Exhaust fumes emanating from fuel burning vehicles are highly toxic and can result in sickness and even death, if not ventilated properly. By monitoring the concentration of these fumes and ventilating based on their concentration, a safe environment can be ensured.
Most mechanical codes indicate that ventilation for parking structures should be continuous, 24/7, unless a system is in place to ensure hazardous gas concentrations are kept below prescribed levels. By ventilating only when sensors indicate a requirement, there is an opportunity for very significant energy savings, both from reduced fan operation, and potentially also from any related heating or cooling costs.
For gasoline, natural gas, or propane powered vehicles, carbon monoxide is the primary gas of concern. For gas detection applications in which diesel engines are employed, nitrogen dioxide should also be measured to ensure adequate coverage.
Vehicle exhaust presents an interesting challenge when designing a gas detection system. Because of the wide variety of applications including underground parking garages, bus barns, maintenance facilities, tunnels, train stations, airports, loading docks, car and truck dealerships and warehouses, a number of different gas detection solutions exist to satisfy the varying requirements. Therefore, a number of factors must be taken into consideration when determining what type of system should be specified.
- Size of space to be monitored: For most vehicle exhaust applications, systems are designed to have one sensor per 7000 ft2 feet. In larger, wide-open spaces, where air is free to move freely, this can be expanded to 9000 ft2. In areas comprised of dividers, sections, corners and other barriers to free movement of air, this should be condensed to one sensor per 5000 ft2. Keep in mind that a sensor the square footage is based on a circle, with the sensor at the center. A radius of 50 ft. is equivalent to 7850 ft2 Therefore, if sensors are mounted on outside walls, sensors should be spaced accordingly to ensure proper coverage.
- Sensor Placement: When laying out sensors in a space, care should be taken to keep them away from areas which may have an affect on readings. These include overhead doors (entrances and exits) as well as areas close to the outside air intake or exhaust fans.
- Number of areas or zones: If an area is to be partitioned into distinct areas for the purpose of ventilation, alarming, or other control functions, sensors can be lumped together in a per zone basis.
- Output requirement: Are alarm contacts required, or will the user be providing his/her own control functions, therefore requiring only sensor/transmitters with analog (4-20 mA) output.
- Type of fuels being used: If vehicles being monitored are gasoline, propane or natural gas powered only, then CO monitoring is sufficient. If however, diesel vehicles are being used, nitrogen dioxide (NO2) sensors should be used as well.
- Desired sequence of operations: The required sequence of operations for activation of auxiliary equipment must be taken into consideration when specifying systems. Are individual sensors to initiate particular operations, therefore requiring individual relay outputs, or will multiple sensors initiate single operations, therefore requiring only single, common relay outputs.
Once these factors have been determined, a selection of the most appropriate AMC system must follow. Here is a basic outline of different AMC exhaust monitoring systems, and when and where they should be used.