WHY DO MEDICAL NEBULIZERS DIFFER IN THEIR OUTPUT AND PARTICLE-SIZE CHARACTERISTICS

Previous work done on the characterisation of nebulizers has focused o n gravimetrical output and particle/droplet size distribution at vario us air flow rates. This paper investigates six different nebulizers, w ith regard to droplet generation and separation properties, at a singl e air flow rate. Droplet generation and separation properties were mea sured with laser diffraction and impactor techniques. For each of the nebulizers the air velocity was calculated and both liquid and air vol umetric flow rate was measured. The primary generated droplets (nebuli zer without impaction baffle) had a mean size of between 15 mu m and > 500 mu m. The secondary generated droplets (nebulizer with baffle) we re in the size range of 1 to 10 mu m. It was found that the baffle sys tem of the investigated nebulizers could be described according to ord inary impaction theory. The mass median diameters (MMD) of these nebul izers were found to be dependent on air velocity (v(g)), and ratio of liquid to air volumetric flow rate (Q(l)/Q(a)) In all of the nebulizer s, between 93 % and 99% of the generated primary droplets were caught by the baffle system which resulted in a very low output. Thus, the ne bulizers examined do not appear to be optimised, with regard to observ ed droplet generation and baffle arrangements, if secondary droplets w ith a size approximate to 1 run are desired. By changing the design at the primary generation point, air velocity (v(g)) and liquid to air f low rate (Q(l)/Q(a)) the primary generated droplet size could be decre ased. This would result in an increase in the number concentration of smaller droplets that pass the baffle system. As changes in v(g) affec ts Q(l)/Q(a), and both v(g) and Q(l)/Q(a) separately influence the siz e distribution and thus output, nebulizer performance becomes difficul t 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 outp ut.