2. DISEÑO DEL CENTRO DE PRODUCCIÓN DE TELEVISIÓN DE ALTA
2.3. EQUIPOS DE ENERGÍA AC Y DC
2.3.1. EQUIPOS DE ENERGÍA DE CORRIENTE ALTERNA
Diffusion batteries have been used to sample particles in a size range of approximately 0.006 to 0.2 pm. The separation o f the particles from the sampled air is based on the fact that gases have diffusion coefficients which are generally orders of magnitude higher than those o f particles (Keith, 1996). Furthermore, the diffusion coefficient of the separated particles decreases with increasing particle size, resulting in a fractional capture of particles through the battery.
Diffusion batteries consist o f multiple layers made o f a bundle o f equal diameter capillary tubes (e.g. 14500 tubes of 0.2 mm diameter and 3.8 cm length) or equally spaced flat plates or very fine wire mesh. Air is drawn through a number o f sampling ports and the particles depending on their diffusion coefficient penetrate through the battery’s levels where they deposit and subsequently are measured using nucleus
Chapter 3 Review o f Techniques fo r Monitoring Particulate Pollutants
The advantage o f this technique is that it gives particle size distribution and is very efficient for measuring very small particles. However, the latter is one o f the reasons that diffusion batteries have not been widely used in ambient measurements since at the moment the focus is on slightly bigger particles (minimum 2.5 pm).
3.3.2.T. Im pingem ent Samplers
In impingement samplers, the solids-gas mixture is first accelerated though a jet and then directed to an abrupt obstruction where the particles, due to their high momentum, do not follow the abrupt change o f direction of the gas streamlines but deposit on the obstruction (Hinds, 1982). Subsequent analysis o f the particle size distribution may be made by gravimetric methods, macroscopic inspection, attenuation o f radiation, or light scattering (Strauss, 1978).
Particle size distributions are normally calculated from experimental data by relating the mass collected on various impaction stages to the theoretical or empirical size cut points associated with those stage geometries (Strauss, 1978).
Three types o f impingement samplers exist. These include a cascade impactor, virtual impactor and cyclone sampling system which are depicted in figure 3.7. A brief description o f each technique is given in the following paragraphs.
( i ) Cascade im pactors . Cascade impactors use multiple stages in series so that the sampled particles are divided into sub-fractions o f specific size with the finest particles (smaller than 0.3 pm) collected on a back-up filter. Figure 3.7.a shows a schematic representation o f the device (Strauss, 1978, Lundgren et al., 1979).
Commercially available cascade impactors have 2-8 stages and can be used as high volume (0.6-1.1 mVmin) or low volume samplers (0.01-0.04 mVmin). The cut-off sizes o f different stages can be determined theoretically or experimentally and are dependent on geometrical parameters and air flow velocity. It is therefore important
Chapter 3 Review o f Techniques fo r Monitoring Particulate Pollutants
that the sampling flow rate is maintained constant during the entire sampling period (Harrison et al., 1986).
The use o f impactors requires skill, experience and proper calibration in order to take accurate particle size distributions. Furthermore, it is important to collect sufficient mass o f samples for further analysis. The latter might require long sampling times (Strauss, 1978). Cascade impactors suffer also from inlet and wall losses as well as re- entrainment losses occurring when particles bounce off overloaded collection surfaces. This problem can be reduced to a certain extent by coating the impaction surface with an inert adhesive material (Hinds, 1972).
( ii ) V irtual im pactors . The operating principle in a virtual impactor is the same as that o f the conventional impactor i.e. based on inertial separation. As it can be seen in figure 3.7.b, the only difference is that in a virtual impactor the impaction plate of a conventional impactor is replaced by a region of relatively stagnant air. This virtual surface (which are practically deflected streamlines) is created in the cavity of collection probe so that the particles coming out o f the nozzle do not impact in any surface but penetrate into the cavity. In the cavity the larger particles which remain airborne are removed by a small flow o f air, known as minor flow, whereas the smaller ones that do not penetrate too deep into the cavity, reverse direction and exit the collection probe near the walls along with the majority o f the air, known as the major flow (Marble et al.,1995). A device like this classifies the aerosol sample into two particle size fractions and therefore it is also called dichotomous sampler. Virtual impactors have similar size range to that of cascade impactors.
Marple et al. (1995) reports some of the advantages and disadvantages of using virtual impactors. In some applications, virtual impactor provides an ideal solution due to the following reasons: First, both the fine and the coarse particles remain airborne in the major and minor flow respectively. Therefore, is not necessary to stop the sampling operation in order to collect the different particle segments for further analysis.
Chapter 3_________________________Review o f Techniques fo r Monitoring Particulate Pollutants
Second, the cut-off characteristics o f the virtual impactor can be as sharp and as accurate as those o f the conventional impactor and far better than the corresponding characteristics o f the cyclone.
Finally, by not having an impaction plate, the problem o f particle re-entrainment causing errors in the size distribution is eliminated.
On the other hand the internal losses are greater in the virtual impactor than in the conventional one. Another inherent disadvantage of the virtual impactor is that the coarse particles segment contains a small fraction of fine particles.
(iii ) Cyclone Sampling Systems. Cyclones have been used less frequently than impactors for obtaining particle size distribution measurements because they tend to be bulky and give poorer resolution. However, when larger samples are required or when continuous operation is needed cyclones are more suitable than impactors (Strauss, 1978).
Figure 3.7.C depicts the most commonly used design: the reverse flow cyclone. In this design the gas-solids mixture enters the top chamber tangentially and spirals down to the apex o f the conical section. It then moves upward in a second, smaller diameter spiral, and exits at the top through a central vertical pipe. The solids move radially to the walls, and after the impaction slide down towards the bottom o f the cyclone to the collection area (Coulson, 1991). Strauss (1978) reports a design by Chang (1974)
comprising a system o f parallel cyclones that separates particles into four size fractions. This system is used for in situ measurement o f source emissions but because o f its size a probe for sample extraction is used.
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Chapter 3 Review o f Techniques fo r Monitoring Particulate Pollutants
SECTIONAL VIEW - DIMENSIONS IN mm
STAINLESS STEEL