Thermal design is the rating: choose a flow arrangement and geometry that deliver Q at U_dirty with an acceptable LMTD, Ft, and pressure drop. It is iterative with mechanical design — a tubesheet that will not calculate, or a span that will vibrate, sends the thermal layout back. Software (HTRI, Aspen EDR, or equivalent) implements the methods; it does not replace a complete process case.
Duty cases, not a single point
Name the rating case (normal flows and temperatures), any summer/winter utility cases, start-of-run vs end-of-run if fouling or catalyst age changes duty, and turndown. Overdesign should be defined: extra area, extra duty, or a stated Uc/Ud. Condensers and reboilers need zone models (desuperheat, condensing, subcooling; boiling with a heat-flux limit). A single lumped U on a mixed-phase duty will mis-size the unit even if the arithmetic looks tidy.
Film coefficients
Tube-side h follows internal flow correlations (Dittus–Boelter / Gnielinski family for turbulent liquids; separate methods for laminar, two-phase, and condensing inside tubes). Shell-side h is dominated by cross-flow over the bundle, corrected for windows, leakage, and bypass (Delaware-type methods as implemented in rating tools). The controlling resistance is the side with the small h or the large Rf. Raising velocity on the controlling side is usually worth more area than polishing the already-high coefficient.
- Sensible liquids: velocity and viscosity dominate.
- Gases: density and allowable ΔP usually cap h; area grows.
- Condensing: vapour shear, inundation, and non-condensables matter more than a textbook steam coefficient.
- Boiling: critical heat flux and fouling on the boiling surface must be stated by process, not assumed.
Fouling as a thermal resistance
Fouling resistances are added in the U calculation so the unit still makes duty after a deposit forms. They are not a substitute for velocity, metallurgy, or cleaning access. TEMA published values are starting allowances by service class. Plant measurements from a similar service are better. Heavy crude, polymerising streams, and poorly treated cooling water can exceed table values; treated boiler feed and clean solvents may foul less. Thermal design should report both clean and dirty performance so operations know how the unit will behave on day one.
Velocity: heat transfer, erosion, and fouling
Tube-side velocity is a design variable via pass count and tube diameter. Too low: particulate and biological fouling, laminar films, poor h. Too high: erosion-corrosion, vibration of the tube, and ΔP that the pump cannot afford. Shell-side cross-flow velocity is set by baffle spacing and cut. Inlet ρv² is checked for impingement. There is no single Ohm velocity table; limits come from the fluid, MOC, and purchaser specification.
If allowable ΔP is left blank, the thermal designer will assume a value. That assumption may not match your pump. Always write shell-side and tube-side allowable ΔP on the datasheet.
Vibration is a thermal-layout check
Baffle spacing chosen only for ΔP can leave tubes unsupported across a span that TEMA class does not allow, or that fluidelastic and vortex-shedding checks fail. Gas and two-phase shell-side services are the usual victims. Thermal design must iterate spacing, cut, intermediate supports, or no-tubes-in-window layouts until vibration screening is acceptable. Treating vibration as 'mechanical will catch it later' produces a bundle that meets duty on paper and sheds tubes in service. See the dedicated article on tube vibration and baffle spacing.
Fluid allocation and pass layout
Allocation (which fluid is in the tubes) is thermal as well as mechanical: the fouling fluid you intend to rod belongs in straight tubes; the high-h utility can sit on the other side; condensing steam often prefers the shell. Pass layout is thermal: more tube passes raise h_i and ΔP_i and usually lower Ft. Shell type (E, F, J, K, X) is a thermal choice before it is a TEMA letter on the GA.
RFQ inputs for thermal design
Send complete stream properties or names and compositions, both-end temperatures or duty, flows, pressures, allowable ΔP, fouling, phase-change details (quality, bubble/dew points), and any licensor U or area constraint. Attach HTRI input if the EPC already ran it. Submit at /rfq?product=heat-exchangers. Ohm will rate from that pack for the tubular types we manufacture (fixed-tubesheet ± expansion joint, U-tube, double-pipe, kettle). We will not invent a U to make a deficient datasheet look complete.
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