Book - School of Science and Technology

Air humidification 493Withdrawal(a) Cylindrical(b) Panel (section)Figure 17.20 Adsorption filter, single cellAdsorption filter. The service life of an activated carbon filter is between 6 months and 2years, depending on the type of installation and the concentration of contaminants beingadsorbed. An efficient particulate filter is recommended for use upstream. Normally, thecells are re-chargeable. Typical cell arrangements are shown in Figure 17.20. Thepressure drop across the unit is about 70 Pa and the face velocity of the order 1.5 m/s.Air humidificationDuring winter months the outside air has a low moisture content and when introduced toa conditioned space tends to reduce the relative humidity. To counter this effect, it is oftennecessary to introduce moisture into the space to maintain the required conditions forcomfort or for a process.Air humidifiers may be classed as either direct or indirect.Direct types. Introduce moisture into the space to be treated and their use is normally limitedto industrial or horticultural applications: these types are outside the scope of this book.Indirect types. Introduce moisture into the supply air either at the plant or within theductwork, dependent upon the type of system.A further sub-classification of the indirect type is storage or non-storage. The storagetypes incorporate a small tankunder the humidifying apparatus from which water isintroduced into the air stream, in one of a variety of ways. The water level in the tankismaintained by a water supply via a ball valve. With the non-storage type moisture, in theform of steam or atomised water particles, is injected directly into the air stream.Concern as to humidifier-related illness has changed the approach to selection and useof the associated equipment quite radically. Storage-type units are no longer acceptablefor application to systems in hospitals, where steam injection is recommended, a trendseen also in commercial practice. Where storage types exist or are envisaged, particularcare in cleaning, maintenance and inspection is required at frequent intervals. Furtherrecommendations relate to the need for biocide treatment of the tankwater and drainagethereof to waste, at regular intervals, preferably daily. The tankmust be drained andcleaned when the humidifier is to be out of use for any extended period.** CIBSE, Minimising the Risk of Legionnaires' Disease, CIBSE Technical Memorandum TM13, 1991(revised 2000). CIBSE, Legionellosis (Interpretation of Approved Code of Practice: The Prevention orControl of Legionellosis, CIBSE Guidance Note GN3, 1993.