HP Modular Cooling System White Paper

Cooling strategies for IT equipment

Technology brief

Introduction ......................................................................................................................................... 2

Limits of traditional cooling practices ...................................................................................................... 2

Free cooling strategies.......................................................................................................................... 4

Air-side economization ..................................................................................................................... 5

Water-side economization ................................................................................................................ 6

Benefits and disadvantages of free air cooling .................................................................................... 6

Air containment strategies ..................................................................................................................... 6

Cold-aisle containment ...................................................................................................................... 7

Hot-aisle containment ....................................................................................................................... 8

Closed-loop cooling systems .................................................................................................................. 9

HP Modular Cooling System .............................................................................................................. 9

HP Performance-Optimized Datacenter ............................................................................................. 11

Choosing the best cooling strategy ...................................................................................................... 12

Cooling decisions based on facility characteristics ............................................................................. 12

Cooling decisions based on room layout .......................................................................................... 13

Cooling decisions based on server density/power per rack ................................................................ 13

Managing data center cooling ............................................................................................................ 14

Managing ITE cooling with HP Systems Insight Manager .................................................................... 14

Managing the data center environment with HP Environmental Edge .................................................... 15

Conclusion ........................................................................................................................................ 15

For more information .......................................................................................................................... 16

Call to action .................................................................................................................................... 16

ITE racks

Introduction

If you’re deploying the latest generation of information technology (IT) equipment or adding more servers to a crowded data center, conventional data center cooling methods and systems will likely be inadequate. The most effective cooling solution for any computing facility depends on the specific characteristics of the facility, equipment layout, and server density.

This technology brief first explains limitations of traditional cooling practices. Then it describes a range of systems you can choose from to modify or supplement your existing cooling system to get the cooling capacity your data center requires.

Limits of traditional cooling practices

Enterprise data centers have most often used an open-area approach to cool racks of servers and storage systems. With this approach, one or more computer room air handlers (CRAHs) are placed on the periphery of the data center room. IT equipment (ITE) racks are arranged in a cold-aisle/hotaisle layout (Figure 1). Cool air is forced through a raised floor plenum and up through vented floor tiles in the cold aisle toward the front of the ITE racks. The cool air is drawn through the ITE racks, and warm air is vented out the rear of the racks and upward toward the ceiling. Air circulation works on the basic strategy of providing cool air at the floor level and collecting warm air near the ceiling.

Figure 1: Traditional open-area data center cooling

Basic room requirement: Raised floor

CRAH

The open-area strategy is generally adequate for racks using up to 10 kilowatts of power and lets data centers scale relatively easily. However, some of the warm air mixes with the cool air, reducing cooling system efficiency. Some equipment generates excessive heat, creating hot spots that need supplemental cooling or specific air channeling. The typical remedy has been to set the cooling system to run colder to compensate for the hot spot.

(20 servers @ 423.9 W per server)

8.47 kW

20 servers

10 servers

4.23 kW

16.1 kW

8.0 kW

42 servers

21 servers

Server blade systems and 1U servers let you assemble high-density infrastructures; however, these systems create much more heat per square foot of floor space and more hot spots. In instances like this, the open-area approach cannot keep up with the demand for cool air.

Figure 2 compares two examples of server loading for a 42U rack. In Figure 2A, our rack contains 20 HP ProLiant DL380 G5 servers, each consuming approximately 424 watts for a total rack consumption of 8.47 kW. In Figure 2B, our rack contains 42 dual-processor DL160 G6 servers consuming 383 watts per server for a total rack consumption of over 16 kW of power.

Figure 2: Examples of power consumption in 42U IT equipment racks with different server loads

A: ProLiant DL380 G5 2U servers

Power

consumption

B: ProLiant DL160 G6 1U servers (42 servers @ 383 W per server)

Power

consumption

Higher power consumption produces more heat. Virtually all power consumed by rack-mounted equipment is converted to sensible heat, which increases the temperature in the environment. The sensible heat load is typically expressed in British Thermal Units per hour (BTU/hr) or watts, where 1 W equals 3.413 BTU/hr. The rack’s heat load in BTU/hr can be calculated as follows:

Heat Load = Power [W] × 3.413 BTU/hr per watt

For example, the heat load for a ProLiant DL160 G6 server in a 2P configuration consuming 383 watts is calculated as follows:

383 W × 3.413 BTU/hr/W =1307 BTU/hr

This means that the heat load of our fully-loaded 42U rack of DL160 G6 servers is 54,901 BTU/hr. In the United States, cooling capacity is often expressed in "tons" of refrigeration, which is derived by dividing the sensible heat load by 12,000 BTU/hr per ton. Therefore, the cooling requirement for our rack of DL160 G6 servers is computed as follows:

54,901 BTU/hr ÷ 12,000 BTU/hr per ton = 4.58 tons

Cold/hot

Closed-loop

Supplemented

The increasing heat loads created by the latest server systems require more aggressive cooling strategies than the traditional open-area approach. Such strategies include the following:

• Free cooling

• Air containment

• Closed-loop cooling

Figure 3 identifies cooling strategies and implementations based on kilowatts per rack and on the server density per rack.

Figure 3: Cooling strategies based on server density/power per rack

data center

cooling

Traditional

open-area

cooling

aisle

containment

cooling

Chassis/component

level cooling, future cooling

technologies

Density (nodes per rack)

Power (kW per rack)

Free cooling strategies

Free cooling strategies use ambient outside temperature to remove heat from inside a data center. You can implement free cooling as either a primary or secondary system. Free cooling systems take advantage of regional climate conditions for more efficient operation, but they do include mechanical and electrical equipment. Free cooling is generally applied using one of two methods: air-side economization or water-side economization.

ITE racks

Air-side economization

Air-side economization uses a system of supply and return fans, filters, and dampers that maintain positive air pressure with respect to the IT equipment exhaust, and positive data center air pressure with respect to the outside. Air-side economization may also include supplemental cooling coils that you can switch in and out of circuit as needed for changes in climate. You can implement air-side economization as either a direct or indirect system. In direct air-side economization, outside air is filtered, stabilized for relative humidity, and then delivered to the ITE racks (Figure 4). The ITE rack layout and exhaust venting is similar to a hot-aisle containment strategy.

Figure 4: Air-side economization (direct method)

Exhaust

Input Filters/fans

Filter/fan

Exhaust air to outside

Air from outside

Room requirements: Air circulation system built into facility

Air-side economization using the direct method is more applicable in cool regions where outside air requires little or no refrigeration.

With indirect air-side economization, outside air cools data center air through an air-to-air heat exchanger. This method uses two air circulation circuits: one for outside air and one for inside air. Outside air is collected, cooled, and then used to cool re-circulated indoor air in a heat exchanger that is common to both air circulation circuits. The two air circuits share the heat exchanger, but indoor and outside air flows are segregated. Even though it requires two fan systems, indirect air-side economization can be more energy efficient and may be the preferred method for warmer climates.

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