Heat exchanger unit for heat recovery steam generator

Abstract: A tubular type heat exchanger unit which utilizes upper inlet (12) and outlet (22) headers which are connected to at least three lower headers (14,16,18,20) by multiple elongated vertically-oriented U-shaped tubes (17). The U-shaped tubes are provided in adjacent banks and rows within a thermally-insulated casing. Preferably 4-12 lower headers each having a bank of 4-2⊘ vertical tubes are provided. Such heat exchanger unit is enclosed within a thermally-insulated and is suitable for use as an economizer and superheater in heat recovery steam generators (HRSG).

Description: This invention pertains to tubular type heat exchangers for use in heat recovery steam generators, and particularly pertains to such heat exchangers units utilizing inverted U-shaped tubes connected to elongated parallel headers for economizers and superheaters used in such generators. Tubular type heat exchangers such as used in economizers or superheaters in heat recovery steam generators usually utilize pairs of upper and lower headers which are connected together by multiple vertically-oriented tubes, so that hot gases such as derived from a gas turbine exhaust can flow transversely across the tubes to heat a fluid flowing vertically in the tubes, so as to generate pressurized steam therein. Such heat exchangers having various tube configurations are known, for example, as disclosed by U.S. Patent Nos. 4,644,⊘67 to, 4,685,426 and 4,944,252. However, such heat exchanger designs utilizing pairs of upper and lower headers are thermodynamically less efficient and are undesirably expensive, so that improved configurations and designs for such heat exchangers have been sought. EP-A-353404 and UK-182773 both show heat exchangers where only upper or lower headers are used and these are connected together by U-shaped tubes where limbs are vertically oriented. According to the invention there is provided a tubular type heat exchange unit adapted for transferring heat from a hot gas to a cooler fluid flowing in tubes of the unit, comprising at least three intermediate elongated headers oriented adjacent and parallel to each other, each intermediate header being connected to an adjacent intermediate header by multiple elongated vertically-oriented U-shaped tubes to provide flow paths for the fluid through the unit, the individual arm portions of the U-shaped tubes being aligned in a tube bank above each header, and being provided in at least four parallel rows each aligned transverse to the intermediate headers with individual vertical arm portions of the U-shaped tubes being aligned in a tube bank above each header, an inlet conduit and an outlet conduit, the intermediate headers being lower headers positioned beneath the U-shaped tubes, characterised in that an upper inlet header and an upper outlet header are provided above the said intermediate headers, the inlet conduit and the outlet conduit being connected to the inlet header and outlet header, respectively, vertically-oriented tubes joining the upper inlet header and the first of the intermediate headers, and vertically-oriented tubes joining the last of the intermediate headers and the upper outlet header. In an alternative heat exchanger unit arrangement which is usually preferred, dual upper inlet and outlet headers are also provided which are each connected to at least two lower headers by means of the multiple vertically-oriented U-shaped tubes, which are provided in at least two adjacent rows of tubes. Preferably four to twelve adjacent lower headers are used, for which the lower adjacent headers are connected together by the multiple inverted U-shaped tubes provided in at least four and usually 6-2⊘ adjacent rows of tubes. The tubular heat exchanger units and thermally-insulatad casing according to this invention advantageously provides a heat exchanger assembly suitable for use as an economizer or superheater in a heat recovery steam generator. Such heat exchanger unit and assembly provides uniform heat transfer to a fluid such as water or steam flowing in the vertical tubes and also increases velocity of the fluid flowing therein for improved heat transfer. The invention will be further described by reference to the following drawings, in which: Fig. 1 shows a plan view of one embodiment of a tubular type heat exchanger unit and assembly according to the invention and is suitable for use in a heat recovery steam generator ; Fig. 2 shows an elevation sectional view taken through one row of vertically-oriented tubes at line 2-2 of Fig. 1; and including both upper and lower headers; Fig. 3 shows another elevation sectional view taken through an adjacent row of vertically-oriented tubes at line 3-3 of Fig. 1 with the lower headers being structurally supported from below; Fig. 4 shows a plan view of a tubular heat exchanger assembly similar to Fig. 1 but including a second unit having its headers located adjacent one end and aligned with a first unit headers, with each unit having separate inlet and outlet conduits; and Fig. 5 shows an enlarged detailed elevation view of a tubular type heat exchanger unit similar to Fig. 2 and 3, and including upper and lower casing and support structure. As is shown by Fig. 1, a heat recovery generator unit 1⊘ includes inlet conduit 11 connected to an inlet upper header 12, which is connected by multiple vertically-oriented tubes 13 to a lower header 14. Lower header 14 is connected by multiple vertically-oriented U-shaped tubes 15 to an adjacent parallel header 16. In the Fig. 1 drawing, four adjacent lower headers 14, 16, 18 and 2⊘ are shown, which headers are connected together by multiple U-shaped vertical tubes 15, 17 and 19, which are aligned in tube banks for each header, as is additionally shown in Figs. 2 and 3. The last bank of tubes 21 are connected to an outlet upper header 22, which is connected to outlet conduit 23. It is seen that the four lower headers 14 to 20 are flow connected together and to the inlet and outlet upper headers by additional rows and parallel banks of vertically-oriented tubes 13 and 21, as shown in Figs. 2 and 3. The inlet conduit 11 and outlet conduit 23 can be located anywhere along the length of the header to which they are each connected, considering space limitations or restrictions for a particular installation. A hot gas stream 3⊘ such as combustion exhaust gases from a gas turbine can flow transversely across the heat exchanger tube banks and rows, so as to transfer heat to a fluid such as pressurized water or steam flowing inside the multiple tubes. The headers and tubes for heat exchanger unit 1⊘ are all enclosed within a casing 32, which has internal thermal-insulation at 33 and usually has a rectangular-shaped cross-section shape. The lower headers 14 to 20 are supported by structural beams 34 which are spaced apart along the length of the headers, and can be embedded within the insulation 33. The header and U-shaped tube configuration for the heat exchanger unit of this invention assures uniform heat transfer from the hot flowing gases to the fluid (water or steam) flowing inside the tubes, which increases the fluid velocity inside the tubes, and permits significant reduction of up to 5% in the heat transfer surface area required for a particular heat duty in a heat recovery steam generator. This invention is useful for economizer units heating pressurized water and for superheater units heating steam for heat recovery steam generators. The tubes are 1 to 3 inch (approx 25.4 to 76.2 mm ) outside diameter and 2⊘ to 6⊘ ft (approx 6.1 to 18.3 m) long. The tubes are each welded pressure-tightly at each end into the upper and/or lower headers, so as to form adjacent tube banks in each header and tube rows connecting the adjacent headers. The headers each have 3 to 6 inch (approx 76.2 to 152.4 mm) outside diameter and are 6 to 14 feet (approx 1.8 to 4.3 m) long depending upon the needs of a particular heat recover generator installation. The desired spacing between adjacent lower headers and their connected tubes in the direction of gas flow is 4 to 10 inches (approx 102 to 254 mm), and the desired spacing between adjacent tubes in the direction parallel to the headers and perpendicular to the gas flow is 4 to 10 inches (approx 102 to 254 mm). Exhaust gas passing transversely across the tube banks may have superficial velocity of 2⊘ to 5⊘ ft/(approx 6.1 to 15.2 m/s), temperatures of 2⊘⊘ to 16⊘⊘°F (approx 93 to 871°C), and fluid pressures in the tubes may be 5 to 27⊘⊘ psig (approx 2.4x10 2 to 1.3x10 5 Ns/m 2 ). The number of lower headers and tube banks used for this invention will depend upon the amount of heat to be extracted from the hot exhaust gas. For practical use at least two and not exceeding twelve lower headers and associated tube banks are used, as heat exchange units having twelve headers is usually the maximum shipping size limit. If desired, two or more heat exchange units 1⊘ can be provided in parallel alignment within a single casing, the units being arranged in a tandem flow arrangement for the hot gas stream 3⊘. In another alternative arrangement, two heat exchange units 1⊘ and 1⊘a can be provided within a common casing 4⊘, which has internal thermal-insulation at 41, as is shown by Fig. 4. For the second heat exchange unit 1⊘a, the headers 14a and 2⊘a are located adjacent one end and in alignment with the headers 14 to 2⊘ of the heat exchange unit 1⊘. Although not shown in Fig. 4, the second heat exchange unit 1⊘a is supported similarly to the arrangement shown in Fig. 3. The heat exchanger unit 1⊘ is shown in greater detail by Fig. 5, in which the lower headers 14 to 2⊘ are each structurally supported by horizontal I-beams 34, which beams are each in turn supported by beams 36 which are located external to the lower side of casing 32. A suitable thermal insulation material 33 is provided between the lower headers 14 to 2⊘ and the casing 32 lower side, and is also provided between the upper headers 12, 22 and the casing 32 upper side. The internal thermal insulation 33 can be provided by a rigid refractory material, or preferably can be a ceramic fiber blanket material covered with a thin metal inner liner 33a such as stainless steel, so as to retain the fibre insulation reliably in the flowing hot gas stream 3⊘. The casing 32 and steel beam structures 36 can be supported in any convenient manner, such as being attached to reinforced concrete structures 38. A drain connection 39 is usually provided from each lower header 14 to 2⊘. During operations of the heat exchange units lo in a heat recovery steam generator, a hot combustion gas 3⊘ flows through the elongated thermally-insulated casing 32 and transversely past the tubes at a superficial velocity of 3⊘ to 5⊘ ft/(approx 9.1 to 15.2 m/s), and thereby heats the fluid such as water or steam flowing inside the tubes. For tubes having length exceeding about 1⊘ feet (approx 3m), the tubes are usually stabilized against lateral vibrations by close-fitting anti-vibration support members or ties 42, which extend between adjacent tubes and fit closely around the tubes. For tubes exceeding about 3⊘ feet (approx 9.1 m) length, two ties 42 spaced about 8 to 1⊘ ft (approx 2.4 to 3m) apart along the tube length should preferably be used. The tubes and headers are usually made of carbon steel or an alloy steel depending upon the operating temperature and pressure required, with the tubes being metal arc welded pressure-tightly into the upper and lower headers of the heat exchanger units. The invention advantageously provides a tubular heat exchanger unit arrangement for use as either economizer or superheater units in heat recovery steam boilers or generators, in which the multiple tubes are located thermally in parallel so as to achieve good mixing of the gas flow and minimize the heat transfer surface requirements, while also providing good temperature balance and minimum thermal stresses developed in the tubes. The invention will now be further described by the following typical Example, which should not be construed as limiting in scope. EXAMPLE A heat recovery steam generator is constructed according to the invention in which a heat exchanger unit having upper and lower headers connected to banks and rows of vertically-oriented tubes are provided within a rectangular-shaped casing, which is internally thermally insulated. The tubes are metal arc welded pressure-tightly into an inlet and outlet upper header, and into lower headers, as generally shown in Figs. 2 and 3. Hot combustion gas such as derived from combustion of natural gas, fuel gas, or oil in a gas turbine, with or without auxiliary burners, can pass transversely across the multiple banks and rows of tubes. For an economizer unit, pressurized water can be introduced into the first upper header for the tubes, and water heated in the tubes by the hot gas can be withdrawn from the last upper header. The heated water can be passed to other heat exchange units such as a superheater in which pressurized steam is passed through the headers and tubes and heated by the hot gas. The superheated steam is then expanded in a high pressure turbine for generating power. Some important typical characteristics and dimensions for the heat exchanger units used as an economizer or superheater are provided in Table 1 below: Header and Tube Characteristics Economizer Superheater Header outside diameter, in.(mm) 4 (102) 4 (102) Header length, ft.(m) 1⊘ (3.0) 1⊘ (3.0) Tube outside diameter, in.(mm) 2(51) 2 (51) Tube length, ft. (m) 5⊘ (15.2) 5⊘ (15.2) Spacing between adjacent lower headers, in.(mm) 5 (127) 5 (127) Spacing between adjacent tubes in direction of gas flow, in.(mm) 5 (127) 5 (127) Spacing between adjacent tubes perpendicular to gas flow direction, in.(mm) 5(127) 5(127) Hot gas inlet temperature, °F (°C) 6⊘⊘ (316) 1,1⊘⊘ (593) Combustion gas superficial velocity, ft/s (m/s) 3⊘-4⊘ (9.1-12.2) 3⊘-4⊘ (9.1-12.2) Pressure in tubes, psig.(Ns/m 2 ) 1,5⊘⊘ (7.2x10 4 ) 1,5⊘⊘ (7.2x10 4 ) Fluid inlet temp., °F (°C) 23⊘ (11⊘) 1,⊘⊘⊘ (538) Fluid exit temp., °F (°C) 575(302) 1,2⊘⊘ (649)

Claims: A tubular type heat exchange unit (10) adapted for transferring heat from a hot gas to a cooler fluid flowing in tubes of the unit, comprising at least three intermediate elongated headers (14,16,18,20) oriented adjacent and parallel to each other, each intermediate header being connected to an adjacent intermediate header by multiple elongated vertically-oriented U-shaped tubes (15,17,19) to provide flow paths for the fluid through the unit, the individual arm portions of the U-shaped tubes being aligned in a tube bank above each header, and being provided in at least four parallel rows each aligned transverse to the intermediate headers (14,16,18,20) with individual vertical arm portions of the U-shaped tubes being aligned in a tube bank above each header, an inlet conduit (11) and an outlet conduit (23), the intermediate headers (14,16,18,20) being lower headers positioned beneath the U-shaped tubes (13), characterised in that an upper inlet header (12) and an upper outlet header (22) are provided above the said intermediate headers (14,16,18,20), the inlet conduit (11) and the outlet conduit (23) being connected to the inlet header (12) and outlet header (22), respectively, vertically-oriented tubes (13) joining the upper inlet header (12) and the first of the intermediate headers (14), and vertically-oriented tubes (21) joining the last of the intermediate headers (20) and the upper outlet header (22). A heat exchanger unit as claimed in Claim 1 in which the tube outside diameter is 1 to 3 inches (approx. 25.4 to 76.2mm), the header outside diameter is 3 to 6 inches (approx. 76.2 to 152.4mm), and the tube length is 20 to 60 feet (approx. 6096 to 18288mm). A heat exchanger unit as claimed in Claim 1 or Claim 2 in which the individual vertical arm portions of the U-shaped tubes (15,17,19) each have substantially equal outside diameter and substantially equal spacing in each row. A heat exchanger comprising a unit as claimed in any preceding claim positioned within a casing (32) which has internal thermal-insulation (33) adapted to support the intermediate headers (14,16,18,20) and encloses the headers (14,16,18,20), U-shaped tubes (15,17,19) and vertically oriented tubes (13,21) so as to provide a heat exchanger assembly. A heat exchanger as claimed in Claim 4 in which the casing (32) has a rectangular cross-sectional shape. A heat exchanger as claimed in Claim 4 or Claim 5 in which the internal thermal insulation (33) is a blanket of ceramic fibre material covered by a thin metal liner. A heat exchanger as claimed in any of claims 4 to 6 in which a second heat exchange unit is provided having headers which are adjacent to one end and in alignment with the first headers. A heat recovery generator assembly in which at least two heat exchangers according to any of claims 4 to 7 are provided in tandem gas flow arrangement within a rectangular-shaped thermally-insulated casing (32).

Inventors: POLCER JOHN, POLCER, JOHN
Applicants: FOSTER WHEELER ENERGY CORP, FOSTER WHEELER ENERGY CORPORATION
Application year: 1993
Main classification: F22B1
IPC codes: F28D7, F22B1, F22G3, F22B37