Challenging the Design Process: High Electrical Cabinet Cooling Requirements vs. Stringent Airborne Noise Requirements

Abstract

An operationally critical, manned control station’s design was challenged to meet stringent airborne noise requirements. Through an innovative, interdisciplinary engineering approach, a solution was implemented, resulting in the first production unit meeting airborne noise requirements. The manned control station was initially designed with nine tubeaxial fans continuously operating at maximum rated speed to provide sufficient cooling to internal components at the station’s worst-case maximum ambient operating temperature of 122°F. Initial testing of a prototype unit showed airborne noise requirements were exceeded by as much as 22 dB (re 20 micro-Pascals). Fan speed reduction and installation of temporary internal acoustic absorption material were tested to measure effectiveness, but were not sufficient to meet airborne noise requirements. The normal operating condition of the control station was reviewed and detailed modeling and testing of the station’s actual cooling requirements under various operating scenarios was conducted. During this process, it was recognized that under normal operating conditions, the worst-case ambient operating temperatures were improbable and that a balancing of cooling requirements and acoustic requirements was possible. A new cost-effective method of fan control in conjuction with the installation of permanent internal absorption material and a redesign of ventilation ducting was implemented. In normal ambient temperature conditions, six of the nine internal fans operate at minimum rated speed. If ambient temperatures rise above normal (the control station’s airborne noise requirments only apply at normal “non-casualty” ambient temperatures), three additional exhaust fans automatically turn on at minimum rated speed to minimize additional airborne noise contributions. If ambient temperatures continue to rise, all nine fans are automatically switched on at maximum rated speed to protect the internal components of the operationally critical control station. Acoustic performance evaluations predicted these improvements would result in meeting the stringent airborne noise requirements. These predictions were validated during testing of the first production unit, which passed all airborne noise requirements.