Engineering Manual o.. - HVAC.Amickracing

BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONSLAB AIRFLOWCONTROLLER PANELAIRFLOWSENSORDAMPER ACTUATOROR AIR VALVECRACK AREA = 0.5 FT 2DAMPERACTUATOROR AIRVALVEAIRFLOWSENSORSUPPLYAIREXHAUSTAIRSUPPLYAIR TOCORRIDORGENERALEXHAUSTAIRSUPPLYAIR TOLABAIRFLOWSENSORSASHSENSORFig. 46. Airflow Tracking Control.DAMPERACTUATORAIR VALVESUPPLY800 CFMDOOR 20 FT 2FUME HOODEXHAUST1000 CFMORVELOCITYSENSORM12215Airflow sensors located in all supply and exhaust ductsprovide flow signals which can be compared by a controller.Sensor locations must meet the manufacturers minimuminstallation guidelines, such as velocity range and length ofstraight duct before and after the sensor, to ensure accuracy.Materials and finishes for sensors in exhaust ducts exposed tocorrosive fumes must be carefully selected.If future flexibility and changing lab configurations areimportant considerations, then flow sensor location, duct size,supply airflow rate, and control system design should all includecapabilitiy to be modified in the future.A characteristic of airflow tracking is stability of the systemin the face of breaches to the lab envelope. This is most oftenlab door openings. In a laboratory maintained at a negativepressure, the space static pressure increases and the air velocitythrough all openings drops significantly when a door opens.Figure 47 shows a laboratory example with a single fume hood,a single door 36 in. wide x 80 in. high (20 ft 2 ), and a crack areaestimated at 0.5 ft 2 . If the fixed airflow tracking differential is200 cfm, the average velocity through the cracks would be 400fpm which is more than adequate for containment. However,when the door opens, the average velocity in this exampledecreases to 9.8 fpm which is marginal to inadequate forcontainment.DIFFERENTIAL = EXHAUST – SUPPLY= 200 CFMDOOR CLOSEDVELOCITY = 200÷0.5= 400 FPMDOOR OPENEDVELOCITY = 200÷20.5= 9.8 FPMC2637Fig. 47. Airflow Tracking Example withDoor Closed and Opened.However, the ability of the tracking system to quickly (usuallywithin several seconds) react and compensate for door openingsand other breaches is a positive characteristic of this controlmethod.Supply duct pressure and building pressurization control aresimpler and more stable with airflow tracking because they areless affected by this type of unexpected upset. The supply ductpressure control remains stable due to fewer disruptions.Building pressurization, defined as the difference between totalair leaving the building and the total air entering, remains thesame.Direct pressure control (Fig. 48) provides the same controlfunction as airflow tracking but its characteristics are quitedifferent. Direct space pressurization control senses thedifferential pressure between the space being controlled and areference space which is usually an adjacent space or hallway.Figure 49 shows a similar example of negative spacepressurization utilizing direct pressure control. If the airflowthrough the hood is 1000 cfm and the pressure control reducesthe supply airflow when the door is opened, the average velocitythrough openings drops from 400 fpm to 48.8 fpm.ENGINEERING MANUAL OF AUTOMATIC CONTROL270