In order to control the movements of contaminants it is useful to be able to see how both the contaminant and the induced airflows move. A number of flow visualization methods have been developed; some are more suitable for laboratory research applications whereas others are quite widely used in industrial situations. We are primarily interested in this latter category. The methods involve releasing a tracer (for example gas, aerosol, or heat) and making visible its path. While in most cases the methods are subjective, their use is invaluable. Ideally the tracer should be nontoxic, nonirritating, inexpensive, and highly visible at low concentrations. The system should be easily portable, self-contained, easy to use, and be controllable.
A review of flow visualization techniques suitable for use in hospital laboratories was carried out several years ago.17
Chapter 12 contains more on flow visualization and measurement techniques.
Probably the most widely used method for visualizing air movement in the workplace is the smoke tube. They are commercially available and consist of a sealed glass tube that contains a granular medium. Sulfuric acid or titanium tetrachloride is absorbed into the inert granules. To use, the ends of the tube are broken off and air is driven through the tube by means of an aspirator, producing a cloud of white acid fume. A relatively small quantity of smoke is emitted in a fine filament that can be directed to areas of specific interest. Air is often passed through the smoke tube from a small rubber bulb, but a continuous filament of smoke can be generated by using a small positive-pressure pump. The method has several advantages: it is inexpensive, portable, and does not require a power source, and the smoke output is controllable and can be seen without auxiliary lighting. The drawbacks are that the smoke is corrosive and an irritant, although it is generated in small amounts and is not usually a problem. (A battery-powered device, that uses small oil-filled tubes to generate an oil mist has been marketed, but from personal experience its operation is not entirely satisfactory.) Acetic acid smoke tubes are available and
Are somewhat less corrosive and irritating; however, they cost approximately twice what titanium tetrachloride tubes cost.
Smoke pellets are produced in a range of sizes and are commonly used tor the testing of household flues and chimneys. The pellet is ignited and will burn for about 10 seconds producing a dense white smoke. Because this is a combustion process there are obvious restrictions on its use (nonflammable atmospheres, nonflammable surfaces, etc.). In addition the smoke is buoyant because of the heat generated. The smoke can also be an irritant and/or toxic. The production of smoke cannot be controlled, but pellets are inexpensive, easy to use, and readily available, and the smoke is produced in sufficient quantities to make them useful in the evaluation, for example, of fume cupboards and booths.
These generators vaporize a liquid (oil/mineral oil or glycol and water), which then condenses into a fine aerosol on contact with cooler air. The amount of smoke produced should be controllable by the liquid feed rate and the temperature of the heating chamber, but in practice the output is not easy to control. They will, however, produce a large amount of smoke over a long period. The generators are relatively expensive (several hundred ECUs), are bulky, are not generally portable, and require an electrical connection.
10.5.3.5 Titanium Tetrachloride
This is a clear liquid that vaporizes and, on contact with damp air, combines with water to produce a dense acid mist. Titanium tetrachloride can be painted on to surfaces, such as fume cupboard sills, from which it will evaporate over a period of several seconds showing the airflow patterns close to the surface. (Airflow patterns close to a surface could also be visualized by fastening short filaments of wool or cotton to the surface). Titanium tetrachloride can also be used, when soaked onto a cotton swab, in a similar way to a smoke tube. It is a simple and inexpensive method but the production of smoke, which is toxic and corrosive, is uncontrollable.
Kennedy17 describes a method using an ultrasonic nebulizer to generate a fog of water droplets which is used in the same way as smoke to visualize airflows. Several types of nebulizers are available but they require an electrical connection and are not hand-held. Food dye can be added to the water to produce colored fog. The nebulizers are expensive (about 1500 ECU) but have negligible operating costs. Although the amount of smoke produced is small, it: is nontoxic and nonirritating.
Bubble generators are commercially available which will produce small neutrally buoyant soap bubbles for use in the visualization of the general flow patterns in rooms. The bubbles are about 3 or 4 mm in diameter and are filled with a helium/air mixture. In practice, it is difficult to make the bubbles truly
Neutrally buoyant and a strong light source is required to view them because of the limited forward scatter of light from the bubbles. They give a good qualitative view of the general flow and are both nonirritating and nontoxic. An investigation by Kerho and Bragg18 reports on the accuracy of helium bubbles as flow tracers.
10.5.3.8 The Dust or Tyndall Lamp
The above methods use a tracer introduced into a flow to visualize the movement of the air. The dust lamp uses a parallel beam of light to illuminate fine particles normally invisible under diffuse lighting conditions. The fine particles scatter light most intensely in a forward direction (the “Tyndall” effect) and by looking toward the parallel beam at an angle of 5-15 degrees from the centerline, the dust cloud can be seen clearly. The fine particulate cloud will appear as a hazy smoke cloud. Observation of very low-concentration clouds or small leaks may require some suppression of the ambient background light but, although strong sunlight or other sources of bright light should be suppressed, other clouds should be made visible under normal lighting conditions. Dust lamps are commercially available, relatively inexpensive, and, being battery powered, are quite portable. The technique is easy to use and gives good results. Further information on the theory and use of the dust lamp can be found in HSE.19
The movement of gases and vapors is more difficult to visualize than that of particulates. However, most gases and vapors have strong absorption peaks in the infrared band. If a flat screen, heated to some 15 “C or more above ambient temperature, is positioned on one side of a source with an infrared camera and filter on the other side, then the gas cloud will absorb a certain amount of infrared. Although the basic method is simple, special equipment (camera and filters) is required.
10.5.3.10 Schlieren and Shadow Graphs
These methods are based on the visualization of small density differences and gradients due, for example, to temperature or concentration differences. Density differences cause localized changes in the refractive index of the gas. If a parallel beam of light passes the area, the beam will be deflected by the localized changes. If the beam is focused onto a knife edge, the deflections will cause more or less light to fall onto the edge. A camera or viewing screen can be used to observe these changes. The method is basically a research or test room technique. It requires high quality mirrors to produce the parallel beam and setup is time-consuming and delicate. It has been used by Clarke20 on investigations of outbursts from microbiological safety cabinets.
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