Thermal Design Aspects of Shell and Tube Type Heat Exchanger
Subodh Kumar
Manager
Brahmaputra Cracker and PolymerLimited
(BCPL)

Tamagna Ghosh
Manager
Brahmaputra Cracker and Polymer Limited
(BCPL)

Roseleen Ahmed
Manager
Brahmaputra Cracker and Polymer Limited
(BCPL)

Heat transfer is the most important and applied operation in chemical and allied industries. Economy and Energy of plant operations are controlled by effective utilization of heat energy. From the viewpoint of engineering, an optimum design is obtained via maximum heat transfer rate atminimum cost. Shell and tube type is the most dominant type of heat exchanger in process plants today and in many cases it is the best or only option. On the other hand, too often shell-and tubes are selected almost "by default" because of familiar technology. The design procedure depends on using the acceptable pressure drops in order to minimize the thermal surface area for a certain service, involving discrete decision variables. Designers need to consider some additional aspects such as geometrical features and velocity conditions for an efficient design. The optimization algorithm is based on an exploration of different factors involved including the tube count where the established constraints and the investigated designers are employed to eliminate non optimal alternatives, thus the number of rating run simulations executed get reduced which also stands for time management .

Shell and tube heat exchangers (STHE) are important components in energy conversion systems in oil, Refinery, Petrochemical and chemical industries. In these industries, the heat transfer rate and the total cost of the shell and tube exchangers significantly affect system designs. Due to their compact design, these heat exchangers contain a large amount of heat transfer area and also provide a high degree of heat transfer efficiency. A good working knowledge of the mechanical features of STHEs and their influence in thermal design serves as an effective tool for the designer.

The principal components of an STHE are shell, shell cover, tubes, channel, channel cover, tube sheet, baffles and nozzles including some other components include tie-rods and spacers, pass partition plates, impingement plate, longitudinal baffle, sealing strips, supports, and foundation. For designing of any STHE, critical information like flow rates of both the streams, operating pressures, inlet and outlet temperatures, allowable pressure drops, heat duty, line size, etc. has to be collected from the process licensor. Additional factors like fouling resistance, physical properties of the fluids, and material of construction are required before designing the exchanger. Further, specific instructions given in the project specifications are also considered as inputs. Although thermal design of STHE is done by sophisticated simulation software, a good understanding of the underlying principles of exchanger design, STHE classification, tube layout, exchanger components, etc. is needed to reach at an optimal design .

TEMA Selection & Main Thermal Design Aspects

The TEMA (Tubular Exchanger Manufactures Association) Standards recognize three classes of heat exchanger construction.

Class R: for the severe requirements of petroleum processing (and usually including most large scale processing applications).
Class C: for general commercial application.
Class B: for chemical process service.

Based on the end connection and shell type, the STHE are classified using three letter codes; the first letter indicating the front-end head type, the second the shell type, and the third the rear-end head type.

Know Your Heat Duty:

The duty specified should be consistent for both the shell side and the tube side. The heat duty is calculated by the formula:

Q= UD Ao Δ Tm
where,
UD= Required Overall heat transfer coefficient.
Ao = Effective heat transfer surface based on tube outside area.
ΔΔTm= Effective mean temperature difference.

Next step is to determine U actual based on shell side and tube side film coefficient. For optimal geometry U actual should be greater than U required .

Generally Heat Exchangers are over designed and the amount of over design is arrived at using the following formula:

(Uactual - Urequired) % Over design = ------------------- X 100

(U required) For an optimal design the overdesign is maintained around 10-15%, unless there is a specific requirement from the process licensor.

Fluid Allocation and Pressure Drop Calculation:

The fluid allocation for a heat exchanger is often governed by requirements which have implications on the safety, cost, and ease of maintenance of the heat exchanger. Generally highly viscous fluids having low heat transfer rate are passed through the shell side and dirty, corrosive fluids which contain non condensable gases & solids are passed through the tube side.

























Figure 1: Flow pattern and pressure drop across shell side

Shell Side Pressure Drop

Δ P shell side= Δ P cross+ ΔΔP window + ΔΔP Nozzle +ΔΔP end

Tube side Pressure Drop
ΔP tube side= ΔP tube+ ΔP nozzle

Fouling Resistance:

Fouling resistance to heat transfer is caused by dirt, sludge, polymer, and other deposits which form on the inner and outer walls of heat exchanger tubes while in service. The values to be used in design represent the resistance expected during normal operation, assuming reasonable service time between cleanings, which are given in TEMA Standard for a variety of process services

Corrosion Allowance:

Corrosion allowances are required for the various heat exchanger components to allow for the material loss due to corrosion in service. Corrosion allowances are specified according to the severity of corrosion and the corrosion resistance property of the material.

Film Co-efficient:

The heat transfer coefficient of tube side and shell side fluid is very important and the individual heat transfer coefficients must be high enough to attain high overall heat transfer coefficient. The individual film coefficients of Shell side and Tube side fluid are taken from simulator output sheet.

Mean Metal Temperature:

The determination of the mean metal temperatures should be based on the operating temperatures of both the shell side and tube side fluids with due consideration given to such factors as the relative heat transfer coefficients of both fluids , the relative heat transfer area of parts in contact with both fluids, etc.

Shell side mean metal temp= (Shell side inlet temp + Shell side outlet temp) /

Tube side mean metal temp= (Shell side O/L skin temp + Tube side O/L skin temp)/ 2

Design Pressure:
  • Has to refer BEDP or Standard of Equipments.
  • If the exchanger is downstream of a vessel then has to consider design pressure of the associated vessel plus high liquid level static head.
  • If the exchanger is upstream of a vessel has to consider the design pressure of the associated vessel plus liquid static head plus line loss at rated flow from the exchanger to the vessel.
  • The low pressure side of the exchanger may be considered for 10/13 of high pressure side design pressure or provide PSV for tube rupture case.
  • All exchangers in steam service or subjected to vacuum in upset condition are designed for full vacuum.
Design Temperature:

The design temperature shall be based on the operating temperatures of the shell side and tube side fluids, and the Design temperature for equipment operating above ambient temperatures shall be 15 deg C higher than the maximum operating temp. The maximum operating temp shall be the highest possible temperature during operation, start up, shut down and upset conditions of the plant. For Exchanger below ambient temperature the design temp shall be the lowest operating temperature, unless process consideration requires otherwise.






















Figure 2: Tube Layout Pattern

Geometrical Design:

Tube diameter & Pitches:
Tube Diameter range = ¾" (19.05 mm) to 1" (25.4 mm)
Minimum Pitch = 1.25 times of outside diameter of tube.

Tube Pattern: Depending on type of fluid handled, pressure drop and co-efficient the There are four tube layout patterns:
triangular (30o), rotated triangular (60o), square (90o), and rotated square (45o).

Tube Passes: The number of tube passes will be optimized from a single pass, 2, 4, 6, 8, 10, 12, 14 and 16 passes. Increasing the number of passes in tube side, increases tube side velocity which will give high heat transfer coefficients and a higher pressure drop.

Tube Length: Tube length may be used for both straight and U tubes are 2438 mm, 3048 mm, 3658 mm, 4877mm, and 6096 mm. The optimum tube length to shell diameter will usually fall within the range of 5 to 10 .For -tubes the length shall be taken as the straight length from end of tube to bend tangent. Effective tube length mainly indicates the tube length to be used to calculate the heat transfer surface area but also will be used to tube side pressure drop or baffle spacing calculation.

Shell: Up to about 24" (610 mm) shells are normally constructed from standard, close tolerance, pipe; above 24" (610 mm) they are rolled from plate. The shell diameter are commonly used is 8", 10", 12", 16"…….60".

Baffle: Baffles must generally be employed on the shell -side to support the tubes, to maintain the tube spacing, and to direct the shell-side fluid across or along the tube bundle in a specified manner. Baffle cut is the segment opening height expressed as a percentage of the shell inside diameter or as a percentage of the total net free area inside the shell. The types of baffles are -
  • Segmental Baffle: Simple and most frequently used and is strongly recommended when baffle cuts between 20 to 35 % is employed. Baffle cuts are Horizontal cut (Perpendicular cut) which is used mostly in single phase & Vertical cut (Parallel cut) mostly used in condensers.
  • Double Segmental Baffle: For the same total flow rate of the shell side fluid, double segmental baffles give the pressure drop of 1/3 of the segmental baffles having the same spacing and cut.
  • Impingement Baffle: Inlet impingement plates are used to prevent the incoming fluid from directly impinging upon the tube bundle. An impingement plate should be provided depending on the value of ρV2 of the incoming fluid.

















Figure 3: Types of Baffles

Baffle Spacing

Baffle spacing is the center line-to-center line distance between adjacent baffles. Total baffle and support spacing is equal to the effective tube length (Le). Segmental baffles normally should not be spaced closer than 1/5 of the shell inside diameter, or 2" (50 mm), whichever is greater.

Vibration Analysis

Selected values on the vibration report of simulator appear with an asterisk (*).These values exceed lower safe limits. Following steps need to be followed to reduce these.























Figure 4: Nozzle location and cut orientation
























Figure 5: A Typical tube sheet layout

  • Decrease tube span.
  • Provide support plate in Bundle.
  • Provide full tube support for U tube bundle.
  • Increasing the shell diameter.
  • Install Impingement plate or increased tube pitch.
  • Vibration potential is predicted in the middle zone of an E shell, change the shell design into either an X shell or J shell.
  • Avoid too small or too large baffle cuts.
  • Use double or multiple segmental baffles to divide the flow into two or more streams.
  • If optional, No tubes in Window (NTIW).
  • Increase height under nozzle minimum ¼ of Nozzle Nominal Diameter.
Conclusion

On the basis of above study it is clear that a lot of factors need to be considered for optimized designing of a SHTE which can affect the performance of the heat exchanger. During optimization it must be noted that the advantage in one of the output parameter can affect the other parameters , which can lead to increase in initial or operating cost.

These days since the end users are expecting more cost competitive and schedule driven projects, detailed engineering consultants are expected to correctly and completely specify all the technical requirements/details in the requisite document to avoid changes at a later stage which may results in project cost and time over-run. A close coordination among various engineering disciplines, Process Licensor and Client can help in early finalization of the selection and design of the exchanger and minimize changes during the detailed engineering stage.