Patent Application
20120073520
The invention relates to a once-through evaporator for a horizontal-type waste heat steam generator, having a first evaporator heating surface comprising a plurality of essentially vertically disposed first steam generator tubes through which a flow medium flows from the bottom to the top, and another second evaporator heating surface disposed downstream of the first evaporator heating surface in respect of the flow medium direction and comprising a plurality of other essentially vertically disposed second steam generator tubes through which the medium flows from the bottom to the top.
In a gas and steam turbine plant, the heat contained in the expanded working medium or heating gas from the gas turbine is used to generate steam for the steam turbine. Heat transfer takes place in a waste heat steam generator mounted downstream of the gas turbine and in which a plurality of heating surfaces for water preheating, steam generation and steam superheating are normally disposed. The heating surfaces are connected into the water/steam circuit of the steam turbine. The water/steam circuit nonually contains a plurality of, e.g. three, pressure stages, each pressure stage possibly having an evaporator heating surface.
For the steam generator mounted downstream of the gas turbine in the heating gas path as a waste-heat steam generator, a number of alternative design concepts are suitable, namely configuration as a once-through steam generator or as a recirculating steam generator. In the case of a once-through steam generator, the heating of steam generator tubes provided as evaporator tubes results in evaporation of the flow medium in the steam generator tubes in a single pass. In contrast, in the case of a natural or forced circulation steam generator, the circulating water is only partly evaporated as it passes through the evaporator tubes, the water that is not evaporated being is re-fed to the same evaporator tubes for further evaporation after separation of the generated steam.
In contrast to a natural or forced circulation steam generator, a once-through steam generator is not subject to any pressure limitation. A high live steam pressure promotes high thermal efficiency and therefore low CO2 emissions from a fossil-fired power plant. In addition, a once-through steam generator has a simple type of construction compared to a recirculating steam generator and can therefore be manufactured particularly inexpensively. Using a steam generator of once-through design as the waste heat steam generator of a gas and steam turbine plant is therefore particularly advantageous for achieving a high overall efficiency of the gas and steam turbine plant without constructional complexity.
A once-through steam generator designed as a waste heat steam generator can basically be implemented in one of two alternative types of construction, namely vertical or horizontal. A horizontal-type once-through steam generator is designed to provide an approximately horizontal flow path for the heating medium or heating gas, e.g. the exhaust gas from the gas turbine, whereas a vertical-type once-through steam generator is designed to provide an approximately vertical flow path for the heating medium.
In contrast to a once-through steam generator of vertical design, a horizontal-type once-through steam generator can be manufactured using particularly simple means and with particularly low production and assembly costs. In both types, particularly in the downstream (in flow medium direction) steam generator tubes of the second evaporator heating surface, an uneven distribution of the two-phase flow medium to the steam generator tubes may occur within each individual row of tubes, resulting in temperature imbalances and mechanical stresses caused by differential thermal expansion. In order to prevent damage to the waste heat steam generator, expansion bends, for example, have therefore hitherto been fitted to compensate for these stresses. However, this measure can be comparatively complex technically in the case of a horizontal-type waste heat steam generator.
The object of the invention is therefore to specify a once-through evaporator for a waste heat steam generator as described in the introduction, which permits a particularly simple type of construction while providing a particularly long service life.
This object is achieved according to the invention by providing a plurality of second steam generator tubes with an internal profile.
The invention is based on the consideration that a particularly simple design of the waste heat steam generator or more specifically of the once-through evaporator could be achieved by dispensing with the hitherto customary expansion bends. In doing so, however, another way must be found of reducing the mechanical stresses caused by temperature imbalances in the parallel connected steam generator tubes of each individual tube row. These occur particularly in the second evaporator heating surface to which the water/steam mixture is applied. The temperature imbalances are caused by different proportions of water and steam in the flow medium at the inlet of the individual tubes of a tube row and a resultant differential flow through these tubes. It was realized that this differential flow in the tubes is caused by the frictional pressure drop in the steam generator tubes being low compared to the geodetic pressure drop. In fact, a flow with a high proportion of steam in the flow medium flows comparatively quickly through individual steam generator tubes with low frictional pressure drop, whereas a flow with a high proportion of water is disadvantaged because of its higher geodetic pressure drop due to weight and may tend to stagnation. In order to make the flows uniform, the frictional pressure drop must therefore be increased. This can be achieved by a plurality of second steam generator tubes having an internal profile which causes an additional frictional pressure drop of this kind.
In order to a achieve a particularly high additional frictional pressure drop, the laminar boundary layer on the inside of the tubes must be reduced. This can be achieved by producing turbulence in the tube. This effect can be amplified still further by swirling of the flow medium. Swirling of this kind can be produced by advantageously making the internal profile helical-spring-shaped.
Said frictional pressure drop must be appropriately determined using the normal operating parameters such as tube geometry, the dimensions of the heating gas path and the temperature conditions. Advantageously, the geometry of the particular internal profile must then be selected such that the predefined frictional pressure drop of the flow medium is obtained via the respective second steam generator tube. This provides an even better means of preventing temperature imbalances.
In an advantageous embodiment, the particular internal profile is provided in the respective second steam generator tube as a kind of internal finning, thereby allowing a particularly simple once-through evaporator or rather waste heat steam generator design.
In order to facilitate retrofitting to existing steam generators and/or achieve greater flexibility in the design of steam generators in respect of the tube geometries, the particular internal profile is advantageously inserted in the respective second steam generator tube as a fitted component. The internal profile is therefore implemented as a separate component and disposed in the steam generator tubes.
In an advantageous embodiment, a plurality of second steam generator tubes are connected in series in the heating gas direction as tube rows. This enables a larger number of parallel connected steam generator tubes to be used for an evaporator heating surface, which means a better heat input due to the enlarged surface. However, the steam generator tubes disposed in series in the heating gas flow direction will be differentially heated in this case. The flow medium will be comparatively strongly heated particularly in the steam generator tubes on the heating gas inlet side. However, as a result of the described design of the second steam generator tubes with an internal profile, a through-flow matched to the heating can also be achieved in these steam generator tubes, thereby ensuring a particularly long service life for a waste heat steam generator of simple constructional design.
In an advantageous embodiment, the first evaporator heating surface is connected downstream of the second evaporator heating surface in respect of the heating gas direction. The advantage of this is that the second evaporator heating surface connected downstream in respect of the flow medium direction and therefore designed for further heating of already evaporated flow medium is also in a comparatively more strongly heated region of the heating gas path.
A once-through evaporator of this kind can be usefully installed in a waste heat steam generator and the waste heat steam generator employed in a gas and steam turbine plant. Said steam generator is advantageously connected downstream of a gas turbine in respect of the heating gas direction. In this configuration, supplementary firing equipment can be usefully connected downstream of the gas turbine to increase the heating gas temperature.
The particular advantages achieved by the invention consist in that, by providing the second evaporator tubes with an internal profile, an improvement is achieved in the distribution of the flow and therefore a reduction in the temperature differences between parallel connected second steam generator tubes and in the resultant mechanical stresses, thereby ensuring a particularly long service life of the waste heat steam generator. The appropriate provision of the internal profile enables other complex technical measures such as expansion bends to be dispensed with, while at the same time allowing a particularly simple, cost-saving design of the waste heat steam generator or more particularly of a gas and steam turbine power plant.
An exemplary embodiment of the invention will now be explained in greater detail with reference to the accompanying drawings in which:
Identical parts are denoted by the same reference characters in all the figures.
The once-through evaporator 1 for the waste heat steam generator 2 as shown in
The evaporator heating surfaces 8, 10 shown in
The flow medium W which can be applied to the evaporator system constituted by the evaporator heating surfaces 8, 10 is evaporated in one passage through the evaporator system and, after exiting the second evaporator heating surface 10, is discharged as steam D. The evaporator system formed by the evaporator heating surfaces 8, 10 is connected into the water/steam circuit (not shown in greater detail) of a steam turbine. In addition to the evaporator system comprising the evaporator heating surfaces 8, 10, a number of other heating surfaces 20 schematically indicated in
The second steam generator tubes 14 now have a helical-spring-shaped internal profile 22 which is illustrated in
The effect of the internal profile 22 on the temperature differences is illustrated in the graphs in
By reducing the temperature differences of tubes with differing steam content at the flow inlet, the mechanical stress loading of the waste heat steam generator 2 is reduced and a particularly long service life is ensured, while at the same time allowing a simple type of construction by eliminating the hitherto customary expansion bends.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2009 024 587.1 | Jun 2009 | DE | national |
This application is the U.S. National Stage of International Application No. PCT/EP2010/055886, filed Apr. 30, 2010 and claims the benefit thereof. The International Application claims the benefits of Getman application No. 10 20009 024 587.1 DE filed Jun. 10, 2009. All of the applications are incorporated by reference herein in their entirety.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP10/55886 | 4/30/2010 | WO | 00 | 12/6/2011 |
