Proper Design of Shell and Tube Heat Exchangers
Atul Choudhari
General Manager
TATA Consulting Engineers Ltd

This paper provides an overview of key design parameters and recommended practical engineering tips for properly designing shell and tube heat exchangers.

Shell and tube heat exchangers are widely used equipment for heat transfer applications. Thermal design of heat exchangers is generally carried out using specialized softwares like HTRI, HTFS, etc. While these softwares use rigorous design techniques, user needs to carefully configure the problem, optimize the solution and analyze the design outputs. A consistent design approach is required to standardize the shell and tube heat exchanger designs.

Input Data Review:
Adequacy, consistency and completeness of process data for initiating the thermal design should be verified. Below are few important parameters that shall be reviewed firmly at the onset of thermal design.

Physical properties: It is crucial that the physical properties are available over the entire temperature range for both hot side and cold side. In case of presence of multiphase mixtures or for phase change cases, properties of relevant phases should be stated separately. The missing physical properties required to carry out the thermal design can be estimated using steady state simulation softwares. The thermal design software has a limited thermodynamic ability for mixture property predictions.

Temperature cross: For exchangers having a temperature cross, multiple shells in series would be required.

Heat release data: For reboilers, condensers or any other heat exchanger, wherein, phase change is taking place, heat release data is required as the enthalpy would be different at different locations in the exchanger. When only pure components are involved then the heat release will be linear and as the phase change occurs at the same temperature, heat release data is not required. For multicomponent mixtures, if the mixture is of 'close boiling range' type, then heat release can be assumed to be linear. For multicomponent mixtures with wide boiling ranges, the heat release data impacts the heat transfer area calculations. When the phase change is occurring over a range of pressure then heat release data at multiple pressure points would improve the thermal design accuracy. For other cases wherein the pressure range is small, heat release data at one single pressure point is adequate.

Cleaning requirements:Based on the cleaning requirements, shell type, tube layout pattern, tube diameter can be selected during thermal design. As an example, if shell side cleaning is required then use of square pitch is recommended. Similarly, for low shell side Reynolds number, tube pattern of 45 deg is preferred and for moderate to high shell side Reynolds number pattern 90 layout is preferred. If no shellside cleaning is desired then shell type can be selected as fixed tubesheet else a floating head needs to be considered.

Fluid allocation: For fluid allocation, many times the criteria below contradict with each other but broadly serves as a screening guide while allocating hot and cold side fluids to shell side and tube side.
  • High temperature streams preferred in tube side.
  • High pressure fluids in tube side.
  • Viscous liquids are better handled in shell side.
  • High allowable pressure drop streams on tube side.
  • Dirtier fluids are preferably placed in tube side.
  • More corrosive fluid through tube side is preferred.
  • Low flow rate streams are better handled in tube side.
Process design margins: The process margins are generally specified on heat duty and flow rates. Sometimes, the overdesign on heat duty is stated as 110% and overdesign on flowrates is specified as say 120%. This means that the heat exchanger to be thermally designed for 110% of specified duty and at the same time, the allowable pressure drop is not exceeded at 120% flow rate. User has to provide adequate design margin on the surface area over and above the process margins specified in the process data sheet.

Basis of design:
Generally, the basis of design is unique for each project. Preparation of basis of design is a first step during the thermal design. Following parameters, as a minimum, are addressed in basis of design:

Fouling factors: Fouling factor varies with the given service application. To maintain consistency across the thermal designs a uniform fouling factor for the given fluid should be used.

Shell type: Shell type: Shell type selection is generally based on the cleaning requirements and fouling factors. Below is the generic recommended practice. Various shell types and its TEMA designations are provided in Figure 1.

>Figure 1. TShell side flow streams

  • Floating head is used if both shell side and tube side fouling factors are more than 0.0002(hr-m2-OC/kcal).
  • Fixed tube sheet is used if the shell side fouling factor is less than 0.0002(hr-m2-OCkcal) and tube side fouling factor is greater than 0.0002 (hr-m2-OC/kcal).
  • U bundle is used if the shell side fouling factor greater than 0.0002 (hr-m2-OC/kcal) with tube side fouling factors less than 0.0002 (hr-m 2-OC/kcal)
  • For the services having both side fouling factors less than 0.0002 (hr-m2-OC/kcal) then either a fixed tube sheet or U bundle are recommended configurations.
  • For vacuum services, to keep shell side pressure drops to minimum; X shell is recommended.
  • Horizontal thermosiphon reboilers are often employed with J, G or H type of shells.
  • Floating head or U tube is sued to avoid the bellow otherwise required for thermal expansion.
Cooling water velocity: Salts in the cooling water starts precipitating due to reverse solubility at higher temperatures. Due to high film temperatures in the heat exchanger, this precipitation leads to scaling and fouling of the heat exchanger. The exchanger performance degrades due to fouling. It is thus necessary to maintain certain minimum velocity for all cooling water services. As a general practice, cooling water velocity is to be maintained at minimum 1 m/s for lowest possible steady continuous long operation flow rate. Steady and long operation flow rate does not include the start up cases but sometimes includes turndown cases. It is practically very difficult to design exchanger for minimum 1 m/s velocity when turndown is too low.

Bundle diameter: The maximum bundle diameter is limited lifting machinery (crane) specifications. In order to pull the bundle, the maximum bundle diameter restrictions are applied. The number varies from case to case basis. It is recommended that the thermal designer obtains this number before the start of the design and should be part of basis of design.

Shell Diameter: Restrictions to shell diameter apply based on maximum weight that the lifting machinery can take at the given site for erection and dismantling purpose. Thermal designer should restrict the designs within the defined constraints of maximum shell diameter. For the cases wherein the surface area requirements are larger than those defined by these constraints, then multiple shells in parallel should be used.

Tube lengths: Tube lengths are either selected in multiples of 1000 mm or as standard TEMA lengths which are basically rounded of values in feet. Basis of design should state the tube lengths selection to maintain consistency of all exchanger designs for the given project. The basis of design should also specify the maximum tube lengths permitted by the plot and layout constraints. This is generally restricted to 6 or 9 metre.

Tube diameter, pitch and pattern:Basis of design should address the considerations for selecting tube geometry including tube wall thickness. The tube wall thickness can be considered as per BWG or it can be rounded of in multiples of 0.5 mm. Tube OD is function of fouling factors and cleaning requirements. Preferred tube ODs based on fouling factor can be standardized. As a recommended practice, if the tube side fouling factor in excess of 0.0004, minimum 1" tubes are used. Basis of design should clearly state if tube ODs are based on standard in inches or standard in mm. (eg,3/4" and 1" or 20 mm, and 25 mm)

Overdesign on surface area: The design margin on surface area is required to account for inaccuracies and limitations to the empirical correlations used during rigorous thermal design. Typically, the design margins on surface area are kept at 6% to 8%.

Design Pressure and design temperature:Preferably, the low pressure side design pressure should be 10/13 times design pressure of high pressure side. The design pressure of pumped liquids should be based on (estimated) pump shut-off pressure. All the exchangers with phase change service should also be designed for full vacuum condition. Steam-out conditions should be specified separately. Many times it is likely that the steam-out design condition turns out to be governing criteria for exchanger design. All possible alternate operations of the equipment should be considered before specifying design conditions. When cold side fluid is in tubes, its design temperature should be equal to design temperature of shell side.

TEMA class: Basis of design document should specify the applicable TEMA class(R, C or B) for the exchangers in the given project. Many design parameters like tube pitch, corrosion allowance; mechanical clearances are based on the TEMA class selected.

Data Entry:
Nozzles: Nozzle size and number are required for accurate prediction of pressure drop. Specify the vapour and liquid outlet nozzle sizes separately for partial condensers. For thermosiphon reboilers, ignoring this data entry can sometimes have large impact on the resistance calculations.

Design Pressure: Though the thermal design softwares have the ability to estimate the design pressure and tube sheet thickness, shell thickness, baffles thickness, etc, it is recommended that user specifies the design pressure so that the estimations of mechanical design from the program are closer to actual designs.

Mechanical clearances: In case of rating an existing exchanger, make sure that all the mechanical tolerances as shown on fabrication drawing are inputted to the program. For new designs, these fields can be left blank. However, after the mechanical design is carried out, it is recommended to input these clearances and re-run the thermal design program for verifying exchanger performance.

Impingement plate: To avoid tube rupture due to high velocity of fluids at bundle entrance and exit, impingement plates are required. Impingement plate occupies a significant portion of the shell. The shell diameter required for given number of tube increases with presence of impingement device.

Figure 2: Exchanger Shell Type and Designations as per TEMA

Tube Layout: As a general experience, for a given shell diameter the thermal design program accommodates more number of tubes than the actual mechanical design permits for. After mechanical design is completed, it is recommended that original run is revisited and performance of the exchanger reviewed before released for vendor enquiry or for construction.

Baffle design: Center to center baffle minimum spacing should be 1/5th of the shell diameter subject to minimum 150 mm which is mechanical fabrication limitation. Generally, the baffle spacing is roundedoff in the multiple of 5 mm. Baffle cut orientation may vary based on application. Vertical cut is provided for total condensers. The baffle spacing and baffle cut design depend on stream analysis and vibration analysis. User to avoid baffles being placed under the nozzle. For low pressure drop and vacuum service applications, "no tubes in window" option can be utilized.

Review and analyze output data:
Controlling coefficient: Observe the individual shell side and tube side heat transfer coefficients and the thermal resistances from the output. Enhancement in overall heat transfer coefficient can be targeted by first identifying the controlling resistance. Check if the case is fouling controlled. The side having lower heat transfer coefficient will be controlling side. Try to enhance the governing coefficient to the possible extent by consuming most of the allowable pressure drop.

Enhancing tube side heat transfer coefficient is relatively easy. Tube diameter, tube length and number of tube passes are the variables available. Please note that velocity affects pressure drop more strongly than it affects heat transfer Coefficient. Within the permissible limits of pressure drop, try to reduce the tube count or increase the tube passes to enhance tube side Coefficient. As allowed by basis of design, reduction in tube diameter can help sometimes.

Shell style, baffle geometry, tube layout pattern and tube pitch are the variables available to enhance the shell side heat transfer co-efficient. Use multiple shells in series for a temperature cross or to increase shell velocity and heat transfer coefficient. Multiple shells in series reduce the penalty due to temperature profile distortion. Decrease the centre to center baffle spacing and reduction in shell diameter enhances the heat transfer coefficient. Baffle type selection also has impact on shell side coefficient as the leakage pattern changes from the selection. As an example, double segmental vs single segmental baffles, heat transfer co-efficient in the later case is generally found to be more.

Pressure drop: Try to consume as much of the allowable drop as possible. Any increase in velocity causes increase in heat transfer co -efficient. Thus increase in pressure drop increases heat transfer co -efficient and in turn lowers the required heat transfer area. Tube side pressure drop can be increased by decreasing tube dia. or increasing the tube passes. Shell side pressure drop can be increased by changing baffle configurations or putting multiple shells in series.

Velocity: Check that the velocity restrictions if any stated in the datasheet and/or design basis are satisfied. As a general rule of thumb, for liquids in the shell side, minimum velocity should be 0.2 m/s.

Design heat duty: Many times, user leaves the outlet temperatures or flow rates to be calculated by the program. For thermosiphon reboilers, make sure that the absolute quantity of vapours at reboiler outlet matches with process data sheet. Check that heat duty multipliers if applicable for the given case are adequately added and the exchanger is designed to meet all the operating condition specified in the process datasheet.

Stream Analysis: There are 5 types of flow streams defined for a shell and tube exchanger. See Figure 2 for details. These streams are defined as below.
  • "A" stream is tube to baffle hole leakages. The magnitude of tube -tobaffle clearance affects size of the Aleakage stream. Because the stream is thermally effective, a significant A stream does not have a large negative impact on thermal performance of the exchanger.
  • "B" stream is main cross flow. The cross flow as indicated by B -stream should be minimum 45%.
  • "C" stream is bundle to shell bypass: This clearance has a strong effect on the tube count.
  • "E" stream is baffle to shell leakage: Because the E-stream is not thermally effective, a large E-stream has a large negative impact on the exchanger's thermal performance. If user specifies a fouling layer thickness, it has no effect on this clearance or on the E-stream calculation. Being thermally inactive stream, this should be always less than 20%. Very large amount of C and E streams causes temperature profile distortion due to bypass and leakage.
  • "F" is pass partition line bypass. The F stream travels along tube pass partition lanes. Because these bypass streams can affect heat transfer and pressure drop performance, they must be modelled accurately. The F-stream, the leakage stream that flows through the pass lane partitions in multiple tube pass bundles, is only partially effective for heat transfer. Use F-stream seal rods to reduce the F -stream flow fraction.
Tube Layout: The software program generally has the ability to produce a tube layout for given configuration. Ensure that the number of tubes specified does not exceed number of tubes calculated for the given shell geometry. This layout is only suggestive and indicative. Final tube layout shall be based on mechanical design. Based on past experience, as a general rule of thumb, specify 2% to 3% of less number of tubes than programs default count to minimize design iterations.

Percent over design: This is the margin on surface area over and above the process heat duty margins. This margin is applied to account for inaccuracies, programming limitations and empirical correlations used by the program. Maintain this margin as per the design basis.

SVibrations: Vibration analysis is integral part of thermal design. For two phase services and low pressure gas applications in particular, special user attention is required to avoid exchanger vibrations. Thermal designer must ensure that the design is vibration-free. Parameters that affect various type of vibration includes tube thickness, baffle spacing, clearance under nozzle, nozzle size, bundle entrance and exit velocity. Flow induced tube vibrations and acoustic vibration are two common types of vibrations encountered in thermal design.

Flow induced vibrations: Tube unsupported span is the key to flow induced vibrations. It can be reduced gradually and no resonance would occur. Various shell types with different baffle configurations can be tried to get rid of flow induced vibrations. The suggested approach in the order of preference is as below.
  • E shell and single segmental baffles
  • E shell and double segmental baffles
  • J shell with single segmental baffles
  • J shell with double segmental baffles
  • No tubes in window configuration
  • X Shell
  • Use of rod baffles
Below is the suggested approach to avoid the vibrations.
  • Cross flow velocity should be less than 0.8 times critical velocity at all locations. Higher the cross flow velocity, higher the turbulent buffeting frequency.
  • Fluid elastic instability is characterized by tubes vibrating in whirling manner. This occurs when cross flow velocity is larger than critical velocity.
  • Vortex shedding frequency is described by Strouhal number. Tube natural square of tube unsupported span. Vortex shedding and turbulent buffeting requirements to be 0.8 times natural frequency.
  • Unsupported tube span is less than 0.8 times TEMA limit
Acoustic vibrations: Most of the problems occur to 45 degree tube layout. The use of 60 degree layout often eliminates acoustic vibrations. Acoustic vibrations can be avoided by using de-resonating baffles.

Mean metal temperature: For fixed tube sheet type of exchangers, mean metal temperatures decide the need for a bellow to take care of thermal expansion. Process engineer should analyze the failure scenarios to arrive at design mean metal temperature values. This includes, mean metal temperatures when one of the fluids is lost, start up conditions, upset conditions, Turndown requirements, etc. For Floating head or U tubes, however, it is not necessary to provide mean metal temperatures.

Concluding Remarks:
Shell and tube heat exchanger thermal design is generally carried out using specialized softwares. These softwares follow a rigorous design methodology and the technology provides the opportunity to select the exchanger configuration in a quicker manner. However, it is very vital that the design approach followed for the shell and tube exchangers on a given project is highly consistent. In addition to the optimized configuration, A uniform design methodology and design standardization helps in maintaining lesser inventory, better maintenance planning.