There are various ROT and, at least, one rigorous procedure to determine the disengagement space:
[•] The greater of 1 ft or 1.6 times the bundle diameter above the liquid level taking into account the head over the baffle (weir).
[•] The bundle shouldn't be higher than 60% of the shell diameter.
[•] Long units better have more than one vapor outlet nozzle
[•] In some cases a demister is located in a dome over the shell.
[•] For a more serious estimation of this height see Tammami's article in OGJ of July 15, 1985 titled: Simplifying reboiler entrainment calculations.
The sizing of the vapor space is dependent on the values in Stoke's Equation. You are concerned with the maximum velocity which you can rum and not allow for liquid carry over. These values are based on the relative differences in the liquid and vapor densities and the surface tension of the liquid.
Process and chemical engineers use a different set of criteria than refrigeration engineers. They allow for a certain percentage of liquid entrainment, where as a refrigeration system has to have ZERO entrainment to the compressor inlet.
You also have to consider the location and method of liquid entry into the space as well as the location and number of vapor outlets.
