Spiral Heat Exchanger

What a Spiral Heat Exchanger Is and What Is Being Decided

The definition is mechanical and worth stating precisely, because the market sells several spiral-named things. A spiral heat exchanger starts as two long metal strips with spacer bars that set the channel width; the strips are wound together around a mandrel, and the edges are welded so that each channel is closed along one side for its full length and open along the other. The wound body is closed by front covers over the open faces, each cover sealing with its own gasket, and the fluids enter and leave through nozzles at the center and the periphery. Because the channel walls are welded, the two media are separated by steel with no gasket in between, and the body is a code pressure vessel, stamped and hydrostatically tested like any other.

Three boundaries keep the purchase honest. The spiral wound gasket is a flange sealing element, a metal strip and filler wound into a ring for high class flanges, and it has nothing to do with heat transfer. Spiral duct is round sheet metal air ductwork for ventilation, and it has nothing to do with pressure vessels. And this page does not replace the shell and tube heat exchanger page or the plate heat exchanger page: those remain the references for their own domains, general purpose pressure duty and compact gasketed duty respectively, and the welded spiral family occupies the specific territory where its single channel construction wins. A specifier who writes the word spiral should therefore attach the duty, because three different product worlds answer to it.

What is actually being decided in this purchase is four commitments. First, that the media fouls or carries solids enough to justify the single channel: digester sludge, black liquor, latex, molasses, fibrous stock, crystallizing fluids, sour water. Second, the channel pitch, from about 5 to 30 mm, which trades heat transfer density against the freedom of the passage. Third, the orientation and cover arrangement, because cleaning access and condensate drainage follow from them. Fourth, the materials package, plate and weld fill and cover gaskets, chosen against the chemistry including the cleaning chemistry. A spiral bought on surface area alone is a guess, and the guess is usually wrong in the expensive direction: too fine a pitch and the unit becomes a cleaner, too coarse and the capital is wasted.

The exchanger is bought as part of a station, and the RFQ should name the station. The hot or dirty side is pushed by a centrifugal pump sized for the viscosity and solids, never gravity fed. Heat rejection sits behind the clean side: a cooling tower where the site has water, a chiller where the process wants temperatures below ambient, or an air-cooled heat exchanger where water is scarce. Steam heated duties take their supply through a properly trapped drip arrangement, a steam trap at the low point with a condensate pot ahead of the return, so the heating side never logs up. On the connected piping, thermal growth is absorbed by an expansion joint near the nozzles so the vessel is not used as a pipe anchor, and the instrumentation package, covered in chapter 4, closes the loop. Naming the whole station in the RFQ lets one supplier check nozzles, drains, and instrument taps in a single pass instead of four arguments at delivery.

The standards frame is short and firm. The body is built as a pressure vessel to ASME VIII Division 1 with a stamp where the jurisdiction demands it, or to PED practice in the European market, and the hydraulic test follows the code. Materials carry EN 10204 3.1 certificates, plate and weld fill both, because the channel welds are pressure parts and the chemistry of the fill matters in acid and chloride duty. Performance is not governed by a universal test standard, which is exactly why the duty statement and the allowable pressure drop belong in the purchase order: the buyer defines the duty, and the vendor sizes and demonstrates against it. The source literature at the foot of this page, the Alfa Laval welded spiral brochures and the Danfoss SONDEX SonSPV sheet, shows how the major makers state these parameters, and the RFQ should demand nothing less specific.

How the Single Curving Channel Transfers Heat

The flow arrangement follows from the winding. In the counter current liquid to liquid form, one fluid enters at the periphery of its channel and flows inward to the center outlet while the other enters at the center and flows outward, so the two streams pass each other in opposite directions across the welded wall along the entire spiral. Each fluid has one channel of constant cross section, a passage maybe 10 to 20 mm wide and the width of the strip tall, and there is no bundle, no baffle, and no bypass lane: every particle of fluid travels the same curving path at essentially the same velocity. Co current arrangements exist for special duties, and the condensing forms arrange the vapor lane differently again, but the counter current spiral is the volume product.

The self cleaning behavior is the reason the design exists. In a single channel, any deposit that begins to build narrows the local passage, the local velocity rises with the blockage, and wall shear rises with it, so the deposit erodes back off toward an equilibrium film; the system is self limiting rather than self accelerating. The curvature adds a second mechanism: flow around a curved path generates secondary vortices that sweep particles across the channel and scrub the walls, keeping solids in suspension instead of letting them settle. And there is nowhere quiet for material to hide, because a channel of constant section has no baffle windows, no dead corners behind tubes, and no low velocity end zones. Media that coat and bake, polymerize, or crystallize on hot walls still foul a spiral, but they foul it slowly, evenly, and reversibly, and the covers open onto the whole length of both channels.

The thermal numbers come from the true counter current arrangement. Because both streams flow strictly opposite each other along the whole surface, the temperature profile is genuinely counter current, the logarithmic mean temperature difference needs almost no correction factor, and the practical result is that temperature approaches under 3 deg C are achievable on liquid to liquid duty, tighter than a baffled shell pass typically delivers once its bypass and leakage streams are paid for. The same curvature that scrubs the walls also triggers turbulence at moderate Reynolds numbers, so heat transfer coefficients run high for the pumping power spent, and the holdup volume is small, which matters on batch and product changeover duty where the heel of a shell body has to be flushed out between runs.

Pressure drop and condensing behavior deserve their own paragraph. The single channel is a long passage of moderate section, so the clean pressure drop is predictable and the fouled pressure drop rises gradually as the equilibrium film thickens; the allowable drop on each side is a purchase specification, not a discovery. In condensing form the argument is stronger: vapor enters a single unobstructed spiral lane, condensate drains by gravity along the spiral to the bottom outlet, and non condensables vent at the defined point, with total pressure drop of only a few millibar at overheads duties. That is why vacuum column overheads, evaporator duties, and reflux condensing are native spiral applications: on a column running at 40 mbar, an exchanger that eats 10 mbar is a tax on the whole plant, and the spiral lane is the geometry that does not eat it.

Mechanically, the channel walls are the pressure parts. Each strip is welded to its neighbor along the spiral seam for the full length of the channel, and those welds carry the pressure differential between media and from media to atmosphere; this is why the weld fill appears in the materials line of the RFQ and why thermal fatigue of the channel welds, covered in chapter 6, is the failure mode that mechanical design guards against. The covers close the open faces: flat covers on moderate duty, dished covers where the code calculation wants it, hinged or bolted, and lifted by davits on the larger bodies. Gaskets sit only on the cover faces, one gasket per fluid, each exposed to one medium, and the selection of those gaskets against media and cleaning chemistry is a real specification decision, not a consumables afterthought.

Types, Construction, and Duty Boundaries

Three families cover the market, and the table at the end of this chapter compresses them. The liquid to liquid spiral, the standard SHE class for liquid to liquid duties in the source literature, runs both media in counter current spiral channels and owns the general heating and cooling of fouling liquids, sludge, and viscous product. The spiral condenser, including the surface condenser form, gives one channel to a condensing vapor with top entry and gravity condensate drainage, and owns vacuum and reflux duty. The third family, vaporizers and reboilers and the mounted direct condenser versions, is smaller in catalog volume and is specified against the maker's thermal sheet rather than from family rules, so this chapter states it honestly as a family and moves on.

The liquid to liquid family splits further by media severity. Clean liquid duty permits the fine pitches at the 5 to 8 mm end of the range, where the spiral packs the most surface per cubic metre and behaves thermally like a very good compact exchanger. Sludge, fibrous stock, and crystallizing fluids demand the middle and coarse pitches, 12 to 20 mm and up, with velocity held high enough to keep solids moving; on digester heating duty, the classic application, the sludge channel pitch is chosen so that the unit survives between cleaning campaigns set by the plant schedule, not by the exchanger's own fouling rate. Viscous duty adds the argument from chapter 2, curvature induced turbulence at moderate Reynolds numbers, which lets the spiral transfer heat in laminar regimes that would leave a shell side almost inert.

The condenser family splits by vacuum severity and mounting. The standard spiral condenser takes vapor at the top, condenses in the spiral lane, drains at the bottom, and vents non condensables at a defined point; it serves reflux drums, evaporators, and vacuum column overheads at pressures where shell-side bundles lose pressure budget to nozzle losses and baffle turns. The direct mounted versions bolt straight onto the column or vessel flange, eliminating the vapor line entirely, and they are specified as a pair with the column nozzle loads in mind. Condensing duty in a spiral is vertical in most installations, because condensate must drain along the spiral by gravity, and the orientation line in the RFQ follows from the duty rather than from plant layout convenience.

Construction and materials follow the chemistry. Carbon steel serves utility water and non corrosive heating; 316L is the volume standard for process and dirty water duty; duplex grades buy chloride and stress corrosion margin; 904L and the Hastelloy family serve acid duty; titanium serves seawater, brine, and wet chlorine service; and the plate thickness, cover design, and weld fill are chosen together because they are one pressure envelope. Nozzles end as flanges to the line standard or as direct weldments, and the body carries lifting lugs, davits, and support saddles sized for the filled weight, which is substantial: a spiral body is a solid cylinder of steel and fluid, dense enough that structural engineers check the steel before the piping engineers check the nozzles.

Boundaries are worth stating bluntly. Above roughly the 25 bar class, and certainly above the 40 bar ceiling of the heaviest small-diameter designs, shell construction takes over and the spiral stops being the honest quotation. Above the 400 deg C class, plate and cover design push the same direction. And for clean, non fouling, moderate pressure liquid duty, the spiral pays a premium it cannot earn back: a gasketed plate pack or a plain shell unit does that duty for less, and chapter 5 puts the comparison in its place. The spiral is bought for media and vacuum behavior, not for compactness alone.

Key Specification Parameters

The table below collects the parameters a complete datasheet must state, and each row is a place where weak quotations hide.

The duty statement belongs in the RFQ, not in the vendor's head, and it reads as numbers: hot and cold side mass or volume flow, inlet and outlet temperatures, and allowable pressure drop on each side, plus a frank media description with solids loading, fiber content, viscosity at temperature, and the cleaning chemistry. On that statement hang two of the instruments in chapter 6: the approach across the unit is trended from a temperature transmitter on each outlet, and the fouling trend is read as rising differential from a pressure transmitter tap pair, or as flow drift on a flow meter when the pumping is on a curve. Sludge and slurry sides are measured with an electromagnetic flowmeter, which tolerates solids that foul other metering, and the clean side trim runs through a control valve on the cooling or heating line so the approach holds at turndown.

Steam heated units carry one more specification line. Where saturated steam is the heating medium, the supply arrives at the top nozzle and the condensate leaves at the bottom with a steam trap arrangement per the station design, and where the site steam carries superheat beyond what the plate and gasket envelope wants, a desuperheater on the supply brings it back toward saturation before the spiral sees it. The cover gasket compounds, not the welded channels, set the steam side temperature ceiling in practice, and the RFQ should name the compound rather than accept a generic high temperature grade.

The duty table below compresses the pitch and orientation decisions into the form a specifier can argue from in a meeting.

Selection Decision Factors and RFQ Language

The decision sequence below runs as a checklist in a specification meeting, and each step eliminates a class of quotations before any price comparison starts.

1. State the media honestly on both sides: solids loading, fiber, crystallization, viscosity at temperature, and the cleaning chemistry the plant will actually run.
2. State the duty as numbers: flows, inlet and outlet temperatures, and allowable pressure drop per side, at maximum and turndown conditions.
3. Decide whether the duty is fouling liquid, condensing, or both, and let that fix the family and the orientation.
4. Set the channel pitch against the fouling description, using the duty table in chapter 4 as the opening position and the vendor thermal sheet as the closing one.
5. Choose materials for plate, weld fill, and cover gaskets against the worst chemistry including clean in place, and demand EN 10204 3.1 certificates.
6. Fix orientation, cover type, and davit or hinge arrangement so the cleaning campaign is a shift, not a shutdown.
7. Protect the inlets: a Y strainer on clean side inlets and a basket strainer on large foul inlets where strings and rags are credible.
8. Fix the verification deliverables: thermal sheet, code stamp, hydrotest certificate, and the pressure drop and approach guarantees at stated conditions.

Against the neighboring heat transfer equipment, the comparison a specifier actually runs is about fouling, pressure budget, and cleanability, and it belongs here in the selection chapter. The shell and tube heat exchanger handles the broadest duty and the highest pressures, but its shell side fouls in baffle windows where velocity dies, and in situ cleaning of a dirty bundle is poor: the spiral answers fouling and cleaning access, the shell answers pressure and universality. The plate heat exchanger is compact and cheap per square metre, but its gaskets bound its temperature and chemistry, and its narrow corrugated passages plug with solids and fibers the spiral swallows. The air-cooled heat exchanger rejects heat to air where water is scarce and belongs upstream of the water loop question entirely. The desuperheater conditions steam, it does not exchange liquid duty. The chiller is the machine that creates the cold side, not the exchanger that transfers to it, and the cooling tower rejects the heat the spiral transfers into the cooling circuit. Each of those is a purchase with its own page; the spiral is bought when the media and the vacuum behavior are the deciding facts.

The application map below turns that logic into a table the project file can carry.

The RFQ block should be written once and reused. A complete block reads: hot and cold side duties, flow, inlet and outlet temperatures, at maximum and turndown; allowable pressure drop each side; media description with solids, fiber, viscosity, and the cleaning protocol; design pressure and temperature for the body; materials for plate, weld fill, and cover gaskets with certificates; channel pitch stated or negotiated from the vendor thermal sheet; orientation and cover arrangement with davit or hinge; nozzle ends, flange standard and facing included; code stamp requirement and test basis; and the guarantee points, approach and pressure drop at stated conditions. Add the station hardware in the same document: pump, valves, strainers, drip and vent arrangements, and the instrument taps, so the supplier sees the whole station. A quotation returned without the thermal sheet behind the pitch is incomplete at any price, because the pitch is the product.

Installation, Standards, Failure Modes, and Maintenance

Installation follows the orientation decision. Condensing bodies mount vertical so condensate drains along the spiral by gravity; sludge and liquid to liquid bodies mount horizontal in most plants, set on saddles with the covers accessible, davit clearance above, and space in front of each face for the hydroblast lance. Nozzle loads go to the vessel design, not the pipe: the connected runs bring their thermal growth to an expansion joint near the unit, and each flange make-up uses its specified gasket and bolt torque, because a leaked cover is the failure mode in chapter 4's gasket row and the fix is correct assembly, not chemistry. The drip and vent arrangements from the station design land in the same package: the heating side drain with its steam trap and condensate pot arranged for service, and the vacuum side vent line run short and warm so it never logs.

Commissioning proves the duty, not just the absence of leaks. Flush both circuits before first heat, through a Y strainer on the clean side and, where the media warrants, a basket strainer opened and cleaned after the flush; construction debris is the one contaminant a self cleaning channel cannot forgive, because a rag wedged at the first turn holds fibers forever. Hydrotest per code, then record the baseline: outlet temperatures from a temperature transmitter on each side, differential pressure from the pressure transmitter taps at design flow, and the flow reading itself, an electromagnetic flowmeter on the sludge side or the station's inline flow meter on the clean side, logged so every later comparison is made at the same point. Stroke the cooling water control valve across its range and confirm the approach holds at turndown, and verify the clean in place circuit reaches the whole channel at the design velocity.

Operating discipline guards the channel welds. Rapid thermal cycling is the enemy of a welded spiral body: quenching a hot unit with cold water at full flow puts shock into the seam welds, and the cumulative damage appears as fatigue cracking years later, so ramp limits, typically a stated temperature change per unit time from the maker, belong in the operating procedure. Periodic reverse flush, driving the channel backward at elevated velocity for a short period, moves fibrous accumulations that normal flow merely tolerates. Abrasive media set a velocity ceiling at the channel inlet, and where solids load is high the inlet area is protected by an upstream diffuser or simply by running the centrifugal pump at a derated speed with the pitch doing the fouling work instead of velocity doing the erosion work.

Failure modes follow a short and predictable list. Cover gasket leakage: weeping at a cover face, cured by correct compound, clean seating surfaces, and a re-torque after the first thermal cycle. Channel blocking from stringy or fibrous media: rising differential across one side, cured by reverse flush and prevented by the strainers named in chapter 5. Thermal fatigue at the channel welds from rapid cycling: found as a hard-to-find internal leak between channels, two outlet temperatures drifting toward each other with no external sign, prevented by ramp discipline. Erosion at the channel inlet from high velocity abrasive service: addressed at design with diffusers and velocity limits. External corrosion under insulation: a CUI program item like any warm vessel, checked by opening the insulation at the supports and nozzles on the inspection schedule. Each of these has a specification answer in chapters 4 and 5, which is the point of writing them down.

Maintenance is a campaign, not a constant. At the interval the fouling trend sets, commonly 6 to 24 months by media, isolate, drain, and open the covers: with the faces off, both channels are accessible for their full length, hydroblast from the outside in or rod the spiral mechanically where deposits have baked, inspect the weld seams for cracking and pitting, renew both cover gaskets, and re-torque in the pattern at the specified value. There is no bundle to pull, no confined space entry, and no tube plug map to maintain, which is the whole maintenance argument over shell construction on fouling duty. Between campaigns, trend the differentials and the approach, keep the strainers and the trap on their schedules, and file the readings against the thermal sheet, because the trend that predicts the next cleaning date is also the evidence that sizes the next RFQ, and a purchasing file that carries numbers ages better than one that carries adjectives.