Laser Diffraction Particle Analysis

The Sympatec HELOS laser diffraction (LD) and other LD particle analyzer instruments quickly and accurately assess drop size distributions at either a local or global location within the spray plume.

Laser Diffraction Overview
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 can 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 pass the nozzle spray plume through the measurement region to obtain a 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. Spraying Systems Co. primarily uses the Sympatec HELOS instrument, but also has multiple Malvern units.

Method of Analysis with Sympatec HELOS LD

This 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 semicircular detectors that measure the diffraction pattern as it is passed by and becomes centered on the array. Smaller droplets project a larger diffraction pattern due to their large surface curvature, while larger droplets have a smaller curvature and a smaller diffraction pattern. After many droplets pass through the measurement region, a droplet size distribution is developed.

Laser Diffraction Benefits

Laser diffraction particle analyzer units are well equipped to quickly measure the droplet size distribution for various nozzles and / or operating conditions. A comparison can then be quickly conducted to determine the optimal setup. These instruments are also well suited for repeatable measurements due to the limited user optimization parameters, auto-setup tuning and 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 refocus 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 only droplet size information. Due to the diffused laser light source, this method is best suited for spray patterns with a low droplet concentration.

Examples of Laser Diffraction Analysis for Customers

Here are just a couple examples of how laser diffraction brings value to spray operations.

  • Nozzle Selection and Operating Condition Optimization
    A pharmaceutical company wanted to determine the optimal operating conditions for the nozzle it was using. There were specific requirements for drop size in order to obtain the desired final product, but the liquid flow rate was predetermined. Spraying Systems Co. tested numerous nozzles over a range of atomizing air flow rate conditions to determine the best one and which operating conditions achieved the desired drop size characteristics using the customer’s spray material.

  • Spray Dry Droplet / Powder Measurement
    In the spray dry industry, products are typically mixed in liquid form, then passed through a drying tower to generate a dry-powder final product. The laser diffraction instrument can measure 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.

Example Data

Sympatec Data
 
Drop Size Comparison
 
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