TUBE ARRANGEMENT

Tube Layout

 Tube layout arrangements are designed so  as to include as many  tubes as possible within the shell to achieve maximum heat  transfer area. There are four tube layout patterns, as shown in Figure 6: triangular(30°), rotated triangular (60°), square(90°), and rotated square (45°).

A triangular (or rotated triangular) pattern will accommodate more tubes than a square (or rotated square) pat- tern. Furthermore, a triangular pattern produces high turbulence and therefore a high heat-transfer coefficient. However, at the typical tube pitch of 1.25 times the tube O.D., it does not permit mechanical cleaning of tubes, since access lanes are not available. Consequently, a triangular layout is limited to clean shellside services. For services that require mechanical cleaning on the shellside,square patterns must be used. Chemical cleaning does not require access lanes, so a triangular layout may be used for dirty shellside services provided chemical cleaning is suitable and effective.  A rotated triangular pattern seldom offers any advantages over a triangular pattern, and its use is consequently not very popular. For dirty shellside services, a square layout is typically employed. However, since this is an in-line pattern, it produces lower turbulence.  Thus, when the shellside Reynolds number is low (< 2,000),it is usually advantageous to employ a rotated square pattern because this produces much higher turbulence, which results in a higher efficiency of conversion of pressure drop to heat transfer.

As noted earlier, fixed-tubesheet construction is usually employed for clean services on the shellside, U-tube construction for clean services on the tubeside, and floating-head construction for dirty services on both the shellside and tubeside. (For clean services on both shellside and tubeside, either fixed-tubesheet or U-tube construction may be used, although U-tube is preferable since it permits differential expansion between the shell and the tubes.)

Hence, a triangular tube pattern may be used for fixed-tubesheet exchangers and a square (or rotated square) pattern for floating-head exchangers. For U-tube exchangers, a triangular pattern may be used provided the shellside stream is clean and a square (or rotated square) pattern if it is dirty.

Tube Pitch

Tube pitch is defined as the shortest distance between two adjacent tubes. For a triangular pattern, TEMA specifies a minimum tube pitch of 1.25 times the tube O.D. Thus, a 25-mm tube pitch is usually employed for 20-mm O.D. tubes. For square patterns, TEMA additionally recommends a minimum cleaning lane of 4 in. (or 6 mm) between adjacent tubes. Thus, the minimum tube pitch for square patterns is either 1.25 times the tube O.D. or the tube O.D. plus 6 mm, whichever is larger. For example, 20-mm tubes should be laid on a 26-mm (20 mm +6 mm) square pitch, but 25-mm tubes should be laid on a 31.25-mm (25mm × 1.25) square pitch. Designers prefer to employ the minimum recommended tube pitch, because it leads to the smallest shell diameter for a given number of tubes. However, in exceptional circumstances, the tube pitch may be increased to a higher value, for example, to reduce shellside pressure drop. This is particularly true in the case of a cross-flow shell.

The selection of tube pitch is a compromise between a close pitch for increased shell-side heat transfer and surface compactness, and a larger pitch for decreased shell-side pressure drop and fouling, and ease in cleaning.  In most shell and tube exchangers, the minimum ratio of tube pitch to tube outside diameter (pitch ratio) is 1.25. The minimum value is restricted to 1.25 because the tube-sheet ligament (a ligament is the portion of material between two neighboring

tube holes) may become too weak for proper rolling of the tubes into the tubesheet.  The ligament width is defined as the tube pitch minus the tube hole diameter;
this is shown in Fig

.

TUBESHEET

A tube sheet is an important component of a heat exchanger. It is the principal barrier between the shell-side and tube-side fluids. Proper design of a tube sheet is important for safety and reliability of the heat exchanger. Tube sheets are mostly circular with uniform pattern of drilled holes. Tube sheets of surface condensers are rectangular shape. Tube sheets are connected to the shell and the channels either by welds (integral) or with bolts (gasketed joints) or with a combination thereof. Tube-sheet connection with the shell and channel for fixed tube-sheet exchanger can be categorized into two types:

1.  Both sides integral construction,

2.  Shell-side integral and tube-side gasketed construction

Tube-sheet connection with the shell and channel for floating heat exchanger and U-tube heat exchangers can be categorized into three types:

1.  Both sides integral construction

2.  One side integral and the other side gasketed construction

3.  Both sides gasketed construction

Pass Arrangements for Flow Through Tubes

The simplest flow pattern through the tubes is for the fluid to enter at one end and exit at the other. This is a single-pass tube arrangement. To improve the heat-transfer rate, higher veloci-ties are preferred. This is achieved by increasing the number of tube-side passes. The number of  tube passes depends  upon the  available pressure drop, since higher velocity  in  the  tube results  in  higher heat-transfer coefficient,  at  the cost of  increased pressure drop. Larowski et al. suggests the following guidelines for tube-side passes:

1.  Two-phase flow on the tube side, whether condensing or boiling, is best kept in a single straight tube run or in a U-tube.2.  If  the  shell-side heat-transfer coefficient  is significantly lower  than  on  the tube side,  it  is not  advisable to increase  the  film coefficient on  the tube side at  the cost of  higher  tube-side pressure drop, since this situation will lead to a marginal improvement in overall heat transfer coefficient.

Number of Tube Passes

The number of tube-side passes generally ranges from one to eight. The standard design has one, two, or four tube passes. The practical upper limit is 16. Maximum number of tube side passes are limited by workers’ abilities to fit the pass partitions into the available space and the bolting and flange design to avoid interpass leakages on the tube side. In multipass designs, an even number of passes is generally used; odd numbers of passes are uncommon, and may result in mechanical and thermal problems in fabrication and operation. Partitions built into heads known as partition plates control tube-side passes. The pass partitions may be straight or wavy rib design. There are some limitations on how the different types of heat exchangers can be partitioned to provide various number of passes. They are summarized here.

1.  Fixed tube-sheet exchanger-any practical number of passes, odd or even. For multipass arrangements, partitions are built into both front and rear heads.

2.  U-tube exchanger-minimum two passes; any practical even number of tube passes can be obtained by building partition plates in the front head.

3.  Floating head exchangers: With pull through floating head (T head) type and split backing ring exchanger (S head), any practical even number of passes is possible. For single-pass operation, however, a packed joint must be installed on the floating head. With outside packed floating head type (P head), the number of passes is limited to one or two. With externally sealed floating tube sheet (W head), no practical tube pass limitation.

4.  Two-phase flow on the tube side, whether condensing or boiling, is best kept within a single pass or in U-tubes to avoid uneven distribution and hence uneven heat transfer.

 Tube to Header Plate Connection: Tubes are arranged in a bundle and held in place by header plate (tube sheet).The number of tubes that can be placed within a shell depends on Tube layout, tube outside diameter, pitch, number of passes and the shell diameter. When the tubes are too close to each other, the header plate becomes too weak.