Transpiration of Water Vapour In Dry Air Systems

All materials are permeable to water vapour, as the water molecule is extremely small compared to the structure of solids, especially when compared to the polymer construction of plastics, and even to the crystalline structure of metals.

Daltons law states that the overall pressure of a gas equals the sum of pressures exerted by its constituents. Air at 1 BarA may contain 75% Nitrogen, 21% Oxygen, 3% Carbon Dioxide, the rest made up of other gases including water vapour, hence 0.75 BarA is exerted by Nitrogen, 0.21 BarA exerted by oxygen etc... At an ambient of 20 oC and a relative humidity of 50%, water vapour comprises 1.2% of the atmosphere. The partial pressure exerted on the outside of a compressed air line will be 11.7 mBarA. If the dry air inside is pressurised to 8 BarA at -50 oC Dewpoint, the partial pressure exerted by the compressed air’s water vapour is 0.039 mBarA. We now have atmospheric water vapour at 11.7 mBarA pushing through a porous medium against a dry air water vapour pressure of just 0.039 mBarA. Naturally water will migrate into the pressurised air line! This is transpiration.

Another factor that will affect dry air systems is adsorption and desorption. Materials will act like sponges to moisture to varying extents. A plastic air line will adsorb moisture onto its structure, and will release it into dry air. Initially when dry air flows through plastic pipe, it will collect a lot of water, the pick up will slow down over time but never stop as transpiration takes over.

Over a long pipe run, water will inevitably migrate into any line, even through the most resistant materials; it should not be assumed that the moisture on the outlet to line will be the same at on the inlet.

The best materials to resist these effects are stainless steel and PTFE. Galvanised Steel and Copper are satisfactory at lower dew points. Another consideration with high purity systems are the integrity of all connections. If a small leak occurs in a high pressure line of course gas will leak out, but vortices at the leak point and a negative vapour pressure differential will allow water vapour to contaminate the flow.

It is important then designing a dry air system that all these factors are taken into consideration. In order to maintain a good repeatable dry air system, we recommend:-

Careful consideration is taken when selecting materials.
For flexible lines only ever use PTFE, - NEVER PLASTICS OR RUBBERS.
Maintain good quality connectors
Use the smallest bore pipe possible to ensure the volume is swept frequently.

Article Source
This article is property of Ideal Group UK – Cleaning and Restoration Company in Kent. For more information on our commercial and domestic cleaning and restoration services, visit our website www.idealgroupuk.co.uk.
Re-distribution of this article is permitted only if the following conditions are met:

Do not remove the link to the article source
No changes are made to the article content or article source.
You agree and understand that Ideal Group UK Ltd is the owner of the exclusive copyright of this article.

Articles

Transpiration of Water Vapour In Dry Air Systems