Book - School of Science and Technology

494 Fans and air treatment equipmentThe types of indirect humidifier may be summarised as follows:. Non-storagesteam injectionmechanical separators.. Storagespray washerscapillary washerspan humidifierssprayed coilsultrasonic atomisers.Steam injectionControlled steam injection may be supplied either from a central boiler plant via a spargepipe, Figure 17.21, or from a local packaged unit as in Figure 17.22. The former, unlessserved from an independent steam generator for the purpose, has limited use due to theodour and oil traces characteristic of central boiler installations. The latter type ofequipment is mounted on or close to the air-conditioning plant and consists, in essence,of a small electrode steam generator, sized to suit the moisture requirement of the plant,controls and a water supply: steam heated units are also available. The standard equipmentavailable is not suitable where the static pressure in the duct is over 1250 Pa. Goodaccess is required for all humidifiers and sparge distribution pipeworkshould be arrangedto be self-draining. Saturation efficiency is about 80%.Mechanical separatorsThese commonly operate via use of spinning discs, as illustrated in Figure 17.23. It mustbe remembered that, in the process of atomisation and evaporation, any mineral saltsdissolved or suspended in the water supply will be released and deposited: some watertreatment may be required. Saturation efficiency is claimed to be as high as 90%.Control valveSteaminletSteamdischargepipeSeparatorDrainAir flowFigure 17.21 Mains steam humidifier, injection type (Spirax Sarco)