Both horizontal and vertical vessels are widely used, and it is often the physical characteristics of the machine room that has a major influence on the choice of orientation. Many designers prefer a vertical vessel for a mechanically pumped recirculation system, because the net-positive-suction head is more easily achieved. The vertical vessel usually requires less floor space than the horizontal, but does require greater vertical space in the machine room. For recirculation packages that incorporate the vessel and the pumps on a skid, the vessels are usually oriented horizontally to meet head room limitations on shipping.
The mechanism for separation of liquid from vapor in horizontal vessels is somewhat different than the process in a vertical vessel, but some of the same principles apply. In the horizontal vessel of Fig. 10.6, the horizontally flowing vapor carries liquid drops while at the same time the drops have a vertical component of velocity because of the gravitational force. If the drops are initially assumed to have no horizontal or vertical velocity when they enter the vessel, the drops accelerate horizontally due to drag force of the vapor on the drops while they accelerate vertically due to gravity. If a drop descends to the liquid surface before being carried out, it will be captured.
A key term is thus the time T for the drop to fall a distance y and settle to the liquid level. This time must not exceed the time to traverse the separating length L in Fig. 10.7. Thus arises the concept of T as the minimum residence time. Furthermore,
whereT = minimum residence time, s
L = separating length, m (ft)
V = horizontal vapor velocity, m/s (ft/s)
A = flow area, m2 (ft2)