Exhaust performance, airflow

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. 

Figure 16.33 shows a schematic of a simple gas turbine. The machine is essentially a rotary compressor mounted on the same shaft as a turbine. Air enters the compressor where it is compressed before entering a combustion chamber. Here the combustion of fuel increases its temperature. The mixture of air and combustion gases is expanded in the turbine. The input of energy to the combustion chamber allows enough power to be developed in the turbine to both drive the compressor and provide useful power. The performance of the machine is specified in terms of the power output, airflow rate through the machine, efficiency of conversion of heat to power and the temperature of the exhaust. Gas turbines are normally used only for relatively large-scale applications, and will be dealt with in more detail in Chapter 23. 

The airflow through the face may range between 75 and 100 fpm. An aerodynamic shape at the entrance or suction slots near the edge to ensure the flow of air is inward aids in the performance of the cabinet, while a baffle at the rear provides that some of the inlet air will flow directly across the work surface as the remainder is drawn upward through the cabinet to the exhaust portal. It is primarily the directional flow of air which ensures that the agents of concern stay within the cabinet. 

A TSR requirement to verify that Zone 1 and Zone 2A ventilation exhaust HEPA and charcoal filters are in-service vwll be implemented to assure that exhaust gases are being filtered when the HCF ventilation system is in operation. A TSR requirement to verify the ventilation system fan sequencing interlock is operable vvnil be implemented to ensure that proper building airflow patterns are maintained in the event of exhaust fan failures. The ventilation system exhaust ducting provides only an inherent passive safety function (i.e., confinement) and no specific TSR controls are required to ensure continued performance of this function. 

It is important not to disturb the performance and airflow pattern of the fume cupboard. Staff standing or walking in front of the fume cupboard will disturb the air exhaust. The operator must have sufficient space in front of the fume cupboard to work easily and comfortable. Room doors that open frequently during operation in a fume cupboard and (strong) inflowing air fi om the room ventilation system (in the direction of the fume cupboard) may have a negative impact on the performance of the fume cupboard. Airflow visualisation with smoke is illustrative and often necessary. 

In addition to the convertible Type A cabinet used in the Type B mode, there are two additional versions of the Class II Type B cabinet. These units differ from the Type A and B3 units mainly in the airflow velocities and proportion of air recirculated, as well as in certain other performance specifications. Class II Type B1 cabinetry allows a little more flexibility in working with volatile, toxic, or radioactive substances, since its exhaust is connected to an exhaust duct that exhausts the cabinet air directly outside the building (Figure 9.6). Because 70 percent of the circulating air in the cabinet is exhausted to the outdoors, most nonexplosive or nonflammable chemicals may be used safely in low concentrations. Microgram quantities of toxic, carcinogenic, or radioactive compounds may be handled in the Class II Type B1 cabinet, provided that the work is performed in the direct exhaust portion (behind the smoke split) of the work surface. 

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. 

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