Central cooling and heating plants are defined by the consolidation of major chilling and heating equipment in a single facility. These plants can either be integrated within the building they serve or situated in remote stand-alone structures. Typical components of a central plant include water-chilling equipment, pumps, and specialty items for water systems. In many cases, boilers are housed separately from refrigeration equipment to comply with safety standards such as ASHRAE Standard 15. Additionally, these plants often incorporate cooling towers and pumps in water-cooled systems.

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1. Simultaneous Cooling and Heating: Central plants can provide primary cooling and heating simultaneously, independent of the operation mode of equipment beyond the central plant. This ensures consistent thermal comfort throughout the facility.
2. Operational Efficiency: Using larger, fewer pieces of equipment typically reduces overall operation and maintenance costs. Central plants allow for wider operating ranges and more flexible operating sequences, enhancing efficiency.
3. Enhanced Accessibility: Centralizing the location of equipment minimizes restrictions on servicing accessibility, making maintenance and repairs more straightforward.
4. Energy-Efficient Design: Central plants facilitate the implementation of energy-efficient design strategies, energy recovery, thermal storage, and energy management. These strategies can be more cost-effective in a centralized system compared to decentralized setups.
5. Multiple Energy Sources: Central plants can utilize a variety of energy sources, such as electricity, natural gas, oil, coal, solar, geothermal, and waste heat. This flexibility provides leverage in purchasing fuel and ensures continued operation during fuel shortages.
6. Standardization and Redundancy: Standardizing equipment within a central plant enhances redundancy and simplifies stocking of replacement parts. Strategic selection of different-sized equipment can improve part-load capability and efficiency.
7. Scalability: Central plants can be economically expanded to accommodate future growth, such as adding new equipment or buildings to the service group.
8. Load Diversity: Load diversity in central plants can substantially reduce the total installed equipment capacity requirement, optimizing energy usage.
9. Submetering: Secondary distribution submetering allows for individual billing of cooling and heating uses, providing financial transparency and accountability.
10. Acoustic and Vibration Control: Grouping major vibration and sound-producing equipment away from occupied spaces simplifies acoustic and vibration controls. Treating a single location is often more effective than managing multiple separate locations.
11. Centralized Emissions Management: Issues such as cooling tower plume and plant emissions are centralized, enabling more economical and aesthetically acceptable solutions.

1. Lead Time for Equipment: New or replacement equipment of the required capacity may not be readily available, resulting in long lead times for production and delivery.
2. Complexity and Expertise: Central plant equipment can be more complex than decentralized systems, requiring more knowledgeable operators and maintenance personnel.
3. Space Requirements: A central location within or adjacent to the buildings served is necessary, and additional equipment room height may be required for larger equipment.
4. Fuel Storage and Delivery: Depending on the fuel source, large underground or surface storage tanks may be required on site. For example, coal requires storage bunkers and regular large deliveries.
5. Emissions and Permitting: Fossil-fuel-fired heating plants need chimneys or flues, special emission treatment, permits, and ongoing monitoring. Special permitting may also be required for central plant operations.
6. Complex Control Systems: The control logic for central plants can be complex, necessitating advanced control systems and skilled personnel for proper management.
7. Higher Initial Costs: The first costs of central plants can be higher compared to alternatives like rooftop units (RTUs), water-source heat pumps (WSHPs), and self-contained systems.
8. Increased Safety Requirements: Central plants come with increased safety requirements, particularly in managing high-pressure steam boilers and other critical components.
9. Extensive Piping Systems: A large pipe distribution system is often needed, which can be a disadvantage in terms of initial cost and installation complexity, but may be beneficial for some applications.

Central cooling and heating plants offer numerous advantages, including operational efficiency, energy savings, and scalability. However, they also present challenges such as higher initial costs, complexity, and space requirements. Careful consideration of these factors, along with a thorough understanding of the specific needs of the facility, is essential for making informed decisions about HVAC systems. By leveraging the strengths of central plants and addressing their limitations, facility managers and engineers can ensure optimal performance and sustainability in building operations.

Understanding Chilled Beam Systems: Passive and Active

Chilled beams are quiet, with less maintenance and air-side infrastructure, but pumping energy is higher and humidity control is a concern.

To truly evaluate how a four-pipe fan coil system stacks up against its newer counterparts, let’s compare the “chilling effects” of each system. Let’s start with chilled beam systems. A passive chilled beam consists of a fin-and-tube heat exchanger, contained in a housing (or casing), that is suspended from the ceiling. Chilled water passes through the tubes. Warm air from the space rises toward the ceiling, and the air surrounding the chilled beam is cooled, causing it to descend back toward the floor, creating convective air motion to cool the space. This allows a passive chilled beam to provide space cooling without the use of a fan. 

An active chilled beam also consists of a fin-and-tube heat exchanger contained in a housing that is suspended from, or recessed in, the ceiling. The primary difference is that an active chilled beam requires a primary air supply. This primary air passes through nozzles, which induce air from the space up through the cooling coil. This induction process allows an active chilled beam to provide much more cooling capacity than a passive chilled beam. The primary air also provides the code-required outdoor ventilation air to the space. For this reason, active chilled beams are more commonly used.

Both passive and active chilled beams are designed to provide sensible cooling only (i.e., no dehumidification), so dehumidification must be provided by a separate dedicated outdoor air unit. For active chilled beam systems, the dedicated outdoor air units will also provide primary air to the chilled beam. Because the chilled beams are designed for sensible cooling only, the chilled water supply temperature to the chilled beams must be raised several degrees above the design space dew point temperature to prevent condensation on the chilled beam.

Chilled beams have a range of advantages: 

• Chilled beams have no terminal unit fan, so overall system fan energy is lower when compared to the other systems requiring terminal equipment fans (four-pipe fan coil unit, DOAS, fan-powered VAV).

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• Chilled beams have no condensate pan or filter, so the overall system maintenance requirements are reduced compared to systems with terminal equipment filters or condensate pans (four-pipe fan coil units, DOAS, fan powered VAV).

• The system requires minimal air-side infrastructure because central air systems are only required for outdoor air ventilation/primary air. The result is reduced floor space requirement for central air handling equipment and vertical duct risers (shafts).

• Because there is no terminal equipment fan noise, chilled beam systems are quiet.

But there are several drawbacks to chilled beam systems:

• Because the entering chilled water temperature must be several degrees above the space dew point, the water side delta T is generally significantly lower than a fan coil unit or variable volume air handling unit delta T. This results in increased pumping energy for the chilled beam system compared to those systems.

• A dedicated outdoor air unit is relied upon to provide building humidity control during the cooling season. This is critical to the performance of the chilled beam. The dedicated outdoor air unit must continually provide subcooled/low dew point ventilation air to the building spaces to maintain acceptable humidity levels and prevent condensation at the chilled beams. Any condensation will be noticeable by the occupants and could permanently damage adjacent building finishes.

• To prevent condensation at the chilled beams during morning start-up, humidity control during unoccupied building hours (night) is required during the cooling season.

• A single chilled beam cannot serve multiple rooms like a variable air volume terminal, dedicated outside air system terminal or fan coil unit. At least one chilled beam is required for every room.

• Separate heating systems (e.g., finned tube, radiators, electric baseboard, duct heaters) are often required in heating-dominated climates with a chilled beam system. Chilled beams utilize the reduced density of cold air to induce flow, and are therefore less effective at providing heat to a space.

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