Comparative analysis of shell-and-tube and plate-type water-to-water heat exchangers

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Comparative Analysis of Shell-and-Tube and Plate-Type Water-to-Water Heat Exchangers Author: Ding Yanxiang. Looking at the power plants already built in China, there are two types of water-to-water heat exchangers used in closed-loop cooling water systems: one is the shell-and-tube heat exchanger, and the other is the plate heat exchanger. Shell-and-tube heat exchangers are a commonly used type of heat exchanger, and they have been widely applied in power plant design. In China, plate heat exchangers are more frequently used in power plants equipped with imported units, gas-steam combined cycle power plants, and nuclear power plants. There is growing interest in plate heat exchangers due to their compact size, light weight, and high heat transfer efficiency. This article compares shell-and-tube and plate heat exchangers, and provides guidance for selection. 1 Introduction to the Structures of Shell-and-Tube and Plate Heat Exchangers (1) Shell-and-Tube Heat Exchanger A shell-and-tube heat exchanger consists of a front water chamber, a tube bundle, a shell, and a rear water chamber. The tube bundle is of a withdrawable type, and it consists of front and rear tube sheets, baffle plates, tie rods, spacers, and heat exchange tubes. The tie rods are connected to the tube sheet and flow divider plates via threads, while the heat exchange tubes are joined to the tube sheet by expansion jointing followed by seal welding. Anti-scour plates are installed on the tube bundle at the shell-side water inlet to prevent the heat exchange tubes from being directly scoured by the cooling water. To reduce the friction when the tube bundle is inserted into or removed from the cylinder, slide rails are provided on the tube bundle. To inspect the garbage, sediment, and blockages in the pipes within the cleaning chamber, inspection holes are provided on the end covers of the front and rear water chambers. To monitor the operation of the water-to-water heat exchanger, temperature and pressure measurement points are installed at the inlets and outlets on the cooled water side (desalinated water side) as well as on the cooling water side (seawater side). In addition, exhaust and drain connections are also provided. (2) Plate heat exchanger: A plate heat exchanger is composed of a set of parallel metal plates with corrugations; there are channel holes at the four corners of each plate. The plates are clamped within a frame made up of a fixed plate with connection pipes on one side and a movable clamping plate, and they are secured using clamping bolts. These connection pipes are aligned with the channel holes on the plate, and are connected to the external pipelines of the two liquids involved in heat exchange. The heat transfer plate and the movable clamping plate are suspended beneath the top support beam, and are aligned and positioned by the bottom crossbeam. The heat transfer plate itself has a specific shape and is sealed with gaskets to prevent external leakage, allowing the two liquids involved in heat exchange to flow alternately in counterflow through the channels between another pair of heat transfer plates. The ripples on the plate not only increase the turbulence of the fluid but also create numerous contact points to withstand normal operating pressures. The flow rate, physical properties, pressure drop, and temperature difference of the fluid determine the number and size of the plates. 2 Design conditions for heat exchangers: The design of heat exchangers must meet the operational requirements of the power plant under various loads from startup to maximum output, with a certain margin of safety. This ensures that the heat exchangers can still carry out the specified cooling tasks within the prescribed maintenance intervals, even under conditions of maximum load, highest inlet water temperature, and maximum fouling heat resistance. Taking domestically introduced 300 MW coal-fired units as an example, the cooling equipment requires that the inlet temperature of the cooling water not exceed 37.5°C; the maximum temperature of the cooled water exiting the cooling equipment is around 42.8°C. The basic parameters are as follows:
**Cooled water**: Deionized water
Design pressure: 1.0 MPa
Flow rate: 1800 m3/h
Inlet and outlet temperatures: 42.8/37.5°C
Pressure drop: ~0.06 MPa

**Cooling water**: Seawater (alternating between seawater and river water)
Design pressure: 0.5 MPa
Inlet temperature: 33°C
Outlet temperature:
Flow rate: 0.05–0.06 MPa

**Comparison of shell-and-tube and plate heat exchangers**
3.1 Comparison of design parameters
Based on the design requirements for heat exchangers, the following three options were considered:
Option 1: 2 shell-and-tube heat exchangers at 100% capacity each
Option 2: 2 plate heat exchangers at 100% capacity each
Option 3: 3 plate heat exchangers at 50% capacity each
The parameters for each option are shown in Table 1. Table 1 3.2 Comparison of Equipment Selection for Open-Circuit Circulating Cooling Water (Cooling Water Side of Water-Water Heat Exchangers) Depending on the different structural designs of shell-and-tube and plate heat exchangers, as well as the volume of cooling water required, different electric strainers and open-circuit circulating cooling water pumps must be selected; details are provided in Table 2. 3.3 Comparison of fluid heat transfer designs The tubes in a shell-and-tube heat exchanger are the fundamental components of such exchangers; they provide a heat transfer surface between one fluid flowing inside the tubes and another fluid flowing outside them. The tube material is determined based on the properties of the fluids flowing on both sides. Corrosive seawater with poor quality is allowed to flow inside the tubes, while demineralized water of better quality flows on the outside of the tubes. In this way, titanium tubes resistant to seawater corrosion are sufficient, and cleaning the tubes of dirt becomes easier. From the perspective of heat transfer fluid dynamics, using smaller-diameter tubes within a given shell allows for a higher surface density; however, most fluids tend to form a layer of dirt on the tube surfaces. Especially when the cooling water inside the tubes has poor quality, the presence of sediment, dirt, and marine organisms can lead to deposits on the tube walls, which deteriorates heat transfer and makes regular cleaning necessary. The minimum diameter for tubes that can be cleaned is approximately 20 mm, while titanium tubes are usually used with a diameter of Φ25 mm. For a given fluid, dirt formation is primarily influenced by the tube wall temperature and flow velocity. To achieve a reasonable maintenance cycle, the flow velocity of the water inside the tubes should be around 2 m/s (depending on the required pressure drop). Since seawater, river water, etc. are commonly used as cooling water, they tend to cause scaling. For shell-and-tube heat exchangers, a rubber ball cleaning device should be installed for regular cleaning depending on the sand content in the water quality. In a plate heat exchanger, the cooling water and the fluid to be cooled convect on opposite sides of the corrugated plates. The corrugations are of a herringbone pattern, and the corrugations on these heat transfer plates intersect at an angle; in other words, adjacent heat transfer plates have corrugations with the same inclination angle but in opposite directions. The cross-sectional area along the flow direction remains constant, but turbulence is induced by changes in the channel shape due to the continuous variation of the flow direction. The corrugation depth of a typical heat transfer plate is 3–5 mm, and the flow velocity in the turbulent zone is approximately 0.1–1.0 m/s. The corrugated plates are very thin, with a thickness of 0.6–1 mm. There must be many contact points between adjacent plates in order to withstand normal operating pressures. The adjacent plates have herringbone grooves in opposite directions; the intersection points of these two types of grooves form the contact points. This arrangement also helps to eliminate vibrations, and while promoting turbulence and heat exchange, it prevents internal leaks caused by fatigue cracks. Herringbone corrugated plates have a high level of turbulence, and this high turbulence helps to enhance the cleaning effect, allowing for an effective reduction of accumulated dirt. However, these plates have many contact points; when the water quality is poor, containing suspended solid particles, debris, and aquatic plants, the narrow gaps between the plates mean it is essential to ensure that all particles larger than 2 mm are filtered out before they can enter the heat exchanger. If the filters do not function properly, blockages can easily occur. 3.4 Comparison of heat transfer coefficients: In shell-and-tube heat exchangers, one fluid flows transversely across the tubes and exchanges heat with another fluid flowing inside the tubes through the tube walls; as these fluids flow perpendicularly to each other, the heat transfer coefficient is generally in the range of 1000–3000 W/(m2·K). In plate heat exchangers, the cooling water side and the fluid to be cooled flow in uniform turbulence; the two fluids flow in opposite directions. Turbulence is induced by the wavy structure, resulting in a high heat transfer rate, a high pressure drop, and a high shear stress field. This helps to prevent the formation of fouling on the heat transfer surfaces. Its heat transfer coefficient is generally 3500–5500 W/(m2·K), which allows for a reduction in the heat exchange area required for the heat exchanger. 3.5 Terminal Difference Comparison: The heat transfer terminal difference in shell-and-tube heat exchangers (i.e., the difference between the inlet temperature of the cooling water and the outlet temperature of the water being cooled) is around 5°C. Due to its structural features, plate heat exchangers can achieve an end temperature difference as low as 1°C economically. 3.6 Comparison of cooling water volume: In shell-and-tube heat exchangers, the ratio of the cooling water volume to the volume of the fluid being cooled is generally 1.2–2.5∶1. Plate heat exchangers, due to their basically identical flow channels for the two media and high heat transfer efficiency, can **reduce the amount of cooling water required. Generally, the ratio between the amount of cooling water and the amount of water to be cooled is 0.8–1.1:1, which helps to lower the installation and operating costs associated with pipes, valves, and pumps. 3.7 Comparison of installation and maintenance: Plate heat exchangers are characterized by their small size and light weight, which facilitates maintenance; no special lifting equipment is required, thus they require less space for installation. Manual maintenance of plate heat exchangers involves disassembling the entire unit, cleaning the plates and gaskets using a water gun and brush, inspecting them, and replacing the plates and gaskets if necessary. Plate heat exchangers should generally be cleaned once a year, and this should be done regardless of whether it is actually necessary. When cooling water with poor quality, such as river water or seawater, is used, the presence of sediment and dirt, along with the rapid growth of microorganisms, poses a risk of surface contamination and blockage. Abroad, when river water is used as cooling water, the cleaning frequency is very high, at an average of 3.3 times per year. A shell-and-tube heat exchanger is composed of a tube bundle; it has a large weight and volume. When performing maintenance by removing tubes, a distance equal to the length of the tube bundle must be left, which results in high space requirements. Additionally, necessary lifting and maintenance equipment is required. The design life of a shell and tube heat exchanger is generally 30 years, with a major maintenance cycle of 4 years. When a leak occurs in the heat exchanger – either due to a leak between the tubes and the tube sheet or as a result of tube rupture – it is possible to restore its operational capacity in a short time by plugging the faulty tubes. Shell and tube heat exchangers allow for a 7% allowance for such plugged tubes. For the cleaning of the pipes, a rubber ball cleaning device can be used for regular mechanical cleaning as needed. 4 Operation of heat exchangers in domestic power plants (1) The 2 x 362 MW units at Huaneng Yueyang Power Plant were manufactured in the UK, and the plate heat exchangers were supplied as part of the main equipment. The power plant is located along the Yangtze River, and the circulating water comes from this river. The water quality of the Yangtze River in this area is characterized by low levels of coarse sand, high levels of fine sand, and an abundance of aquatic plants. To deal with these issues, three filters are installed before the circulating water reaches the steam turbine room. However, the plant reports that the plate heat exchangers tend to get clogged. The reason for this, according to analysis, is the poor sealing of the rotary filters, which allows aquatic plants to get inside; the fundamental problem lies in the inadequate effectiveness of the three filtering systems. (2) The main unit of Phase 6 of the Shanghai Wujing Power Plant is a 300 MW imported unit manufactured in Shanghai. In the closed cooling water system of this unit, the cooling water for the shell-and-tube water-to-water heat exchangers is supplied by a circulating water system, whose water comes from the Huangpu River – water that contains a large amount of debris and impurities. Therefore, two open rotary filters are installed before the inlet of the water-to-water heat exchangers. The filters originally intended for Unit 11 were imported foreign equipment with a pore size of 3–4 mm; due to the poor quality of the water in the Huangpu River, these filters often became clogged, and it was not possible to clean them automatically during operation. After several attempts at troubleshooting that proved ineffective, manual disassembly and cleaning were required during operation. This process was labor-intensive and affected the safe operation of the unit; as a result, manual cleaning had to be carried out almost every other day. The main reason analyzed is that the pore size of the filter screen is too small, and its design, among other aspects, is not suitable for the water quality conditions in our country. To address the aforementioned issues, a new automatic backwash electric filter was adopted; with a filter pore size of ψ6 mm, it operated well without any blockages occurring. Most of the closed-loop cooling water systems for the 300 MW coal-fired units put into operation in our country at an early stage adopted shell-and-tube water-to-water heat exchangers, and they have performed well. In recent years, due to continuous technological advancements and the need for improved design, the disadvantages of shell-and-tube water-to-water exchangers – namely their large footprint and the need for ample space for maintenance – have become more apparent in the optimization of main plant layouts. In some units where circulating water is used as secondary cooling water, considering the relatively good quality of the cooling water in such exchangers, with fewer impurities and less contamination, as well as the ongoing improvements in filter structures, plate heat exchangers are also employed in closed cooling water systems. 5 Technical and economic analysis: Taking domestically introduced 300 MW units as an example, based on the design requirements of the water-to-water heat exchangers and the specifications for closed-cycle cooling water systems, shell-and-tube and plate-type manufacturers provided preliminary quotes; estimates were only given for other major auxiliary equipment. A comparison is shown in Table 3. The plate heat exchangers use imported equipment; their price has been converted into RMB using the exchange rate at the time of quotation, with only value-added tax taken into account. The maintenance and repair costs are not included in the table above, as they are difficult to estimate and can only be analyzed qualitatively. For shell-and-tube heat exchangers, these costs mainly refer to the removal of contaminants from the water chamber, as well as the expenses associated with sealing leaks. The maintenance of plate heat exchangers includes the cleaning of the plates and the replacement of gaskets; since they need to be cleaned more frequently than shell-and-tube heat exchangers, and the gaskets must be replaced after 2–3 years of use, the maintenance costs for plate heat exchangers are higher. From the above comparison, it can be seen that the investment costs for Option 1 and Option 2 are roughly the same. 6 Conclusions Through the comparison of shell-and-tube and plate heat exchangers, the following conclusions can be drawn: Plate heat exchangers feature high heat transfer efficiency, small size, low weight, and are easy to assemble and disassemble; when the quality of the cooling water is good, they represent an ideal type of heat exchange equipment. However, when there is a large amount of sediment, dirt, water plants, and the like in the cooling water, the filter screen cannot function effectively and tends to get clogged, requiring frequent cleaning and thus affecting the safe operation of the unit.