Steam coils installed in commercial buildings in the 1990s or earlier are now 25 to 35 years into their service lives. Many of them are still operating. Some are operating correctly. Others are operating with degraded performance that nobody has quantified because the air handling unit has not been opened in years and there is no baseline to compare against. And some are at a point where the combination of tube corrosion, fin pack damage, and casing deterioration makes replacement the right decision, even though the coil is technically still producing heat.
Replacing a steam coil after 20 or more years of service is a different project from replacing a coil after 5 years. The original documentation may not exist. The system conditions may have changed. The air handler may have been modified. And the coil itself may have developed failure modes that are specific to long-term steam service rather than to normal wear.
What steam does to coils over time
Steam coils accumulate damage through mechanisms that are specific to the steam condensate cycle and that are different from the corrosion and fouling mechanisms in a chilled water or heating hot water coil.
Oxygen pitting is the most common long-term failure mode in steam coil tubes. When the vacuum breaker fails or is missing, condensate is drawn back into the coil at shutdown and stands in the tubes until the next startup. That standing condensate contains dissolved oxygen that attacks the copper tube surface. Over hundreds or thousands of cycles, the pitting accumulates until tube wall thickness is reduced to the point where the tube fails under steam pressure.
Water hammer damage is the second common failure mode. Condensate that is not properly draining from the coil accumulates in the tube circuit and is accelerated by steam entering behind it. The water hammer that results produces mechanical stress at the tube-to-header connections that is different from the normal operating stress the coil was designed for. Over years of repeated water hammer events, the tube-to-header joints develop fatigue cracks that eventually leak.
Casing corrosion in aging steam coils is often more severe than the coil fins and tubes would suggest because the casing is exposed to the condensate environment on the outside of the coil while also being exposed to the airstream. Casings that have been insulated with wet or deteriorated insulation have been holding moisture against the metal for years, accelerating corrosion from the outside in.
Why the original specification may not be appropriate
A coil that was installed 25 years ago was specified for the system conditions that existed when the air handler was commissioned. In the intervening decades, the system conditions may have changed in ways that are not reflected in any documentation.
The steam supply pressure may have changed. The airflow through the coil may have changed. The condensate return system configuration may have changed. Each of these affects either the replacement coil’s pressure rating, its thermal performance, or its susceptibility to the failure modes described above. A replacement that replicates the original coil without accounting for current conditions may not perform correctly and may fail on an accelerated schedule.
What to assess before the replacement is specified
The tube condition assessment should include a visual inspection of accessible tube surfaces for pitting, corrosion, and discoloration, and where tube condition is uncertain, an ultrasonic wall thickness measurement at locations where pitting is suspected.
The vacuum breaker should be located, confirmed to be at the correct installation location at the highest point of the coil, and tested for correct operation. A vacuum breaker that is missing, incorrectly located, or mechanically failed has allowed condensate flooding at every shutdown for however long it has been in that condition. The extent of the oxygen pitting that has resulted is one of the key factors in whether the coil is repairable or needs replacement.
The steam trap should be inspected and tested. An aging trap that has failed open has been passing live steam into the condensate return system, pressurizing the return lines and preventing condensate from draining. A trap that has failed closed has been flooding the coil with standing condensate. The combination of a failed trap and a failed vacuum breaker is the scenario most likely to produce both oxygen pitting and water hammer damage.
The replacement specification
The replacement coil needs to be specified for the current system conditions rather than the original design documentation. Tube material and gauge selected for the actual steam supply pressure. Condensate drainage designed to minimize standing water at shutdown if oxygen pitting has been a problem. And the vacuum breaker replaced as part of the coil replacement regardless of its apparent condition, with its correct installation location documented for future maintenance reference.
For aging steam systems where the boiler plant is also approaching end of life, the GP Energy Products team handles commercial steam boiler assessment across Pennsylvania, New Jersey, Delaware, and Maryland. Visit gpenergyproducts.com for more. For condensate return pump systems in aging steam plants, the Merion Pump Company team handles condensate return and boiler feed pump assessment. Visit merionpump.com for more.
HX Coils fabricates custom replacement steam coils for commercial and institutional HVAC applications across the Mid-Atlantic region. Reach out before the replacement is specified and we will work through the condition assessment and replacement specification for the specific system.
References
1. ASHRAE. HVAC Systems and Equipment Handbook, Chapter on Coils. Covers steam coil selection, freeze protection, vacuum breaker specification, and replacement criteria for commercial HVAC. ashrae.org
2. Spirax Sarco. Steam Engineering Tutorials, Module 12. Covers steam coil failure modes including oxygen pitting, water hammer damage, and condensate drainage requirements. spiraxsarco.com
3. AHRI Standard 410. Forced-Circulation Air-Cooling and Air-Heating Coils. Governs performance testing and certification for replacement steam coils in commercial HVAC applications. ahrinet.org
4. ASME. Boiler and Pressure Vessel Code, Section VIII. Covers pressure vessel design requirements applicable to steam coil tube pressure ratings and wall thickness minimums. asme.org
