Engineering Manual o.. - HVAC.Amickracing

CONTROL FUNDAMENTALSlosses. The exhaust-air system may be incorporated into the airconditioning unit, or it may be a separate remote exhaust. Supplyair is heated or cooled, humidified or dehumidified, anddischarged into the space.DAMPERRETURN FANEXHAUSTAIRRETURNAIRDAMPERSFILTER COIL SUPPLY FANOUTDOORAIRMIXEDAIRSUPPLYAIRC2712Fig. 15. Ventilation System Using Return Air.Ventilation systems as shown in Figures 14 and 15 shouldprovide an acceptable indoor air quality, utilize outdoor air forcooling (or to supplement cooling) when possible, and maintainproper building pressurization.PLEATED FILTERFor more information on ventilation, refer to the followingsections of this manual:— Indoor Air Quality Fundamentals.— Air Handling System Control Applications.— Building Airflow System Control Applications.FILTRATIONAir filtration is an important part of the central air handlingsystem and is usually considered part of the ventilation system.Two basic types of filters are available: mechanical filters andelectrostatic precipitation filters (also called electronic aircleaners). Mechanical filters are subdivided into standard andhigh efficiency.Filters are selected according to the degree of cleanlinessrequired, the amount and size of particles to be removed, andacceptable maintenance requirements. High-efficiencyparticulate air (HEPA) mechanical filters (Fig. 16) do not releasethe collected particles and therefore can be used for clean roomsand areas where toxic particles are released. HEPA filterssignificantly increase system pressure drop, which must beconsidered when selecting the fan. Figure 17 shows othermechanical filters.CELLBAG FILTERFig. 17. Mechanical Filters.Other types of mechanical filters include strainers, viscouscoated filters, and diffusion filters. Straining removes particlesthat are larger than the spaces in the mesh of a metal filter andare often used as prefilters for electrostatic filters. In viscouscoated filters, the particles passing through the filter fiberscollide with the fibers and are held on the fiber surface. Diffusionremoves fine particles by using the turbulence present in theair stream to drive particles to the fibers of the filter surface.AIR FLOWPLEATED PAPERFig. 16. HEPA Filter.C2713An electrostatic filter (Fig. 18) provides a low pressure dropbut often requires a mechanical prefilter to collect large particlesand a mechanical after-filter to collect agglomerated particlesthat may be blown off the electrostatic filter. An electrostaticfilter electrically charges particles passing through an ionizingfield and collects the charged particles on plates with an oppositeelectrical charge. The plates may be coated with an adhesive.14ENGINEERING MANUAL OF AUTOMATIC CONTROL