Raised floors are common in data centers, used to provide a space through which cool air is delivered to the data center. Unfortunately, the space under the raised floor is also often used to run cabling. It’s the “out of sight, out of mind” mentality, and it can lead to some significant energy loss.
Under-floor cabling contributes to energy loss in three ways:
- Blockage of air due to cables
- Cool air leakage through rack cable cutouts
- Cool air leakage through power distribution unit (PDU) cutouts
1. Blockage of air due to cables
Data centers are always changing – and usually expanding. As a result, you may need to add new network and power cables. Too often, new cables are simply added to the clutter of cables that already exists underneath the raised floor. Only rarely do companies remove old unused cables, either for lack of time or because they don’t want to risk pulling the wrong cable and causing a disruption.
The resulting mass of cables can and does cause blockages in air flow. With not enough cool air flowing into IT equipment, companies are forced to add more air conditioning – usually unaware of the root cause of the problem.
2. Cool air leakage through rack cable cutouts
Cables stored underneath a raised floor naturally have to punch through at some point to connect to IT and network equipment. The typical way this is done is through cutouts in floor tiles that measure about 20×20 cm, or 8 x 8 inches. These cutouts will be only partially filled with cables, leaving space through which cold air can escape from the under-floor plenum and into the data center. Assuming the data center uses a hot aisle/cold aisle layout, this cool air will usually enter the hot aisle, which is at the back of the server racks.
The cool air mixes with the warm air – exactly the opposite of what you’re trying to accomplish with the hot aisle/cold aisle configuration and with air containment systems. This air mixing lowers the temperature of the hot air and reduces the capacity of the computer room air handler (CRAH) to remove heat. For example, let’s say a CRAH unit with 27°C (80°F) return air temperature provides 70kW of heat removal capacity. At a return air temperature of 22°C (72°F), the heat removal capacity drops to 43kW. That lost capacity may create hot spots – which are sometimes addressed by adding more CRAH units.
3. Cool air leakage through power distribution unit (PDU) cutouts
PDU cutouts are an even greater potential source of cool air loss. Many PDUs require a large opening – 0.8-1.5 m square (9-16 square feet) – in the raised floor through which individual circuits are fed to IT racks, along with large input conductors that feed the PDUs. As with the cable cutouts, these spaces are not entirely filled and allow cool air to escape into the data center, causing the same issues as described above.
The solution to each of these problems is to place cables in overhead cable trays. The reduced clutter under the floor will improve air flow and reduce leakage. Modeling and analysis performed by Schneider Electric show that the decision to place network data and power cabling to overhead cable trays can lower cooling fan and pump power consumption by 24%.
To learn more, check out the Schneider Electric white paper, “How Overhead Cabling Saves Energy in Data Centers.”