Why Do Medical Nebulizers Differ in Their Output and Particle Size Characteristics?

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Abstract
Previous work done on the characterisation of nebulizers has focused on gravimetrical output and particle/droplet size distribution at various air flow rates. This paper investigates six different nebulizers, with regard to droplet generation and separation properties, at a single air flow rate. Droplet generation and separation properties were measured with laser diffraction and impactor techniques. For each of the nebulizers the air velocity was calculated and both liquid and air volumetric flow rate was measured. The primary generated droplets (nebulizer without impaction baffle) had a mean size of between 15 μm and > 500 μm. The secondary generated droplets (nebulizer with baffle) were in the size range of 1 to 10 μm. It was found that the baffle system of the investigated nebulizers could be described according to ordinary impaction theory. The mass median diameters (MMD) of these nebulizers were found to be dependent on air velocity (vg), and ratio of liquid to ah volumetric flow rate (Ql/Qa). In all of the nebulizers, between 93% and 99% of the generated primary droplets were caught by the baffle system which resulted in a very low output. Thus, the nebulizers examined do not appear to be optimised, with regard to observed droplet generation and baffle arrangements, if secondary droplets with a size ≈ 1 μm are desired. By changing the design at the primary generation point, air velocity (vg) and liquid to air flow rate (Ql/Qa), the primary generated droplet size could be decreased. This would result in an increase in the number concentration of smaller droplets that pass the baffle system. As changes in vg affects Ql/Qa, and both vg and Ql/Qa separately influence the size distribution and thus output, nebulizer performance becomes difficult to control in a desired way. This suggests that it will be difficult to change the droplet size and at the same time control the mass output.

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