District energy systems and campus distribution networks serve a fundamentally different design brief than standalone building HVAC. A coil in a single building air handler serves one space, at one set of conditions, connected to one piping circuit. A coil in a campus distribution system serves a building that is one node in a network carrying fluid at pressures and temperatures that reflect the demands of the entire campus, not just the building it is in.
The coil selection decisions that are routine in standalone building applications become more consequential at campus and district energy scale. Getting them right requires understanding how the campus network affects the conditions the coil will see in service, and how the coil’s performance affects the network it is connected to.
Pressure requirements at campus scale
Campus and district energy distribution systems operate at higher pressures than typical standalone building hydronic systems. A central plant serving a large campus may distribute chilled water or steam at pressures that exceed the standard working pressure ratings of commercial HVAC coils. The coil selection needs to account for the actual operating pressure of the campus distribution system, not the nominal pressure of the building’s internal system.
In steam distribution systems, the pressure at the building connection point reflects both the central plant operating pressure and the pressure drop across the distribution network. Campus steam systems frequently operate at medium pressure, typically 15 to 150 PSI, which requires coils rated for medium-pressure steam service. Standard low-pressure steam coils rated for 15 PSI service are not appropriate for medium-pressure campus steam applications, and a coil installed at the wrong pressure rating is both a performance issue and a safety concern.
For chilled water distribution, the pressure at the building connection reflects the pump head of the central plant plus any static pressure from elevation differences across the campus. Large campuses with significant topographic variation can produce chilled water supply pressures at low points in the network that exceed the pressure ratings of standard commercial coils. The building isolation heat exchanger or the coil itself needs to be selected for the actual supply pressure, not the building design pressure.
Temperature conditions in campus distribution
Campus distribution systems operate at temperatures that may differ significantly from what a standalone building system would use. Central plant chilled water supply temperatures are often colder than what a conventional building chiller produces, because serving a large distribution network with significant pipe losses requires a lower supply temperature to deliver adequate cooling capacity at the most remote buildings.
For steam systems, the supply temperature at the building connection depends on the steam pressure, which varies across the campus network. A building close to the central plant sees higher steam pressure and temperature than a building at the end of the distribution network. The coil selection for each building needs to reflect the actual steam conditions at that building’s connection point, not the central plant design conditions.
High-temperature hot water campus systems, which operate above 250 degrees Fahrenheit, require coils designed specifically for HTHW service. The material selection, pressure ratings, and thermal expansion accommodation requirements for HTHW coils differ significantly from standard hydronic coil specifications. A campus transitioning from steam distribution to HTHW distribution needs coils that are designed for the HTHW application, not adapted from standard hydronic specifications.
Performance documentation for campus energy modeling
Campus and institutional facility owners increasingly require detailed coil performance documentation for building energy modeling, LEED certification, and campus energy master planning. A coil that delivers adequate heating or cooling capacity is not sufficient if the facility engineer cannot document what the coil’s performance contribution is to the overall building energy model.
The documentation requirements for campus coil applications typically include certified performance data at multiple operating conditions, pressure drop data at design flow and at reduced flow conditions for variable flow applications, and in some cases independent third-party performance verification. AHRI Standard 410 governs performance testing for forced circulation coils, and an AHRI-certified performance rating provides the documented basis for energy modeling that campus facility engineers require.
For coil replacements in existing campus buildings, the replacement coil documentation needs to match the format required by the campus facility management system. A coil that was originally specified with a particular set of performance parameters and installed in a building that is part of a campus energy monitoring program needs to be replaced with a coil whose performance is documented in a way that allows the campus energy model to be updated accurately.
Coil size and custom fabrication requirements
Campus and district energy coil applications frequently require coil sizes that are outside the standard commercial catalog range. Large air handling units serving auditoriums, laboratories, athletic facilities, and other high-occupancy campus spaces have coil face areas and capacity requirements that custom fabrication addresses more reliably than standard catalog products.
Custom fabrication also allows the coil geometry to be optimized for the specific operating conditions of the campus distribution system. A coil designed for the actual supply and return temperatures, the actual flow rates, and the actual pressure conditions at the specific building connection point will perform more consistently than a standard catalog coil adapted to conditions it was not designed for.
HX Coils reviews every campus and district energy application before fabrication begins. For applications where performance documentation is required for energy modeling or accreditation, the documentation package is defined before fabrication so the test data and certifications are available when the coil ships. For campus applications where the boiler plant or central steam system is also part of the project, the GP Energy Products team handles the boiler and steam system side of the conversation. Visit gpenergyproducts.com for more on GP Energy’s commercial and industrial boiler capabilities.
HX Coils manufactures custom coils for campus and district energy applications across the Mid-Atlantic region. If you have a campus distribution project in development, reach out before the coil is specified and we will make sure the selection accounts for the actual conditions the coil will see in service.
References
1. ASHRAE. District Cooling Guide. Covers chilled water distribution system design including pressure and temperature conditions at building connection points. ashrae.org
2. ASHRAE. District Heating Guide. Covers steam and hot water distribution system design for campus applications. ashrae.org
AHRI Standard 410. Forced-Circulation Air-Cooling and Air-Heating Coils. Governs performance testing and certification for 3. HVAC coils including campus applications. ahrinet.org
4. International District Energy Association. Campus Energy Systems Design Guide. Covers coil selection and performance documentation requirements for district energy applications. districtenergy.org
