A perimeter cooling system is a conventional method for managing thermal loads in data centers, where cooling units are strategically placed along the outer walls of the data hall. This approach primarily distributes cold air under a raised floor to cool the IT equipment housed within the space.
Understanding the Core Concept
Historically, perimeter cooling has been a prevalent method for data center cooling. It relies on dedicated cooling units, often Computer Room Air Conditioners (CRACs) or Computer Room Air Handlers (CRAHs), situated at the perimeter of the IT environment. These units cool the air and then push it into the plenum created by a raised floor. From there, the cold air rises through perforated floor tiles directly in front of or beneath IT equipment racks.
A defining characteristic of this method is the distribution of cold air under a raised floor with no form of containment. This design often results in what is known as targeted supply and flooded return air distribution. While cold air is targeted to reach IT equipment, the hot air expelled by the servers mixes freely within the room (a "flooded return") before being drawn back into the perimeter cooling units.
How Perimeter Cooling Works
The operational mechanism of a perimeter cooling system can be broken down into these steps:
- Air Intake: Perimeter cooling units (CRACs/CRAHs) draw warm air from the data center environment.
- Cooling & Dehumidification: Inside the unit, the air passes over cooling coils (containing refrigerant or chilled water) to lower its temperature and sometimes remove humidity.
- Cold Air Distribution: The now-cooled air is discharged, typically downwards, into the subfloor plenum created by the raised floor system.
- Targeted Supply: Cold air then travels horizontally beneath the raised floor and exits upwards through perforated floor tiles, usually positioned in the "cold aisles" directly in front of server racks.
- IT Equipment Cooling: The cold air is drawn into the front of IT equipment (servers, storage, networking gear) to cool internal components.
- Flooded Return: Hot air, exhausted from the rear of the IT equipment, rises into the open data center space. This warm air then circulates back towards the perimeter cooling units to complete the cycle.
Key Components
- Computer Room Air Conditioners (CRACs) or Computer Room Air Handlers (CRAHs): These are the primary cooling units, typically large cabinets that condition the air.
- Raised Floor System: Creates a plenum (a sealed compartment) beneath the main floor for cold air distribution and often for power and data cabling.
- Perforated Floor Tiles: These specialized tiles allow the cold air from the underfloor plenum to enter the data center space and cool the IT equipment.
- Airflow Management Accessories: While often lacking full containment, some systems might use blanking panels in racks or brush strips in cable openings to improve airflow to a limited extent.
Advantages and Disadvantages
While perimeter cooling has been a historical staple, its effectiveness varies with modern IT demands.
Advantages:
- Simplicity: Relatively straightforward to design and implement for lower-density environments.
- Initial Cost: Can have a lower upfront cost compared to more complex containment or in-row cooling solutions.
- Flexibility (for initial setup): Provides a broad "blanket" of cold air, which offered some flexibility for IT equipment placement in the past.
- Space Utilization: Cooling units are along the walls, potentially leaving the central floor area open for IT racks.
Disadvantages & Limitations:
- Hot Spots: The lack of containment and the "flooded return" approach often lead to inefficient air mixing. Cold air can bypass IT equipment, and hot exhaust air can recirculate back into equipment inlets, creating hot spots, especially in high-density racks.
- Energy Inefficiency: This mixing of hot and cold air leads to significant energy waste. Cooling units must work harder and cool to lower temperatures than necessary to compensate for poor airflow management.
- Bypass Air: Cold air escapes through unused perforated tiles or cable openings, returning to the CRAC without cooling IT equipment.
- Recirculation: Hot exhaust air from IT equipment is drawn back into the inlets of other (or even the same) IT equipment.
- Scalability Challenges: Struggles to efficiently cool modern, high-density IT racks (e.g., >5-7 kW per rack).
- Limited Control: Difficult to precisely control and direct airflow to specific IT equipment, leading to over-cooling in some areas and under-cooling in others.
- Higher Operating Costs: Due to inefficiency, operating costs (energy consumption) are typically higher than more optimized cooling solutions.
Comparison with Modern Cooling Approaches
The evolution of data center technology, particularly the increase in power density per rack, has highlighted the limitations of traditional perimeter cooling. Modern data centers often incorporate airflow containment strategies to improve efficiency.
| Feature | Perimeter Cooling System (Traditional) | Modern Cooling System (e.g., Containment, In-Row) |
|---|---|---|
| Cooling Units | Along the perimeter of the room (CRACs/CRAHs) | Along the perimeter, in-row units, or rear-door heat exchangers |
| Air Distribution | Under raised floor, cold air from perforated tiles (Targeted Supply) | Under raised floor, overhead plenums, or direct from in-row units |
| Air Return | Flooded return (hot air mixes in the room) | Contained return (hot aisle or cold aisle containment) |
| Containment | No form of containment | Hot aisle containment or cold aisle containment |
| Efficiency | Lower (prone to hot spots, bypass air, recirculation) | Higher (minimizes air mixing, targets cooling) |
| Density Support | Low to moderate power densities | High to ultra-high power densities |
| Energy Use | Higher operating costs | Lower operating costs due to optimized cooling and reduced waste |
Evolution to Enhanced Efficiency
Due to its inherent inefficiencies, perimeter cooling, especially without any form of containment, is less common in new, high-density data center designs. The industry has largely moved towards solutions that separate hot and cold air streams more effectively, such as:
- Hot Aisle Containment (HAC): Encloses the hot aisle to prevent hot exhaust air from mixing with the cold air in the room.
- Cold Aisle Containment (CAC): Encloses the cold aisle to deliver a dedicated supply of cold air directly to equipment inlets.
- In-Row Cooling: Places cooling units between server racks, closer to the heat source, for more precise cooling.
- Rear-Door Heat Exchangers: Attach directly to the back of racks to remove heat immediately at the source.
While perimeter cooling systems are still found in many existing data centers, especially older or lower-density facilities, their effectiveness can be significantly improved by implementing basic airflow management practices like blanking panels, sealing cable openings, and, most effectively, by adding hot or cold aisle containment.
