EXH400 shell and tube exchanger design and selection

This section contains information on TEMA nomenclature, selecting the most economic exchanger configuration for a defined service, allocating the streams to shell or tube side, specifying appropriate mechanical components, defining baffle layout, deciding if a small predesigned exchanger is appropriate, and estimating the size and cost of shell and tube exchangers.

Chevron Corporation 400-1 December 1989

400 Shell and Tube Exchanger

Design and Selection

Abstract

This section contains information on TEMA nomenclature, selecting the most economic exchanger configuration for a defined service, allocating the streams to shell or tube side, specifying appropriate mechanical components, defining baffle layout, deciding if a small predesigned exchanger is appropriate, and estimating the size and cost of shell and tube exchangers

410 TEMA (Tubular Exchanger Manufacturers Assoc.) Nomenclature 400-2

451 Front Head Design

481 Step by Step Procedure

482 Surface Area Calculations

483 Tube Count and Number of Tube Passes

484 Shell Diameter

485 Exchanger Investment Cost

410 TEMA (Tubular Exchanger Manufacturers Assoc.) Nomenclature

The Tubular Exchanger Manufacturers Association (TEMA) has developed clature for describing shell and tube heat exchangers It includes a simple code for designating the size and type of the exchanger In addition, standard terminology has been set up to specify typical parts and connections

nomen-TEMA size is the shell inside diameter in inches rounded to the nearest integer, followed by the straight length of the tubes in inches rounded to the nearest integer The two dimensions are separated by a hyphen (-)

For kettle reboilers, the port diameter in inches precedes the shell inside diameter The two dimensions are separated by a slash (/) Port diameter is the size of the opening the bundle slides through

TEMA type consists of three letters describing the stationary or front end head, shell, and rear head, in that order The letter designations are shown on

Figure 400-1

For example, a 20-foot straight length U-tube bundle, 3-foot shell diameter, with a single shell pass and removable shell cover would be a TEMA SIZE 36-240 TYPE AEU The same bundle installed in a 5-foot diameter kettle reboiler would be a TEMA SIZE 36/60-240 TYPE AKU

Standard terminology to describe components and connections of shell and tube exchangers is provided in Figure 400-2

TEMA sets mechanical standards for three classes of exchangers reflecting the severity of the service For most refinery services, the most restrictive class is used—TEMA Class R For other services (chemical plants for example), TEMA Class C or B exchangers are used In general, Class R exchangers have thicker shells, larger and thicker heads, thicker tubes, and larger miscellaneous parts TEMA requirements are noted where appropriate throughout this manual

420 General Design Considerations

Single- and two-phase exchangers and most condensers have very similar tions The typical layout is summarized in the following list and shown in Figures 400-3 and 400-4 (Steam generators (2 types), reboilers, and condensers are described in Sections 340, 350, 360 and 370.)

configura-The typical shell and tube exchanger geometry includes the following items:

• TEMA E shell style

• U-tubes for rear head type with full support plate at tangent

• TEMA A-type front head

• Single segmental baffles with cut of 18 to 25% of shell I.D and with cut oriented vertically

• Baffle spacing of 20 to 100% of shell I.D

Heat Exchanger and Cooling Tower Manual 400 Shell and Tube Exchanger Design and Selection

Fig 400-1 Heat Exchanger Nomenclature (TEMA, Figure N-1.2) (Courtesy of TEMA)

Fig 400-2 Heat Exchanger Components (1 of 3) (TEMA, Table N-2 and Figure N-2) (Courtesy of TEMA)

10 Shell Flange—Stationary Head End

11 Shell Flange—Read Head End

12 Shell Nozzle

13 Shell Cover Flange

14 Expansion Joint

15 Floating Tubesheet

16 Floating Head Cover

17 Floating Head Flange

18 Floating Head Backing Device

19 Split Shear Ring

20 Slip-on Backing Flange

21 Floating Head Cover—External

22 Floating Tubesheet Skirt

23 Packing Box

24 Packing

25 Packing Gland

26 Lantern Ring

27 Tierods and Spacers

28 Transverse Baffles or Support Plates

Heat Exchanger and Cooling Tower Manual 400 Shell and Tube Exchanger Design and Selection

Fig 400-2 Heat Exchanger Components (2 of 3) (TEMA, Table N-2 and Figure N-2) (Courtesy of TEMA)

Fig 400-2 Heat Exchanger Components (3 of 3) (TEMA, Table N-2 and Figure N-2) (Courtesy of TEMA)

Fig 400-4 Typical Cross Section, Shell and Tube Exchanger

Heat Exchanger and Cooling Tower Manual 400 Shell and Tube Exchanger Design and Selection

• 3/4-inch O.D., 14 BWG (average) thickness (0.584 inch I.D.) carbon steel tubes

• Tube length variable with one or two tube passes depending on service

• 45 degree rotated square layout with tube pitch = 1.25 × tube O.D for liquid and two-phase hydroprocessing shell side service

• 90 degree square layout with tube pitch = 1.25 × tube O.D for boiling, condensing, and single-phase gas shell side service

• Two or more pairs of sealing strips (bars)

• Dummy tubes in pass partition lane when two tube passes

• Two rows of impingement rods at inlet nozzle when warranted

Overall Exchanger Configuration

The Company preference is a TEMA AEU exchanger for most services U-tubes are the cheapest rear head type that allows for thermal expansion of the tubes The TEMA A type front head has a removable channel cover This allows for inspection and cleaning of the tube side without pulling spool pieces in the piping

Shell Side Nozzle Placement

Single inlet and outlet shell side nozzles are normally located at opposite ends of the exchanger with one on the top and one on the bottom of the shell This arrange-ment allows vents and drains to be located in piping

Route two-phase flow based on the following rule: “Heat up and cool down.” This means hot fluid being condensed should enter on the top and exit on the bottom of the exchanger Likewise, cold fluid being boiled should enter on the bottom and exit on the top The “Heat up and cool down” rule does not apply to single-phase flow

Transverse and Support Baffles

The normal configuration for the tube side consists of U-tubes with a full support plate at the tangent This is shown in Figure 400-3 The plate blocks flow over the U-bends Otherwise, the bends must be supported to protect against vibration.For baffles, use single segmental baffles with a cut of around 18 to 25% of the shell I.D for most efficient conversion of pressure drop to heat transfer The baffle cut should be vertical for best drainage of the shell side at shutdown Baffle thickness is set by TEMA

Baffle spacing should be 20 to 50% of the shell I.D It is usually set to maintain good heat transfer (economic pressure gradient or shear controlled flow regime) Guidelines for economic exchanger velocity and pressure drop are provided in Section 220 of this manual In some cases (particularly for gas and two-phase flow shell side), additional supports may be required to prevent vibration See

Section 260 of this manual for more information

Tube Selection

Tubes are normally 3/4-inch outside diameter, 14 BWG (minimum) thickness inch inside diameter), and made of carbon steel Length is limited by the plot space for pulling the bundle and standard bundle pulling equipment TEMA has named 8,

(0.56-10, 12, 16 and 20 feet as standard tube lengths Other lengths are possible

Alloy tubes are appropriate for some services The cost of upgrading to alloy tubes should always be weighed against possible process adjustments to permit carbon steel construction Section 800 of this manual discusses materials selection for different services

Tubepass Layout

Most exchangers should be limited to one or two tube passes Using U-tubes with two passes is best and cheapest, however some services dictate 1 pass with a more expensive rear head (vertical thermosiphon reboilers or crude/overhead condensers, for example)

Tube Pitch

For liquid and two-phase services, use 1-inch, 45 degree rotated square pitch This promotes mixing Use 1-inch, 90 degree square pitch for boiling, condensing, and single-phase gas on the shell side For boiling, the vertically oriented lanes promote circulation For condensing and single-phase gas, in-line tubes minimize pressure drop without sacrificing heat transfer Both 45 and 90 degree pitch provide 0.25-inch inspection and cleaning lanes through the bundle

Preventing Shell Side Flow Bypassing

Single- and two-phase exchangers with impingement protection typically include two pairs of sealing strips (bars) The bars block the leakage stream flowing around the baffles between the bundle and shell (“C” stream shown in Figure 200-3 in Section 213) For vertical cut baffles, the bars straddle the nozzles (located at the top and bottom of the bundle) Note that the bars on the bottom act as skid bars for bundle removal

For an exchanger with two tube passes, the single pass partition lane runs ular to the baffle cuts Dummy tubes are positioned in the pass partition lane to block flow bypassing (“F” stream shown in Figure 200-3 in Section 213) Dummy tubes are spaced four to six tube rows apart between baffle cuts and are the same diameter as the tubes

Tolerances and Clearances

All tolerances and clearances are TEMA

Heat Exchanger and Cooling Tower Manual 400 Shell and Tube Exchanger Design and Selection

430 Stream Placement

Allocating the streams to the shell or tube side is determined by weighing factors which sometimes conflict These factors include stream temperature, pressure, rela-tive flowrate, viscosity, corrosiveness, relative heat transfer film coefficient, and pressure drop limitations Guidelines for allocating the streams to the shell or tube side are given in Figure 400-5

Fig 400-5 Allocating the Streams for Shell and Tube Heat Exchangers

In Order of Decreasing Priority:

X Enhance outside surface to raise

limiting side coefficient phase gas only)

Treated Cooling Tower Water X Corrosion inhibitors effective

tube-side; otherwise use alloy tubes

good heat transfer at low Reynold’s number

Alloy Required for Corrosion X Allows cheaper shellside

compo-nents Very Low System Pressure or ∆ P

Available

X Can use J or X shell style to

shorten flow path and reduce sure drop

however, tube rupture design sometimes controls

High ∆ T across one Bundle (Over

200°F)

X Excessive ∆ T in stationary

tubesheet if placed on tubeside

Deposits Too Hard to

Hydroblast (Rare)

X Use floating rear head for straight

tubes Complete Tube Plugging (Rare) X Use floating rear head for straight

tubes

440 Pass Arrangements and Multiple Shells

The appropriate stream pass arrangements for a particular service are based on:

• Providing economic pressure gradient on both sides of exchanger

• Operating in shear controlled flow regime for two-phase flow

• Limiting pressure drop

• Controlling temperature efficiency

On the tube side, the pressure gradient is adjusted by changing the number of tubes per pass To get more area, increase the flow path length either by using longer tubes, by adding more shells in series, or by increasing the number of tube passes.Note that two tube passes are typical because more passes dramatically increase pressure drop Not only does the pressure drop increase proportionally to the increase in flow path length, but to the square of velocity For example, going from two to four passes increases the pressure drop by a factor of eight with the tube count held constant

On the shell side, the pressure gradient is adjusted by changing the baffle spacing

To get more area, the exchanger (tube length) is made longer When more area is needed and the tube length is maximum, add another shell with the shell side flow

in series

The shell style is changed from a TEMA E-type to a TEMA J- or X- type when the resulting pressure drop is too large at the target pressure gradient This shortens the flow path allowing the pressure gradient to be maintained

Use parallel exchangers only when a single exchanger is too large, and the pressure drops can not be increased at the target pressure gradients Exchanger size is limited by the manufacturer’s fabricating equipment and the user’s maintenance equipment Space availability may also limit size, especially when modifying an existing unit

Parallel units with isolation valves have been used to provide an installed spare or when flow rates will vary more than 50% from normal When the flow rate varies, the number of units onstream is changed to maintain reasonable operating pressure drop

Consider using a mixed parallel/series arrangement of shell and tube passes in multiple units only when required to meet pressure drop restrictions The overall temperature efficiency of the units is reduced Note that the F-factor described in Section 211 of this manual is the common measure of temperature efficiency.Temperature efficiency will vary with service Area is most effectively used when shell and tube side stream routing approaches pure countercurrent flow (F-factor of 1.0) Going to multiple units in series increases the temperature efficiency Keep the F-factor above approximately 0.85

When performance is limited by a temperature pinch between the streams (small local temperature difference reflected as low F-factor), multiple shells become cost

Heat Exchanger and Cooling Tower Manual 400 Shell and Tube Exchanger Design and Selection

effective by reducing the total area requirement Countercurrent flow of both fluids through the shells maximizes efficiency

For condensing services, significant subcooling loads are usually processed in a separate exchanger following the condenser This allows the geometry to be changed to accommodate the much lower volumetric rate of the liquid As a result, the area needed for subcooling is reduced

450 Bundle and Tubesheet Arrangements

This section covers front head selection, fixed tubesheet applications, U-tubes versus floating rear heads, and TEMA F shells (two shell pass exchangers)

451 Front Head Design

The TEMA Type A front stationary head is normally used It has a removal channel cover so the tube side can be inspected without disconnecting nozzles or removing pipe spools The bonnet channel (Type B) is cheaper and is appropriate for small exchangers with small easily removed pipe spools For operating pressures above

1000 psig, a special front head is required Options are discussed in Section 532

452 Fixed Tubesheets

Fixed tubesheets are the cheapest type of head They are typically used when the shell side service is nonfouling and noncorrosive, and the metal temperature of the shell and tubes operate within 50°F (including startup, shutdown and steam out conditions) The bundle is not removable

The shell side is not accessible for inspection or mechanical cleaning since the tubesheets are seal welded to the shell If the temperature difference is larger than 50°F, an expansion joint may be required in the shell

Steam generators with very high (1000°F and above) process side temperatures and water on the shell side must have fixed tubesheets See Section 350 of this manual for more information

453 U-tubes Versus Floating Rear Heads

U-tube and floating head bundles are removable Both permit thermal expansion of the tubes The various types of rear heads are shown on Figure 400-1

U-tubes (TEMA Type U) are the cheapest of the two types and are preferred The bends can be mechanically cleaned by hydroblasting for typical fouling deposits—

as long as complete plugging does not occur

One disadvantage of U-tube bundles is that corrosion is difficult to monitor imen tubes can only be taken from the outside perimeter of the bundle

Spec-TEMA Type S and T floating rear heads cost more than U-tubes Maintenance is complicated by the added bundle flange Floating heads can be taken apart and the