VERTICAL MULTIPLE PASSAGE DRAINABLE HEATED SURFACES WITH HEADERS-EQUALIZERS AND FORCED CIRCULATION
The present invention discloses improved heated surfaces (HS) with vertical multiple passage panels or vertical serpentine coils from straight tubes with connections between them by top and bottom bends. In HS with tube bends there is not any mixing headers—each circuit has a single tube from inlet header to outlet header. This increases mass velocity of flow and improves stability and temperature regulation of tubes. The bottom bends have holes. The bottom bend holes of the adjacent passes are connected with drain header by drain stubs. Each header serves to drain the adjacent tube passes and as equalizing header of pressure/flow. It will help to decrease multivaluedness and maldistribution of flow between parallel tubes of the module. Such design of HS noticeably decreases the corrosion of tubes.
The present application claims priority to U.S. provisional application No. 62/062,055 filed on Oct. 9, 2014.
BACKGROUND OF THE INVENTION1. Field of the Invention
The present invention is the heated surfaces (HS) for application in the different technical fields—power industry, metallurgy, chemical industry, etc. Below the invention is considered in the context of power industry.
2. Background Art
For units of power industry which are based on Brayton and Rankine cycles the general tendency is increasing, the initial parameters—pressure and temperature to improve, the thermal efficiency (TE) and decrease carbon dioxide emissions. In the modern conventional units the pressure is well above the critical pressure. The boilers of such units are once through type.
The same tendency with pressure increasing can be seen in the case of Combine Cycle Power Plants (CCPP) as well. At present time the pressure in steam generators of CCPP is below of critical pressure. However in the future heat recovery steam generators (HRSG) the pressure will be above critical pressure. The cycle of CCPP with HRSG of supercritical pressure (SCP) can have the thermal efficiency well above 60%. The SCP HRSG could be realized as once through boilers (OTHB) only. The subcritical conventional boilers and HRSGs can be as once through (circulation ratio is equal one) units and with forced circulation (circulation ratio is above one) as well. These two types of units are the subject of invention. The conventional boilers and the HRSGs of subcritical pressure with natural circulation are not considered here based on essence of invention.
The main problems of once through units and units with forced circulation are corrosion/erosion, temperature regime of heated surfaces, and hydrodynamic instability. In accordance with Electrical Power Research Institute data the main failures of boilers and HRSGs are as result of corrosion of heated surfaces. The suggested invention can improve the effectiveness and reliability of multiple passage heated surfaces and increase TE of whole units.
SUMMARY OF THE INVENTIONA principal peculiarity of suggested heated surfaces with forced circulation are the vertical multiple passage panels (usually used in conventional boilers) and vertical serpentine coils (usually used in HRSGs) from some rows of straight tubes with connections between them by top and bottom bends. Such types of HS can be used for different elements of boilers and HRSGs—water preheaters (WPHTR), economizers (EC), evaporators (EV), superheaters (SH), and reheaters (RH).
In HS with tube bends there are not any mixing headers—each circuit goes like a single tube from inlet header to outlet header. It allows increasing mass velocity of steam/water or steam-water mixture for subcritical pressure (or supercritical fluid) and improves stability of flow and temperature regime of tubes.
To have opportunity for water drain at the lower point of bottom bends there are the holes. The bend holes of the adjacent passes are connected with drain header by pipe stubs. Each header serves for drain of the adjacent tube passes. At the same time the header can serve as equalizing header of pressure/flow. In the case of the different thermal-hydraulic characteristics of parallel tubes there is opportunity for bypass flows. It will help to decrease multivaluedness and maldistribution of flow between parallel tubes of the module. Besides such design of HS noticeably decreases the corrosion when the units are not in operation.
The present invention will now be discussed in further detail below with reference to the accompanying figures in which:
All types of thermal power plants are emitting a lot of waste heat into the atmosphere. To improve the thermal efficiency and decrease carbon dioxide emissions a noticeable portion of waste heat can be captured and used for the different goals. Let's consider the issue in the context of Combine Cycle Power Plants (CCPP) as one of the most effective cycles in power industry. In this case the heat of exhaust gas after gas turbines (GT) can be used in heat recovery steam generators. The steam after HRSG goes into steam turbine (ST) which drives electrical generator. The thermal efficiency of such combined cycle (Brayton and Rankine cycles) can be above 60%. In contrast a single ST cycle of subcritical pressure is limited around 40%, the best modern GTs have thermal efficiency about 45%. The modern units with ST cycle of supercritical pressure have achieved 43-44%. In this respect the future tendency in this branch of power industry should be the CCPP with HRSG of supercritical pressure (SCP). The SCP HRSG could be realized as once through boilers (OTHB) only (it means a forced circulation in all elements of boiler including zone of maximum thermal capacity). At the same time the conventional boilers of supercritical pressures are in operation during many years already.
There are some publications regarding SCP HRSG with horizontal design of heated surfaces and vertical gas flow. However there is no information about any practical application of this approach. The most common technology of subcritical horizontal OTHB is Benson HRSG licensed by Siemens (
One of the most important and key element in once-through boilers and HRSGs is evaporator (pseudo EV in the case of SCP). It is a reason for consideration below of HS as the EVs in the first turn. In subcritical EV there is two-phase flow (steam/water mixture) in all range of operational parameters. In supercritical boilers two-phase flow could be under partial loads. Two-phase flows have very complex structure, hydrodynamics, and very complex behavior of heat transfer and hydraulic resistance coefficients. The stratification of two-phase flows, deposition of salts, corrosion/erosion issues, critical heat fluxes, and instability are the major concerns for designers of once-through HRSGs and conventional boilers.
There is a good experience in design and operation of conventional once through units and units with forced circulation (
The tube panels of conventional boilers with forced circulation (EC, EV, SH and RH) have the very different layouts. For example, on
In the coils of HRSGs there is similar problem. For example, in the earliest types of serpentine coils there were some non-drainable bottom bends. Because of strong corrosion the bottom bends were replaced with bottom headers (
The tubes of HRSG heated surfaces are the finned tubes usually. In the case of relatively big heat fluxes some rows of HRSG can be manufactured from the bare tubes. A principal peculiarity of suggested heated surfaces for HRSG conditions is the vertical serpentine coil from some rows of straight tubes with connections between them by top and bottom U-bends 506 (
Let's consider the specific of suggested HS for EV of HRSG because the physics of two-phase flow is more complex as the single flows. In case of EV with tube bends (
At the same time the header can serve as equalizing header of pressure/flow. In the case of the different thermal-hydraulic characteristic of parallel tubes there is opportunity for bypass flows. It will help to decrease multivaluedness and maldistribution of flow between parallel tubes of the module. There is sense to underscore that bottom portion of coil (from the hole in bottom bend to drain pipes) will be filled in with water (for subcritical EV) or heavy phase (for supercritical EV). It is a result that in bottom bends there is a centrifugal force, which in many times more than force of gravitation. Besides in the bottom bend the both vectors of centrifugal force and force of gravitation are coincided (both are acting in downward direction). Both forces are proportional to density of medium. It means that integral force for water will be more than for steam (for subcritical pressure) or for heavy phase will be more than for light phase (for supercritical pressure). Besides, in vertical downward two-phase flows, as rule, velocity of droplet is more than velocity of steam. It means that centrifugal force for droplet in bottom bend will be more than for steam. This results in that all operation conditions the bottom portion of coil (from drain stubs and below) will be filled in with water (for subcritical EV) or heavy phase (for supercritical EV). This is very important for thermal-hydraulic processes in the coil.
In some HRSG design conditions drain system is situated in the area of relatively hot exhaust gas. To avoid thermal-mechanical problems the drain system should have the proper temperature regime. It can be achieved by circulation of small amount of water (or supercritical liquid) through drain system. For this goal two adjacent drain lines are connected by drain cross over (drain bypass) pipe (
Temperature regime of drain system can be reliable under small water bypass if it is situated out of the main exhaust gas flow (in the area of relatively stagnant gas flow). Such scenario can be realized with help of gas baffle plates (
In the case, when the coil is situated in very high range of gas temperatures, the temperature regime of drain system can be kept on reliable level with help of water cooled walls. The walls could be fabricated from membrane tubes to minimize gas bypass. Water for the wall is used after EC before going to EV. It is possible as well to take water for the wall between the sections of EC. Thermodynamic efficiency is taken into account in each case. A direction of water flow in the wall could be counter flow, parallel flow, or perpendicular with exhaust gas flow. Designer has to take into account the peculiarities of temperature regimes of the coil and water cooled wall in the contact area of tubes with different wall temperatures. In the case of multi wide HRSG the gas baffle plates should be installed between the water cooled boxes of the different modules. In most cases the water cooled walls will not be necessary.
Temperature regime of once-through HRSG EV tubes could be different from HRSG EV tubes with natural circulation. In OTH HRSG EV subcritical or supercritical pressures could be a zone with deterioration of heat transfer. It means that under any enthalpy of fluid there is a jump in tube wall temperature. The value of temperature jump depends on parameters of exhaust gas flow, as well of water pressure, mass velocity, and heat flux for given geometry of coil tubes. For any combination of these parameters temperature jump could be strong enough. To improve the tube temperature regime the different types of intensificators can be used (rifled tubes, inserts, etc.).
A special attention should be paid to bottom bends and connections with drain stubs (
The HRSG can be operated under supercritical and subcritical pressures. Under nominal conditions a unit can be supercritical but under part loads the pressure in system can be subcritical. Besides on the EV outlet a two-phase flow could be, but not superheated steam. To prevent a steam-water mixture going in superheater the special separators should be on the outlet of EV (see
Relatively simpler thermo-mechanical situation is in the header—equalizer and the drain system of multiple passage panels of conventional boilers (
Special attention should be paid to the position of header—equalizer. The header should be a horizontal orientation to avoid the effect of possible gravitational component in pressure drop (it is specific of vertical and inclined headers). Header should be situated below the lowest row of the panel. It can guarantee that header will be filled in with water (for subcritical pressure) or heavy fluids (for supercritical pressure). This provision is very important for stability of flow.
The length of heated tubes and pipe stubs of panel different circuits in the connection points with headers could be different. In this respect the special attention should be paid to equal pressure drop. The possible way to do it is to adjust the sizes of the holes in the bottom bends and in the header—equalizers in such manner to have the equal pressure drop in all parallel circuits.
While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, and examples herein. The invention should therefore not be limited by the above described embodiment, and examples, but by all embodiments within the scope and spirit of the invention.
Claims
1. Heated surfaces (HS) with forced circulation of liquids/gases, comprising:
- a vertical multiple passage panel or serpentine coil comprising rows of vertical straight tubes with connections between them by top and bottom bends;
- each bottom bend having a drain stub;
- each drain stub being connected to a header; all headers are located below the bottom bends;
- each header being connected to at least one bypass line for circulating water;
- thus providing a stable operation of heat exchange elements of boilers and steam generators; wherein
- each HS absorbing heat from exhaust gas.
2. The heated surfaces of claim 1, wherein the drain stub is connected to the header, which is an elemental header; further comprising: at least one integral header receiving the bypass from the elemental header and outputting at least one drain pipe for water draining into a water accumulator.
3. The heated surfaces of claim 2, further comprising drain stubs entering the integral header.
4. The heated surfaces of claim 2, wherein the accumulator is located outside a casing; and the drain pipes go through the casing via holes.
5. The heated surfaces of claim 4, wherein the holes in the casing have larger diameter than the drain pipes allowing thermal expansion of the drain pipe.
6. The heated surfaces of claim 2, wherein both types of headers—integral and elemental serve as equalizers thus regulating a pressure and a flow liquid/vapor in parallel circuits.
7. The heated surfaces of claim 2, wherein water always passes through the headers to cool down the bypass lines and the drain pipes.
8. The heated surfaces of claim 1, wherein a diameter of the stub is smaller than a diameter of the bottom bend.
9. The heated surfaces of claim 1, wherein a diameter of the stub is at least twice smaller than a diameter of the bottom bend.
10. The heated surfaces of claim 1, further comprising: a drain box covering the bottom bends, the drain stubs, the drain pipes, the bypass lines and the headers; the box including water cooled wall tubes to keep proper temperature conditions in the drain box.
11. The heated surfaces of claim 1, wherein each water cooled wall tube is connected to an inlet header and an outlet header.
12. The heated surfaces of claim 1, wherein a pressure inside the panel or the coil is a subcritical pressure.
13. The heated surfaces of claim 1, wherein a pressure inside the panel or the coil is a supercritical pressure.
14. The heated surfaces according to claim 1, where the sizes of holes in the bottom bends and in the headers of multiple passage panels or serpentine coils are adjusted in such manner to have an equal pressure drop in all parallel circuits.
Type: Application
Filed: Oct 8, 2015
Publication Date: Apr 14, 2016
Inventors: Vladimir S. POLONSKY (Moscow), Stanislav V. POLONSKY (Moscow)
Application Number: 14/878,186