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

Compressed Air Plumbing (Wet vs Dry)

• A well designed compressed air tank will have a a Design for Cleaning -esque Cone Bottom Tank and a fitting allowing some means of drainage
  • A 90 Degree Fitting and a simple Ball Valve can suffice
    • This is a very loud process, and also typically sends a stream of (Potentially Rusty) PCW spewing (especially if it hasn’t been drained in a while) so AT MINIMUM having to face away from the operator, and having them wear safety glasses+hearing protection CAN WORK
      • It should also be in an area where the PCW can be allowed to evaporate off (such as a concrete pad/inside etc, Rags/Oil Adsorbent Pads can also be used)
      • Ideally though, some sort of Compressed Air Tank Vent Muffler would be used, and the PCW would be separated + sealed
  • Automatic Compressed Air Tank Drains CAN be used
    • Main issues are:
      • Unless they have a pre-vent alarm/notification etc, it can catch people off guard (Although proper HAZCOM / a mention in the Sight Specific Safety Orientation (for all (new) workers, and any contractors ) could alleviate this, along with the aforementioned Muffler
      • A more subtle one is the role Routine Manual Draining has in terms of incidentally adding a Visual Inspection / “Look, Smell, Listen” check to things. Automated systems are convenient yes, but also can allow for forgetting of systems that would be better observed/maintained
        • Simply opening a ball valve (especially if it is designed in an Accesible Manner, ie Large Handle Ball Valve, not too close to the floor/covered in dirt etc) isn’t TOO HARD of a task too, so the need for automation isn’t that great (unless the site would be without people for long stretches of time)
• ”Wet” Compressed Air Lines:
  • Shall be sloped such that Condensate flows downhill to a central drain location, or at least flows under a Blowdown procedure sufficiently
  • They also shall be designed to resist any sort of corrosion/clogging/Vapor Lock that can occur
• ”Dry” Compressed Air Lines may have the advantage of simpler plumbing due to the pressurized gas not needing any sort of slope/special considerations
  • Although as per earlier, from a Design for Component Failure / Anti-Fragility perspective, doing so anyways may be worthwhile
    • If not however though, this may justify the cost for cleaning/drying systems and plays a role in that planning process/math

Internal Links

• Compressed Air
  • Compressed Air Storage
  • Compressed Air Plumbing
  • Compressed Air Hose
    • Compressed Air Hose Reel
  • Compressed Air Fittings
• Dry Nitrogen
  • Dry Nitrogen vs Dry Compressed Air
  • Open Source Pressurized Dry Storage Box