Chilled water coil replacement looks straightforward from the outside. The old coil comes out, a new one goes in, the system returns to service. That sequence works correctly when the replacement coil is specified for what the system actually is today, not for what it was when the original coil was installed. When those two things are different, and in many commercial buildings they are, the replacement coil that arrives may not perform the way the facility expects.
This article covers what needs to be confirmed before a chilled water coil replacement is specified, where the common specification gaps occur, and what HX Coils reviews before fabrication begins.
What a chilled water coil does and how the design variables interact
A chilled water coil removes heat and moisture from the airstream passing over it by circulating chilled water through the tubes. The leaving air condition, the temperature and humidity at which the air exits the coil, is determined by the interaction of four variables: the entering air condition, the chilled water supply temperature, the chilled water flow rate, and the coil’s heat transfer geometry.
Those four variables are not independent. Changing one changes the relationship between the others. A coil that achieves a leaving air temperature of 55 degrees Fahrenheit at a chilled water supply temperature of 44 degrees Fahrenheit and a flow rate of 6 GPM per ton may not achieve the same leaving condition if the chilled water supply temperature has risen to 46 degrees or the flow rate has dropped because a pump was replaced with a smaller unit. Understanding how the variables interact is what allows a replacement specification to account for the current system conditions rather than simply replicating the original coil dimensions.
Entering air conditions
The entering air condition is the temperature and humidity of the air arriving at the coil face. For a recirculating system serving interior spaces, the entering air condition is relatively stable and well-defined. For an air handling unit serving spaces with significant outdoor air fractions, the entering air condition varies with outside weather and with the outdoor air fraction set by the economizer controls.
The replacement coil needs to be selected for the design entering air condition, which is typically the peak cooling load condition. In a system where the outdoor air fraction has increased since the original coil was installed, either by design change or by economizer control modification, the entering air condition at peak load may be warmer and more humid than the original coil was designed for. A direct replacement will be undersized for the current load.
For air handling units serving spaces with variable occupancy or variable outdoor air requirements, the entering air condition at peak load should be confirmed against current system documentation before the replacement is specified. If current documentation is not available, the design engineer should be consulted.
Chilled water supply temperature and flow rate
The chilled water supply temperature and flow rate at the coil connections determine how much cooling capacity the coil can deliver for a given heat transfer geometry. Both of these variables can change over the life of the system in ways that are not always reflected in the as-built documentation.
Chilled water supply temperature may have changed if the chiller plant has been upgraded, if the system set points have been modified for energy conservation, or if the building has been connected to a campus chilled water system that operates at different supply conditions than the original building chiller. A coil specified for a 44-degree supply temperature will perform differently at 46 or 48 degrees, and the difference may be enough to prevent the coil from meeting the leaving air condition required for the spaces it serves.
Chilled water flow rate at the coil is determined by the pump serving that coil circuit and the hydraulic resistance of the piping between the pump and the coil connections. If the pump has been replaced, if the piping configuration has been modified, or if control valves have been added to the circuit since the original installation, the flow rate through the coil may be different from what the original coil was designed for. Confirming the current flow rate requires either field measurement or a hydraulic analysis of the current system configuration.
Coil geometry and face conditions
The physical geometry of the coil, its face dimensions, row count, fin spacing, tube diameter, and tube spacing, determines the heat transfer surface area available and the face velocity of the air passing through the coil.
Face velocity is the velocity of the air approaching the coil face, calculated as the airflow in cubic feet per minute divided by the coil face area in square feet. Face velocity affects both the heat transfer performance of the coil and its moisture removal capability. At low face velocities, below approximately 300 feet per minute, the coil removes less moisture per unit of cooling capacity because the air spends less time in contact with the cold coil surface. At high face velocities, above approximately 550 feet per minute, moisture carryover becomes a risk as condensate is carried off the coil surface rather than draining from the fin pack.
For replacement coils, the face dimensions are typically constrained by the air handler casing dimensions. The row count and fin spacing can be adjusted within those dimensional constraints to optimize the coil’s performance for the current operating conditions. A replacement with the same face dimensions but a different row count or fin spacing can deliver meaningfully different performance at the same entering conditions.
What changes in aging systems
Beyond the operating condition variables, aging systems often have physical changes that affect the replacement coil specification. Connection sizes and orientations that were modified during a previous repair. Casing dimensions that changed when the air handler was relined or repaneled. Control valve configurations that affect the flow direction through the coil.
For coils where the original documentation is unavailable, field measurements of the actual coil in place are the starting point. Face height and width measured at the coil itself. Connection size confirmed at the actual connections. Row count confirmed by counting rather than assumed from a specification that may reflect the original installation rather than what is currently in place.
For systems where the coil has been replaced before, the previous replacement record may reflect a specification that was itself adapted from the original. Building up a replacement specification from a previous replacement without verifying against current system conditions can propagate specification errors across multiple replacement cycles.
The application review before fabrication
HX Coils reviews every chilled water coil application before fabrication begins. The review confirms the entering air conditions against current system documentation or field data, the chilled water supply temperature and flow rate against current chiller plant and pump system parameters, the coil face dimensions from field measurements rather than nameplate data, and the row count and fin spacing against the current airflow and leaving air condition requirements.
For chilled water coil applications where the pump system serving the coil circuit is part of the review, the Merion Pump Company team handles chilled water pump selection and can provide flow rate and pressure data that informs the coil selection. Visit merionpump.com for more on Merion’s commercial pump capabilities. For coil applications that are part of a larger air handling unit replacement or packaged mechanical system project, the FabPro Systems team integrates coil specifications into factory-assembled packages. Visit fabprosystems.com for more.
HX Coils manufactures custom chilled water coils for commercial and industrial HVAC applications across the Mid-Atlantic region. Reach out before the replacement is ordered and we will confirm the specification is right for the current system before fabrication begins.
References
1. ASHRAE. HVAC Systems and Equipment Handbook, Chapter on Coils. Covers chilled water coil selection, entering air conditions, and face velocity requirements. ashrae.org
2. AHRI Standard 410. Forced-Circulation Air-Cooling and Air-Heating Coils. Governs performance testing and certification for chilled water coils. ahrinet.org
3. ASHRAE. Fundamentals Handbook, Chapter on Heat Transfer. Covers the heat transfer principles governing chilled water coil performance. ashrae.org
4. SMACNA. HVAC Systems Commissioning Manual. Covers field measurement procedures for existing coil installations. smacna.org
