This diagram illustrates a standard Class A data center underfloor air distribution system.
Below is the detailed description corresponding to many ventilation products:
Dampers & Valves
1 - Fresh Air Damper
Location: AHU outside air intake
Function: Main on/off control for the fresh air system. It interlocks with AHU fan operation – opens when the fan runs, closes when the fan stops. It can also be manually closed for equipment isolation during maintenance.
Industry Note: Standard inlet component for all air handling units.
5 - CAV Damper (Constant Air Volume Damper)
Location: AHU supply air outlet section
Function: Maintains a constant fresh air supply volume regardless of duct pressure fluctuations, via a mechanical self-compensating mechanism. It stabilizes data hall positive pressure, preventing unfiltered air from infiltrating through gaps and cable penetrations.
Industry Note: Standard for data center fresh air systems, preferred over VAV (Variable Air Volume) dampers because data center fresh air demand is a fixed value, not variable.
6 - Fire Damper (70°C)
Location: Where the fresh air duct penetrates the fire-rated partition between the AHU room and main data hall
Function: Fire safety component mandated by code. The fusible link melts at 70°C, automatically closing the damper via spring force to block the duct, preventing smoke and flame from spreading through the ventilation system.
Industry Note: Mandatory per GB 50016 Fire Code for ducts crossing fire compartment boundaries.
7 - Return Air Damper
Location: Return air inlet of each CRAH unit
Function: Regulates CRAH return air volume to match supply airflow; can be closed to isolate the unit during maintenance; prevents backflow of hot air when the unit is off.
Industry Note: Standard fitting for large-capacity CRAH units.
12 - Pressure Relief Damper
Location: Data hall perimeter wall, discharging to corridor or outside
Function: Core component for data hall positive pressure control. It automatically opens to exhaust excess air when internal pressure exceeds the setpoint (typically 5–10 Pa), and closes when pressure drops below the threshold. It maintains stable positive pressure to prevent infiltration of unfiltered air.
Industry Note: Standard positive pressure control device for data centers; installed independently from the AHU, and exhausts air directly from the data hall.
Terminal Air Distribution & Organization
10 - Perforated Floor Tile
Location: Raised floor within cold aisles
Function: Terminal supply air diffuser for underfloor air distribution. Perforated openings deliver conditioned air upward from the underfloor plenum into the cold aisle. Available in multiple open area ratios (25%, 40%, 60%, etc.) to match different rack heat loads.
Industry Note: Standard terminal delivery device for raised floor underfloor air systems.
Supply Air Plenum
Location: Entire space below the raised access floor
Function: Pressurized mixing and distribution chamber. It mixes and equalizes supply air from CRAH units and the AHU, creating uniform static pressure across the entire floor. This ensures consistent airflow to all perforated tiles and cold aisles, eliminating airflow imbalance between near and far units.
Industry Note: Core design element of underfloor air distribution; eliminates the need for extensive ductwork.
Return Air Plenum
Location: Ceiling plenum space above the data hall
Function: Return air collection chamber. It collects hot exhaust air rising from all hot aisles and distributes it evenly to the return inlets of the CRAH units. It utilizes the natural buoyancy of hot air, eliminating the need for dedicated return ductwork and reducing construction cost.
Industry Note: Standard configuration for underfloor supply / overhead return air systems.
Cold Aisle / Hot Aisle
Location: Formed by alternating rows of server racks in the data hall
Function: Airflow organization strategy. Racks are arranged front-to-front and back-to-back, creating dedicated cold aisles (supply air) and hot aisles (exhaust air). This physically separates cold supply and hot exhaust air, preventing air mixing and improving cooling efficiency by over 30%.
Industry Note: Industry standard airflow management practice for modern data centers.
Air Purification Products (Graded Filtration Along Airflow Path)
The system adopts a 6-stage graded filtration architecture arranged in the direction of airflow.
2 - G4 Pre-filter
Location: Inlet section of the AHU, on the fresh air intake side
Function: Serves as the first-stage coarse filtration barrier for incoming outside air. It captures large particles (≥5μm) such as dust, lint, fibers and debris, protecting downstream medium filters, chemical filters and cooling coils from fouling and clogging.
Industry Note: Standard primary filtration for all ventilation systems; a low-cost, regularly replaceable consumable.
3 - F8 Medium Filter
Location: Inside the AHU, after the G4 pre-filter and upstream of the cooling coil
Function: Second-stage fine filtration for fresh air. It removes PM2.5 and fine particles (≥1μm), ensuring the cleanliness of supply fresh air meets data center requirements.
Industry Note: Standard configuration for Class A data center fresh air systems; the core guarantee for intake air cleanliness.
4 - Chemical Gas Phase Filter
Location: Inside the AHU, downstream of the cooling coil (arrangement compliant with ASHRAE TC 9.9 guidelines)
Function: Gaseous contaminant removal. It adsorbs corrosive gases such as hydrogen sulfide, sulfur dioxide, nitrogen oxides, salt spray and VOCs, preventing corrosion on server circuit boards, connectors and metal components, and extending hardware service life.
Industry Note: Mandatory for high-tier data centers, especially facilities located in coastal, industrial or chemical zones; optional for standard data centers.
8 - G4 Pre-filter (Return Air)
Location: Return air inlet of each CRAH unit
Function: First-stage coarse filtration for recirculated indoor air. It captures airborne dust, cable debris and particulates within the data hall, protecting the CRAH cooling coil and fan from dust buildup which would degrade heat transfer efficiency.
Industry Note: Standard return air filtration for all precision cooling units.
9 - F8 Medium Filter (Supply Air)
Location: Inside the CRAH unit, downstream of the cooling coil and upstream of the supply outlet
Function: Second-stage fine filtration for recirculated air. It removes fine particulates to maintain the data hall air cleanliness at ISO Class 8 level (required by GB 50174 standard). It forms a two-stage filtration loop with the return G4 pre-filter.
Industry Note: Standard configuration for downflow CRAH units; the core driver of indoor air cleanliness.
11 - Rack Air Filter
Location: Server rack air inlet face (cold aisle side)
Function: Final terminal filtration barrier at rack level. It blocks fine dust and small debris from entering the server chassis, protecting internal CPU, memory and storage components from dust accumulation that would impede heat dissipation.
Industry Note: Standard accessory for server racks; coarse-grade terminal protection.
Core Air Handling Units
Air Handling Unit (100% Outside Air)
Location: Dedicated AHU Room, separate from the main data hall
Function: Dedicated fresh air processing unit operating on 100% outside air. It integrates filtration, cooling, dehumidification and fan sections. It treats outdoor air to required cleanliness and temperature/humidity conditions, then delivers it to the underfloor supply plenum. It is responsible for makeup air, positive pressure maintenance and occupant ventilation requirements, and operates completely independently from the indoor cooling loop.
Industry Note: Mandated by GB 50174 to be installed as a separate, dedicated system.
CRAH Unit (Computer Room Air Handler)
Location: Along both sides of the main data hall
Function: Core unit of the indoor cooling loop. It uses a top-return / down-supply configuration: draws hot return air from the ceiling plenum, filters and cools it, then discharges cooled air downward into the underfloor plenum. It drives the entire indoor air circulation and maintains constant temperature and humidity in the data hall.
Industry Note: Provides over 90% of the data hall air circulation; primary cooling equipment for the raised-floor hot/cold aisle configuration.
