The Importance of Rack Hygiene in High Density Data Centers

No matter how well a containment system is built and installed, its benefits can be reduced or negated if good “rack hygiene” practices are not followed. (Source: TechnoGuard)









































































































































































































This week we complete our series of articles on the challenges of cooling IT equipment in high-density environments by exploring the importance of rack hygiene and the energy efficiency cost of air leakage.

Get the full report.

No matter how well a containment system is built and installed, its benefits can be reduced or negated if good “rack hygiene” practices are not followed. Poor practices can lead to extreme cases of cabinet recirculation, causing ITE inlet temperatures to rise 10 to 30°F above the cold aisle supply temperature.

Here are the basics that are relatively easy to implement, and at little to no cost.

  • Use horizontal blanking plates for all voids between ITE
  • Whenever possible, keep the ITE at the bottom of the rack (especially important for unconfined or partial containment (end of aisle doors).
  • Use vertical blanking components (soft seals or brushes if the cables from the back must pass through the front of the ITE)
  • Practice rear cable management to avoid blocking airflow. Use shorter cables (power and network) whenever possible.
  • Install brushed collars (or similar devices) for all underfloor openings for power and network cables

In cases where there is a significant disparity in watt density and exhaust airflow between opposing cabinets in a common hot aisle, the higher volume and velocity of airflow exiting cabinets at high wattage can prevent low-density cabinets from allowing hot air to fully escape. cabinets that can cause them to overheat. If this happens, an upward facing rear baffle on the higher density cabinet (or both cabinets) will help alleviate this problem.

If the cabinet overheats, an upward facing rear baffle on the higher density cabinet (or both cabinets) will help alleviate this problem.

Air Leakage Energy Efficiency Cost

MoFlo, courtesy of TechnoGuard

This is because any supply airflow to the cooling system that does not pass solely through the ITE is considered bypass airflow and a waste of cooling and ventilation energy. This is also called overfeeding or overventilation. Although it is nearly impossible to accomplish this in practice, to ensure that ITE airflow requirements are met, a certain amount of oversupply is required. However, unnecessary and avoidable leaks should be controlled to the extent possible or practical in high density environments.

Although we have discussed the technical aspects of airflow management, this also directly translates to an increase or decrease in fan power consumption and energy cost (see Fan Laws ).

Fan Laws

There are three basic fan laws, which express how the change in relative fan speed, static pressure, and horsepower (horsepower) interact.

fan law 1

The change in fan speed is proportional to the change in airflow

fan law 2

The change in static pressure (increase/decrease) with the square of the change in airflow

fan law 3

Fan power (increases/decreases) with the cube* of the change in airflow (fan speed). (*in reality, the exponent of the actual fan curve may be slightly lower than the exponent of a “3” cube)

The graph in Table 9 shows how bypass airflow can impact fan energy.

high density

Table 9 – A hypothetical example of relative airflow versus fan power, simplified for clarity* (in reality, the exponent of the actual fan curve will be slightly lower than the exponent of the cube “3 “)

  • Assumptions (simplified for clarity)
    • Cooling system design Rated critical load of 1000 kW
    • 12 Units Full N+2 Redundancy
    • Normal conditions: Total of 12 CRAH
    • Failure mode: 10 CRAHs required for critical cooling unit specifications
    • Cooling design capacity of cooling unit 100 kW – Fan rated at 15,000 CFM fan power 10 kW
    • Cooling load 100% DT 25°F (126 cfm/kW) 126,000 CFM: no leaks
    • 90% load IT (900 kW) IT DT 25°F (126 cfm/kW) total required 113,400 CFM
  • Design versus actual operating conditions: As shown in Table 9, a bypass airflow of 20% increases power consumption and fan cost by 83% (compared to a theoretical perfect match between CFM and DT of the computer equipment cooling system). While in reality it is virtually impossible for a perfect airflow match to occur in an actual data center, the exponential nature of the fan’s third law shows the significant penalty or benefit in mitigating leaks. which increase bypass airflow.

Download the full article, “High Density IT Cooling – Breaking Thermal Boundaries” Courtesy of TechnoGuard, for exclusive content including what to look for when selecting a data center provider.

Ramon J. Espinoza