Step 8. Read off an overall heat transfer coefficient H o from Fig. 33.
Calculate the LMTD over the section from the temperatures used in the previous
section on pumparound. Then calculate the total area of tower required for the
heat transfer from the expression
Q ¼ AH o Δt m
(16)
where
Q = Heat duty in Btu/h.
A = Heat transfer area in sqft.
H o = Overall heat transfer coefficient in Btu/sqftÁhÁ
F.
Δt m = Log mean temperature difference in
F.
Step 9. Calculate the theoretical number of trays required by dividing the total area
calculated in step 8 by the design cross-sectional area (step 5). Multiply this
number of trays by the selected HETP to give the height of packing.
Step 10. Calculate the pressure drop across the grid using the actual vapor velocity
in cuft/s in the equation
K ¼ V a
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi
ρ v = ρ l À ρ v
ð
Þ
p
(17)
(Eq. 17) to determine the constant K. Then read off the pressure drop in inches of
hot liquid per foot height from Fig. 34.
To express this pressure drop in mmHg, multiply by the SG of the hot liquid and
1.865.
Appendix 1: Pressure-Temperature Curves for Hydrocarbon
Equilibria
Figs. 35 and 36.
Appendix 2: ASTM Gaps and Overlaps
Fig. 37.
Appendix 3: Valve Tray Design Details
Table 14.
Appendix 4: Chord Height, Area, and Length
Fig. 38.
194
D.S.J. Jones