A hollow-flight screw conveyor heats or cools bulk material as it conveys: thermal media enters a rotary union, flows through the length of the hollow flights, and returns through a center pipe and syphon tube [S2].
Operating envelopes published by manufacturers span 1,200°C in high-temperature alloy builds [S4] and roughly 700°F (about 371°C) for standard carbon-steel industrial units [S2], with screw speeds of 30 to 60 rpm typical and trough fill held to 15–40% [S1].
How a Hollow-Flight Screw Transfers Heat
Heat moves by direct conduction, not convection, because the thermal media contacts the inside of the flights while the bulk material contacts the outside, eliminating the air-gap losses typical of jacket-only designs [S2]. The hollow flight and the center pipe both contribute surface area, and rotating the screw at low speed tumbles the bed so a larger fraction of particles sees the hot or cold wall on each pass [S2].
Construction is flat flights welded to a helicoid wrap with cupped flights, with the wrap continuous around the center pipe for the full screw length; the flights are not welded to the center pipe, so the two expand and contract independently, lowering thermal stress and extending equipment life [S2]. A rotary union on the tail shaft admits and returns the media while the assembly turns, and the union is sized case by case for speed, media type, temperature, and pressure [S2].
A 2010 patent application (US20100051233A1, Therma-Flite Inc.) classifies these units as F28F 5/06 hollow-screw conveyors within the B65G 33 family of enclosed screw conveyors for fluent solids [S5], which is the same classification cited in the manufacturer's product literature for heat-transferring hollow-flight screws [S5].
Operating Envelope and Throughput Geometry
Standard screw conveyors run a few feet up to 40 feet long, with 150 feet achievable for special units, and a 30% trough fill is common for normally flowing materials, with the practical range 15–40% to keep material off the hanger bearings [S1]. Screw speed is typically 30 to 60 rpm and can reach 150 rpm for some materials and applications [S1].
Throughput is governed by cross-sectional area × linear speed × bulk density × slope factor, where the cross-section is π/4·D²·%fill and the linear speed is the pitch distance times the rotation speed [S1]. Upward inclination cuts capacity hard: about 5% loss at 10°, 10–40% at 20°, and 20–70% at 30°, which is why horizontal installation is preferred and inclined runs are compensated with a larger diameter or higher rpm [S1].
Heat-transfer duty is sized from the screw's available surface area and the system flow balance, not just the trough jacket [S2][S8]. Manufacturer guidance states that the hollow-flight principle achieves a far greater transfer performance than energy transfer through the shaft tube and housing alone, which lets a smaller machine handle a given duty or, conversely, pushes the upper capacity ceiling of a given envelope higher [S4].
Material Selection for the Wetted and Heated Surfaces
Carbon steel is the default for industrial hollow-flight screws operating below 700°F; 304, 316, Inconel, and duplex stainless steels are offered for higher temperatures, corrosive bulk solids, or food-grade duty [S2]. KWS notes that material of construction is selected from this same short list based on bulk-side chemistry, media-side chemistry, and peak metal temperature [S2].
For aggressive high-temperature service, Tiermax publishes a 1,200°C ceiling on its cooling-screw line, using alloyed flights and a U-trough, pipe-trough, omega, or double-omega housing to keep the bulk out of contact with the casing [S4]. Common bulk-side feedstocks documented in vendor case work include coal bottom ash, fly ash, plastics, sewage sludge, salts, cyanide, metal oxides, cement raw meal, and pyrolysis residues [S4].
For food-grade heating, the closed envelope plus media-side isolation addresses the rigorous-separation requirement between the cooling or heating medium and the bulk product, which is the main reason cooling-screw designs are specified over jacket alternatives in food plants [S4].
Typical Industrial Applications and a Sizing Example
Documented applications include cooling fly ash from coal-fired boilers, heating lime for asphalt-shingle operations, maintaining a set temperature on food products, and cooling drill cuttings downstream of a rotary dryer [S3]. Activated-carbon cooling at 80 cubic feet per hour is a published duty point for one hollow-flight thermal processor, with the design sized by the surface area available for sensible-heat removal and the residence time set by screw speed [S6].
Heat-transfer media in field use include cool water, hot oil, and steam; steam-heated hollow-flight screws are the common case for simultaneous conveying and heating, and condensate handling is part of the standard scope [S3][S5]. The thermal media choice cascades directly into rotary-union and gasket selection, so the media specification (not just the bulk-side temperature) is what locks the metallurgy and seal set on the drawing [S2].
For orientation against other conveying options, a screw conveyor running an inclined layout will pay a steep capacity penalty past 20°, while a belt conveyor handles the same duty with much less throughput loss on a slope but cannot heat or cool the bed inline. The hollow-flight screw is the right pick when the duty is heat transfer plus a short horizontal or mildly inclined run, not long-haul conveying or steep lifting.
Limits, Failure Modes, and Common Misapplications
Hollow-flight screws cannot curve or bend; a direction change means a drop from one screw to a second one [S1]. Hanger bearings every 10 ft or so support the shaft and keep it off the trough, and those bearings create local cold or hot spots that the %fill limit is set to avoid [S1]. Bulk materials that bridge or form an insulating layer on the flights will steadily lose heat-transfer efficiency until the layer is cleaned, which is why self-cleaning flight options and trough geometry choices (U, pipe, omega) are part of the spec conversation [S4].
Screw conveyors in general are not the right tool for free-flowing materials on steep inclines, since the same fall-back that hurts throughput also caps heat-transfer residence time [S1]. When the duty demands both long horizontal reach and inline heating, the hollow-flight screw is usually paired with a separate bulk bag feed or a preconditioning stage rather than stretched on its own.
Steam-heated hollow-flight screws carry a documented condensate-management requirement, and the patent literature treats condensate handling as a first-class part of the screw geometry rather than a piping afterthought [S5]. Rotary-union service life is the most common scheduled-maintenance item and is keyed to media cleanliness as much as to pressure or temperature [S2].
Sourcing Notes and Standards Watch
Hollow-screw heat exchangers are classified under F28F 5/06 (hollow screw conveyors) within F28 heat-exchange apparatus, and within the conveying family sit under B65G 33 for enclosed screws handling fluent solids [S5]. Manufacturer literature and patents both anchor the geometry to these classification codes, which is the cleanest way to map vendor brochures to procurement language [S2][S5].
Material-grade language (304, 316, Inconel, duplex) and the 700°F carbon-steel ceiling come from KWS design guidance for the standard industrial product [S2], while the 1,200°C ceiling and the omega/double-omega housing geometry come from Tiermax's high-temperature cooling-screw specification [S4]. CMT Process Solutions documents the steam, hot-oil, and cool-water media set and lists the four common bulk-side duty classes in its heat-transfer screw conveyor line [S3].
Trackable signals to watch on the next sourcing cycle: vendor-published U-values or kW per square meter of flight area for specific media (none of the surveyed pages publish a numeric overall heat-transfer coefficient, only qualitative claims of higher efficiency versus jacket designs [S2][S4]); ASME B31.3 process-piping alignment for the media-side circuit, which is implicit in any steam or thermal-oil build but not named in the surveyed sources; and a published CEMA 350 or equivalent safety standard for the screw envelope, which the surveyed literature does not cite by number. For broader context on the conveying family, see the lead screw and ball screw reference pages, which are unrelated mechanically but useful for distinguishing power-transmission screws from bulk-handling screws in cross-disciplinary specs.
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