8 Heat Pipe Structures Every Thermal Engineer Should Know

Introduction

The standard cylindrical sintered-wick copper-water heat pipe — the configuration most thermal engineers picture when "heat pipe" is mentioned — represents only one of many heat pipe structures in active production today. Heat pipe technology has evolved into a family of distinct device categories, each optimised for specific application requirements: ultra-thin form factors for smartphones, planar spreading for AI accelerators, flexible geometries for wearables and aerospace systems, sub-millimeter scales for chip-level cooling, oscillating channels for high-flux compact applications, conductance modulation for satellite thermal control, gravity-driven loops for HVAC and refrigeration, and microgrooved configurations for spacecraft thermal management.

This article presents eight major heat pipe structures used across modern thermal management. For each, we cover the operating principle, the application categories where it excels, the wick or channel architecture that defines its performance, and the manufacturing equipment required to produce it at scale. The objective is to give thermal engineers, procurement teams, and thermal component manufacturers a structured reference for understanding which heat pipe configuration matches which application — and what production capability is required to manufacture each one reliably.

1. Ultra-Thin Heat Pipes (UTHP)

Ultra-thin heat pipes are flattened heat pipe configurations with a finished thickness below 2.0mm. The category exists because of one fundamental constraint in modern consumer electronics: the Z-axis space available for thermal solutions has shrunk faster than thermal duty has decreased. Smartphones, ultra-thin laptops, smartwatches, and tablets all require thermal components that fit into chassis thicknesses where standard cylindrical heat pipes physically cannot be used.

Typical Specifications by Application

• Laptop and tablet UTHPs: 0.8–2.0mm thickness, 20W+ heat transport capability
• Smartphone UTHPs: 0.4–0.6mm thickness, 5W+ heat transport capability
• Smartwatch UTHPs: Sub-0.5mm thickness, 1–3W heat transport capability

Operating Principle

UTHPs operate on the same vapor-liquid phase change principle as cylindrical heat pipes, but with cross-section geometry optimised for low-profile applications. The internal vapour space is reduced, the wick structure must remain functional at very thin walls, and the working fluid charge must be precisely controlled to avoid both dry-out and flooding at the smaller internal volumes.

Manufacturing Implications

Ultra-thin heat pipe production has stricter process tolerances than standard cylindrical heat pipe production. The flattening process (hot pressing) must achieve dimensional accuracy of ±0.05mm with 15-tonne hydraulic pressure to produce flat sections without wall thinning or wick damage. Water injection accuracy must reach ±0.05g to maintain proper fill ratio in reduced internal volumes. Pipe shrinking precision before sealing must hold to ±0.02mm to ensure consistent weld geometry at the smaller scales.

The 11-step production sequence used for cylindrical heat pipes applies to UTHPs, but each step requires equipment configured for the tighter tolerances. For manufacturers entering the ultra-thin category, equipment selection — particularly hot press, automatic welder, and degassing-injection station precision — directly determines yield economics.

2. Flat Plate Heat Pipes (Vapor Chambers)

Flat plate heat pipes — commonly called vapor chambers (VCs) — are sealed planar devices with internal sintered wick structures bonded to both interior faces and a small working fluid charge held under vacuum. Unlike cylindrical heat pipes that transport heat from point to point along an axis, vapor chambers spread heat two-dimensionally across an entire planar surface, delivering near-isothermal performance from a localised heat input to the full condenser face.

Where Vapor Chambers Are Specified

Vapor chambers are the dominant thermal solution in applications where:

• Heat flux density exceeds standard heat pipe capability (>50–100 W/cm²)
• The heat source area is large relative to the available cooling footprint
• Multiple heat sources must share a single thermal solution
• Z-axis space constrains form factor below standard heat pipe array thicknesses
• Temperature uniformity across the contact surface is a design requirement

Modern AI accelerator coolers, high-end GPU thermal solutions, gaming laptops, and high-power server CPUs increasingly specify vapor chambers — often combined with heat pipes for remote heat transport to a fin stack.

Manufacturing Implications

Vapor chamber production uses different equipment from cylindrical heat pipe production. The sealed flat-plate construction requires VC sealing machines, secondary degassing equipment, diffusion bonding furnaces (for some VC architectures), and continuous resistance welding stations. The process is more complex than tube-based heat pipe construction, which is one reason VCs cost more per unit despite using similar working fluids and base materials. For thermal solution manufacturers planning capacity in both heat pipe and vapor chamber production, equipment investment requires planning around two distinct manufacturing process flows rather than a single shared production line.

For a deeper comparison of when to specify vapor chambers versus heat pipes, see our Vapor Chamber vs Heat Pipe Comparison Guide.

3. Flat Plate Micro Heat Pipes

Flat plate micro heat pipes are flattened phase-change devices similar to vapor chambers in geometry, but with a fundamental architectural difference: the evaporator and condenser sit on opposite sides of a single wick layer with no internal vapour cavity. The wick is in direct contact with both heat input and heat output surfaces.

Where They Excel

The absence of a discrete vapour cavity allows flat plate micro heat pipes to be manufactured at thicknesses below those achievable with vapor chambers, while maintaining isothermal performance characteristics over the device area. They are specified primarily in applications where:

• Available thickness is below 1.0mm
• Heat flux is moderate-to-high but local rather than distributed
• Direct die contact is required without a separate spreader

This category includes some of the thinnest production thermal management devices in modern electronics, used in flagship smartphones, foldable displays, and ultra-thin computing devices.

Manufacturing Implications

The production process for flat plate micro heat pipes overlaps with both cylindrical heat pipe manufacturing (for wick sintering, working fluid handling, and helium leak testing) and vapor chamber manufacturing (for flat-plate sealing). Specialised equipment is required for the very thin form factor — particularly precision sintering for thin wick structures and high-resolution sealing technology for the planar enclosure. Production yields are typically lower than for thicker UTHPs due to the manufacturing precision required at the sub-millimeter scale.

4. Ultra-Thin Loop Heat Pipes (LHPs)

Loop heat pipes operate on the same evaporation-condensation cycle as standard heat pipes but with separated evaporator and condenser sections connected by smooth-walled vapour and liquid lines. The loop architecture provides several advantages over inline configurations: the smooth-walled return line reduces flow resistance, the separated condenser eliminates vapour entrainment of the returning liquid, and the loop can transport heat over much longer distances (commercial loop heat pipes reach 23+ meters).

Ultra-thin loop heat pipes are flattened loop configurations developed for mobile device thermal management and battery thermal control applications. They retain the long-distance transport capability of standard loop heat pipes while fitting into form factors typical of consumer electronics.

Wick Architecture

Loop heat pipes concentrate the wick structure in a compensation chamber adjacent to the evaporator, rather than distributing it along the full tube length. This architectural choice — wick only where capillary pumping is needed — is what enables the long transport distances without the capillary limit issues that constrain inline heat pipes.

Manufacturing Implications

Loop heat pipe manufacturing introduces production challenges not present in standard heat pipe manufacturing. The compensation chamber requires precision sintered wick fabrication separate from the main loop. Vapour and liquid line construction requires precision tube forming and welding at multiple connection points. Working fluid charging is more sensitive than in inline heat pipes — fill ratio errors propagate through the loop architecture and are harder to recover from. Helium leak testing is critical because the loop has multiple welded joints, each a potential failure point.

For manufacturers considering loop heat pipe production, the equipment investment is significantly higher than for cylindrical heat pipe production. The complete loop heat pipe production line includes specialised stations for wick assembly, multi-section welding, and loop integrity testing in addition to the standard 11 manufacturing steps used for cylindrical heat pipes.

5. Flexible Heat Pipes

Flexible heat pipes are heat pipe configurations designed to bend or flex along part of their length without compromising thermal performance. The flexibility is typically located in the adiabatic section between the evaporator and condenser, allowing the relative position of heat input and heat rejection to be adjusted during installation or under operational conditions.

Three Construction Types

Metal flexible heat pipes use either copper's natural ductility or metal bellows in the adiabatic section. They achieve good thermal performance and pressure containment but are limited in maximum flex angle (typically 30–45°) by the metal's mechanical limits.

Polymer flexible heat pipes use flexible polymer materials as the casing. They achieve much greater flex angles (90°+) and conform to complex surface geometries, but the lower thermal conductivity of polymers limits heat transport capacity to lower-power applications.

Metal-polymer hybrid flexible heat pipes combine metal evaporator and condenser sections (for thermal performance) with polymer flexible sections (for bending capability). This architecture addresses both requirements in a single device.

Application Categories

Flexible heat pipes are specified in:

• Aerospace systems requiring vibration tolerance and lightweight construction
• Foldable consumer electronics (foldable smartphones, dual-screen laptops)
• Robotic systems with moving thermal connections
• Wearable electronics with non-rigid form factors
• Medical devices requiring conformability to human anatomy

Manufacturing Implications

Flexible heat pipe production requires specialised processes not used in rigid heat pipe manufacturing. Metal-polymer interface bonding must achieve hermetic integrity across the dissimilar materials. Bellows manufacturing for metal-flexible designs introduces forming tolerances and weld points absent from standard tube production. Polymer casing fabrication requires polymer-compatible working fluids (alternatives to water in many cases). For manufacturers entering the flexible heat pipe category, equipment investment goes well beyond a standard heat pipe production line and may require partnership with polymer processing specialists.

6. Micro Heat Pipes

Micro heat pipes are sub-millimeter-scale phase-change devices that operate on the same principle as conventional heat pipes but with one critical architectural difference: they typically have no internal wick structure. Instead, capillary pumping is provided by sharp-corner regions in the channel cross-section — triangular, rectangular, or non-standard polygonal channels — where surface tension of the working fluid generates sufficient driving force for return flow.

Three Configurations

Single micro heat pipes are individual micro-scale tubes typically 100–1,000μm in diameter and 10–60mm long, with cross-sections ranging from triangular to non-standard polygonal forms designed to enhance capillary pumping.

Micro heat pipe arrays are clusters of parallel micro channels in a single substrate, used as heat spreaders for chip-level thermal management. The array architecture provides spreading along the axial direction of all channels simultaneously.

Micro grooved flat plate heat pipes extend the array concept by interconnecting the vapour channels — the cross-channel connections enable two-dimensional heat spreading instead of the one-dimensional spreading of independent arrays. This category bridges micro heat pipe technology with vapor chamber-style planar performance.

Manufacturing Implications

Micro heat pipe production requires fabrication technologies not used in conventional heat pipe manufacturing. Channel formation typically uses precision micromachining, plasma etching, or laser micromachining rather than the tube cutting and shrinking processes of cylindrical heat pipe production. Substrate materials are often non-metallic (silicon, polymer) rather than copper. Hermetic sealing at the micro scale requires specialised bonding processes — anodic bonding, eutectic bonding, or polymer adhesion. The capital equipment investment differs fundamentally from conventional heat pipe production lines.

7. Oscillating (Pulsating) Heat Pipes

Oscillating heat pipes (OHPs), also called pulsating heat pipes, represent a fundamentally different approach to phase-change heat transport. Instead of using a wick structure to drive working fluid circulation, OHPs use a continuous serpentine capillary tube where the working fluid naturally distributes as alternating vapour-liquid plugs. Heat input causes the plugs to oscillate or circulate through the serpentine path, transferring heat from the heated end to the cooled end.

Two Loop Architectures

Closed-loop OHPs form a complete circuit, allowing the working fluid to circulate continuously in one direction. This architecture has received the most research attention because the directional circulation produces stable, repeatable thermal performance.

Open-loop OHPs have a serpentine path with closed ends, relying purely on oscillation rather than circulation. They are simpler to manufacture but have less stable thermal performance.

Performance Enhancement Variants

Recent OHP developments include:

• Check valve OHPs — incorporating one-way valves (including Tesla-type valveless designs) to force directional circulation
• Non-uniform channel OHPs — varying channel cross-sections to generate additional capillary driving forces
• Wicked OHPs — adding wick structures to selected sections to enhance latent heat transfer
• Three-dimensional OHPs — extending serpentine channels into 3D geometries for higher channel density per footprint

Application Categories

OHPs are specified in waste heat recovery systems, aerospace thermal management, electronics cooling, battery thermal control, and other applications where their unique combination of simple structure, low cost, and gravity-independent operation provides advantages over wick-based heat pipes. Effective thermal conductivity of OHPs can exceed solid copper by tens of times.

Manufacturing Implications

OHP production is fundamentally different from wick-based heat pipe production. The core process is continuous serpentine tube forming, vacuum charging with precise working fluid volume, and hermetic sealing — without the powder filling, sintering, and wick development steps that dominate cylindrical heat pipe production. The production equipment investment is correspondingly different. For manufacturers considering OHP production capacity, the process flow more closely resembles tube bending and refrigeration system charging than conventional heat pipe manufacturing.

8. Variable Conductance Heat Pipes (VCHP)

Variable conductance heat pipes (VCHPs) are heat pipe configurations designed to maintain near-constant operating temperature across varying thermal loads — addressing the limitation that standard heat pipes cannot self-regulate when heat input changes. The temperature regulation is achieved by introducing a non-condensable gas (NCG) charge into a reservoir at the condenser end.

Operating Principle

In normal operation, the NCG forms a "gas plug" in the condenser that occupies part of the condenser volume, reducing the effective condensation area. When thermal load increases, the higher vapour pressure compresses the gas plug, exposing more condenser area and increasing thermal transport capacity. When thermal load decreases, the lower vapour pressure allows the gas plug to expand, reducing condenser area and thermal transport. The result is that operating temperature stays nearly constant across a wide range of thermal duties — a critical capability for temperature-sensitive applications.

Construction Variants

Basic gas reservoir VCHPs add a gas chamber at the condenser end. They provide temperature regulation but suffer from vapour diffusion into the reservoir, where it condenses and reduces gas charge effectiveness over time.

Wicked reservoir VCHPs add a wick structure to the reservoir, absorbing condensed liquid and stabilising the gas-vapour interface. This design improves long-term stability but adds manufacturing complexity.

Hot reservoir VCHPs locate the gas reservoir adjacent to or within the evaporator section, thermally coupling it to the heat source. This reduces gas temperature fluctuation due to environmental conditions and improves temperature regulation precision.

Active feedback VCHPs add temperature sensors, electronic controllers, and reservoir heaters to actively modulate the vapour-gas interface position. They achieve the tightest temperature control but require additional electronic systems and power.

Passive mechanical feedback VCHPs use bellows-type reservoirs that mechanically respond to source temperature, modulating the vapour-gas interface without electronic controls. They are simpler than active systems while providing better regulation than basic reservoir designs.

Application Categories

VCHPs are specified in aerospace thermal management (geostationary satellite radiator panels), spacecraft electronics cooling, sodium-sulphur battery temperature control (high-temperature operation), petrochemical process control, and any application where temperature stability of the heat source is more critical than maximum heat transport capacity.

Manufacturing Implications

VCHP production adds requirements beyond standard heat pipe manufacturing: precision NCG charging at calibrated volumes, gas-tight reservoir sealing, and (for active variants) integration of sensors, heaters, and control electronics. The non-standard NCG charging process requires equipment beyond standard water injection stations, and quality control must verify both working fluid charge and gas charge separately. For manufacturers considering VCHP production, the equipment investment is incremental on top of a standard heat pipe production line — not a separate manufacturing process flow.

9. Two-Phase Loop Thermosyphons (TPLT)

Two-phase loop thermosyphons are gravity-driven phase-change devices that transport heat through a closed loop using natural circulation. Unlike heat pipes, TPLTs do not require capillary wick structures — gravity returns the condensed liquid to the evaporator. The simpler construction (no wick) makes TPLTs cheaper to manufacture and capable of longer transport distances than wick-based heat pipes, but it constrains operation to configurations where the condenser is positioned above the evaporator.

Construction

TPLTs typically include an evaporator (positioned at the heat source), a condenser (positioned above the evaporator at the heat rejection location), a vapour line transporting vapour from evaporator to condenser, and a liquid line returning condensed working fluid by gravity. Common materials include copper, stainless steel, aluminum alloys, and glass. Working fluids are selected based on operating temperature: water, acetone, ethanol, CO₂, hydrogen, and various refrigerants are all in production use across different application categories.

Application Categories

Refrigerator cold storage management: TPLTs paired with phase change materials (PCM) provide stable temperature regulation for fresh food compartments, allowing the compressor cycle to operate at steady state for higher efficiency.

Air conditioning systems (TPLT/AC): Integrated TPLT and vapour compression systems share evaporator and condenser hardware. The system operates in AC mode when ambient temperature exceeds the switch reference temperature, and switches to TPLT (passive) mode when ambient is cool enough for natural circulation cooling — saving energy in spring/autumn conditions.

Heat pump systems (TPLT/HP): Similar dual-mode integration for solar-assisted or air-source heat pumps, where the TPLT mode transfers heat passively when source temperature is sufficient, and the HP mode activates when source temperature is too close to or below the target water temperature.

Data center cooling: Free-cooling architectures in cold climates use TPLTs to transfer heat from server racks to outdoor heat exchangers without active refrigeration during cool ambient conditions.

Manufacturing Implications

TPLT production differs from standard heat pipe production primarily in the absence of wick structure manufacturing and the addition of system-level integration. Tube forming, welding, and helium leak testing are common to both. Working fluid charging is similar but typically at larger volumes (TPLTs use more working fluid because the loop volume is larger). For manufacturers, TPLT production capacity is best understood as a complementary capability to standard heat pipe manufacturing — using related equipment and skills but serving different application categories.

10. Axial Grooved Heat Pipes (Spacecraft Applications)

Axial grooved heat pipes use micromachined grooves running along the inner tube wall as the capillary structure, replacing the sintered powder wick of standard heat pipes. This architectural choice — high-permeability grooves instead of high-capillary-pressure sintered wicks — produces heat pipes with specific advantages for spacecraft thermal management applications: high reliability under mechanical loads, low sensitivity to gas plug formation, strong heat transport capability, high effective thermal conductivity, excellent isothermality, and relatively straightforward manufacturing compared to alternative spacecraft thermal solutions.

Four Groove Configurations

Porous grooved configurations combine longitudinal grooves with porous capillary structures (sometimes screen-mesh covered) to balance high capillary force with adequate liquid transport.

Single-groove and re-entry groove configurations use re-entrant groove geometries optimised for liquid transport. Performance is determined by groove count, width-to-radius ratio (which affects automatic gas venting), and circumferential threading patterns.

Axial/hybrid groove configurations use advanced trapezoidal groove geometries or hybrid combinations of rectangular and re-entrant grooves to achieve low dryout risk with high evaporator capillary pressure.

Single-slit groove configurations use circumferential grooves on the vapour space inner wall connecting the vapour space to the liquid grooves — reducing vapour entrainment of working fluid. Liquid charge volume is critical: small reductions in fill volume produce significant capillary pumping pressure decreases and substantial heat transport capacity loss.

Application Categories

Axial grooved heat pipes are the dominant configuration in spacecraft thermal control systems — communication satellites, scientific spacecraft, space telescopes, and orbital platforms. Their advantages over alternative thermal solutions in space environments are significant: reliability under launch mechanical loads, gravity-independent operation in microgravity, long service life without maintenance, and predictable performance over the wide temperature ranges encountered in orbit.

Manufacturing Implications

Axial grooved heat pipe production requires precision groove machining technology not used in sintered-wick heat pipe production. Grooves are typically formed by drawing, plough-extrusion, electric discharge machining (EDM), or similar precision metal forming processes. The grooved tube production replaces the powder filling and sintering steps that dominate sintered-wick production lines. For aerospace-grade heat pipes, additional process controls — surface finish requirements, dimensional tolerance verification, working fluid purity standards — extend beyond commercial heat pipe manufacturing requirements. For manufacturers considering axial grooved heat pipe production, the equipment investment is fundamentally different from sintered-wick heat pipe manufacturing capacity.

How Heat Pipe Structure Selection Affects Manufacturing Equipment Investment

The eight heat pipe structures above use overlapping but distinct manufacturing process flows. For thermal solution manufacturers planning production capacity, equipment investment decisions need to be matched to the target product mix.

Production Process Overlap Map

Equipment Investment Patterns

Standard cylindrical, ultra-thin, and variable conductance heat pipes share most of the 11-step production sequence. A manufacturer producing all three categories can use a single integrated production line with marginal additional equipment for VCHP gas charging and UTHP-grade precision components.

Vapor chambers and flat plate micro heat pipes require separate flat-plate sealing infrastructure but share sintering, leak testing, and performance verification equipment with cylindrical heat pipe production.

Loop heat pipes and oscillating heat pipes require specialised tube forming and integration equipment that overlaps less with standard production. They typically justify dedicated production capacity.

Micro heat pipes use entirely different fabrication technology (lithography, etching) and are a separate manufacturing investment from any of the other categories.

Two-phase loop thermosyphons and axial grooved heat pipes share working fluid handling and leak testing with standard production but use distinct upstream forming processes.

For thermal solution manufacturers planning capacity expansion, the heat pipe structure selection directly determines what equipment investment is required. For more detail on the standard 11-step heat pipe production process, see our Heat Pipe Production Process: 11 Manufacturing Steps.

Conclusion: Match Structure to Application, Match Equipment to Production Plan

The eight heat pipe structures covered above span thermal duties from sub-watt smartphone cooling through 100kW+ spacecraft heat rejection, and form factors from sub-millimeter chip-level devices through multi-meter aerospace systems. Each structure exists because no single configuration is optimal across all applications — and the choice of which structure to specify, like the choice of which production capability to invest in, is an engineering optimisation rather than a quality judgment.

For thermal solution designers, accurate understanding of each structure's strengths enables correct specification decisions for new applications. For thermal component manufacturers, accurate understanding of each structure's manufacturing requirements enables correct equipment investment decisions for capacity expansion. The overlap between application engineering and production engineering — and the importance of matching equipment capability to product requirements — is what determines whether a thermal component supplier can serve a target application category profitably.

At CoolingThermal, we manufacture automation equipment for heat pipe and vapor chamber production covering most of the structures discussed above — including standard cylindrical heat pipes, ultra-thin heat pipes, variable conductance heat pipes, and vapor chambers. Our equipment supports the full production process from pipe cutting through performance testing, with line deliveries to thermal module manufacturers supplying tier-1 consumer electronics, server, and EV brands worldwide. Contact our engineering team to discuss the production equipment specifications for your heat pipe manufacturing requirements.

• Written by

  CoolingThermal Engineering Team

  CoolingThermal is an automation equipment manufacturer based in Kunshan, China, specializing in heat pipe and vapor chamber production equipment since 2017. Our engineering team designs, builds, and commissions complete production lines covering forming, degassing, welding, testing, and assembly processes. The technical content on this blog is written by the same team that develops the equipment — based on real production experience, not secondary research.