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Heat Exchangers

In almost any chemical, petrochemical or refining system, heat exchangers are used to transfer heat from one fluid to another. The most basic and the most common type of heat exchanger construction is the tube and shell type.

Pacific Engineering Pty Ltd designs and supplies shell and tube type heat exchangers based on local and international standards. We design reactors, columns, strippers, condensers, coolers, heaters, reboilers, distillation columns, falling film evaporators, finned tube air coolers or modify your existing heat exchanger and provide thermal / mechanical designs and detail drawings.

Alterations to existing heat exchangers:

We can make the following alterations to your existing exchangers:

  • redesign thermally and mechanically to meet new process conditions
  • interchange flowing media between the tube and shell side
  • design a new bundle to conform to an existing shell in-service
  • increase number of passes to control fouling
  • modify tube layout to allow cleaning
  • material change / improvement on either side

Pacific Engineering Pty Ltd can provide detailed design including themal and mechanical calculations, detail drawings, third party design verification, design registration and external fabrication inspection services for an exchanger.

Some of the recent projects:
- DBO Bundle Refurbishment for BlueScope Steel Pty Limited
- Kettle Reboiler (Clean Stem Generator) for APT Eng –1000 kg/hr, Stainless steel. Designed to TEMA & AS1210
- Ethylene Vessel for BECHTEL / STEWART TOOL CO - Australia Pacific LNG Project. Designed to ASME including Finite Element Analysis (FEA) on Headplate nozzles.

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TEMA... The Tubular Exchanger Manufacturers Association, Inc. (TEMA) is trade association of leading manufacturers of shell and tube heat exchangers and is a worldwide accepted authority on shell and tube heat exchanger design.

HEAT EXCHANGER DESIGN CONSIDERATIONS

Fluid allocation
Tube side is preferred under the following circumstances:
- Fluids which are prone to foul, corrosive or toxic
- Streams with low flow rates, very high design pressure or very high design temperature, streams with a low allowable pressure drop.

Tube layout
- Triangular pitch is better for heat transfer and surface area per unit length. Square pitch is required for shell side mechanical cleaning.
- For the identical tube pitch and flow rates, the triangular layout will have the highest heat transfer coefficient and the highest pressure drop. The square layout will have the lowest heat transfer coefficient and the lowest pressure drop.
- The triangular pitch provides a more compact arrangement, resulting in smaller shell and the strongest tubesheet for a specified shell-side flow rate.
- Square pitch simplifies cleaning and has a lower shell side pressure drop.
- Tube pitch is usually chosen so that the pitch ratio is between 1.25 & 1.5.

Tube size & length
- The most common plain tube sizes are 15.88, 19.05, and 25.40 mm outside diameters. Smaller diameter tubes yield higher heat transfer coefficients and result in a more compact exchanger. However, larger diameter tubes are easier to clean. For mechanical cleaning, the smallest practical size is 19.05 mm. For chemical cleaning, smaller sizes can be used.
- Standard tube lengths are 4877, 6096, 10972 & 12192 mm for carbon steel tubes.

Baffle cut & spacing
- Segmental baffles normally should not be closer than 20% of shell inside diameter or 50.8 mm whichever is greater.
- Baffle and support plate spacing are determined by mechanical strength and vibration.
- Baffle cuts can vary between 15% and 45%.
- Baffle spacing determines the effective shell side velocity and hence has the greatest influence on shell side pressure drop.
- To achieve good vapour distribution the vapour velocity should be as high as possible consistent with satisfying pressure drop.

 

MAJOR TEMA EXCHANGERS TYPES, THEIR ADVANTAGES & LIMITATIONS

U-Tube Exchangers (AEU, BEU):
Removable tube bundle. Suitable for high pressure / temperature and larger thermal shock applications. Does not require expansion joint. Recommended for clean process fluids and those subject to chemical removal on tube side, limited mechanical cleaning on the shell side. Requires full re-tubing (partial re-tubing is very limited). Draining the tubes is difficult when mounted in the vertical position with the tubesheet side up. Wall thickness at the u-bend is thinner than at straight portion of tubes due to bending. U-bends may require stress relieving.

Fixed Tubesheet Exchangers (AEM, BEM, NEN):
Tubesheets integral with the shell (no removable bundle). Recommended for low fouling fluids on shell side (mechanical cleaning is not possible), suitable for low and high pressure. May require expansion joint (bellows) which may limit high pressure applications on shell side. High degree of protection against contamination of streams. This type is the less costly TEMA design per square meter of heat transfer surface (provides maximum surface area) comparing removable bundle designs.

Floating Head with Backing Ring (AES, BES)
Bundle is removable. Suitable for applications requiring frequent tube-bundle removal for inspection and cleaning as well as large differential temperatures between shell and tube sides. Suitable for high pressure applications. Recommended for non-hazardous fluids. Mechanical cleaning is possible on both sides (square tube layouts). Partial re-tubing is possible. Failure of gasket or tube leakage on floating head side is not easy to detect. Shell cover, backing device and floating head cover must be removed to remove the tube bundle. More costly to hydro test tube side, as it requires test rings.

Exchangers with Externally Sealed Floating Tubesheet (AEW, BEW):
Bundle is removable. Suitable for low pressure, low temperature applications. Recommended for non-hazardous fluids. Mechanical cleaning is possible on both sides. Maximum of two tube side passes is possible which may limit to utilise the tube side allowable pressure drop.

Outside Packed Floating Head Exchangers (AEP, BEP):
Bundle is removable. Differential expansion is provided by the packing. Suitable for low pressure, low temperature applications on shell side due to its packing. Suitable for high pressure on tube side. Mechanical cleaning is possible on both sides (square tube layout). Recommended for non-hazardous fluids on shell side. Large thermal shock applications should be avoided.

Pull Through Floating Head Exchangers (AET, BET):
Bundle is removable. Suitable for low pressure, low temperature Mechanical cleaning is possible on both sides. Failure of seal or tube leakage on floating head side is not easy to detect. Recommended for non-hazardous fluids. Expensive to build comparing the other TEMA types.

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