Over the past two months, we have encountered steadily raising temperatures within the prototype Helsinki Chamber. After some investigation, we had to conclude that both the exhaust and supply air flows were insufficient. During experiments, two interesting new phenomena occurred
- Supply underpressure, whenever the supply is unsufficient
- Exhaust overpressure, whenever the exhaust is insufficient
During supply underpressure, the servers are simply provided insufficient supply air. This causes suction on the supply chamber, which forces some of the exhaust air to reverse flow through the servers and back into the supply chamber. Exhaust overpressure works similarly, but the cause is the insufficient air flow out of the exhaust chamber.
Both effects can occur on their own, and both can be verified by allowing the HC to run without either the front or back covers. Moreover, both effects have their counterparts in production environments using either cold or hot aisle containment. Supply underpressure can happen whenever the CRACs do not supply enough cool air into the cold aisle. Exhaust overpressure is more elusive, but will happen if a hot aisle has insufficient exhaust air suction.
These effects were initially very depressing, because from an engineering perspective they signalled that only a full “wind tunnel” would be ideal for server air flow. Such a tunnel would be very tricky to construct without also allowing rain or snow inside the chambers. This meant that a certain researcher had to return to the design whiteboard.