Breathing, bathing, clothes washing, kitchens bathrooms all generate moisture. Water vapour produced by these means is carried by warm air.
When the air cools the water vapour is deposited as water droplets - especially against colder surfaces.
Fungal spores, ever present in the atmosphere, will germinate on damp surfaces and gather together as black or grey ‘mould’ on damp surfaces. Thus giving rise to the typical condensation damp appearance.
As buildings become better insulated and more airtight in response to the drive for better thermal performance and reduced carbon dioxide emissions, their proper ventilation becomes more and more of an issue…
Heat loss resulting from uncontrolled ventilation can become a major proportion of a building’s total heating demand and,
Air leakage through the fabric cannot be relied upon to provide enough background ventilation to keep our buildings healthy.
Designed to provide what is known as background ventilation. Ensures that the building is provided with fresh air from the outside as well as allowing for the removal of internally generated low levels of vapour and pollution caused by occupation or emanation from construction materials.
It should be capable of being reduced or closed completely when the building is unoccupied, no significant amount of moisture is being generated and no polluting equipment is running.
Typically includes trickle vents in windows and background ventilators through walls, but also can be provided by means of passive stack ventilation or as part of a complete mechanical system.
Designed to provide fast removal of high concentrations of pollutants, such as those that occur when decorating or as a result of accidents (e.g. burning food), or if a room has been unoccupied for a period with the whole-building ventilation closed down. It can also be used to improve thermal comfort in summer.
Purge ventilation is usually provided by opening windows or switching on local extract fans if provided.
Driven by temperature differences between the hot air in the occupied space and the cooler external air. Warm air rises upwards in the building and exits through one or more high level openings. The air displaced from the building causes cooler fresh air to be drawn into the building through low-level openings, such as windows and vents.
Enhanced by increasing the height of the ventilation stack or by providing a small fan in the extract path for hot still days.
The driving force is the pressure difference between the inlet and extract segments of the ventilator.
The terminal can be in a variety of shapes, although the most common are square and circular. The inside of the terminal is divided into four separate chambers, which act as the main inlet and extract ducts.
Generally used in environments where air becomes directly contaminated by a particular activity or process. The main benefit of mechanical extract over natural ventilation is a constant and predictable ventilation rate. For many applications this is an essential requirement; the use of natural ventilation would not be appropriate.
Primarily used in high volume industrial and commercial buildings with a large floor to ceiling space. The units provide a mixture of fresh and re-circulated air that can be warmed in the winter to provide space heating.
Comprises a central air handling unit (AHU), typically containing separate supply and extract fans, an air filter and/or a heating coil supplied with hot water from the building boiler system.
To save energy in cold weather, some systems are designed to recirculate a portion of the warm extracted air back into the supply. This cuts down the amount of outside air that needs to be heated.
A heat recovery device can also be incorporated into the AHU. This takes heat from the exhaust air and transfers it back into the supply air without mixing the two air streams. This is important in some industrial and medical applications where contaminated extract air must not be reintroduced into the space.
Secondary filters (classes F5 to F9) have an average efficiency ranging from 40 – 95%. Average efficiency is defined at the weighted average of the efficiencies for the different specified dust loading levels up to final pressure drop.
Secondary filters are effective in trapping pollen, bacteria and dust.
Secondary filters can be of the extended media bag type, pocket type or a rigid bag.
Can be highly efficient in filtering out particulate contaminants like smoke, dust and pollen.
Thin wires suspended in the air stream are charged with a high positive voltage. The positive ions created in the ionising field become attached to particles which then pass into the collecting plate section. This section consists of a series of parallel plates, with alternative plates positively charged and the other plates at ground potential. The electric field forces the charged particles on to the oppositely charged plates.
The retention of particles on the collector plates
Activated carbon is highly porous and can be used to absorb volatile chemicals. Practical applications can include the removal of volatile organic compounds and odours from an air stream.
Activated carbon is usually combined with conventional filter techniques to provide filtration for larger particulates. Activated carbon filters are usually used in combination with secondary filters to prevent the pores becoming blocked with particulate matter.
Can be used to neutralise micro-organisms as they are sensitive to the effects of invisible light at certain wavelengths (the DNA structures of the micro-organisms are damaged and are not able to replicate).
Ultraviolet irradiation has been used in hospital applications based on the following options:
Portable forced air re-circulation equipment utilising
both UVGI and HEPA filtration to trap and kill pathogens
by introducing an air current to provide room air mixing.