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Oil-Free Air Compressors

Part 2: The Modern Alternative of Centrifugal Cooler Design


The Impact of Adverse Operating Conditions on Cooler Life

As we mentioned in Part 1: Cooler Design Considerations for Centrifugal Compressors, it’s essential to understand the cooler design of your compressor system, as some cooler designs perform better than others under less than ideal operating conditions. Examples of how adverse operating conditions can negatively impact cooler life include:

  • Oil-free compressor condensate is aggressive, eroding tube sheets on the shell side. In typical industrial/process applications the condensate is acidic, in the 4.0 – 5.0 range on the pH scale (7.0 pH is neutral).
  • Erosion of tube sheets can occur inside the tubes if cooling water chemistry is not maintained.
  • Thermal expansion/contraction rates of various materials differ, which impacts cooling efficiency as fin contact with the tubes is compromised.
  • The type of expansion joint between the tube and tube sheet can weaken the roll joint, creating a failure point. Serrated tube sheets with pneumatically rolled joints are a better design than straight tube hole rolled joints.
  • Fin thermal expansion and the flattening (fins rolled over) damage that occurs during pulling and cleaning will degrade cooling capability and shorten the life of the cooler bundle.

Typical conditions after three to five years includes hard scale builds up on tube sheets, extreme corrosion, and removal/installation damage to fins are all apparent. Small 3/8” diameter tubes are prone to clog quickly and require regular rodding to remove both soft sludge and hard scale buildup. The U-bend design can be very difficult to clean, often requiring the use of harsh chemicals, especially if the service interval is too long.  U-bundle designs have a high replacement rate. In the cases illustrated below, it was less expensive to replace the bundles with new units than to clean and repair them.

The Modern Alternative: Stainless Steel Centrifugal Cooler Design

Stainless steel cooler designs provide much longer service interval and life for centrifugal compressors. The best designs include these features:

  • Large stainless-steel tubes – ½” OD
  • Stainless steel tube sheets of similar material as the tubes
  • Aluminum fins for efficient cooling
  • Tubes welded to tube sheets to ensure stable expansion and eliminate joint failure (no rolled joints)
  • Hydro-formed expansion of tubes the entire length of the bundle
  • Proper tube-to-tube sheet and tube-to-fin contact to provide stability, optimize heat transfer, and extend cooler life

Advantages of Stainless Steel Cooler Design

There are significant advantages of stainless steel design, such as:

  • Superior corrosion resistance is more forgiving of less than ideal water chemistry.
  • Superior corrosion resistance extends the cleaning/replacement cycle and minimizes degradation of compressor performance during cooler life.
  • Straight tube design allows for easier cleaning of soft sludge buildup in an open tower system.
  • Shroud provides better fin protection, reducing the chance of damage during cleaning.

Don’t Overlook the Coolers!

When evaluating a new compressor, it’s important to evaluate on-site performance, up-front cost and maintenance costs, but it’s equally important to evaluate how well the compressor will perform over its entire life. Cooler design and materials are key factors to consider when evaluating long term performance and total cost of ownership.

Reach out to our compressed air experts today for assistance with your cooler design!

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