In the bubble chart yesterday, we looked at rack top temperatures over the course of a week. I mentioned that with bubble charts, it’s not easy to compare two parameters such as rack top and rack bottom. However, what you can do is look at the delta temperature (Rack Top Temp – Rack Bottom Temp) for each location. The bubble chart above is showing this difference.
The story that this data is telling us is incredible and extremely useful in determining airflow inefficiencies.
Here is some background. This facility has an under floor air distribution system with cold aisle/hot aisle layout. Cool air is delivered through perforated tiles to the rack inlet. Generally, the difference in air temperature between the bottom and top of the rack should be around 8°F due to the heating as the air rises and the typical mixing that occurs.
- Rack 6, 8, 9, and 29 have greater than a 10°F delta T. This may indicate areas where lots of mixing is occurring. Most likely air from the hot aisle is sneaking over the top or through the racks affecting temperatures. Blanking panels may help.
- There are many racks with a negative delta T, meaning that the top of the rack is colder than the bottom of the rack. Potential causes are hot air from the hot aisle infiltrating the cold aisle low to the ground due to missing blanking panels, through holes in the floor, or through the IT equipment itself. The negative delta T may also indicate that IT equipment is installed backwards exhausting hot air into the cold aisles.
- Most of the racks have a delta T that changes over the course of the week as indicated by multiple bubbles on a single vertical line. However racks 12, 16, 17, and 18 show only one large bubble – meaning that they are operating at exactly the same delta T for the entire time. How could this be ?
- The real value of looking at delta T in a bubble graph is to identify airflow issues.. This graph would most likely be used when hot or cold spots are identified to determine the root cause.