Modernizing the Wrigley Field chilled-water system
Selecting and specifying the right chiller is generally dictated by capacity, and there are many philosophies on the best way to control, operate, and calculate system operational costs.
- Review methods to specify an air-side economizer, water-side economizer, and modified chiller efficiencies.
- Understand the control systems that are part of chilled-water systems.
- Assess a case study in which a chilled-water system was implemented.
With Wrigley Field reaching 100 years old, the Chicago Cubs Baseball Organization embarked on a number of HVAC improvements inside and outside the historic ballpark.
Throughout the Wrigley Field redevelopment design, the project team took steps to incorporate energy-efficient measures while meeting the client needs and overcoming all challenges. Three strategies—an air-side economizer, water-side economizer, and modified chiller efficiencies—are worth highlighting.
Air-side economizer options
According to the Pacific Northwest National Laboratory, “An air-side economizer is a duct/damper arrangement in an air handling unit (AHU) along with automatic controls that allow an AHU to use outdoor air to reduce or eliminate the need for mechanical cooling.” Certain outdoor-air (OA) and building cooling-load conditions justify the use of the AHU being put into air-side economizer mode. Generally, this would be when the outdoor dry-bulb air temperature is less than the return-air (RA) temperature; however, depending on the OA humidity for the region, this may create humidity-control issues.
There are four common control types for air-side economizers, with each presenting various advantages and disadvantages:
- Fixed dry-bulb temperature control
- Differential dry-bulb temperature control
- Fixed enthalpy with fixed dry-bulb temperature control
- Differential enthalpy with fixed dry-bulb temperature control.
Fixed dry-bulb temperature control is the least complex control type to implement and operate. A high-limit shutoff temperature setpoint is determined during system design, and the system operates based on that setpoint. If the outdoor-air temperature is below that setpoint, the air-side economizer is enabled, thus the RA damper and OA damper modulate together to meet load.
If the OA temperature is lower than the RA temperature, the air-side economizer is enabled, the RA damper is opened, and the OA damper is reduced to the minimum position. If the AHU system is capable, the OA damper position can vary in this mode based on system carbon dioxide (CO2) levels and building occupancy. To prevent short cycling of the economizer, a time duration for all control types is usually implemented into the AHU economizer-controls sequence.
Differential dry-bulb temperature control is still simple to implement and operate, but it requires additional controls programming from the fixed dry-bulb temperature control. To operate the economizer using this type of control, the AHU compares the OA temperature with the RA temperature being sensed by the RA temperature sensor. If the OA temperature is less than the RA temperature, the economizer is enabled; if the OA temperature is more than the RA temperature, the economizer is disabled.
While both fixed and differential dry-bulb temperature control methods are simple to implement and operate, neither incorporates the OA humidity into operating the air-side economizer.
Fixed enthalpy with dry-bulb temperature control introduces a new OA condition to be met to enable the economizer. Controlling the economizer based on the enthalpy in addition to the dry-bulb temperature further limits the usage of the economizer, but allows for better control of the system. By using fixed enthalpy with dry-bulb temperature control, the AHU control system assesses the OA enthalpy and dry-bulb temperature against the high-limit shutoff setpoints and disables if setpoints are exceeded.
The differential enthalpy with fixed dry-bulb temperature control operates with the strictest requirements, similar to the differential dry-bulb temperature control. In this control method, the AHU control system assesses the economizer mode based on the OA enthalpy and dry-bulb temperature against the RA enthalpy and dry-bulb temperature. If either of the respective OA air properties is above either of the RA air properties, economizer mode remains disabled.
When the 2013 edition of ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings was released, the standard no longer permitted economizer control evaluating dewpoint and fixed dry-bulb temperature. Whichever control type is decided for the project, sensor accuracy and reliability can have a significant impact on long-term operation. As the high-limit variable of an economizer determines if the economizer is enabled or disabled, this sensor can increase cost and energy consumption if not selected properly.
Designing a system with air-side economizer to comply with energy code can present several challenges depending on the building occupancy type and program requirements. Often, the most difficult design conditions are when there are high airflow requirements, such as in commercial kitchens, and/or narrow humidity control, such as in laboratories. For these types of building occupancies, a water-side economizer may be the optimal solution.
During the design of the Wrigley Field Plaza Building, a robust central cooling plant with a water-side economizer was planned and, therefore, an air-side economizer was not required. Due to the proximity of the two 50,000-cfm base-building AHUs located on the seventh-level mechanical penthouse to the outdoors, outdoor- and exhaust-air ductwork was routed through the roof and an air-side economizer was implemented for these AHUs. All other systems in the building rely on the water-side economizer.
Water-side economizer options
When evaluating whether a water-side economizer is the best solution, one must consider several conditions, beginning with the building usage and occupancy type. If the usage for the building requires tight humidity control, such as in museums, hospitals, or laboratories, the AHUs may not have the capability of providing adequate control of the space during the use of an air-side economizer. Furthermore, if the building is to have a central cooling plant, the OA-condition restrictions of the local climate may be so restrictive that the run time for the water-side economizer would exceed that of the air-side economizer. This would make a water-side economizer a more enticing option as it would be used more frequently.
There are arguments that could be made against water-side economizers due to their high use of make-up water. However, due to the low cost of water in Chicago as compared with the national average, a water-side economizer for this application had a clear advantage.
A potential justification for designing a water-side economizer is if the OA and exhaust louver locations established by the project are restrictive. Often for air-side economizer projects, the AHU needs to be near the exterior façade to enable a short connection to the outdoors. This often forfeits the sought-out perimeter offices for executives. A water-side economizer minimizes this because, instead of large ductwork resulting in lower ceilings or perimeter offices becoming mechanical rooms with a view, the ductwork is sized for minimum OA and the economizer requirement is met by the heat exchanger at the central cooling plant.
Per ASHRAE Standard 90.1-2016 Section 184.108.40.206, Fluid Economizers, if the project team elects to design a water-side economizer, the cooling towers/fluid economizer needs to “be capable of providing up to 100% of the expected system cooling load at OA temperature of 50°F dry-bulb/45°F wet-bulb and below.” This typically requires the engineer to calculate the economizer capacity using the load calculation with loads on the building broken down by OA temperature.
One exception for this section involves computer rooms; there is a table to evaluate the requirements based on the climate zone. The other exception for this section includes dehumidification requirements that cannot be met at a 50°F dry-bulb/45°F wet-bulb outdoor condition and where the expected system cooling load at 45°F dry-bulb and 40°F wet-bulb is met by the water-cooled fluid economizers. Standard 90.1 also requires the fluid economizer to have integrated economizer control.
When designing the water-side economizer with plate and frame heat exchangers, there are two piping arrangements for which the heat exchangers can be configured in relation to the chillers: parallel (Figure 3) and series (Figure 4). If the building system only has the option of a water-side economizer, then series arrangement with integrated control is the only option when following Standard 90.1. To have an integrated control of a water-side economizer per Standard 90.1, the system is responsible for not only providing free cooling when the outdoor wet-bulb temperature is 45°F and below, but also for prolonging the free cooling period by precooling the condenser-water temperate and warming the return chilled-water temperature.
In some cases, the air-side economizer option has been integrated into the system design. Unlike data centers for an office building, hotel, or mixed-used complex commercial building (such as a high-rise), the variation between the summer load versus winter load can be large—from 4:1 to as high as 10:1.
For most cooling tower manufacturers, cooling towers or cooling tower cells can be winterized by using weir dams or maintaining 50% or more of peak summer condenser flow at the nozzles to prevent scaling during winter operation. Depending on specific cooling tower selections, some manufacturers can reduce winter condenser flow even more than 50%.
Therefore, when choosing cooling towers and chillers for the project, the main selection criteria included the winter load, summer load, winter condenser flow, summer peak condenser flow, chiller capacity, and each cooling tower or cell capacity. The integrated/series application can start to perform free cooling at 47° to 50°F wet-bulb with better approach temperatures. The cooling tower approach temperature ideally should be limited to 5°F and operation for said cooling tower can be optimized if variable-speed drives (VSDs) are included on the cooling tower fans and condenser-water pumps.
If cooling tower size is based on the gallons-per-minute performance requirement for both summer and winter loads with an implemented air-side economizer in system design, along with weather data and winter cooling capacity, it is a comprehensive effort to validate the heat exchanger and winter cooling tower cell capacity.
It also is critical that the appropriate heat-exchanger configuration is reviewed and vetted. Cross-flow cooling towers start to have inner-cell ice forming with ambient temperature ranges from 40° to 45°F dry-bulb. It is recommended to slightly increase the temperature of chilled water and warm side of the heat exchanger during winter operation. This increases the condenser-water flow across the cooling tower fill, which helps prevent tower icing.
An additional criterion that must be met if a water-side economizer is used involves preheating the domestic hot-water system. Per ASHRAE Standard 90.1 Section 220.127.116.11, facilities that operate 24 hours a day where the total installed heat-rejection capacity of the water-cooled system exceeds 6 million Btu/hour of heat reject and the design service water-heating load exceeds 1 million Btu/hour, the domestic hot-water system must be preheated. The heat exchangers will seasonally switch over from water-side economizer to preheating of domestic hot water, which needs to be reviewed as an integrated building energy savings.
Standard 90.1 compliance as well as maintaining a desirable condenser temperature at 55°F and above for chiller operation are the key ways to truly save energy, especially when the application applies to hospitality/residential/hospital buildings. This temperature should be used as a starting point at the beginning of the condenser-water evaluation and should be adjusted depending on the age and type of chillers. Over the past 30 years, chiller performance has improved by effectively operating at lower conditions. Existing chillers may only perform effectively down to 60°F, whereas newer chiller technology has allowed some chillers to perform effectively down to as low as 45°F.
Designing a mixed-use building
During the planning and design of the Wrigley Field Plaza Building located adjacent to Wrigley Field, the design team was tasked to provide an architecturally appealing office building that would serve as the office location of the Chicago Cubs Baseball Organization. This proved to be challenging because all four sides of the office tower were planned for retail storefront, which left minimal facade for intake and exhaust louvers. At this point, the design team decided to integrate a water-side economizer into the central cooling plant to serve chilled water to the Wrigley Field Campus.
The Wrigley Field Plaza Building is a 220,000-sq-ft building with retail tenants on the ground floor and parts of the 2nd floor, a conference center on the 2nd floor (Level 2), tenant offices on Level 3, the Chicago Cubs Baseball Organization on levels 4 through 6, and a mechanical penthouse on Level 7. Below the 7-story mixed-use building are two subgrade basement levels of equal gross area. These two subgrade levels comprise the central chilled-water plant, Wrigley Field concession storage, a central kitchen commissary responsible for all food preparation of Wrigley Field, staff dining and locker rooms, and the clubhouse, player locker rooms, laundry, strength and conditioning, and hydrotherapy suite.
With the large number of locker rooms, laundry, and kitchen areas in the subgrade levels demanding large exhaust airflows (thus, OA for make-up), intake and exhaust were challenging. These conditions made designing the central cooling plant with a water-side economizer the optimal solution to provide the owner with desired spaces and ceiling heights.
The central cooling plant at the Wrigley Field campus is made up of three 585-ton centrifugal chillers in Level B2 with equally sized cooling towers on the roof. One cooling tower is winterized to allow for water-side economizer operation with heat tracing on outdoor piping and isolation valves on the condenser-water supply, return, and equalizer piping. Due to the limiting heights of the screened wall, coordination of the cooling tower piping connections and elevations of the piping to the support structure was critical. The chilled-water system is piped in a constant primary variable secondary system with two separate secondary systems.
One secondary loop serves the Plaza Building and is routed in the core of the building up to the AHUs in the penthouse, serving levels 2 through 6. The other secondary loop serves Wrigley Field and is routed underground to the ballpark; it runs overhead on the concourse level. Currently, the piping serves the concession stands, but there are plans in future phases to serve additional loads depending on future programming still in design.
During the design, one of the critical performance metrics was the minimum design load of the chilled-water system. Determining the peak cooling load for the campus was straightforward; during July, when a building in Chicago typically peaks, building usage is at its peak. A baseball game with a fully occupied office building, retail restaurants, and kitchen commissary at full usage is not uncommon for a typical July day at Wrigley Field.
When calculating the low load, however, the opposite was the case—the calculated load occurs during the winter when occupancy loads are less clear. Important criteria the design team assessed when determining the minimum cooling load included whether the team would be using the facilities for training during the offseason, the loading of the intermediate-distribution frame closets throughout the campus (which would be substantially lower as the office tower and ballpark operations would be minimal), and whether the kitchen commissary refrigerators and freezers would be emptied at the end of the season.
Additionally, the maximum load on the water-side heat exchanger and cooling tower cells also was assessed to determine the heat-exchanger capacity. Important criteria the design team assessed when determining the maximum water-side cooling load, besides the Standard 90.1 requirement, included whether there would be a winter event at the ballpark (i.e., outdoor hockey game), occupancy loads and uses, refrigeration equipment usage, and computer room cooling loads.
Among other cold-climate chilled-water systems, careful consideration is required to prevent continuous operation through the winter months to prevent freezing. During the design of the Wrigley Field Plaza Building, there were a number of precautions taken. These included heat tracing outdoor piping used in water-side economizer operation, isolation of valves not winterized for system drain-down, condenser-water bypass through the cooling tower basin, and variable-speed condenser-water pumps.
Modified chiller efficiencies
ASHRAE Standard 90.1-2013 Table 6.8.1-3 dictates minimum efficiency requirements for water-chilling packages that all projects are required to meet. Effective Jan. 1, 2015, the requirements were further restricted where water-cooled, electrically operated centrifugal chillers of 400 to 600 tons required 0.560 kW/ton at full load (FL) and 0.500 kW/ton (integrated part-load value; IPLV) for Path A. Additionally, Path B required 0.585 kW/ton (FL) and 0.380 kW/ton (IPLV) for the same chiller type and load range.
Both requirement paths are based on AHRI Standard 550/590 testing criteria. The difference is Path A is intended for applications where most of the load is full load while Path B is intended for applications where most of the load includes partial loads. Commonly, Path B includes chillers with VSDs whereas Path A includes chillers without VSDs; however, this is not a requirement. During the design of the chillers at the Wrigley Field Plaza Building, the load profile was concluded to be dynamic, thus VSDs were to be used for the chiller operation and compliance Path B was taken.
Furthermore, compliance with ASHRAE Standard 90.1-2013 Table 6.8.1-3 can be met if the chiller is designed toward AHRI 550/590 testing criteria. This requires that the chiller be designed by the manufacturer for an evaporator-leaving water temperature (LWT) of 44°F and flow rate of 2.4 gpm/ton. Additionally, the chiller is required to be designed for a condenser-entering water temperature (EWT) of 85°F and flow rate of 3 gpm/ton. If the desired chiller criteria are to vary from the prescribed compliance requirements, a calculation can be made based on a formula from Standard 90.1 that calculates alternative compliance efficiencies based on the design parameters. For the Wrigley Field campus chilled-water system, this was the case.
Design conditions for the chillers supplying the Wrigley Field campus were established to have an LWT and EWT of 42° and 56°F, respectively, on the evaporator. Design conditions for the chiller condensers were established to have an LWT and EWT of 100° and 85°F, respectively. Due to these variations from AHRI 550/590, the chillers were designed around the calculated full load and IPLV from the formula in ASHRAE Standard 90.1-2010 Section 18.104.22.168.1. ASHRAE Standard 90.1-2010 was used while the project was being designed and permitted because it was the adopted code for Illinois.
Full-load efficiency was calculated to be 0.706 kW/ton and nonstandard part-load value (NPLV) was calculated to be 0.470 kW/ton. Based on these results, the chillers were selected and far exceeded the Standard 90.1 requirements. The installed chillers have a full-load efficiency of 0.648 kW/ton and an NPLV of 0.379 kW/ton, which exceed the FL requirement of 0.706 kW/ton by 8.2% and the NPLV requirement of 0.470 kW/ton by 19.4%.
Suzan Sun-Yuan is a vice president at Environmental Systems Design Inc. She has extensive experience in the design of commercial, institutional, and educational facilities. She serves as the point of contact for clients, consultants, and contractors, with responsibility for design quality assurance, coordination of the project team, and scheduling of phases.