Air purity
Ve once took air-conditioning for granted, but the incidence of Legionnaire’s disease has concentrated attention on the problems that can arise if air-conditioning is fitted and forgotten. In addition, there is an energy wastage aspect to consider when air-conditioning is run continuously, and the problem of the noise which, even at a low level, is always present and which can be heard even in some concert halls.
Many buildings that use air-conditioning do not need the full flow to be operating all the time, because the extent of occupancy can be very variable, particularly in cinemas, restaurants, kitchens and other spaces. The answer is an air-quality sensor, using a semiconductor surface whose conductivity will alter when contaminants are present in the air. The types of contamination that can be sensed include perspiration, tobacco smoke, oil and grease vapours (from cooking) and similar noxious fumes. The semi- conductor surface is used at an elevated temperature so that there is no tendency for contaminants to condense on the surface.
The calibration of an air-quality sensor is fairly arbitrary, and its positioning is important. Doors, windows, fireplaces, corners and recesses need to be avoided because the air samples will not be typical, and the preferred height for fitting the sensor is 1.5 m above floor level. Vhen the sensor is first installed, its output should not be used, because it will take up to 2 days of operation to stabilize the unit. Once this has been done, the sensitivity has to be set with reference to clean air, so that the output switches when the air becomes noticeably contaminated. Once the sensitivity is set, the sensor can be coupled to the air-conditioning system to trigger the fans when the contamination level is reached.