The design of a system of ventilation by dilution is based on the hypothesis that the concentration of the pollutant is the same throughout the space in question. This is the model that chemical engineers often refer to as a stirred tank. If you assume that the air that is injected into the space is free of the pollutant and that at the initial time the concentration within the space is zero, you will need to know two facts in order to calculate the required rate of ventilation: the amount of the pollutant that is generated in the space and the level of environmental concentration that is sought which hypothetically would be the same throughout.
Under these conditions, the corresponding calculations yield the following equation:. Although occasionally the index of the quality of ventilation is regarded as practically equivalent to that value, the above equation clearly shows that its influence is limited to controlling the speed of stabilization of the environmental conditions, but not the level of concentration at which such a steady state will occur.
That will depend only on the amount of the pollutant that is generated a , and on the rate of ventilation Q. When the air of a given space is contaminated but no new amounts of the pollutant are generated, the speed of diminution of the concentration over a period of time is given by the following expression:. Expressions can be found for calculations in instances where the initial concentration is not zero Constance ; ACGIH , where the air injected into the space is not totally devoid of the pollutant because to reduce heating costs in the winter part of the air is recycled, for example , or where the amounts of the pollutant generated vary as a function of time.
This value will be established by regulations or, as an ancillary norm, by technical recommendations such as the threshold limit values TLV of the American Conference of Governmental Industrial Hygienists ACGIH , which recommends that the rate of ventilation be calculated by the formula. A value of K between 1 and 10 must be selected as a function of the efficacy of the air mixture in the given space, of the toxicity of the solvent the smaller c lim is, the greater the value of K will be , and of any other circumstance deemed relevant by the industrial hygienist.
The ACGIH, among others, cites the duration of the process, the cycle of operations and the usual location of the workers with respect to the sources of emission of the pollutant, the number of these sources and their location in the given space, the seasonal changes in the amount of natural ventilation and the anticipated reduction in the functional efficacy of the ventilation equipment as other determining criteria.
In any case, the use of the above formula requires a reasonably exact knowledge of the values of a and K that should be used, and we therefore provide some suggestions in this regard. The amount of pollutant generated may quite frequently be estimated by the amount of certain materials consumed in the process that generates the pollutant. So, in the case of a solvent, the amount used will be a good indication of the maximum amount that can be found in the environment. As indicated above, the value of K should be determined as a function of the efficacy of the air mixture in the given space.
This value will, therefore, be smaller in direct proportion to how good the estimation is of finding the same concentration of the pollutant at any point within the given space. This, in turn, will depend on how air is distributed within the space being ventilated. According to these criteria, minimum values of K should be used when air is injected into the space in a distributed fashion by using a plenum, for example , and when the injection and extraction of air are at opposite ends of the given space.
On the other hand, higher values for K should be used when air is supplied intermittently and air is extracted at points close to the intake of new air figure This air then mixes with the stream and slows it down, creating measurable turbulence as well. As a consequence, this process results in intense mixing of the air already in the space and the new air that is injected, generating internal air currents. Predicting these currents, even generally, requires a large dose of experience figure In this way, the areas where air moves at high speeds are kept as small as possible.
The stream effect just described is not produced near points where air escapes or is extracted through doors, windows, extraction vents or other openings. Air reaches extraction grates from all directions, so even at a relatively short distance from them, air movement is not easily perceived as an air current. In any case, in dealing with air distribution, it is important to keep in mind the convenience of placing workstations, to the extent possible, in such a way that new air reaches the workers before it reaches the sources of contamination.
When in the given space there are important sources of heat, the movement of air will largely be conditioned by the convection currents that are due to density differences between denser, cold air and lighter, warm air.
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In spaces of this kind, the designer of air distribution must not fail to keep in mind the existence of these heat sources, or the movement of air may turn out to be very different from the one predicted. The presence of chemical contamination, on the other hand, does not alter in a measurable way the density of air. While in a pure state the pollutants may have a density that is very different from that of air usually much greater , given the real, existing concentrations in the workplace, the mix of air and pollutant does not have a density significantly different than the density of pure air.
Furthermore, it should be pointed out that one of the most common mistakes made in applying this type of ventilation is supplying the space only with air extractors, without any forethought given to adequate intakes of air. In these cases, the effectiveness of the extraction ventilators is diminished and, therefore, the actual rates of air extraction are much less than planned. The result is greater ambient concentrations of the pollutant in the given space than those initially calculated. To avoid this problem some thought should be given to how air will be introduced into the space.
The recommended course of action is to use immission ventilators as well as extraction ventilators. Normally, the rate of extraction should be greater than the rate of immission in order to allow for infiltration through windows and other openings. In addition, it is advisable to keep the space under slightly negative pressure to prevent the contamination generated from drifting into areas that are not contaminated. As mentioned above, with ventilation by displacement one seeks to minimize the mixing of new air and the air previously found in the given space, and tries to adjust the system to the model known as plug flow.
This is usually accomplished by introducing air at slow speeds and at low elevations in the given space and extracting it near the ceiling; this has two advantages over ventilation by dilution. In the first place, it makes lower rates of air renewal possible, because pollution concentrates near the ceiling of the space, where there are no workers to breathe it. The average concentration in the given space will then be higher than the c lim value we have referred to before, but that does not imply a higher risk for the workers because in the occupied zone of the given space the concentration of the pollutant will be the same or lower than a c lim.
In addition, when the goal of ventilation is the control of the thermal environment, ventilation by displacement makes it possible to introduce warmer air into the given space than would be required by a system of ventilation by dilution. This is because the warm air that is extracted is at a temperature several degrees higher than the temperature in the occupied zone of the space. The fundamental principles of ventilation by displacement were developed by Sandberg, who in the early s developed a general theory for the analysis of situations where there were nonuniform concentrations of pollutants in enclosed spaces.
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This allowed us to overcome the theoretical limitations of ventilation by dilution which presupposes a uniform concentration throughout the given space and opened the way for practical applications Sandberg Even though ventilation by displacement is widely used in some countries, particularly in Scandinavia, very few studies have been published in which the efficacy of different methods are compared in actual installations.
This is no doubt because of the practical difficulties of installing two different ventilation systems in a real factory, and because the experimental analysis of these types of systems require the use of tracers. Tracing is done by adding a tracer gas to the air ventilation current and then measuring the concentrations of the gas at different points within the space and in the extracted air. This sort of examination makes it possible to infer how air is distributed within the space and to then compare the efficacy of different ventilation systems.
The few studies available that have been carried out in actual existing installations are not conclusive, except as regards the fact that systems that employ ventilation by displacement provide better air renewal. In these studies, however, reservations are often expressed about the results in so far as they have not been confirmed by measurements of the ambient level of contamination at the worksites.
One of the chief functions of a building in which nonindustrial activities are carried out offices, schools, dwellings, etc. The quality of this environment depends, to a large degree, on whether the ventilation and climatization systems of the building are adequately designed and maintained and function properly. These systems must therefore provide acceptable thermal conditions temperature and humidity and an acceptable quality of indoor air. In other words, they should aim for a suitable mix of outside air with indoor air and should employ filtration and cleaning systems capable of eliminating pollutants found in the indoor environment.
The idea that clean outdoor air is necessary for well-being in indoor spaces has been expressed since the eighteenth century. Benjamin Franklin recognized that air in a room is healthier if it is provided with natural ventilation by opening the windows. The idea that providing great quantities of outside air could help reduce the risk of contagion for illnesses like tuberculosis gained currency in the nineteenth century.
Studies carried out during the s showed that, in order to dilute human biological effluvia to concentrations that would not cause discomfort due to odours, the volume of new outside air required for a room is between 17 and 30 cubic metres per hour per occupant. In standard No. An absolute minimum of 8. This same organization, in standard No. In its standard No. It also recommends increasing this value when the air brought into the building is not mixed adequately in the breathing zone or if there are unusual sources of pollution present in the building.
In contrast with existing recommendations for ventilation standards, this guide does not specify volumes of ventilation flow that should be provided for a given space; instead, it provides recommendations that are calculated as a function of the desired quality of indoor air. Existing ventilation standards prescribe set volumes of ventilation flow that should be supplied per occupant.
The tendencies evidenced in the new guidelines show that volume calculations alone do not guarantee a good quality of indoor air for every setting. This is the case for three fundamental reasons.
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First, they assume that occupants are the only sources of contamination. Recent studies show that other sources of pollution, in addition to the occupants, should be taken into consideration as possible sources of pollution.
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Examples include furniture, upholstery and the ventilation system itself. The second reason is that these standards recommend the same amount of outside air regardless of the quality of air that is being conveyed into the building.
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And the third reason is that they do not clearly define the quality of indoor air required for the given space. Therefore, it is proposed that future ventilation standards should be based on the following three premises: the selection of a defined category of air quality for the space to be ventilated, the total load of pollutants in the occupied space and the quality of outside air available.
The quality of indoor air can be defined as the degree to which the demands and requirements of the human being are met. Basically, the occupants of a space demand two things of the air they breathe: to perceive the air they breathe as fresh and not foul, stale or irritating; and to know that the adverse health effects that may result from breathing that air are negligible.
It is common to think that the degree of quality of the air in a space depends more on the components of that air than on the impact of that air on the occupants. It may thus seem easy to evaluate the quality of the air, assuming that by knowing its composition its quality can be ascertained.
This method of evaluating air quality works well in industrial settings, where we find chemical compounds that are implicated in or derived from the production process and where measuring devices and reference criteria to assess the concentrations exist. This method does not, however, work in nonindustrial settings. Nonindustrial settings are places where thousands of chemical substances can be found, but at very low concentrations, sometimes a thousand times lower than the recommended exposure limits; evaluating these substances one by one would result in a false assessment of the quality of that air, and the air would likely be judged to be of a high quality.
But there is a missing aspect that remains to be considered, and that is the lack of knowledge that exists about the combined effect of those thousands of substances on human beings, and that may be the reason why that air is perceived as being foul, stale or irritating.
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The conclusion that has been reached is that traditional methods used for industrial hygiene are not well-adapted to define the degree of quality that will be perceived by the human beings that breathe the air being evaluated. The alternative to chemical analysis is to use people as measuring devices to quantify air pollution, employing panels of judges to make the evaluations.
Human beings perceive the quality of air by two senses: the olfactory sense, situated in the nasal cavity and sensitive to hundreds of thousands of odorous substances, and the chemical sense, situated in the mucous membranes of the nose and eyes, and sensitive to a similar number of irritating substances present in air. It is the combined response of these two senses that determines how air is perceived and that allows the subject to judge whether its quality is acceptable. One standard person is an average adult who works in an office or in a similar nonindustrial workplace, sedentary and in thermal comfort with a hygienic standard equipment to 0.
Pollution from a human being was chosen to define the term olf for two reasons: the first is that biological effluvia emitted by a person are well-known, and the second is that there was much data on the dissatisfaction caused by such biological effluvia. Any other source of contamination can be expressed as the number of standard persons olfs needed to cause the same amount of dissatisfaction as the source of contamination that is being evaluated.
Figure The curve is based on different European studies in which people judged the quality of air polluted by over a thousand people, both men and women, considered to be standard. Similar studies conducted in North America and Japan show a high degree of correlation with the European data. The decipol unit The concentration of pollution in air depends on the source of contamination and its dilution as a result of ventilation.
Perceived air pollution is defined as the concentration of human biological effluvia that would cause the same discomfort or dissatisfaction as the concentration of polluted air that is being evaluated. One decipol from the Latin pollutio is the contamination caused by a standard person 1 olf when the rate of ventilation is 10 litres per second of noncontaminated air, so that we may write.