Page 256 - Applied Process Design For Chemical And Petrochemical Plants Volume III

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218 Applied Process Design for Chemical and Petrochemical Plants

From the chart, read p long per baffle, as psi. To obtain Finned Tube Exchangers
total longitudinal drop, multiply by the number of baf-
fles. The procedures for designing exchangers using the
finned tubes are generally specific to the types of fins under
2. Calculate loss due to cross-flow through the tube bun- consideration. The 16 and 19 fins-per-in. low fin tubes (Fig-
dle; use Figure 10-142. ure 10-10A and 10-10B) are uniquely adaptable to the con-
ventional shell and tube exchanger 16, 127 (see Table 10-39)
2
G1cross-flow2 10.04 W2>1B2 1M2, lb>sec 1ft 2 (10-231) and are the type of tubes considered here.
These low-fin tubes can be installed and handled in the
where B baffle pitch or spacing, in. same manner as plain tubes. The larger diameter fins (5 or
M net free distance (sum) of spaces between tubes from more per in.) are usually used in services with very low out-
wall to wall at center of shell circle, in. side coefficients of heat transfer and require a unit design to
accommodate the tube’s installation.
B is held to a 2-in. minimum or / 5 shell diameter (I.D.) Other finned tube configurations are shown in Figures
1
3
and is 26 in. maximum for / 4 -in. tubes and 30 in. for 1-in. 10-10A, 10-10E, 10-10G, and 10-10H and represent
tubes. Refer to TEMA for tube support and baffle spacing increased external finning possibilities. Internal ribs, Fig-
recommendations. ures 10-10K and 10-10M, can certainly help the film trans-
Read pressure drop factor, F p , from Figure 10-142. fer coefficient, provided fouling is not a prominent factor.
Other finned designs (number of fins/in.) are available
1F t 21Fp21N c 121n c 2
p c 1cross-flow2 , psi (10-232) from most manufacturers, and in order to use them in heat

transfer designs, specific data needs to be available from
the manufacturer. The literature cannot adequately cover
where F t tube size factor, from table on Figure 10-142 suitable design data for each style of tube. Pase and O’Don-
F p pressure drop factor, Figure 10-142 203
nell present the use of finned titanium in corrosive ser-
N c number of baffles 206
vices. One of the outstanding books by Kern and Kraus
n c number of rows of tubes in cross-flow
density of fluid, lb/ft 3 covering the entire topic of Extended Surface Heat Transfer
includes detailed theory and derivations of relations plus
practical applied problems for finned and compact heat
The number of tube rows that will be crossed as the fluid
exchangers.
flows around the edge of one baffle and then across and
The longitudinal finned tube usually is adapted to double
over to the next baffle is used as for conventional designs.
pipe exchangers but is used in the conventional bundle
design with special considerations.
n c 0.9 1total tube rows in shell at center line2
Other finned tube references of interest are Hashizume 208
and Webb. 209

Low Finned Tubes, 16 and 19 Fins/In.

This tube has a ratio of outside to inside surface of about
3.5 and is useful in exchangers when the outside coeffi-
cient is poorer than the inside tube coefficient. The fin
efficiency factor, which is determined by fin shape and size,
is important to final exchanger sizing. Likewise, the effect
of the inside tube fouling factor is important to evaluate
carefully. Economically, the outside coefficient should be
1
about / 5 or less than the inside coefficient to make the
finned unit look attractive; however, this break-even point
varies with the market and designed-in features of the
exchanger.
Process applications are primarily limited to low-finned
Figure 10-142. Pressure drop in fluid flowing across tube banks with
segmental baffles. (Used by permission: Buthod, A. P. Oil & Gas Jour- tubing, although the high-finned tubes fit many process gas
nal, V. 58, No. 3, ©1960. PennWell Publishing Company. All rights designs that require special mechanical details. This test lim-
reserved.) its the presentation to the low-finned design.