Maximize data center cooling efficiency by controlling airflow

Cool air is expensive, and wasting it is inefficient. Maintaining hot and cool air separation maximizes cooling effectiveness, energy efficiency and air conditioner performance.

Air remains the primary cooling method in most data centers, despite the trend toward liquid cooling. There are two goals to make air cooling effective: maximize air delivery to the IT equipment and keep hot air discharged from ITE separate from the cool air that enters.

There are different cooling types to separate hot and cold air, but not every data center has the same methods as others. Admins must understand the different types of air-cooling methods and the importance of controlling return air to maximize cooling efficiency.

The benefits of air separation

Separating air of different temperatures accomplishes two tasks:

  1. It maximizes the amount that gets to ITE.
  2. It maximizes air conditioner performance with mechanical cooling and maximizes free cooling hours when outside air is in use.

Types of air mixing

Air mixes when it passes through openings between devices in equipment racks or when it circulates over the tops and around the ends of cabinet rows. These methods are classified as bypass air and recirculated air.

With bypass air, cooled air goes past ITE rather than through it. Cool air mixes with hot discharge air, which lowers the temperature of air that returns to the air conditioners and reduces cooling capacity.

With recirculated air, hot air discharged from ITE returns to the front and mixes with the cooled supply air. This raises the temperature of air that enters ITE, reduces cooling efficiency and can shorten ITE service life.

Separating hot and cold air

Through hot and cold aisle design, cabinets are set up face to face and back to back. Snap-in filler panels go over open spaces between ITE devices in racks and cabinets to close gaps. With this method, however, air still leaks over cabinet tops and around row ends.

Forms of aisle containment

Aisle containment separates the two air temperatures with barriers. This includes hot aisle, cold aisle, full and partial containment:

  • Hot aisle containment. Encloses ITE discharge air, which leaves the rest of the room at supply air temperature. It is generally the easiest to implement in new construction and is energy-efficient. This method does use some cooling air for the rest of the room.
  • Cold aisle containment. Encloses the ITE supply air, which leaves the rest of the room at hot aisle temperature. This maximizes cool air usage if air balance is under control. Unless there is already a return air plenum ceiling, cold aisle is easier to retrofit in existing rooms.
  • Full containment. Uses doors at each end of the contained aisle and solid panels between cabinet tops and ceilings. Fillers at cabinet bases keep air from flowing underneath.
  • Partial containment. Uses plastic strip closures instead of doors and panels. They may only be at aisle ends, not above cabinets. Partial containment is usually more effective, less expensive and easier to implement in existing data centers than full containment.

All forms improve air control and energy efficiency. The major caution is fire protection. Full containment requires discharge heads in every aisle. Existing data centers may have heads in alternate aisles. Partial containment avoids repiping around working ITE. All doors and panels should be fire-rated in accordance with the National Fire Protection Association 75 standard.

Three types of cooling methods in a data center are dependent on the architecture: raised access floors, overhead cooling and close coupled cooling.

Conserving air with different cooling types

Three types of cooling methods in a data center are dependent on the architecture: raised access floors, overhead cooling and close coupled cooling.

Raised access floors

Raised floors are full of holes -- those in air flow panels, as well as those for cable and piping passthroughs. Air flow panels with integral or supplemental vanes are available to control air quantity and direct air to the correct cabinets, often more efficiently than legacy tiles. High-flow tiles are helpful, but don't use too many. More air cannot be delivered to ITE than the supply under the floor.

These floors convey air in accordance with manufacturers' standards for air plenum floors. They leak air and leak more as people walk on them and admins remove and replace the panels. To limit leakage, professionally clean underfloor spaces every few years, and relevel tiles in the process.

Remove no more than two adjacent tiles at a time to avoid destabilizing the floor and as few as possible to maintain maximum airflow throughout the floor. Replace tiles exactly as they were removed to maintain the seal.

Significant leakage occurs at the room perimeter and around large equipment, like air conditioners and uninterruptible power supplies. Seal all edges with closed-cell foam. Also, seal piping passthroughs with fire-stop material. Floor cable openings are also big leakage points. Grommet or brush-type seals are available to use for new and existing cables. Don't use fiberglass, mineral wool or any other product that can flake off and get into the air and equipment.

Overhead cooling

Many newer data centers, whether they have raised floors or not, use overhead cooling. Ductwork moves air into the cold aisle from conventional perimeter air conditioners or distributes it from individual cooling units mounted in the aisle, on top of cabinets or directly above them. This works as denser cool air falls, which displaces the warmer air and forces it upward.

If admins select and supply diffusers to maximize air delivery, ducted systems can deliver quite uniform temperatures throughout the aisle. Air conditioner fans must also have the capability to operate against duct static pressures. Electronically commutated fans, which are energy-efficient and have become relatively standard, may not be able to supply enough air through long ducts.

Direct overhead cooling requires special refrigerant piping and delivers sensible cooling with no humidity control. It is usually used for high-density spot cooling for cabinets.

Close coupled cooling

With this method, cooling units deliver air as close to ITE intakes as possible and pull hot exhaust air back into themselves from nearby cabinets before it can escape the hot aisle. The usual form is in-row coolers, although direct overhead units are also classified as close coupled.

The importance of controlling return air

Computer room air conditioning (CRAC) controls both the quantity and temperature of the supply air based on return air temperature. Bypass air reduces return air temperature, so CRAC controls measure that less cooling is necessary. ITE inlet temperature then rises, also raising return air temperature, so CRAC units now provide more cool air until the return air temperature goes back down and the cycle repeats. This is called short cycling and results in poor cooling, poor humidity control and accelerated equipment wear.

A benefit of well-controlled return air is a thermodynamics characteristic that surprises most IT specialists. Air conditioners can deliver more cooling when hotter air returns to their coils. Below are two examples, using the same air conditioner with 65% more cooling capacity:

  1. A return air temperature of 75 degrees Fahrenheit (23.9 degrees Celsius) leads to an air conditioner rating of 20 tons (t) or 75 killowatts (kW) of cooling.
  2. A return air temperature of 95 degrees Fahrenheit (35 degrees Celsius) leads to an air conditioner rating of 33 t or 114 kW of cooling.

Raising the return air temperature requires raising the inlet temperature to ITE, which also saves cooling energy.

Legacy data centers operate at 55 degrees Fahrenheit (12.8 degrees Celsius). A 20 degree Fahrenheit (11.1 degree Celsius) temperature rise through ITE (known as the delta T, DT or ΔT) is standard, which makes the discharge temperature 75 degrees Fahrenheit (23.9 degrees Celsius).

Raise the inlet temperature to 75 degrees Fahrenheit (23.9 degrees Celsius), and the discharge temperature should rise to 95 degrees Fahrenheit (35 degrees Celsius).

The ASHRAE-recommended range for ITE inlet temperature is a maximum of 80.6 degrees Fahrenheit (27 degrees Celsius). Therefore, 75 degrees Fahrenheit (23.9 degrees Celsius) is well within the parameter. No cabinet should exceed the maximum, even with temperature variations within the aisle. Air conditioners now have more cooling capacity, which saves energy.

Robert McFarlane is principal in charge of data center design for the international consulting firm Shen Milsom and Wilke LLC. McFarlane has spent more than 35 years in communications consulting and has experience in every segment of the data center industry.

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