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Advantages of Inrow Cooling Systems Against Perimeter (CRAC&CRAH) Cooling Systems

White Paper 101

by Kıvanc ILALCTO Canovate Group

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The main goal of any data center cooling system is to remove the heat generated by the IT equip-ment in most energy efficient way. Most of IT equipment in a datacenter can tolerate ASHRAE’s recommended maximum temperature of 27˚C. Thus we can enable a well functioning and envi-ronmentally balanced datacenter with higher room temperatures rather than the low room tem-peratures we are used to with perimeter type CRAC&CRAH cooling systems. We can bring the cooling capacity required close to the racks where the actual heat is generated. Therefore we will not waste energy to cool the entire room and most of the cooling capacity will be focused on the racks which is the actual source of the heat.

Inrow cooling systems are recommended to be used in combination with cold&hot aisle contain-ment to achieve the best and energy efficient cooling performance. To ensure the inrow cooling unit properly captures the hot air from the IT equipment, the DC room must be designed with a cold /hot aisle containment system. This is a fundamental data center design criteria that must be met in order for the inrow cooling units to provide maximum level of effective cooling.

Cold Aisle Containment

Chiller

Figure 1Hot Aisle Containment + Inrow Cooling Units Cold Aisle Containment + Inrow Cooling Units

Figure 2

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The fundamental purpose of in-row cooling is to capture heat from the IT equipment in the hot aisle and neutralize the heat before it mixes in the room. Hot air mixing within the data center is the fundamental cause of thermal issues and hot spots. A metric for measuring the effectiveness of how hot IT exhaust air is captured by cooling equipment (or how cold air is supplied to IT air inlets) is called the capture index(1). Capture index is based solely on the airflow patterns associat-ed with the supply of cold air to, or the removal of hot air from a rack. The capture index is typically a rackbased metric and has values between 0% and 100%. Higher values mean good cooling performance and scalability of a cooling architecture. Inrow coolers has the highest hot air capture index values since they are placed closer to the heat source just in between the racks and can capture most of the hot exhaust air before mixing with room air.

Inrow coolers are variable capacity devices that are capable of over-supplying cold air to the cold aisle. When the system is in balance, the supply temperature from the cooler is equal to the IT inlet temperature. When ancillary equipment (i.e. stand-alone devices) is initially added outside of the containment, the temperature of the overall room environment increases due to hot air mixing. In this case row-based cooling units sense this increase and respond by increasing cooling ca-pacity to neutralize the hot air. This increased cooling capacity is achieved by increased air flow (row coolers increase fan speed, EC controlled fans) and or lower supply air temperature (row cooler increase chilled water valve flow).

Figure 3 Figure 4Inrow Cooling Unit Airflow

Cold Air

Hot Air

Airflow within Cold Aisle Containment

(1) J.W. VanGilder, S.K. Shrivastava, Capture Index: An Airflow-Based Rack Cooling Performance Metric, Published in ASHRAE Transactions 2007, Volume 113, Part 1

Advantages of Inrow Cooling Systems against Perimeter (CRAC&CRAH) Cooling Systems

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Cooling with inrow coolers compared to a perimeter coolers (CRAC&CRAH) has following advantages:

1. Suitable for high density applications - Inrow cooling system is the best cooling option for high density datacenters where, you need minimum cooling capacity of above 6-7 kW per rack. Traditional perimeter cooling systems (CRAC&CRAH) can be a good solution for densities below 5KW, but they are not enough to provide efficient cooling capacity for high density racks. Current IBM blade servers of 9U requires minimum 7kW cooling alone and you can install 2-3 such units in a single 42U rack. Thus by using high density inrow cooling units you can increase your server capacity per rack, and hence you can reduce the total footprint of your datacenter. In return you can save from datacenter space, rent expense, and extra cabling in between the racks.

2. Allows elimination of the raised floor – Perimeter coolers generally distribute cold air to IT equipment under the raised floor. The cost associated with purchasing, installing and maintain-ing the raised floor can be eliminated when inrow coolers are used. Cabling can be down via Canovate’s special cable channels mounted on the top of the racks.

3. Lower CAPEX – Over-conservative designing of a data center with perimeter coolers results in significant capital expense waste. Due to unblanced distribution of cold air, datacenter de-signers are conservative while defining the required cooling capacity. Actually they assume more cooling capacity than required to eliminate balancing and hot spot problems in future. Up to 30-40% of this expense can be eliminated by relying on inrow coolers to cool the entire data center load.

Figure 5 Figure 6CRAC&CRAH based cooling requires raised floor Cold air flow in CRAC based room cooling under the

raised floor

Advantages of Inrow Cooling Systems against Perimeter (CRAC&CRAH) Cooling Systems

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4. Eliminates distance effect - If the distance of the rack rows are getting longer from the perim-eter cooling units, the pressure of the cold air will be not enough to cool the additional rows in further distance. This will result in uneven&unbalanced distribution of cold air and hot spots. Since inrow cooling units are placed just in between the racks, there is no pressure and cooling capacity loss.

5. Lower OPEX and energy costs – Over-provisioning of coolers results in added energy ex-pense, especially if the unnecessary coolers have fixed-speed fans, which is very common in perimeter units supplying cold air via raised floor.

6. Uniform distribution of cold air - The inrow cooling units provide uniform cooling capacity and pressure on the entire rack level. CRAC units provide enough cooling capacity on the bot-tom level of the rack, but the upper level of the racks can not be sufficiently cooled due to the decreasing pressure of incoming cold air under the raised floor.

Figure 7

Figure 8

Distance effect problem

Upper level hot spots shown on thermal analysis

Advantages of Inrow Cooling Systems against Perimeter (CRAC&CRAH) Cooling Systems

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7. Eliminates the negative effect of dense cabling under the raised floor - Dense cabling under the raised floor can result loss of the most pre-planned cooling capacity. DC designers mostly don’t consider the air circulation blocking effect of dense cabling under the raised floor. Sometimes they have to consider an over-conservative design to limit this problem by adding more cooling capacity than the actual requirement.

8. Flexible design - An inrow-cooling based datacenter design allows cooling capacity and re-dundancy to be targeted to the actual requirements of containments. For example, one contain-ment of racks can run on high density applications such as blade servers,pizza boxes etc , while another containment will be used lower power density applications such as telecommunication or cabling enclosures. Furthermore, N+1, 2N , 2N+1 redundancy can be targeted at specific containments.

9. Modular & Scalable - Inrow cooling & aisle containment systems are modular and you can easily add or remove cooling capacity depending on your changing cooling requirements. You can also add capacity later on by adding additional racks and cooling units. Pay as you grow! In contrast perimeter cooling systems should be designed in advance with the maximum cooling capacity.

10. Simpler redundancy – If a data center has a redundancy requirement (i.e. N+1, 2N, 2N+1), you can make this redundancy in smaller increments. For example if you need 120KW cooling requirement, you can provide n+1 redundancy with 5 X 30 KW inrow units (4+1). For CRAC &CRAH system you have to assume combinations like (120KW+120KW, or 2x 60KW +60 KW)

Figure 9Loss of cooling pressure due to dense cabling

Advantages of Inrow Cooling Systems against Perimeter (CRAC&CRAH) Cooling Systems

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Advantages of Inrow Cooling Systems against Perimeter (CRAC&CRAH) Cooling Systems

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11. Seggregation of risk - By using multiple seperate inrow cooling units instead of big&bulky CRAC&CRAH units you can decrease the amount of the affected racks incase of the failure of a bigger unit which serves more racks per unit than inrow cooling units

12. Better free cooling potential and lower PUE levels - Canovate’s chilled water based inrow cooling units operate with higher inlet water temperatures (14-18˚C). Therefore these cooling units have higher free cooling potential which will results in lower PUE levels below 1.2 in appropriate climate conditions. Furthermore your chiller units will consume less ener-gy to provide chilled water of 14-18 ˚C compared to 5-7 ˚C required for chilled water based CRAH units.

13. Green IT and environment friendly - Due to the advantages explained in items 5-12 inrow cooling units provide significant energy savings compared to CRAC systems and generate less amount of gasses to harm our environment.

14. Lower maintenance costs – Using inrow coolers means no additional maintenance con-tracts for perimeter coolers, reducing the operating expenses in the data center. Inrow cool-ing units have hot-swappable EC fans and easily replacable control units. Thus any problem can be easily solved on site and in most cases you don’t even need to stop the cooling units while making these changes.

15. Less vendor interactions – Often times when hybrid designs are considered, designers look to multiple vendors to supply different systems. Using additional vendors means addi-tional complexity in maintaining the data center operation. Inrow cooling system is a pack-age solution together with racks and aisle containment system.

Hot-swapple EC fan

Control Unit