1. When selecting and designing a plate heat exchanger for a certain process, factors such as its design pressure, design temperature, medium characteristics, and economy must be considered.
Selection of veneer area If the veneer area is too small, the number of plates is large, the floor area is large, and the resistance drop is reduced. On the contrary, if the veneer area is too large, the number of plates is small, the area is small, and the resistance drop is increased. However, it is difficult to ensure an appropriate plate-to-plate flow rate. Therefore, in general, the veneer area can be replaced with an angular hole velocity of about 6m / s.
The choice of flow velocity between plates affects the heat transfer performance and pressure drop. High flow velocity, high heat transfer coefficient and increased resistance drop; otherwise, the opposite is true.
Generally, the flow velocity between the plates is 0.2-0.8m / s, and the speed between the two fluid plates should be consistent as much as possible. When the flow velocity is less than 0.2m / s, the fluid cannot reach the turbulent state, and it will form a large dead zone. If the flow velocity is too high, the resistance drop will increase sharply, and the flow velocity between the gas plates is generally not greater than 10m / s.
When the flow of the fluid on both sides is determined to be approximately the same, it should be arranged as equal as possible; when the flow of the fluid on both sides is greatly different, the side with the smaller flow is arranged in multiple processes or plate heat transfer with unequal cross section channels Device. In addition, when the temperature rise or temperature drop of a certain medium is large, multiple processes can also be used. The side where the phase change occurs is generally a single process, and the interface mode is upward, downward, and out. In multi-process heat exchangers, generally the same number of flow channels should be used for each process in the same fluid. The correction coefficient of the pressure drop of the heat exchanger is 1.2 ~ 1.4 for single process, 1.8 ~ 2.0 for 2 ~ 3 process, and ~ 2.6 ~ 2.8 for 4 ~ 5 process. (4) Flow direction selection For single-phase heat exchange, the countercurrent has the largest In general, in the design of the plate heat exchanger, the fluid should be arranged as countercurrent as possible. When the fluid on both sides is an equal flow, it is countercurrent; when the fluid on both sides is an unequal flow, the cocurrent and countercurrent occur alternately, and the average temperature difference is smaller than that of the pure countercurrent.
Plate heat exchanger type calculation method:
Heat exchanger selection calculation formula Q = K · F · △ tm where: Q-heat flow (W) △ tm log average temperature difference (℃) F-heat transfer area (m2) Plate heat exchanger in actual operation Due to the effects of dirt and uneven water flow, a correction coefficient β (usually 0.7 to 0.9) needs to be introduced in the above formula. Therefore, in actual use, the above formula is: Q = β · K · F · △ tm
Estimation method When the velocity between the plates is 0.3-0.5m / s, water (steam) -water K = 3000-7000; water (steam) -oil K = 400-1000; oil-oil K = 175-400.