A shell-and-tube heat exchanger is two pressure circuits sharing a tube wall. Tube-side fluid enters a channel (front head), is distributed into the tubes, and leaves through the same channel or a rear head depending on pass count. Shell-side fluid enters a nozzle on the cylinder, is forced back and forth across the tubes by baffles, and leaves at the opposite nozzle. Product detail for the configuration Ohm manufactures is at /products/heat-exchangers.
Tube-side flow path
The channel is partitioned so that fluid makes one or more passes through the bundle. One pass: fluid enters one end and leaves the other (or, in a U-tube, returns through the other leg of the U). Two, four, or six passes fold the tube-side path in the channel with pass-partition plates. Each extra pass raises velocity, film coefficient, and pressure drop. Straight tubes with a Type A removable cover can be inspected and rodded from the channel; a Type B bonnet is cheaper but the piping or the bonnet must come off to see the tubesheet.
Shell-side flow and baffles
Segmental baffles are plates with a cut (commonly 20–25% of shell diameter, set by the rating). Fluid crosses the bundle, turns in the window, and crosses again. That cross-flow is what produces most of the shell-side heat transfer — and most of the shell-side ΔP. Leakage streams (tube-to-baffle holes, bundle-to-shell bypass, pass-lane leakage) steal effectiveness if sealing strips and TEMA clearances are ignored.
Baffle spacing is not a default pitch. Closer spacing raises velocity and ΔP and shortens unsupported tube span (vibration). Wider spacing saves ΔP and can put the bundle into a vibration or settling-fouling regime. Double-segmental, helical, or no-tubes-in-window layouts exist for specific hydraulic and vibration cases; they are specified, not assumed.
Passes: tube side, shell side, and temperature cross
An E-shell is one shell pass: fluid travels the length of the shell once (zig-zagging across baffles). Multiple tube passes in an E-shell create a mix of co-current and counter-current zones; the LMTD correction factor Ft drops, and a temperature cross may become impossible in one shell. Two E-shells in series, or an F-shell with a longitudinal baffle, restore counter-current behaviour when the process needs it. Do not ask for a temperature cross in a single multi-pass E-shell and expect the rating to close.
| Arrangement | What it does | Watch for |
|---|---|---|
| 1 shell pass / 1 tube pass | True counter-current if nozzles are opposed | Needs two tubesheets or a U with one pass per leg |
| 1 shell pass / 2+ tube passes | Common; Ft < 1 | Temperature cross may force series shells |
| F-shell (longitudinal baffle) | Two shell passes in one cylinder | Baffle leakage if the longitudinal seal is poor |
| J / G / H / X shells | Split, divided, or cross flow | Low ΔP condensers and special duties — not a default cooler |
LMTD, conceptually
The driving temperature difference is not the arithmetic average of the two ends. For counter-current flow, LMTD = (ΔT1 − ΔT2) / ln(ΔT1/ΔT2), where ΔT1 and ΔT2 are the temperature differences at the two ends of the exchanger. Co-current flow uses the corresponding end differences and always gives a smaller LMTD for the same terminal temperatures. Multi-pass geometry applies Ft (from TEMA/Bowman charts or rating software) so that Q = U A (LMTD_cf · Ft).
If Ft falls below about 0.75–0.80, the arrangement is usually wrong: add a shell in series, change pass count, or accept that the specified outlets cannot be met in that layout. LMTD is a process property of the terminal temperatures and flow arrangement — not a vendor 'performance number'.
State which temperatures are fixed and which are results. A rating that treats both outlets as fixed when the plant cannot control one of them will look precise and still be the wrong machine.
Nozzles, vents, drains, and impingement
Nozzles set velocity into the channel and onto the bundle. TEMA requires impingement protection when inlet ρv² exceeds the class threshold — a plate, dummy tubes, or an annular distributor — so the first tube rows are not eroded. Shell inlet and outlet orientation must match the piping study; rotating a nozzle after fabrication is a new drawing.
High-point vents and low-point drains belong on both circuits if the plant will hydro, chemically clean, or de-inventory the unit. Condensers need a defined non-condensable vent path. None of these are ornaments: a missing vent leaves air in a hydro test; a missing drain leaves a chloride puddle in a stainless channel.
Expansion: why the rear end matters
Tubes and shell see different metal temperatures. A fixed-tubesheet unit welds both tubesheets to the shell; differential growth loads the tube-to-tubesheet joints unless a shell expansion joint is fitted. A U-tube bundle flexes at the bends and can be withdrawn. A floating head (TEMA S/T/W) lets the rear tubesheet move — that is a TEMA option purchasers specify; it is not a type Ohm manufactures. See /products/heat-exchangers for the fixed-tubesheet and expansion-joint construction Ohm builds.
RFQ data that makes the flow path real
Send both streams (identity, phase, flow, T_in/T_out or duty), design and operating pressures, allowable ΔP including whether nozzles are in the budget, fouling resistances, preferred tube OD if the plant standard has one, orientation, and nozzle-location constraints from the plot. Name TEMA type if you already know it; otherwise describe fouling and ΔT and ask for a recommendation. Attach the pack on /rfq?product=heat-exchangers. Ohm will rate and mechanically design to the named code (TEMA practice; ASME VIII-1 or IS 2825 when specified) at the Vadodara works — not from a catalogue kW.
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