KNOWLEDGE CENTER

Technical Glossary

Terms used on Ohm heat exchanger and process equipment pages. Each entry links to a related guide or product page where useful.

LMTD

Log Mean Temperature Difference — the effective driving temperature difference in a heat exchanger.

For counter-current or co-current flow with constant heat capacities, LMTD is (ΔT1 − ΔT2) / ln(ΔT1/ΔT2), where ΔT1 and ΔT2 are the temperature differences at each end. F-correction factors are applied for multi-pass shells that are not pure counter-current. Duty Q = U A F LMTD (with consistent units). Send both inlet and outlet temperatures on the RFQ so LMTD can be checked.

Fouling factor

A design allowance (thermal resistance) for dirt, scale, or deposits on heat-transfer surfaces.

Fouling factors are specified by the process licensor, plant standard, or TEMA typical values — they are not a guarantee of run length. Over-specifying fouling oversizes the exchanger and can increase fouling further by lowering velocity. Under-specifying causes early approach-temperature loss. State the fouling factor you want used; we do not invent one for an undocumented fluid.

Shell side

The fluid flowing around the tubes, inside the shell, directed by baffles.

Shell-side flow is usually the lower-pressure or fouling-prone stream when a removable bundle is specified, but the assignment is a process decision. Baffle cut, spacing, and sealing strips control cross-flow, pressure drop, and vibration risk.

Tube side

The fluid flowing inside the tubes, entering through the channel or bonnet.

Tube-side velocity affects heat transfer, erosion, and fouling. Number of tube passes is chosen to hit velocity and pressure-drop limits. Mechanical cleaning of straight tubes is possible with a removable channel cover; U-tubes cannot be rodded through the bend.

TEMA

Tubular Exchanger Manufacturers Association — mechanical standards for shell-and-tube exchangers.

TEMA defines type codes (three letters: front, shell, rear), construction classes R/C/B, and mechanical details. It is a design standard, not a company certification. See the TEMA standards page.

Heat duty

The heat-transfer rate the exchanger must deliver, usually in kW or MMkcal/h.

Duty is calculated from m · Cp · ΔT for sensible duties, plus latent heat for boiling or condensing. If you do not have duty, send flows and temperatures for both streams. Do not leave both duty and stream data blank.

Pressure drop

Allowable loss of pressure on each side of the exchanger, set by the process.

Allowable ΔP is a process constraint. Tight ΔP forces more area (lower velocity, more fouling risk) or a different shell type. High ΔP can cause vibration or pumping cost. Always state shell-side and tube-side allowable pressure drop on the RFQ.

U-value

Overall heat-transfer coefficient — the combined conductance of films, wall, and fouling.

U is not a material property of the vendor. It follows from fluids, velocities, wall conductivity, and fouling. Published 'typical U' tables are only a starting guess. Final U is calculated from the duty case.

Condensation

Duty in which a vapour is cooled through its dew point and condensed against a coolant.

Condensers may be total or partial, horizontal or vertical, shell-side or tube-side condensing. Inerts (NCG) change the required area and often need a vent path. Send vapour composition, pressure, and coolant data.

Ejector

A jet device that uses a motive fluid (steam or liquid) to compress or entrain a suction fluid — no moving parts in the jet.

Steam-jet ejectors are used for process vacuum. Liquid-jet (venturi) ejectors handle mixing, pumping, or fume extraction. Ohm manufactures both steam-jet vacuum systems and non-metallic liquid-jet ejectors as secondary product lines to heat exchangers.

Vacuum level

The absolute pressure to be maintained at the process suction nozzle.

State vacuum as absolute pressure (mbar a, mmHg a, or torr), not as 'inches of vacuum' without a reference. Staging of steam-jet systems depends on suction pressure, load, and available motive steam.

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