In the process of fluid flowing through the plate heat exchanger, there are various flow resistances. The size of the flow resistance is related to the physical property of the fluid, the flow rate and the geometrical characteristics of the flow passage of the plate heat exchanger. In order to meet the flow and velocity of the fluid required by the plate heat exchanger, a certain pressure difference must be established between the flow in and out of the space in order to overcome the flow resistance when the fluid flows through the flow passage of the heat exchanger. The change of fluid pressure in the flow passage of heat exchanger will also affect the heat transfer in some cases.
1. The purpose of flow resistance calculation of heat exchanger
(1) calculate the pressure drop of fluid flow through the heat exchanger as the basis for selecting the pump.
(2) when there is a phase change in the heat exchanger, the change of working pressure of the fluid caused by flow resistance will change the saturation temperature of the fluid, change the temperature difference between it and the other top grade, and affect the heat transfer.
Pumping fluid flows through the heat exchanger power pump Ⅳ is proportional to the pressure drop of fluid flowing through the heat exchanger AP, namely:
Ⅳ: G - delta P (W) (2-2) 10 type
G -- mass flow rate of fluid (kg/s);
Pressure drop of fluid between inlet and outlet of AP- heat exchanger (Pa);
10 - fluid density (kg/m3).
2. Composition of flow resistance
(1) single flow
1) Frictional resistance
The resistance caused by friction between the fluid and the solid wall surface, due to the mutual displacement between the viscosity of the fluid and fluid particles. The higher the velocity, the greater the viscosity, the rougher the wall surface, and the longer the flow, the greater the friction resistance.
2) local resistance
Resistance caused by changes in the direction or speed of fluid flow due to various local obstacles.
The size of local resistance is related to the geometry, size, flow pattern and wall roughness of local obstacles.
(2) dual-directional flow
factors affecting the flow of vapor-liquid include flow resistance (friction, local resistance, acceleration resistance and gravity resistance) in addition to multiple factors such as the mutual slip of vapor-liquid due to the difference in the density of vapor-liquid.
1) the frictional resistance
Frictional resistance is greater than that of single-phase flow due to the influence of factors such as liquid phase increasement caused by vapor phase mixing and sliding speed of vapor phase flow on turbulent effect caused by liquid film. In fact, the frictional resistance of two-phase flow is often solved by multiplying the frictional resistance of the two-phase flow when there is only the liquid component in it with corresponding multiple.
2) local resistance
The local resistance of two-phase flow is more complex than that of single-phase flow. For example, when the fluid passes through the bend, the single-phase flow is caused by the eddy current and the change of flow field, and the two-phase flow is caused by the change of phase separation and slip ratio between the two phases, so the calculation formula is much more complicated than the single-phase formula.
3) acceleration resistance
Acceleration resistance is the force loss caused by the change of density and velocity of two-phase flow in the flow process. Generally, the acceleration resistance is small, compared with the friction resistance and gravity resistance. But in the vapor-liquid two-phase flow with high heat load, the acceleration resistance increases to a degree comparable to the friction resistance.
4) gravity resistance
Gravity resistance is the loss of resistance due to the difference in height in the vertical flow passage.
3. Effect of pump power consumption
(1) The fluid pump power required by the plate heat exchanger depends on a large extent on the size of the pressure drop, which is related to the physical property of the fluid and the equivalent diameter of the flow passage. For high-density fluid (such as liquid), the pump power consumption is small, the impact of pressure drop on the design is small. On the contrary, for low density fluid (such as gas), the required power is very large. Therefore, heat exchanger design must pay attention to the pressure drop.
(2) Fluid pump power is proportional to the mass velocity or Reynolds number to the third power. Therefore, it may be economically unreasonable to obtain a slightly higher heat transfer coefficient by increasing the flow rate.
4. The influence of pressure drop on the distribution uniformity of velocity in the flow passage
If the partial pressure drop of the heat exchange accounts for the main part of the total pressure drop, the velocity of each part of the heat exchange surface is more uniform. If the pressure drop of the inlet Angle hole is very large, the velocity distribution of each part in the surface flow passage will be uneven, which will affect the heat transfer performance of the heat exchanger.
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