Many processes in the production of drugs in the pharmaceutical industry produce significant quantities of dust which can cause serious health related problems to workers handling the materials. Additional problems of these operations are the loss of valuable material and the serious risk of cross contamination of drugs. Operations such as scooping, tablet processing, blending, milling, and tray dumping are all potential sources of hazardous levels of dust generation. While some machines may come with a dust or fume handling device incorporated, it is often necessary to retrofit or supplement existing hardware in order to meet increasingly stringent exposure level requirements.
An effective solution to this problem is the installation of an LEV system. Local Exhaust Ventilation (LEV) is the concept of capturing airborne particles as close to the source of emission as the workspace will allow. This is generally achieved by means of an exhaust hood, ductwork, fan, filters, and controls which remove the particles before they have had a chance to escape the local workspace. The efficiency of this system depends greatly on the total system. However, the design and function of the most important element, the exhaust hood itself, is often underappreciated. A good design should not only consider how much material, such as dust or fumes, it can effectively remove, but it must be mounted and located in such a way as to reduce interference with the operation or process while not being affected by cross-currents and disturbances caused by worker or machine movements. These cross-currents can greatly diminish the influence of the exhaust hood and reduce its effectiveness if not properly designed. The hood works by controlling air patterns near the area of highest concentration of contaminated air. This is accomplished by balancing the requirements of holding the intake air volume to a minimum while maximizing the removal of air contaminated with dust produced by the handling of bulk compounds. By achieving this balance cost is held to a minimum by lowering the energy required and reducing the potential of material loss by removing only the airborne material and not the bulk material.
The design study of local exhaust has its beginnings as far back as the 1930’s and 1940’s as concern for worker safety began to grow. These early studies focused on quantifying the air patterns and velocity contours upstream from the exhaust inlet.
Much of this work was experimental and it was a tedious, timely process to collect data for even the simplest of hoods. One of the most notable findings of these early studies was that for a given hood shape the air contour pattern around the hood and streamlines were always the same regardless of the exhaust airflow volume. Design of hoods followed as much on these early design guidelines as it did on intuition.
More recently, with the advent of computers and air flow modeling software, the engineer has been able to apply the same principles that govern the design of ship hulls and airplanes to the design of exhaust hoods. Specifically, the equations of conservation of mass and momentum are solved for a specific geometry and set of boundary conditions. With this, the flow patterns, such as velocity vectors and pressure gradients, can be modeled and the hood design optimized. This has allowed for unprecedented detail and insight into what can be achieved by a carefully designed and mounted LEV hood.
The results of a sample computer simulation for an LEV hood with an exhaust volume of 200 cfm placed over a partially filled drum is shown in figure 1a. In this example the velocity field for a plane passing through the centerline of the hood and drum is plotted. The full 3D hood is shown in figure 1b. Factors affecting the field, such as the level of material inside the drum and the placement of a dome over the hood, can be studied. Regions of high velocity are shown in red, and low velocity is shown in blue. From this simulation it can be clearly seen how the air dips into the drum as it moves from right to left towards the exhaust slot. If the exhaust volume is not carefully specified (i.e. with excessive exhaust volume) the air can dip into the drum and “scoop” valuable material out of the drum. It can also be seen that near the edge of the drum (on right side) there exists a region of high velocity air. This region is highly unstable, fluctuating from a laminar to turbulent state and can result in “bursts” of small highly concentrated dust clouds escaping the control volume. With careful analysis of the simulation these affects can be minimized and mitigated.
Additional design tools employed in the design of hoods is the use of 3D CAD software (figure 1b). By using the latest in 3D modeling software it is possible to customize the fit of an LEV hood on virtually any operation such that there is minimal interference with the operation and surrounding equipment.
Experimental validation is conducted by using smoke trails or fog for visualizing the flow patterns generated by the LEV hood, as shown in figure 2. These tests can reveal areas of flow stagnation or excessive turbulence which may act to actually discharge material into surrounding air. These qualitative tests can assist in the design and field start-up of the hood and validate the results of the simulations. Manufactures of hoods will have done extensive testing of their hoods to ensure the greatest efficiency possible for a given operation. This may be followed up with ambient sampling of air passed through filter cartridges worn by an operator working in the control zone of the LEV system during the actual operation.