A heat exchanger is a pressure-containing piece of process equipment that transfers heat from a hotter fluid to a colder fluid across a metal wall. The streams do not mix. The wall is usually a tube; one fluid is inside the tubes and the other is outside them, in a surrounding shell. That arrangement is the industrial default because it can be designed for high pressure, named codes, mechanical cleaning, and phase change — heating, cooling, condensing, or boiling — from the same family of drawings.
Duty is the starting number
Duty (Q) is the heat transferred per unit time, usually in kW. For a single-phase stream with no composition change it is Q = m · Cp · ΔT: mass flow times specific heat times the temperature change of that stream. For condensation or boiling, latent heat dominates and the temperature may be nearly constant. The exchanger must deliver that duty at the fouled condition, within the allowable pressure drop on each side, and without mixing the two fluids.
If outlet temperature is specified on both sides, duty is determined and the designer sizes area. If one outlet is a result, duty still has to be stated or calculable from the other stream. An RFQ that names only 'a cooler' without flows or temperatures cannot be engineered.
How heat actually moves
Heat leaves the hot fluid by convection to the metal, conducts through the tube wall (and through any fouling film), and enters the cold fluid by convection. The overall heat-transfer coefficient U bundles those resistances. Area A, the log-mean temperature difference (LMTD), and a flow-arrangement correction Ft then set Q = U · A · LMTD · Ft. Fouling, velocity, and metallurgy change U; geometry and pass arrangement change LMTD and Ft. None of those are marketing parameters — they are the rating.
Shell side versus tube side
In a shell-and-tube unit the two circuits are independent. The tube side is the fluid inside the tubes and the channel (front head). The shell side is the fluid in the cylindrical shell, directed across the bundle by baffles. Allocation is a process decision:
There is no universal 'hot fluid on the shell' rule. Write both fluids, pressures, fouling character, and cleaning method. Allocation follows those constraints.
- High-pressure fluid often goes in the tubes so the shell can stay thinner.
- The stream that must be mechanically rodded goes on the tube side of a straight-tube exchanger with a removable channel cover.
- Condensing steam is often placed on the shell for distribution — unless the specification says otherwise.
- The more corrosive or exotic metallurgy is isolated to tubes and tubesheets when that saves a solid-alloy shell.
Where plants actually use them
Chemical plants use exchangers on reactor feed and effluent, distillation reboilers and condensers, and utility water or steam. Pharmaceutical plants add solvent-recovery condensers and specified hygienic utility loops. Oil and gas and fertiliser duties often arrive as TEMA-classed datasheets. Food and dairy plants use them for process heating and cooling where hygienic fittings are written on the order — not assumed. The same physical machine also recovers heat that would otherwise go to a cooling tower.
What Ohm Engineers manufactures
Ohm Engineers designs and fabricates heat exchangers at the Vadodara works (GIDC Manjusar-Savli) under ISO 9001:2015. The manufactured types are four:
| Type | What it is | Typical reason to specify |
|---|---|---|
| Shell-and-tube, fixed tubesheet | Two tubesheets welded to the shell; optional shell expansion joint | Clean shell side, high joint integrity, modest metal-temperature difference — or ΔT relieved by bellows |
| U-tube | One tubesheet; U-bends; removable bundle | Differential expansion or shell-side bundle pull; tube side relatively clean |
| Double-pipe (hairpin) | Pipe-in-pipe, often in hairpin banks | Modest area, high pressure, or modular add-on |
| Kettle reboiler | TEMA K-type enlarged shell with weir / disengagement space | Column reboil or vaporising duty that needs liquid hold-up |
What this page is not claiming
Plate exchangers, air-cooled (fin-fan) units, and floating-head rear ends exist in the industry and are specified every day. Ohm does not manufacture plate packs or air-cooled bays. Floating head (TEMA S, T, W) is a rear-end family a buyer may write on a TEMA code; Ohm's removable-bundle offering is U-tube, and expansion on a fixed tubesheet is handled with a shell expansion joint when the duty requires it. Do not read a TEMA letter table as an Ohm SKU list.
What to send for an RFQ
Start from process data, not a model number. Send hot and cold fluid identities, flows, inlet and outlet temperatures (or duty), operating and design pressures, allowable pressure drop on each side, fouling factors if the licensor or plant standard has them, preferred materials, named code (TEMA class, ASME VIII-1 or IS 2825 when those apply), quantity, and delivery location. Upload a datasheet, GA, or P&ID if you have one. Use the technical RFQ form at /rfq?product=heat-exchangers — that pack is enough to start engineering review at Vadodara.
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