KR20050086420A - Once-through evaporator for a steam generator - Google Patents

Abstract

A steam generator (A) has a once-through evaporator (14) which converts liquid water into steam in tubes (30) over which hot gases flow. Each tube contains a metal tape (40) which is twisted into a helical configuration to induce turbulence in the mist produced by the boiling, and the turbulence insures that the mist wets the inside surfaces of the tubes, thus producing good heat transfer and moderate temperatures in the tubes.

Description

Perfusion evaporator for steam generators {ONCE-THROUGH EVAPORATOR FOR A STEAM GENERATOR}

Technical Field

The present invention relates to a steam generator, and more particularly, to an evaporator for a steam generator, and a tube for such an evaporator.

Background

Steam is widely used in the industry, and the most important application is probably power generation. Typically, in many cases the hot gases generated in the combustion pass through a steam generator that converts water into superheated steam. Representative examples of these facilities are waste heat recovery steam generators (HRSGs) used to extract heat from the hot gases emitted by gas turbines driving electrical generators. The extracted heat generates steam, which passes to a steam turbine that powers another electricity generator.

Conventional steam generators, in the most basic form, include three additional parts, superheaters, evaporators, and economizers or feed water heaters, apart from the ducts through which the hot gas passes, which parts relate to the flow of gas in the ducts. Are arranged in order. The water flows in the opposite direction, is heated through the economizer but is still in the liquid phase, and then passes through the evaporator to convert mostly into saturated steam, which is then passed through the superheater and the saturated steam becomes superheated steam.

Evaporators are classified into two types, circulating and perfusion, each having its own advantages and disadvantages. Both structures have a series of tubes in the duct through which the hot gas passes.

In the case of the circulation type, the tube is in a given circuit with a steam drum on top of the tube. The drum receives the water, which flows from the drum through the downcomer into the tube where some of the water is converted to steam, but the vapor is present as air bubbles in the water and then returned to the steam drum through the riser. . Here, the saturated vapor separates from the liquid water and moves to the superheater. Saturated steam is replaced with feed water to the drum. The tubes of the circulating evaporator are always kept wet, ie liquid water is present over the entire inner surface. This improves good heat transfer. In addition, this keeps the tube at a relatively mild temperature, thus eliminating the need to use a hot alloy in the tube.

Circulating evaporators have their own disadvantages. Perhaps the biggest drawback is the cost for the steam drum, the large downward flow path, and the header for supplying water to the tube. In addition, the water stored in these parts requires time to reach the boiling point, which increases the startup time in the circulating evaporator.

The perfusion evaporator does not require a down flow path or drum, and the manufacturing cost is low. In addition, the water stored in the downward flow path or drum stays in the tube itself and the feed header from which the tube extends. Due to this, the perfusion evaporator can be operated more quickly than a conventional circulation evaporator. However, the perfusion evaporator must completely convert the water into steam, so only steam escapes from its tube and flows to the superheater. Liquid water should not leave the evaporator. The evaporator relies on a feed pump located upstream of the water circuit to circulate the water at a controlled rate, i.e., at a rate that allows steam to leave in a saturated or slightly overheated state if correct.

Thus, in a perfusion evaporator, the tube wall closest to the water inlet extends wet, as in the circulation evaporator, because these ends of the tube can only recognize liquid water. In that tube, however, the water is converted to a mist and then to saturated steam. In a fog flow situation, the water deviates from the inner surface of the tube wall, so that the fog is present in the core extending through the center of the tube. The walls around these cores extend in a dry state. This causes the temperature of the tube wall to be higher, resulting in poor heat transfer efficiency. The higher the temperature, the more capable of withstanding the high temperatures, in other words, expensive high alloy steels are needed.

Brief description of the drawings

1 is a schematic cross-sectional view of a steam generator with a perfusion evaporator constructed in accordance with the present invention and embodying the present invention;

2 is a perspective view of an evaporator;

3 is a cross-sectional view taken along line 3-3 of FIG. 2;

4 is a partial cross-sectional view of the end of one of the evaporator tubes showing the twisted tape secured to the tube;

FIG. 5 is a partial cross sectional view similar to FIG. 4 but rotated 90 °; And

FIG. 6 is a fragmentary, cross-sectional view of one of the evaporator tubes cut away to show flow in the evaporator.

Best Mode for Carrying Out the Invention

Referring now to the drawings, the steam generator A (see FIG. 1) is basically provided with a duct 2 having an inlet end 4 and an outlet end 6. The inlet end 4 is connected to a hot gas source, such as a gas turbine or incinerator, which flows through the duct 12 and exits the duct at the outlet end 6. The steam generator A also includes a superheater 12, an evaporator 14, and a feed water heater or economizer 16, which components are in duct (in sequential order from inlet end 4 to outlet end 6). 2) is arranged. Thus, the hot gas first flows through the superheater 12, then through the evaporator 14 and finally through the economizer 16. Water flows in the opposite direction. For this purpose, the economizer 16 is connected to a water feed pump 18 which transfers the water supply to the economizer 16. The economizer extracts heat from the hot gas and transfers the heat to the liquid water flowing through the economizer, which raises the temperature of the water, but the water still remains in the liquid phase. After exiting economizer 16, liquid water flows into evaporator 14 and passes through the evaporator. Evaporator 14 converts water into steam, mostly saturated steam. Steam flows to superheater 12, which raises the temperature of the steam, converting the steam into superheated steam that can be used to power a turbine or some industrial process or to heat a building. Superheater 12, evaporator 14 and economizer 16 are basically tube banks. Evaporator 14 operates according to the perfusion principle. Indeed, the steam generator A may have one or more evaporators 14.

The evaporator 14 has a feed header 26, an outlet header 28 and a tube 30 extending between these two headers 26, 28 (see FIG. 2). The feed header 26 has an inlet port 32 connected to the economizer 16 to receive the heated water from the economizer 16, in which water is actually under the head generated by the pump 18. Is transferred to the inlet port. The outlet header 26 has an outlet port 34 connected to the superheater 12 through which the steam, which is saturated or slightly overheated, is directed to the superheater 12. The tube 30 has fins 36 to facilitate extraction of heat from the gas flowing through the duct 2.

Within the tube 30, the heated water from the feed header 26 is converted to steam, which is collected in the discharge header 28 and then transferred to the superheater 12. Thus, the portion of each tube 30 closest to the feed header 20 receives liquid water, while the portion closest to the outlet header 28 contains saturated or possibly superheated steam. In the middle portion of each tube 30, liquid water undergoes a phase change and becomes vapor. Here, the water boils and becomes a mist or a mixture of water and saturated steam. Thereafter, the mist becomes more saturated steam, and finally the saturated steam can become superheated steam even though it is slightly overheated. If there is actually superheated steam, the superheated region of the tube 30 is quite short. The tube 30 is formed of carbon steel or chromium-molybdenum steel.

Each tube 30 receives a helical tape 40 (see FIGS. 3-5) extending from the inlet of the tube, which is the end of the tube connected to the feed header 26 through the area where the fog is present. . Since the width of each tape 40 is slightly smaller than the inner diameter of the tube 30 from which it extends, the tape 40 can be inserted into or withdrawn from the tube 30 without interfering with the tube 30 itself. . Preferably, the width of each tape 40 is at least about 1/16 inch smaller than the inner diameter of the tube for at least 2 inches of inner diameter. Since the tape 40 is twisted multiple times between both ends, the edges of the tape form a helix, which lies along the inner side of the tube 30. Indeed, the entire 360 ° twist of the tape 40 should occur within a distance of length to diameter of 5 to 25. For example, for a tube 30 with an inner diameter of 2 inches and a length to diameter ratio of 5 to the twist of the tape 40, the overall 360 ° twist of the tape 40 is within 10 inches of the tape 40. Will happen. At the end of the tape 40 staying at the inlet of the tube 30 is provided an anchor bar 42 extending transversely across the inlet end of the tube 32. The anchor bar 42 is welded to the end of the tube 30 and the tape 40, thereby fixing the tape 40 with the tube 30. The tape 40 is formed of a metal capable of withstanding the temperature associated with the slightly superheated vapor, which is compatible with the metal of the tube 30 in the sense of minimizing the electrolyte reaction. Stainless steel is suitable when the tube 30 is carbon steel.

In the operation of the steam generator A, the hot gas flowing through the duct 2 passes through the tubes of the superheater 12, the evaporator 14 and the economizer 16 in order, each time through each component. The temperature is lowered. Feed pump 18 forces pumping water into economizer 16, where water extracts heat from the gas flowing over the tubes of economizer 16. Although the temperature of the water increases, the water still remains in the liquid phase. Under the head generated by the pump 18, water flows from the economizer 16 to the feed header 26 of the evaporator 14 and then to the tube 30 of the evaporator 14. Within the tube 30, the water will experience much higher temperatures derived from the gas passing through the duct 2. Indeed, the gas passing through the evaporator 14 raises the temperature of the tube 30 to a high temperature sufficient to convert the water in the tube 30 into steam. The water initially remains liquid when it enters the tube 30, but as it passes through the tube, it begins to boil and produce fog. The tape 40 extends through the region of fog flow and generates good measures to induce turbulence in the fog as the fog flows toward the discharge header 28. Turbulence causes the fog, ie, the water particles, to abut the inner surface of the tube 30 (see FIG. 6), resulting in a better and more efficient heat transfer between the gas flowing over the tube 30 and the mist in the tube 30. . This also prevents the tube 30 from overheating. Without the tape 40, the fog is likely to be maintained in the center of the tube 30 and is surrounded by saturated or superheated steam along the inner surface of the tube 30, so that the tube 30 is hotter in the region of the fog. Will work on. As the mist in the tube 30 approaches the exhaust header 28, the mist may be converted to saturated steam and even to superheated steam even if slightly overheated. However, the area of the tube 30 which seeks only superheated steam is short and is maintained at a relatively mild temperature because heat is conducted from these areas to areas occupied by fog and liquid water.

Instead of securing tape 40 to tube 30 at feed header 26, the tape may be secured to outlet header 28, in which case the tape will extend towards feed header 26. The tape may extend over the entire length of the tube 30 through which they pass and may only pass through the region of fog flow. Instead of having the tubes 30 arranged in a single bank, the evaporator 14 may have tubes consisting of a plurality of banks.

Claims (18)

A perfusion evaporator for a steam generator, A feed header for receiving liquid water; A discharge header disposed at intervals from said supply header to receive steam; Tubes extending between and connected between the supply header and the outlet header, the tubes capable of flowing water from the supply header toward the outlet header and converting into steam by the heat applied to the tubes; And Tapes in at least some of the tubes to induce turbulence in the mist generated in such tubes when water is converted into steam in such tubes A perfusion evaporator comprising a steam generator. The perfusion evaporator of claim 1, wherein each tape is twisted such that the edges of the tape form spirals, the spirals lying along the inner surfaces of the tubes on which the tapes are placed. The perfusion evaporator of claim 2, wherein each tape is about 5 to 25 in length to diameter for a twist of 360 °. The perfusion evaporator of claim 1, wherein the tape is anchored at one end of the tube and extends through the tube. 3. The tube of claim 2 further comprising a bar extending transversely across each tube containing the tape at an end of the tube to which the tape is fixed, the bar attached to the tube and extending across the tube. And the tube tape is fixed to the bar. 5. The perfusion evaporator of claim 4 wherein each tape is secured to the tube and extends through the tube at its end of the tube in the feed header. The perfusion evaporator of claim 2, wherein the width of each tape is less than the inner diameter of the tube and each tape extends through the tube. A duct through which hot gases pass, an economizer located within the duct, a superheater located upstream of the economizer with respect to the flow of the gases, and a pump for transferring liquid water to the economizer In the steam generator, An improved perfusion evaporator, located in the duct between the superheater and the evaporator, connected to the economizer and the superheater, the water from the economizer flows into the evaporator and is converted into steam after being converted into a mist flow in the evaporator, In the perfusion evaporator, this steam is directed to the superheater and exits the superheater as superheated steam. Tubes lying in the duct, the tubes lying in the duct, through which the hot gases pass; And Twisted tape located in each tube in the region of fog flow Comprising, a perfusion evaporator. 9. The combination of claim 8, wherein the tape is twisted so that its edges form a helix and the helix lies along the inner side of the tube. 9. The combination of claim 8, wherein each tape is secured to one end of the tube from which the tape extends. 11. The apparatus of claim 10 further comprising a bar extending across the bar, the bar being fixed to each tube at the end to which the tape is fixed, Wherein the twisted tape in the tube is attached to the bar. The method of claim 8, wherein the liquid water in each tube is converted to a mist and then to saturated steam, And the tape for the tube lies at least in the region of the fog. 13. The combination of claim 12, wherein the twisted tape in each tube extends from the inlet and extends at least through the region of the tube where the fog is present. As a combination used in a perfusion evaporator, A tube having an inlet end and an outlet end, And a twisted tape having a helical edge positioned within the tube and lying along the inner side of the tube. The combination of claim 14, wherein the tape is secured to the tube at one end of the tube. The combination of claim 14, wherein the tape has a length to diameter ratio of about 5 to 25 for a total 360 ° twist. The combination of claim 14, wherein the width of each tube tape is slightly smaller than the inner diameter of the tube. 15. The combination of claim 14, further comprising water in one end of the tube, steam in the other end, and a mist flow region between the water and the steam, wherein the tape lies in the mist flow region.