Dry vs “Wet” Compressed Air

Basics

• This page aims to cover the differences between the Compressed Air as it comes out of most Air Compressors (particularly Oil Sealed/Lubricated Reciprocating Piston based compressors), and how that compares to properly filtered/“dry” air (and to a lesser extent Dry Nitrogen )
• The distinction is largely important for making sure it is compatible with the intended end use (or aiding in selecting Compressed Air Filters / Compressed Air Dryers etc), as well as for design of Compressed Air Plumbing for a Centralized System

Considerations

Preface

• As stated previously, this page aims to largely cover Oil Sealed/Lubricated Piston Based Compressors
• As the air exits most of these it contains Oil Mist and Water Vapor
• The Oil Mist isn’t the WORST for most things, but can be problematic for some uses, in particular Supplied Air Respirators etc
• The Water Vapor can introduce some issues in terms of Long Term Storage use, or especially later when it cools and forms condensate (which can lead to Vapor Lock / Steam Hammer type issues and other issues of a system intended to be Single Phase Flow becoming Two Phase Flow )

Oil Mist

• This is (for the most part) the easier to solve issue
• Compressed Air Oil Mist Filters are quite common
  • Demister / Mist Remover can be related terms for larger refinery sized units etc
• A Canister of Granulated Activated Carbon may be of use as well for “cleaning” any residual
• Finally for certain uses such as Supplied Air Respirators or SCUBA Tank Filling (or SCBA as well) verification of the Compressed Air’s cleanliness is also required
  • Air Systems International ‘s “Breather Box™️” is the COTS industry standard of sorts, although they aren’t TOO DIFFICULT to DIY
    • Short of Liability/Traceability etc, especially for IDLH use

Water Vapor

• Drying can be done BEFORE compression, or after
• As of this edit, this page will largely focus on Post-Compression Water Vapor Removal (although feel free to add more info!)

Post-Compression Water Vapor Removal

• Several systems are possible, but mainly:
  • Silica Gel (or other Desiccant ) Canisters
    • Either a sensor, or Moisture Indicating Silica Gel are useful for this system to be able to determine when the media is Saturated and needs dried out
  • Condensation Based Systems
    • Depending on desired dryness, and local conditions, a system more akin to a Condensing Furnace may suffice
    • Other systems may require submersion in cooling water (or some other Heat Transfer Fluid ) or use of a Vapor Compression Refrigeration system
  • Hybrid systems can also be used, where a primitive condenser is used to “take some of the load off” of the desiccant

Being able to use “standard” air

• Due to the complexity/bulk, as well as COST of the systems, especially for (High Level) Drying, selecting and designing systems+procedures such that they can tolerate usage of unprocessed “straight out of the compressor” air is beneficial
• Furthermore from a Design for Component Failure / Anti-Fragility perspective, there may even be some advantage to having ALL compressed air lines be plumed in a way that can handle operation in a “wet” mode