Introduction. mode systems that incorporate mechanical and

Introduction.This essayhighlights the necessity of mechanical ventilation in buildings. Ventilation isthe supply and removal of air in a building and it is used to obtain thecorrect fresh air quantity and to prevent and remove air borne contaminants.1 Althoughnatural ventilation systems can be effective, this essay will highlight thatthere are circumstances where only mechanical ventilation systems are appropriate,such as unacceptable levels of pollution.

Types of ventilation systems will beintroduced, along with: ventilation and external conditions; ventilation andthe indoor climate; and ventilation and comfort. All of which highlight thatthere will always be a need for mechanical ventilation in buildings.Types of ventilation systems.Ventilation canbe provided naturally, mechanically, or using mixed mode systems thatincorporate mechanical and natural elements.  Natural ventilation systems circulate airwithout mechanical assistance. Fresh air is drawn into a building throughtemperature difference and/or wind.2 Windowsare utilised and when installed on multiple sides, cross ventilation can occur,which can achieve high levels of ventilation.

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3 Stackventilation relies on internal and external temperature differences, whichgenerates convection currents. The air entering a building is warmed by heatemitters, equipment, etc., causing it to lose density and be distributed up thebuilding, finally being discharged through vents.4 Astemperature difference is key, wind is not essential. However, in windyconditions, combined stack and cross ventilation is very effective.5 Mechanical ventilation systems vary dependingon requirements. Extract only systems use extract fans to remove air, butsupply is delivered naturally. This would be considered mixed mode ventilation.

These systems are often used in domestic kitchens and bathrooms where air gets contaminated.6 Supplyonly systems distribute fresh air typically by fan-coil units. After filteringout contaminants they can heat or cool the incoming air depending onrequirements.7Supply and extract systems are more complex, requiring a central Air HandlingUnit (AHU). The AHU usually has separate supply and extract fans, filters, anda heating coil.

Ductwork is used to circulate the air. Due to the high controllevels, reliable rates of ventilation can be achieved.8 Looking at systems alone, developers would choosenatural ventilation. It is cheaper to run and install; simpler to operate; and likemechanical systems, compatible with building management systems (BMS).

However,as buildings are designed to meet different needs, mechanical ventilation isstill required. Ventilation and External Conditions When choosing a ventilation system, afundamental criterion is local pollution levels, which includes noise andcontaminated air. If pollution levels are unacceptable, natural ventilation isnot an option.9The reliance on windows paves the way for contaminants and noise to enter. Thisis particularly the case in urban environments, where pollution levels tend tobe higher. To combat contaminated air, mechanical supply systems, such as perimeterfan coil units, force air through filters before dispersing it into a room.10  There are numerous types of filters, such as drybag; viscous; and electrostatic.

Dry bag filters are simple; they inflate whenthe system is in use, and create a large surface area that traps incomingcontaminants. Viscous filters are coated in oil that ensures contaminants likedust stick to it. Electrostatic filters are very effective for preventing theentrance of even pollen and smoke due to their ability to positively charge incomingparticles. These particles are then attracted to negative plates. Finally, theelectrostatic filter employs an activated carbon filter to eliminate anyunpleasant odours.11  Approved Document F of the BuildingRegulations, is concerned with ventilation, and includes ways mechanical supplysystems can be positioned to minimise the ingress of contaminants. Therefore, theworkload of the air filters is reduced and the systems overall efficiencyincreases.

Air intakes are positioned as high as possible to avoid the directinfluence of pollution such as traffic fumes. They should also be placed on theside of the building furthest from sources of pollution. Furthermore, they mustbe upstream and away from exhaust outlets. This is so wind does not push contaminatedair back into the building.12 Ventilation and the Indoor Climate.As air can becontaminated by internal activities, BSRIA, the Building Services Research andInformation Association, state that natural ventilation is sometimes notappropriate. Mechanical ventilation systems are designed to produce the airchanges required to supply fresh air and remove impurities.13 Factoriesand industrial buildings need mechanical extract systems in place to remove thedust, fumes, and hazardous gases that amass.

14 These extractfans are positioned away from supply inlets. Like supply systems, extract systems havefilters. Commercial kitchens produce large amounts of hot, greasy, andcorrosive fumes. These substances cannot be allowed to build up and spread.

Therefore,kitchen extract systems contain activated carbon filters to absorb thesehazards. They are linked to a bifurcated axial flow fan that forces air out ofthe building.15These fans are used due to their protective inner casing that prevents hot andcorrosive fumes from damaging the motor. This increases the efficiency of the system.16 Ventilation is key in limiting moisture.  Approved Document F, prioritises moistureremoval.17The level of moisture in the air is a measure of humidity, (the amount of watervapour).

As it is a gas, vapour increases as the temperature rises. Warm air,like in an active kitchen, holds more moisture than cooler air.18 However,once the temperature drops, or when the air meets cooler surfaces, condensation,a form of dampness occurs, leading to mould growth.19 Consequently,mechanical extract fans are used in bathrooms and kitchens to remove moisture. Ventilationrates are therefore designed dependent on requirements. In domestic kitchenswith cooker hoods, a ventilation rate of 30 l/s is required to remove moistureproduced at 2000g/h.20 Thisventilation rate would be higher in a commercial kitchen as more moisture isgenerated. Mechanical extract is also vital in removingsmoke.

Either smoke is drawn up through roof-mounted extracts, or a centralisedextract unit with ductwork, forcing air out via numerous extract grills.21 Theseextract systems are a vital part of fire safety systems, as smoke inhalation isfires biggest killer.22  Appropriate fresh air supply and air changerates are integral in ventilation. The supply rate is dependent on theactivities taking place. An office would need a minimum of 10l/s per person butin contaminated environments, like factories, the rate could be as high as36l/s per person.23 Themore air required means more air changes per hour.

The office would need 2-6air changes whereas factories need 20-30.24 As itcan be designed to suit, mechanical ventilation systems can provide the airchanges required whereas natural systems cannot.  For energy efficiency in cold weather, mechanicalventilation systems often incorporate re-circulation and direct heat recovery. Therefore,some of the warm return air is redistributed. In contaminated environments, heatrecovery devices are installed to take the heat from the outgoing air to warm upthe supply without the two meeting.

25 Bydoing this a mechanical supply and extract system can provide heating andventilation, improving the occupants thermal comfort.Ventilation and ComfortThe thermalcomfort of occupants is fundamental and ventilation can assist with some of thephysical variables: air temperature; air movement; and humidity. Air temperature can be affected by sensibleheat gains that are generated from solar radiation, lighting, machinery, people,etc. In addition, latent heat gains from moisture given off by people andactivities can also increase temperatures.26 Toremove these gains, airflow is utilised. Air movement increases the rate ofheat loss due to convection currents.

27Dependant on the needs, the fan speed in mechanical ventilation systems can beadjusted. Natural ventilation systems cannot achieve this due the dependence onexternal conditions.  Excessive heat loss, particularly in winter,is a problem for natural ventilation. Mechanical ventilation with heat recovery(MVHR) systems can be installed, to counteract this problem.28 TheMVHR has a flat plate heat exchanger that recovers 90% of the heat from theextract air.29To maintain efficiency the ductwork is insulated to avoid heat loss once airleaves the MVHR unit.

 Another limitation of natural ventilation isthat it has the potential to cause draughts.30 Mechanicalventilation systems can pre-heat supply air to the same temperature asspecified to avoid cold draughts.31 Anotherway is through controlling air diffusion patterns. Supply air is distributedout of supply air diffusers. These are often positioned along the perimeter or centreof the ceiling, enabling set airflow patterns that prevent draughts, unlikewindows.32Mechanical ventilation systems are also capable of providing air curtains. Thisis when warm air is distributed across external door openings to prevent coldair from entering.

33 Theseare particularly useful in buildings with constant foot traffic likesupermarkets. The fan power is set to ensure that warm air reaches the floor,and sometimes it is used to generate turbulence that prevents insects fromentering.34ConclusionAs evidenced bythe preceding sections, there will always be a need for mechanical ventilationin buildings. Polluted environments, like busy urban areas, mean onlymechanical ventilation can be used as they can filter out contaminants.  As buildings have multiple uses, there can beno uniform air supply and air change rates in general.

Mechanical supplysystems can be adapted to suit, be it rooms with low occupancy and low heatgains, or those with much higher. Factories, industrial buildings andcommercial kitchens must have mechanical extract systems to remove thecontaminants and hazards. Natural ventilation would not be able to cope asthese impurities must be removed at the source. As previously discussed,mechanical extract systems are also vital to remove moisture and are anintegral part of fire safety systems. In terms of comfort, mechanical ventilationsystems can be utilised to remove heat gains and prevent heat loss via the useof MVHR units. Therefore, aiding to the occupants’ thermal comfort and abuildings overall energy efficiency.  Overall, mechanical ventilation provides avaried and integral role in buildings.

Of course, where permissible, naturalventilation can be a great choice. However, considering over half the world’spopulation lives and works in urban environments, and with this figure set toincrease an average of 1.63% between now and 203035,mechanical ventilation will continue to play a vital role. 1 Chadderton, D.V, (2013), Building Services Engineering, 6thEdition, p 75 2 BSRIA (2017), The Illustrated Guide to Mechanical Building Services, p 223 Ibid, p 244 Hall, F and Greeno, R, (2013), Building Services Handbook, 8thEdition, p 2465 BSRIA (2017), The Illustrated Guide to Mechanical Building Services, p 266 Ibid, p 277 Ibid, p 288 Ibid, p 299 CIBSE (2016), CIBSE Guide B2: Ventilation & Ductwork, p 2-5310 BSRIA (2017), The Illustrated Guide to Mechanical Building Services, p 2811 Hall, F and Greeno, R, (2013), Building Services Handbook, 8thEdition, pp 267-27112 HM Government, (2010), The Building Regulations 2010: Ventilation, Approved Document F: F1Means of Ventilation, p 5613 Hall, F and Greeno, R, (2013), Building Services Handbook, 8thEdition, p 25214 BSRIA (2017), The Illustrated Guide to Mechanical Building Services, p 2715 Hall, F and Greeno,R, (2013), Building Services Handbook, 8thEdition, p 27216 Ibid, p 25717 HM Government, (2010), The Building Regulations 2010: Ventilation, Approved Document F: F1Means of Ventilation, p 4418 McMullan, R, (2012), Environmental Science in Building, 7th Edition, pp 88-8919 Ibid, p 9720 HM Government, (2010), The Building Regulations 2010: Ventilation, Approved Document F: F1Means of Ventilation, p 4421 Ibid, p 2722 Ready.gov, Home Fires, https://www.ready.gov/home-fires, (Accessed 20thDecember 2017)23 Hall, F and Greeno, R, (2013), Building Services Handbook, 8thEdition, p 24124 Ibid, p 24225 BSRIA (2017), The Illustrated Guide to Mechanical Building Services, p 2926 Chadderton, D.V, (2013), Building Services Engineering, 6thEdition, p 8627 Health and Safety Executive, The Six Basic Factors, http://www.hse.gov.uk/temperature/thermal/factors.htm#radiant(Accessed 20th December 2017)28 BSRIA (2017), The Illustrated Guide to Mechanical Building Services, p 3029 Ferguson, A, (2011), Information Paper 12/11 Part 1 Sustainable refurbishment of the BREVictorian terrace – Part 1: Design philosophy, p 1330 BSRIA (2017), The Illustrated Guide to Mechanical Building Services, p 2231 Hall, F and Greeno, R, (2013), Building Services Handbook, 8thEdition, p 28732 Ibid, pp 274-27533 Ibid, p 27634 Hall, F and Greeno, R, (2013), Building Services Handbook, 8thEdition, p 27635 World Health Organisation, Urban Population Growth, http://www.who.int/gho/urban_health/situation_trends/urban_population_growth_text/en/,(Accessed 5th January 2018)

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