1-805-484-2992
Quotes - Engineering - SalesSingle Spiral Exchangers
Spiral Flow on Both Sides, Counterflow and Self-Cleaning
Both media follow curved channels in true counterflow. Self-cleaning on both sides, capable of crossing temperature approaches, and entirely at home with slurries, sludges and fibrous media that would block anything else.
The single spiral is the original and most general-purpose spiral configuration. Two long metal strips are rolled around a central core with spacer studs between them, forming two concentric spiral channels. One fluid enters at the periphery and travels inward; the other enters at the center and travels outward.
Self-cleaning in both channels. Because each circuit is one continuous channel, all the flow must pass through every part of it. A deposit narrowing the channel raises local velocity, and the higher shear scrubs it away. With spiral flow on both sides, both streams get that protection, which is why single spirals are used for the genuinely difficult liquids — sewage sludge, paper stock, mineral slurries, fibrous effluents, viscous emulsions — that would blind a plate pack in hours.
Crossing temperature approaches. True counterflow with no crossflow component means no correction factor and no limit on temperature cross. A single spiral will take the cold stream out hotter than the hot stream leaves, in one unit, which makes it very effective on heat recovery duty where two process streams are being interchanged.
Condensing service. Single spirals condense well at pressures at or above atmospheric, where the vapor volumetric flow is manageable within a spiral channel. Steam heating and vapor condensing are both standard duties for the configuration. The limitation is the same one that constrains plate exchangers: as pressure falls, volumetric flow rises and a spiral channel becomes restrictive. For vacuum condensing the crossflow arrangements exist instead.
Access. Removable covers on both faces give access to the heat transfer surfaces for inspection and manual cleaning when a duty genuinely requires it, which is a useful backstop even on a self-cleaning design.
For large vapor volumes and vacuum see cross-flow spiral; for column-mounted reflux duty see counterflow spiral.
Configuration
- Flow: Spiral both channels
- Arrangement: Counter or co-current
- Orientation: Vertical or horizontal
- Covers: Removable both faces
Features
- Self-cleaning both sides
- Crossing approach capable
- Handles slurries and sludge
- Compact for the duty
- Removable access covers
Typical Condensing Duty
- Condensing above atmospheric
- Steam heating duty
- Fouling liquid recovery
- Slurry and sludge service
- Close approach heat recovery
| PRODUCT | STYLE | FLOW | SELF-CLEANING |
| Condenser / Cooler | Single Spiral | Spiral both sides | Both channels |
Single Spiral Selection
The Fouling Argument in Detail
It is worth understanding why the self-cleaning effect is real rather than marketing. In any exchanger with parallel flow paths — a tube bundle, a plate pack — fouling is unstable in the wrong direction. A path that starts to foul gains resistance, so flow diverts to cleaner paths, so velocity in the fouling path drops, so it fouls faster. The process runs away.
A single channel cannot behave that way. There is nowhere for flow to divert to, so a restriction raises velocity through itself, and shear stress rises with it. The fouling process is self-limiting instead of self-accelerating. That difference is why spiral exchangers survive in services that destroy other constructions, and it is worth paying for when the stream is genuinely dirty.
Orientation and Condensate
Single spirals can be installed vertically or horizontally, and on condensing duty the choice matters. Vertical installation with vapor entering at the top lets condensate drain with gravity assisting the flow, which keeps the channel clear and helps subcooling as the condensate continues along the cooler part of the path.
Horizontal installation can be appropriate where headroom is limited or where the duty is primarily liquid with modest condensing. The consideration is the same as in any condenser: liquid must be able to leave without accumulating and reducing the area available to vapor. On a duty with a large condensate load, vertical is usually the safer arrangement.
Where the Single Spiral Is Not the Answer
Being straightforward about the limits: single spirals are not the choice for deep vacuum condensing, because the spiral channel imposes too much pressure drop on a large volumetric vapor flow — the crossflow configuration exists for exactly that reason. They are not usually the cheapest option for a clean, simple, high-volume duty that a plain shell and tube would handle. And because they are a welded construction, major internal repair is not comparable to swapping a tube bundle.
Where they excel is the awkward middle ground: dirty streams, close or crossing approaches, limited plot space, and duties where a conventional exchanger would need cleaning far more often than the plant can accommodate.
Spiral exchanger basics
In practice it means each fluid travels a single channel that may be 50 to 100 ft long and only 0.25 to 1 in wide, wound into a unit a few feet in diameter. The channel width is set by the spacer studs and is chosen for the fluid: a wide channel for sludge or fibrous stock, a narrow one for clean liquids where velocity and heat transfer are wanted.
Because the two channels are separated only by the spiral plate, the entire plate is active surface on both faces, which is why a unit the size of a small drum can carry the surface of a much longer shell and tube. The feature that catches people out is the single pass: there are no multiple passes to adjust, so changing velocity means changing channel width or unit diameter, not pass count.
The effect holds only while the velocity in the channel stays high enough to produce useful shear, which for most slurries and sludges means keeping it above roughly 3 ft/s at the lowest flow the unit will see. At turndown the scrubbing weakens along with the velocity, so a spiral designed for full rate and run at a third of it will foul like any other exchanger.
Hard scale is the other limit: calcium carbonate or silica that bonds to the plate is not sheared off by velocity and has to be removed chemically or by opening the covers. What the geometry defeats is soft, settling or fibrous fouling, where a deposit begins as a loose bed and the local velocity increase lifts it before it consolidates. Stating minimum flow as well as design flow on the datasheet is how we make sure the effect is there when you need it.
At moderate vacuum, down to roughly 300 mmHg absolute, a single spiral can still condense acceptably if the channel is wide and the unit is sized for the pressure drop rather than for the heat transfer. Below that the trade turns against it quickly, because the vapor volume at 100 mmHg is about seven times what it is at atmospheric pressure and the spiral channel offers no way to open the flow area without lengthening the path.
One arrangement that extends the range is vertical installation with the vapor entering the wide outer turn and condensate draining down the channel, so the vapor volume shrinks along the path as condensation proceeds toward the center. For the pressures that vacuum distillation, deodorizing and freeze drying run at, the cross-flow element is the correct choice and we would not quote a single spiral.
Vertical is the default for any duty where condensate exceeds a small fraction of the flow, and horizontal is reserved for liquid-to-liquid service or for condensing where the condensate is a few percent of the vapor mass. On a vertical unit the lower cover carries a drain connection at the bottom of the channel, so condensate leaves continuously and the channel cross-section stays available to vapor. On a horizontal unit the condensate runs along the bottom of each turn and has to climb out through the outlet nozzle, which only works when there is little of it.
Headroom is the usual reason to compromise: a vertical spiral 4 ft in diameter needs about 6 ft of clear height plus cover removal space, and a mezzanine installation sometimes cannot offer that. In that case a horizontal unit tilted a few degrees toward the outlet is a workable middle ground.
Yes, and opening one is a two-person job of an hour or two rather than a bundle pull: the cover is unbolted, lifted, and the whole length of one channel is in view from the face. Each cover seals its channel with a full-face gasket, so an inspection also means a new gasket, and the gasket material has to suit the fluid and temperature, which on a hot condensing duty usually means a graphite or PTFE-based material rather than rubber.
What you can see from the open face is the entire channel wall, so a fouling pattern can be mapped and photographed and any pitting or erosion found early. What you cannot do is replace the spiral plate: a perforated plate means a weld repair from the open face or a new unit, which is the one place a tube bundle has the advantage.
Yes, and that is the duty the geometry suits best, because both channels self-clean and true counterflow lets the cold stream leave hotter than the hot stream leaves. A typical case is hot effluent from a cooker or digester at 180 F preheating incoming feed at 60 F: a single spiral can bring the feed to 160 F while cooling the effluent to 80 F, a 20 F approach at both ends in one unit, where a shell and tube would need two or three shells in series to reach the same point.
The pressure drop for both streams tends to be higher than in a tube bundle, often 5 to 15 psi per side at design velocity, which has to be available from the pumps. Sludge, stillage and fibrous process water are the usual candidates; the payback is the steam not spent on the feed heater.
