TECHNICAL FIELD
This invention relates in general to heat exchangers and more particularly to a heat exchanger having an improved drain system, to the drain system itself; and to a heat recovery steam generator containing the heat exchanger.
BACKGROUND ART
The gas turbines that power electrical generators discharge exhaust gases at extremely high temperatures. Heat recovery steam generators (HRSGs) extract the heat from the gases to produce steam that powers steam turbines that in turn drive more electrical generators.
The typical HRSG includes multiple heat exchangers located one after the other in the flow of hot exhaust gases from a gas turbine. Among heat exchangers are an economizer for elevating the temperature of feed water, an evaporator for converting the high temperature water discharged by the economizer into saturated steam, and a superheater for converting the saturated steam into superheated steam. Many HRSGs have more than one economizer, evaporator, and superheater operating at different pressures.
An economizer operates with a full charge of water. Indeed, it simply heats the water so that the evaporator to which it is connected consumes less heat converting the water to saturated steam. The superheater contains only steam during operation of the HRSG, but when the HRSG is taken off line and shut down, the steam within the superheater condenses and water collects in its lower regions. From time to time, economizers and superheaters require servicing, and many service procedures involve draining the water that remains in those components. Likewise, some HRSGs operate in environments that experience temperatures below the freezing temperature of water. When those HRSGs are taken out of service, the economizers and superheaters should be drained to prevent water from freezing in them. Hence, the economizers and superheaters have drain systems at their lower regions for removing water from them.
Every HRSG includes a duct-like structure, called a casing, having an inlet and an outlet. Hot gas from a gas turbine or other source enters the casing at the inlet and flows through the casing, discharging at the outlet. Within the casing the gas encounters at least one superheater, at least one evaporator, and an economizer, generally in that order with respect to the flow of the gas. While some of the components of the drain systems for both the economizer and the superheater exist within the casing, the remaining components lie outside the casing.
Economizers and superheaters include coils that can consist of multiple rows of tubes, with the tube rows being arranged one after the other in the flow of hot gases (FIG. 2). Within each row of tubes the tubes are connected to a lower header that extends horizontally and also transversely with respect to the gas flow. The lower headers facilitate circulation through the coil and enable it to be drained. Drain piping cannot be connected directly between headers because water will transfer from header to header without passing through the tubes. This bypass from tube row to tube row to row will diminish the coil performance. But each coil requires drain piping that includes numerous small bore pipes and fittings, both internal of the HRSG casing and external to it as well. The latter require penetration seals in the floor of the casing.
The typical coil (FIG. 2) has a drain pipe leading away from each lower header, through the casing to a valve located outside of the casing. Beyond its valve, each drain pipe opens into a drain manifold also located external to the casing, and it contains a common drain valve to provide redundancy. This requires multiple penetrations of the casing, and each penetration requires a seal that is welded to the pipe and to the floor of the casing. It also requires making welds within the confines of the casing where each drain pipe connects with its lower header. More welds are required externally of the casing at couplings and elbows in the drain pipes as well as at the valve in each drain pipe. And of course still more welds are required to join each drain pipe to the drain manifold into which it opens. All of this work takes place in the field where the HRSG is assembled, not in the shop where the coil is manufactured, and much of it requires workman to perform their tasks in extremely confined spaces.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a longitudinal sectional view of a heat recovery steam generator having heat exchangers constructed in accordance with and embodying the present invention;
FIG. 2 is a perspective view of a drain system for a prior art heat exchanger;
FIG. 3 is a perspective view of a coil for the heat exchanger of the present invention;
FIG. 4 is a sectional view of the coil shown in FIG. 3;
FIG. 5 is a perspective view of the drain system for the heat exchanger of the present invention; and
FIG. 6 is another perspective view of the drain system for the heat exchanger.
BEST MODES FOR CARRYING OUT THE INVENTION
Referring now to the drawings (FIG. 1), a heat recovery steam generator (HRSG) A includes a casing 2, which is essentially a large duct having an inlet 4 and an outlet 6. Between the inlet 4 and the outlet 6 the casing 2 has a floor 8 that supports several heat exchangers for converting subcooled water into superheated steam. A hot gas, typically the exhaust of a gas- or oil-fired turbine, enters the casing 2 at its inlet 4, passes through the several heat exchangers which extract heat from it, and leaves through the outlet 6 at a substantially reduced temperature. Among the heat exchangers are an economizer 12 for bringing subcooled water, which might be the condensate from a steam turbine, to a higher temperature, an evaporator 14 for converting the heated subcooled water derived from the economizer 12 to saturated steam, and a superheater 16 for converting the saturated steam to superheated steam. The HRSG may have more than one economizer 12, evaporator 14 and superheater 16 organized in groups that operate at different pressures. Each group has a pump located before or after its economizer 12, and that pump controls the pressures at which the group operates. In terms of the flow of gas through the casing 2, for any group the superheater 16 precedes the evaporator 14 and the evaporator 14 precedes the economizer 12. The economizer 12 and the superheater 16 will retain water when the HRSG
A is not in operation, and this water can interfere with maintenance procedures. Moreover, if the HRSG A is subjected to freezing temperatures, the retained water can freeze and damage the economizer 12 and superheater 16.
Using the economizer 12 as an example, it includes (FIGS. 3 & 4) a coil 18 that is located in the casing 2 and a drain system 20 that is located below the coil 18 and for the most part inside the casing 2. The coil 18 has tubes 22 that extend vertically and are organized in multiple rows 24 that extend transversely with respect to the flow of gas through the casing 2. Above the tubes 22 the coil 18 has a supply header 26 that extends over the row 24 of tubes 22 located downstream in the flow of the exhaust gas through the coil 18 and is provided with an inlet port 28. At the upstream rows 24 of tubes, a discharge header 30 extends over the tubes 22, and it has an outlet port 32. The supply header 26 connects with and opens into the upper end of every other tube 22 of the endmost downstream row 24. The remaining tubes 22 of the endmost downstream row 24 connect with the upper ends of adjacent tubes 22, that is to say with every other tube 22, in the next row 24 through U-shaped transitions 34. The upper ends of the remaining tubes 22 in the next row are connected at their upper ends to the upper ends of adjacent tubes 22 in the next row 24 upstream from it through still more transitions 34. The same pattern of transitions 34 at the upper ends of the tubes 22 continues through the coil 18 to the discharge header 30. There the upper end of every other tube 22 in the next to endmost row 24 and the upper ends of all of the tubes 22 in the endmost row 24 open into the discharge header 30. The lower ends of the tubes 22 open into lower headers 36 that extend transversely with respect to the flow of gas, there being a separate lower header 36 for each row 24, except at the upstream end where a single lower header 36 services the two endmost upstream rows 24. Each lower header 36 has a drain port 38.
Subcooled water, such as the condensate from a steam turbine, enters the coil 18 at the inlet port 28 of its supply header 26. The header 28 distributes the water to every other tube 23 of the endmost downstream row 24 through which it flows downwardly into the lower header 36 to which those tubes 22 are connected. The water rises out of the lower header 36 at the remaining tubes 22 of the endmost row 24. The water flows into every other tube 22 of the next row 24 through the U-shaped transitions 34 that connect those tubes 22. The water circulates through the coil 18 from one row 24 to the next in a like manner and at the endmost upstream rows enters the discharge header 30. It leaves the header 30 at the outlet port 32 and flows on to the evaporator 14. As the water circulates through the coil 18 its temperature increases, but even so it leaves the discharge port 32 in a subcooled condition.
The lower headers 36 lie above the floor 8 of the casing 2, while the drain system 20 for the most part occupies the space between the lower headers 36 and the floor 8. The drain system 20 includes (FIGS. 5 & 6) drain pipes 40 that extend downwardly from the discharge ports 38, to which they are connected, and then transition horizontally. Thus, each drain pipe 40 has a generally upright segment 42 and a generally horizontal segment 44. The discharger ports 38 are located near the ends of their respective headers 36, with the ports 38 for alternating headers 36 being located at opposite ends of those headers 36. Thus, the horizontal segments 44 of the drain pipes 40 for alternating headers 36 approach the bottom center of the coil 18 from opposite directions. Here they open into a common drain manifold 46 that extends horizontally beneath the headers 36. The drain pipes 40 are provided with check valves 48 that are oriented such that water will flow through the drain pipes 40 to the manifold 46, but not in the opposite direction. The check valves 48, which may be located in either the upright segments 42 or the horizontal segments 44, allow water to drain from the coil 18 and lower headers 36 when the manifold 46 is opened, but prevent water from circulating from header 36 to header 36 through the drain system 20 and thus bypassing the tubes 22. Actually, all of the drain pipes 40 should contain a check valve 48, except the drain pipe 40 that connects with the endmost header 36 at the downstream end of the coil 18. That drain pipe 40 need not contain a check valve 48, although it may. Each check valve 48 should open under a low head, that is to say with a small pressure differential across it. Preferably it should take the form of a swing-type valve. If a check valve 48 does not close completely, only minor bypassing from header 36 to header 36 will occur.
The drain manifold 48 has a common drain port 50 that opens downwardly. All that resides above it, including the tubes 22, the headers 26, 30, 36 as well as the drain pipes 40, the check valves 48 and the drain manifold 46, may be assembled in a shop and shipped as a unit to the site where the HRSG A is assembled. This includes most of the drain system 20. Contrast this with a conventional heat exchanger where the drains and their valves are installed at the site of the HRSG at considerable expense and inconvenience.
Once the coil 18 is set onto the floor 8 of the casing 2, at the site where the HRSG is assembled, a few additional procedures complete the drain system 20. To this end, the drain port 50 of the drain manifold 46 is connected to a common drain line 52 that extends downwardly from the manifold 46 and penetrates the floor 8 of the casing 2 at a single seal 54. Below the casing 2 the drain line 52 contains two drain valves 56 arranged in succession, with the last of the two providing redundancy. This eliminates the congestion of piping found below the floors of casings for HRSGs equipped with traditional drain systems.
While the coil 18 with the improved drain system 20 serves as the economizer 12 in the HRSG A, the coil 18 and its drain system 20 may be used on other heat exchangers having multiple lower headers 36 in which water may collect. For example, the superheater 16, while in operation contains only steam, but when taken out of operation, that steam may condense and occupy the lower regions of its tubes 22 and lower headers 36. The superheater 16 may take the form of the coil 18, although with fewer tubes 22 and lower headers 36, and may have the improved drain system 20. Also, a feedwater heater, to the extent that it may differ from an economizer, could be furnished with the drain system 20.
Patent Application
20130048245
