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.

06/27/2017


This article is peer-reviewed.

Learning Objectives

  • 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.

Figure 1: This photo shows the centrifugal chiller in the Wrigley Field campus chilled-water plant. All graphics courtesy: Environmental Systems Design

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.

Figure 2: The plate and frame heat exchangers in the Wrigley Field campus chilled-water plant allow for the chilled-water system to operate in water-side economizer.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.

Figure 3: A water-side economizer configuration is shown with a heat exchanger in a parallel piping configuration. 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

Figure 4: This water-side economizer configuration has the heat exchanger in series piping configuration.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 6.5.1.2, 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.


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