| NEM is a recognized leader in the field of heat recovery steam generators (HRSGs) behind gas turbines. Indeed, that is almost an understatement. Looking around the world, we have supplied more than 200 steam boilers for a whole raft of applications, and for a wide range of gas turbine types and classes right up to 250 MW.
Configurations range through one- two- and three-pressure systems with or without supplementary firing, including re- and/or superheating. And then there’s the option of f orced or natural circulation. Whatever the case, we have a proven track in finding the ideal, cost effective technical solution for the specific situation.
Forced circulation
In forced circulation heat recovery steam generators the horizontal gas flow from the turbine is channeled vertically through the boiler, with the flue gases streaming along horizontal fin tubes. Pumps maintain the water/steam flow through the tubes.
Horizontal natural circulation
 In this type, the flue-gases flow horizontally along vertically-positioned fin tubes. Circulation through the evaporators from the drum positioned above the boiler is via down-corners.
Vertical natural circulation
In this type the vertical gas flow is channelled along horizontal positioned fin tubes. Natural circulation is ensured by selecting the correct combination of pressure-less over the evaporators, circulation ratios and relevant drum height vis-à-vis the evaporator.
Vertical boilers- the mechanical concept
Selection of the module or "bundle" concept depends on the size of the boiler and on the local situation. With the first approach, the boiler is fully pre-built as modules in the factory, and the partitions, tube bundles, tube support plates and headers are all linked up.
With the "bundle" concept, only the partitions, tube bundles, tube support plates and headers are linked up in smaller units. Later, on-site, these are positioned in the steel construction. The side walls of the gas path are then fitted into place. In both cases the entire boiler is suspended from above in a steel construction, with the stack supported on this.
Horizontal boilers-the mechanical concept
In the case of smaller boilers we built complete modules with an integral steel construction, which is positioned directly on the foundations. We build larger boilers using pre-fabricated "bundles" of upper- and lower- headers, with intervening fin tubes. Once all this has been suspended in a steel construction, the walls of the gas path are fitted on. This method fully supports the boiler from above.
Three insulation options
Two factors dictate the type of insulation used-customer preferences and the situation on the spot. With vertical boilers there’s a choice of internal or external insulation- or internal insulation only in high-temperature areas. Horizontal boilers are always insulated internally (there’s usually no need for external insulation).
Building on experience
Establishing the optimal dimension for an HRSG demands in-depth expertise on heat transfer by fin tubes. And this must be matched by know-how on the cost-effective design of the heating surface. NEM has all this expertise in-house. We establish the design of the tube diameters, fin height and- for instance- fin distances, for every operational setting. We can rely here on a rich source of experience and practical data from past projects. The same resources are invaluable in other contexts-like an iteration process to define the optimal geometry of a boiler.
Painstaking calculation of dynamic processes
 Powerful, advanced hard- and software now enables highly accurate determination of the dynamic process in STEG installations. All the gas, water and steam transfer elements-including control aspects- are seamlessly melded into a single program. And inputting complex circuits between several gas turbines, boilers and steam turbines is no problem either. Dynamic calculations are a crucial factor in NEM’s constant quest for excellence and leadership in:
Optimizing utilization. To this end, NEM designs fast start-up installations. Those dynamic calculations enable us to give meaningful advice on start-up and cool-down procedures.
Optimising life-span of pressure parts. The fact that we’ve determined the static and dynamic load on pressure parts enables accurate predication of the life-span for the most critical sections. So we can design with this in mind- to a far greatest degree than ever before in the field of boiler construction.
Optimisation of process control. NEM skills plus dedicated control software programs enable fast calculation and evaluation of dynamic behaviour of installations.
Flow tests
 Dimensions are crucial for inlet ducts. They have to be right for flue gases to flow evenly along fin tubes.
This explains all the time we devote to the inlet ducts. For most designs we will use model-based calculations.
Easy access maintenance
We design and supply HRSGs with a long working life and low-cost maintenance in mind. In principle, the only parts needing regular inspection are pumps and diverter valves.
Manhole covers give access to inspection areas between the "bundles". From here, the maintenance team can check the fin tubes with ease.
For more information please contact Hofung Technology or send an email to sales@hofung-technology.com.
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