Case study: Office chiller and cooling plant controls replacement
Chillers, pumps, and plant controls were replaced at a high-rise commercial office building in downtown Chicago
A prominent downtown Chicago high-rise office building was facing high repair and maintenance costs for the three 28-year-old chillers that provided cooling for the more than 30 floors of commercial office tenants. Given the increasing cost of maintenance, risk of extended downtime and poor efficiency, the decision was made to replace the chillers and modernize the cooling plant infrastructure in 2016.
The original cooling plant design used constant-speed pumping to distribute chilled water to air handlers. Each air handler had two-way control valves and a central plant bypass valve allowed excess flow to recirculate in the plant. The existing configuration lent itself well to a conversion to variable primary flow chilled water distribution, where the existing bypass was changed to a minimum flow bypass.
The three chilled water pumps and three condenser water pumps were beyond their typical useful service life. Each pump was replaced with new, variable frequency drive driven pumps on a common header. New magnetic bearing oil-free chillers were selected for their high efficiency and low turndown capability. The cooling tower fans had previously been retrofitted with VFDs.
An existing supervisory building automation system controlled the old plant, as well as the air handling systems and terminal units throughout the building. As part of the bid package, a cooling plant optimization controller with BACnet communication was specified to control the new cooling plant equipment including the chillers, chilled water pumps and condenser water pumps. To support consistency for the operations team, the graphic user interface was developed on the existing BAS with point integration via BACnet.
The CPOC is mounted adjacent to the chillers in the basement mechanical room. The plant has now been in operation for four cooling seasons and is viewed as a success with the operations team. The controller handles chiller staging, pump staging and a variety of temperature and pressure resets. The cooling tower fans continue to be managed by the base building automation system.
Like most VFD driven centrifugal chillers, a 700-ton centrifugal chiller operates most efficiently when partially loaded. As the condenser water temperature increases, the increase in chiller lift results in more work for the same amount of cooling. Note that ASHRAE 90.1-2019: Energy Standard for Buildings Except Low-Rise Residential Buildings section 126.96.36.199 requires that either the leaving chilled water temperature be reset based on load or outdoor air temperature, with an exception allowing for constant leaving water temperature if the chilled water flow varies with load. The 2018 and 2021 editions of the International Energy Conservation Code has similar requirements under section C403.4.4.
In contrast, a non-VFD driven chiller performance map has a very different profile. Unlike the case study system, a plant with multiple types of chillers or different vintages of chillers presents a challenging scenario for staging based on efficiency without the help of sophisticated software.
To demonstrate that there is a notable difference in plant performance when staging chillers based on efficiency rather than capacity, we can analyze the performance map from the case study VFD driven chiller using AHRI 550/590 part load distributions. Take Table 1 where chillers are staged on only when necessary to meet load as compared with Table 2, where chillers are staged on earlier. The data is provided at a constant leaving chilled water temperature of 42°F as part of a variable flow system. Assuming a partially occupied building peak cooling load of 1,785 tons and 1,500 occupied cooling hours per year, we can estimate the chiller energy use at the two scenarios.
Comparing the two scenarios, the CPOP saves an estimated 63,655 kilowatt-hour, a 9.1% improvement on chiller energy alone. The savings could be better given real-time decision-making factoring in load, condenser water temperature and variable speed pump control. During the design process, cooling load software was used to iterate and optimize chiller selections and perform a cost-benefit analysis from different manufacturers. Entering manufacturer chiller performance maps and analyzing plant staging options in energy modeling software can provide insight into the performance for different equipment configurations.
In the case study project, the CPOC controls both chiller staging, pump staging, resets and operation items like modulating the low flow chilled water bypass. Given that all new plant controls were required as part of the project, the incremental increase in cost for a CPOC was justified when factoring in the cumulative savings of improved chiller staging, optimal pump staging, pump pressure setpoint resets and cooling tower fan control.
While this case study emphasized the opportunity in VFD driven chillers, a 2016 case study conducted by Pacific Northwest National Laboratory for the General Services Administration at a facility with constant speed compressors indicated that “this technology, as demonstrated, may be cost-effective because the energy savings justify the installed costs …”