Fresh air requirements for human comfort
Over the last few years considerable research has been conducted both here and in the USA to determine what pollutants exist in the ambient atmosphere and within buildings. These studies have principally looked at the prevalence of volatile organic compounds, pesticides, carbon monoxide and particulates.
Whilst we are all aware of reports of increased numbers of persons suffering, for example, from asthma, it is extremely difficult to correlate these with an increase in particular pollutants. A number of scientists and engineers have concluded that most people are likely to have the greatest contact with potentially toxic pollutants not outside but within buildings.
If we consider the gross amounts of one particular carcinogen
I. e., benzene released into the atmosphere, the greater portion comes from automobile fuel (82%). The next highest sources are industry (14%) and domestic usage (3%). Cigarette smoke only contributes around 0.1 % of the total. However, within the confines of a building, approximately 45% of the total exposure may come from smoking, 36% from inhaling petrol fumes and other common products and only 3% from industrial processes.
The corollary of these figures is to conclude that improved ventilation with an increase in fresh air levels and in association with improved filtration is a must. It could greatly improve the indoor air quality and hence reduce the risk of disease. Buildings could then claim to be more friendly to health.
Similar conclusions can be reached for many other chemicals found at quite high concentrations inside buildings — even the “perc” (perchloroethylene) used by dry cleaners or the deodorisers used by ever increasing numbers of the population, have been reported to cause cancer at high concentrations. If we add the potential risks due to carbon monoxide (from incomplete combustion in kitchens and elsewhere) and radon (a natural radioactive gas seeping from foundations and brickwork), one wonders why we should spend up to 90% of our lives within buildings. Is it just a coincidence that the author’s long living grandfathers (a fisherman and a farmer) spent most of their waking hours outdoors?
The need for improved indoor air quality has persuaded many ventilation engineers that the way forward is to increase the amounts of outside fresh air circulated within buildings. But if that outside air is far from fresh there is a definite problem. It’s more than a possibility that I can open the windows and door of my Suffolk pub for a cooling breeze. Provided I am not too disturbed by the smell of silage or fertilisers, this is the most environmentally friendly way to ensure that I don’t breathe in Joe’s Old Holborn tobacco fumes or whatever. It doesn’t require any fan power and doesn’t therefore increase the C02 emissions from the nearby power station at Eye.
In a big city building there are a different set of problems. The air outside is often worse than that inside. If it’s not the dust, it’s the petrol fumes. And if it is not the petrol fumes, then there is that other pollutant, noise.
There are, therefore, four techniques which can be used to achieve an acceptable indoor environment:
A) dispersion
B) dilution
C) filtration
D) absorption
These techniques result in the following strategies:
A) Smoking areas should be separated from non-smoking areas. Fresh air inlets should be adjacent to the non-smoking areas whilst extract should be adjacent to smoking areas.
B) Fresh air should be well above the minimum specified for preventing the build up of carbon dioxide and should be used to dilute the smoke produced from cigarettes.
C) Filtration can be incorporated to reduce the amounts of fresh air necessary by allowing air to be recirculated back to the areas of occupancy. This will also ensure that air which has been heated is not completely rejected to atmosphere, thus saving on heating bills.
D) Whilst not practised to date, it is not impossible to include gas absorbers in a ducted ventilation system so that pollutants such as carbon monoxide and petrol fumes can be removed with or without the aid of catalysts.
Human beings may be considered as heat engines, taking in food (fuel) and air to produce energy and waste. To sustain life, oxygen is necessary for the metabolism of food. Human beings breathe in air (with its oxygen content) and exhale air (with a significant amount of its oxygen converted to carbon dioxide). Trees on the other hand take in carbon dioxide and change it back into oxygen. Hence the animal and vegetable worlds are in balance and rely on each other. The carbon and hydrogen in foods are “burnt” to produce carbon dioxide and water and these are rejected by the body either by exhaling or as waste. Foods can be principally classified as:
• carbohydrates
• fats
• proteins
The ratio of carbon to hydrogen in each is different. We can measure the amount of carbon dioxide produced by a person to the oxygen consumed, and this is defined as the respiratory quotient (RQ). RQ varies according to diet and is shown for typical foods below:
0. 71 for a diet of 100% fat
0. 8 for a diet of 100% protein
1. for a diet of 100% carbohydrates
A value of 0.83 is taken as a reasonable average for a normal dietary mix.
The rate at which oxygen is consumed and carbon dioxide is generated depends on physical activity. A simple equation gives the outdoor air flow rate needed to maintain carbon monoxide levels at a constant level:
V0= n, rN Equ21.1
60(Cs-Co)
Where:
V0 = outdoor airflow rate per person (I/s)
N = C02 generation rate per person (l/min)
Cs = C02 concentration in the space (ppm)
C0 = C02 concentration in outside air
The C02 generation rate will depend on the amount of physical activity (see Figure 21.1).
For a recreational activity such as a pub, exertion will be in talking animatedly, whilst raising one’s drinking arm, this will be
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Physical activity, metric units Figure 21.1 C02 generation with physical activity |
About 0.3 l/min. If there is the chance of dancing or other forms of organised hooliganism as in a discotheque, then this could increase by a factor of 4. If the maximum space concentration is to be held at 100 ppm, and the outdoor concentration is 30 ppm, then the amount of air needed per person:
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V„ = |
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-7 I/s |
0.3
Equ21.2
60(0.001-0.0003)
It should be noted that the percentage change in carbon dioxide is more significant than the reduction in oxygen level.
The calculated figure is only just below the currently recommended 8 I/s as given in many Codes of Practice. It takes no account of the additional air required for cigarette combustion, dilution of pollutants etc.
How then, are we to know what is an acceptable indoor air quality? The American Society of Heating Refrigeration and Air — Conditioning Engineers (ASHRAE) defines this as air in which “there are no known contaminants at harmful concentrations as determined by cognisant authorities and with which a substantial majority (80% or more) of the people exposed do not express dissatisfaction”. That is indeed a tough target, but one to which we can all surely agree. How to calculate it without a substantial statistical survey is the real problem.
If this definition is accepted, and the air quality within a building is as good as that experienced in the external ambient atmosphere, then no one can expect more. This is not an impossible target and no doubt surprise will be expressed that tobacco smoke is not necessarily the dominant criteria. However, it must again be emphasised that the ventilation rates are higher than those which are currently specified. Let us therefore ensure that all public places have at least 8 I/s of fresh air per person but recognise that higher rates are necessary for any further improvement of indoor air quality towards that enjoyed outside in the fresh air. Unless air cleaning is also practised it is suggested that we should think in terms of:
• 30 litres/second of fresh air per smoking person and
• 12 litres/second of fresh air per non-smoker.
Suffice it to say, such a level will take care of all the other indoor pollutants.
Further, our systems should be designed for the true maximum occupancies of a particular area and these can well be above:
1 person per 1 m2.
To repeat, ventilation at these rates would achieve an acceptable level of air quality not only in respect of tobacco annoyance, but also in other particulates, odours, carbon dioxide levels, biological aerosols, formaldehydes, radon etc.
The fan manufacturers will of course be more than happy to supply the increased numbers of fans suggested, but if larger more efficient units are selected, energy consumption will not rise disproportionately and, per air change, may even be less. The air cleaner manufacturers should also be happy, as with their equipment installed, the quantities of fresh air can be reduced to non-smoker levels and heating costs correspondingly contained.
One of the axioms in ventilation is that it is always best to deal with a problem as near to its source as possible. Thus we have noted that extract should be close to the smokers.
An alternative approach is to pass the ventilation air through some sort of filter. This will result in a reduction in the amounts of fresh air necessary and also save on the heating bills. Do not however assume that ventilation can always be reduced to 8 I/s. That would only exchange one set of problems for another — Environmental Tobacco Smoke (ETS) for an increase in C02 levels and possible problems from the heat generated by closely packed bodies. The author’s conclusion is that one should still consider a minimum fresh air level of 24 I/s.
There are a number of different types of filter available, each with its own respective merits. The restaurant in the author’s “local” uses an electrostatic precipitator — it’s brilliant. The combination of high ceilings and filter means that smokers and non-smokers are mixed without problem, and the air is perfectly clear. Precipitators consist of an initial bank of positively charged ionising wires between co-planar grounded electrodes followed by a bank of grounded collection plates. Between each pair of collection plates is a positively charged repulsion plate. Using relatively low voltages and currents of positive polarity, ozone formation is reduced to a minimum.
Gas absorbers which burn off carbon monoxide is the presence of a catalyst can also be installed — most easily in a ducted system.
Other types of filter which are available include absolute paper High Efficiency Particulate Air (HEPA) filters and those using other types of media.
Efficient ventilation is a definite plus in the promotion of all public places. It can provide an atmosphere which should satisfy the most fastidious. ETS is unpleasant, even if some of one’s friends are smokers, and one wishes to continue to have the opportunity to socialise with them. It is more than possible that ventilation can be installed that will allow one to do this in perfect safety even if ETS is proved to be a danger.
In the light of the current debate, it should be emphasised that hot and cold tobacco smoke are very different both physically and chemically. Certainly, a well-designed and maintained ventilation system can ensure that the atmosphere within a public space need be no worse than that in the surrounding streets. The work of Dr Andrew Geens at the University of Glamorgan show this to be the case. The proviso is that such systems must
Be well maintained. Many ventilation systems are not kept in good working order. A step change in the mechanical ability of publicans, hotel management, office management etc is most desirable.
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Figure 21.3 Direct drive centrifugal unit with vertical discharge |
In any case, ventilation is essential to provide oxygen for breathing and to remove the heat generated by the occupants. If the fans used are of the variable speed type with appropriate controls, then the rate of ventilation can be adjusted to meet the demand, according to occupancy and activity. More and more systems are in fact being provided with sensors and the control of both heating and ventilating systems can then be integrated. This is especially the case with the larger ducted system where in-duct cleaning can also be incorporated.
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Figure 21.4 Direct drive propeller unit with side discharge |
It should be noted that the mathematics in this Section has been concerned solely with the amounts of fresh air required. Much larger quantities of air may be re-circulated in air conditioning systems for the removal of heat.
To repeat, all such systems however, must be cleaned and maintained regularly if they are to continue to function satisfactorily. As a connoisseur of fine bitters, the author knows the value of keeping the ducts clean! It’s no different with a ventilation fan or a filter!
Posted in Fans Ventilation A Practical Guide