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Recent studies suggest that we are spending the majority of our time indoors meaning that we should take care that the environment that we live in is not detrimental to our wellbeing. Modern lifestyles introduce a number of risks to health in the form of exposure to humidity, carbon dioxide, nitrous oxides, volatile organic chemicals as well as pollutants from outside that pass into the space we are occupying. For this reason having an appropriate and effective ventilation strategy for the built environment is essential in maintaining occupant health and productivity.
When considering a new building whether for residential, commercial or industrial use, there are a number of directives and regulations in place that govern the energy it uses during its working life. Foremost in affecting the design of the building are the various approved documents that make up the building regulations. For ventilation, there are two the relevant documents that set out the minimum volume flow rates required to maintain a healthy environment (Approved Document F) and the energy involved in delivering the air into or extracting it from the building (Approved Document L).
In its most simplest of forms, the calculation of a volume flow rate first requires a calculation of the volume of room, occupied space or entire building. When we know the volume of air within the building we can use guidance on how many air changes are required based on the activity that takes place within the space to maintain a healthy environment.
For example:
Library
Occupants of a reading room in a library are usually sedentary with low respiration rates hence the room would usually require a ventilation rate of 4 air changes per hour. If the room has dimensions of 5 metres x 5 metres x 3 metres (75m3), the ventilation system should be capable of delivering 300m3/hour into the room…
Volume Flow = (5 x 5 x 3) x 4 = 300 m3/hour
Welding Room
If we consider a similar sized room in a fabrication plant where welding is the chief activity for the occupants within the room the volume flow rate increases significantly due to the presence of combustion gasses, smoke and airborne particulates. need for 22 air changes per hour
Volume flow = (5 x 5 x 3 ) x 22 = 1650m3/hr
Typical air change rates based on the activity within the occupied space are shown on the table below:
Room |
Number of air changes per hour |
Room |
Number of air changes per hour |
Commercial Kitchen |
33 |
Canteens |
10 |
Bakeries |
25 |
Laboratories |
10 |
Boiler Room |
22 |
Lavatories |
10 |
Engine Room |
22 |
Factories |
9 |
Foundries |
22 |
Bathroom |
8 |
Welding Shop |
22 |
Offices |
8 |
Hospital Theatre |
20 |
Mushroom Houses |
8 |
Domestic Kitchen |
18 |
Hospital Ward |
7 |
Shower room |
18 |
Battery Rooms |
7 |
Paint Shop |
15 |
Banks |
6 |
Public Houses |
14 |
Assembly Rooms |
6 |
Dance Halls |
14 |
School Room |
6 |
Cinema / Theatre |
12 |
Squash Court |
5 |
Cafes |
11 |
Living Room |
4 |
Restaurant |
10 |
Library |
4 |
Other ventilation requirements can also be stated for residential properties where normal, boost, trickle and purge ventilation rates are specified. In educational establishments the ventilation rate may be specified per occupant and are stated in a flow rate of a number of litres per second per person.
Ventilation and Energy Efficiency
In temperate climate zones the emphasis is on providing sufficient heat to maintain a comfortable environment however in hotter climates the emphasis changes to providing sufficient cooling. For the UK, Approved Document L deals with the energy performance of the building and focuses mainly on minimising the amount of heat that is lost through the fabric and construction elements that comprise the building structure. The latest issue of this document encourages greater air tightness in building construction which provides a number of challenges to overcome to provide a healthy indoor environment.
Improving the air tightness of a building will reduce the amount of energy required to heat or cool the occupants inside however it carries the risk of a build-up of contaminants inside. High levels of humidity can lead to mould growth which can lead to respiratory conditions such as asthma. This has led the ventilation industry to adopt the maxim ‘build tight, ventilate right’. The fewer air leaks a building structure has, the greater the emphasis on finding a solution that provides adequate ventilation to avoid unhealthy conditions. When minimising leaks and controlling the path the air takes to ventilate a building, consideration is needed to determine how much resistance there is in the system to deliver the required flow rate
About the author
Dan Hopkins is the Technical Manager at ebm-papst UK Ltd, the world’s leading manufacturer of high efficiency fan and motor products.