EP3175092A1

EP3175092A1 - Heat recovery system and method

Info

Publication number: EP3175092A1
Application number: EP15745283.0A
Authority: EP
Inventors: Nicholas F. Urbanski
Original assignee: ExxonMobil Upstream Research Co
Current assignee: ExxonMobil Upstream Research Co
Priority date: 2014-07-31
Filing date: 2015-06-24
Publication date: 2017-06-07
Also published as: AU2015296988A1; WO2016018531A1; SG11201610460SA; US20160033170A1

Abstract

The present techniques are directed to a heat recovery system, such as a waste heat recovery system (WHRU) that receives and passes a vapor across a heat exchanger (300) to transfer heat from the vapor to a heating medium in the heat exchanger. The vapor may be an exhaust gas from a source outside of the heat recovery system. The heat recovery system includes a collection system (202) to deinventory the heating medium from the heat exchanger during abnormal operation of the heat recovery system.

Description

HEAT RECOVERY SYSTEM AND METHOD

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the priority benefit of United States patent application number 62/031,720 filed July 31, 2014 entitled HEAT RECOVERY SYSTEM AND METHOD, the entirety of which is incorporated by reference herein.

FIELD

[0002] The present techniques relate generally to heat recovery such as waste heat recovery, and more particularly to a heat recovery system having a heating-medium collection system for abnormal operation to reduce thermal degradation of the heating medium.

BACKGROUND

[0003] This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.

[0004] Waste Heat Recovery Units (WHRU) and similar systems transfer energy from higher temperature vapor or gas streams to a heating medium (HM) fluid, typically a liquid. The HM is typically utilized as a heat source for other heat transfer equipment (users or load) in a target or desired process. The source of the high-temperature vapor stream may be a gas turbine, fired heater, steam generator, and other sources. The high-temperature vapor may be a hot exhaust gas, for example, from one or more of these sources. At the WHRU, the high- temperature vapor typically flows through ductwork or other enclosure housing coils or a bundle of tubes. The HM flows through the coils or tubes and absorbs heat from the high- temperature vapor stream passing over the coils or tubes.

[0005] In normal operation, the HM fluid typically flows through the coils or tubes at a flow rate and relatively-short residence time that prevents excessive elevated temperature of the HM thus may avoid significant thermal degradation of the HM or excessive deposition of HM impurities, and the like. Unfortunately, during an abnormal operation or shutdown condition, the HM flow may slow or stop. Thus, the HM fluid retained in the coils or tubes may experience elevated temperatures and thermally degrade. Indeed, at elevated temperature over time, glycols or other similar components of the HM may form sludges having corrosive acids. Further, certain HM fluids and components, e.g., water, may exceed typical deposit rates of impurities on the tube wall at elevated temperature over time, fouling the tubes.

[0006] United States Patent Application Publication No. 201 1/0067742 entitled "Thermoelectric -Based Power Generation Systems and Methods" to Bell et al. describes a waste heat recovery system with a cylindrical shell that contains an exhaust fluid. Two heat exchangers extend into the cylindrical shell to exchange heat with the exhaust fluid. A control valve determines when exhaust fluid may pass through the second heat exchanger.

[0007] United States Patent Application Publication No. 2011/0167865 entitled "Air- Conditioning Apparatus" to Morimoto et al. describes an air-conditioning apparatus in which the refrigerant exchanges heat with a HM in at least one intermediate heat exchanger. The document also describes the circulation systems of both the refrigerant and the HM.

[0008] United States Patent No. 4,669,530 entitled "Heat Exchanger Method and Apparatus" to Warner describes a dual-heat exchanger heat removal system for cooling sulfur trioxide. The design protects the first heat exchanger from corrosion by not permitting the sulfur trioxide to condense within its assembly. Furthermore, the design protects the second heat exchanger from thermal damage by cooling the sulfur trioxide in the first heat exchanger.

[0009] United States Patent No. 4,737,531 entitled "Waste Heat Recovery" to Rogers describes a heat exchange unit that recovers waste heat from carbon black smoke by preheating oxygen-containing gas such as air passed to a carbon black producing reactor. A control loop is provided that automatically diverts the oxygen-containing gas being preheated through a by-pass line if the temperature of the effluent smoke removed from the heat exchange is below a minimum temperature value to minimize the deposition of carbon on the heat exchange surfaces in the heat exchange zone.

[0010] United States Patent No. 6,984,292 entitled "Water Treatment for Thermal Heavy Oil" to Kresnyak et al. includes initial steps of capturing the waste heat energy from high- pressure steam separator located downstream of steam generators. It further describes operating conditions that promote a 1% to about 50% mass vapor in the stream returning to the heated separator to prevent fouling and scaling.

[0011] United States Patent No. 7,823,628 entitled "Passive Back-Flushing Thermal Energy System" to Harrison describes a thermal energy system having a heat exchanger for transferring thermal energy between a source and a load. The heat exchanger has a primary side associated with the source and a secondary side for conducting a fluid associated with the load. The secondary side of the heat exchanger is back-flushed via a specially-designed back-flush valve upon a consumption of a portion of the fluid. The abstract explains that passive back flushing prevents fouling of the heat exchanger due to sediments, scale, and mineral deposits that may be present in the circulating fluid.

[0012] United States Patent No. 8,470,097 entitled "Silicon-Containing Steel Composition with Improved Heat Exchanger Corrosion and Fouling Resistance" to Chun et al. describes a method of providing sulfidation corrosion resistance and corrosion induced fouling resistance to a heat transfer component surface.

[0013] A problem in a heat recovery system including a WHRU may be excessive temperature and associated thermal degradation of the HM, such as glycol, in the waste heat recovery system.

SUMMARY

[0014] An embodiment relates to a heat recovery system having a tube bundle with multiple tubes. The heat recovery system include an enclosure housing at least a portion of the tube bundle, wherein the enclosure is configured to receive a vapor and pass the vapor over the tube bundle. A pump configured to flow a heating medium through the tube bundle to one or more users. A control system is configured to detect an abnormal operation of the heat recovery system, wherein the abnormal operation includes a reduction of flow of the heating medium below a predetermined range of flow values. Lastly, the heat recovery system includes a collection system configured to reduce thermal degradation of the heating medium during the abnormal operation by evacuating the heating medium from the tube bundle during the abnormal operation.

[0015] Another embodiment relates to a waste heat recovery unit having a heat exchanger and a ductwork enclosing at least a portion of the coils, wherein the ductwork is configured to receive and pass an exhaust gas over the heat exchanger. The waste heat recovery unit includes a pump configured to circulate a heating medium through an internal flow path of the heat exchanger to absorb heat from the exhaust gas, and wherein the pump is configured to circulate the heating medium from the waste heat recovery unit to users of the heating medium. Further, the waste heat recovery unit includes a collection system configured to remove the heating medium from the heat exchanger during an abnormal operation to reduce thermal degradation of the heating medium in the heat exchanger. The collection system includes a collection vessel configured to receive heating medium removed from the heat exchanger during the abnormal operation, and wherein the abnormal operation includes a cessation of flow or substantial reduction in a flow rate of the heating medium through the heat exchanger.

[0016] Yet another embodiment relates to a method of operating a waste heat recovery unit, the method including circulating a heating medium through multiple tubes of a tube bundle in the waste heat recovery unit and to users of the heating medium outside of the waste heat recovery unit. The exemplary method includes passing a vapor over the tube bundle to transfer heat from the vapor to the heating medium. Further, the method includes detecting, e.g., via a control system, an abnormal operation of the waste heat recovery unit, the abnormal operation including loss of circulation of the heating medium through the multiple tubes. Lastly, the method includes removing the heating medium from the multiple tubes to a collection vessel in the waste heat recovery unit during the abnormal operation to reduce or avoid thermal degradation of the heating medium.

[0017] Yet another embodiment relates to a method of constructing or retrofitting a waste heat recovery unit (WHRU) to prevent or reduce thermal degradation of heating medium during abnormal operation of the WHRU, the method including adding a conduit downstream of a tube bundle configured to transfer heat from an exhaust gas to the heating medium, wherein the conduit is configured to divert flow of the heating medium in the WHRU during the abnormal operation of the WHRU, the off-specification operation including cessation of normal flow of heating medium through the tube bundle. The exemplary method includes installing a collection vessel coupled to the conduit and configured to receive heating medium drained from the tube bundle via the conduit during the abnormal operation. Lastly, the method includes installing a valve on the conduit, wherein the valve is configured to isolate the collection vessel during normal operation of the WHRU and to place the collection vessel in service during the abnormal operation. DESCRIPTION OF THE DRAWINGS

[0018] The advantages of the present techniques are better understood by referring to the following detailed description and the attached drawings, in which:

[0019] FIG. 1 is a block flow diagram of an operational system having a waste heat recovery unit (WHRU) and users of the HM circulating from the WHRU;

[0020] FIG. 2 is a block flow diagram of an exemplary WHRU of FIG. 1 having an HM collection system;

[0021] FIG. 3 is a schematic diagram of the exemplary WHRU of FIG. 2 having a tube bundle coupled to the HM collection system;

[0022] FIG. 4 is a simplified process flow diagram of the exemplary WHRU of FIG. 3 having an exemplary HM collection system;

[0023] FIG. 5 is a block diagram of an exemplary method of evacuating HM from a WHRU tube bundle during abnormal operation; and

[0024] FIG. 6 is a block diagram of retrofitting a WHRU to incorporate an exemplary HM collection system for abnormal operation.

DETAILED DESCRIPTION

[0025] In the following detailed description section, specific embodiments of the present techniques are described. However, to the extent that the following description is specific to a particular embodiment or a particular use of the present techniques, this is intended to be for exemplary purposes only and simply provides a description of the exemplary embodiments. Accordingly, the techniques are not limited to the specific embodiments described below, but rather, include all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims.

[0026] As used herein, "substantially", "predominately" and other words of degree are relative modifiers intended to indicate permissible variation from the characteristic so modified. It is not intended to be limited to the absolute value or characteristic which it modifies, but rather possessing more of the physical or functional characteristic than its opposite, and preferably, approaching or approximating such a physical or functional characteristic.

[0027] "Exemplary" is used exclusively herein to mean "serving as an example, instance, or illustration." Any embodiment described herein as "exemplary" is not to be construed as preferred or advantageous over other embodiments.

[0028] The term "gas" is used interchangeably with "vapor," and means a substance or mixture of substances in the gaseous state as distinguished from the liquid or solid state. Likewise, the term "liquid" means a substance or mixture of substances in the liquid state as distinguished from the gas or solid state. As used herein, "fluid" is a generic term that may include liquid and either a gas or vapor.

[0029] Embodiments of the present techniques are directed a thermal energy system such as a heat recovery system, waste heat recovery unit (WHRU), or similar system. The thermal energy system has a heat exchanger for transferring thermal energy between thermal energy content and a load. The thermal energy content may include a high temperature vapor, such as an exhaust gas, while the load may include a heat transfer fluid, a HM, and the like. The high-temperature vapor may be provided or produced by gas turbines, fired heaters, steam generators, and so forth.

[0030] In examples, the heat exchanger has a primary side associated with the high- temperature vapor, and a secondary side for receiving and outputting the load, e.g., the secondary side receives a fluid to be heated and outputs a heated fluid. The heat exchanger may reside, for instance, in a ductwork or other enclosure transporting the energy source fluid. As discussed below, auxiliary equipment and a control system may provide for shutdown conditions and evacuation of the secondary side, e.g., tube-side, of the heat exchanger.

[0031] A bundle or coils of tubes may form the assembly of the heat exchanger transferring energy between the high-temperature vapor and load fluids. The tubes of the bundle, or coil of tubes, of the heat exchanger may be plain, externally finned, internally finned, continuously finned and may possess numerous other heat-transfer enhancements or features. The conduits or tubes of the bundle/coil of tubes may have a downward slope to aid drainage of HM fluid. The upward movement or escape of formed HM fluid vapor may also be assisted. The HM will generally exchange energy with the high-temperature vapor while remaining in the liquid phase. The system may include tanks, valves, and associated piping to evacuate the HM fluid from the heat exchanger in the case of failed circulation of the HM fluid. [0032] Further, air or ambient air may be introduced during the shutdown period to reduce the temperature of the high-temperature vapor flowing across the primary side, e.g., the exterior, of the heat exchanger. The reduced temperature from the introduction of air may decrease the thermal degradation of the HM fluid on the secondary side of the heat exchanger. The system may also introduce an inert gas purge through the tube side of the heat exchanger to remove any residual heating-medium film on the inner surface of the secondary side of the heat exchanger. This may also mitigate the formation of a solid and corrosive fouling layer on either side of the heat exchanger, due to oxidation.

[0033] In the WHRU's transfer of energy from higher temperature vapor streams to a HM fluid, typically a liquid, the HM may be utilized as a heat source for other heat transfer equipment, such as users or other loads, in a desired or target process. As discussed, in the WHRU, the high-temperature vapor that is carried in the ductwork typically passes over a bundle of tubes or coils through which the HM fluid flows. Various physical states and compositions of the HM fluid may be employed but the HM fluids may generally thermally degrade or deposit impurities or scale when maintained at excessive elevated temperatures over time. Thermal degradation of glycols, for example, a component of certain HM fluids, may form sludges of corrosive acids. With continued exposure to the effects of the high- temperature vapor on the outer tube wall, the formed corrosive sludge may continue to degrade and transform into a substantially solid layer of material on the inner tube wall. The solid material may negatively affect the mechanical integrity and operational performance of WHRU tube bundle and associated process equipment or users of the HM.

[0034] In normal operation, the HM fluid flows inside the tubes at a rate that may substantially avoid thermal degradation of the HM fluid, including at the inner tube wall that typically exhibits the highest temperature in the tubes. However, during off-specification or abnormal operating conditions, the HM fluid flow inside the tubes may slow or even stop. If the tube bundle remains full of stagnant HM fluid exposed to the high-temperature vapor flow, thermal degradation of the HM fluid inside the tubes may begin, for example, starting at the inner tube wall. Again, the thermal degradation may lead to the formation of corrosive sludge that negatively impacts and reduces the mechanical integrity of the tubes, and may lead to formation of an associated fouling layer that generally worsens the performance of the WHRU. Off-specification or abnormal operating conditions may result in a reduction of or stopped HM fluid circulation flow, higher vapor temperatures than design on the outside of the tubes, and failure of a control damper within the WHRU assembly to divert the high- temperature vapor flow away from the tube bundle when desired, and so forth. In the event that the HM circulation decreases below an acceptable range and the high-temperature vapor is not diverted away from the WHRU coils either by design or system failure, the described apparatus and generalized strategy of its operation is expected to protect the HM from thermal degradation and, therefore, protect its associated equipment.

[0035] FIG. 1 is a block flow diagram of an operational system 100 having a heat recovery unit such as a waste heat recovery unit (WHRU) 102. In general, a WHRU recovers heat from hot exhaust vapor in the system 100 before discharging the hot exhaust vapor. In the recovery, the WHRU transfers heat from the incoming hot exhaust vapor to a heat- transfer fluid. Thus, the recovered heat is used to increase the temperature of the heat- transfer fluid. The heated heat-transfer fluid may be supplied as a utility HM to process equipment in the system 100.

[0036] In operation in the illustrated embodiment, the HM circulates through the WHRU 102 to absorb heat from hot exhaust vapor and then through one or more users 104 in the system 100 to provide heat to the users 104. The users 104 may be heat exchangers or other types of equipment that absorb heat from the HM. The overall system 100 may be an industrial, commercial, or public facility or complex involved in the manufacturing, production, processing, treatment, handling, and so forth, of one or more products (not shown).

[0037] In embodiments, the WHRU 102 circulates the HM through a heat exchanger to absorb heat from an incoming hot exhaust vapor to provide a HM supply 106 to the users 104. The WHRU 102 receives a HM return 108 from the users 104. The HM return 108 is typically cooler than the HM supply 106. Again, the users 104 may be heat exchangers or other process equipment that absorb heat from the HM supply 106.

[0038] In some embodiments, the WHRU 102 receives a vapor 110 from one or more sources 112, and discharges a cooled vapor 114. The cooled vapor 114 may be discharged to atmosphere or to a recovery system, and so on. The inlet vapor 110 is generally a high- temperature vapor, such as a hot exhaust gas, and the like. The sources 112 of the inlet vapor 110 may be gas turbines, fired heaters, steam generators, and other sources of hot vapor or gas. In examples, the inlet vapor 110 may be waste gas from the source 112. An exemplary temperature range of the inlet vapor 110 is about 300 °C to about 800 °C. An exemplary temperature range of the cooled vapor 114 discharging from the WHRU 102 is about 200 °C to about 550 °C. Of course, other temperature range values for the inlet vapor 110 and the cooled vapor 114 are applicable. In certain embodiments, the inlet vapor 110 is a discharge or exhaust gas or vapor from one or more sources 112 outside of the WHRU 102. In other words, the WHRU 102 may be a utility system that receives inlet vapor 110 from process or production systems or other utility systems. In fact, the WHRU 102 may be a shared utility in the overall system 100 at a facility or site, for example. As appreciated by one of ordinary skill in the art, the WHRU 102 may be a utility system outside the battery limits of the source(s) 112. Indeed, in examples, the WHRU 102 may be an outside battery limits (OSBL) system to the source(s) 112.

[0039] Further, the overall system 100 and/or WHRU 102 may include a control system 116, such as a distributed control system (DCS), programmable logic controller (PLC), and so on. The control system 116 may have a human interface (HMI) and facilitate control of the system 100 including the WHRU 102. The control system 116 may direct operation of equipment, control valves, and the like, in the WHRU 102. The control system 116 may include instrumentation, computers, computer memory, a processor, and so forth. The control system 116 may include logic or code stored in memory and executable by the processor to implement or facilitate the control actions disclosed herein. Lastly, as discussed below, the WHRU 102 may include a shutdown HM collection system (that may work in concert with the control system 116) to reduce thermal degradation of the HM during abnormal operations of the WHRU 102.

[0040] FIG. 2 is a block flow diagram of an exemplary WHRU 102 of FIG. 1 having an exhaust stack 200 and a HM collection system 202. Like numbered items are as described with respect to FIG. 1. The exhaust stack 200 may house a heat exchanger (not shown) having coils or a tube bundle to transfer heat from the inlet vapor 110 to a HM, e.g., to provide a heated HM supply 106. The HM collection system 202 is coupled to the tube bundle in the stack 200 via a first or supply conduit 204 and a second or return conduit 206. As discussed below, the supply conduit 204 and return conduit 206 may accommodate bidirectional flow during abnormal operation.

[0041] The HM collection system 202 may both facilitate circulation of the HM including during normal operation, and facilitate the evacuation and collection of HM from the heat- exchanger tubes or coils in the stack 200 during a shutdown or abnormal operation. The HM collection system 202 may include the pump and hydraulic expansion vessel that circulates the HM. Thus, the HM collection system 202 may provide the HM supply 106 to users 104 (FIG. 1) and receives the HM return 108 from the users 104. In the illustrated embodiment during normal operation, the HM supply 106 flows through the supply conduit 204 from the tube bundle or coils in the stack 200, and the HM return 108 flows through the return conduit 206 to the tube bundle of coils in the stack 200.

[0042] During abnormal operation with the HM circulation slowing or stopping, and with the high-temperature vapor 110 continuing to flow across the heat-exchanger tubes or coils in the stack 200, the HM collection system 202 may facilitate evacuation of the HM from the tube coils in the stack 200. The evacuation of the HM from the tube coils may reduce thermal degradation of the HM during the shutdown or abnormal operation.

[0043] In some configurations, the HM collection system 202 includes a collection vessel 208 to accumulate HM drained via the supply conduit 204 from the coils during the abnormal or off-specification operation. The collection vessel may be sized to at least the volume of the heat-exchanger tubes or coils in the stack 200. Also, in certain examples, any vaporized HM from the coils during the off-specification operation may flow through the return conduit 206 to the HM collection system 202. If desired, the HM vapor may further discharge through a vent 210 from the collection system 202.

[0044] Additionally, the HM collection system 202 may flow an inert gas 212 purge through the coils or tubes in the stack 200 via the supply conduit 204, for example, to facilitate removal of residual HM film from the inner surface of the coils or tubes. The purge may reduce the formation of a solid and corrosive fouling layer in the tubes. Moreover, a gas 214 such as ambient air may be introduced into the stack 200 during the abnormal operation or off-specification operation to reduce the temperature of inlet vapor 110 flowing across the tubes or coils in the stack 200. Thus, the introduction of the gas 214 may contribute to the reduction of the thermal degradation of the HM fluid in the tubes or coils. Lastly, the WHRU 102 may employ the control system 116 to facilitate implementation of the HM evacuation from the tubes and the associated aforementioned operations. For example, abnormal operation may include a shutdown or a decrease in flow that may cause the HM fluid to reach temperatures that may result in degradation. Accordingly, a measurement of the temperature of the HM may be used to determine whether the HM needs to be drained from the coils, alone or in combination with other indications, such as a failure of a circulation pump. [0045] FIG. 3 is a schematic diagram of the exemplary WHRU 102 of FIGS. 1 and 2 having a heat exchanger as a tube bundle 300 in the stack 200. Like numbered items are as described with respect to FIGS. 1 and 2. The tubes or coil of tubes of the bundle 300 of the heat exchanger may be plain, externally finned, internally finned, continuously finned and may possesses other heat-transfer enhancements or features. The WHRU 102 may have one or more heat exchangers, each having respective tube bundles 300, disposed in series, parallel, or both.

[0046] In the illustrated embodiment, the tube bundle 300 is disposed in a primary stack 302 and coupled to the HM collection system 202. The coupling provides for operational or fluidic coupling of the HM collection system 202 to the tube-side of the tube bundle 300. The stack 200 includes the primary stack 302 and a bypass stack 304. The primary stack 302 and the bypass stack 304 may be ductwork and/or other types of enclosures and conduits. If ductwork is employed, the ductwork may be a series of ducts. As appreciated by the skilled artisan in this context, a duct may be an enclosure, conduit, tube, or pipe for conveying the vapor 110. The material of construction of the ductwork may be selected for relatively high temperature to accommodate the temperature of the vapor 110. In certain embodiments with the primary stack 302, the ductwork or portions of the ductwork conveys vapor 110 or other fluids across the tube bundle 300, and also houses or encloses, or partially encloses, the tube bundle 300.

[0047] The primary stack 302 is generally employed during typical operation, whereas the bypass stack 304 may generally be placed in operation during atypical operation. To activate the bypass stack 304 in the illustrated embodiment, a damper 306 may be opened to divert vapor 110 flow from the primary stack 302 to the bypass stack 304. The diverted vapor 110 may discharge from the bypass stack 304, as indicated by arrow 308, to the atmosphere or for additional processing. In embodiments, the bypass stack 304 may include various operational units 310, such as a silencer or exhaust treatment catalyst bed.

[0048] As mentioned, the primary stack 302 houses the tube bundle 300. The inlet vapor 110 enters the primary stack 302 and passes across the tube bundle 300. The WHRU 102 employs the tube bundle 300 to transfer heat from the vapor 110 to the HM flowing through the tubes of the tube bundle 300. A cooled vapor 114 flowing from across the tube bundle 300 discharges from the primary stack 302 to the atmosphere or additional processing.

[0049] The HM having absorbed heat from the vapor 110, and thus, a higher temperature, discharges from the tube-side of the tube bundle 300 into the supply conduit 204 and is provided as HM supply 106 to users 104 (FIG. 1). The HM return 108 from the users 104 flows through the return conduit 206 to the tube-side of the tube bundle 300 to be reheated in the tube bundle 300.

[0050] During an abnormal operation, such as a pump failure or system shutdown that involves the HM circulation slowing or discontinuing, the HM collection system 202 may facilitate evacuation of HM from the tube-side of the tube bundle 300. Such evacuation may reduce thermal degradation of the HM during the shutdown or abnormal operation, especially while the high-temperature vapor 110 is continuing to flow across the tube bundle 300 during the abnormal operation. The continued flow of the high-temperature vapor 110 across the tube bundle 300 may be intended or by design, or may be due to mechanical failure or faulty operation of the damper 306, problems with the bypass stack 304, and so forth.

[0051] As discussed, the HM collection system 202 may include a collection vessel 208 to accumulate HM drained via the supply conduit 204 from the tube bundle 300 during the abnormal or off-specification operation. The tube bundle 300 and the supply conduit 204 may be configured (e.g., a mild slope) to facilitate flow or draining, e.g., via gravity, of the HM from the tube bundle 300 to the collection vessel 208.

[0052] In some embodiments, a back-flushing of the tube bundle 300 to displace or remove HM from the tube bundle 300 is not required. Indeed, certain embodiments of the HM collection system 202 are configured wherein back-flushing is not employed to remove HM from the tube bundle 300 during the abnormal operation.

[0053] Moreover, any vaporized HM from the tube bundle 300 during the shutdown or off-specification operation may flow from the tube bundle through the return conduit 206 to the HM collection system 202 and discharge as a vent 210. This HM vapor flow (if present) is generally in the opposite direction of the typical HM return 108 liquid flow through the return conduit 206 during normal operation.

[0054] As also mentioned, the HM collection system 202 may include piping to introduce an inert gas 212 into the tube-side of the tube bundle 300 via the supply conduit 204 or other conduit. The purging of the tubes with the inert gas 212 may facilitate removal of residual HM film from the inner surface of the coils or tubes, reduce formation of a solid and corrosive fouling layer in the tubes, and the like. In examples, the inert gas 212 may flow through supply conduit 204 (in opposite direction of normal flow of the HM supply 106), through the tubes of the tube bundle 300, and then through the return conduit 206 (in opposite direction of normal flow of the HM return 108) to be discharged from the HM collection system 202 in the vent 210.

[0055] FIG. 4 is a simplified process flow diagram of the exemplary WHRU 102 of FIGS. 1-3 having an exemplary HM collection system 202. In this example, the HM collection system 202 is coupled to the tube bundle 300 via the supply conduit 204 and return conduit 206. In the illustrated embodiment, the HM collection system 202 drives both circulation of HM during normal operation and the collection of HM from the tube bundle 300 (e.g., to a HM collection vessel 208) during certain abnormal operations of the WHRU 102.

[0056] The HM collection system 202 includes a HM expansion vessel 402 and a pump 404 to circulate the HM including during normal operation. The pump 404 provides a motive force to circulate the HM through the HM circuit including through the tube bundle 300 and the users 104. In examples, the pump 404 is a centrifugal pump. The HM expansion vessel 402 accommodates thermal expansion of the HM flowing through the hydraulically-full HM circuit. In particular, the liquid level 406 in the expansion vessel 402 may rise and fall to account for thermal expansion of the HM in the circuit.

[0057] A vent valve 408, such as a control valve, pressure control valve, or flow control valve, among others, which is located on the expansion vessel 402 overhead may be modulated to prevent excessive accumulation of uncondensed components in the system and to control pressure of the expansion vessel 402 to a desired pressure set point. The valve 408 may vent noncondensable or uncondensed components, as vent 210, that accumulate in the circuit. In operation, the vent valve 408 may modulate the flow rate of a vent 210 stream from the vapor space of the expansion vessel 402 to control pressure in the expansion vessel 402, and thus, facilitate control of suction pressure of the HM circulating pump 404.

[0058] In the illustrated embodiment, the expansion vessel 402 receives the HM return 108 from the users 104 (FIG. 1). The HM return 108 flows from the expansion vessel 402 via pump suction piping 410 to the pump 404. The pump 404 discharges the HM return 108 through pump discharge piping 412 and return conduit 206 to the tube-side of the tube bundle 300 in the primary stack 302. Isolation valves may be disposed around the circulation pump 404 on the suction piping 410 and the discharge piping 412, respectively. In the illustrated example, an isolation block valve 416 is disposed on the pump discharge piping 412. The valve 416 may additionally be a modulating control valve to adjust flow rate of the HM. On other hand, in the case of the valve 416 as only an isolation valve, an additional valve (not shown) as a control valve may be disposed on the pump discharge piping 412 or the return conduit 206 to modulate the flow rate of the HM. Any such control valves, as well as pump recirculation line 418 and recirculation valve 420, may promote hydraulic stability of the pump 404 operation. Typically, the pump 404 provides motive force in the HM circulation for transport of the HM supply 106 from the WHRU 102 to the users 104. In operation, the HM supply 106 flows from the tube bundle 300 through the supply conduit 204, supply piping 422, and any supply valves 424 and 426 on the supply piping 422, to the users 104.

[0059] As discussed, removal of the HM fluid from the tube-side of the tube bundle 300 may be implemented during certain abnormal operations of the WHRU 102 to avoid or reduce thermal degradation of the HM. To implement such removal, an HM collection vessel 208 disposed on the downstream side of the tube bundle 300 may be placed in service, such as by opening valves 428 and 430, e.g., block valves, around the collection vessel 208. Of course, other valve and piping configurations to place the collection vessel 208 in operation may be applicable. Moreover, in some embodiments, one or more optional valves 424 and 426 on the piping 422 that transports the HM supply 106 to users during normal operation may be closed in the shutdown or off-specification scenario.

[0060] Typically, during normal operation, the HM collection vessel 208 is isolated, e.g., with valves 428 and 430 in a closed position, and is not utilized during normal operation of the WHRU 102. Again, the HM supply 106 during normal operation flows through supply piping 422 and with any supply valves 424 and 426 in an open position. However, as mentioned, during an abnormal operation of the WHRU 102, the collection vessel 208 and collection piping 432 placed in service to receive HM fluid from the tube bundle 300 to the collection vessel 208 via the supply conduit 204 and collection piping 432.

[0061] Indeed, in abnormal operation where the circulating HM fluid slows or stops, such as due to a pump 404 failure, and which may include examples with the high-temperature inlet vapor 110 continuing to flow across the tube bundle 300, the HM fluid may be evacuated from the tube bundle 300 to the collection vessel 208. Otherwise, the stagnant HM fluid in the tube bundle 300 may reach elevated temperatures and thermally degrade. Therefore, in such abnormal conditions, the HM fluid may be removed, e.g., drained, to the collection vessel 208 via collection piping 432.

[0062] As indicated, to place the collection vessel 208 in service, at least one block valve around the collection vessel 208 may be opened. In the illustrated embodiment, to place the collection vessel 208 in operation, an inlet valve 428 to the collection vessel 208 and an outlet valve 430 from the collection vessel 208 may each be opened. Thus, a collection conduit or collection piping 432 and the collection vessel 208 are placed in service for the abnormal operation and to remove HM from the tube bundle 300. Further, in addition to the collection vessel 208 and collection piping 432 placed in operation to receive HM fluid evacuated or drained from the tube bundle 300, one or more valves 424 and 426 (if present) on the HM supply 106 piping 422 to the users 104 may be optionally placed in a closed valve position.

[0063] Further, in embodiments, HM fluid vaporized (if present) in the tube bundle 300 may be removed. For example, such HM vapor may flow through return conduit 206 and recirculation line 418 to the expansion vessel 402, and discharged by the vent valve 408. Moreover, it should be noted that if a pump recirculation line 418 is not included in the WHRU 102, a similar line 418 and valve 420 may be installed to provide a route for HM vapor to reach the vapor space of the expansion vessel 402 from the tubes or coils during the abnormal operation. In addition, an inert gas 212 may be purged through the tube bundle 300. The inert gas 212 may reach the tube bundle 300 through the supply conduit 204, for example. Of course, other various pipes or conduits may be utilized to introduce the inert gas 212 purge through the tube bundle 300.

[0064] Thus, in an event that the HM circulation rate decreases below an acceptable value, the aforementioned techniques may protect the HM fluid from substantial thermal degradation. This may be especially useful if the high-temperature vapor, e.g., hot exhaust gas, is not diverted from the WHRU tubes or coils. This shutdown technique may reduce fouling of the WHRU coil or tubes, and also users of the HM from the WHRU.

[0065] A control system may be configured to detect an abnormal operation of a heat recovery system or the WHRU, wherein the abnormal operation may include a reduction of flow of the HM below a predetermined range of flow values. An inadvertent or intentional shutdown of the circulation pump 414 may be the cause of the reduced or no circulation flow in the abnormal operation. An inadvertent shutdown of the pump 414 may involve an operational mistake and/or equipment failure. Again, the pump 414 may be a pump configured to flow a HM through the tube bundle 300 to one or more users.

[0066] In one example of abnormal operation, the HM circulation pump 404 may be electrically tripped and thus stopped. In other words, the electrical supply to the motor of the pump 404 is stopped and thus the pump 404 does not provide a motive force for flow of the HM through the tube bundle 300. This represents a viable cause of reduced HM circulation rate and/or cessation of HM circulation. Of course, other causes may result in a reduced or ceased HM circulation.

[0067] To implement a shutdown and HM evacuation technique to reduce thermal degradation of the HM in the tube bundle 300 or coils, the pump 404 may be isolated from the HM fluid in the tube bundle 300. Further, the recirculation valve 420 may be placed in an open or full open position to facilitate any potentially expanding HM liquid or vapor in the tube bundle 300 to travel in a reverse flow direction through the return conduit 206. HM vapor generated in the tube bundle 300 during the abnormal operation and shutdown may pass through the recirculation valve 420 and out through the vent valve 408 as needed. Optionally, a block valve 434 may be employed to the users from the HM expansion vessel 402 during the shutdown period.

[0068] With the flow of the primary HM fluid circuit stopped, and potentially blocked via the valve 434 at the expansion vessel 402, the valve 428 to the HM collection vessel 208 may be opened to facilitate HM fluid inventory within the tube bundle 300 to drain freely (by gravity) into the HM collection vessel 208. Opening this side stream to the collected vessel 208 may reduce the amount of time the HM fluid is subject to the harsh process conditions in the tube bundle 300, such as with continuation of the high-temperature vapor 110 flowing over tube bundle 300 while the pump 404 is shut down and with no normal flow of HM through the tube bundle 300. This opening of the valve 428 to the collection vessel 208 may reduce the vaporization of the HM fluid, the degradation of the HM fluid, and the formation of corrosive sludge in the non-flowing HM fluid, and so on. The tubes of tube bundle 300 may possess a downward slope to aid downward HM fluid liquid drainage to the HM collection vessel 208 and facilitate upward escape of formed HM fluid vapor escape to the HM expansion vessel 402 in this shutdown arrangement.

[0069] A gas 212 such as ambient air may be introduced into the stack 302 during the shutdown period to dilute and lower temperature of the high-temperature vapor 110, e.g., hot exhaust gas, flowing over the tube bundle 300. Such ambient air introduction may aid the mitigation of the thermal degradation of the HM fluid within the tube bundle 300 during the off-specification operation and shutdown period. Furthermore, the aforementioned inert gas 212 purge may pass through the tubes of the tube bundle to assist in purging any film of HM fluid that remains on the inside walls of the tubes.

[0070] Lastly, these exemplary configurations specifically related to exemplary valve and equipment arrangements are not meant to limit the present techniques. Other arrangements within the scope of the present techniques may implemented to circulate the HM, and also to reduce HM fluid thermal degradation and associated damage to equipment in response to various abnormal operation and circuit shutdown causes.

[0071] For example, the pump 404 may be located on the HM return portion of the circuit before the inlet to the coils or tube bundle 300, as shown, or may instead be disposed on the HM supply portion of the circuit after the outlet of the tube bundle 300. Indeed, in some embodiments, the pump 404 may generally be disposed at locations throughout the HM system and circuit. Further, in certain examples, the pump 404 may not be a component of the collection system 202 but instead be disposed outside of the collection system 202. Furthermore, in some embodiments, the collection vessel 208 may be generally in addition to the expansion vessel 402, as shown, wherein the collection vessel is not the expansion vessel. Moreover, the HM flow through the coils or tube bundle 300 may be downward or upward with respect to ground. Also, the enclosure or primary stack 302 routing the inlet vapor 110 may be vertical, horizontal, or inclined, or any combination thereof.

[0072] FIG. 5 is a block diagram of a method 500 of operating a heat transfer system such as a waste heat recovery unit. At block 502, the method includes circulating a HM through multiple tubes or coils of a tube bundle in a waste heat recovery unit and to users of the HM. At block 504, the method includes passing a vapor over the tube bundle to transfer heat from the vapor to the HM. The vapor may be an exhaust gas from an upstream source.

[0073] At block 506, the method includes detecting, e.g., via a control system, an abnormal operation of the waste heat recovery unit. The abnormal operation may include loss of normal circulation of the HM through the multiple tubes or coils, an excessive temperature of the HM in or exiting the tube bundle, and so on. For instance, the HM flow may slow or stop through the tubes. In examples, the loss of circulation of the HM is caused by the failure or shutdown of a pump that circulates the HM. Further, the abnormal operation may include continued flow of the vapor over the tube bundle or coils. In some circumstances, the reduced flow may be caused by an operational error from a user 104 (FIG. 1), for example, closing a valve in a separate process unit.

[0074] At block 508, the method includes evacuating the HM from the multiple tubes or coils to a collection vessel during the abnormal operation to reduce thermal degradation of the HM. The waste heat recovery unit has a HM collection system having the collection vessel. Evacuating of the HM may include draining by gravity the HM from the multiple tubes to the collection vessel. The evacuation of the HM may substantially deinventory or evacuate the HM from the multiple tubes or coils. Moreover, the evacuating of the HM from the tubes or coils may include venting HM vaporized in the multiple tubes or coils during the abnormal operation.

[0075] At block 510, the method may include diluting the vapor with air to reduce temperature of the vapor during the abnormal operation. The air may be ambient air introduced into the enclosure or ductwork receiving the vapor and housing the tube bundle. At block 512, the method may include purging the multiple tubes or coils with an inert gas during the abnormal operation.

[0076] FIG. 6 is a block diagram of a method 600 of constructing or retrofitting a waste heat recovery unit (WHRU) to reduce thermal degradation of HM during abnormal or off- specification operation of the WHRU. In particular, the method 600 may involve construction of a new WHRU or retrofit of an existing WHRU to provide for deinventory or evacuation of HM from tubes or coils in the WHRU during off-specification or abnormal operation to reduce thermal degradation of the HM. For example, the new construction or retrofit may allow HM in the tubes or coils to be drained during the abnormal operation. At block 602, the method includes adding a conduit downstream of a tube bundle in the WHRU that transfers heat from an exhaust gas to the HM, wherein the conduit to divert flow of the HM in the WHRU during the abnormal operation of the WHRU. The abnormal operation may include cessation of normal flow of HM through tubes or coils of the tube bundle, such as with shutdown or failure of a HM circulation pump, closure of a valve on the HM circuit, and so forth. At block 602, the method includes installing a collection vessel coupled to the conduit and to receive HM drained from the tube bundle via the conduit during the abnormal operation. The collection vessel may be sized to hold at least the amount of HM in the tubes or coils of the tube bundle during normal operation. Moreover, the collection vessel and associated piping and valves, may be carbon steel, stainless steel, nickel alloys, exotic metals, fiberglass reinforced plastic (FRP), and so forth. At block 604, the method includes incorporating a valve on the conduit to facilitate isolating the collection vessel during normal operation of the WHRU and to facilitate placing the collection vessel in service during the abnormal operation. For example, the valve (see, e.g., valve 428 in FIG. 4) in a closed position may isolate, at least in part, the collection vessel during normal operation of the WHRU, and the valve in an open position may facilitate the collection vessel to be placed in service during the abnormal operation of the WHRU. At block 606, the method includes installing an inert-gas purge inlet to provide an inert gas purge (e.g., nitrogen) through tubes or coils of the tube bundle during the off-specification operation. At block 608, the method includes installing another conduit to receive vaporized HM from the tube bundle during off- specification operation. At block 610, the method includes installing an ambient-air inlet on ductwork receiving the exhaust gas and housing the tube bundle. Lastly, at block 612, the method may include integrating or programming a control system (e.g., DCS, PLC, etc.) to detect the abnormal or off-specification operation, and to automatically implement operation of the aforementioned features. To do so, the control system may have a processor and code or logic stored on memory and executable by the processor. The control system via instrumentation may detect, for example, a HM circulation flow rate falling below a predetermined value, an "off operation of the HM circulation pump, a HM temperature above a predetermined value, and so forth. In response, the control may be configured (including programmed) to automatically adjust opening positions of valves to place the collection in service to drain HM liquid from the tubes or coils.

[0077] In summary, Waste Heat Recovery Units (WHRU) and similar systems transfer energy from higher temperature vapor or gas streams to a HM fluid, typically a liquid. The HM is generally utilized as a heat source for other heat transfer equipment (users or load) in a target process. The source of the high-temperature vapor stream may be a gas turbine, fired heater, steam generator, and other sources. At the WHRU, the high-temperature vapor typically flows through ductwork or other enclosure housing coils or a bundle of tubes. The HM flows through the coils or tubes and absorbs heat from the high-temperature vapor stream passing over the coils or tubes.

[0078] In normal operation, the HM fluid may flow through the coils or tubes at a flow rate that provides a relatively-short residence time, which prevents excessive elevated temperature of the HM, significant thermal degradation of the HM, excessive deposition of HM impurities, and the like. Unfortunately, during an abnormal operation or shutdown condition, the HM flow may slow or stop. Thus, the HM fluid in the coils or tubes may experience elevated temperatures and thermally degrade. Indeed, at elevated temperature over time, glycols or other similar components of the HM may thermally degrade to corrosive acids and sludge. Further, certain HM fluids and components, e.g., water, at elevated temperatures over time may exceed typical deposit rates of impurities on the tube wall, fouling the tubes.

[0079] In conclusion, embodiments herein may provide for a heat recovery system with a tube bundle having multiple tubes or coils. The system includes an enclosure (e.g., ductwork) housing at least a portion of the tube bundle, wherein the enclosure to receive and pass a vapor over the tube bundle. The vapor may be hot exhaust gas from an upstream source. The system includes a pump to circulate a HM through the multiple tubes or coils to absorb heat from the vapor, and the pump to circulate the HM to users of the HM. A control system is employed to detect an abnormal operation of the heat recovery unit, the abnormal operation including a cessation or substantial reduction of a flow rate of normal flow of the HM through the multiple tubes. The abnormal operation may involve a shutdown of the pump. Further, the abnormal operation may include continued flow of the vapor across the tube bundle in the enclosure.

[0080] The heat recovery system includes a collection system to deinventory or evacuate the HM from the multiple tubes during the abnormal operation to reduce thermal degradation of the HM during the abnormal operation. The collection system may include a collection vessel to accumulate the HM removed from the multiple tubes during the abnormal operation. The collection vessel may be sized to hold at least the amount of HM in the multiple tubes during normal operation. The collection system may also receive and vent HM vaporized in the multiple tubes during the abnormal operation. Additionally, the collection system may have an inert gas inlet to provide an inert gas purge through the multiple tubes during the abnormal operation. Also, the heat recovery system may have an ambient air inlet on the enclosure to introduce ambient air into the enclosure to dilute and reduce temperature of the vapor during the abnormal operation.

[0081] Other embodiments provide for a WHRU having a heat exchanger (e.g., coils, tube bundle, tubes, etc.) and ductwork enclosing at least a portion of the heat exchanger, wherein the ductwork to receive and pass an exhaust gas over the heat exchanger. The WHRU includes a pump to circulate a HM through an internal flow path of the heat exchanger to absorb heat from the exhaust gas, and the pump to circulate the HM to users of the HM. Further, the WHRU includes a collection system to remove the HM from the coils during an abnormal operation to reduce thermal degradation of the HM in the heat exchanger. The collection system has a collection vessel to receive HM removed from the heat exchanger during the abnormal operation. The collection system typically includes a conduit to drain by gravity the HM from the heat exchanger to the collection vessel. The abnormal operation may include a cessation in flow or a reduction in a flow rate of the HM through the heat exchanger. The abnormal operation may include or be caused by a shutdown of the pump, an accidental closure of a valve by a user, and the like. Moreover, during the abnormal operation, the exhaust gas may continue to flow across the coils. Lastly, the WHRU may include a control system to detect the abnormal operation, and to automatically isolate the waste heat recovery unit from the users and to facilitate implementation of removal of the HM from the heat exchanger during the abnormal operation.

[0082] While the present techniques may be susceptible to various modifications and alternative forms, the exemplary embodiments discussed above have been shown only by way of example. However, it should again be understood that the techniques is not intended to be limited to the particular embodiments disclosed herein. Indeed, the present techniques include all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims.

Claims

CLAIMS What is claimed is:

1. A heat recovery system comprising:

a tube bundle having multiple tubes;

an enclosure housing at least a portion of the tube bundle, wherein the enclosure is configured to receive a vapor and pass the vapor over the tube bundle;

a pump configured to flow a heating medium through the tube bundle to one or more users;

a control system configured to detect an abnormal operation of the heat recovery system, wherein the abnormal operation comprises a reduction of flow of the heating medium below a predetermined range of flow values; and a collection system configured to reduce thermal degradation of the heating medium during the abnormal operation by evacuating the heating medium from the tube bundle during the abnormal operation.

2. The heat recovery system of claim 1, wherein the enclosure comprises ductwork.

3. The heat recovery system of claim 1 or claim 2, wherein the vapor comprises exhaust gas from a source outside of the heat recovery system.

4. The heat recovery system of claim 1 or any of claims 2 to 3, wherein the abnormal operation comprises a shutdown of the pump.

5. The heat recovery system of claim 1 or any of claims 2 to 4, wherein the abnormal operation comprises continued flow of the vapor across the tube bundle in the enclosure.

6. The heat recovery system of claim 1 or any of claims 2 to 5, wherein the collection system comprises a collection vessel configured to accumulate the heating medium removed from the multiple tubes during the abnormal operation.

7. The heat recovery system of claim 1 or any of claims 2 to 6, wherein the collection system is configured to evacuate the heating medium from the tube bundle without back- flushing the multiple tubes.

8. The heat recovery system of claim 1 or any of claims 2 to 7, wherein the collection system is configured to receive and vent heating medium vaporized in the tube bundle during the abnormal operation.

9. The heat recovery system of claim 1 or any of claims 2 to 8, wherein the collection system comprises a purge gas inlet configured to pass purge gas through the tube bundle during the abnormal operation, wherein the purge gas is inert.

10. The heat recovery system of claim 1 or any of claims 2 to 9, comprising an ambient air inlet on the enclosure configured to introduce ambient air into the enclosure to dilute and reduce temperature of the vapor.

11. The heat recovery system of claim 1 or any of claims 2 to 10, wherein the tube bundle comprises multiple tubes comprising coils.

12. A waste heat recovery unit comprising:

a heat exchanger;

a ductwork enclosing at least a portion of the coils, wherein the ductwork is configured to receive and pass an exhaust gas over the heat exchanger;

a pump configured to circulate a heating medium through an internal flow path of the heat exchanger to absorb heat from the exhaust gas, and wherein the pump is configured to circulate the heating medium from the waste heat recovery unit to users of the heating medium;

a collection system configured to remove the heating medium from the heat exchanger during an abnormal operation to reduce thermal degradation of the heating medium in the heat exchanger, wherein the collection system comprises a collection vessel configured to receive heating medium removed from the heat exchanger during the abnormal operation, and wherein the abnormal operation comprises a cessation of flow or substantial reduction in a flow rate of the heating medium through the heat exchanger.

13. The waste heat recovery unit of claim 12, wherein the abnormal operation comprises a shutdown of the pump.

14. The waste heat recovery unit of claim 12 or claim 13, wherein the abnormal operation comprises continued flow of the exhaust gas across the heat exchanger.

15. The waste heat recovery unit of claim 12 or any of claims 13 to 14, wherein the collection system comprises a conduit configured to drain the heating medium from the heat exchanger to the collection vessel.

16. The waste heat recovery unit of claim 12 or any of claims 13 to 15, comprising a control system configured to detect the abnormal operation, and configured to automatically implement removal of the heating medium from the coils during the abnormal operation.

17. A method of operating a waste heat recovery unit, the method comprising:

circulating a heating medium through multiple tubes of a tube bundle in the waste heat recovery unit and to users of the heating medium outside of the waste heat recovery unit;

passing a vapor over the tube bundle to transfer heat from the vapor to the heating medium;

detecting via a control system an abnormal operation of the waste heat recovery unit, the abnormal operation comprising loss of circulation of the heating medium through the multiple tubes; and

removing the heating medium from the multiple tubes to a collection vessel in the waste heat recovery unit during the abnormal operation to reduce or avoid thermal degradation of the heating medium.

18. The method of claim 17, wherein the vapor comprises exhaust gas.

19. The method of claim 17 or claim 18, comprising diluting the vapor with air to reduce a temperature of the vapor during the abnormal operation.

20. The method of claim 17 or any of claims 18 to 19, wherein removing comprises draining the heating medium from the multiple tubes to the collection vessel when the heating medium is a liquid and venting the heating medium from the multiple tubes to the collection vessel when the heating medium is a vapor.

21. The method of claim 17 or any of claims 18 to 20, comprising purging the multiple tubes with an inert gas during the abnormal operation.

22. A method of constructing or retrofitting a waste heat recovery unit (WHRU) to prevent or reduce thermal degradation of heating medium during abnormal operation of the WHRU, the method comprising:

adding a conduit downstream of a tube bundle configured to transfer heat from an exhaust gas to the heating medium, wherein the conduit is configured to divert flow of the heating medium in the WHRU during the off-specification operation of the WHRU, the abnormal operation comprising cessation of normal flow of heating medium through the tube bundle;

installing a collection vessel coupled to the conduit and configured to receive heating medium drained from the tube bundle via the conduit during the abnormal operation; and installing a valve on the conduit, wherein the valve is configured to isolate the collection vessel during normal operation of the WHRU and to place the collection vessel in service during the abnormal operation.

23. The method of claim 22, comprising installing an inert-gas purge inlet configured to provide an inert gas purge through tubes of the tube bundle during the abnormal operation.

24. The method of claim 22 or claim 23, comprising installing another conduit configured to receive vaporized heating medium from the tube bundle during the abnormal operation.

25. The method of claim 22 or any of claims 23 to 24, comprising installing an ambient- air inlet on ductwork receiving the exhaust gas and housing the tube bundle.