Examples for Heat Exchanger Design

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Examples for Heat Exchanger Design

Lauterbach Verfahrenstechnik

GmbH

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Calculation Examples

1. Water- Water Heat Exchanger

Basics

The WTS program consists of several single modules, calculating one or more values for the design of a heat exchanger. The Ga module for example calculates the heat transfer in pipe flow and the RDV module provides the tube-side pressure drop in shell-and-tube heat exchangers.

All required input values for the calculation of a heat exchanger are concentrated on the WTS input mask, which allows full control over the calculation.

Task

A heat exchanger is to be designed with the following requirements:

Tube side Shell side

Medium: Water Water

Pressure: 4 bar 3 bar

Temperature in: 80 °C 20 °C

Temperature out: 60 °C 53 °C

Mass flow: 20 kg/s

Boundary conditions:

 Only standardized tube or shell diameters shall be used  Steel tubes shall be used

 A maximum of 3 meters for the bundle length shall not be exceeded.

 Velocity in tubes shall be at least 1 m/s, to avoid an excessive fouling in the tubes.

1. Start the WTS program

The dialog mask ‘Basic data selection’ appears. This form shows default settings for the design of a shell and tube heat exchanger with segmental baffles.

If the option ‘Default values’ is set default dimensions are used or a shell diameter with the required minimum diameter is selected from the list.

The option 'Distance Tube sheet – 1. baffle > Nozzle Diameter' and entering the nozzle diameters on the shell side, the program checks if the distance between the tube sheet and the first baffle is larger than the nozzle diameter plus a run-out length depending on the nozzle diameter. If this is not the case, the program enlarges this distance. This avoids that a baffle is located under the inlet or outlet nozzle. This can lead to a correction of the required tube length because of the adjustment in the end region.

2. Selection of basic data

First of all select shell-side and tube-side media.

2.1 Tube dimensions

Geometrical data for different tube are integrated. The user might add data of his own frequently used tubes (see manual ‘operating the program’ or press the ‘HELP’ button).

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2.2 Tube pitch

The usual tube pitch for the integrated tubes may be selected under ‘Tube pitch’. In our example we will use a pitch of 26 X 60°

2.3 Shell dimensions

Shell dimensions of different standard shells are stored in the program. Shell dimensions can be added (see manual or press the ‘HELP’ button)

If the dimensions are already known (e.g. recalculation of a heat exchanger) the according shell can be specified here. Mostly however the shell dimension is unknown when starting the calculation. Please select

‘Free Input / Design’. By choosing this option, the program selects an appropriate shell from the list of integrated shells.

The criterion for this selection is the ‘Desired tube-side velocity’, which is pre-defined with 1.8 m/s for water in the field ‘V tube-side’. The user can overwrite this value. The program now selects a shell in which the velocity is not exceeded while completely tubed. The ‘desired tube-side velocity' is seen as a limitation (avoiding of erosion and cavitation).

In our example we will choose ‘Free Input / Design’

2.4 Bundle type

‘Straight tube’ is selected.

2.5 Installation position

The installation position (horizontal or vertical) is not taken into consideration while calculating flow of single-phase media (liquid or gas). It is necessary for condensation processes only.

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2.6 Desired Tube-side velocity

If you have chosen ‘Free input / design’ for the shell, the program automatically selects a shell from the list of shells during calculation of the heat exchanger. The selection criterion is the desired flow velocity in the

tubes.

The program decides not to exceed the desired velocity in the tubes when the shell is completely tubed. The desired velocity is an upper limit for the velocity in the tubes. The velocity is limited by cavitation and erosion effects. The default desired flow velocity is 1.8 m/s for liquids and 30 m/s for gases.

2.7 Desired Shell-side velocity

The ‘Desired shell-side velocity’ is the criterion for the distance between baffles. The velocity is limited by tube vibration, cavitation and erosion. It is pre-defined with 1 m/s for water and 30 m/s for gases.

For further information press the ‘HELP’ button.

Confirm your selection with ‘OK’. The form ‘Visual WTS’ is starting up.

3. Visual WTS

The bundle type is predefined with one tube-side pass and one shell-side pass. Enter now the known values.

WTS is not limited to specific input values. In our example the shell side and tube side inlet and outlet

temperature and the tube side mass flow are given. The shell side mass flow is calculated via the heat balance. If you entered for example the ‘Absolute thermal performance W’, the two inlet temperatures and the mass flows, the program would calculate the two outlet temperatures.

After having entered all known values confirm with ‘OK’. The calculation is started and the program switches to the WTS input mask.

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4. WTS Input mask

Shell Dimensions

The heat exchanger has been calculated and the entered and calculated values are now displayed in the WTS input mask. The program selected a shell with 273 x 6.3 mm and a number of tubes of 61. This results in a tube-side velocity of 1.668 m/s.

Number of tubes

Flow velocity tube side

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5. Results and evaluation

Very important!

The program makes a difference between a required (theoretical) bundle length which is necessary to transfer the entered heat performance and the final (actual) bundle length, which is to be manufactured.

To start the calculation you must always enter the final bundle length!

By entering the final bundle length, the exchanger is recalculated and a new required bundle length is calculated if necessary. Adjust the final bundle length to the recalculated required bundle length.

The difference between these values is the reserve of the heat exchanger.

In our example the bundle length is limited to 3 m. That’s why we enter 3 m in the final bundle length. A comparison between final bundle length (3 m) and required bundle length (3.968 m) shows that the exchanger area is too small. Overdesign = - 24.4%

Required bundle length = 3.968 m Final bundle length = 3 m

 Heat transfer area not sufficient! Overdesign= -24.4%

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6. Optimization

6.1 Increasing the number of tubes

You may increase the number of tubes while performing the calculation. Either you select a bigger shell in the menu ‘Basic input’ / ‘basic data’ or you overwrite the value in the WTS mask with a bigger one.

Enter now 65 for the number of tubes and confirm with ‘ENTER’. The WTS program now tries to put

65 tubes into a shell, which is integrated in the WTS12.tab.

The program selects a shell of 323.9 x 7.1 mm. 91 tubes can be put into this shell. The required bundle length is calculated as 3.11 m, this means the exchanger is still too small.

Therefore increase the number of tubes again and enter for example ‘95’ as number of tubes. The WTS program selects a shell of 355.6 x 8 mm in which 121 tubes can fit. The required bundle length is now 2.45 m. With a final (actual) bundle length of 3 m it shows an overdesign of about 22 %. Due to increasing the number of tubes the flow velocity in the tubes is now 0.84 m/s. The exchanger does still not meet our requirements (at least 1m/s in the tubes).

Required bundle length = 2.452 m Final bundle length = 3 m

Heat transfer area sufficient. Overdesign ca. 22%

Flow velocity in the tubes =0.8407 m/s

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6.2 Changing number of tube-side passes

The WTS program is capable to calculate heat exchangers with 1, 2, 3, 4, 6 and 8 tube-side passes.

In our example the tube-side flow velocity is 0.8407 m/s. To avoid an excessive fouling in the tubes a velocity of at least 1 m/s is required. Let’s change the number of tube-side passes from 1 two 2.

Click on the field ‘number of passes (tube side), select ‘2-Passes Type 1’ and confirm with ‘ENTER’.

The exchanger is recalculated and re-dimensioned. The calculation results in a required bundle length of 2.657 m. The overdesign is 12.9 %.

2 Tube-side passes

Final bundle length = 3 m

Adjusted to required bundle length

Flow velocity in the tubes = 1.199 m/s

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6.3 Thermal conductivity of tubes

The thermal conductivity  for steel is pre-defined in WTS as 52 W/(m·K). If your tubes are made of another material, overwrite the value for the thermal conductivity with the one for your material.

Now select Stainless steel with a thermal conductivity of 15 W/(m·K).

The exchanger is recalculated again. The required bundle length increases to 3.38 m. The heat exchanger is 11.28% underdesigned!

Increase the number of tubes to 110. A new shell is selected with 406.4 x 8.8 mm. The number of tubes in this shell is 142 and the overdesign is 5.8%.

Thermal conductivity = 15 W/(m · K)

Final bundle length = 3 m

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Select the input field 'Thermal conductivity of tube material' and press F3 to get thermal conductivities of different tube materials dependent on the temperature.

Right-click the input field 'Thermal conductivity of tube material' and select 'values' to receive thermal conductivities of different materials.

6.4 Number of baffles

The program has calculated the number of baffles. The criterion for this calculation was a velocity between the baffles of approximately 1 m/s (see 2.6 Shell-side velocity)

The number of baffles shall now be decreased from 24 to 19. Overwrite the value for the number of baffles with 15.

Due to the decreased number of baffles the heat transfer coefficient decreased as well and the required bundle length increased. Please check if the final bundle length is still sufficient!

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7. Calculation of pressure drop

The shell-side and tube-side pressure drop is calculated after having entered the diameter of the nozzle at inlet and outlet. If you don’t know them yet, you may also enter the nozzle velocity.

The program calculates the nozzle diameter. This calculated nozzle diameter might be used as approximate value for a nozzle according to DIN or ANSI. If you have determined a nozzle, overwrite the calculated values in the WTS mask. The inlet and outlet velocities are recalculated.

Select DN 125 for the tube-side inlet and outlet nozzle and DN 100 for the shell-side inlet and outlet nozzle.. The pressure drops are calculated.

Shell-side pressure drop p = 0.21 bar Tube-side pressure drop: p = 0.81 bar

Tube-sideide pressure drop

The total tube-side pressure drop is a composite of:

 Pressure drop in inlet nozzle

 Pressure drop in outlet nozzle

 Pressure drop of tube inlet, tube outlet and turnaround in case of multi-pass shells. In this case it is taken

into account whether the flow is guided by a U-tube or a turnaround.

 Pressure drop by friction.

The distribution of the tube-side pressure drop is shown in the RDV module!

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Shell-side pressure drop

The total shell-side pressure drop is a composite of:

 Pressure drop in the cross-flow zone, between the edges of the baffles

 Pressure drop in both end zones below the nozzles

 Pressure drop in the window zone

 Pressure drop in the nozzles

The distribution of the shell-side pressure drop is shown in the LAE module!

If the exchanger is limited by the pressure drop (for example a gas-gas heat echanger) it is necessary to know where the pressure drop can be found to be able to optimise the exchanger.

If the pressure drop arises in the cross-flow zone, the number of baffles must be reduced. If the presure drop arises in the window zone, the height of the window must be increased.

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8. Tube sheet

The tube sheet maybe diplayed directly in the WTS module but cannot be edited graphically. Changes in the WTS input mask however effect the graphics dynamically.

Switch to the SPIE mask by clicking on the tab ‘2 SPIE’ . Here you can find further important values for the tube sheet. To display the tube sheet click on the menu item ‘Display tubesheet’ in the ‘Tube sheet’ menu.

This tube sheet can now be edited graphically. You may move or delete single tubes or complete tube rows. (See SPIE manual).

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9. Further details of the calculation

To obtain further details of the calculation switch to the individual modules by clicking the according tab strip.

1 WTS Thermal and hydraulic design of shell and tube heat exchangers

2 SPIE Design of tube sheets, determination of tube sheet data

3 H2O Properties of water

4 H2O Properties of water

5 GA Heat transfer in pipe flow

6 GH Shell side heat transfer in baffled shell-and-tube heat exchangers

7 ZELL or FN Determination of the correction factor (FN factor) for the logarithmic mean temperature

difference (LMTD) for different exchanger types

8 RBSA Tube bundle vibration analysis

9 RDV Tube side pressure drop in shell-and-tube heat exchangers

10 LAE Shell side pressure drop of shell-and-tube heat exchangers

11 WTSC CAD extension

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2. Heat Exchanger with Floating Head, AET

Type

(pull through floating head)

Task

For an easy cleaning of the shell side a tube pitch of 45° is selected. Fouling must be considered

Input

Tube side Shell side

Water Thermal oil Transcal N

180 m³/h 1100 m³/h

Tin = 20 °C Tin = 185 °C

Tout = 90 °C Tout = 160,7 °C

Fouling = 0,00018 Fouling = 0,00053 m²·K/W

Target



Calculation of the bundle-shell distance cause of the floating head according TEMA



Correction factors for the heat transfer and the pressure drop



Bypass flow, leakage flow, changing flow directions



Heat transfer correction for unequal baffle spacing at inlet and outlet



Sealing strips



Size of window, baffle distance

3. U-Tube Heat Exchanger

Input

Tube side Shell side

Water Water

m = 50 kg/s m = 50 kg/s

Tin = 80 °C Tin = 20 °C

Tout = 50 °C

Tubes 25 x 2 mm

Tube-side flow velocity > 1 m/s to avoid fouling

Target



Cross-Over of outlet temperatures

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Check List for Shell and Tube Heat Exchangers

1. Recognizing the problem

Criteria for selecting the correct type and determining the transfer area.

Criterion Influence

Performance, Temperature profile, required area

Type / number of exchangers / flow pattern limits outlet temperature Design pressure and temperature of media,

maximum conditions Type limited by mechanical design limits.

Vibration behaviour Determines e.g. maximum unsupported tube length or leads to

special construction types

 No tubes in window

 Twisted Tube Bundle.

Fouling behaviour of media Limits the life cycle.

 In operation cleaning possibilities or overdimensioning

 Consider tube pitch, angle

Corrosion behaviour of media Material selection

Installation possibilities Geometrical data of construction type

 Length  Diameter  Weight

Safety against outside environment Type of gaskets, ‘exchanger in vessel’

Easy maintenance, repair Accessibility

 Type of floating head

 Type of cover

Cause of failure on production Might result in high cancellation expenses, which justify parallel

exchangers in stand-by. Causes of failure in upstream stages or

changing the operating parameters Mechanical and thermal design

Uncertainty of basic data Mechanical and thermal design

Local design regulations Determines:

 Max. tube length, type

 Tube diameter

 Tube pitch, angle

 Bundle type, head type

 Fouling factors

 Tube material

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2. Rating the design

Criterion Hint

Create a true-to-scale sketch with baffles Optical appearance and engineering judgement

e.g. Outlet nozzles in shell on the wrong side or extreme results.

 Check for input errors

 Have you used substitute values without judging?

Desired nozzle position Number of baffles determines the nozzle position

Fraction of tube-side and shell-side heat transfer coefficient at overall heat transfer coefficient.

Is one value extremely high?

Check tube-side and shell-side flow distribution.

 Check fouling factors

 Check thermal conductivity of tube material

Usage of pressure drop Is the actual pressure drop used to optimize the heat transfer?

Distribution of pressure drop Do not decrease pressure in zones without heat transfer.

 Inlet and outlet nozzles not more than 10% of total pressure

drop.

 Check velocity in window zone.

Pressure drop ΔPF not greater than 2 x ΔPC

Shell-side pressure drop too high  Baffle pitch too small

 Tube pitch too small

 Nozzles too small

 Window zone too small

 Not enough shell-side passes

Tube-side pressure drop too high  Too many passes

 Nozzle too small

Check correction factors Rl / Rb and Fl / Fb

for bypass flow . Product Fl x Fb must be >= 0,5  High bypass flows by high pressure drop over the bundle.

 Mainly caused by low baffle distance

 A higher baffle distance may result in better performance.

Small values of Rl und Rb caused by high

bypass flows Change internal temperature profile.  Basis of CLMTD gets invalid.

 Results uncertain.

Shell-side heat transfer coefficient too small Fw = Fc * Fl * Fb * Fr should be ca. 0.6

 Check baffle pitch

 Shell-Bundle distance too high

 Use sealing strips with viscous media and big shell-bundle

distance Sudden change from

Laminar-Turbulent Turbulent -Laminar

Intermediate area is interpolated. No Nu-equation available.

 Consider Re-number at Inlet/outlet

 Check two exchangers in series

Exchanger area very big  Check heat transfer coefficient

 Check Fn factor

 Check cross over

Tube vibration by high shell-side velocities. Mostly two-phase flow (turbulent vibration) or gas flow (acoustic vibration).

Baffle pitch too high

Perform a vibration analysis from pitch > 0.7 x L b,max on. Instead of analysis:

 No tubes in window

Examples for Heat Exchanger Design