Aerosols velocity

Aerosols generated from the Respimat inhaler have been characterized as having a prolonged aerosol cloud duration compared to MDIs and have a slower cloud velocity as measured using video camera imaging. Hochrainer et al. (2005) measured the cloud duration of the Respimat aerosol to be 0.2-1,6 s compared to less than 0.2 s for HFA and CFC MDIs. Aerosol velocities have been reported as less than lm/s for the Respimat, compared to 6-8m/s for CFC MDI inhalers [283], While a degree of patient coordination is required to actuate the Respimat and to inhale,... 

Hochrainer, D., Holz, H., Kreher, C., Scaffidi, L., Spallek, M., and Wachtel, H. (2005), Comparison of the aerosol velocity and spray duration of respimat soft mist inhaler and pressurized metered dose inhalers, /. Aerosol Med., 18,273-282. 

Aerosol properties, such as particle size distribution, aerosol velocity, and hygroscopicity, affect aerosol deposition in the human lungs. Aerosol size distribution, including mass median aerodynamic diameter (MMAD) and geometric standard deviation (GSD), is one of the most important variables in governing the site of droplet or particle deposition in the lungs. ... 

The reformulation of beclomethasone MDI formulations using HFA propellants has provided us with some insight as to the relationship between the particle size and the aerosol velocity of corticosteroids delivered by MDI and their clinical effect. Particles generated from an HFA-MDI containing the newly formulated beclomethasone aerosol primarily are composed of 1.1- to 2.1-pm particles, compared to 3.3- to 4.7-pm particles generated by the older CFC-MDI (59). At the same time. 

A change in particle size and aerosol velocity as a result of the HFA reformulation could also lead to an alteration in the site of deposition of beclometha-sone (BDP) within the lung compartment, perhaps favoring the lung periphery to a greater extent than the CFC formulation. Such a change in the site of deposition is suggested from data by Seale et al., who showed that the area under the cnrve (AUC) of 200 pg of the HFA formulation of BDP was similar to that of 400 pg of the CFC formulation in patients with mild to moderate asthma (61). 

It is well known that only about 10-20% of the dose that is generated by a me-tered-dose inhaler (MDI) penetrates beyond the oropharynx and deposits in the lungs (5, 73-75). This is because there is not enough time for the propellant to evaporate, so particle size is large, and there is only a small distance between the patients s mouth and the aerosol actuator, so aerosol velocity remains high. Both these features enhance impaction and loss of aerosolized medications in the oropharynx, as discussed previously in this chapter. One way to slow down the high initial droplet speed and allow liquid propellant to evaporate is to actuate the aerosol into a holding chamber or spacer device. 

Impaction accounts for most of the large particle trap effect of the spacer. This is illustrated in Fig. 4, which shows the fine and coarse particle doses obtained from a budesonide pMDI (Pulmicort, AstraZeneca, Sweden) actuated into a metal spacer. The spacer was initially 23 cm long, and subsequent reduction in the length of the spacer caused a reduction in both the coarse- and eventually also the fine-particle doses. Even the initial shortening of the spacer reduced the coarse particle dose, whereas the fine-particle dose was unaffected thus, the total particle dose was reduced, but the ratio between fine and coarse particles was improved by shortening the spacer. Thus, the spacer length is critical for the fine-particle dose and the ratio of fine to coarse particles. Different pMDIs have different vapor pressures and therefore different aerosol velocities and volumes as a result, the optimal spacer length is specific to a particular pMDI. Moreover, the spacer should be adapted to the particular aerosol jet. For this reason, the op-... 

As a first stage, the stream of liquid from an HPLC eluant is passed through a narrow tube toward the LINC interface. Near the end of the tube, the liquid stream is injected with helium gas so that it leaves the end of the tube as a high-velocity spray of small drops of liquid mixed with helium. From there, the mixture enters an evacuation chamber (Figure 12.1). The formation of spray (nebulizing) is very similar to that occurring in the action of aerosol spray cans (see Chapter 19). 

In pneumatic nebulizers, the relative velocity of gas and liquid first induces a reduction in pressure above the surface of the liquid (see the calculation in Figure 19.4). The reduction in pressure is sufficient to cause liquids to flow out of capillary tubes, in accord with Poiseuille s formula (Figure 19.5). As the relative velocity of a liquid and a gas increases — particularly if the mass of liquid is small — this partial vacuum and rapid flow cause the surface of the liquid to be broken into droplets. An aerosol is formed. 

The size of the droplets formed in an aerosol has been examined for a range of conditions important in ICP/MS and can be predicted from an experimentally determined empirical formula (Figure 19.6). Of the two terms in the formula, the first is most important, except at very low relative flow rates. At low relative velocity of liquid and gas, simple droplet formation is observed, but as the relative velocity increases, the stream of liquid begins to flutter and to break apart into long thinner streamlets, which then break into droplets. At even higher relative velocity, the liquid surface is stripped off, and the thin films so-formed are broken down into... 

In a concentric-tube nebulizer, the sample solution is drawn through the inner capillary by the vacuum created when the argon gas stream flows over the end (nozzle) at high linear velocity. As the solution is drawn out, the edges of the liquid forming a film over the end of the inner capillary are blown away as a spray of droplets and solvent vapor. This aerosol may pass through spray and desolvation chambers before reaching the plasma flame. 

This arrangement provides a thin film of liquid sample solution flowing down to a narrow orifice (0.007-cm diameter) through which argon flows at high linear velocity (volume flow is about 0.5-1 1/min). A fine aerosol is produced. This particular nebulizer is efficient for solutions having a high concentration of analyte constituents. 

The sample solution flows onto a piece of fritted glass through which argon gas flows. The flow of argon is broken down into narrow parallel streams of high linear velocity, which meet the thin film of liquid percolating into the pores of the frit. At the interfaces, an aerosol is formed and is blown from the top of the frit. 

For a longitudinal disturbance of wavelength 12 pm, the droplets have a mean diameter of about 3-4 pm. These very fine droplets are ideal for ICP/MS and can be swept into the plasma flame by a flow of argon gas. Unlike pneumatic forms of nebulizer in which the relative velocities of the liquid and gas are most important in determining droplet size, the flow of gas in the ultrasonic nebulizer plays no part in the formation of the aerosol and serves merely as the droplet carrier. 

Plutonium solutions that have a low activity (<3.7 x 10 Bq (1 mCi) or 10 mg of Pu) and that do not produce aerosols can be handled safely by a trained radiochemist in a laboratory fume hood with face velocity 125—150 linear feet per minute (38—45 m/min). Larger amounts of solutions, solutions that may produce aerosols, and plutonium compounds that are not air-sensitive are handled in glove boxes that ate maintained at a slight negative pressure, ca 0.1 kPa (0.001 atm, more precisely measured as 1.0—1.2 cm (0.35—0.50 in.) differential pressure on a water column) with respect to the surrounding laboratory pressure (176,179—181). This air is exhausted through high efficiency particulate (HEPA) filters. 

The AeroSizer, manufactured by Amherst Process Instmments Inc. (Hadley, Massachusetts), is equipped with a special device called the AeroDisperser for ensuring efficient dispersal of the powders to be inspected. The disperser and the measurement instmment are shown schematically in Figure 13. The aerosol particles to be characterized are sucked into the inspection zone which operates at a partial vacuum. As the air leaves the nozzle at near sonic velocities, the particles in the stream are accelerated across an inspection zone where they cross two laser beams. The time of flight between the two laser beams is used to deduce the size of the particles. The instmment is caUbrated with latex particles of known size. A stream of clean air confines the aerosol stream to the measurement zone. This technique is known as hydrodynamic focusing. A computer correlation estabUshes which peak in the second laser inspection matches the initiation of action from the first laser beam. The equipment can measure particles at a rate of 10,000/s. The output from the AeroSizer can either be displayed as a number count or a volume percentage count. 

Elastic scattering is also the basis for Hdar, in which a laser pulse is propagated into a telescope s field of view, and the return signal is collected for detection and in some cases spectral analysis (14,196). The azimuth and elevation of the scatterers (from the orientation of the telescope), their column density (from the intensity), range (from the temporal delay), and velocity (from Doppler shifts) can be deterrnined. Such accurate, rapid three-dimensional spatial information about target species is useful in monitoring air mass movements and plume transport, and for tracking aerosols and pollutants (197). 

Cyclone mist eliminators and collectors have virtually the same efficiency for both liquid aerosols and solid particles. To avoid reentrainment of the collected liquid from the walls of the cyclone, an upper limit is set to the tangential velocity that can be used. The maximum tangential velocity should be limited to the inlet velocity. Even at this speed, the liquid film may creep to the edge of the exit pipe, from which the liquid is then reentrained. 

Other Applications Very small, very low-flow, and relatively high-velocity exhaust inlets, similar to LVHV nozzles, have been used successfully to control fumes from electric soldering irons." " Some investigations have been made into small, point-control exhaust ventilation for aerosols generated by high-speed dental tools. However, such low-volume point-control ventilation systems have not seen widespread use. 

In aerosol theory, is the velocity of free fall of a particle, and by extension in the current work is an empirical velocity related to the buoyancy of the contaminant in air. We further assume that the overall fluid flow pattern is unaffected by the minor quantity of the buoyant contaminant. 

A venturi scrubber is a venturi-shaped air passage with water introduced just ahead of or into the venturi throat. The liquid-gas contact is at a maximum in the venturi throat. The relative velocity between gas and liquid aerosol droplets is high, with the gas velocities in the range of 50-100 m/s. The particles are conditioned in the throat, and condensation is the important collection mechanism. After the particles in the gas have been deposited on droplets, a comparatively simple device such as a cyclone collector can be used to collect the wetted dust. 

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