Entrained Gases in the System and Related Problems
Gases present in mechanical and hydronic systems are often invisible, poorly understood, and routinely underestimated. Yet their effects on performance, efficiency, and reliability are profound. This article provides a structured overview of how gases enter systems, the forms they take, and the cascading problems they cause in heating, cooling, and mechanical installations.
Key Takeaways
| Question | Short Answer |
|---|---|
| Are gases in closed systems unavoidable? | Yes. They enter during filling, operation, and through ongoing diffusion. |
| Do small gas volumes really matter? | Yes. Even thin gas layers significantly reduce heat transfer. |
| Are air vents sufficient? | No. Static vents only address free gas, not dissolved or entrained gas. |
| What is the biggest operational risk? | Hidden efficiency loss combined with long term corrosion. |
| Can gas problems be managed continuously? | Yes, with appropriately designed gas separation strategies. |
1. What Is Meant by “Gases in the System”
In mechanical and hydronic systems, “gases in the system” refers to the presence of non liquid phases within a fluid circuit.
These gases are typically air or its constituent components, such as oxygen and nitrogen, present in different physical forms depending on pressure, temperature, and flow conditions.
2. The Three Forms of Gas in Mechanical Systems
Gases appear in three primary forms.
Dissolved gas exists at the molecular level within the fluid. Entrained gas consists of microscopic bubbles carried by the flow. Free gas forms visible pockets that collect at high points or low velocity zones.
3. How Gases Enter Closed Loop Systems
Gas ingress occurs at multiple stages.
Initial filling introduces air, while ongoing operation allows gases to enter through diffusion, make up water, pressure fluctuations, and maintenance activities. Temperature changes continuously drive gas release from solution.
4. Gas Behaviour Under Temperature and Pressure Changes
Gas solubility in water is governed by pressure and temperature.
As fluid temperature increases or pressure decreases, dissolved gases come out of solution, forming microbubbles. These bubbles migrate, coalesce, and often become trapped in heat exchangers and emitters.
5. Impact on Heat Transfer Performance
Gas layers act as thermal insulation.
Even a thin film of gas on a heat transfer surface significantly increases thermal resistance, reducing effective heat output, increasing warm up times, and forcing systems to operate at higher temperatures.
6. Hydraulic and Flow Related Problems
Entrained and free gases disrupt stable flow.
They cause noise, cavitation risk in pumps, flow imbalance, and erratic control valve behaviour, undermining system controllability and comfort.
7. Corrosion and Material Degradation
Dissolved oxygen is a primary driver of corrosion.
Over time, corrosion products form sludge and deposits, restricting flow paths, fouling heat exchangers, and accelerating component failure.
8. Why Traditional Venting Is Often Insufficient
Manual and automatic air vents address only free gas.
They do not remove dissolved gas and are ineffective against entrained microbubbles circulating within the system, leaving the root cause largely untreated.
9. System Level Consequences of Unmanaged Gas
Unchecked gas accumulation leads to compound failures.
Efficiency loss, rising energy consumption, corrosion damage, increased maintenance, and shortened asset life reinforce one another, often without obvious early warning.
10. Managing Gases as a Continuous Process
Effective gas management requires continuous separation, not periodic venting.
Modern approaches focus on creating conditions where gases are encouraged to separate from the liquid phase and be removed under normal operating conditions.
Conclusion
Gases in mechanical systems are not incidental defects but inherent by products of operation.
Understanding how gases enter, behave, and interact with system components is essential to maintaining efficiency, reliability, and longevity. Addressing gas related problems at their source transforms hidden losses into measurable performance gains.