Book - School of Science and Technology

502 Fans and air treatment equipmentair by conventional means. These systems particularly suited to displacement ventilationsystems where the supply air temperature is approximately 19 C.For the regeneration of the desiccant wheel to take place extract air returns to the plantat approximately 25 C db having gained moisture from the occupied space. It is firstpassed through an evaporative humidifier to reach a condition of 17 C db and 94% RH.This air then picks up sensible heat only from the thermal wheel from which it exits at55 C db and 12% RH. Finally this air is then reheated to approximately 65 C and 7%RH before it enters the moisture laden side of the desiccant wheel. Having regenerated thedesiccant material the air is discharged to atmosphere. The 2001 edition of Guide SectionB2 provides further information on desiccant cooling systems.In winter operation the thermal wheel simply operates in reclaim mode using theextract air to heat the supply air to the desired temperature.Specialised packaged plant is manufactured for the process and whilst use has beenlimited historically to industrial process applications, the system described above has beenused increasingly for comfort air-conditioning applications. Examples of office buildingsexist where the combination of the technologies of desiccant treatment with other lowenergy cooling or heating techniques has produced some interesting highly energy efficientsolutions.Packaged air handling plantThe various components previously described may be built up to provide a complete plantin a number of ways. They may be connected using sheet metal ducts or by incorporationwithin a masonry chamber: the former method tends to be untidy and space consumingand the latter is somewhat cumbersome and suspect as to permanent air-tightness unlesslined with sheet metal.Current practice favours the use of factory packaged plant where individual componentsare housed in modular casings for site assembly. The better designs for suchmodules are so arranged, as shown in Figure 17.29, that they may be assembled in anumber of different ways and thus fitted to the building space available.All the arrangements in Figure 17.29 indicate the supply fan at the discharge of the unitcreating `draw through' solutions. Whilst this arrangement is the most common inapplications where high levels of filtration are required there can be benefit in placingthe fan closer to the air intake, creating a `blow through' solution with air handling unitunder positive pressure.A recent and logical extension of this principle has been the production of thesemodules in weather protected form such that they, themselves, are plant rooms for, say,roof mounting without the need for architectural enclosure.In addition to the components shown, packaged equipment is available with air-to-airheat recovery devices built in, and with packaged air cooled refrigeration plant to servedirect expansion cooling coils (these are sometimes operated as heat pumps).Adequate access for maintenance must be provided between the component elements ofthe plant; this is particularly important where the process uses water, such as in humidifiers,or where dehumidification may occur on cooling coils. Space must be allowed alongthe length of the plant to enable control sensing devices to be fitted and sufficient spacealongside the enclosure for components to be withdrawn; the width for withdrawal beingequivalent to the width of the plant itself unless provision is made for components to beremoved in sections.