Page 305 - Fluid mechanics, heat transfer, and mass transfer

SHELL AND TUBE HEAT EXCHANGERS
286
. What are the disadvantages of square tube pitch? 3. Use of finned tubes with staggered arrangement.
& Poor turbulence on the shell side. & 2. In-line square pitch.
& Lower shell side heat transfer coefficients, requiring 1. Triangular staggered pitch.
larger heat transfer surface area and hence number 3. Finned tubes with staggered arrangement.
tubes for the same duty. . ‘‘Arrangement of tubes in a heat exchanger with square
& Consequently, more expensive larger size shell will pitch is better suited for access for cleaning rather than
be required for the same heat duty. triangular pitch.’’ True/False?
. Give an example where square pitch is advantageous?
& True.
& When boiling liquids are on the shell side. Square
. ‘‘Shell side heat transfer coefficients are higher
pitch allows vapors to rise through spaces between
for square pitch compared to triangular pitch.’’ True/
the tubes (Figure 10.17).
False?
. What type of tube pitch arrangement is normally
& False.
recommended for fouling fluids?
. What are the normally recommended values of tube
& Square pitch is amenable for mechanical cleaning on
pitch?
the shell side. It must be recognized that square pitch
& Values of tube pitch depend on the tube diameter.
does not produce much turbulence that promotes
For example, TEMA specifies a minimum tube pitch
greater fouling deposit rates. When shell side
of 1.25 times the tube O.D. for triangular pattern.
Reynolds numbers are low (<2000), a rotated square
Thus, a 25 mm pitch is usually used for 20 mm O.D.
pitch is advantageous as it produces much higher
tubes.
turbulence.
& For square patterns, TEMA additionally recom-
& However, triangular pitch can be used if chemical
mends a minimum cleaning lane of 6 mm (4 in.)
cleaning is suitable and effective, as it does not
between adjacent tubes. Thus, the minimum tube
require straight access lanes.
pitch for square patterns is either 1.25 times the tube
. ‘‘In U-bundle heat exchangers, ease of cleaning of tube
O.D. or the tube O.D. plus 6 mm, whichever is larger.
exteriors is more if triangular tube pitch is used instead
For example, 20 mm tubes should be laid on a 26 mm
of square pitch.’’ True/False?
(20 mm þ 6 mm) square pitch, but 25 mm tubes
& False. should be laid on a 31.25 mm (25 mm 1.25 mm)
. Arrange the following in the order of increasing DP for square pitch.
flow outside tube banks at the same flow rate. & Normally the minimum specified values of tube pitch
1. Tube arrangement with triangular pitch (staggered). are used in the design as it leads to the smallest size
2. Tube arrangement with square pitch (in-line). shell for a given number of tubes.
& However, in exceptional circumstances, the tube
pitch may be increased to a higher value,for example,
to reduce shell side pressure drop. This is particularly
true in the case of cross-flow shell.
& Normally used values of tube pitch are 25, 30, and
38 mm.
. What are the advantages and disadvantages of using
closer tube pitch?
& Advantages
➢ Larger number of tubes is fitted in a given shell or
for a given number of tubes; a smaller diameter
shell will be required.
➢ Reduces initial cost.
& Disadvantages
➢ Closer pitch.
➢ Sagged tubes, particularly in hot service, touch
each other after some use.
Vapor bubbles rising through boiling liquid inside - When the tubes are arranged too close to each
FIGURE 10.17
a heat exchanger with square tube pitch. other, while more tubes can be accommodated