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data center quick reference guide

December 2008



3. Location of the ComputerRoom Air Conditioning

(CRAC) Unit

It is important to note that the location of the CRAC unit has an impact on airflow. In

a raised floor environment, the CRAC unitshould ideally be placed at the end of thehot aisle for a more uniform airflow across

the data center floor. The closer the CRAC

is to the hot return air, the less air mixing.Illustration #4 demonstrates the airflowimpact of placing the CRAC in the hot vs.

cold aisle. The red bars (CRAC in hotaisle) show a uniform air distribution and

the blue bars (CRAC in cold aisle) show anuneven distribution and negative closeproximity to the CRAC.

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1. Data Center Floor Layout

The most efficient floor layout in a data center is hot aisle/coldaisle. Hot aisle/cold aisle means that the enclosures face eachother creating one aisle with cold air in front of the enclosure

and one with hot air. This way you separate the hot air from the cold air and effectively prevent air mixing. If all the racks

face the same direction, then the hot exhaust air from row 1

will flow into the front of the servers of row 2 and so forth.

The cumulative effect causes equipment to overheat.

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2. Height of the Raised Floor

The height of the raised floor has a big impact on how wellyou can cool the enclosures. Generally speaking, the higher

the floor the more static pressure that can develop and themore air you can push through the perforated floor tiles.Illustration #3 shows how different heights affect the airflow

through a tile.

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Illustration 1. Hot aisle / cold aisle.

Illustration 2. Facing same direction.

Illustration 3. Source: Uptime Institute

Illustration 4. Source: Uptime



4. Venturi Effect (based on Bernoulli principle)

A phenomenon called the Venturi effect causes the perforated tiles closest to the CRAC unit to have a lower or negative airflow

as shown in the illustrations.As the cold air is being pushed outof the CRAC unit under the raised floor, the velocity (speed) of

the air is high and has very little static pressure.This is why thefirst few perforated tiles down the aisle have lower or negative

airflow (as shown in illustration 5).The best analogy for this is the“shower curtain effect”.When you close the shower curtain and

turn the water full on, you will see that the shower cur tain will be

drawn towards the water.This is true because the velocity of thewater, causes low pressure and pulls air towards the bottom and

sucks in the shower curtain.

This effect can easily be overcome by moving the CRAC unitfrom the cold aisle (directly in front of the perforated tiles) to

the hot aisle and thereby limiting the effect.The most common

mistake is to place high density enclosures closest to the CRAC

unit thinking it will receive more cold air (as shown in illustration4). This will not work; it is better to move these racks or enclo-sures away from the CRAC unit.

5. Cabling and Obstructions Underneath the Raised Floor

The raised floor is primarily used for air distribution, but in some places it isused for cable distribution as well. However, the downside is that cables under-

neath the raised floor create air dams blocking airflow and causing non-uniformairflow across the floor. It is also important to note that cable distribution

underneath the raised floor is more expensive and less flexible than overhead,because of the tedious job of laying them underneath the floor in conduit

rather than cable ladders overhead.

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Illustration 8. CRAC pushing cold air

underneath the

raised floor.

Illustration 9. Cable bun-dles causing blockage.

Illustration 6. Cable clutter creating air dams

Illustration 7. Raised floor steel structure.

Illustration 5



6. Enclosure Alignment on Raised Floor

In order to bring the IT equipment residing within theenclosures as close to the cold air stream as possible,

the front of the enclosures should be placed on the

edge of the perforated floor tiles.The farther from thecold air source the harder it is for the equipment to“grab” the cold air (the strength of the fans are typically no more than 20 CFM). One perforated tile per enclo-

sure is required. Edge alignment also ensures a minimalnumber of floor tiles utilized so access to the subfloor is

not impeded.

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7. Perforated versus Grated Tiles

and the Weight Constraints

There are a few different types of raised floor tiles - wooden,steel and concrete core - that dictate the amount of load you canput on them. Remember that most enclosures are rated for 2,000

lbs (equivalent to 286 lbs/sq ft.) and most buildings are rated for 500 lbs/sq. ft. Even if a steel core raised floor is used (equivalent

to 625 lbs/sq. ft.), it doesn’t mean that you can add more equip-ment in the rack. Note that 60% of the weight of an enclosure is

centered around the two front leveling feet/casters/studs asshown in illustration #13.

8. IT Equipment Cooling Methodology

You will find that the majority (>95%) of all EIA-310Dmounted IT equipment is cooled front-to-back. Some

equipment such as older SUN® and Cisco® equipmentwill be cooled side-to-side or bottom-to-top.

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9. Perforated Side Panels

Some customers will request perforated side panels using the

philosophy that the more “outlets” in the rack the more heat youcan exhaust.That is true to some extent, but the problem is that

heat escapes out the side and re-circulates around to the front of the cabinet, causing air mixing and the inlet temperature to r ise.Youwant to ensure that airflow is strictly front-to-back to separate the

hot air and cold air.

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Illustration 10

Lined up with tile.

Front Rear Front Rear

Illustration 11

Not lined up with tile.

RICC

Core Max. Load Solid Perforated Grated

Wooden 1000 lbs. 0% Open 25% Open 65% Open

OCFM 500-800 CFM 1400 - 1900 CFM

Steel 1500 lbs. 0% Open 25% Open 65% OpenOCFM 500-800 CFM 1400 - 1900 CFM

Concrete 2500 lbs. 0% Open 25% Open 65% Open

OCFM 500-800 CFM 1400 - 1900 CFM

Illustration 16

Illustration 14

Illustration 15

Illustration 12. Source: AccessFloors.com

Illustration 13


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10. Blanking Panels

The American Society of Heating, Refrigeration and

Air Conditioning Engineers (ASHRAE) recommends the

use of blanking panels for open U space in the rack toavoid hot air from re-circulating to the front of the rack.

The customer will experience a significant drop in inlet temperature by adding blanking panels. Major server

OEMs like HPand IBM clearly state in their productmanual that to ensure proper airflow in a rack, blanking

panels should be used.

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11. Simplified Data Center Layout L

12. Leakage from the Raised Floor

A leakage can be caused by a missing tile, a perforated or grated tile placed in an area where no cooling is required or cut-outs in the

floor for cable distribution. It is important that holes are covered (i.e. brush-strips in the floor for cable distribution) or perforated/grated tiles are moved to the front of the racks.

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Illustration 17

Illustration 19

Illustration 18

Transformer

Maintenance bypass

panel (switchgear)

Power Distribution

Unit

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13. Raised Floor or Not

A raised floor may not necessarily be used in the data center. In this case the air distribution is done on the concrete slab.This layoutof the CRAC units is typically referred to as a “bowling alley” because the cold air is being pushed down the cold aisle and the hot air returned along the ceiling back into the CRAC unit.The biggest difference between the two layouts is that in a raised floor environ-

ment, the CRAC units are placed in the hot aisle to be as close to the hot air as possible and in a non-raised floor environment, the

CRAC is placed in the cold aisle to provide cold air as close to the cold aisle as possible.

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14. Various Air Distribution Models L

Return

Illustration 20 Illustration 21





16. Chimney Effect

It us understood that the Chimney effect is when one or

more fans are located in the top panel of the enclosure. Thiseffect impedes the cold airflow to the front of the servers,

starving them from cold air. If the front of the enclosure isnot properly isolated (using blanking panels), the top mounted

fans will draw both hot air from the rear as well as cold air from the front. The result is an enclosure running hot,because the servers are no longer fed enough cold air.

15. Turning Vanes / Scoops on CRAC

A scoop or turning vane mounted on the bottom of theCRAC was regularly deployed in data centers up until 1999and 2000. The idea was that a turning vane would help the

air travel from the bottom of the CRAC underneath the

floor. However, the turning vane actually increases the velocity (speed) of the air as it exits the CRAC and only directs itin one direction. Eliminating the vane helps reduce velocity

and naturally increases pressure.

17. Equipment Environment Specifications

Inlet Air (conditioned air)

ASHRAE talks about “allowable” and “recommended” inlet temperature ranges.Allowable represents the minimum and maximum inlet temperature you can expose the equipment to without causing damage. Recommended represents the optimal inlet temperature, that

will help preserve and increase the reliability of the equipment.

Temperature Humidity

Allowable Recommended Allowable RecommendedTier 1 Data Center 59º - 90º F 68º - 77º F 20% - 80% 40% - 55%

Outlet Air (hot air)

On the outlet side (rear of the rack enclosure or IT equipment) you should see the following temperatures as the conditioned inlet air temperature will rise as it flows across the server.

Temperature Humidity

Allowable Recommended Allowable Recommended

Tier 1 Data Center Max. 110º F 88º - 97º F 20% - 80% 40% - 55%

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Illustration 26

Illustration 27

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Thermal Basics

• The colder the room, the drier (low humidity) and vice versa

• Keep hot air from the cold air.

• Heat flows from hot to cold: heat will not come out of a rack unless the air outside is cooler.

• Air takes the path of least resistance.

• Cold air falls and hot air rises.

• Convection is when moving air (i.e. fans) carries heat away.

• CRAC units don’t make cold air, they remove the heat and cold air is the by-product.

• The higher the difference in temperature, the more efficient the CRAC unit operates.

• Air velocity must slow down before static pressure can develop.

• One perforated tile at minimum per every rack in the cold aisle. None in the hot aisle.

• Cable management arms on the back of servers prevent airflow.

• Recommended aisle spacing as described in Americans with Disability Act should be four-six feet in the cold and two-four feet in the hot.

• Having the coldest data center does not mean an efficient data center, there may still be hot spots.

• Stratification is when temperature increases as the air rises.

Numbers and Statistics

• A data center building usually lasts for 15-20 years.

• Average IT equipment lasts for 2-3 years.

• A standard US raised floor tile is 24" x 24" (2' x 2'). Outside North America - 23.6" x 23.6".

• For every 18 degree Fahrenheit increase in temperature, life of hardware is reduced by 50%.

• An average perforated floor tile under optimum conditions may deliver up to 800 CFM of cold air, but a number between 300-500 CFM is typical.It all depends on the height of the raised floor and air dams underneath.

• To measure airflow on a floor tile, put your business card in the center of the tile. If it floats approx. 1" above, you have approx. 500 - 600 CFM of air

• With perforated front and rear doors, 160 CFM of chilled air will cool approx. 1,000W of heat (20 degree temperature change across the server).

• With Tower of Cool, 80 CFM of chilled air will cool approx. 1,000W of heat (40 degree temperature change across the server).

• The average heat load in today’s rack is approx. 1,500-2000W/rack.

• Americans with Disability Act (ADA) recommends a 36" wide cold/hot aisle for accessibility.

• Most server manufacturers require at least 63% of open area (perforation on the front and rear door). Some will dictate square inch of open space(at least 800 sq. in.)

• When Relative Humidity (RH) drops below 30%, failures tend to increase as Electrostatic Discharge (ESD) issues arise.

• When RH goes too high, Conductive Anodic Failures (CAF) occur as condensation issues arise within the enclosure and equipment stored within.

• Power in equals heat out: 1,000W of power into the rack equals 1,000W of heat out of the rack.

• Watts and BTUs: 1,000W equal 3,412 BTU/hour.

• BTUs and Refrigeration: 12,000 BTU/hour equals one ton of refrigeration.

• Convert watt/sq. ft. to watt/rack.

• Network Equipment Building System (NEBS) is the strictest standard covering earthquake protection. Having a zone 4 rating means that the

equipment will stay in the rack, working through the earthquake as well as afterwards.

• Uniform Building Code (UBC) is not nearly as strict as NEBS but follows the zone 0 through 4. It will basically guarantee that the rack will stay in thesame place throughout an earthquake.

• The TIA/EIA-310D standard is the primary standard for rack-mount equipment (1U is 1.75").

All Products and company names are trademarks or registered trademarks of their respective holders.

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