HP BladeSystem enclosures are designed to handle airflow differently from other blade architectures and rack servers. Air flows from front to back in a low-flow, high pressure drop configuration that results in a larger air temperature delta (increase) than other designs.
Most other blade enclosures are cooled with a simplier low-pressure configuration that depends on a high volume of air.
The high pressure scheme means three significant things:
- Less air needs to be pushed by the data room AC system. Result: Less AC equipment and air ducting are needed in the data center.
- The fans don't have to spin as fast. Result: The fans use less power. In a comparison from an HP lab, the fans in 16 1U rack server collectively used 512 watts, while the fans on 16 blade servers used 264 watts under the same workload.
- The exhaust air will be hotter. (The "slow" air that flows through BladeSystem effectively picks up more heat per unit volume of air). Result: More efficient CRAC unit operations, because they're better off at a relatively high temperature differential.
Here's a cut-away view of a c7000 enclosure, showing the cool air pulled in from the front (blue), picking up heat, then flowing out of the rear (red):
Ultimately, these three things save power and cost. They can also prevent you from having to do unnatural things (like empty rack cabinets) so full cabinets can get air from multiple floor tiles.
How does the high-pressure system work? The c7000 carefully controlling air through every blade and enclosure. Server blades themselves present a high impedence path to air. There's no direct path from front to back inside a blade; the walls, components, and air baffles figuratively confront the air, forcing it to areas where it picks up heat. To the left is a picture of an individual server blade showing that impedence.
It’s important to properly manage the hotter air coming out of the back of the enclosure and keep it from finding its way back to the front of the rack, mixing with the cooler inlet air, and raising the inlet air temperature. This is addressed in two ways: design of the BladeSystem (and rack cabinets) to prevent direct-return paths of exhaust air to inlet air; and the low-speed flow allows natural convection to return the hot air to the chiller. Other designs with larger volumes of high speed air won’t feel as hot to the casual observer standing in the hot aisle, but it’s more likely that this heat will infiltrate the cold aisle.