10.5. Sieve Tray Layout and Tray Hydraulics

Tray layout is an art with its own rules. This section follows the presentations of Ludwig (1997), Bolles (1963), Fair (1963, 1984, 1985), Kister (1990, 1992, 2008), and Lockett (1986), and more details are available in those sources. The holes on a sieve plate are not scattered randomly on the plate. Instead, a detailed pattern is used to ensure even flow of vapor and liquid on the tray. The punched holes in the tray usually range in diameter from 1/8 to 1.0 inch. In the range from ½ to 1 inch, efficiency is constant and at its highest level. The 1/8-inch holes with the holes punched from the bottom up are often used in vacuum operation to reduce entrainment and minimize pressure drop. In normal operation, holes are punched from the bottom down since this is much safer for maintenance personnel. In fouling applications, holes are ½ inch or larger. For clean service, ½ inch is a reasonable first guess for hole diameter.

A common tray layout is the equilateral triangular pitch shown in Figure 10-20. The use of standard punching patterns will be cheaper than use of a non-standard pattern. The holes are spaced from 2.5d_o to 5d_o apart, with 3.8d_o a reasonable average. The region containing holes should have a minimum 2- to 3-inches clearance from the column shell and from the inlet downcomer. A 3- to 5-inch minimum clearance is used before the downcomer weir because it is important to allow for disengagement of liquid and vapor. Since flow on the tray is very turbulent, the vapor does not go straight up from the holes. The active hole area is considered to be 2- to 3-inches from the peripheral holes; thus, the area up to the column shell is active. The fraction of the column that is taken up by holes depends upon the hole size, the pitch, the hole spacing, the clearances, and the size of the downcomers. Typically, 4% to 15% of the entire tower area is hole area. This corresponds to a value of β = A_hole/A_active of 6% to 25%. The average value of β is between 7% and 16%, with 10% a reasonable first guess.