Introduction to Shell and Tube Heat Exchangers.pdf

7/23/2019 Introduction to Shell and Tube Heat Exchangers.pdf

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TEMA Shell and Tube Heat Exchangers

© Copyright Progressive Thermal Engineering All rights reserved.Page 1-1-1

Introduction to Shell and TubeHeat Exchangers

Introduction

General Description

Identifying Major Components

TEMA Standards Vibration





Shell and Tube Heat Exchangers

Tubeside flow (four passes)

Typical Major Components

Floating head

Stationary head

Pass partitions

Pass partitionShellside flow,

one pass

Tubesheet

Baffle

Tube

Shell





General Description

The variety of designs and configurationsare almost limitless

Common features: – A collection of tubes manifolded together to

form a “tube bundle”

– A chamber formed around the outside of thetube bundle, the “shell”

One stream flows inside the tube bundle,the other outside the tube bundle,contained by the shell

Identifying Major Components

Tubesheets

Channels

Fixed and floating heads Shell covers

Bundle





Tubesheets

Within the scope of TEMA, tubes aremanifolded together with tubesheets orU-bends

A tubesheet is a flat, circular plate drilledto allow the tubes to be inserted

U-bends are used to connect pairs oftubes together to remove the need for atubesheet at one end of the exchanger

Tubesheets / U-bends





Straight Tube / U-tube

An exchanger in which two tubesheetsare used is called a straight tubeexchanger

An exchanger with one tubesheet and U-bend returns is called a U-tubeexchanger

Channels

In order to direct the tubeside flow in andout of the tubes, a chamber is attachedto the tubesheet, called a channel

Depending on the design this may alsobe called a bonnet or waterbox

Selection of channel type is based onbalancing access requirements formaintenance against cost





Fixed / Floating Heads

Tubesheet may be fastened to the shell,or free to move relative to the shell – A tubesheet which is fastened is called a

fixed head

– A tubesheet which is free to move is called afloating head

A fixed-tubesheet exchanger has bothtubesheets fixed

A floating-head exchanger has one fixedhead and one floating

Shell Cover

In a floating head heat exchanger, aremovable cover may be incorporatedinto the shell at the floating end

Allows access to the floating headwithout disturbing the fixed head





Bundles

The tube bundle comprises:

– tubesheets / tubes / U-tubes

– baffles / support plates

– tie-rods / spacers

The tubes may be arranged for singlepass or multi-pass, using pass partition

plates in the channel(s)

Bundle Fabrication





TEMA Size and Type

TEMA Designations

The TEMA standard contains a codingsystem to allow the size andconfiguration of a heat exchanger to be

expressed in a concise manner

This description system is widelyaccepted and understood





TEMA Type Code

A three letter code is used to describethe configuration:

B E MB E M

Front head

Shell type

Rear head

TEMA Shell Types

Selection of shell type is primarily aprocess/thermal design decision

Different types create different flow pathsthrough the shell





TEMA Shell Types

Divided flow

One-pass shell Two-pass shell,longitudinal baffle

E F

J

TEMA Shell Types

Double-split flowSplit flow

Crossflow

X

H

Kettle reboiler

K

G





TEMA Front Head Types

Selection of front head type is primarily amechanical design decision

Different types provide various levels ofaccess for maintenance

Cost and pressure-integrity become

factors at higher pressure

TEMA A Type

Channel and

removable cover





TEMA B Type

Bonnet (integral cover)

TEMA C Type

Channel integral with tubesheet and removable cover





TEMA N Type

Channel integral with tubesheet and removable cover

TEMA D Type

Special High Pressure Closure





TEMA Rear Head Types

Selection of rear head type is primarily amechanical design decision

Different types provide various levels ofaccess for maintenance

Types L, M and N imply a fixed-tubesheet construction

Types P, S, T, U and W are floating headtypes (bundle free to expand relative toshell)

TEMA L, M and N Type

L M

N





TEMA U Type

TEMA P, S, T and W TypesML

P

S T

W





TEMA Size Code

A two-number code

First number is the shell id to the nearestwhole inch

Second number is the tube length to thenearest whole inch

Example: TEMA size 31-240(31” ID shell with 240” (20’) tubes)

TEMA Size Code

For U-tube exchangers, the tube length isthe length of the straight leg

For kettle reboilers, the shell diameter is

expressed as two numbers, the port IDthen the shell ID:

Length

Port IDShell ID

E.g. Size 17/43 - 192





Tube Bundle Construction

Heat Exchanger Tubes





Heat Exchanger Tubing

Welded vs Seamless Tube

Average vs Minimum Wall

U-tube bending

Tube pitches

Seamless vs Welded

Drawn from a solidbillet or forged

cylinder

Rolled from a flat stripand welded alonglongitudinal seam





Which to use?

Welded tube is cheaper and more readilyavailable

Quality of welded tube can be very high:

– continuous testing of weld

– Individual tube pressure testing

– heat treatment of weld

– no significant weld bead outside tube section

Use seamless only for lethal service, veryhigh integrity, or very high pressure

Wall Thickness Tolerance

Important to understand the differencebetween minimum wall and average walltubes

Tolerances are controlled by the ASTMmaterial spec. (e.g. SA-213 is a minimumwall spec., SA-249 is an average wallspec.)

Min. wall usually -0% +20% thickness

Av. wall usually -10% +10% thickness





U-tube Bending

The process of forming a U-bend from astraight tube will:

– thin the wall on the outside of the bend

– tend to flatten the tube on the outside of thebend

– tend to crimp the inside of the bend

– work-harden the tube material

These effects limit the diameter of thebend to a minimum value (typically 3Do)

Heat treatment may be required

U-Bend Forming

TEMA limits wall thinning to 17% for non-work hardening materials (equivalent toR = 1.5Do)

Crimping

Flattening

Thinning

R

Do





Tube Pitches

60° layout

Pt

30° layout

Pt

90° layout

Pt

45°

layout

Pt

Tube Pitch

A minimum value of Pt must bemaintained:

– To retain mechanical strength in tubesheet

– To allow any welding of tube end

– To allow cleaning outside tubes

Typical: Pt = 1.25 x Dt





Tube-to-Tubesheet Attachment

Tubes need to be attached to: – prevent interstream leakage

– transmit mechanical loads

Attachment may be by welding,expansion or both

Type of attachment affects tube pitch

requirement Attachment type has impact on ease of

re-tubing

Types of Attachment

Expansion: – Used as an attachment process providing

both sealing and strength functions

– Light expansion used to remove crevice atback face of tubesheet

Welding: – Seal weld creates a better seal than

expansion, especially at high temperatures

– Strength weld (weld strength ≥ tube strength)provides both sealing and strength

Explosive expansion and welding arealso used





Welded Only

Tubesheet

Tube

Weld

Expanded Only

Tubesheet

Tube





Welded and Expanded

Tubesheet

Tube

Weld

Back-Face Welding

Tubesheet

Tube

Weld





Tube Passes

Tubepasses

Multiple passes are used to:

– increase tubeside velocity

– reduce overall length

– allow U-tube/floating head designs

Single pass designs used to retaincounter-current flow





Pass Partitions

Pass partition

2-Pass, B-Type Head

Tubesheet

Drain hole

Weld

Gasket

Pass Lane

Pass Arrangements

4-Passquadrant

6-Passribbon-banded





Pass Arrangements

Quadrant(good for U-tubes)

Ribbon-banded(good pass lane

orientation)

1 2

4 3 1

2

3

4

5

6

1

2 3

45

6

H-banded(good tube count

distribution)

Pass Considerations

Number of passes and arrangement ismainly driven by thermal design

Limitations:

– construction of pass partitions

– thermal gradients

– effect on tubecount (total and pass-to-pass)

– gasket seating

– shellside bypassing





Baffles

Shellside Baffles

Baffles have two main purposes:

– To direct the shellside fluid in crossflow, toimprove heat transfer

– To support the tubes against sagging andvibration

Described by type, cut and pitch





Baffle Types

SingleSegmental

DoubleSegmental

Baffle Pitch and Cut

Pitch

hCut % = (h/Ds) x 100

End Space(often greater than pitch) Ds





No-Tubes-in-Window

Standard Single Segmental

No-Tubes-in-Window

Baffle Construction

Triple Segmental





Tie-Rod / Spacer

This system allows accurate alignment ofthe baffles during construction

Tie-rod Spacer tube

Baffle

Permits condenserdrainage

Better end zonedistribution

Prevents separationor stratification

Baffle Orientation

Perpendicular cut(perpendicular to nozzle centreline)

Nozzle

Baffle cut

Parallel cut(parallel to nozzle centreline)

Baffle cut

Nozzle





Baffle Selection

Selection depends on

– Pressure drop requirements

– Tube support requirements

– Heat transfer requirements

Typical Tubesheet Layout




Tubesheet Layout

Documents

Introduction to Shell and Tube Heat Exchangers.pdf