Economizers and feedwater heaters
Yuri Rechtman, Nooter/Eriksen Inc., Fenton, MO, United States
Abstract
Two distinctly different approaches to the physical design of an economizer exist in today’s heat recovery steam generator (HRSG) business. One is driven by design considerations, another by manufacturing reasons. A custom design allows theoretical flexibility to satisfy thermal and hydraulic process requirements. A standard design requires all panels to be the same for ease of manufacturing and utilizes crossover jumpers to connect panels and build the flow circuitry. This arrangement often requires more heating surface due to the mix of cross- and counterflow arrangements. Both custom design and standard design economizers have operated successfully in HRSGs since the 1970s.
Keywords
Standard design; custom design; flow accelerated corrosion; steaming; feedwater heaters; heat recovery steam generator
Two distinctly different approaches to the physical design of an economizer exist in today’s heat recovery steam generator (HRSG) business. One is driven by design considerations, another by manufacturing reasons. A custom design allows theoretical flexibility to satisfy thermal and hydraulic process requirements. A standard design requires all panels to be the same for ease of manufacturing and utilizes crossover jumpers to connect panels and build the flow circuitry. This arrangement often requires more heating surface due to the mix of cross- and counterflow arrangements. Both custom design and standard design economizers have operated successfully in HRSGs for over 40 years.
5.1 Custom design
A custom designed economizer is shown in Fig. 5.1. The optimal water velocity is achieved by varying flow circuitries and tube diameters. This design results in the most effective utilization of heating surface, superior flexibility, and high reliability.
High heating surface efficiency is achieved by using true counterflow arrangement, i.e., the hottest exhaust gas faces the hottest economizer outlet feedwater and the coldest exhaust gas exits where the feedwater is the coldest.
The two most commonly used arrangements in economizers are full and half circuit.
5.1.1 Full circuit
Every tube in the inlet tube row is connected to both the inlet and the lower header as shown in Fig. 5.1. The second row of tubes exits from the same lower header and carries the entire water flow up. Return bends redirect the feedwater flow up at the top to the next row of tubes and down to the lower header.
Designs with return bends at the top, as shown in Figs. 5.1 and 5.2, have superior mechanical flexibility when compared to standard designs where the tubes are restrained between two headers as described in the next section.
5.1.2 Half circuit
Every other tube in the inlet row is connected to the inlet header, as shown in Fig. 5.2. These tubes carry the entire economizer flow into the lower header. All tubes in the inlet row are connected to the lower header. Half of the lower header tubes, not connected to the inlet header, carry the entire economizer flow up into return bends and into the second economizer row. Feedwater makes two passes, once up and once down within each row.
5.2 Standard design
5.2.1 Full circuit
A typical standard design full circuit economizer is shown in Fig. 5.3.
It has headers at the top and at the bottom of every tube row. Feedwater enters into one or two full tube rows at the top with jumpers connecting these rows to following rows at the bottom as shown in Fig. 5.3.
Feedwater velocities could be lower than in the custom design, since the total flow is distributed to one or two full rows, unless the headers use divider plates so that the flow can make multiple passes within a row. When water velocities are low, a vent system is necessary to remove air that is released from the water that enters the coil.
5.2.2 Half circuit
The feedwater flow entering the economizer, shown in Fig. 5.4, is distributed to one half of tubes in the inlet row connected at one end of the header. The feedwater flow crosses over through the lower header to the other half of tubes in the same row. A divider plate separates the two passes of water flow in the upper header. If more than two passes of water flow occur in a row of tubes, divider plates are required in both the upper and the lower headers. The flow out of the tubes at the bottom of the header converges and flows through the header from one pass to another. The flow then diverges and enters the tubes in the next pass and flows upward to the top header. The flow is similar through all rows to the outlet header. Vents are connected to the ends of the headers.
The principles described above also apply when two rows of tubes are attached to each upper and lower header.
5.3 Flow distribution
Uniform distribution of the feedwater flow in the tubes is necessary to achieve the desired thermal performance, provide strong cooling, and maintain uniform tube temperatures in a row.
Good flow distribution is dependent on the pressure drop within a coil. The higher the pressure drop, the better the distribution. Every HRSG manufacturer develops velocity guidelines for tube side fluid velocity and designs economizer circuitries to achieve that velocity in their designs.
Feedwater flow distribution in a standard design is not as uniform as in a custom design due to configuration of the circuitry. Poor flow distribution affects tube wall temperatures resulting in increased tube stresses, reduced performance, and a potential for steaming.
Excessive velocities within economizers can result in flow accelerated corrosion (FAC) issues. Custom designed economizers, shown in Figs. 5.1 and 5.2, have not had FAC problems.
Inadequate velocities within economizers can result in severe maldistribution, which causes uneven heating of tubes leading to reduced performance and mechanical failures.
The converging and diverging water flow encountered as the water leaves a row of tubes, flows in a header, and enters another row of tubes can make good flow distribution difficult to achieve.
A typical completed custom designed economizer module is shown in its shipping position in Fig. 5.5.
5.4 Mechanical details
5.4.1 Tube orientation
Economizer tubes are arranged horizontally in a vertical HRSG (exhaust flows vertically) and usually vertically in a horizontal HRSG (exhaust flows horizontally). Horizontal HRSGs may also have a horizontal tube arrangement. That could occur when height restrictions are present at the job site, so the width of the HRSG is greater than its height. For example, a 20 ft W×10 ft H economizer would have sixty 10-ft-long vertical tubes per row if a 4-in. tube spacing is used (20 ft×12 ÷ 4 in.) while there would only be thirty 20-ft-long tubes if the tubes were horizontal. A horizontal economizer arrangement in this example would result in a more economical design. Horizontal tube economizers are easier to vent through the vertical headers.
A vertical tube economizer has a limited capability for circuitry variation due to the industry standard requirement that each HRSG coil have the ability to be completely drained. A horizontal economizer has almost an unlimited choice of the circuitry.
Horizontal economizer tubes in a vertical HRSG, which usually has a long side and a short side, may run in either direction depending on water velocity needs. For example, in a district heating application, where the water flow is very high, a large number of short tubes will have a lower pressure drop than a small number of long tubes.
5.4.2 Venting
Upper return bends in custom design economizers can get vapor locked, resulting in reduced or even no flow in several circuits. Economizer performance may significantly degrade due to vapor locked circuits with no water flow. A minimum tube side flow must therefore be established for each custom configuration to assure that water velocity is high enough to clear tubes of any trapped vapor or air.
Standard design economizers have upper headers, but venting from jumper pipes requires vapor or air to rise to the top of the jumper through buoyancy forces while water is pumped in to fill the coil. Ends of headers are away from the header nozzle or jumper connections and could result in trapped vapor or air at these points.
5.4.3 Steaming
Steaming is a phenomenon that can occur at the hot end of any economizer, especially at startup or during load swings. Steaming can reduce performance by deactivating the heating surface if the steam is not released from the tubes.
Using several up-flow rows of tubes for steam venting is a unique feature of custom designed economizers. Any steam generated in the hottest rows would flow up into the steam drum.
Standard designs use a vent connecting the last one or two economizer headers to the steam drum. The vent may have an automatic valve that can be remotely opened when steaming conditions exist. This does not help any down-flow tubes where steam buoyancy forces are countered by flow forces. Once the valves are closed, there is no provision for venting.
Many users are not comfortable with steaming in economizers. Two simple techniques can be utilized to prevent steaming in economizers:
• The feedwater control valve is usually located at the outlet of the feedwater pump before the condensate enters the economizer in a typical HRSG arrangement. This control valve could be located at the outlet instead of the inlet of the economizer. Such an arrangement could operate at a higher pressure with a saturation temperature that is above the exhaust gas temperature at the economizer outlet location. Increasing the economizer saturation temperature above the exhaust gas temperature at the economizer outlet eliminates the possibility of steaming. Steaming will then occur in the economizer outlet piping at the feedwater control valve outlet where the pressure is reduced. Feedwater control valves with cavitational trim are typically provided in order to extend the control valve life. A safety valve may be required at the economizer outlet piping since the economizer can be manually isolated by the inlet and the outlet valves. Locating the feedwater control valve at the economizer outlet costs more than a conventional setup, due to thicker tubes and headers required for operation at a higher pressure.
• A partial water side bypass can eliminate most of the economizer steaming. A certain percentage of the incoming feedwater, as shown in Fig. 5.6, bypasses the cold end of the economizer. The outlet feedwater temperature is controlled by the difference between the saturation temperature in the steam drum that is being fed by the economizer and the economizer feedwater outlet temperature. The temperature differential is typically set to less than 5°F, so the economizer does not steam throughout most of the operating modes.
5.4.4 Corrosion fatigue
The Electric Power Research Institute’s Heat Recovery Steam Generator Tube Failure Manual [1] states that corrosion fatigue is one of the leading causes of HRSG tube failures. All inlet headers experience some stress because of abrupt temperature changes when flow is established at startup. Stress and less-than-optimal water chemistry will lead to corrosion fatigue failures at header connections.
As can be seen in Fig. 5.4, differential growth between the inlet row and the following row will create stress at the lower jumper pipes because of the rigidity of the large bore pipes connecting the rows.
The arrangement shown in Fig. 5.4 has additional stress associated with the tubes in the down-flow pass within a row being a different temperature than the adjacent up-flow pass especially at startup. The stress is greatest in the two center tubes where one has downward flow and the other has hotter upward flow. This stress is further magnified by the moment created by the tube bends. This additional stress can be a main contributor to corrosion fatigue in this type design.
5.5 Feedwater heaters
5.5.1 Concerns
Feedwater heaters are low-pressure and low-temperature economizers. Due to the low water temperature and the location of the feedwater heater at the cold end of the HRSG they can be prone to internal and external corrosion concerns. There are a number of solutions to reduce or eliminate corrosion issues.
• Exhaust from combustion turbines operating on natural gas often contains traces of sulfur and thus will have a dew point temperature of approximately 140°F. Tubes whose surface temperatures are below the dew point will experience water condensation, sulfuric acid formation, and resultant corrosion of tubes. To prevent this, the condensate entering the feedwater heater should be preheated to a temperature that is equal to or higher than the dew point temperature. Condensate entering the feedwater heater at an elevated temperature keeps tube wall temperatures above the dew point effectively eliminating dew point conditions on the tube surface. The industry accepted minimum condensate inlet temperature is 140°F.
Various methods of condensate preheating to prevent sulfuric acid corrosion in feedwater heater tubes are utilized in the HRSG industry.
• Oxygenated condensate supplied to feedwater heaters exposes tubes to internal corrosion. A common solution to internal tube corrosion is the use of stainless or duplex stainless steel tubes.
Several arrangements described below are used in HRSGs to resolve the external tube corrosion concern in feedwater heaters.
5.5.2 Feedwater heater arrangements
An HRSG with a feedwater heater must satisfy the specified performance requirement. Feedwater heaters in different arrangements reviewed here are all designed to achieve the same performance goal.
• Basic Feedwater Heater
The feedwater heater in Fig. 5.7 is designed to preheat condensate from 95°F to 320°F with exhaust gas entering the coil at 365°F and leaving at 185°F. This is a simple arrangement where no consideration is made for sulfur corrosion concerns on the feedwater heater tube surfaces. The incoming condensate enters the inlet row of tubes without any preheating. The metal temperature of the inlet tubes in the feedwater heater shown in Fig. 5.7 with 95°F condensate inlet temperature will be between 105°F and 115°F, which is well below 140°F. These tubes will corrode in a relatively short time.
• Water Recirculation
One common practice today is to utilize a feedwater heater arrangement with recirculation as shown in Fig. 5.8. Condensate is delivered to the feedwater heater at the same temperature as in the arrangement in the previous example shown on Fig. 5.7. It is mixed with a portion of the feedwater heater outlet water that is recirculated back to the inlet until the mix reaches an acceptable (140°F) feedwater heater tube inlet temperature. Condensate temperature is monitored at the feedwater heater inlet. A temperature controller adjusts the control valve position at the recirculation pump outlet. The recirculation pump is sized to provide sufficient flow at maximum HRSG production conditions. A small percentage of condensate is bypassed from the inlet of the feedwater heater to its outlet if the maximum recirculation flow the pump can generate is lower than that required to preheat the condensate to 140°F feedwater heater inlet temperature.
The heat balances for the feedwater heater arrangements shown in Figs. 5.7 and 5.8 are identical. Condensate is delivered at 95°F and leaves the feedwater heater at 320°F. An HRSG equipped with either feedwater heater will produce the same amount of steam. The advantage of Fig. 5.8’s arrangement is that sulfur dew point conditions are not present on the surface of even the coldest tubes of the feedwater heater. The arrangement with recirculation in Fig. 5.8 requires more heating surface than the basic unit in Fig. 5.7.
• External Heat Exchanger
The patented feedwater heater arrangement shown in Fig. 5.9 utilizes an external heat exchanger instead of a recirculation pump.
Condensate enters the cold path of the external heat exchanger (located outside the HRSG casing) at 95°F and leaves it at 140°F. The preheated condensate enters the coldest tubes of the feedwater heater at a temperature that is above the sulfur dew point. The cold end of the feedwater heater is split in two sections parallel to each other and both perpendicular to the exhaust flow. Feedwater is preheated in Coil 1 from 140°F to 185°F and fed into the hot path of the external heat exchanger for preheating the incoming condensate. Water from the exchanger’s hot path outlet temperature is fed into Coil 2 of the feedwater heater at 140°F. Coil 2 outlet flow enters Coil 3 of the feedwater heater for the final preheating to 320°F, or the temperature required by the process. Conditions for sulfuric acid formation are eliminated from the exhaust stream, where corrosion may occur, and moved into the external heat exchanger, where no sulfur is present.
Benefits of the Fig. 5.9 arrangement as compared to feedwater heater arrangement in the Fig. 5.8 are:
• reduced initial cost: less heating surface
• reduced operating cost: no pump motor power loss
• reduced maintenance cost: no rotating equipment
The feedwater heater energy balance shown in Fig. 5.9 is identical to the energy balances in Figs. 5.7 and 5.8.
• An alternate feedwater heater arrangement with an external heat exchanger that does not violate the patent utilized in Fig. 5.9 is shown in Fig. 5.10. This arrangement accomplishes the same task as the arrangement in Fig. 5.9 except the unit is larger, as the heating surface efficiency is not as good due to lower-temperature water entering in the middle of the coil, causing a drop in the exhaust gas temperature for the remainder of the coil.
A patented feedwater arrangement shown in Fig. 5.11 can be utilized when oxygen is present in the incoming condensate and sulfur is present in the fuel. The oxygen-rich condensate enters the cold side of the external heat exchanger at 95°F and is preheated to 185°F before entering the deaerator. Deaerated water flows to the hot side of the external heat exchanger, where it is cooled down to 140°F and pumped into the cold feedwater heater coil. The outlet flow of that coil is fed to the hot feedwater heater coil at 230°F for the final preheating to 320°F required by the process. The feedwater heater evaporator is placed in the split between two sections of the feedwater heater to generate the required amount of steam for deaeration.
All feedwater heater arrangements shown above satisfy the same process requirement of preheating the incoming condensate from 95°F to 320°F. The arrangements in Figs. 5.8, 5.9, and 5.10 or the arrangement in Fig. 5.11 should be used in HRSGs with condensate preheating to eliminate cold end corrosion. The arrangements in Figs. 5.9 and 5.10 provide reliable operation by replacing the recirculation pump with a heat exchanger. Each arrangement could feed a low-pressure evaporator operating at 120 psig with the corresponding saturation temperature of 350°F. The temperature difference of 30°F between the low-pressure evaporator saturation and the feedwater heater outlet temperature is required for the deaerating process to occur when a nondeaerated condensate is introduced to the HRSG in a conventional arrangement. The arrangement shown in Fig. 5.11 is designed to deaerate the incoming condensate within the feedwater heater, so that a higher-pressure LP system (the next pressure level forward in the HRSG) would require no temperature difference between the low-pressure drum saturation and the feedwater heater outlet temperature. That allows more low-pressure steam to be generated in the low-pressure system, since a 0°F temperature difference between the economizer outlet temperature and the drum saturation temperature can be utilized to increase the HRSG efficiency in a cost-effective manner. The incoming condensate is deaerated in the integral deaerator, so carbon steel tubes can be used instead of stainless steel tubes in the feedwater heater, hence the lower cost. Thus the arrangement in Fig. 5.11 is referred to as the high-efficiency arrangement.
The feedwater heater evaporator drum water can be chemically treated with solid alkalis, such as phosphates or caustics, reducing the possibility of FAC.
5.5.3 Dew point monitoring
The patented dew point monitor shown in Fig. 5.12 may further improve the HRSG performance. A conductivity meter is installed outside of the feedwater heater casing. One wire from the meter is attached to the feedwater heater inlet piping. The other wire is attached to a clamp that is attached to a tube in the coldest row of the feedwater heater. There is an electric insulator between the tube and the clamp. Moisture formed on the insulator bridges the gap between the tube and the clamp when the dew point conditions occur. Plant operating personnel can use dew point monitoring to minimize condensate temperature at the inlet of the feedwater heater by experiment. The unit could operate with condensate temperature controlled to 130°F or lower instead of 140°F as designed, if no moisture is detected on tubes at the reduced temperature. The controlled temperature may be adjusted seasonally depending on the ambient temperature.
