Filtration Airflow: Too Much of a Good Thing?
CNC plasma cutting machines are used extensively in metalworking. They give manufacturers the ability to quickly, accurately and repeatedly cut complex shapes out of metal sheets, ranging in thickness from a quarter to several inches. A high energy plasma cutting torch will generate significant volumes of metal debris, with some of the fine particles capable of becoming airborne. At the same time, an oily smoke plume is also created; these contaminants need to be captured to keep the workspace clean.
The solution to this dust and smoke generation is to cut on top of a downdraft table with a large external fan extracting air during the cutting process. Heavy particulates will fall directly into waste containers under the table, while the fine particulates and smoke will be drawn through the duct work to the fan. A cartridge style dust collector is installed directly before the fan to capture these contaminants.
Proper airflow control is required to match the dust collector and plasma table performance. If there is too little airflow, not all of the smoke will be captured, resulting in a fog-filled room. Excess airflow can be an issue as it decreases spark trap performance and can cause particles to deeply embed into the filter media, making them resist pulse cleaning, and ultimately shortening filter life.
With new and clean filters, the fan will deliver too much flow. This initial high air-to-media ratio decreases filter life and increases pressure. In this installation, initial testing showed duct velocities of more than 4,500 fpm and 3,500 fpm was the target rate for this application. The dust collector has an adjustable damper to restrict airflow, however, there is not a way for the plant to know the airflow rate. Another option to an adjustable damper is a motor with a variable frequency drive. In addition, over time the filters will build up pressure drop reducing airflow.
A series of duct traverses were carried out using a Fluke digital airflow meter and pitot tube. Drag forces cause the air at the wall of the duct to flow slower than the air in the middle. A traverse is a test method that creates a flow profile of the ductwork and determines the average velocity. Samples were taken at 4 points in the duct, starting at the centerline and working to the wall; with 1 set traversing horizontally and 1 vertically (8 data points overall). The test was repeated 5 times, with the fan damper being incrementally closed each time. Static pressure in the duct is also recorded.
Using this data, we were able to create a table that indicated the Air Velocity, Air Volume and Static Pressure for each setting of the fan damper. It was quickly noted the unrestricted air velocity was much higher than required. Adjusting the damper to position #3, they were able to bring the velocity down to 3,800 fpm, which will improve filter life and decrease motor energy requirements. Over time, as the dust collector filters become fouled, they will simply open the damper by one position, restoring the airflow to the target range.