Environmental Monitoring Equipment & Supplies • Sales, Rental & Service by Enviro-Equipment — Charlotte, North Carolina USA

Zephyr Rotary Claw

This article is part five of the "Remediation Secret Knowledge" series which draws upon my personal experiences working in the field with compressors which have a number of uses on remediation projects including:

  1. Operation of pneumatic pumps for groundwater extraction or chemical injection
  2. Air sparging or bio-sparging where compressed air is injected into the subsurface to remediate petrochemical saturated soil or impacted groundwater
  3. Ozone injection where compressed air is used to create ozone for injection

A number of parameters need to be determined prior to selecting a compressor for a specific purpose. Some of the typical parameters are as follows:

  • Pressure required
  • Flow required at a specific pressure
  • Moisture content allowed
  • Hydrocarbon content allowed
  • Outlet temperature allowed
  • Noise level allowed
  • Compressor runtime based on pressure and flow requirements
  • Expected service life of compressor components
  • Maintenance required
  • Cost of maintenance compared to other suitable compressor types
  • Power available to operate the compressor
  • Material composition and friction loss of piping to be connected to the compressor
  • Expected duration of the project
  • Budget allowed for purchase

These days a lot of remediation projects are awarded based solely on the low bid. For example, I remember a project we lost because of cost which the winning bidder had combined six small rotary vane compressors to create the required flow for the project compared to our one large, much more expensive, screw compressor. Unfortunately for them, about six months into the operation the compressors began to fail but they got the project and we did not. So needless to say, if you are going to use "or equivalent" in bids you are accepting, make sure you are getting an equivalent performance for the expected project duration.

Compressors and Air Sparging

Air sparging is a very common remediation method that requires the compressor to run continuously.  The projects typically require between six months and three years to complete depending on the size and concentration of the plume.  Rotary screw compressors have been the typical choice for these applications until the last decade where the rotary claws have become more popular.  Reciprocating and rotary vane compressors typically do not hold up well under continuous run applications for long periods of time (i.e. years) and reciprocating compressors are typically used in small product recovery pumps systems where compressor run times are short.

There are some really good quality vane compressors but they cost a lot more than the dry carbon vane type I've seen used on many remediation work sites. Their use would typically be similar to that of the rotary screw compressors.

Air outlet temperature is a concern for most air sparging applications.  Air\air after-coolers are used to cool the air prior to going through the regulators, flow meters and low temperature manifold piping typically used on these projects.  Oil mist and moisture are also a concern on most environmental applications.  Water and oil coalescing filters are used to remove oil and water where oil flooded rotary screw or reciprocating compressors are used.

Oil less air compressors are available but typically do not last as long as oil flooded units.  On short duration projects they may be fine and typically have lower cost. Some of the pros and cons of the compressors being discussed herein are presented below:

Rotary Screw Compressors

Pros:

  • High pressure and high flow available
  • No oil mist units available
  • Made for continuous run applications
  • Long service life
  • Available with noise reducing enclosures
  • Available with weather proof enclosures for outdoor installation

Cons:

  • High purchase price
  • High pressure not required on most environmental applications
  • Large footprint
  • Heavy
  • Outlet air typically requires temperature correction (after coolers) and moisture removal
  • Larger horsepower motors required to run compressor use more energy

Reciprocating Compressors

Pros:

  • Low cost
  • High Pressure
  • Oil less units available
  • Available in vertical and horizontal configurations
  • Outdoor units available

Cons:

  • Low available flow compared to others
  • Not typically for continuous run applications
  • Oil less units have short service life on remediation sites
  • High maintenance cost compare to others
  • High noise level
  • High pressure not required on most environmental applications
  • Outlet air typically requires temperature correction (after coolers) and moisture removal

Rotary Vane Compressor (Dry Carbon Vane Type)

Pros:

  • No oil mist in air
  • Provides higher flow at lower pressures than reciprocating compressors
  • Low Cost
  • Small foot print

Cons:

  • Short service life
  • Typically only provide up to 25 PSI of pressure
  • Carbon vanes can break if accidently rotated backwards on three phase units
  • Carbon vanes create black dust and filter require frequent cleaning
  • Air is typically hot

Rotary Claw Compressors

Pros:

  • High flow and up to 31 PSI of oil free air
  • Small foot print
  • Requires less horsepower to produce same or higher air flow as compared to the other units
  • Lower maintenance cost if well maintained
  • Long service life if well maintained
  • Produces sufficient pressure for most environmental applications
  • Does not require a storage tank
  • Very high outlet temperature on vacuum applications which is a pro if the outlet is going to an oxidizer    

Cons:

  • Very high outlet temperature requires after cooler for air sparging projects
  • Typically requires metal piping connections due to high temperature operation
  • Relatively loud requiring outlet silencers
  • Requires good ventilation to keep motor from overheating
  • Higher cost typically German made equipment
  • Longer lead time for parts and new units compared to the other compressors  

Environmental remediation projects typically take longer to complete than early estimates predict.  It is important to get good equipment from the beginning as O&M budgets seem to get used up pretty quickly and additional costs associated with equipment problems are likely not in the usual estimate.  In this environment of the "low bid wins no matter what" make sure you specify quality equipment that will last the life of the project (when properly maintained).

Look for Part 6 of our "Remediation Secret Knowledge" series when I'll discuss pumps including the centrifugal, diaphragm, progressive cavity, and liquid ring types.

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