Laser Diffraction Particle Analysis (LD)
The laser diffraction particle analyzer instruments provide a method to quickly and accurately assess the drop size distributions at a local, or global, location within the spray plume. A laser unit is used to provide a collimated light source and, as droplets pass through the laser beam, a receiving unit detects the diffraction pattern. This method is able to quickly measure many droplets and the information is used to develop a drop size distribution. A typical measurement practice at Spraying Systems Co. is to slowly passes the nozzle spray plume through the measurement region to obtain an single drop size distribution for the entire spray, at a given spray distance. In this manner, the drop size for multiple nozzles, or multiple operating conditions may be quickly tested and compared.
While the methodology is nearly identical, there are various commercial instruments available from companies such at Sympatec and Malvern; Spraying Systems Co. primarily uses the Sympatec HELOS instrument, but does have multiple Malvern units as well.
The laser diffraction method utilizes a collimated laser light source to provide diffuse parallel light. As droplets pass through the laser beam, a diffraction pattern is created by interference of various light waves passing through, or around, a droplet or particle. This results in high and low intensity light waves that are projected toward a receiving detector diode-array. The diode array is a series of semi-circular detectors, which measure the diffraction pattern as it is passed by and becomes centered on the array. Smaller droplets will project a larger diffraction pattern due to their large surface curvature, while larger droplets will have a smaller curvature and a smaller diffraction pattern. After many droplets pass through the measurement region, a droplet size distribution is developed.
The Laser Diffraction Particle Analyzer units are well equipped to quickly measure the droplet size distribution for various nozzles and/or many operating conditions. A comparison can then be quickly conducted to determine the optimal setup. These instruments are also very well suited for repeatable measurements due to the limited user optimization parameters, auto-setup tuning, and relatively straightforward data post-processing requirements. Additionally, the diffraction technique may be applied to liquid droplets as well as solid particulates (powders) for size assessment.
The droplet size range is limited by the receiving optics that re-focus the diffraction pattern onto the diode array detector. In order to optimize this process, the Sympatec laser diffraction unit is equipped with multiple lenses and is able to auto-setup the focus and alignment of each lens package.
The laser diffraction method provides droplet size information only. Also, due to the diffused laser light source, this method is best suited for spray patterns with a low droplet concentration.
Nozzle selection and operating condition optimization
A pharmaceutical company contacted Spraying Systems Co. and was interested in determining the optimal operating conditions for the Spraying Systems Co. nozzle that they were using. There were specific requirements for drop size in order to obtain the desired final product, but the liquid flow rate was pre-determined. Numerous candidate Spraying Systems Co. nozzles were selected and tested over a range of atomizing air flow rate conditions to determine the best possible nozzle & operating conditions to achieve the desired drop size characteristics using the customer’s spray material.
Spray-Dry droplet/powder measurement
In the SprayDry industry, it is typical to mix a product in liquid form, then pass it through a drying tower to generate a dry-powder final product. The Laser Diffraction instrument is capable of measuring both the initial droplet form generated by the nozzle, as well as the downstream dried powder form of the product. Thus, the nozzle parameters may be directly correlated, and ultimately optimized, to provide the target dried product to the customer’s process.