referred to as coil boilers for this reason. Other types of steam generator include vertical tube boilers, but work in a similar way.
Typical applications for rapid steam generators include the food sector for ovens, jacketed pans or micro-breweries and distilleries, direct steam injection and cooking in vessels and providing hot water through heat exchangers. Other applications include humidification loads during dry days in winter (and perhaps not at all during summer) typically for government facilities, universities, or pharmaceutical R&D installations. There are also many applications within the health care sector, such as with autoclaves and other sterilisation applications, or providing heat and humidity for operating theatres in hospitals, for instance.
One of the characteristics of all these load profiles is their variable or intermittent nature. This is a key factor when it comes to specifying steam-raising plant and is a point emphasised by Carl Knight, managing director of boiler manufacturer Fulton: “It’s virtually impossible to get a steady load. Steam loads vary enormously dependent on the type of equipment that they actually supply.”
The need for speed
Chris Horsley, managing director of boiler manufacturer Babcock Wanson, explains why this kind of application is well suited to the performance of the coil-type rapid steam generator: “One of the key advantages of the coil steam generator is it’s quick in raising steam from a cold start. Most fire tube boilers, if you’re lucky, take an hour minimum to respond from cold, and this can easily extend to several hours for the larger ones, before they will start to produce steam. Coil types will very, very quickly generate steam due to the inherent low water content. Typically in three to four minutes it’s producing steam, and in five to seven minutes it’s producing steam at 10 bar.”
He continues: “The reason they can raise steam so quickly is a coil type boiler contains a very small amount of water during operation. If you’ve got an enormously peaky load, they’ll also react quickly, as there’s no stored thermal mass in the boiler.”
Horsley uses the growing craft brewing industry as an example: “Because they’re doing small batches, they want to switch the boiler on to heat the process and then switch it off when they are finished. Because there’s no getting away from it: running steam boilers consumes vast amounts of energy.”
(However, Knight argues that new-generation fire tube boilers are capable of raising steam in as little as 10-15 minutes, and offer considerable benefits in meeting variable demand when compared with coil designs: “Being able to start from cold offers no advantage when dealing with varying steam loads.” He notes: “Fire tube or shell boilers have stored energy in their water volume, helping to meet variable demand for steam. This ‘reservoir’ of energy does not exist in coil boilers.”)
Typically, coil boilers generate saturated steam at up to 10 bar and at a saturation temperature of circa 185°C, though both higher and lower pressures are possible, depending on the application. Outputs range from 20-30kg of steam per hour. For example, Horsley says of Babcock Wanson: “We’re working from 150 kg/h on our smallest model right up to 10,000 kg/h on the largest one, and everything in between.”
Thermal expansion can also generate enormous stress within the fabric of a boiler during an operational cycle. While they are certainly designed to manage these loads, this kind of thermal cycling can introduce fatigue which can reduce the equipment lifespan.
Though the laws of physics mean that coil designs also expand and contract during a thermal cycle, they are inherently less susceptible to fatigue. “The coil is an immensely strong structure; we can easily run a coil boiler to 100 bar in both closed and open circuits should the customer require it, and at even higher pressure. The metal will expand and you have to take account for expansion, but you can easily build an expansion facility into the design. Like a spring, we can move,” says Horsley.
Furthermore, a coil boiler installation requires a far less substantial foundation than a fire tube boiler, as the volume of water, and therefore operating mass, is far smaller.
Although there are a number of advantages making it ideal for, say, batch process steam production, the nature of the coil boiler does present some issues for operations engineers.
Knight explains: “Within a coil generator, there is a small area where the water transitions into steam. Because it happens at the same place on the coil all the time, that part of the coil is very vulnerable. You need to make sure that the feedwater is correctly treated.”
This is a point picked up by Horsley too, who says: “All boilers need water treatment, they all follow the same basic requirements.” But he adds a note of caution for coil steam generator operators here: “Coil boilers are in one way more tolerant of poor feed conditions, but can also be susceptible to poor water treatment. If you are abusive to the boiler for long enough, there is a good chance you’ll damage [it], and the results may well be seen quicker than with a fire tube boiler. That’s the con. The pro is it’s easy to repair, and more importantly there is very limited risk from the results of the failure, and really no steam side explosion risk even from a major failure of the coil.”
(At the same time, Knight argues that the latest generation of fire tube boilers are also easy to repair. “With no refractory metals being used in modern boilers, repairing or replacing damaged parts is easier and more cost-effective.”)
The impact of water quality clearly has a profound impact on the lifespan of all types of boiler. To help addressthis risk, Babcock Wanson offers an extended corrosion warranty, on condition that water treatment is conducted by the company. Says Horsley: “We’ll do your water treatment and we’ll guarantee you will have no corrosion failures. We provide the customer with a digital app. Every day, the customer tests the water and enters the figures, and we feed back the results to adjust the water treatment accordingly. It’s a simple system that is proven to provide excellent results over many years.”
Existing safety requirements may work against coil steam generators’ natural advantages. Knight explains: “Typically, boilers tend to be oversized, and when they’re oversized, it means they are likely to be cycling a lot. The burners will switch off and switch back on again, and this can occur many, many times a day.
“Legislation requires that when boiler equipment is restarted, it must purge the combustion chamber for safety reasons. If you do that many, many hundred times a day, a lot of cold air is being blown through the boiler, which effectively cools it. Every time you do that, you lose energy up the chimney.”
An approach to address this issue which is gathering traction in the industry is to enable a high turn-down capability in rapid steam generators. In this scenario, rather than shutting down and cycling the steam generator, it is instead operated at minimal load during low demand periods. The potential savings are significant.
He adds: “When you talk about turn-down, the efficiency gains can be massive. We’ve seen areas where the fuel savings have been more like 22-25% because of turn-down, because the boiler doesn’t actually switch off. When the load diminishes, the boiler ramps down and the boiler can sit at that low rate for long periods. As the steam load appears on the system, it can rapidly modulate to full fire to meet that load. The boiler will still switch off at times, but instead of switching off up to say 150 times a day it may only switch off five or 10 times a day,” adds Knight.
This potential efficiency gain is further emphasised by Knight, who says: “For example, there’s a project in South Wales where a burner manufacturer has been called in to upgrade some burners on some very large boilers. They’ve managed to achieve an eight-to-one turn-down ratio, and the savings have been nearly a quarter of a million pounds a year.”
He adds: “The movement going forward is to increase turn-down ratio. It’s to get the boilers to run with more digitalization so you can interrogate them from a distance; that’s the way we’re moving now.”
Indeed, the emergence of digital controls is also having a profound effect on the performance of rapid steam generators. As Horsley says: “With the drive of automation, all boilers these days are bristling with electronics, whether it’s a fire tube boiler or a coil steam generator.”
The array of sensors and controls is not only improving efficiency and turn-down capabilities, but is also helping to control emissions, another key driver in the development of coil steam generators.
Late last year, the Medium Combustion Plant Directive (MCPD) became law in Europe, stipulating stringent emissions limits of NOx and SOx (see also www.is.gd/jovexe). This meant considerably better sophistication in control of combustion in burner systems over 1 MW. Knight explains: “Mostly it’s through design of circulation and design changes to the burner, the way you control the flame in the burner to get the NOx levels below a hundred milligrams. It’s about driving up efficiency and driving down emissions.”
Reflecting these trends, last year Fulton introduced its VSRT vertical steam boiler, which boasts turn-down rates of up to 10:1 and what it calls a unique design using a spiral-rib heat exchanger to improve efficiency and durability.
“The low-emissions requirement, turn- down, flexibility and sophistication of the controls, how you manage them more precisely: those are trends that we’re expecting to see more of,” says Knight.
He adds: “Some of our customers want the very cheap and cheerful boilers that last, and they don’t all want sophistication. But it’s a trend that we’re certainly seeing. In terms of the larger systems above one megawatt, there is legislation. MCPD demands low NOx levels.”
Looking ahead, it is clear that all types of boilers and steam generators are becoming more sophisticated and more controllable. As Horsley observes: “It’s all about automation, turning the plant off when you don’t need to use it, turning it on quickly and safely when you do; matching the steam generation to the process needs as close as you can.”
While there are evident commercial and environmental benefits, smarter steam generators place greater demand on smarter operators too. As Horsley concludes: “For operations engineers, the important thing these days is that boilers are getting quite complex. Training is key now in terms of cost saving.”
BOX OUT: New steam generator training now available
The Boiler Operator Accredited Scheme (BOAS) training has been expanded in a new category 5, for steam generators. Training is at two levels: steam generator operator and engineer/manager. The one-day coil boiler class for operators is now being organised by Babcock Wanson (www.is.gd/uneluv). A three-day engineer/manager course is being organised. More information: www.is.gd/keneso