【數據中心設計】CDCP 學習筆記 - 數據中心 (Part 9-3) - Data Centre Design - Cooling Infrastructure

Up Flow or Down Flow

• Up Flow
• Can be installed with or without a raised floor
• Limited airflow guidance (need to use ducting)
• Down Flow
• Can 'only' be used with raised floor
• Allows for air flow guidance through a raised floor

Wall flow

• A new trend is a wall-flow concept
• CRAHs are mounted in the service corridor and are blowing the air horizontal into the data centre
• Racks are positioned directly on the slab
• Racks can be taller and heavier
• Return air is ducted to the service corridor
• No need for the raised floor

在Service Corridor上安裝CRAC unit，不需要架空地板，為了避免冷熱空氣的混合，需要確保熱空氣通過管道或者通過假天花被引回Service Corridor。

架空地板冷卻方式與非高架地板冷卻方式哪個好？

• 關於哪種方案是最好的，沒有統一的答案，取決於以下因素：
• ICT設備的類型
• slab-to-slab height
• required cooling capacity
• flexibility
• 架空地板並不再是一種趨勢，因為未必所有建築都考慮過有架空地板的配置。

Cooling Concepts (1) —— with Raised Floor

• Hot- and Cold-Aisle setup
• Racks are placed Front-to-Front and Back-to-Back
• Cold- and Hot-Air areas are separated
• Some hot air will still flow into the cold air areas

• A suspended ceiling can aid in guiding the hot air back to the air-conditioner without mixing with the cold-air areas
• Increased efficiency

• Air conditioners should be placed perpendicular to the hot aisle
• Allow fast hot air to return
• Allow more evenly equalised air pressure under the raised floor

Placement of equipment in the rack 

• With traditional downflow cooling, the bottom of the rack will be colder than the top
• Put high heat load approx. 30 cm above raised floor and not above half the rack height
• Higher possible if you have enough CFM

在架空地板配置中，機架底部會有更多的冷氣供應量。因此，把最高的熱負荷的設備儘量放在底部。在機架的頂部，冷氣量愈少，因此冷卻能力較低。把較低的熱負荷的設備儘量放在較高部分。

Avoid leakage and short circuit air

• Air leakage from the floor - Use grommets
• Air leakage within the racks (back-wash) leads to in-efficiencies - Use blanking panels

The maximum throw of the air conditioner

• Consider the maximum throw of your air conditioner (if using perimeter cooling)
• Typically 12 - 18m
• Too long rooms require placement of perimeter air conditioners on both sides

Cooling Concepts (2) —— with Non-Raised Floor

In-Row cooling

• In-row cooling can be deployed when using a non-raised floor setup
• Cooling close to the heat load leads to good efficiencies for airflow
• Fewer racks per sqm inside the computer room
• In-row cooling units are
• Direct expansion
• Chilled water
• Fluid cooled

Overhead duct

• Overhead, ducted, cooling dumps cold air directly in front of the racks and extract the hot air from the back
• Ducts have often louvres/vents to regulate CFM/CMH
• Ducts must be well designed
• Ensure enough air volume can be dumped and extracted at the right locations
• Air conditioner redundancy must be taken into account
• Do not paint the ducts as paint might splinter over time causing particulates to contaminate the room
• Inspect and clean ducts on a regular basis

Cooling Concepts (3) —— Supplemental Cooling

High-Density Cooling: Floor mount

• Collects cool air from underfloor
• Increases CFMto the rack
• Flexible, Movable
• Neighbouring racks could have a potential cooling impact

High-Density Cooling: Hot air fans

• Collect hot air at the point of generation
• Route directly to CRAC
• Snap-on retrofit
• Variable speed: as needed
• Flexible, movable
• Only assist with removal of heat
• Does not assist in increasing the cold air supply

High-Density Cooling: Overhead supplement air

• Traditional CRAC / HVAC downflow/throw units with raised floor principle need to be extensively extended
• Ducted return to air conditioner direct from the rack
• Top flow rack units in addition to raised floor cooling
• Limit 10-25 kW

High-Density Cooling: Self-contained racks

• Fully ducted supply and return, local rack cooler
• Collect equipment exhaust air
• Provide cooling coils in the rack
• Specialized rack
• Heat removal by water or refrigerant
• Flexible / movable
• Fire suppression considerations
• Limit: 18-35 kW per rack
• Cooling principles: water/dielectric/Liquid CO2/Refrigerant

Cold Aisle vs Hot Aisle Containment？

Cold Aisle Containment

• The cold aisle area is contained
• Cold air only goes where it needs to be, being the air-intake of the equipment
• Air volume required for a cold aisle can be calculated by reviewing the CFM requirements of all the equipment inside the contained area
• Typically recommended when containment is applied to the entire room
• It is required that proper redundancy measures are taken as very little buffer air might be available in case of an air conditioner failure
• Slight overpressure needs to be created
• Ideally, air pressure is measured in the cold aisle area and connected to air conditioners to regulate the CFM/ CMH output
• Be aware that CFM / CMH is variable in most of today's equipment

Hot Aisle Containment

• The hot aisle area is contained
• Hot air is separated from the cold air and is guided directly back to the air conditioner or outside of the building
• Cold air is flowing throughout the entire computer room
• Typically recommended when only a small area needs to be contained
• Cold air is going through the entire room 
• Cold air should only go where it is required being the intake of the equipment
• The whole room is now acting as a buffer area
• Works well with in-row cooling
• Hot aisle areas can sometimes be very hot and noisy 
• Review local regulations for personnel working in such environments