It is assumed the reader possesses basic knowledge in thermal design, and therefore many fundamental concepts are not explained here as this blog is not intended to be an educational website. This page is under construction, new contents will be added in the near future.
- Heat Duty:
i. Q = UAF(LMTD) use F=xx when LMTD correction factor is not readily available for S&T exchanger.
or
ii. Q = m Cp DT
- Minimum Approach Temperature
i. The smaller the approach temperature, the higher the cost of heat exchanger.
ii. One should carefully evaluate the selected utility and its corresponding approach temperature, before proceeding with sizing. The following values of approach temperature are deemed acceptable.
| Utility | Acceptable approach temperature 0C |
| Cooling Water | x |
| Air | xx |
| Steam | xx |
| Refrigerant | x |
- Rough guide for selection of heat exchanger types:
i. Double pipe: less than xx m2
ii. Multi-tube hairpin: less than xx m2
iii. Plate heat exchanger: Design P typically limited to xx barg, design T limited to xxx oC
- Recommended Tubeside Velocity
(under construction)
- Many Oil Major have imposed fouling resistance limits that precludes the use of U-bundle and fixed tubesheet.
i. U-bundle: xxx m2 oC/W (tubeside fouling resistance)
ii. Fixed tubesheet: xx m2 oC/W (shellside fouling resistance)
- Tube Layout Selection
i. Rotated square pitch: It is preferred as it induces turbulence which produces a higher heat transfer coefficient.
ii. Square pitch: To facilitate mechanical cleaning (if shellside fouling resistance > xx m2 oC/W
- HTRI flow streams
i. F stream: should be less than xx% otherwise confidence of HTRI result decreases.
(under construction)
- Pressure Drop Guidelines
i. Pressure drop across inlet & outlet nozzles should not exceed xx% of total pressure drop.
- Tube Thickness for Various Materials of Construction
i. Many Oil Major has specified minimum tube AWG for various material of construction of tube..
- Baffle Design
i. For condensers, vertical baffle cut is typically used.
ii. For single phase on shellside, horizontal baffle cut is used.
iii. For F shell, vertical baffle cut is preferred for ease of fabrication / bundle assembly.
iv. Baffle cut should not be reduced below xx% to increase shellside heat transfer coefficient. On the other hand, It should not be increased above xx% to reduce shellside pressure drop.
v. By default, HTRI limits baffle cut to xx% (min) & xx% (max) for NTIW if baffle cut is not specified. Additionally, HTRI will calculate the baffle cut such that the rhov2 in window is less than xxxx.
vi. Minimum baffle spacing of xx of shell I.D. or x”, whichever is greater.
vii. As baffle spacing is reduced, shellside pressure drop increases at a much faster rate than does the heat transfer coefficient for both laminar & turbulent flows. The optimum ratio of baffle spacing-to-shell I.D., that will result in the highest efficiency of conversion of pressure drop to heat transfer, is normally between xx to xx shell I.D.
- Air Fin Cooler Design Considerations
i. Forced draft is preferred when exit air temperature exceeds xx degC or inlet process temperature exceeds xxx degC. High exit air temperature may damage certain fan blades, bearings and V-belts if induced draft fan is used during fan-off or low-flow operation. Tube bundle of induced draft will need to be wider if fan shaft is designed to penetrate the bundle so that motor, gear and V-belt is located below the bundle.
ii. Gear drive is required when motor power is above xx kW.
iii. Design face velocity
a. In general, face velocity should be greater than x m/s.
b. The larger the number of tube rows, the higher the design face velocity. Typically ranges from x m/s to x m/s.
iv. While increasing fan pitch angle will increase the air flow, vibration level at gearbox and motor inboard bearing will correspondingly increase as the fan support structure might not be sufficiently rigid. Will need to ensure that resonance frequency of structure is higher than the excitation frequency of fan.
- Condensers Design Considerations
i. Shellside pressure drop in J- & X-shell is normally 1/x that of E-shell.
ii. Good design practice is to remove inerts & condensate at outlet.
iii. May need to consider the use of variable baffle spacing in order to promote shear-controlled flow.
iv. Use integral flash for xx condensing applications. Integral flash will achieve large xxx.
v. Use differential flash for xx condensing applications. Differential flash will achieve smaller xxx.
vi. Froude number < X: weir type condensate drainage at outlet nozzle
Froude number > Y, flooded drain pipe is predicted at outlet
- Thermosiphon Reboilers Design Considerations
i. Recommended range of vaporization is xx to xx% by weight. If fouling resistance is less than xx m2 oC/W, vaporization may be increased to xx % .
ii. Pressure drop in outlet reboiler piping should be less than xx% of the total pressure drop.
iii. P & T of heating curves required for simulation:
a. Pressure at column bottom, pressure at inlet to tube bundle, and pressure in-between the two aforementioned pressures.
b. Temperature for sub-cooled, bubble and dew points of liquid to avoid any extrapolation.
- Thermal Design Checklist (under construction):
| S/N | Description | Check Points | Remarks |
| 1 | Duty & Mass Balance | Correctness of duty (m. Cp. dT) & heat curve | |
| 2 | Number of Units | If pressure drop limit & limiting dimensions have not been reached, we can explore exchangers in series. Shells in series are mandatory if a temperature cross is observed. | |
| 3 | Overdesign Margin | Check if overdesign is required on area, duty or flow rate? Margin is necessary dur to following reasons: i. Very low fouling factor ii. Less confidence in fluid properties data. iii. Very close temperature approach iv. Eqpt is critical to unit operation. It shall be specified by Process Group. | Generally, limit overdesign margin to x%, and based on governing case. |
| 4 | Critical Pressure Data | Boiling cases require this data to calculate boiling coefficient | |
| 5 | Nozzle Sizes | To increase nozzle size if rho.v2 is too high. dP across nozzle should be limited to xx% of total. | |
| 6 | Horizontal or Vertical | Shell orientation should be advised by Process team. | |
| 7 | Loss of Flow on One Side | | If leakage cannot be tolerated, then consider U-bundle with tubes expanded in double groove & seal welded to tubesheet. |
| 8 | Bundle Type | i. Fixed tube bundle is used when shellside fluid is clean | |
| 9 | Shell Type | | |
| 10 | Channel Type | | |
| 11 | Tube Diameter | | |
| 12 | Tube Length | | |
| 13 | Tube Gauge | | |
| 14 | Tube Pitch | | |
| 15 | Baffles | | |
| 16 | Baffle Spacing | | |
| 17 | Tube Layout | | |
| 18 | Number of Tube Passes | | |
| 19 | Impingement Plate | | |
| 20 | Tie-rod, Spacer & Sealing Devices | | |
| 21 | Shell Nozzle Orientation | | |
| 22 | Condensation Cases | | |
| 23 | Reboiler Cases | | |
| 24 | Vibration | | |