This equation also has a slope of Lm/Vm although it cuts the y = x line at x = Wxw/ (W — £) as shown in Figure 11.49. When x = xw, y = 0, corresponding to the composition of the vapour, steam, rising to the bottom plate. For a given external reflux ratio and feed condition, Lm/Vm will be the same whether direct or indirect steam is used and the lower operating line must cut the y = x line at the same value in each case, though, of course, xm for the indirect steam will be higher than xw for direct steam.
When stepping off the theoretical stages, the bottom step should start at y = 0, x = xw. in this way, the use of direct steam, although eliminating the still, dilutes the bottom material, and so increases the number of stages required in the lower part of the column.
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