Indoor air quality and energy-optimized ventilation.
Popa, Monica ; Sirbu, Dana Manuela ; Curseu, Daniela 等
1. INTRODUCTION
The Constitution of World Health Organization (WHO, 1948) defines
health as "a state of complete physical, mental and social
well-being and not merely the absence of disease or infirmity".
Modern people spend 90 % of the day indoors, a good indoor air quality being a human right. With the experience of sick building syndromes, the
outdoor intake is considered a very important technical solution and the
indoor air quality is an important criterion to evaluate the
environment. Energy saving is another important item to take into
account. The main purpose of most ventilation systems is to provide a
health and comfortable indoor climate for the building's occupants.
One of the main criticisms of an air conditioned space is the indoor air
quality. Subjective response includes a lack of freshness, dryness and
poor temperature control. Objective measurements indicate temperature
outside the normal recommendations, high levels of pollutants, low
relative humidities. The following parameters have a critical influence
on the ventilation rates (the volume of outside air actually introduced
into the space): the impairment of the air quality caused by occupants,
emissions from materials in the room, activities such as smoking, the
quality of outside air, the thermal load (Popa, 2003).
2. ARTIFICIAL ENVIRONMENTS
If an air-conditioning system is installed in a room, it
automatically changes into an artificial environment and an appropriate
control of all environmental elements should be carried out:
a. Ventilation efectiveness considers the mixing of supply air in
an occupied space. It describes the fraction of fresh air delivered to
the space that reaches the occupied zone. The proportion of fresh air
that is delivered to the occupied zone of a space depends on the room
air distribution which is determined by the location of the air delivery
and the geometry of the space (ASHRAE, 1989). Ideally 100% of fresh air
should be delivered to the breathing zone, but often the air
short-circuits between the supply and extract points. Ventilation
efficiency is the term used to quantify ventilation effectiveness, being
a measure of the ability of a ventilation system to exhaust the
pollutants generated within a space. For a specific pollutant it is the
ratio of its concentration at the point of extract to the mean
concentration level of the pollutant throughout the occupied zone.
Ventilation efficiency (E):
E = [C.sub.e]/[C.sub.l] (1)
Where [C.sub.l]--concentration of pollutant at a location;
[C.sub.e]--concentration of pollutant at exhaust
If there is a significant level of a pollutant in the supply air
then this should be subtracted from the internal concentration levels
(Ott, 2007.).
b. Exposure to indoor air pollutants
The dose of a man is expressed by the equation
D = q [integral] C(t)dt (2)
Where D--the dose, q--rate of respiration
C(t)--concentration of the contaminant, t--time
The concentration in a room can be expressed as following if the
instantaneous mixing is postulated:
C = Co + M/Q (3)
Where [C.sub.o]--outdoor concentration M--contaminant generation
rate Q--ventilation rate
From equation (3) it derives the increase of indoor concentration
over that of outdoor air as
[DELTA]C = C - Co = M/Q (4)
Where [DELTA]C--indoor concentration in excess over outdoor air
concentration,
c. Related elements
The equation (4) shows that the increase on indoor concentration
compared to the outdoor air is proportional to the generation rate and
inversely proportional to the ventilation rate. This postulates a
constant state and the average over a long period of time. These
elements are the function of time and influenced by the changes of
climate and human behaviour.
The generation rate M is also a function of human activities,
quantity of contaminant existing and climatic conditions. The
ventilation is also dependent on the life-style of residents, weather
conditions and quality of building (Popa et al., 2008).
The quantity of existing pollutant is often decided by climatic
conditions or by other conditions as in the case of fungi and mites
growing on building materials or air-conditioning apparatus.
3. POLLUTANTS AS INDICATORS
The existing pollutants from an environment can be either
independent, additive or synergetic.
In environments where industrial hygiene is applied, there are few
significant pollutants originated from the production processes and they
are monitored to protect the workers. If ventilation is applied, it
usually dilutes other contaminants generated at the same time. In an
ordinary environment, the pollutants are generated at low levels,
without significant amounts of toxics. If the health effect is additive
for each component, the relation between the concentration and the
maximum permissible concentration of each pollutant is expressed as
follows:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)
If a material [not equal to] is selected as indicator, the equation
(5) can be converted into the next equation:
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6)
The concentration allowed to the indicator is much lower than the
concentration specified as maximum allowable concentration.
Equation (3) emphasizes that the indoor concentration of a
pollutant is caused by two elements: the introduced outdoor pollutant
and the indoor generation, the last having a greater impact on the
indoor pollution.
If a puff of high concentration is transported from a source to a
building, a certain part of the pollutant will enter the building
depending on the time of stay at the site and the rate of infiltration
or outdoor air intake. If the ventilation rate is reduced the amount
introduced indoors is relatively small, leading to low concentrations
especially if the pollutants are of high adsorption characteristics
(Yanagisawa et al., 1996).
On the contrary, the indoor generated contaminants are present in
the building even at low concentrations, until removed by ventilation.
The equation to predict the concentration of pollutants in indoor
air postulate instantaneous uniform distribution. Although the
prediction of spatial distribution is possible with numerical analysis,
more research is needed.
The actual non-uniform distribution of pollutants indoors
emphasized a variety of modes by rooms' characteristics. In
ordinary air-conditioned rooms, the mixing of air is not significant
after the supplied air was distributed throughout the room. If a puff of
pollutant was generated from one point of a room, it will be transported
by the general flow pattern to the exhaust opening as a diffusing puff.
If the contaminant is continuously generated it will create a belt of 1
to 1.2 m in width where the concentration is consequently much higher
than the average value (Godish, 2004).
4. ENERGY-OPTIMIZED VENTILATION
Apart from maintaining thermally comfortable indoor conditions, the
principal aim of an indoor air system is to ensure good indoor air
quality while consuming the minimum of energy. The principal measures
for reducing energy consumption in ventilation and air conditioning systems are as follows:
--Establish constructional, operational and organizational
conditions which will facilitate low energy consumption in the system.
--Carefully investigate the need for the proposed application
--Determine sizing criteria in accordance with actual demand.
--Use components with high efficiency levels across the whole
operating range.--Design systems for demand-controlled operation and
operate them accordingly. Demand controlled ventilation systems reduce
costs even more substantially and without reducing the required degree
of comfort.
--Ensure that the relevant operating parameters and energy
consumption can be measured and undertake these measurements regularly
during operation.
One of the most effective measures for the optimization of energy
consumption for the distribution of air is to separate the thermal
conditioning of the air (heating and cooling) from the air renewal
process, by using radiator-type systems for heating or cooling. This
means that the flow rate of the air discharged into a space can be
limited to the outside air flow rate actually necessary for hygiene
purposes. From the point of view of energy consumption, the outside air
flow should be based on the required volumetric flow rate per person
(Fitzner, 2000).
Energy-optimized solutions are possible if:
--Sources of pollution are avoided, or the emissions are at least
extracted locally;
--The movement of the air (from the supply vent to the extract
vent) is optimized (ventilation effectiveness) or if the air is
discharged in the vicinity of people
--Smoking is prohibited. If this is not possible, the design flow
rate for the outside air must be based on the estimated number of
cigarettes per hour (50-130 [m.sup.3] per cigarette);
--The volume of air per personne is relatively large (time delay
for the dynamic response of the air pollution load in a mixing system);
--Tobacco smoke pollution in small spaces (up to 100 m3) can be
reduced with air purifiers.
In air-only systems where the thermal load is such that supply air
volume required is greater than the outside air rate dictated by hygiene
requirements, the additional air should be supplied in form of
recirculated air. Recirculated air should be used only where there is
little or no pollution caused by work processes, unless the recirculated
air is suitably filtered so that it is of the same quality as the
outside air.
5. CONCLUSIONS
The target of environmental control is the health of people
according to the definition of health by WHO. A necessary activity of
professionals is to establish criteria that ensure that indoor air
quality is adressed and hopefully prevented. All requirements must be
feasible in practice. Outdoor air intake is a basic measure to cover the
unknown pollutants. The energy-optimized ventilation must adapt the
influencing variables both intrinsically and in relation to the overall
system.
6. ACKNOWLEDGEMENTS
The considerations presented in this paper are part of a large
scale environmental study in frame of the National Research Grant IGLOB
42-117 PN- II 2008.
7. REFERENCES
Fitzner, K. (2000). Control of pollutants in air handling systems,
Proceedings of Healthy Buildings 2000, Seppanen & Sateri (Eds.),
pp.21-34, ISBN 952-5236-06-4, Espoo, Finland, August, 2000.
Godish, T. (2004). Indoor Air Quality, In: Air Quality, 4th ed.,
pp. 351-390, Lewis Publ., ISBN 1-56670-586-X, U.S.A.
Ott, W.R. (2007). Mathematical Modeling ofIndoor Air Quality, In:
Exposure Analysis, Ott, Steinemann, Wallace (Eds.), pp.411-441, CRC Press, ISBN 1-56670-663-7, USA
Popa, M.S. (2003). Unkonventionelle Technologien und
Fertigungseinrichtungen, Technologien fur Feinmechanik, U.T. Press, ISBN
973-8335-76-0, Cluj-Napoca, Romania
Popa, M; Sirbu, D.; Curseu, D.; Popa, M.S. (2008). The individual
health risk assessment, Proceedings on CD of International Technology,
Education and Development Conference, IATED (Ed.), ISBN:
978-84-612-0190-7, Valencia, Spain, March 2008.
Yanagisawa, Y.; Lee, K. & Spengler, J.D. (1996). Evaluation of
mitigation measures to indoor air pollution from global environmental
aspects, Proceedings of the 7th International Conference on Indoor Air
Quality, Yoshizawa S. (Ed.), pp.51-59, ISBN4-9900519-1-2, Nagoya, Japan,
July, 1996.
*** ASHRAE 62-1989 (1989). Ventilation for Acceptable Indoor Air
Quality
*** WHO (1948). The Constitution of World Health Organization