In most cases, the goal is to establish some sort of ventilation control, such that a balance can be achieved between energy conservation and guarding against toxic exposure. Carbon monoxide is usually chosen as the target analyte, unless the vehicular traffic will be largely diesel. While diesel engines do emit CO, it is less than in gasoline-powered vehicles. In such cases, nitrogen dioxide is monitored.
An online search will produce several references specifying the area that can be monitored by one sample-draw monitoring point. A good mid-range estimate is 300 square meters (3229 square feet). For diffusion sensors, this number is usually 30-40 percent less. Thus, more diffusion sensors would be needed in the same application.
Consideration should be given to the number and location of the exhaust fans. Good practice is to monitor near the intake register, thus sampling the “oldest” air in the facility. A simple “on/off” scheme to control the fans is discouraged these days, in favor of a so-called demand-controlled ventilation (DCV) scheme. As the name implies, the system is designed to keep the garage fans running continuously and vary motor speeds based on CO concentrations in the garage.
As per the reference linked…
A proven effective variable flow CO system is one that designs in or syncs variable frequency drive (VFD) technology with a control strategy that:
- Enables the motors to run continuously at low speeds—when CO levels are de minimis—while adhering to code/design ventilation rate requirements.
- Creates a reservoir of fresh air in the garage so CO concentrations are prevented from exceeding pre-defined sensor trip points for an extended period of time, minimizing the number of times the motors must ramp to flush out the garage.
- Incrementally increases fan motor speeds, (the ventilation rate) whenever CO concentrations near pre-set trip points. Said another way, the motors don’t instantly ramp from low to high speeds, but rise proportionally (in speed) to counter CO concentrations with an equivalent amount of fresh air.
The result is that property owners can continuously ventilate their garages in an energy-efficient manner while ensuring the health and safety of building occupants and visitors.
Ventilation control schemes must also consider the specific gas concentrations that will trigger the desired actions. In larger installations, algorithms can be created whereby a combination of concentration data from two or more sampling points can trigger differing control actions.
As to packaging, the number and location of the sampling points, along with the size of the facility will determine if individual monitoring units, central enclosure(s), or a combination of the two would be the best approach. Some locations may be determined to be more critical than others for a variety of reasons—and this must also be taken into account.
Both parking garages and tunnels will have peak traffic times, and it may be prudent to adjust the “resting” low fan speed in these cases.
We realize that these applications can present challenges, and invite you to contact us for further information.