In vertical separators, a minimum diameter must be maintained to allow liquid drops to separate from the vertically moving gas. The liquid retention time requirement specifies a combination of diameter and liquid volume height. Any diameter greater than the minimum required for gas capacity can be chosen. Figure 4-15 shows the model used for a vertical separator.

*Gas Capacity*

Derivation of Equation 4-9

For the droplets to fall, the gas velocity must be less than the terminal velocity of the droplet. Recall that:

Determine gas velocity, Vg. A is in ft2, D in ft, d in inches, Q in ft3/s.

*Liquid Capacity*

where h = height of the liquid volume, in.

Derivation of Equation 4-10

t is in s, Vol in ft3, Q in ft3/s, h in inches

*Seam-to-Seam Length and Slendemess Ratio*

The seam-to-seam length of the vessel should be determined from the geometry once a diameter and height of liquid volume are known. As shown in Figure 4-16, allowance must be made for the gas separation section and mist extractor and for any space below the water outlet. For screening purposes the following approximation has been proven useful. Use the larger of the two values:

As with horizontal separators, the larger the slenderness ratio, the less expensive the vessel. In vertical separators whose sizing is liquid dominated, it is common to chose slenderness ratios no greater than 4 to keep the height of the liquid collection section to a reasonable level. Choices of between 3 and 4 are common, although height restrictions may force the choice of a lower slenderness ratio.