and through the down pipe to the plate below. The vapor flows up through the liquid on the plate and to the space above. For these flows to follow the desired pattern, the necessary pressure drops and
Fig. 16-1. Schematic cross-sectional Other hand, down pipes of large diagram of bubble-cap plate column. crogs section reduce the available plate area for vapor liquid contact. In flowing across the plate the liquid decreases in depth owing to the frictional and kinetic effects giving the so-called hydraulic gradient. An overflow weir is employed to maintain the liquid level at approximately the desired level. These various factors are considered in detail in later paragraphs.
Pressure Drop for Vapor Flow. The vapor meets its main resistance in passing through the bubble cap and the liquid on the plate. Consider the section of a cap shown in Fig. 16-2. The vapor from the plate
hydraulic heads must be available.
The liquid meets resistance in the down pipes, in flowing across the plate, and in flowing over the weirs. The frictional resistance in the down pipes is handled by making them of adequate cross section and height to take the liquid load. For a given cross section, in general the liquid-handling capacity of the down pipes will increase with increasing height but it is desirable to keep the height low in order to reduce the plate spacing. On the
Fig. 16-1. Schematic cross-sectional Other hand, down pipes of large diagram of bubble-cap plate column. crogs section reduce the available plate area for vapor liquid contact. In flowing across the plate the liquid decreases in depth owing to the frictional and kinetic effects giving the so-called hydraulic gradient. An overflow weir is employed
Fig. 16-2. Cross section of bubble cap.
Fig. 16-2. Cross section of bubble cap.
below enters the riser at (1) and encounters a pressure drop due to the reduction in cross section. There is a frictional drop in the riser from (1) to (2), a reversal loss (2) to (3) between the top of the riser and the cap, and then a frictional drop against the ca£ from (3) to (4). The vapor then passes through the slot and up to the vapor space above the liquid. It is convenient to group these into three pressure drops: the pressure drop through the riser and cap, hc; the loss in pressure in flowing through the slots, h8; and the pressure drop due to the liquid head above the slots, hL.
Pressure Drop through Risers and Cap. This loss is chiefly a kinetic velocity effect due to the changing cross-sectional areas. The pressure drop in inches of the liquid equivalent to the kinetic head is
¿gc pl where hn = kinetic head, inches of liquid of density pL
Vr = maximum velocity in riser, between top of riser and cap, or in annulus between riser and cap, f.p.s. gc = conversion constant
« 32.2[(ft.)(lb force)]/[(sec.2)(lb. mass)] pv = density of ga&, same units as pl The actual loss in pressure, hc, from (1) to (4) should be a function of hhr. The data of Mayer (Ref. 24), Schneider (Ref. 28), and Dauphin6 (Ref. 5) on several 3-, 4-, and 6-in.-diameter caps with risers from 2 to 4 in. in diameter gave ratio of hjhn from 4.7 to 6.3. Souders (Ref. 34) gave results indicating a ratio of 2.9, Kirkbride (Ref. 20) recommended 3.2, and Edminster (Ref. 7) suggested 7.8 but included the pressure drop through the slots. In view of the extensive data by the first three investigators, a value of the ratio equal to 6.0 will be used giving ho = 1.1 — (16-2)
Pressure Drop through Slots. The pressure drop through the slot, h8, is evidenced by the liquid level in the cap being lower than the top of the slots. The value of h8 will be taken as equal to the difference in the pressure of the vapor in the cap at position (4) and the liquid outside the cap at the top of the slot. The slot action varies with the rate of flow. At low rates of flow an intermittent type of bubbling action is obtained. Owing to the surface tension, the pressure within the cap rises until the liquid under the cap is depressed an appreciable distance below the top of the slots. When the pressure is sufficient to overcome the surface tension, there is a rapid flow of vapor reducing the pressure, and the cycle is then repeated. Thus a pressure drop across the plate greater than the height of the liquid above the slots is necessary to initiate vapor flow. At higher rates of vapor flow, bubbling becomes continuous, and the pressure drop through the slots becomes greater than that necessary to initiate flow. At still higher vapor rates, the vapor blows open channels through the liquid. For all cases, the pressure drop through the slots is greater than for flow through the slots on a dry plate. The calculation of h* is also complicated because the velocity of the vapor in the cap approaching the slots may be equal to or greater than the slot velocity, and a simple orifice-type equation is not applicable.
A definite value of h, is necessary to initiate flow. It then increases slowly with the rate of vapor flow as the slot opening increases, and when the slots are completely open, the pressure drop increases rapidly with increase in vapor rate through the slots.
For values of h8 less than the height of the slot, the data of Carey (Ref. 3), Griswold (Ref. 14), Mayer, Schneider, and Dauphin^ can be correlated by h9 - 0.12 i + K (iCV. J—")MT (16-3)
where h9 - slot opening or pressure drop through slots, in. y = surface tension, dynes/cm. pL = liquid density, lb. per cu. ft. pY = vapor density, lb. per cu. ft. h* = slot height, in.
Va = velocity based on total slot area, f.p.s. K = see Table 16-1 h*V
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