Low-Momentum Supply Systems
Low-momentum air supply systems designed for local ventilation purposes mainly use vertically downward airflow. Some systems with an inlet of low-momentum horizontal airflow for a whole workroom are on the borderline between general and local ventilation and are therefore briefly described here. A more complete description will be found in chapters 7 and 8 dealing with general ventilation.
Horizontal displacement ventilation (see Chapters 7 and 8) is a ventilation principle mainly applied to general ventilation of workrooms. In some instances, a local ventilation problem may be solved by building a separate enclosure or a room around the workstation and arranging for a general ventilation in that enclosure. An example where that principle has been utilized is the control of emissions of and worker exposure to styrene vapors during
Lamination of pleasure boats made of glass fiber reinforced polyester plastic.- The “room” is then built with one wall using a low-impulse inlet air unit and an exhaust of the same capacity on the opposite wall. The emitted air contaminants (low — or medium-impulse sources) will then be swept away by the predominantly horizontal airflow to the exhaust. With a well-designed system it is possible to reach a very low level of mixing, both horizontally and vertically. If work is also planned and carried out considering the special airflow patterns, a very high degree of reduction of the workers’ exposure to the air contaminants may be achieved.
10.3.3.2 Vertically Downward Airflow Direction
The aim with air showers is to create locally improved ventilation efficiency. They are characterized by the fact that surrounding air is mixed into the supply air to a very low extent when it reaches the worker and the principle is used with the purpose to create a cleaner or cooler zone without having to consider the heat or contaminant loads in all other areas of the room. Compared to the situation when only general ventilation is used a reduction of workers’ exposure to air contaminants from 50% to more than 90% can be achieved by an air shower. Air showers make it possible to reach a chosen target value for local contaminant exposure or climate control with 30 to 70% cost savings compared to mixed ventilation. When this ventilation principle is utilized to reach a defined target level for air contaminant exposure or climate, the investment requirements in general are not higher than compared to the alternative.3
Principle Including Sketches/Figures
The basic idea is to create a volume of clean air that includes the breathing zone of one worker by introducing a vertically oriented, low-momentum airflow covering a limited area under the unit with clean air (Fig. 10.48). The airflow can enter with a velocity high enough or slightly chilled to ensure that the introduced, clean air reaches the level where the air contaminant concentration or the temperature is to be controlled. The vertically downward airflow generated by the shower must, therefore, be high enough to counteract the effect of the vertically upward convection airflow around the worker caused by the heat of the body. The area covered by the air shower is often limited to the area in which the worker is mainly moving, normally about 1 m2, but may be expanded by combin ing two or more units.
Applicability of Sources
The system may be used to control exposure to air contaminants when the source strength is moderate and has a low-momentum, as is typical in manual handling of processes causing release of gases, vapors, smoke, or dust. Examples are use of glue, paint, printing ink, or other resin-containing products emitting organic solvents and/or gases, soldering, and handling of pulverous raw material as in chemical industries or bakeries. Air showers may also be used to create a high degree of comfort in terms of the freshness of the air or C02 in a room with moderately warm sources, e. g., computer workstations.
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FIGURE 10.48 The air shower principle creates a zone of clean air around the worker and pushes the contaminant plume away from the breathing zone.
The air shower principle is effective but the effect may be drastically reduced if specified design parameters are not followed and maintained. If the system is designed for a supply air temperature of 2 °C below general room temperature, a major divergence from that may drastically reduce the desired effect. A large temperature difference will result in a high acceleration of the supply air and therefore a reduced protected volume, especially when a textile tube is used as the inlet air unit. It is also important to avoid high temperature differences because of the risk that drafts may cause discomfort for workers.
A small temperature difference of the supply air relative to the surrounding air will not result in a protection effect since the density difference will be too low to result in the expected vertical air transport. If the temperature of the supply air is too low problems will result due to drafts on the worker, which will increase the risk that he or she will move from the protected zone if possible.
Design Equations andlor Parameters
The flat supply air unit can be built of, for example, sheet metal as a box with one side perforated. The area of the perforated side must be slightly wider than the area to be protected since the acceleration of the plume (if colder supply air than the surrounding air is used) will cause the cross-section to be reduced. A uniform velocity profile is important to achieve the desired result; divergences may result in negative effects to such a degree that the aim will not be reached. The use of a fabric felt filter downstream of the perforated plate to produce a more uniform velocity profile is therefore recommended.4
The unit shaped as a half sphere is a little more complicated but also gives more possibilities to adjust the shape of the protected volume. A factory-made unit5 was designed according to the following criteria.
To prevent the surrounding air from mixing with the supply air, the air shower should use air-permeable filter material with a harder, load-bearing, nonflammable outer shell and an inner layer of softer material with a high air resistance. This design requires that the supplied air be filtered.
The air shower should have such a shape (plane or hemispherical) as to ensure that the downward airflow from the air shower has a sufficiently wide diameter to counteract the natural tendency of the colder air to form a narrow cylinder of air.
The airflow rate to the unit is determined from the desired vertical air velocity in a cross-section of the airflow. Air velocities between 0.14 and 0.2 m s~:l are suggested4^6’7 when using slightly chilled supply air. This corresponds to an airflow rate of 500 to 720 m3 hr1 per m2 of supplied area. Systems without cooling have used air velocities between 0.08 and 1.0 m s-1 s>9 or from 290 to 3600 m3 fr1 per m2 of supplied area. The chilling of the supply air is important to achieve a good result.3’4,6’7 The temperature is recommended to be 1.5 to 3 °C below the surrounding air (A0). Figure 10.50 illustrates how a worker’s exposure to a gas emitted from a worktable changes when A0 is changed from 0 to 1.5 °C. A good effect may also be achieved if the supply air velocity is high enough (0.8-1.1 m s-1) or if the air shower is combined with an exhaust unit that forces the airflow in a downward direction.
If the heat generation in the workstation is low, an additional heating may be necessary to compensate for the heat loss induced by chilled supply air
FIGURE 10.50 Worker exposure to tracer gas emitted from worktable as a function of time when the temperature difference between room and inlet air AQ was gradually changed from 0 to 1.5 °C.
From the air shower. This may be arranged by heating the additional air supply outside the area affected by the air shower5 or by adding a radiant heating panel close to the worker.9 It is important to ensure that the worker avoids leaning over the source during work. Raising the source (the worktable) and ensuring good lighting may reduce this risk.
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