CN1703599A - 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

Straight-through evaporators for steam generators

Relevant applications before and after

This application is derived from, and claims priority from, earlier US application Serial No. 60/416,083 filed October 4, 2002.

technical field

The present invention relates to a steam generator, in particular to an evaporator for a steam generator and a tube arrangement for such an evaporator.

technical background

Steam is used extensively in industry, perhaps its most important use being to generate electricity. Typically, in many cases, hot gases resulting from combustion flow through a steam generator to convert water into superheated steam. Representative of these devices is the heat recovery steam generator, which absorbs heat from the hot gas exhausted by a gas turbine driving an electric generator. This heat is used to generate steam which is passed on to a steam turbine driving another electric generator.

The most basic structure of a typical steam generator, in addition to a pipeline through which hot gas passes, also includes three additional structures arranged in sequence corresponding to the flow of gas in the pipeline, namely superheater, evaporator, and feed water heater. The water flows in opposite directions: first into the feedwater heater to be heated, but still in a liquid state; then into the evaporator, where the water is mostly converted to saturated steam; then into the superheater, where the saturated steam is converted into superheated steam.

Evaporators come in two basic types: recirculation and pass-through, each with advantages and disadvantages. Both types of evaporators have a set of tubes installed in the line through which the hot gas passes.

In a circulation type evaporator, the above-mentioned tubes are arranged in a circulation with a steam cylinder above them. There is water in the steam cylinder, and water flows into the above-mentioned pipe from the steam cylinder through a downpipe, and a part of the water in the pipe is converted into steam. But the steam exists in the water as bubbles and returns to the steam cylinder through a riser. The saturated steam in the steam drum is separated from the liquid water and enters the superheater. After the saturated steam leaves the steam drum, water is then injected into the steam drum by the water feeder. The tubes of a recirculating evaporator are wet all the way, that is, liquid water fills the inside of the tubes. This helps to produce good heat transfer and keeps the tubes at a relatively moderate temperature, eliminating the need for high temperature alloys in the piping fittings.

But circulating evaporators also have drawbacks. Probably the greatest drawback should be attributed to the expense of the following structures: steam cylinders, large downpipes and supply pipes to feed the pipes. And it takes time to make the water storage part therein reach the boiling temperature, so that the start-up time of the circulating evaporator is prolonged.

Straight-through evaporators require no downpipes or steam drums, are relatively cheap to manufacture, and have water stored directly in the tubes, and in supply lines that communicate with the tubes. These conditions make the straight-through evaporator run faster than the natural circulation evaporator. However, the straight-through evaporator must completely convert the water to steam so that only steam, which does not contain liquid water, flows from the tubes into the superheater. The evaporator requires the aid of an injection pump upstream of the water flow to circulate water through the evaporator at a controlled rate which, if moderate, allows the steam to leave the evaporator in a saturated or slightly superheated state.

Therefore, in a straight-through evaporator, as in a circulating evaporator, the walls of the tubes are wetted closest to the water inlet, because only liquid water flows through the ends of these tubes. But in the next step, the water will be converted in the tubes into mist and further into saturated steam. In the area where the mist flows, the water is isolated from the inner surface of the tube wall, so the mist exists in the area outside the center of the tube and extends forward. The tube walls surrounding this central region are dry, making the tube walls warmer and less efficient at exchanging heat. Higher temperatures will require pipes to be made of a high-temperature resistant metal, an expensive high-alloy steel.

Description of drawings

Fig. 1 is a structural sectional view of a steam generator, which is equipped with a straight-through evaporator constituted and defined according to the present invention;

Figure 2 is a perspective view of the evaporator;

Fig. 3 is a sectional view along line 3-3 in Fig. 2;

Figure 4 is a cross-sectional view of one end of an evaporator tube showing a helical band secured to the tube;

Fig. 5 is a sectional view similar to Fig. 4 after the sectional view in Fig. 4 is rotated by 90°;

Figure 6 is a cross-sectional view of the structure of an evaporator tube partially cut away in cross-section, showing the flow in the tube.

best practice

The steam generator A shown in FIG. 1 mainly includes a pipeline 2 having an input port 4 and an output port 6 . The inlet 4 is connected to a hot gas source, such as a gas turbine or a burner. The hot gas enters the pipeline 12 from the input port 4 and is discharged from the output port 6 . In addition, the steam generator A also includes a superheater 12 , an evaporator 14 and a feed water heater 16 , which are sequentially installed between the input port 4 and the output port 6 of the pipeline 2 . Thus, the hot gas flows first through heater 12 , then through evaporator 14 , and finally through feedwater heater 16 . The water flows in the opposite direction to the hot gas. The feed water heater 16 is connected with a water injection pump 18, and water is sent into the feed water heater 16 by the water injection pump 18. The feedwater heater 16 absorbs the heat of the hot gas and transfers the heat to the liquid water flowing through the feedwater heater 16, thereby increasing the temperature of the water, but the water is still in a liquid state. Liquid water flows from the feedwater heater 16 to the evaporator 14 . The evaporator 14 converts the water to steam, mostly saturated steam. The steam then enters the superheater 12 and is converted into superheated steam after being heated, which can be used to drive turbines for some industrial production, and can also be used for heating in buildings. The superheater 12, the evaporator 14, and the feedwater heater 16 are basically in the form of a tube group. The evaporator 14 operates in a straight-through manner. In practice, this steam generator A may have a plurality of evaporators 14 .

As shown in FIG. 2 , the evaporator 14 includes a supply tube 26 , a discharge tube 28 and a plurality of tubes 30 extending between the supply tube 26 and the discharge tube 28 . The supply pipe 26 has an input port 32 connected to the feedwater heater 16 for receiving heated water from the feedwater heater 16 . In fact, water flows from the feedwater heater 16 through the input port 32 into the evaporator 14 under the pressure generated by the injection pump 18 . The outlet pipe 26 has an output port 34 connected to the superheater 12, through which the saturated steam or slightly superheated steam converted from water enters the superheater 12 directly. Fins 36 are attached to the tube 30 , and these fins 36 help to absorb the heat of the gas flowing through the line 2 .

After the hot water enters the pipe 30 through the supply pipe 26, it is converted into steam, and the steam then flows into the superheater 12 through the discharge pipe 28. Thus, the portion of each tube 30 closest to the supply pipe 20 contains liquid water, and the portion closest to the discharge pipe 28 contains saturated steam and slightly superheated steam. In the middle part of each tube 30, the liquid water is in the transition state and becomes steam. Here, the water boils, transforming into mist or a mixture of water and saturated steam. Further on, the water mist further becomes saturated steam, and the saturated steam eventually becomes superheated steam, albeit only slightly superheated. In the superheated region of the tube 30, if it does contain superheated steam, this region is also very short. The material of construction of the pipe 30 is carbon steel or chromium molybdenum alloy steel.

As shown in Figures 3-5, each tube 30 has a helical band 40 inside, and one end of the helical band 40 starts from the input port of the tube 30 connected to the supply tube 26 and extends through the area where the water mist is present. The width of each helical band 30 is slightly smaller than the inner diameter of the tube 30 where it extends, which facilitates the insertion and extraction of the helical band 40 in the tube 30 without being hindered by the tube 30 . The preferred width of each spiral band 40 is less than 1/16 inch of the inner diameter of the tube 30, which has an inner diameter of at least 2 inches. The helical ribbon 40 is helically bent several times between its ends so that its edge follows the inner surface of the tube 30 in a helical shape. In fact, a full 360° helix of the helical band 40 occupies a length in the tube 30 that is 5-25 times the diameter of the tube 30 . For example, for a pipe 30 with an internal diameter of 2 inches, a helix of the helical band 40 occupies 5 times the diameter of the pipe 30, i.e. a 360° full-angle helix of the helical band 40 will occupy 10 inches of the pipe 40 . One end of the spiral band 40 located at the inlet of the pipe 30 is fastened to a strut 42 which passes transversely through the inlet of the pipe 32 . Said strut 42 is welded to one end of the tube 30 and to the helical band 40 , thus fixing the helical band 40 to its tube 30 . The helical ribbon 40 is formed of a metal which is resistant to the temperature of the slightly superheated steam and which is compatible with the metal of which the tube 30 is constructed, minimizing sensitive electrochemical reactions. If the tube 30 is a carbon steel material, stainless steel is a suitable choice.

During the operation of the steam generator A, the hot gas flowing through the pipeline 2 passes through the tubes of the superheater 12, the evaporator 14 and the feed water heater 16 in sequence. During this process, the temperature of the hot gas gradually decreases. The water injection pump 18 sends water into the feed water heater 16, and when the water flows through the pipes of the feed water heater 16, it absorbs the heat transferred from the hot gas flowing through the feed water heater. The temperature of the water rises, but it remains liquid. Water flows from the feed water heater 16 into the supply pipe 26 of the evaporator 14 and then into the pipe 30 of the evaporator 14 under the pressure generated by the water injection pump 18 . While the water is in the pipe 30 , the pipe 30 encounters a hot air stream of higher temperature flowing through the pipe 2 . In fact, the hot gas flow, passing through the evaporator 14, raises the temperature of the tubes 30 sufficiently to convert the water in the tubes 30 into steam. The water initially enters the tube 30 in a liquid state, but as it passes through the tube 30 it starts to boil, creating a mist. The helical ribbon 40 extends across the region where the water mist is present, creating a certain amount of turbulence during the flow of the water mist to the discharge pipe 28 . As shown in Figure 6, the eddy current drives the water mist, that is, the water particles hit the inner surface of the tube 30, so that the hot gas flowing through the tube 30 can exchange heat with the water mist flowing in the tube 30 better and more effectively, This also prevents the tube 30 from overheating. Without the helical ribbon 40, the mist would exist in the center of the tube 30 surrounded by saturated or superheated steam along the inner surface of the tube 30. This will result in a higher temperature of the tube 30 in the area where the water mist is present. When the water mist in the pipe 30 flows to the discharge pipe 28, the water mist is transformed into saturated steam, even superheated steam, although only slightly superheated. The region of tube 30 containing only superheated steam is short and is able to maintain a relatively moderate temperature because heat is conducted from the region where superheated steam is present to the region where water mist and liquid water are located.

If the spiral bands 40 are not fastened to the pipe 30 at the supply pipe 26 , they can also be fastened to the pipe 30 at the discharge pipe 28 so that the spiral tape 40 will extend in the direction of the supply pipe 26 . The spiral tube 40 may extend the full length of the tube 30, through or only through the area through which the mist flows. The tubes 30 of the evaporator 14 may not be installed in a single group, but may be composed in a plurality of groups.

Claims (18)

1. A straight-through evaporator for steam generator, characterized in that, said evaporator comprises: a supply pipe for receiving liquid water; a discharge pipe separate from the supply pipe for receiving steam; a plurality of pipes connecting and extending between the supply pipe and the discharge pipe so that water may flow from the supply pipe to the discharge pipe and be converted into steam by the heat of said pipes; A strip for creating a vortex in the water mist in at least some of the above-mentioned tubes, the water spray generated by the conversion of water into steam in said tubes. 2. An evaporator according to claim 1, wherein each of said bands is helical such that the edges of the bands form a helix along the inner surface of the tube in which the bands are located. 3. An evaporator according to claim 2, characterized in that the length of a 360° helix of each belt is 5-25 times the diameter of the tube. 4. An evaporator according to claim 1, wherein the strip extending in the tube is fixed at one end of the tube. 5. An evaporator according to claim 2, characterized in that said evaporator further comprises a strut extending transversely across the tube, and the tube comprises a strap secured to one end of the tube, said A strut is secured to and extends through the tube, and a strap in the tube is secured to the strut. 6. An evaporator according to claim 4, wherein each of said strips secured to the tube at one end of the supply tube extends within the tube. 7. An evaporator according to claim 2, wherein each band extending in the tube has a width less than the inner diameter of the tube. 8. A steam generator, characterized in that it comprises a pipeline flowing through hot gas, a feed water heater located in the pipeline, a superheater located before the feed water heater corresponding to the gas flow direction, a feed water heater The heater feeds liquid water pumps, a modified straight-through evaporator in line between the superheater and the evaporator and connected to the feedwater heater and superheater, such that the water flowing from the feedwater heater into the evaporator is The evaporator is first converted into water mist, and then converted into steam, and the steam directly enters the superheater to become superheated steam. The evaporator includes: a plurality of tubes in the pipeline through which the hot gas flows; A helical band installed in the mist flow area in each tube. 9. A combination according to claim 8, wherein each of said straps is helical such that its edges form a helix along the inner surface of the tube. 10. The combination of claim 8, wherein each strap is secured to one end of a tube through which it extends. 11. The combination of claim 10, wherein said evaporator further comprises a strut extending through one end of each tube and securing a strap to one end of said tube; and said tube The helical bands in are fastened to the struts. 12. The combination according to claim 8, characterized in that the liquid water in each of the tubes is first converted into water mist and then into saturated steam; Area. 13. Combination according to claim 12, characterized in that the helical band in each tube extends from the inlet of the tube at least through the area of the tube where the water mist is present. 14. For the use of straight-through evaporators, the combination includes: a pipe with an input and an output; A helical band embedded in a tube with a helical edge along the inside surface of the tube. 15. A combination according to claim 14, wherein said strap is secured to one end of the tube. 16. The combination according to claim 14, characterized in that, the length of a 360° full-angle helix of the belt in the tube is 5-25 times the diameter. 17. The combination of claim 14, wherein the width of the helical band in each tube is slightly smaller than the inner diameter of the tube. 18. The combination of claim 14, further comprising water in one end of the tube and steam in the other end of the tube, and a region of water mist flow between the water and the region where the steam is present, and said tape is positioned between the areas where the water mist flows in the area.

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