Figure 11. Non-compliant fan has fan intake flowing over the motor. Photo courtesy of Gladstein, Neandross & Associates, NREL 44702 Figure 12. Ground-mounted fan with external ducting. Photo courtesy of Gladstein, Neandross & Associates, NREL 44703 (i.e., plastic or aluminum) to prevent the potential for igniting released gas. In addition, the electric motor that drives the fan must be explosion-proof or located where the fan intake or exhaust does not pass over the electric motor. Figure 11 shows a non-compliant belt-driven fan with the fan intake flowing over the electric motor.
Certification of compliance must be obtained from the manufacturer, and the type of fan selected must be approved by the AHJ. (manual operation is also possible). Another impor tant consideration is that if non-rated electrical equipment is within 18 inches of the ceiling and protection is provided by ventilation only (at a rate of 4 ACH), the fans must be in continuous operation when CNG vehicles are in the building. 3. For systems activated in conjunction with facility lighting, the ventilation system is interlocked with the lighting circuit. The applicable codes and regula tions assume that interior lighting will always be activated whenever a maintenance facility is occu pied.
Similar to the continuous ventilation system, the associated high heating costs may make this type of system cost prohibitive in colder climates. This option should also be implemented with care, as a release could occur when the building is not occupied and the lights are not in use. Also note that the ven tilation required consists of 100% make-up air with no recirculation. Therefore, although this option is compliant with some codes, it is not recommended. In some cases, there is not enough space to install a fan or the roof cannot support the additional load of a new fan.
In these cases, ground-mounted fans and suitable external ducting (Figure 12) can be used to comply with requirements. Operation and control of the ventilation system depend on several factors. The exhaust fans and means for providing makeup air can 1) operate continuously, 2) be controlled automatically using a gas-detection system, or 3) be activated through the facility overhead and work space lighting system. A description of each method for operation and control of the ventilation system follows: Some designers recommend that two levels of ventila tion be employed by utilizing two-speed or variable- speed fan motors.
A two-level system will provide a lower level of ventilation (4 ACH) to reduce noise levels and energy consumption while the second stage will provide rapid dilution. The approach taken must be approved by the AHJ. 1. For ventilation systems that operate continuously, the fans are constantly running. Continuously operating ventilation systems are most commonly employed in warmer climates where ambient air does not require conditioning for worker comfort. These systems require continuous electrical power for the fans and may entail increased fan maintenance costs.
Facility ceiling profiles vary considerably as a result of structural roof members, varying ceiling heights, or other internal configurations. The codes do not require airflow across every square inch of ceiling; rather, the system should be designed such that the airflow eliminates 2. For systems controlled using a combustible-gas- detection system, the fans are only activated when the sensors detect the presence of natural gas SEPTEMBER 2017 13