Introduction. Unit (AHU). The AHU usually has separate

Introduction.

This essay
highlights the necessity of mechanical ventilation in buildings. Ventilation is
the supply and removal of air in a building and it is used to obtain the
correct fresh air quantity and to prevent and remove air borne contaminants.1 Although
natural ventilation systems can be effective, this essay will highlight that
there are circumstances where only mechanical ventilation systems are appropriate,
such as unacceptable levels of pollution. Types of ventilation systems will be
introduced, along with: ventilation and external conditions; ventilation and
the indoor climate; and ventilation and comfort. All of which highlight that
there will always be a need for mechanical ventilation in buildings.

Types of ventilation systems.

Ventilation can
be provided naturally, mechanically, or using mixed mode systems that
incorporate mechanical and natural elements.

 Natural ventilation systems circulate air
without mechanical assistance. Fresh air is drawn into a building through
temperature difference and/or wind.2 Windows
are utilised and when installed on multiple sides, cross ventilation can occur,
which can achieve high levels of ventilation.3 Stack
ventilation relies on internal and external temperature differences, which
generates convection currents. The air entering a building is warmed by heat
emitters, equipment, etc., causing it to lose density and be distributed up the
building, finally being discharged through vents.4 As
temperature difference is key, wind is not essential. However, in windy
conditions, combined stack and cross ventilation is very effective.5

 Mechanical ventilation systems vary depending
on requirements. Extract only systems use extract fans to remove air, but
supply 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 Supply
only systems distribute fresh air typically by fan-coil units. After filtering
out contaminants they can heat or cool the incoming air depending on
requirements.7
Supply and extract systems are more complex, requiring a central Air Handling
Unit (AHU). The AHU usually has separate supply and extract fans, filters, and
a heating coil. Ductwork is used to circulate the air. Due to the high control
levels, reliable rates of ventilation can be achieved.8

 Looking at systems alone, developers would choose
natural ventilation. It is cheaper to run and install; simpler to operate; and like
mechanical systems, compatible with building management systems (BMS). However,
as buildings are designed to meet different needs, mechanical ventilation is
still required.

Ventilation and External Conditions

 When choosing a ventilation system, a
fundamental criterion is local pollution levels, which includes noise and
contaminated air. If pollution levels are unacceptable, natural ventilation is
not an option.9
The reliance on windows paves the way for contaminants and noise to enter. This
is particularly the case in urban environments, where pollution levels tend to
be higher. To combat contaminated air, mechanical supply systems, such as perimeter
fan coil units, force air through filters before dispersing it into a room.10

 There are numerous types of filters, such as dry
bag; viscous; and electrostatic. Dry bag filters are simple; they inflate when
the system is in use, and create a large surface area that traps incoming
contaminants. Viscous filters are coated in oil that ensures contaminants like
dust stick to it. Electrostatic filters are very effective for preventing the
entrance of even pollen and smoke due to their ability to positively charge incoming
particles. These particles are then attracted to negative plates. Finally, the
electrostatic filter employs an activated carbon filter to eliminate any
unpleasant odours.11

 Approved Document F of the Building
Regulations, is concerned with ventilation, and includes ways mechanical supply
systems can be positioned to minimise the ingress of contaminants. Therefore, the
workload of the air filters is reduced and the systems overall efficiency
increases. Air intakes are positioned as high as possible to avoid the direct
influence of pollution such as traffic fumes. They should also be placed on the
side of the building furthest from sources of pollution. Furthermore, they must
be upstream and away from exhaust outlets. This is so wind does not push contaminated
air back into the building.12

Ventilation and the Indoor Climate.

As air can be
contaminated by internal activities, BSRIA, the Building Services Research and
Information Association, state that natural ventilation is sometimes not
appropriate. Mechanical ventilation systems are designed to produce the air
changes required to supply fresh air and remove impurities.13 Factories
and industrial buildings need mechanical extract systems in place to remove the
dust, fumes, and hazardous gases that amass.14 These extract
fans are positioned away from supply inlets.

 Like supply systems, extract systems have
filters. Commercial kitchens produce large amounts of hot, greasy, and
corrosive fumes. These substances cannot be allowed to build up and spread. Therefore,
kitchen extract systems contain activated carbon filters to absorb these
hazards. They are linked to a bifurcated axial flow fan that forces air out of
the building.15
These fans are used due to their protective inner casing that prevents hot and
corrosive fumes from damaging the motor. This increases the efficiency of the system.16

 Ventilation is key in limiting moisture.  Approved Document F, prioritises moisture
removal.17
The level of moisture in the air is a measure of humidity, (the amount of water
vapour). 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. Ventilation
rates are therefore designed dependent on requirements. In domestic kitchens
with cooker hoods, a ventilation rate of 30 l/s is required to remove moisture
produced at 2000g/h.20 This
ventilation rate would be higher in a commercial kitchen as more moisture is
generated.

 Mechanical extract is also vital in removing
smoke. Either smoke is drawn up through roof-mounted extracts, or a centralised
extract unit with ductwork, forcing air out via numerous extract grills.21 These
extract systems are a vital part of fire safety systems, as smoke inhalation is
fires biggest killer.22

 

 Appropriate fresh air supply and air change
rates are integral in ventilation. The supply rate is dependent on the
activities taking place. An office would need a minimum of 10l/s per person but
in contaminated environments, like factories, the rate could be as high as
36l/s per person.23 The
more air required means more air changes per hour. The office would need 2-6
air changes whereas factories need 20-30.24 As it
can be designed to suit, mechanical ventilation systems can provide the air
changes required whereas natural systems cannot.

 

 For energy efficiency in cold weather, mechanical
ventilation systems often incorporate re-circulation and direct heat recovery. Therefore,
some of the warm return air is redistributed. In contaminated environments, heat
recovery devices are installed to take the heat from the outgoing air to warm up
the supply without the two meeting.25 By
doing this a mechanical supply and extract system can provide heating and
ventilation, improving the occupants thermal comfort.

Ventilation and Comfort

The thermal
comfort of occupants is fundamental and ventilation can assist with some of the
physical variables: air temperature; air movement; and humidity.

 Air temperature can be affected by sensible
heat gains that are generated from solar radiation, lighting, machinery, people,
etc. In addition, latent heat gains from moisture given off by people and
activities can also increase temperatures.26 To
remove these gains, airflow is utilised. Air movement increases the rate of
heat loss due to convection currents.27
Dependant on the needs, the fan speed in mechanical ventilation systems can be
adjusted. Natural ventilation systems cannot achieve this due the dependence on
external 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 The
MVHR has a flat plate heat exchanger that recovers 90% of the heat from the
extract air.29
To maintain efficiency the ductwork is insulated to avoid heat loss once air
leaves the MVHR unit.

 Another limitation of natural ventilation is
that it has the potential to cause draughts.30 Mechanical
ventilation systems can pre-heat supply air to the same temperature as
specified to avoid cold draughts.31 Another
way is through controlling air diffusion patterns. Supply air is distributed
out of supply air diffusers. These are often positioned along the perimeter or centre
of the ceiling, enabling set airflow patterns that prevent draughts, unlike
windows.32
Mechanical ventilation systems are also capable of providing air curtains. This
is when warm air is distributed across external door openings to prevent cold
air from entering.33 These
are particularly useful in buildings with constant foot traffic like
supermarkets. The fan power is set to ensure that warm air reaches the floor,
and sometimes it is used to generate turbulence that prevents insects from
entering.34

Conclusion

As evidenced by
the preceding sections, there will always be a need for mechanical ventilation
in buildings. Polluted environments, like busy urban areas, mean only
mechanical ventilation can be used as they can filter out contaminants.

 As buildings have multiple uses, there can be
no uniform air supply and air change rates in general. Mechanical supply
systems can be adapted to suit, be it rooms with low occupancy and low heat
gains, or those with much higher. Factories, industrial buildings and
commercial kitchens must have mechanical extract systems to remove the
contaminants and hazards. Natural ventilation would not be able to cope as
these impurities must be removed at the source. As previously discussed,
mechanical extract systems are also vital to remove moisture and are an
integral part of fire safety systems.

 In terms of comfort, mechanical ventilation
systems can be utilised to remove heat gains and prevent heat loss via the use
of MVHR units. Therefore, aiding to the occupants’ thermal comfort and a
buildings overall energy efficiency.

 Overall, mechanical ventilation provides a
varied and integral role in buildings. Of course, where permissible, natural
ventilation can be a great choice. However, considering over half the world’s
population lives and works in urban environments, and with this figure set to
increase 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, 6th
Edition, p 75

2 BSRIA (2017), The Illustrated Guide to Mechanical Building Services, p 22

3 Ibid, p 24

4 Hall, F and Greeno, R, (2013), Building Services Handbook, 8th
Edition, p 246

5 BSRIA (2017), The Illustrated Guide to Mechanical Building Services, p 26

6 Ibid, p 27

7 Ibid, p 28

8 Ibid, p 29

9 CIBSE (2016), CIBSE Guide B2: Ventilation & Ductwork, p 2-53

10 BSRIA (2017), The Illustrated Guide to Mechanical Building Services, p 28

11 Hall, F and Greeno, R, (2013), Building Services Handbook, 8th
Edition, pp 267-271

12 HM Government, (2010), The Building Regulations 2010: Ventilation, Approved Document F: F1
Means of Ventilation, p 56

13 Hall, F and Greeno, R, (2013), Building Services Handbook, 8th
Edition, p 252

14 BSRIA (2017), The Illustrated Guide to Mechanical Building Services, p 27

15 Hall, F and Greeno,
R, (2013), Building Services Handbook, 8th
Edition, p 272

16 Ibid, p 257

17 HM Government, (2010), The Building Regulations 2010: Ventilation, Approved Document F: F1
Means of Ventilation, p 44

18 McMullan, R, (2012), Environmental Science in Building, 7th Edition, pp 88-89

19 Ibid, p 97

20 HM Government, (2010), The Building Regulations 2010: Ventilation, Approved Document F: F1
Means of Ventilation, p 44

21 Ibid, p 27

22 Ready.gov, Home Fires, https://www.ready.gov/home-fires, (Accessed 20th
December 2017)

23 Hall, F and Greeno, R, (2013), Building Services Handbook, 8th
Edition, p 241

24 Ibid, p 242

25 BSRIA (2017), The Illustrated Guide to Mechanical Building Services, p 29

26 Chadderton, D.V, (2013), Building Services Engineering, 6th
Edition, p 86

27 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 30

29 Ferguson, A, (2011), Information Paper 12/11 Part 1 Sustainable refurbishment of the BRE
Victorian terrace – Part 1: Design philosophy, p 13

30 BSRIA (2017), The Illustrated Guide to Mechanical Building Services, p 22

31 Hall, F and Greeno, R, (2013), Building Services Handbook, 8th
Edition, p 287

32 Ibid, pp 274-275

33 Ibid, p 276

34 Hall, F and Greeno, R, (2013), Building Services Handbook, 8th
Edition, p 276

35 World Health Organisation, Urban Population Growth, http://www.who.int/gho/urban_health/situation_trends/urban_population_growth_text/en/,
(Accessed 5th January 2018)