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Automobiles, trucks and propane powered fork-lifts indoors, mean toxic fumes that need to be measured and exhausted. We have been making toxic gas sensors for this job for over 30 years and we’ve heard it all. “We don’t need gas sensors and automated ventilation because; - fresh air comes in through the garage door every time it opens. We switch on the ventilation manually when we need it. We pump fresh air in 24/7. We had a consultant come in who tested the air (once) and it was OK. We rarely use the parking lot.” Just because you cannot see it, does not mean it is not there, and we could also say, just because it could be there, does not mean that it is. If you heat or air-condition a garage, you need an automated air quality system, complete with gas sensors. Here is the why and the how.
Vehicle emissions are the most common major source of indoor air pollution today for any building with vehicles running inside a closed space. Everything takes a distant back seat; - second hand cigarette smoke, off gassing of volatile organic compounds from carpets, furniture, and cleaning products. Exhaust contains a wide range of chemicals which are bad for your health, including carbon monoxide which impedes our ability to absorb oxygen, nitric oxides, oxides of sulphur, and very fine particulate, or soot, that contribute to respiratory problems. The particulate is so fine that it can stay suspended in the air for hours, eventually to settle, causing corrosion and premature aging of metals, electrical fixtures, concrete and, everything it touches.
OK, so ventilation is a good thing for parking and loading areas, why not continuous ventilation? Because, almost all of the time the ventilation should be off, and the costs to heat and air-condition the exhausted air is shy-rocketing Vehicle emissions are very peaky. When there is vehicle activity, the air is very bad and when there is no activity, the air quality is very good, on the condition that you eliminated the gases when the activity was high. For those times, you need to be aggressive with the exhaust ventilation. This gives the highest level of safety at the lowest possible investment in energy.
Ventilation is typically arranged with motorised dampers at each end of the space being protected or diagonally opposed, with strong exhaust fans at one end. This is duplicated at each level of a multi-level space and often a common make-up air damper is used to reduce the number of components. Heating and conditioning the make-up air directly is optional and consideration should be given to the usage of the space. The temperature can drop for a few minutes on cold days in a parking lot before the heating catches up, but if it is a space where people are working, such as a vehicle maintenance facility, the dips are not acceptable. Keep in mind that the greater the temperature differential between inside and outside, the faster you want to move out the toxic gases and keep the conditioned air inside. There is a false perception that in very cold weather it is more economical to restrict the amount of air flow. The contrary is true.
For gasoline and propane powered vehicles you need to monitor the ambient carbon monoxide levels. The carbon monoxide emissions can vary greatly from one vehicle to another with air temperature, and will depend on whether the vehicle has been running long enough to get the catalytic converter up to speed. This gas is the best indicator of air quality, not just because it is so toxic, but because it is always there, in proportion to the total toxic load. Sensors will typically cover a detection radius of 40 to 50 feet maximum. Placing more sensors than this will improve the response time slightly but not affect the overall efficiency of the system significantly.
Diesel powered vehicles are a special case because there is not enough carbon monoxide in the exhaust to work with. The largest risk associated with diesel powered vehicles is from the floating particulate. This is difficult to measure so the nitrogen dioxide levels must be monitored. This gas is always present due to the high temperature of diesel combustion and gives a very good indication of the total toxic load of the exhaust. Carbon dioxide, for example, is also present in exhaust, but if the motors are mal-adjusted and burning badly, the toxic products can be ten times as great as those from one in top condition. The carbon dioxide levels will remain constant but the levels of carbon monoxide and nitrogen dioxide will go up proportionally.
In summary, do not over-think the control process, as to the time of day, where the gas will go, and how many cars per hour will drive through, etc. Let the gas sensors do their job of measuring and let the ventilation do its job of clearing the pollutants.
Opera have just launched their series 5000 gas detector which can control ventilation autonomously over a twisted pair network. It is also BACnet Testing Laboratory listed for BACnet MS/TP communication up to 76800 baud. The new unit features a rugged splash proof enclosure and inexpensive plug-and-play sensor modules to suit the gas.
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