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Quotes - Engineering - SalesHairpin Double Pipe Exchangers
True Countercurrent Flow in the Simplest Possible Form
One pipe inside another, folded into a U. It is the simplest exchanger in the catalog and on small condensing duties it is frequently the best one — true countercurrent flow, very high pressure capability and surface you can add to a section at a time.
The pipe pair is bent through 180 degrees so both sets of connections come back to the same end. The process fluid flows in the inner tube, the service fluid in the annulus around it, and they travel in opposite directions from end to end.
Countercurrent flow is the defining property. With one channel per side there is no crossflow component and no correction factor on the log mean temperature difference. That means a hairpin can achieve a temperature cross — where the cooling fluid leaves hotter than the process fluid leaves — in a single unit, which a baffled shell and tube cannot do without multiple shells in series. On condensing duty with significant subcooling, where the condensate needs to leave well below the vapor inlet temperature, that capability is directly useful.
Pressure capability comes free. Both sides are pipe, and pipe is by far the cheapest way to contain pressure. A hairpin rated for several thousand PSI is unremarkable; an equivalent shell and tube would need a heavy forged shell and would cost a great deal more. Where vapor has to be condensed at high pressure, this alone often decides the configuration.
Surface grows in modules. Hairpins are built in standard section lengths and manifolded together. Need fifty percent more duty? Add sections. Very few exchanger types allow capacity to be increased without replacing the unit, and for pilot plants, processes still being developed, or duties where the design basis is uncertain, that flexibility has real value.
Cleaning is easy. The closure at the return bend opens, and the inner tube can be cleaned mechanically or simply withdrawn and replaced. For a small duty on a fouling stream that access is worth a great deal, and it is one reason hairpins persist in services where a more thermally efficient exchanger would be harder to maintain.
The limitation is surface area. A single inner tube provides very little, so duty grows only by adding sections, and each section brings its own connections, supports and pressure drop. Past a modest number of sections a shell and tube unit will deliver the same surface more cheaply and in less space. Where that crossover approaches, multi-tube hairpins extend the range considerably.
Configuration
- Construction: Pipe in pipe, U-bend
- Flow: True countercurrent
- Inner Tube: Plain or finned
- Closure: Removable at bend
Features
- Temperature cross capable
- Very high pressure rating
- Modular added surface
- Low hold-up volume
- Simple mechanical cleaning
Typical Condensing Duty
- Sample and analyzer cooling
- Small vent condensing
- Condensate subcooling
- High pressure condensing
- Pilot plant duty
| PRODUCT | STYLE | FLOW | PRESSURE |
| Condenser / Cooler | Hairpin Double Pipe | True Countercurrent | Very High Capable |
Hairpin Exchangers on Condensing Duty
Finned Inner Tubes
When the annulus fluid has a much poorer heat transfer coefficient than the inner tube fluid — a viscous oil, a gas, or a low-flow service stream — the annulus side becomes the bottleneck and adding length is an inefficient way to fix it. Longitudinal fins welded along the outside of the inner tube address this directly by multiplying the area on the weak side.
On condensing duty this is useful when the vapor condenses inside the inner tube and the cooling medium in the annulus is air or a poor liquid coolant. Fin count, height and material are all selectable, and a finned hairpin can carry several times the duty of a bare one in the same envelope. Where both sides have good coefficients, fins add cost without much benefit.
Series, Parallel and Pressure Drop
Hairpin sections can be connected in series, in parallel, or in combinations of both, and the arrangement is a genuine design variable rather than a detail. Series connection maximizes velocity and therefore heat transfer coefficient, and preserves the countercurrent temperature profile end to end, but pressure drop accumulates through every section.
Parallel connection divides the flow, which cuts pressure drop sharply but reduces velocity and coefficient. On condensing duty this matters more than on liquid duty because vapor volumetric flow is large and falls as condensation proceeds. A common arrangement puts the vapor inlet sections in parallel to handle the high volumetric flow, then combines into series sections as the vapor condenses and the volume collapses.
Where Hairpins Beat the Alternatives
It is worth being clear about the niche. A hairpin will not compete with a shell and tube on cost per square foot at any meaningful size. What it offers is a set of specific advantages that matter on particular duties: a genuine temperature cross in one unit; pressure ratings that would make an equivalent shell prohibitive; minimal hold-up for hazardous or expensive fluids; the ability to add surface later; and easy mechanical access to the inner tube.
Sample coolers, analyzer conditioning systems, small tank vent condensers, condensate subcooling sections downstream of a larger condenser, high-pressure gas condensing, and pilot plant service are where those advantages line up. If none of them applies to your duty, a shell and tube is probably the better answer and we will say so.
Hairpin questions
It lets one small unit do a job that would otherwise take two shells in series: condense and subcool a vapor while the coolant leaves hotter than the condensate does. Picture solvent vapor arriving at 220 F and leaving as 120 F liquid, with cooling water entering at 85 F and leaving at 150 F.
That is a 30 F temperature cross. In a conventional two-pass shell and tube the correction factor on the mean temperature difference collapses toward zero for that case and the designer is forced into multiple shells. A hairpin takes it in stride because there is only one path on each side. The practical consequence is that a subcooling duty tacked onto the back of a larger condenser, or a condensate cooler that must hit a tight outlet temperature, can be handled in a unit that fits on a pipe rack.
A hairpin contains pressure the way a pipe does, with wall thickness that grows only in proportion to diameter, so high ratings are a matter of picking a schedule rather than redesigning the vessel. A 2 in Schedule 80 inner pipe in carbon steel carries well over 2,000 psi at moderate temperature, and heavier schedules push that higher without any change to the geometry.
A shell containing the same pressure at 20 in diameter needs a wall ten times thicker and a forged flange to match, which is where the cost goes. The components that actually limit a hairpin are the return-bend closure and the shell-side nozzle flanges, so those are the parts that get specified to the full rating. High-pressure gas condensing, pilot plant reactors and sample coolers off a high-pressure loop are where this shows up in practice.
Fin the inner tube when the annulus coefficient is the bottleneck, which you can tell by comparing the two film coefficients before buying. Condensing vapor inside the tube may run at 1,000 Btu/hr sq ft F or better, while a slow glycol stream in the annulus may give 100 or less, and in that case the annulus controls almost the whole resistance.
Longitudinal fins, typically 20 to 36 of them a half inch or more high, multiply the annulus-side area by about five, which brings that side into balance. The same fins on a duty where both sides already run at several hundred Btu/hr sq ft F add cost and annulus pressure drop for almost no gain. Fins also need to be clean to work, so a fouling annulus fluid is a reason to stay bare and add a section instead.
Decide with the velocity you can afford, because the heat transfer coefficient and the pressure drop move in opposite directions as sections go from series to parallel. Split a liquid stream across two parallel sections and each sees half the velocity, which cuts the coefficient by roughly 40 percent while cutting the pressure drop per section to about a quarter.
On a condensing duty the inlet is the only place that needs the relief, because the vapor volume collapses as it condenses. A common arrangement feeds two or three sections in parallel at the vapor inlet, then collects into single sections in series for the last of the condensing and the subcooling. Draw the pressure drop budget for each side first, then arrange sections to use it, rather than accepting a default series train and discovering the vapor side has no vacuum left.
Somewhere around a few hundred square feet of surface, though the exact point moves with pressure and fluid. A standard 2 in by 3 in section with 20 ft legs offers roughly 25 sq ft of outside surface, so a 150 sq ft duty is six sections, each with its own closure, supports and connecting piping.
At that count the hardware and the field welding start to cost more than a small shell and tube, and the pressure drop across the series train becomes the design constraint. The exception is pressure: at 1,500 psi or more, a hairpin train of eight or ten sections can still undercut the forged shell it would replace. For a duty that is marginal, a multi-tube hairpin is the step before a shell and tube, since one of its sections carries several times the surface of a single pipe.
Estimate it from the duty, an overall coefficient and the mean temperature difference, then round up to whole sections. Condensing 200 lb/hr of low-pressure steam releases about 190,000 Btu/hr; with an overall coefficient near 200 Btu/hr sq ft F and an 80 F mean temperature difference that calls for about 12 sq ft, which a single 2 in by 3 in section with 20 ft legs covers with margin.
Add subcooling to 120 F and the picture changes, because sensible cooling of the condensate runs at a far lower coefficient and a shrinking temperature difference. The subcooling tail can easily need as much surface as the condensing itself, so the same duty may become two sections in series. If the exact condensate outlet temperature matters, say so, because that is the number that decides between one section and two.
