Engineering Manual o.. - HVAC.Amickracing

INDOOR AIR QUALITY FUNDAMENTALSPassive pathways include vertical elevator and mechanicalshafts which permit gravity flow of airborne contaminants, anddrop ceilings, tunnels, and partition walls where mold, mildewand fungus may be growing, spreading, and generating spores.Frequently, exterior walls are found to permit entry of water,moisture, and contaminated outdoor air. In large spaces suchas manufacturing areas and atriums, pressure differentials causethe movement of contaminants.Active pathways for air contamination include HVACsystems, cooling towers designed into the building, and localspot humidifiers, dehumidifiers, fans and heaters added by theoccupants after occupancy. If the contaminants are not trappedor eliminated at the source, the HVAC systems can move themthroughout the building. Since the HVAC system is the primaryactive pathway for contamination it plays a significant role inthe solution to the IAQ problem. Filtration is a significant aspectof the HVAC system where air is recirculated.Cooling towers that are not properly treated can promote thegrowth of legionella which may then be introduced into the HVACsystem and spread throughout the building. Local devices suchas humidifiers and dehumidifiers can promote fungus growth andspread it into the room space where it is picked up by the HVACsystem and distributed throughout the building. While designerscannot control what the occupants place in the building afterturnover, they can provide documentation and education as to thelimits of the systems installed.HVAC systems operating at 100 percent outdoor air produceindoor air concentrations of contaminants that approach outdoorconcentrations. If there is a 50 percent probability that theoutdoor air will not meet NAAQ standards there is a 50 percentprobability the IAQ will be substandard unless filtration systemsare designed for these contaminants. This includes HVACsystems with economizers during moderate weather conditions.REMEDIATING CONTAMINANT LEVELSParticulate Contaminant RemediationMethods of particulate remediation used to maintainacceptable IAQ levels include filters and electronic wireelements. Particulate filters are used to remove contaminantsfrom the air stream. Standard particulate filters with a 75 percentdust spot efficiency will remove all pollen and larger particlesas well as oil smoke. High efficiency HEPA filters having aDOP efficiency rating of 98 percent have been in use in thehealth care industry for many years. These remove viruses andbacteria. Portable and fixed installation HEPA filters are alsoavailable in the residential and light commercial market.Gas Contaminant RemediationGas phase filtration is used to reduce and control gas andvapor contaminants. Gas phase filtration systems utilizing virgincoconut shell carbon activated to 60 percent carbon tetrachlorideactivity should be capable of removing parts per billion ofVolatile Organic Compounds, ozone, nitrogen dioxide, andsulfur dioxide.According to the EPA one half the population of theUnited States lives in areas that do not meet the NationalAmbient Air Quality Standards with ozone being the mostprevalent contaminant. Electronic air cleaners are highefficiency filter devices that produce ozone as a byproduct.Ozone is listed as a harmful contaminant, yet it has beenused to improve IAQ by removing VOCs. If ozone is usedas a filtration or cleaning device it is necessary to followthese devices with activated charcoal or other gas phasefilters to removing any residual ozone.Radioactive particles and gases can be removed from air bydevices such as filters and absorption traps, but the gammaradiation from such material is capable of penetrating solid walls.EFFECTS OF TEMPERATURELack of thermal comfort control is an often ignored cause ofpresumed IAQ problems.Discomfort is often confused with sick building syndrome.Thermal comfort is a function of six parameters: air temperature,mean radiant temperature, relative air velocity, humidity, activitylevel, and clothing thermal resistance. The InternationalOrganization for Standardization has adopted the PredictedMean Vote (PMV) thermal comfort index to provide a methodto index comfort and discomfort levels taking into account thesix PMV parameters. PMV sensors are available on a limitedbasis. These sensors may become commonplace in the future,but for today most people who recognize that discomfort cancreate the perception of IAQ problems are focused on thetemperature and relative humidity aspects of the problem.Air is perceived to be fresher and less stuffy with decreasingtemperature and relative humidity, and there is a directcorrelation between increases in sick building syndrome andrises in room temperatures. The effect of temperature is linearand stronger than humidity. Temperature also effects relativehumidity. For example, if space conditions are 79°F and 16percent relative humidity and the temperature is decreased 7degrees to 72°F, then the relative humidity will rise to 20 percentor a 4 percent increase. These conditions are not uncommon inthe afternoon in the winter in temperate climates. Discomfortattributable to dry air can usually be remedied by lowering theair temperature a few degreesENGINEERING MANUAL OF AUTOMATIC CONTROL148