Exhaust hood performance

It has long been recognized that the presence of a worker close to an enclosure, especially a fume cupboard, can have a significant effect on the exhaust hood performance (see Section 10.2.3.3). However, one aspect of... 

L. M. Conroy. Field Study of Local Exhaust Hood Performance Retdsed Final Report. National Institute for Occupational Safety and Health (NIOSH) Grant 5 KOI OH00078-03. February 19, 1996. 

FIGURE 7.81 Hood performance for different exhaust airflow rates, (a) Target airflow rate q (b) Target airflow rate q < q. (c) Target airflow rate q > q. ... 

Numerical simulation of hood performance is complex, and results depend on hood design, flow restriction by surrounding surfaces, source strength, and other boundary conditions. Thus, most currently used method.s of hood design are based on experimental studies and analytical models. According to these models, the exhaust airflow rate is calculated based on the desired capture velocity at a particular location in front of the hood. It is easier... 

To design an air recirculation system it is necessary to know the performances of fans, air cleaners, and exhaust hoods included in the current system. The equations described here include the source generation rate and the total airflow rate through the room, which could be difficult to measure. The ratio between source rate and flow rate has the unit of concentration and should in fact be equal to the concentration without recirculation. The equations could thus be transformed to include the contaminant concentration without recirculation instead of this ratio. In this way a direct comparison between concentration without and with recirculation is possible. By using the described equations it is then possible to design an air recirculation system to result in the demanded concentration in a workroom. 

Similar to supply inlets, no measurements exist for evaluating the inlets specific influence on contaminant concentration. The available measurements for the combinations are the same as for exhaust hoods, i.e., capture efficiencies and similar measures. Sometimes the performance of a combined system can be approximated from the performance of the incoming supply inlet and exhaust hood. 

On 8 December 1986, 22 students and their teacher in a Connecticut high school chemistry laboratory were carrying out an experiment. This was intended to demonstrate oxidation/ reduction of metals and called for silver oxide to be used. As this was unavailable, mercuric oxide was used instead but the exhaust hoods to remove fumes from the laboratory were not turned on. The students performed the experiment in pairs using 1.75 g mercuric oxide which they placed in a crucible and heated for 15 minutes to drive off the oxygen. When it was realised that the yield of metallic mercury was very low, and that the metal must therefore have vapourised, fhe experiment was stopped. 

Safety shields should be used for protection against possible explosions or splash hazards. Laboratory equipment should be shielded on all sides so that there is no line-of-sight exposure of personnel. The front sashes of conventional laboratory exhaust hoods can provide shielding. However, a portable shield should also be used when manipulations are performed, particularly with hoods that have vertical-rising doors rather than horizontal-sliding sashes. 

Do not modify fume hoods in any way that adversely affects the hood performance. This includes adding, removing, or changing any of the fume hood components, such as baffles, sashes, airfoils, liners, and exhaust connections. 

Removal of filters from exhausts, hoods or air conditioning units should be performed in such a way that the person doing it is at no time in contact with the filter or the filter housing (see Section 2.5 (4)). 

Ventilation — Exhaust hoods at the arc, fans, and open spaces all help to reduce the concentration of hazardous fumes, gases, and dusts, and prevent the accumulation of flammable gases, vapors, and dusts that could cause fire. Know the symptoms of fumes and gases and get out of the area if they develop. Perform atmospheric tests. 

The supply airstream shall not be allowed to impinge on obstructions between it and the exhaust slot in such a manner as to significantly interfere with the performance of the exhaust hood. 

Perform all acid digestions and acid dilutions in an exhaust hood while wearing a face shield. To avoid exposure to acid vapors, do not remove beakers containing concentrated acid solutions from the exhaust hood until they have returned to room temperature and have been diluted or emptied. 

Flammability. The flammability of laminate materials is classified according to Underwriters Laboratories (UL) specifications. All of the tests are performed using a standard test setup under an exhaust hood using a Bunsen burner as a source for the flame. The categories are as follows ... 

Baffle. The back wall of the hood chamber, called the baffle, forms a plenum for exhausting air from above the counter top and the lower area of the chamber. The hood should have adjustable slots in the back wall at the work surface level and at the top of the chamber. These slots are of crucial importance to hood performance (399) and are used to balance the airflow in the hood. They should never be blocked nor the position of the moveable slat changed. 

Environmental concerns with thermal spraying techniques include the generation of dust, fumes, overspray, noise, and intense light. The metal spray process is usually performed in front of a water curtain or dry filter exhaust hood, which captures the overspray and fumes. 

All exterior hoods should be evaluated regularly. The evaluation procedures can be divided into detailed and simple procedures. Detailed procedures need special instruments and competence, whereas simple procedures may be performed daily. Since simple procedures do not directly measure the performance of the exhaust, it is usually necessary to calibrate them using detailed procedures. 

Capture system performance on a nonbuoyant source is influenced by enclosure (hood) design and location of the exhaust point. 

There are no specific design equations for this type of hood. Usually the exhaust flow rate is similar to the flow rate for ordinary fume cupboards. The different recommendations for auxiliary cupboards do not generally agree, most likely because all parameters influencing the performance have not been taken into account. 

Fuller claims that an auxiliary cupboard has better performance than an ordinary cupboard, if properly designed and used. The auxiliary flow rate should be 50% to 75% of the total exhaust flow from the cupboard. Below 50% there is no beneficial effect and above 75% the auxiliary air will aspirate contaminants out of the cupboard. The auxiliary air should enter the hood through the upper one-half to two-thirds of the opening. This should fill the volume between the cupboard operator and the opening and assist in the containment. The auxiliary air should be distributed uniformly across the length of the cupboard for a vertically sliding sash or above only the open sash of a cupboard with a horizontally sliding sash. It should also have a temperature within 1.5 °C of the room temperature and have a constant flow rate without pulsations. 

Fume hoods must be of a type suitable for the service they are intended to perform. For many applications, minimum face velocity is specified by regulations. An installer should always check the velocity when a new hood is placed in operation. It should be rechecked whenever any modification is made to the exhaust system. It is up to the laboratory operator to make certain that a hood is not put to new uses for which it was not designed. 

Other Recirculation or exhaust air from chemical areas is prohibited. No connection between chemical areas and other areas through ventilation system is permitted. Emergency backup power is necessary. Hoods should be tested at least semiannually or after modification or maintenance operations. Operations should be performed 20 centimeters inside hood face. 

A fourth area must be defined if nested-PCR, i.e., a second PCR with amplicons from a first PCR as starting material, needs to be performed. This nested-PCR area can be restricted to a negative-pressure laminar flow cabinet (type fume hood) equipped with an HEPA filter before the air exhaust and with a UV lamp that is switched on for 20 min after use to destroy any possible contaminating DNA. 

Hoods should not be designed to recycle exhaust air into the laboratory. Hoods that do not vent to the outside, but rather filter in-laboratory air, are called laminar flow boxes. These are important for working with dusty radioactive materials, like soils, but are not appropriate for most other laboratory operations. Laminar flow boxes should be clearly labeled so that laboratory policy can limit the analytical operations that can be performed in them. 

The melting of iron and steel in induction furnaces results in low emissions compared to the cupola furnace. Emissions due to the combustion of fossil fuel are especially prevented. An exhaust capture efficiency of up to 95 % is possible using special capture systems, such as side-draughts, movable hoods and partial covering of the furnace. Filtration of the off-gases is mainly performed using dry systems. Dust emission levels below 5 mg/Nm can be obtained [225, TWG, 2003]. Typical emission data are given in Table 3.14. 

In-crucible nodularisation may be performed at a specific stand or location in the melting shop. The crucible with the molten metal is brought to this point after pouring, but before taking it to the casting furnace or station. This allows the installation of a fixed hood for exhaust capture. 

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