The tile is the entity that delivers the air to the servers, but it is only one part of the system. We have broken the airflow ecosystem into four components. These all play a part in the efficiency of the airflow system. Each of these components supports the flow of air while minimizing waste and loss of thermal integrity. Each and every molecule of cool, moving air, should go through or across a server.

These four components are:
1. The under floor space
2. The perforated tile
3. The cabinet / rack
4. The return air system

For this example, we will be focusing primarily on one part of the system; the perforated tile as an airflow delivery device. Through a detailed analysis of perforated tile performance, in an open plenum environment, we have found the need for three distinct performance characteristics.

Triad Vs. Traditional

1. The tile should be constructed in a way that it ensures positive flow out of every part of the tile (no short cycling).

2. It should divert the air to the servers so it can penetrate the boundary layer of heat on the front side of the cabinet or rack.

3. It should be able to “stratify” the air to the top of the cold aisle. Servers need a constant flow of air to keep them cool.

TILEEFFNEW

 

All three of these performance parameters are necessary and lead to consistent temperatures and improved cooling. When analyzing the performance of traditional perforated tiles by applying the stated requirements, we find meeting our performance criteria to be difficult. Traditional tiles have been tested in a duct and were never created to divert the air. When we measure the performance of the tile in an open plenum, we find the design of the tile to be flawed leading to mixing, no dispersion of the air into the servers and an inability to flow to the upper servers.

This flaw comes from the design of a “flat bottom” found on most perforated tiles. The best way to illustrate the impact of this design flaw is to show you how flat bottom tiles are like a car window when a car is moving down the road. With your car windows open, you find inconsistent flow coming into the car. Air going by the window of the car passes by part of the “open area” of the window, providing some air into the car, but pulling air out of the car as well. The only way to get the air to flow directly into the car is to angle the air into the area needing the air. This angling of the air towards the heated item creates wind chill on the surface of the item to be cooled and dissipates heat by letting the air pass by the item.

In other words, we need a vent window to divert the air to the servers. Flat bottom tiles are like a car window. Air passes by part of the tile, which leads to mixing, less directed flow and a low stratification line. These flaws have lead to design considerations that reduce the impact of the flaw found in the tile, but lead to a great deal of inefficient airflow utilization. These airflow system designs have focused on pressure and open space instead of velocity and wind chill.

Consequently, the same study finds that we have 2.6 times more cooling capacity than is necessary.

Airflow Study

In a study on ‘Airflow in the data center’, Dr. Bob F. Sullivan and Kenneth G. Brill estimate that only 28% of the air in the raised floor airflow system actually gets to the servers.

This means 72% of the air is wasted and is only present so as to guide the other 28%. We, in effect, use air to divert air.