Ventilation Systems

Ventilation - a means of changing the air in an enclosed space. The volume of air necessary to provide for human occupancy may be considered under the following principal headings:

- Provide fresh air for respiration; approx. 0.1 to 0.2 l/s per person.
- Preserve the correct level of oxygen in the air; approx. 21%.
- Control carbon dioxide content to no more than 0.1%.
- Control moisture; relative humidity of 30% to 70% is acceptable.
- Remove excess heat from machinery, people, lighting...
- Dispose of odours, smoke, dust and other atmospherics contaminants.
- Relieve stagnation and provide a sense of freshness; air movement of 0.15 to 0.5 m/s is adequate.

Measures for control:

- Health and Safety at Work.
- The Factories Act.
- Offices, Shops and Railway Premises Act.
- Building Regulations, Approved Document F - Ventilation.
- BS 5925: Code of practice for ventilation principles and designing for natural ventilation.

The statutes provide the Health and Safety Executive with authority to ensure buildings have suitably controlled internal environments. The Building Regulations and the British Standard provide measures for application.

Air changes per hour or ventilation rate is the preferred criteria for system design. This is calculated by dividing the quantity of air by the room volume and multiplying by the occupancy.

Some examples of guiding to ventilation rates (air changes per hour) are:
- Boiler plant rooms: 10-30.
- Canteens: 8-12.
- Cinema/theatre: 6-10.
- Hospital wards: 6-10.
- Hospital operating theatres: 10-20.
- Libraries: 2-4.
- Offices: 2-6.

Natural ventilation - Passive Stack Ventilation (PSV)

Natural ventilation is an economic mean of providing air changes in a building. It uses integral components with construction such as air bricks or openable windows. The sources for natural ventilation are wind effect/pressure and stack effect/pressure.

PSV consists of vertical or near vertical ducts of 100 to 150mm diameter, extending from grilles set at ceiling level to terminals above the ridge of a roof.

PSV is energy efficient and environmentally friendly with no running costs. It works by combining stack effect with air movement and wind passing over the roof. It is self-regulating, responding to a temperature differential when internal and external temperatures vary.


Mechanically Assisted Extract Ventilation Systems (MAVS or MEV)

MAVS may be applied to dwellings and commercial premises where PSV is considered inadequate or impractical. A low powered silent running fan is normally located within the roof structure. It runs continuously and may be boosted by manual control when the level of cooking or bathing activity increases. Humidity sensors can also be used to automatically increase air flow.

MAVS are acceptable to Approved Document F1 of the Building Regulations as an alternative to the use of mechanical fans in each room. However, both PSV and MAVS are subject to the spread of fire regulations (Approved Document B). Ducting passing through a fire resistant wall. floor or ceiling must be fire protected with fire resistant materials and be fitted with a fusible link automatic damper.


Mechanical Ventilation with Heat Recovery (MVHR)

MVHR is a development of MAVS to include energy recovery form the warmth in fan extracted moist air. The heat recovery unit contains an extract fan for the stale air, a fresh air supply fan and a heat exchanger. This provides a balanced continuous ventilation system. Apart from natural leakage through the building and air movement from people opening and closing external doors, the building is sealed to maximise energy efficiency. Up to 70% of the heat energy in stale air can be recovered.

Mechanical ventilation

There are three categories of mechanical ventilation systems:

- Natural inlet and mechanical extract.
- Mechanical inlet and natural extract.
- Mechanical inlet and mechanical extract.

Some noise will be apparent from the fan and air turbulence in ducting. This can be reduced by fitting sound attenuators and splitters.

Internal sanitary accommodation must be provided with a shunt duct to prevent smells or other contaminants passing between rooms. In public buildings, duplicated fans with automatic changeover are also required in event of failure of the duty fan.

Basement car parks require at least 6 air changes per hour and at exits and ramps where queueing occurs, local ventilation of at least 10 air changes per hour must be provided.

Ductwork in all systems should be insulated to prevent heat losses from processed air and to prevent surface condensation.

For efficient distribution of air, the uniformity of circular ducting is preferred for the following reasons:

- Less opportunity for turbulence.
- Less resistance to friction.
- Inherent rigidity.
- Lower heat losses or gains.
- Sound transfer generally less.
- Less potential for air leaks.

Where space is restricted under floors or in suspended ceilings, rectangular ducting of high aspect ratio may be required for practical reasons.

Galvanised sheet steel is the most common material used for ventilation and air conditioning ducting.

Flexible ducts are useful for short connections from air distribution boxes or plenums to several diffusers within close proximity. Flexible connections to fans will help to reduce vibration and sound.

Sound attenuation in ducting can be achieved by continuously lining the duct with a fire resistant, sound absorbing material. Where this is impractical, strategically located attenuators/silencers composed of perforated metal inserts or a honeycomb of sound absorbent material can be very effective.

Air velocity within a room or workplace should be between 0.15 and 0.5 m/s, depending on the amount of activity. Sedentary tasks such as desk work will fall into the range of 0.15 to 0.3 m/s, whilst more active assembly work, workshop and manufacturing between 0.3 and 0.5 m/s.

Estimation of duct size and fan rating can be achieved by simple calculations and application to design charts.


Types of Air Filters

Cell or panel: flat or in a vee formation to increase the surface contact area. Available in dry or wet (viscous) composition in disposable format for simple fitting within the ductwork. A rigid outer frame is necessary to prevent flanking leakage of dirty air. Dry filters can be vacuum cleaned to extend their life, but in time will be replaced. The viscous filter is coated with an odourless, non-toxic, non-flammable oil. These can be cleaned in hot soapy water and recoated with oil.

Absolute: a type of dry cell filter produced from dense glass paper. The paper is folded into deep pleats to create a series of vee formations arranged parallel to the air flow to increase surface contact.

Bag: a form of filtration material providing a large air contact area. When the fan is inactive the bag will hang limply unless wire reinforced. It will resume a horizontal profile during normal system operation. Fabric bags can be washed periodically and replaced.

Roller: operated manually or by pressure sensitive switch. As the filter becomes less efficient, resistance to air flow increases. The pressure effects a detector which engages a motor to bring down clean fabric from the top spool.

Viscous: these have a high dust retention capacity and are often specified for application to industrial situations. An improvement on the panel type has close spaced corrugated metal plates continuously sprayed with oil.

Electrostatic unit: this has an ionising area which gives suspended dust particles a positive electrostatic charge. These are conveyed in the air stream through metal plates which are alternatively charged positive and negative. The unit can have supplementary, preliminary and final filters giving an overall efficiency of about 99%.

Activated carbon: otherwise known as activated charcoal. A disposable filter composed of carbon particles resembling pieces of coconut shell and arranged to provide a large surface area. A glass fibre matting is often used to contain the carbon shells. This type of filter is used specifically in commercial cooker hoods and in other greasy, odorous atmospheres, as the carbon is extremely absorbent.



Controlled Environment in Hospitals

There has been a growing concern in the medical community regarding the hazardous effects of poor indoor air quality on the heath of individuals which leads to increased incidence of health related symptoms like headache, dizziness, eye and throat infection, fatigue, memory loss etc. The terminology 'Indoor Air Quality' refers to the nature of the conditioned (heated/cooled) air that circulates throughout the space (2). This refers not only to the comfort, which is affected by temperature, humidity, odour, but also to the harmful chemical and biological contaminants present in the conditioned space.

The basic differences between controlled environment by air conditioning for hospitals to take care of the above mentioned factors and that for other building types stem from :

1. The need to restrict air movement in and between the various departments.
2. The specific requirements for ventilation and filtration to dilute and remove contamination in the form of odour, air-borne microorganisms and viruses, and hazardous chemical and radioactive substances.
3. The different temperature and humidity requirements for various areas; and
4. The design sophistication needed to permit accurate control of environmental conditions.

Air filteration: To prevent the flow of air containing infectious particulates, air filtration is provided in Air Handling Units which filters particles, pathogens and water droplets carried into the air, either from the coils and humidifiers or through leaks in the low-pressure side of the unit. For critical care areas like operation theatres, ICU, emergency and recovery areas normally three-stage filtration is provided.

- Pre-Filters (BS-6540) : These are first stage filters having efficiency 70% down to 10 Microns. These filters are cleanable and washable and installed at inlet of airstream.

- Fine filters (BS-6540-part-I) : Second stage filters having efficiency 99% down to 5 Microns. The pressure drop in dirty conditions should not exceed 20mm WG and the initial drop should be between 6.5 to 8.5mm WG. These filters are washable.

- Hepa filters: With efficiency 99.97% down to 0.3 Microns used for operating rooms and ICU's. These are special high flow types with more media to handle higher