Heat recovery assembly

A combined cycle system has a gas turbine for generating an exhaust gas stream and a heat recovery steam generator with a housing for defining a horizontal exhaust gas flow path for the exhaust gas stream. Positioned within the housing is a heat recovery assembly having vertical rows of horizontally oriented heat transfer tubes transverse to the direction of gas flow and spaced apart in the direction of the gas flow path. The heat transfer tubes are supported by a plurality of vertical support plate assemblies oriented parallel to the exhaust flow path. Each support plate assembly has a plurality of coplanar support plate segments, each support plate segment supporting less than three rows of heat transfer tubes.

FIELD OF THE INVENTION
 This invention relates to the field of combined cycle systems having a gas
 turbine and an associated heat recovery steam generator. More
 particularly, this invention relates to a heat recovery assembly for use
 in a heat recovery steam generator.
 BACKGROUND OF THE INVENTION
 Gas turbines have been widely used to provide electrical power, usually as
 a standby for both peak power and reserve power requirements in the
 utility industry. Gas turbines are preferred because of their rapid
 starting capability and low capital costs. Conventional gas turbines,
 however, operate with reduced thermal efficiency due to the high exit
 temperatures of the exhaust gas stream and the resulting thermal loss.
 Therefore, a gas turbine is often combined with the heat recovery steam
 generator to improve overall system efficiency.
 The heat recovery steam generator can be employed to drive a steam turbine
 for power output or to provide process steam in cogeneration cycles. Heat
 recovery steam generators typically have either a vertical exhaust gas
 flow or a horizontal exhaust gas flow through arrangements of heat
 recovery and air pollution control assemblies. The heat recovery
 assemblies, or heat exchange circuits, conventionally include
 superheaters, evaporators, economizers and preheaters. In heat recovery
 steam generators having vertical exhaust gas flow, the exhaust gas stream
 from the gas turbine flows upward through stacked arrangements of heat
 recovery assemblies and air pollution control assemblies. These heat
 recovery assemblies of the heat recovery steam generators having vertical
 exhaust gas flow employ horizontally oriented heat transfer tubes. The
 horizontally oriented heat transfer tubes have forced circulation of a
 heat transfer fluid therethrough. The use of horizontally oriented heat
 transfer tubes having forced circulation can permit rapid start up of the
 heat recovery steam generator.
 Conventionally, in a heat recovery assembly having horizontal heat transfer
 tubes, the heat transfer tubes extend through vertical pairs of spaced
 apart parallel heat transfer tube support plates. The horizontal tubes are
 arranged in horizontal rows, a conventional heat recovery assembly having
 many rows. Typically, a heat transfer assembly has more than 20 rows of
 heat transfer tubes. The heat transfer tube support plates are suspended
 within the housing. The mechanical load and thermal stresses exerted on
 the heat transfer tube support plate are in the same vertical direction
 when a heat recovery assembly with horizontal heat transfer tubes is
 employed in a heat recovery steam generator with vertical exhaust gas
 flow. The mechanical stress on the support plates is generally along a
 vertical line due to the suspended arrangement of the support plates. The
 thermal gradient and therefore the thermal stresses on the heat transfer
 tube support plates are generally constant along any given horizontal
 line, but vary in the vertical direction. The vertical variation in the
 thermal gradient and therefore the thermal stresses arises from the
 cooling of the exhaust gas during passage through the heat recovery
 assembly.
 The support plates are free to expand down as the heat recovery assembly
 heats up due to the suspension of the support plates in the housing. The
 resulting downward expansion and therefore the thermal stress is in a
 generally uniform manner. The thermal expansion of the upper portion of
 the support plate will be less than the thermal expansion of the lower
 portion of the support plate due to the variation of the thermal gradient
 along a vertical line. Again, however, the thermal expansion along any
 given horizontal line is uniform resulting in a uniform downward expansion
 of the support plate.
 Heat recovery steam generators having horizontal exhaust gas flow have
 vertically upright heat recovery and air pollution control assemblies. The
 heat transfer tubes of the heat recovery assemblies are vertically
 oriented and have natural circulation of the heat transfer fluid
 therethrough. Horizontal exhaust gas flow is particularly preferred for
 heat recovery steam generators having limitations on height or structure
 compared to the height or structure typically required for a vertically
 oriented exhaust gas flow path.
 The use of a conventional heat recovery assembly having horizontally
 oriented heat transfer tubes in a heat recovery steam generator having a
 horizontal gas flow results in distortion or warpage of the conventional
 heat transfer tube support plates. The support plate of a conventional
 heat recovery assembly having horizontal heat transfer tubes is relatively
 wide, supporting many rows of heat transfer tubes. Typically, a heat
 recovery assembly has more than 20 rows of heat transfer tubes. The
 mechanical load on the heat transfer tube support plates is in the
 vertical direction due to the suspension of the support plates within the
 housing. The thermal gradient on the support plate is generally constant
 along a vertical line in contrast to a vertical exhaust gas flow wherein
 the thermal gradient is generally constant along a horizontal line. In the
 horizontal exhaust gas arrangement, the thermal gradient varies along any
 given horizontal line of the support plate as the horizontally flowing
 exhaust gas is cooled by passage through the heat recovery assembly. As a
 result, the portion of the support plate in the upstream direction will
 generally expand vertically downward a greater amount than the support
 plate portion in the horizontal downstream direction due to the upstream
 portion having a generally higher temperature. Therefore, the mechanical
 and thermal stresses within the support plate are perpendicular to each
 other. The result of the non-parallel arrangement of the mechanical and
 thermal stresses is the distortion or warpage of the support plate and the
 potential for failure of the heat transfer tubes.
 SUMMARY OF THE INVENTION
 Briefly stated, the combined cycle system in accordance with the invention
 has a gas turbine and heat recovery steam generator having horizontal
 exhaust gas flow with a horizontal tube heat transfer assembly with
 segmented heat transfer tube support plates for the support of
 horizontally oriented heat transfer tubes. The heat transfer assembly with
 horizontally oriented heat transfer tubes is preferably positioned as the
 first heat transfer assembly in the upstream direction of the exhaust gas
 flow, but can alternately or additionally be positioned in the downstream
 direction of the exhaust gas flow.
 The heat recovery assembly employs a vertically segmented heat transfer
 support plate assembly whereby the support plate segments are sufficiently
 horizontally narrow to minimize thermal gradients horizontally across the
 individual support plate segments and therefore reduce the potential for
 warpage or distortion of the support plate assembly that could affect the
 heat transfer tubes mounted thereto.
 In the preferred form of the invention, the heat recovery assembly has
 multiple vertically arranged rows of horizontally oriented heat transfer
 tubes. The vertically arranged rows are transverse to the direction of the
 gas flow path and are spaced apart in the direction of the gas flow path.
 The support plate assembly is vertically segmented parallel to the
 vertical rows of heat transfer tubes wherein less than three and
 preferably only two vertical rows of the heat transfer tubes are mounted
 to each support plate segment. A width for each support plate segment of
 two vertical rows of heat transfer tubes reduces the thermal gradient
 across the support plate segment. The reduced thermal gradient
 substantially reduces the potential for warpage of the individual support
 plate segments. The reduced warpage of the individual support plate
 segment reduces the potential for mechanical failure of the heat recovery
 assembly.
 The heat transfer assembly of the invention is employed of heat recovery
 steam generators having horizontal exhaust gas flow. The use of the heat
 recovery assembly of the invention having horizontal heat transfer tubes
 and forced circulation of the heat transfer fluid therethrough allows for
 a heat recovery steam generator with horizontal exhaust gas flow having
 rapid start up capabilities compared to conventional heat recovery steam
 generators with horizontal exhaust gas flow.
 An object of the invention is to provide a support plate for use in the
 heat recovery assembly having horizontally oriented heat transfer tubes
 with forced circulation of a heat transfer fluid therethrough.
 Another object of the invention is to provide a heat transfer tube support
 plate having a reduced potential warpage when employed with horizontally
 oriented heat transfer tubes in the heat recovery steam generator having a
 generally horizontal exhaust gas flow. These and other objects of the
 invention will become apparent from review of the specification and
 drawings.

DETAILED DESCRIPTION OF THE INVENTION
 A gas turbine combined cycle system 10 in accordance with the invention has
 a gas turbine 12 and a heat recovery steam generator 14. A duct 16 directs
 the exhaust gas stream 18 from the gas turbine 12 to the heat recovery
 steam generator 14. The heat recovery steam generator 14 has a housing 20
 having a diffuser or inlet portion 22 and a full cross-section portion 24.
 The housing 20 defines a generally horizontal gas flow path therethrough.
 The inlet portion 22 of the housing 20 expands the exhaust gas stream from
 the reduced area of the duct 16 to the full cross-section portion 24 of
 the housing 20.
 Positioned within the full cross-section portion 24 is a horizontal tube
 heat recovery assembly 26. The horizontal tube heat recovery assembly 26
 has multiple horizontally oriented heat transfer tubes 34. The tubes 34
 are oriented across or perpendicular to the exhaust gas stream 18. A pump
 29 circulates a heat transfer fluid through the heat transfer tubes 34.
 The heat transfer tubes 34 are preferably connected for once through
 circulation of the heat transfer fluid. The housing 20 contains additional
 heat recovery assemblies 28, 30 and air pollution control assemblies 32.
 The horizontal tube heat recovery assembly 26 is preferably positioned at
 the first circuit or heat recovery unit in the upstream direction, but can
 be readily employed for heat recovery at any position within the housing
 20.
 The heat transfer tubes 34 are arranged in parallel vertical rows 36. The
 rows 36 extend in the downstream direction of the exhaust gas stream 18.
 The rows 36 of heat transfer tubes 34 are mounted to a pair of
 transversely spaced apart support plate assemblies 38. The support plate
 assemblies 38 are perpendicular to the heat transfer tubes 34 and parallel
 to the exhaust gas stream 18. Each support plate assembly 38 is formed of
 multiple vertically oriented support plate segments 40a,b,c. Each support
 plate segment 40a,b,c supports less than three rows 36 of heat transfer
 tubes 34. Preferably each support plate segment 40a,b,c supports two rows
 36 of heat transfer tubes 34. The support plate segments 40a,b,c are
 suspended from a support member 31 in a conventional manner well known in
 the art. The support plate segments 40a,b,c of a particular support plate
 assembly 38 are preferably coplanar. The support plate segments 40a,b,c of
 a particular support plate assembly 38 are further preferably spaced apart
 in the direction of flow of the exhaust gas stream 18. Plate gaps 41 are
 therefore defined between the support plate segments 40a,b,c to prevent
 interference between the support plate segments 40a,b,c due to thermal
 expansion of the support plate segments 40a,b,c from heating by the
 exhaust gas stream 18. Each pair of opposed support plate segments 40a,
 40a; 40b, 40b; and 40c, 40c of the pair of support plate assemblies 38,
 together with heat transfer tubes 34 mounted to each pair of support plate
 segments 40a, 40a; 40b, 40b; and 40c, 40c, form heat recovery assembly
 segments 27a,b,c.
 During operation of the heat recovery steam generator 14, the hot exhaust
 gas stream 18 passes generally horizontally through the rows 36 of heat
 transfer tubes 34 supported by the support plate assembly 38. The support
 plate segments 40a in the upstream direction of support plate assemblies
 38 typically receive the greatest amount of heating from the exhaust gas
 stream 18. As the exhaust gas stream 18 passes through subsequent heat
 recovery assembly sections 27b,c of the horizontal heat recovery assembly
 26, each pair of support plate segments 40b,c positioned downstream of a
 particular support plate assembly 38 receives a lesser degree of heating
 relative to the upstream support plate segments 40a. Therefore, the
 support plate segments 40a in the upstream direction of the exhaust gas
 stream 18 experiences the greatest thermal expansion and therefore expand
 vertically downward the greatest relative amount. Support plate segments
 40b,c positioned further downstream experience a relatively smaller amount
 of heating and therefore expand vertically downward a smaller relative
 amount.
 The multiple support plate segments 40a,b,c, forming the support plate
 assemblies 38 permit the combination of horizontal gas flow in horizontal
 heat transfer tubes 34 of the horizontal tube heat recovery assembly 26
 without excessive thermal stress on the support plate assemblies 38. Each
 support plate segment 40a,b,c is sufficiently narrow horizontally to
 reduce the potential for warpage due to thermal gradients in the
 horizontal direction across the support plate segments 40a,b,c in the
 direction of the exhaust gas stream.
 While a preferred embodiment of the present invention has been illustrated
 and described in detail, it should be readily appreciated that many
 modifications and changes thereto are within the ability of those of
 ordinary skill in the art. Therefore, the appended claims are intended to
 cover any and all of such modifications which fall within the true spirit
 and scope of the invention.