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Chemical and Process Engineering Resources

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Category: Heat Transfer Technology
Question: How can one determine if their reboiler or vaporizer is operating in the mist flow region?
Keywords: shell,tube,heat,transfer,exchangers,boiling,mist,flow,regime
Answer: This is a flow pattern to avoid in heat transfer. The mist flow region is dependant upon velocity, % vapor and stratification effects. In this type of flow the tube wall is mostly dry and the liquid droplets are carried along in a vapor core. Therefore the heat transfer is much lower because the much higher thermal conductivity of the liquid is in very little contact with the tube wall. The higher the % vaporization, the lower the velocity needs to be to avoid mist flow. For example in a vertical tube where the vaporization is 50 % and the vapor density is 1.0 lb/cu ft, the velocity needs to be below approximately 80 ft/sec. If the vaporization is 75 %, the maximum velocity is approximately 30 ft/sec. This comes from the Fair equation. In a horizontal tube where there can be stratification, these maximum velocities are much lower. If the mist flow region can not be avoided, then twisted tape turbulators can be used to increase the heat transfer. They will throw the liquid in the vapor core toward the tube wall. Source: Gulley Computer AssociatesAdditionally, see the below information for the Fair Equation.