Examples for Heat Exchanger Design

Transcription

1 for Heat Exchanger Design Lauterbach Verfahrenstechnik GmbH 1 / 2011

2 Contents Calculation 1 1. Water- Water Heat Exchanger 1 Basics...1 Task Start the WTS program Selection of basic data Visual WTS WTS Input mask Results and evaluation Optimization Calculation of pressure drop Tube sheet! Further details of the calculation Heat Exchanger with Floating Head, AET Type (pull through floating head) 14 Task...14 Input...14 Target U-Tube Heat Exchanger 14 Input...14 Target...14 Check List for Shell and Tube Heat Exchangers Recognizing the problem Rating the design...16 for Heat Exchanger Design Contents i

3 Calculation 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). In our example we will use the tube 20 X 2 Shell-Bu 12 Bend R

4 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 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 singlephase media (liquid or gas). It is necessary for condensation processes only. -2-

5 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. -3-

6 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 m/s. Number of tubes Flow velocity tube side -4-

7 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 = % Required bundle length = m Final bundle length = 3 m Heat transfer area not sufficient! Overdesign= -24.4% -5-

8 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 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 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 = m Final bundle length = 3 m Heat transfer area sufficient. Overdesign ca. 22% Flow velocity in the tubes = m/s At least 1m/s to avoid fouling -6-

9 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 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 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 = m/s -7-

10 6.3 Thermal conductivity of tubes The thermal conductivity for steel is predefined 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 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 Adjusted to required bundle length -8-

11 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! -9-

12 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. Tube-side pressure drop: p = 0.81 bar Shell-side pressure drop p = 0.21 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! -10-

13 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. -11-

14 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). -12-

15 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 12 KUDO Customer documentation -13-

16 2. Heat Exchanger with Floating Head, AET Type (pull through floating head) Task Input For an easy cleaning of the shell side a tube pitch of 45 is selected. Fouling must be considered Tube side Shell side Water Thermal oil Transcal N 180 m³/h 1100 m³/h T in = 20 C T in = 185 C T out = 90 C T out = 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 T in = 80 C T in = 20 C T out = 50 C Tubes 25 x 2 mm Tube-side flow velocity > 1 m/s to avoid fouling Target Cross-Over of outlet temperatures FN factor -14-

17 Check List for Shell and Tube Heat Exchangers 1. Recognizing the problem Criteria for selecting the correct type and determining the transfer area. Criterion Performance, Temperature profile, required area Design pressure and temperature of media, maximum conditions Vibration behaviour Fouling behaviour of media Corrosion behaviour of media Installation possibilities Safety against outside environment Easy maintenance, repair Cause of failure on production Causes of failure in upstream stages or changing the operating parameters Uncertainty of basic data Local design regulations Expenses Influence Type / number of exchangers / flow pattern limits outlet temperature Type limited by mechanical design limits. Determines e.g. maximum unsupported tube length or leads to special construction types No tubes in window Twisted Tube Bundle. Limits the life cycle. In operation cleaning possibilities or overdimensioning Consider tube pitch, angle Material selection Geometrical data of construction type Length Diameter Weight Type of gaskets, exchanger in vessel Accessibility Type of floating head Type of cover Might result in high cancellation expenses, which justify parallel exchangers in stand-by. Mechanical and thermal design Mechanical and thermal design Determines: Max. tube length, type Tube diameter Tube pitch, angle Bundle type, head type Fouling factors Tube material Determined by economical consideration of the criteria. -15-

18 2. Rating the design Criterion Create a true-to-scale sketch with baffles Desired nozzle position Hint 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? 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? Usage of pressure drop Distribution of pressure drop Check tube-side and shell-side flow distribution. Check fouling factors Check thermal conductivity of tube material Is the actual pressure drop used to optimize the heat transfer? 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. Small values of Rl und Rb caused by high bypass flows 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. 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 Twisted Tube Bundle. -16-