Liquid Filling Rate & Water Injection Volume Determination for Vapor Chambers (VC)

This document addresses the water injection volume for heat pipes and vapor chambers, focusing specifically on copper-water VC structures.

Key Questions Covered

1. What are porosity and liquid filling rate?
2. How much water should be injected, and how is this volume determined?
3. Is vacuum pumping used to remove inert gases? Will injected water be pumped out? Can we pump a vacuum first and then inject water?
4. Why is secondary degassing required?

01 Working Principle

Both heat pipes and vapor chambers (VC) rely on phase change for rapid heat transfer:

 Water absorbs heat, evaporates into vapor, and moves to the cold end under internal pressure.

 The vapor releases heat and condenses back into liquid.

 The liquid returns to the evaporation end via the capillary structure, completing the heat transfer cycle.

From this principle, we can derive these conclusions:

1. A vapor channel is essentialIt provides a path for vapor from the evaporation section to the condensation section. Without it, vapor accumulates in the evaporation section and cannot cool effectively.
2. Overfilling blocks the vapor channelIf the vapor channel is filled with liquid water, vapor flow is blocked, preventing heat transfer.
3. Excess water submerges the evaporation sectionThe evaporation section contains capillary structures. Overfilling leads to pool boiling, while a proper volume enables thin-film evaporation, which has better heat transfer performance.
4. Insufficient water causes dryoutWhen not operating, water distributes within the capillary structures. If the volume is too low, rapid evaporation depletes the capillary supply, leading to dryout of the evaporation section.

02 What is Liquid Filling Rate? How to Determine and Calculate Water Volume?

Since both overfilling and underfilling are problematic, and the required volume varies by product design, the liquid filling rate is used to standardize this parameter.

 Liquid filling rate is defined relative to the volume of the capillary structure. A 100% filling rate means all open pores in the capillary structure are filled with liquid.

 Porosity refers to the ratio of open pore volume to the total volume of the capillary structure.

Steps to Determine Water Injection Volume

1. Measure porosityCommon methods include the weighing method (most widely used), density method, mercury intrusion porosimetry, ultrasonic testing, gas adsorption, and metallographic analysis.The weighing method calculates pore volume by measuring the weight change of the capillary structure before and after being soaked in liquid (e.g., water).
2. Calculate total capillary volumeSum the volumes of all capillary structures inside the VC (e.g., copper mesh, copper powder, powder columns).
3. Determine liquid filling rateBased on product design and performance requirements, a typical filling rate ranges from 60% to 80% to balance heat transfer efficiency and dryout risk.
4. Calculate water volumeMultiply the total capillary volume by the porosity and the target liquid filling rate.

Water volume directly impacts the VC's maximum heat load (Qmax), thermal resistance, startup temperature, and thermal shock reliability.

03 Role of Primary and Secondary Degassing & Impact on Water Volume

Both primary and secondary degassing aim to remove non-condensable inert gases from the VC cavity. If inert gases occupy the vapor chamber, they block vapor flow and prevent condensation, rendering the VC ineffective.

 Primary degassing: Direct vacuum pumping to remove most inert gases.

 Secondary degassing: Heating the lower section of the VC to drive remaining inert gases and water vapor into a dedicated gas storage section of the injection tube, which is then sealed off. Heating also releases gases trapped in the capillary structure.

Key Points:

 Both degassing steps cause unavoidable water loss, which cannot be calculated theoretically and must be determined experimentally. The initial water injection volume must include a compensation for this measured loss.

 The duration of degassing requires optimization. Once a target vacuum level is reached, prolonged pumping will only increase water loss without improving vacuum quality.

 It is impossible to remove 100% of non-condensable gases.

04 Vacuum First or Water Injection First?

In theory, pumping a vacuum first and then injecting water would prevent water from being lost during pumping. However, this presents significant technical challenges:

 It requires precision equipment for seamless switching between vacuum and injection, absolute airtightness, micro-scale volume control, and removal of dissolved gases from the water.

 This approach is not yet commercially viable in the industry.

Current Industry Practice

To reduce water loss during vacuum pumping, the lower section of the VC is submerged in cold water before or during pumping. This lowers the evaporation rate of the water, minimizing loss.

Stamping Process for Radiator Components

Stamping plays a very important role in the manufacturing process of radiator components. Below is a brief list of radiator components that require stamping process:

 Modules: FIN sheets, base plates, hardware brackets, fasteners, etc.

 Heat pipes and vapor chambers (VC): Upper and lower cover blanks, copper meshes, press rivets, semi-finished product trimming, shaping, etc.

05 Basic Processes of Cold Stamping

According to the deformation characteristics of materials, cold stamping processes can be divided into two categories: separation processes and deformation processes.

 Separation process: After the completion of this stamping process, the stress in the deformed part of the material exceeds the fracture stress of the material, resulting in fracture and separation of the material. Examples include punching, blanking, trimming and other processes.

 Deformation process: After the completion of this stamping process, the stress in the deformed part of the material exceeds the yield stress σs of the material but does not reach the fracture stress σb, thereby causing plastic deformation of the material and changing the original shape and size of the material. Examples include bending, drawing, flanging, hole expanding and other processes.

• 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.