Process Steam Generation

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Makeup BFW inlet

Saturated Steam

Makeup BFW inlet

Hot process fluid

Cooler process fluid



Hot process fluid

Cooler process fluid



Process Stream

ProcessSteaminlet

SaturatedSteam out

Boiler Feed WaterMakeup inlet

BFW Blowdown



12

98

143

181716151413

1110



6

63

121110987



Art Montemayor September 30, 2005

Rev: 0

A E L

B F M

C G N

N H PChannel integral with tubesheet

& removable cover.

Channel integral with tubesheet

& removable cover.

Shown: Removable Tube

TEMA DESIGNATIONS

Double split flow Outside, packed floating head

Split Flow Shell Fixed tubesheet; like "C"

stationary head.

Bundle

Stationary head.

Bonnet (Integral Cover) 2-pass shell with longitudinal

baffle

Fixed tubesheet; like "B"stationary head.

Shell TypeFront End Stationary Head Rear End Stationary Head

Channel and removable cover One-pass shell Fixed tubesheet; like "A"

Page 6 of 11FileName: 245450211.xls.ms_office

WorkSheet: TEMA Designations




Rev: 0

D J S

Conventional Front End Heads:

Aor,

B

K T

Other popular rear end head types employed:

U

W

Special, high-pressure closure

lantern ring

(No Rear Head Required)

device (split-ring)

Kettle type of reboiler Pull-through floating head

Divided shell flow

U-tube bundle design

Packed floating tubesheet with

Floating head with backing






Rev: 0Some examples of the TEMA designation for Heat Exchangers are shown below:

Front bonnet (Intergral Cover), with one-Pass Shell and a Fixed Tubesheet rear Bonnet

Fixed tubesheet heat exchanger. This is a very popular version as the heads can be removed to clean the insid

of the tubes. The front head piping must be unbolted to allow front head removal; if this is undesirable, then

this can be avoided by applying a type A front head. In that case only the cover needs to be removed. It is no

possible to mechanically clean the outside surface of the tubes as these are fixed inside the shell. Chemical

cleaning can be used in the shell side. Shown is a version with one shell pass and two tube passes. This is

probably the least expensive of the shell-and-tube designs.

This is the same type of heat exchanger as shown above, except it has only one tube pass

Channel with Removable Cover, One Pass Shell, Fixed Tubesheet Bonnet

This is almost the same type of heat exchanger as the first BEM. The removable cover allows the inside of th

tubes to be inspected and cleaned without unbolting the piping. However, as can be expected, the tradeoff is

that this convenient feature makes it more expensive.

BEM

BEM

AEM






Rev: 0The maintenance feature of having a removable tube bundles requires an exchanger as the following:

Channel and Removable Cover, One Pass Shell, Floating Head with Backing Device

A floating head heat exchanger is excellent for applications where the difference in temperature between the

hot and cold fluid causes unacceptable stresses in the axial direction, between the shell and tubes. The

floating head can move, i.e. it provides the ability to allow tube expansion in the axial direction.

Note that the bundle can not be pulled from the front end. For maintenance both the front and rear end head,

including the backing device, must be disassembled. If pulling from the front head is required a type AET

should be selected.

However, it is wise and prudent to be aware of the inherent trade-offs in this design. Note that the tube-side

fluid can leak through the internal floating head cover gasket and mix (or contaminate) the shell-side fluid.

It is very difficult -and sometimes impossible to mitigate or compensate for the internal bolts tightening the

internal bonnet to remain under constant, steady torque. Hot fluid temperatures make the bolts expand and

the result is a reduction in bolt torque and subsequent leaks through the bonnet gasket. Additionally, it is a

common and expected occurance for maintenance crews to find the internal bolts badly rusted or corroded to

the point where they have to be burned or sawed off in order to extract the "removable" tube bundle.

The chemical engineer has other options to apply when requiring mechanical expansion of a heat exchangertube bundle. Various rear head design also exist that allow for tube bundle expansion. Among these are the

popular (and inexpensive) "U" tube bundle design. A "P" and "W" rear head design will also contribute this

feature without the hazard of internal mixing (or contamination) of the two fluids.

Also, be aware that any TEMA shell and tube design with a removable tube bundle feature has - by nature - a

larger shell diameter (& increased cost) due to the need to be able to pull the rear tubesheet the length

of the exchanger's shell. A larger diameter shell can sometimes also present problems in a lower Reynolds

number (yielding a lower heat transfer) and internal by-passing of the shell fluid around the baffles (this also

reduces the effective heat transferred. All these effects eventually lead to a bigger heat exchanger (more area

and more tubes) in order to do a heat transfer operation.

AES






Rev: 0

Longitudinal Baffles - their application and inherent problems

The employment of longitudinal baffles in heat exchangers - such as the "F", "G", and "H" shell types - can

often resolve both heat transfer and fluid flow problems within the shell and tube exchanger used.

Their application can significantly increase the shell-side Reynolds Number and lead to more efficient shell-s

heat transfer coefficients with a subsequent increase in heat transfer. Additionally, these type of baffles perm

the engineer to incorporate counter-flow heat transfer. True counter-current heat transfer is as efficient

a heat transfer configuration as an engineer can obtain. In some heat recovery applications, this is highly soug

By splitting the shell-side flow, some applications can actually have a significant reduction in shell-side press

drop. This is especially true in partial vacuum process operations where a minimum of pressure drop can be

tolerated.

However, the application of longitudinal baffles should be always carefully scrutinized and used sparingly. T

are, as would be expected, some very important trade-offs involved in the application of longitudinal baffles.

Firstly, if a longitudinal baffle is a process necessity, the baffle should be seal-welded against the inner shell

wall in order to ensure that there will be no internal, by-pass leakage. This positive step negates the possibili

of having a removable tube bundle. Additionally, the welding necessity requires a minimum shell diameter

and this winds up being applicable only to relatively large streams.

By the basic need to establish effective shell-side flow around a longitudinal baffle, one has to accept the

obvious fact that a minimum of shell-side clearances can be tolerated. Once having said and applied these fa

one then has to also accept that the required, small baffle clearances mean extraordinary fabrication technique

and resultant super-human maintenance efforts to extract a removable tube bundle. In far too many actual

field cases, it has been found that the removable tube bundle with a longitudinal baffle is a non-practical devi

Field results have shown that in most cases the tube bundle has resulted in being destroyed in order to remove

This extraordinary and desperate maintenance act labels such a design as non-practical.






Rev: 0



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