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Depending on different application requirements, there are many types of valve body structures, with the following being the most commonly used: (1) Straight-through single-seat valve: This type of valve has one valve core and one valve seat, as shown in Figure 10-6(a). Its advantages include a simple structure, low leakage rate, and ease of closing, allowing for complete isolation of the fluid flow. However, since there is only one valve core, the unbalanced force generated during fluid flow is significant, especially under high pressure differences or with large diameters. Therefore, it is suitable only for applications with low pressure differences. (2) Straight-through double-seat valve: This valve has two valve cores and two valve seats, as shown in Figure 10-6(b). Fluid enters from the left side, passes through the upper and lower valve cores, and then exits from the right side. Since the forces acting on the upper and lower valve cores are in opposite directions and of roughly equal magnitude, they can cancel each other out, resulting in low unbalanced forces. Nevertheless, due to manufacturing limitations, it is difficult to ensure that both valve cores and seats close tightly simultaneously, leading to higher leakage rates. Its flow capacity is moderate. It is suitable for clean fluids with low viscosity, free from fibers and suspended particles, as well as for applications with high pressure differences and high static pressures where some leakage is acceptable. (3) Angle valve: As shown in Figure 10-6(c), an angle valve has a rectangular valve body, with a structure similar to that of a straight-through single-seat valve. Its valve core has a single-directional guiding structure, allowing it to be installed only in one orientation. The flow path of an angle valve is simple, resulting in low resistance. It is suitable for controlling fluids with high pressure differences, high viscosity, and those containing suspended solids or particles, as it helps prevent clogging and fouling, and makes cleaning easier. (4) Three-way valve: This valve has three inlets and outlets connected to pipes. Based on its function, it can be divided into diverging and converging types, as shown in Figures 8-6(d) and (e). A diverging valve has one inlet and two outlets, while a converging valve has two inlets and one outlet. When the valve core moves, the flow rate in one direction increases while it decreases in the other direction, but the total flow rate remains constant. (5) Butterfly valve: Also known as a flap valve, a butterfly valve consists of a valve body, a flap, a flap shaft, and shaft seals, as shown in Figure 10-6(f). It is typically used in conjunction with a long-stroke actuator; a pneumatic signal acts on a lever to rotate the flap shaft, thereby changing the flow area and thus the flow rate. Its advantages include low pressure loss, simple structure, low cost, and long service life. It is particularly suitable for applications involving gases and fluids with suspended solids at low pressure differences, large diameters, and high flow rates. However, its leakage rate is relatively high. The flow characteristics of a butterfly valve are similar to those of a equal percentage curve before 60 degrees of rotation; after 60 degrees, the torque increases, leading to unstable operation and poor performance. Therefore, butterfly valves are usually used within a range of 60 degrees of rotation. They are commonly used in compressor control systems.