Testing hoods

There are many machines which arrive on the factory floor with correctly designed and tested hoods supplied by the machine manufacturers. This is a trend that has grown over the years, and is one to be encouraged. Hoods supplied in this way can have the volume specified, which, in turn, will ensure that the control meets with the appropriate regulations. 

A test has been devised for testing hoods for impact resistance. The mower is run over a bed of 1 inch aluminium cubes, at 0 C for 5 min. There must be no fracture and the missiles must be contained. 

Sodium chloride as a test substance has been used in several countries and the United States. The concentration in the test hood is 15 2 mg/m and the particles have an MMAD of 0.66 0.I2 m, with a geometric standard deviation of 2.15 0.I9. Calibration is done with a flame photometer using propane supplied by an external unk. The photomultiplier analyzes die flame, and the results are processed. 

NOTE In the initial testing of any undesireable interaction between Sodium Azide, Acetic Acid and Sulfuric Acid, I mixed 5mL of each into a small cup underneath my "fume hood". Though I smelled nothing, within seconds my head felt like it was expanding, my heart started racing, and I felt more weak and confused than normal. I just barely escaped and recovered in 15 minutes, but, Needless to say, this procedure is a tad on the dangerous side. You have been warned. ... 

Unless specifically tested, all cyanocarbons should be considered as toxic as sodium cyanide or hydrogen cyanide (see Cyandes). They should be used only in a fume hood, and mbber gloves should be worn. 

In general NYY-O cable is used with a minimum copper cross-section of 2 X 2.5 mm. The cable is connected to the pipeline by a suitable process [5-7], and the connections carefully coated. The cable is usually connected to aboveground test points and covered with hoods, tiles or a cable ribbon. 

Respiratory protective devices - Powered filtering devices incorporating a helmet or a hood - Requirements, testing, marking. Superseded BS EN 146 1992... 

A similar effect was observed for changes in hood flow rate. With a fixed cross-draft velocity, capture efficiency decreased with decreasing hood flow rate. This effect was much more important when freeboard height was small. Their results showed that when hood flow rate was 1.5 m s m, efficiency remained close to 1.0 as long as the cross-draft velocin. was less than 0.45 in s. The most severe conditions tested were a hood flow rate equal to 0.33 m s" nr- and crossdraft velocity equal to 1.15 m s. Under these conditions, capture efficiency was equal to 0.83 for freeboard hei t equal to 0.3 m, but decreasing to 0.4 when freeboard height was decreased to 0.1 m. 

ASHRAE 110-1995. Method of Testing Laboratory Fume Hoods. Atlanta.. American Society of Heating, Refrigeration, and Air-conditioning Engineers, 1995. 

The assumptions that the exhaust flow has a negligible effect and that the offset jet can be treated as an equivalent wall jet were tested by Robinson and Ingham - and found to be reasonable over the majority of the surface of the tank, except close to the jet nozzle and exhaust hood. Far from the surface of the tank, the exhaust flow has a more noticeable effect. 

The procedures and requirements for rhe type and commissioning tests are covered in narional and international standards—for example, for laboratory fume cupboards, welding fumes, and kirchen hoods. 

For example, the type test of a laboratory fume hood includes determination of the concentration at various points across the opening of the hodd by using various tracer source locations inside the hood. The commissioning test could concentrate on the measurements taken at one point in the opening with one source location. 

In-use tests deal with the concentration measurements in the worker s breathing zone for various hood applications. The performance tests presented here are typically in-use tests, where normal work procedures are ongoing. 

The initial performance test for all local ventilation systems is a smoke test, which provides easy airflow visualization between the source and the hood, it helps to identify, with little effort, the main features of airflow patterns. Such a test, recorded by a video camera, allows performance comparisons to be made before and after improvements. Real contaminant or tracer gas measurements are necessary in the case of more detailed testing. 

The capture velocity of a hood is defined as the air velocity created by the hood at the point of contaminant generation. The hood must generate a capture velocity sufficient to overcome opposing air currents and transport the contaminant to the hood. For enclosing hoods, capture velocity is the velocity at the hood opening. In this case, the velocity must be sufficient to keep the contaminant in the hood. In practice, hood shape and the influence of crossdrafts on the measured capture velocity have to be considered. All three velocity components should be measured and used to calculate the magnitude and direction of the total velocity. Other methods used, not as good as the previous one, are to measure the velocity with a directional velocity sensor towards the hood or to measure the net velocity by an omnidirectional velocity sensor. In the last method the main airflow direction should be viewed and evaluated by means of a smoke test (see Sections 10.2.1 and 10.2.2.1). 

For a new process plant, calculations can be carried out using the heat release and plume flow rate equations outlined in Table 13.16 from a paper by Bender. For the theory to he valid, the hood must he more than two source diameters (or widths for line sources) above the source, and the temperature difference must be less than 110 °C. Experimental results have also been obtained for the case of hood plume eccentricity. These results account for cross drafts which occur within most industrial buildings. The physical and chemical characteristics of the fume and the fume loadings are obtained from published or available data of similar installations or established through laboratory or pilot-plant scale tests. - If exhaust volume requirements must he established accurately, small scale modeling can he used to augment and calibrate the analytical approach. 

Type I Flame arresters acceptable for end-of-line applications. Where a Type I arrester is provided with cowls, weather hoods, or deflectors, etc., it shall be tested in each confignration. 

Type II Flame arresters acceptable for in-line applications. Type II arresters shall be specifically tested widi the inclnsion of all pipes, tees, bends, cowls, weather hoods, etc., which may be fitted between die arrester and the atmosphere. Owing to the prohibitive cost of testing deflagration flame arresters for each particular installation, the Type II (in-line) category is generally not enconntered. 

If a flame arrester is provided with cowls, weather hoods, deflectors, etc., it must be tested for the configuration involved if the test is done to meet the UL, USCG, or CEN standard (see Chapter 8). Maintenance of a flame arrester should be performed carefully to avoid any adverse impact on arrester performance (see Chapter 7). All flame arresters should be inspected regularly as operating experience dictates (see Chapter 7). 

Use a thermometer and collect the distillate in a test-tube. Note the boiling-point, and observe if it fluctuates oi remains constant and if any solid residue remains. A low boiling - point generally denotes a low molecular weight. A portion distilling in the neighboui-hood of ioo may indicate the presence of water. 

Most design books continually report that plastics cannot take the heat of metal (steel, etc.) indicating that plastics cannot take heat. As reviewed, by far practically most plastic products do not have to take any more heat then the human body. Practically all plastics easily meet this heat requirement for these type products and in fact many types of these plastics meet the higher heat requirements of plastic products that exist under the engine hood of an automobile, in the trunk of an automobile (excellent user-environmental test), electrical/electronic devices, etc. 

Flammability UL-94 vertical burning test Flammability hood as recommended by UL... 

All bench-mounted equipment should be listed with its dimensions, which can be taken from measurements or from catalog data. It should be noted which instruments may have to be put at a certain minimum distance from other objects in order to avoid interference or allow for servicing. Work space should be allowed next to each instiiiment for samples, notebooks, etc. This space may be considerable in case of an analytical instrument on which many samples are to be tested at one time. Space sharing should be discouraged, with the required work space next to an instrument reserved for that alone. Sinks and fume hoods should also be included in this list, with an allowance of at least 18 inches of free space on each side of a sink. 

Fig. 23.2 Isolators used for sterility testing. The operator works within the hood which is suspended inside the cubicle the hydrogen peroxide generator which is used to sterilize the isolators is shown in the left foreground. (Courtesy of SmithKline Beecham Pharmaceuticals.)...

---