Traditional boilers or steam generators

A modern boiler is not just a very large kettle with added controls, as many still seem to believe. The latest coil-type steam generators are safer, smaller, faster and more precisely controllable than traditional drum or firetube boilers, and they are virtually maintenance free.

Combustion processes are today controlled by high-technology computer-based systems, often using data supplied by extremely precise gas analysers to minimise both fuel consumption and emissions. Feedwater quality to boilers or steam generators is monitored, as are the temperatures of feedwater, air and fuel. Steam quality is closely watched and controlled to eliminate problems such as water carryover into the steam supply. The steam pressure must be constant and the response to changes in demand must be fast and precise.

In these demanding conditions, even in process installations requiring large volumes of steam, the traditional drum and firetube boilers are increasingly being replaced by the latest generation of coil steam generators. These have dramatically improved reliability compared with earlier generations of steam raising equipment, and are often installed at the point of steam use rather than in the plant room. My own company, Babcock Wanson UK, is routinely supplying and maintaining steam generators with outputs as high as 8 tonne/h of steam.

Although many applications require centralised steam generation and distribution, compact steam generators now make point-of-use generation straightforward. This eliminates major energy losses that can occur during distribution and both cuts steam costs and improves steam quality. Savings can be as much as 40 per cent of the energy in the fuel used.

Steam generators can also safely use a much wider range of water qualities with, for example, a high level of total dissolved solids (TDS). This is because, in a shell or drum boiler, the quality of steam produced depends to a large extent on the area and quality of water at the steam/water interface. As the rate of steam take-off increases, the condition of the surface becomes critical. Even with ideally conditioned water, the surface will eventually break up, so that water is carried over with the steam.

To avoid undue priming and carryover in a modern firetube boiler, a limit for TDS within the drum has to be set, generally between 2500 and 3000ppm, depending on the size of the boiler and the operating conditions. To sustain 2500ppm within the boiler drum with feedwater of around 400ppm TDS, a blowdown rate in excess of 15 per cent is needed - with all the attendant losses of heat, effluent and treatment chemicals.

In contrast, in a steam generator the feedwater is pumped under positive volume conditions and is, therefore, not free to prime. Much higher TDS levels can be sustained; the solids being removed from the steam separator in concentrated form - effectively, continuous blowdown.

Steam generators are frequently selected for their ability to tolerate high TDS waters. The energy savings are clearly significant, particularly when waters with TDS in excess of 300ppm are to be handled.

Apart from this tolerance of high TDS, water requirements for a steam generator are generally similar to those for other boilers, as outlined in BS 2486 - hardness, oxygen and alkalinity being the principal areas to be considered. The main difference is that water treatment has to be completed before entry into the steam generator.In general, there are five key design areas for maximum effectiveness in a steam generator. These are the combustion equipment, coil design and heating surface, and the feedwater pump and feed system.

The Babcock Wanson Autoflame, used in the company's larger VPX steam generators (above), is typical of purpose-designed burners, matched precisely to the generator combustion chamber and heating surface. In a steam generator, the combustion chamber or furnace is constructed from the tubular coil and usually forms the first path for incoming feedwater.

The feedwater pump on a steam generator also differs from that on a drum or firetube boiler, as it has to be a positive displacement pump, usually a piston pump of single- or multi-cylinder design. Babcock Wanson's generators, for example, use the its W type rotary piston pump, a hydraulic diaphragm unit with a self-adjusting drive. Firetube boilers will continue to have their place, particularly for very large steam outputs. But the future lies with the electronic control compatibility and efficiency of the steam generator.

Paul Crighton is managing director of Babcock Wanson UK