Page 214 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10F 5/30/2001 4:35 PM Page 177

Heat Transfer 177

Select a unit as follows: 1
2
U 162 Btu>hr 1ft 21°F2
.00618
3 / 4 -inch O.D. 14 BWG 32 ft, 0 in. steel U tubes, assume
1,790,000
effective length is 31 ft, 0 in. for preliminary calculation: Area required 184 ft 2
69 11622
149
Number of tubes 24.5
2
31 .196 ft >ft From Figure 10-27B, the equivalent tube length 31.2 ft
Area available 30 (31.2) (0.196) 188 ft 2
From standard tubesheet layout or Table 10-9, select a
10-in. I.D. shell with 30 tubes, 2 passes, for first trial. The originally assumed unit is satisfactory.
4. Determine condensing coefficient tube loading, Figure It is to be noted that only the steam-condensing coeffi-
10-67A. cient will change (lower tube loading, increasing h io ).
Because an arbitrary maximum value was used for coeffi-
2,000 cients, the overall U will not change nor will the t. There-
G– o 5.6 lb>lin ft 1equivalent2
0.5 31 23 fore, only the available area of the new sized unit needs to be
checked against the previously calculated required area. For
Condensate properties at estimated t w of 300°F a 12-in. shell with 32 tubes available:
Sp. G. 0.916
k a 0.455 Area available 32 31 .196 194 ft 2
0.19 centipoise
A 12-in. shell will be satisfactory:
Because of the low tube loading and physical properties
of condensate, the value of the film coefficient is 194 184 10
S.F. 5.44% with .002 dirt factor
beyond the range of the chart. Therefore, the use of a 184 184
h io of 1,500 is conservative.
5. Determine the boiling coefficient. Area “safety factor” (which may be interpreted as more
Add a “dirt” factor of 0.001 to h io : allowance for fouling):
1 1 1
.001 .001 0.00167
h io h io 1,500 188 149
11002 26%
149
h io 1/.00167 600
For a small unit such as this, 26% over surface is not too
Calculate tube wall temperature, t w . In this example: uneconomical. A smaller unit might be selected; however, if
the tubes are shortened and the shell diameter is enlarged,
t steam 322°F t c 262°F the unit will be more expensive. Note that 24-ft (total
2
length) tubes will give 146 ft of surface. The only safety fac-
First try: tor is in the knowledge that the flux selected, Q/A, appears
to be quite low. If it were doubled (and this could be done),
Assume h o 300 the smaller unit would be a reasonable selection.
600
t w 262 1322 2622
600 300
Boiling: Nucleate Natural Circulation (Thermosiphon)
262 0.6671602 262 40
Inside Vertical Tubes or Outside Horizontal Tubes
302°F
Natural circulation reboilers are effective and convenient
t w 302 262 40°F units for process systems operating under pressure. They are
h o for t w of 40°F is greater than 300, Figure 10-103. usable in vacuum applications but must be applied with
Use 300 maximum (Kern’s recommendation).
care, because the effect of pressure head (liquid leg) on the
boiling point of the fluid must be considered. The tempera-
6. Determine the required unit size.
ture difference between the heating medium and boiling
Add a .001 dirt factor to h o .
point of the fluid may be so small as to be impractical,
regardless of the tube length in a vertical unit.
1 1 1 L w
100 The recommended tube length is 8 ft in vertical units,
U 600 300 k with 12 ft being a maximum. Of course, some designs oper-
.00167 .00333 .001 .0018 .00618 ate with 4- and 6-ft tubes; however, these are usually in