Sample Natural Ventilation Application Mechanical ... - TTMD

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Article1. 3. Acoustic PerformanceThe presence of apparent external sound sources is thefactor that causes greatest difficulty in the applicationof natural ventilation. There are two main solutions tothis issue:- Vent holes are located on the side far from thesource of sound. If the source of sound is traffic, thepositioning of these openings on the side withouttraffic would at the same time ensure clean natural air.- Acoustic curtains may be added to ventilation openings.For schools in particular, it is very important that a goodacoustic performance is secured together with naturalventilation. BB93 includes recommendations forensuring acoustic performance together with naturalventilation. Figure 1.4 demonstrates that ventilationopenings with acoustic protection could be integratedwith window ledge and circumferential heating.consideration of cooling load that may increase due toclimate change or tenant preferences. The projectionto be made in this case should include the openingsto be left in the floor and ceiling to enable installationof additional systems. The cost of additional flexibilityshould be compared with the operating and installationcosts to be incurred by undue air-conditioning, and thedecision should be taken accordingly.b) Zoned mixed-modeThis mode considers that different areas of the buildingserve different purposes of use. Air-conditioning isapplied where it is actually needed. Heating andventilation is applied in areas with low heat gain. This typeof an approach is applied to building sections where fixedheat loss and gain is expected throughout the building’slifetime. Such practices may lead to tension among users.The users of two areas with different conditions may thinkthat other group of users have better conditions and mayresort to actions to claim their rights.c) Changeover mixed-modeThis mode considers that the cooling load of eacharea may differ seasonally. An example of this is themechanical ventilation used under very extremeweather conditions (too hot or too cold). Under mildweather conditions, natural ventilation is used. Thiseliminates the draft effect during winter months.Furthermore, it helps the pre-cooling of building withnighttime ventilation.241. 4. Natural Ventilation and Mixed-ModeThe above discussion addresses the conditionsrequired for application of natural ventilation in abuilding. However, it is not compulsory to ventilate allareas of a building with the same system. Differentstrategies may be applied at different times for differentsections. This is called “mixed-mode” approach. This isexplained in CIBSE AM13 in detail. Various approachesto mixed-mode are outlined below.a) Contingency mixed-modeThis system is used if flexibility is needed in thearea. Such systems should be designed with dued) Concurrent mixed-modeIt ensures concurrent operation of Mechanical andNatural Ventilation. While the mechanical systemmeets the fresh air need, opened windows or openingshelp cooling for summer. Furthermore, mechanicalventilation may be turned on for nighttime cooling,and the security problem in natural ventilation may beresolved. Under very hot weather conditions, energymay be wasted as excessive natural ventilation wouldprovide unnecessary fresh air. Figure 1.5 presents asuperficial flow diagram for users to be used duringselection stage.1.5. Starting with Design:If natural ventilation is found to be feasible based onthe above discussions, a decision needs to be taken asto whether this would be a single type or a mixed-modesystem. The next strategy is to proceed with designconcept. There are three key steps in the design stage.TTMD 2013
Articlea) Modeling of air flow from inlets to outlets:This model varies by the shape and organization ofthe building. In addition, the purpose of use of thebuilding and ventilation, and the position of the land itis located on are also effective. For example, is there isa road with busy traffic on one side of the building, itwould be unreasonable to locate the air inlets on thatside. Air inlets should be located on the other side interms of pollution and air quality.b) Basic drivers required to be analyzed forobtaining the desired air flow model:While many strategies take wind pressures intoconsideration, some utilize temperature differences. Insome cases, fans may be used to assist these naturalforces; a good design should warrant that dominantforces ensure the required flow.c) Sizing of openings to obtain the desired airflow rate and flow pattern:This is performed in three stages.1. Flow rates are calculated by taking intoconsideration thermal comfort conditions and airquality.2. Openings are sized and positioned to ensure theseflow rates under design conditions.3. A control system must be designed so thatthe system runs automatically at various rates ofoccupancy and weather conditions.StartNoIs max. heatgain higherthan 30-40 W/m 2 ?IS it possibleYes to decrease itNoTemporaryNobelowoccupaucy?30-40 W/m 2 ?mixed-modeacceptable?NoNoYesYesCan capacityeffects balance temperatureand inlet airquality oscllation?YesYesYesNoPeakseason?YesYesNarrowbuilding?NoIs it possible todecrease a triumwidth below15 m?NoZonedmixed-mode?Mixed-modeventilationYesYesYesIs this spacefacing exterior?YesNoNoise andcontamination levelacceptable?NoNoYesCan people adjustvarying weatherconditation?NoNecessaryto scrutinizetemperature?±1KYesNecessaryto scrutinizerelative humidity?±10%Yes25NoNoYesNoIs it necessary touse humidificationwinter months?YesNaturalventilationMechanicalventilationMechanicalventilation andhumidificationComfortcoolingFullventilationFigure 1.5: Choice of ventilaiton strategyTTMD 2013
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