Lesson 11 - Jacketed Vessels SAQ

** Why is the heat transfer surface area of a jacketed vessel limited? How can jacketed vessels be designed in a way that overcomes this limitation?

Response:

In a jacketed vessel, the heat transfer surface area is the vessel wall. This means that heat transfer is limited by the size and dimension of the vessel.

There are several ways in which the design of a jacketed vessel can be improved to overcome these limitations. Some of these ways are listed below.

• Increasing the height to diameter ratio of the vessel.

• Increasing the overall heat transfer coefficient by:

  • Agitating the product fluid inside the vessel. One of the largest influences on the overall heat transfer coefficient is the inside film coefficient. Agitated fluids have significantly higher film coefficients. Therefore, agitation is one of the most effective ways of increasing the heat transfer rate in a jacketed vessel.

  • Increasing the turbulence of the utility fluid through the type of jacket selected or by using baffles. Increasing the utility fluid turbulence will increase the outside film transfer coefficient. As with the inside film coefficient, this also has a major impact on the overall heat transfer coefficient.

  • Using a utility fluid that changes phase rather than temperature. This is because when the utility fluid is changing phase, the heat transfer rate is less influenced by the fluid velocity.

  • Minimising the thickness of the vessel wall and selecting a material for the vessel wall that is a good conductor of heat. (However, please note that these parameters only have a marginal impact on the overall heat transfer coefficient.)

• Increasing the temperature driving force by:

  • Agitating the product fluid inside the vessel. Agitating the product fluid improves the uniformity of temperature at the heat exchanger surface, which in turn improves the temperature driving force.

  • In the case of heating, increasing the temperature of the utility fluid. Or in the case of cooling, decreasing the temperature of the utility fluid. This must be balanced off against the risk of increased fouling (in the case of heating) or freezing the product (in the case of cooling). Care must also be taken if the product fluid is heat sensitive.