Operating the program

Start the WTS program by ‘Programs’ / ‘HEAT’ / ‘Shell and tube heat exchanger’

In this menu, you can determine the network paths for a network installation of WTS. WTS basic data (tubes, pitches, and shells), the tubesheet library and the properties module ‘S-TAB’.

(see: which are stored in the program)Tube dimensions, Tube pitches, Shell dimensions, Materials, etc.,

After starting the program, the ‘Basic data selection’ dialog box appears.

Default settings are settings for the design of a shell and tube heat exchanger with segmental baffles.

In the ‘Basic data selection’, you can specify company standards or variables of series and media included in the module.

The conception of the WTS program is that only few mouse clicks are needed to select type and basic design of the heat exchanger as well as tube-side and shell-side medium.

>> You only have to enter seven values (temperatures, pressures, mass flows resp. performance to generate a tubesheet and to design a heat exchanger.

Before starting the calculation, the type and the general layout should be known, if possible. Therefore, the ‘Basic data selection’ dialog box is provided. Furthermore, standard values for dimensions of tube and shell can be used to meet variables of series or company standards for design of the heat exchanger.

The following basic data can be specified:

Several standard tubes according to DIN standards are given. (See Tube dimensions, Tube pitches, Shell dimensions, Materials, etc., which are stored in the program )

The description 25 x 2, Shell-Bu. 12, Bend R 37 means:

·         Tube outside diameter 25 mm

·         Wall thickness 2 mm

·         Minimum distance bundle-shell 12 mm (This distance is not underrun when the shell is completely tubed.
This value can be modified in the input mask. If you have selected 'TEMA Standard' in the 'Selection of basic data', the minimum bundle-shell distance is calculated according to TEMA and cannot be overwritten anymore.

·         If you want to change the value, you need to deactivate the 'TEMA standard' option in the 'Selection of basic data' first.)

·         Bending radius for U-tube 37 mm. The description refers to the smallest bending radius possible. The tube outside diameter, the wall thickness and the tube material essentially determine this radius.

Bending radii for some tubes made from C steel and CrNi steels

If the tubes closest to the exchanger center are bent crosswise or the passes are separated by separating baffles in the bonnets you should enter the pass lane width instead of the bending radius.

The pass lane width is the clearance between the tubes of two passes. The pass lane width is calculated by the bending radius:

Pass lane width = 2 x bending radius - tube outside diameter

Bending radius or pass lane width are only required for two or multiple tube side passes.

DR 13.5 x 0.7 – 26°/0.8 means:

·         DR corrugated tube or KDR cross- corrugated tube

·         Tube outside diameter 13.5 mm

·         Wall thickness 0.7 mm

·         Corrugation angle φ = 26°

·         Corrugation depth h = 0.8 mm

With corrugated tubes, the minimum bundle-shell distance is set to 12 mm by default. This value can be modified in the input mask afterwards.

Distance and arrangement of tubes in the tube bundle.

·         The crosswise pitch, the longitudinal pitch and the pitch angle determine the arrangement of the tubes in the tube bundle.

·         The crosswise tube pitch is the center-to-center distance of two adjacent tubes in a tube row, crosswise to the flow direction.

·         The longitudinal tube pitch is the center-to-center distance of two adjacent tubes in flow direction.

·         Crosswise and longitudinal pitch are depending from the pitch angle.

·         There are four frequently used tube pitches. The most frequently used pitch is the 60° tube pitch. (see Tube dimensions, Tube pitches, Shell dimensions, Materials, etc., which are stored in the program )

32 x 60° means:

Crosswise tube pitch 32 mm

Pitch angle 60°

a) Triangle pitches 60° and 30°

With the triangle pitches you can achieve the highest tube density in the shell. This means that in a shell with a triangle tube pitch there can be 15% more tubes than in a shell with a square tube pitch. Additionally, with triangle pitches the heat transfer is better than with square pitches. This means the same area has a better thermal performance than with a square tube pitch.

Disadvantages of triangle pitches:

No mechanical cleansing of the outer side of the tubes is possible and the pressure drop is higher than with square pitches.

b)  Square pitches 45° and 90°

A square pitch should be used for all heat exchangers with expected greater shell-side fouling.

The 45° pitch leads to a better heat transfer with laminar flow due to better turbulences. If the pressure drop is the limiting element of the heat exchanger the 90° pitch should be used for turbulent flow.

Conversion of pitches

With the 60° pitch, the crosswise tube pitch should be about 1.25 to 1.5 times the tube outside diameter.

By means of using the 'conversion of tube pitch' the crosswise pitch of the according 30°, 45° und 90° pitch can be calculated.

Here you may select the tube material and the according thermal conductivity. The unit of the thermal conductivity is [W/(m·K)].

To save a new material and its thermal conductivity, simply enter the data in the fields 'Tube material' and 'Thermal conductivity' and then press the 'Save' button. (See Tube dimensions, Tube pitches, Shell dimensions, Materials, etc., which are stored in the program )

Under shell diameter, many shells are predefined in accordance to DIN standards 2448 and 2458 for seamless and welded tubes. (See Tube dimensions, Tube pitches, Shell dimensions, Materials, etc., which are stored in the program )

273 x 6.3 Dist. Baffles 100 means:

·         Outside shell diameter 273 mm

·         Wall thickness 6.3 mm

·         Minimum baffle pitch 100 mm (This pitch is not underrun when the heat exchanger is designed. You can modify this value in the input mask afterwards.)

>> The minimum baffle pitch should not under-run 20% of the shell diameter with a minimum of 50 mm.

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 considers not exceeding 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 shell may be changed by selecting either a new shell in the ‚basic data selection‘ or by entering the wall thickness and the shell outside diameter in the WTS input mask. You can also enter the number of tubes in the WTS input mask. The program selects the shell from the database, in which all the tubes fit in. As the shell is always filled completely, the entered number of tubes might not be the same as the calculated number of tubes.

To make the program select the next larger shell, it is sufficient to increase the number of tubes slightly. Tod have a smaller shell, you have to decrease the number of tubes significantly.

If the option Default values is deactivated and you enter a number of tubes, the program determines the shell that is perfectly sized to fit the number of tubes you have entered. Usually this is not a standard shell; this means it has to be custom made.

By the bundle type, you define the design of the tube-side end head. Together with the option ‘TEMA standard’, the calculation of the tubesheet is adjusted to the values of TEMA standard.

The installation position is important only for condensation processes because due to the type of installation of the heat exchanger different correlations have to be used for the thermal calculation.

The desired velocity in the tubes determines the selection of the shell diameter if ‘Free input / design’ has been chosen under shell diameter.

The program tries not to exceed the desired tube-side velocity and selects an appropriate shell from the list of shells under ‘shell diameter’.

If a shell has been selected under ‘shell diameter’ the tube-side velocity is fixed so that indicating a desired tube-side velocity makes no sense and is deactivated.

The user can modify the value for the desired tube-side velocity at any time by mouse-click on the value field.

Typical values for tube-side velocity:

(From: Walter Wagner, Wärmeübertrager)

The velocity is limited due to cavitation and erosion.

To avoid erosion at the inlet and outlet region, the product ρ·v² should also be put into consideration. This dynamic pressure considers also the influence of the fluid mass

The desired shell-side velocity serves for determination of the distance of the segmental baffles. If the option ‘TEMA standard’ has been selected the resulting value is checked according to TEMA standard and corrected if necessary.

The user can modify the value for the desired velocity in the tubes at any time by mouse-click on the value field.

Typical values for shell-side velocity:

The velocity is limited due to vibration, cavitation and erosion.

To avoid erosion at the inlet and outlet region, the product ρ·v² should also be put into consideration. This dynamic pressure considers also the influence of the fluid mass

¹⁾ Velocity means velocity in nozzle

²⁾ Velocity means velocity in first tube row

For all other gases and vapors, including all nominally saturated vapors and for liquid vapor mixtures, impingement protection is required.

The tube-side and shell-side media are selected by mouse-click in the pull-down-menu. The physical state of the media ‘liquid, gas or condensation’ which is important for calculation of the heat transfer is set automatically. The values for the desired tube-side and shell-side velocities are adjusted to the physical state.

If ‘Free input‘, ‘coolants’, ‘Proper’ and ‘S-TAB’ (property tables created by the user) have been selected the physical state has to be indicated by the user. Then, the values for the desired tube-side and shell-side velocities are adjusted to the physical state.

If ‘Free input‘ has been selected the properties have to be entered manually (see section ‘Manual input of properties‘).

The basic WTS package provides several property modules for single substances:

This list can be extended by further property modules, which are not included in the basic package. The material data compiler PROPER (not included in the basic package) allows free selection of the components and their mixtures included in the library.

Furthermore, the free input of properties is possible with the S-DAT module. If both media have been selected with the menu item ‘Basic data‘ the physical properties are calculated automatically by the corresponding modules (e.g., water, air, natural gas). The humid gases are an exception because their properties must be determined with the HX module (not included in the basic package).

The option ‘TEMA Standard’ ensures that the dimensions specified according to TEMA are kept in the calculation. The TEMA module is activated automatically and dimensions according to the TEMA code appear in the WTS user interface. For example, by selecting the bundle type the bundle-shell distance is determined according to the TEMA code.

By default, WTS calculates segmental baffles. The window size should range from 15% to 40% of the shell diameter.

The option 'No-tubes-in-window' creates a tubesheet without tubes in the window zones. (See Segmental baffles)

Application:

·         Avoiding of tube vibrations.

The unsupported tube length is reduced to the baffle distance. The unsupported tube length in the window zone is twice the baffle distance.

·         Significant reduction of pressure drop in window zones

With the option: ‘Disk/doughnut baffles’, shell-and-tube heat exchangers with disks and doughnuts as baffles can be calculated. The hole diameter of the doughnut and the diameter of the disk have to be specified in the WTS input mask.

The options 'No tubes around disk' and/or ‘No tubes in doughnut’ create a tubesheet without tubes around the disk zone and/or in the doughnut zone. (See Disk / doughnut baffles)

Application:

·         Avoiding of tube vibrations.

The unsupported tube length is reduced to the baffle distance. The unsupported tube length in the window zone is twice the baffle distance.

·         Significant reduction of pressure los in window zones

With the option: ‘Shell without baffles’, shell-and-tube heat exchangers without baffles in the shell region can be calculated. In contrary to 'double-pipe heat exchangers' consisting of an external and only one internal tube, a tube bundle is presumed.

Helical baffles have the following advantages over segmental baffles:

·         Improved shell-side haeat transfer with the same pressure loss,

·         lower pressure loss with the same shell-side mass flow,

·         smaller bypass and possibly smaller leakage flows as a result of the low pressure loss,

·         less shell side sedimentation as a result of the reduced dead spaces and

·         less flow-induced vibrations.

The disadvantages of using helical baffles are the complicated manufacturing process and the higher manufacturing costs. This is relativised under certain circumstances when additive manufacturing methods are used.

Compared to the segmental baffles and the Discontinuous helical baffles, the continuous helical baffles have the highest "heat transfer/pressure drop" ratio; the disadvantage is the complicated manufacturing process. In addition, the continuous helical baffles cannot be manufactured up to the centre line of the shell. A central tube is more often used in the centre of the shell space.

 
Schematic diagram of a shell-and-tube heat exchanger with continuous helical baffles and with a central tube

A disadvantage of the simple discontinuous helical baffles is the presence of triangular gaps between the adjacent baffles (see figure), which lead to additional leakage flow between the helical elements. In some designs, these gaps are covered with plates; this improves heat transfer, but at the same time increases pressure loss and manufacturing costs.

 
Schematic diagram of a shell-and-tube heat exchanger with simple discontinuous helical baffles

Rod baffles can be used to reduce flow-induced vibrations in shell-and-tube heat exchangers without baffles. In addition to the advantage of supporting the tubes, rod baffles have the further advantages that they improve the heat transfer in the shell space (not to the same extent as segment baffles) and that the pressure loss on the shell side is smaller compared to that with segmental baffles.

 
Rod baffles in a tube bundle heat exchanger with square tube pitch.

With the option: ‘Internally finned tubes’, heat exchangers with internal longitudinal or helical fins can be calculated. The number and the geometry of the tubes as well as the angle of gradient have to be specified. This angle is 0° for longitudinal fins and for helical fins, it is limited to 30°.

The option ‘Tubes with inserts’ allows the calculation of heat exchangers with spiral inserts (twisted tape) in the tubes. During calculation, WTS automatically switches to the TWIS module. The height, thickness and pitch of the spiral insert have to be entered there.

With the option: ‘Corrugated tubes’, heat transfer and pressure loss for flow in single or crosswise corrugated tubes can be calculated. 21 different types of tubes of Messrs. HDE GmbH, Menden, with diameters ranging from 13.5 to 28 mm can be selected in a pull-down-menu after clicking the option ‘Corrugated tubes’.

With the option ‘Outside low finned tubes’ heat exchangers which have tubes with finned surfaces can be calculated. After setting this option in the WTS input mask you have to enter the fin diameter, pitch and thickness. The calculation is designed for fins with small cross section (low fin).

The tubes are characterized with ca. 630 - 1000 fins / m (16 - 26 fins / in) and a fin height of 1 to 2 mm. The increase of surface compared to a straight tube is about 200 % - 300 %.

>> Those finned tubes should not be used for shell-side condensation.

With the option ‘Outside longitudinally finned tubes” heat exchangers with longitudinal finned tubes can be calculated. The option ‘Shell without baffles’ is then automatically set. After setting this option the geometry and the thermal conductivity of the fins can be entered into the WTS input mask.

The option ‘plate fin-and-tube’ allows the calculation of plate-fin-and-tube heat exchangers. The thermal conductivity, thickness and pitch of the fins have to be entered into the WTS input mask.

With the option: ‘Double-pipe heat exchanger’, a heat exchanger consisting of an internal and an external tube can be calculated.  The internal tube and the shell tube can be selected in a selection box on the left hand side of the basic data selection. The values can be changed in the WTS input mask at any time.

With the button ‘Tubesheet library‘, the values of a tube area created by yourself can be transferred from your tubesheet library into the menu item ‘Basic data‘. After pressing this button, a table is displayed where you have to mark the selected tubesheet by mouse-click.

Transferring tubesheet data

To transfer the values of the tubesheet click ‘Load Tubesheet’. With ‘Finish editing (ESC)’ you return to the ‘Basic data‘ menu. Variables selected before such as diameters of tube and shell are overwritten by the data transferred.

>> Please note that creating, editing and saving of the tubesheet in the tubesheet library is only possible in the SPIE module.

When your selection of basic data is completed, confirm your selection by ‘OK’. The next selection box appears:

By mouse-click on the respective button under ‘Bundle type’ you determine number and arrangement of the tube-side passes.

Under ‘Shell type’, you determine the number of shell-side passes (using a longitudinal baffle).

Already here, it is possible to specify the system data (temperatures, pressures, mass flows resp. thermal performance). These values can be entered in the WTS basic mask later on.

Selection of bundle and shell type

Confirm your selection by ‘OK’.  The WTS basic mask is displayed.

TIP: Conversion of units

A click on the unit or a double click on the input field the conversion of units is activated.

Enter the values in the unit you have at hand e.g. mass flow in kg/h. The value is immediately converted in all other displayed units.

The unit with the activated checkbox is displayed on the mask and will be used for the documentation.

Please confirm your input with 'ENTER'.

Default:  Calculation of cocurrent flow heat exchanger is only performed for:

·  Heat exchangers with one tube-side pass and one shell-side pass (1,1 heat exchanger)

·  Heat exchangers with two tube-side passes and two shell-side passes (2,2 heat exchanger).

Countercurrent flow

Heat exchangers with counter current flow have a higher thermal efficiency than cocurrent flow heat exchangers.

In countercurrent flow heat exchangers it is possible, that the outlet temperature of the cold medium exceeds the outlet temperature of the hot medium.

This is called  'Temperature Cross Over'

Idealized temperature profile in a countercurrent flow heat exchanger with temperature cross-over

Cocurrent flow

In cocurrent flow heat exchangers the outlet temperatures of the cold and the hot medium approach asymptotically.

Idealized temperature profile in a pure cocurrent flow heat exchanger (e.g. double pipe)

The cocurrent flow does not allow a switching of temperatures and the thermal efficiency is lower than in countercurrent flow. However, there are some significant characteristics of cocurrent flow heat exchangers:

1.     Cocurrent flow creates a more constant wall temperature than the countercurrent flow.

2.     The maximum wall temperature is lower than in countercurrent flow.

3.     This avoids unintended over or under temperatures. This is most important for food processing where the temperature of the medium has to stay within a certain range to avoid protein coagulation.

On the WTS user interface the user controls the run of the calculation. From here it is possible to switch to any other module.

The WTS user surface contains all input variables required to calculate a heat exchanger. These input values also appear in the input masks of the selected chapters. They can be entered or modified there as well because all chapters are linked with each other.

To switch to the user interfaces of the single modules click the according symbol.

WTS provides a special menu for basic inputs which is in the menu bar of the input mask.

With this menu item you return to the ‘Selection of basic data’ dialog box to change the tube, tube pitch or to select another shell.

For output of specification sheets to MS EXCEL click the menu item ‘Specification‘. MS EXCEL must be installed before activating this item.

This file is provided in the selected language German, English and French - if purchased. Switch to the according language in the 'Language'  menu of the WTS program.

The KUDO module creates an abridged documentation with the performance characteristics and only a few geometrical data like outside diameter of the shell and tube length. Geometrical data documentation is reduced to outside diameter of the shell.

If a tubesheet is loaded from the tubesheet library, it is always locked. This means, the geometrical variables of the heat exchanger are fixed and it can be neither overwritten in the input mask nor recalculated by the program. The fields concerned are marked in löight grey on the input mask. If modification of such variables is desired click the menu item ‘Tubesheet lock/unlock’ to unlock the tubesheet for editing.

The ‘Default values’ are activated if this menu item is ticked. Click ‘default values’ to deactivate the option or to re-activate it. If the option is activated WTS uses standard dimensions and default values for the design of heat exchanger.

For example, if the user enters a new number of tubes during the optimization, WTS selects the shell which just has enough space for this number of tubes.

If the option is deactivated and a new number of tubes is entered, WTS calculates the exact inside shell diameter resulting from this number of tubes.

If the option is activated again, the heat exchanger is automatically recalculated. WTS uses again standard dimensions. (See Tube dimensions, Tube pitches, Shell dimensions, Materials, etc., which are stored in the program )

The tubesheet is displayed in the graphics window. Modifications in the WTS input mask are dynamically shown in the graphics.

If the heat transfer coefficient in the tubes (alpha i) is known from other sources (such as measurements) and therefore doesn't need to be calculated by WTS, this value can be entered into the field ‘Heat transfer coefficient (inside)’ on the WTS input mask after clicking the option ‘Set alpha tube-side’.

In this case, the user interface ‘Clear value of variable’ is not displayed, i.e. no recalculation to a dependent variable like e.g. the tube diameter, the number of tubes, the flow velocity etc. is done. If you click again the option ‘Set alpha tube-side’ the heat transfer coefficient in the tubes is calculated by WTS.

If the heat transfer coefficient around the tubes (alpha a) is known from other sources (such as measurements) and therefore doesn't need to be calculated by WTS, this value can be entered into the field ‘Heat transfer coefficient (outside)’ on the WTS input mask after clicking the option ‘Set alpha shell-side’.

In this case, the user interface ‘Clear value of variable’ is not displayed, i.e. no recalculation to a dependent variable like e.g. the distance between the baffles, the number of baffles etc. is done. If you click again the option ‘Set alpha shell-side’ the heat transfer coefficient around the tubes is calculated by WTS.

Before entering the shell nozzle the inlet baffle spacing is set equal to the baffle spacing. After having entered the nozzle diameters, the program checks if the distance tubesheet to first baffle is greater than the nozzle diameter plus a run-out length depending on nozzle diameter and shell diameter. If this is the case, the inlet baffle spacing is increased automatically.

Thus, you can avoid that the first or the last baffle is below the inlet or outlet nozzle. The increase of the distance is considered for calculating the heat transfer coefficient. This may result in a correction of the required length or of the overdesign.

You can also enter the inlet baffle spacing as long as it is greater than the baffle spacing or greater than the nozzle diameter plus a run-out length. Smaller values are not accepted by default!

If you activate the option ‚Give inlet spacing‘, every positive value is accepted even if it is smaller than the nozzle diameter. Then, the baffle spacing is always calculated and cannot be entered (the input field is locked). The baffle spacing can then be changed by entering the number of baffles.

If ‘Free input‘ has been selected in ‘Basic data‘ for one of the media, the properties of this medium have to be entered manually. This is possible with the S-DAT module. After entering the inlet and outlet temperature and the pressure on the WTS mask the S-DAT module is activated. Pressure and mean temperature have already been transferred.

Required data:

Liquids and gases (single-phase media)

For calculating the heat transfer and the pressure drop, properties at mean temperature and at wall temperature are required. At first start of S-DAT, the wall temperature is unknown. Please enter only the properties at mean temperature at first. As soon as the data input is complete, the program switches automatically back to the WTS mask. After the wall temperature has been calculated please switch to the S-DAT module by clicking on the according tab strip and complete the properties at wall temperature. If you do not enter the properties at wall temperature, the values are replaced by the values at mean temperature for calculating the pressure drop and the heat transfer. Then the accuracy of the calculation decreases. Especially with viscous media like oils greater errors might occur.

Condensation of pure substances

When condensing pure substances, the properties of the vapour and the properties of the condensate at condensation temperature are required.

As soon as the data input is complete, the program switches automatically back to the WTS mask.

If you have selected ‘Free input’ for both exchanger sides two S-DAT modules are created.

Please note:

Gas preheaters heat natural gas to avoid a temperature decrease under a given limit after the throttling.

The additional module JTHO in the WTS program allows you to calculate the temperature of the natural gas in the tubes before the throttle if you provide the temperature after the throttle. This shows you the outlet temperature of the natural gas.

You need to provide the pressures before and after the throttle and the temperature after the throttle in the JTHO module.

If you have selected ‘Natural Gas (Free Input)’, you also need to specify the concentrations of the gas components.

WTS input mask

The first value block contains the information about the quantities, pressures and temperatures of both media. The values can be entered in any order.

In most cases the mass flows and temperatures are already known, which means that the heat balance is fixed. However, the exact temperatures or quantities are specified only by the properties used in the program, i.e. the last temperature or quantity which remains to be entered is calculated by the program to complete the heat balance.

It is recommended to specify first one of the two media by the mass flow and the temperatures. As the heat balance is already defined by the first medium, the temperatures and the mass flow of the second medium can't be selected freely. As the temperatures are most often the more important target variables, they should be entered now. The balance is completed by the calculation of the second mass flow.

Fouling resistances for water in shell and tube heat exchangers (m²K/W)

© Springer Verlag Berlin Heidelberg 2013

© Springer Verlag Berlin Heidelberg 2013

© Springer Verlag Berlin Heidelberg 2013

© Springer Verlag Berlin Heidelberg 2006

Humid gases are gas mixtures consisting of an inert gas and a condensing component, e.g. humid air
(air + steam). When humid air is cooled down, water is condensing below dew point temperature. The released condensation heat has to be considered for the heat balance. In WTS the heat balance and the properties for cooling humid gases are calculated in the HX module (not included in the WTS basic package). 

After having entered the mass flow, the pressure and the temperatures, the program automatically switches to the HX module and the input mask for entering the vapour and the inert gas is displayed. (See 'Manual for properties modules', section 'HX module').

If you select air, carbon dioxide or nitrogen as inert gas and water steam as vapour, the program switches back to WTS. No further entry is necessary.

If you select 'Free input' for the inert gas, the program switches to the HX input mask.

Enter here for the inert gas:

·         The 'Molar mass' or the 'Specific gas constant'

·         The thermal conductivity and the dynamic viscosity

·         The 'Specific heat capacities at state 1 and 2

 As soon as the data input is complete, the program switches automatically back to the WTS mask.

If you have selected 'Free input' for the vapour and having confirmed by pressing the OK button, the mask 'Antoine constants' is displayed.

The vapour pressure of the condensing component is determined by an Antoine equation in the HX module. If you know the Antoine constants of the vapour, just enter them into the according field in the upper part of the mask. If you don't know the Antoine constants, enter three pairs of values in the lower part of the mask. After pressing the button ‘Calculate Antoine constants’, the constants are determined.

With the end button the program transfers the constants and switches back to the HX input mask

Enter now the molecular weight of the vapour, the evaporation enthalpy, the reference temperature of the evaporation enthalpy and the specific heat capacities of the vapour and the liquid.

After having entered all values, the program switches to WTS.

If no standard dimensions, i.e. no default variables for the tube-side and shell-side dimensions have been specified, all data required for the calculation of the tube area have to be entered. For this the second value block in the WTS input / output mask is provided.

The tubesheet is generated automatically for 1, 2, 3, 4, 6 or 8 tube-side passes and 1 or 2 shell side passes.

If two shell-side and four tube-side passes are specified the program generates a crosswise divided tubesheet with an exactly equal number of tubes in all passes.

Usually, heat exchangers are designed based on fixed standard dimensions. This applies e.g. to the shell diameter, the tube pitch and tube dimensions, the type of the tube-side head and other values specified by a standard (e.g. TEMA).

Additionally, the tube-side and shell-side velocities have to be considered. The shell-side velocity and the TEMA standard determine the baffle pitch. The input of these values into the menu item ‘Basic data‘ of the ‘Basic input‘ menu leads to an exchanger designed according to industrial standards.

Immediately after specification of the thermal performance the calculation of the exchanger starts.

By default, WTS uses shells which are saved in the data base and can be selected in the ‘Selection of basic data’ form.

You can also enter the shell outside diameter and the wall thickness in the WTS input mask. The shell inside diameter is then calculated. The input field for the shell inside diameter is locked for input.

If you deactivate the ‘Default values’ option in the basic menu and then enter the number of tubes, the program calculates the shell inside diameter which has exactly space for the entered number of tubes. In this case the shell outside diameter is calculated. The input field for the shell inside diameter is locked for input.

The calculation ends as soon as the length of the exchanger is determined. This is the required length. The required length is the theoretical length which is necessary to transfer the required performance.. Now enter the standard length (final length). With this length, the exchanger is calculated again and a new required length is determined. The difference between the two lengths shows the ‘reserve’ of the design.

>> Important:  You always have to enter the final bundle length!

Usually, the difference between final size and required size is expressed by a ‘substitute fouling rate’. But this is not very specific. The ‘length reserve’ is directly proportional to the area and represents a tangible measure of the reserve.

Pressure drop:

After you have entered the diameter of the inlet and the outlet nozzle and the final bundle length, the tube-side and shell-side pressure drop is being calculated.

The calculation can be directed by modification of the corresponding variables. The quality of the calculation can be enhanced e.g. by a ‘parameter study’ informing how the exchanger responds when the process parameters are modified.

Pressure drop:

After you have entered the diameter of the inlet and the outlet nozzle and the final bundle length, the tube-side and shell-side pressure drop is being calculated.

For shell-side condensation, NO pressure drop is calculated!

The WTS program can calculate a condenser with condensation of any pure substance.

If the vapour enters the condenser superheated and the condensate leaves the condenser subcooled, the condenser has to be divided into three zones. The desuperheating zone, the condensing zone and the subcooling zone. These zones have to be calculated separately with the WTS program. The complete transfer area is the sum of the heat transfer areas of the single zones.

Please note:

A water steam condenser with desuperheating zone, condensing zone and subcooling zone can be calculated with the KOND program.

If ‘Saturated steam’ or ‘NH3 condenser’ has been selected in the ‘Basic data selection’ dialog box as condensing medium, a condensation of saturated steam (water steam) resp. ammonia is presumed and WTS selects the condensation set-up to calculate the heat transfer. The properties for the condensing medium (water or ammonia) are calculated automatically.

For any pure substance to be condensed ‘Free input‘ has to be selected for the condensing medium and ‘condensation’ for the physical state in the ‘Basic data selection’.

In the Visual WTS form or in the WTS input mask the condensation pressure as well as the condensation temperature have to be entered.

Then the program switches to the S-DAT module. Enter the physical properties of the condensate (boiling  liquid) into the upper part and the physical properties of the vapour plus the heat of evaporation in the bottom part of the mask.

As soon as your data input is complete the program switches back to the WTS input mask.

For shell-side condensation the shell-side pressure drop is not calculated!

Condensation

>> of pure substances in  WTS, KOND and VERD

The condensation of pure substances in WTS, KOND and VERD has been revised completely. The new calculation approaches of the VDI Heat Atlas 11th edition 2013 for condensation of pure vapors in and at vertical and horizontal tubes have been implemented into the programs WTS, KOND and VERD. Additionally now the pressure drop is calculated when condensing in or at vertical and horizontal tubes.

>> in vertical and horizontal tubes (tube-side condensation)

For condensation in vertical and horizontal tubes, the calculation approaches for flowing vapor of the VDI Heat Atlas 11th edition are used for calculating the heat transfer and the pressure drop.  Considered is the increasing film thickness during the condensation as well as the interaction of the gas and liquid phase caused by the shear stress at the film surface, which has a great effect on the heat transfer in the condensate film. As the shear stress decreases in the course of the condensation, the condenser is divided into 10 cells, where 10% of the vapor condenses in each cell. For every cell, the mean heat transfer coefficient, the pressure drop and the required condensation area are calculated and listed in the DOKU module. On the WTS input mask, the total pressure drop, the total required area and an arithmetically averaged heat transfer coefficient is shown.

>> at vertical tubes (shell-side condensation)

For condensation at vertical tubes, the calculation approaches for stationary vapor of the VDI Heat Atlas 11th edition are used. As the film thickness increases over the condensation process, which diminishes the heat transfer, the condenser is also divided into 10 cells, where 10% of the vapor condenses. For every cell, the mean heat transfer coefficient and the required condensation area are calculated and listed in the DOKU module. In the WTS input mask the total required area and an arithmetically averaged heat transfer coefficient is shown.

>> at horizontal tubes (shell-side condensation)

For condensation at vertical tubes, the calculation approaches for stationary vapor of the VDI Heat Atlas 11th edition are used as well. The condenser is not divided into 10 cells.  The increase of the film thickness by draining of the condensate from the upper tubes to the lower tubes and therefor the decrease of the heat transfer in the lower tube rows is considered by a correction factor of the Nu number in the module J1.2.

Nu = Nu_{single tube} · nⁱ

n = number of tube rows

i = -1/6

Comment from VDI Heat Atlas

The experimentally determined exponents for Eq. (42) range from i = –1/6 to 0. For the practical calculation of heat transfer coefficients, the value i = –1/6 is recommended.

Value i = 0 means no correction

Please Note

By using the new approaches for condensation of pure vapors of the VDI Heat Atlas 11th edition, there might occur results differing from the results of older LV program versions when designing condensers with the new program version.   

Depending on the case of application, the condensation area might increase or decrease a little.