Cabinet ventilation flow

Process gas is not the only flow that the controller can monitor for control purposes. Cabinet ventilation flow is also an important parameter and the automatic controller usually monitors this parameter and will shutdown the cabinet system if the air flow through the cabinet is too small. Other signals that should shutdown the entire cabinet include, fire, heat, smoke, or flame detected inside the cabinet, or gas leak detected inside the cabinet. Often cabinet control systems are designed to shutdown if any of these situations occur in the nearby area, or if a separate safety monitoring system sends the cabinet a signal to do so. 

Class II BSC. The Class II BSC (Fig. 9) provides product, personnel, and environmental protection, and is the most common BSC employed in pharmaceutical manufacturing and pharmacy-compounding operations. The Class II BSC has several subclassifications, based upon cabinet ventilation design (Table 1). The Class II BSC (Fig. 8), the most widely used by hospital and home-care pharmacies, features a front access opening with carefully maintained inward airflow for replacement of air exhausted from the cabinet, a HEPA-filtered vertical laminar flow airstream within the entire work area, and HEPA-filtered exhaust air. The vertical laminar flow airstream and front access opening are common to all Class II cabinets, although LAF velocities and patterns, HEPA filter sizes and position, ventilation rates, and cabinet exhaust methods vary considerably in different designs (Fig. 7). 

The exhaust duct of each ventilated containment cabinet must be fitted with an adjustable low flow sensor. Audible and visible alarms must be located near the cabinet, and the silence switch should energize an indicator at the status board. These are local alarms which should not automatically trigger a call for emergency response personnel. 

The capacity per unit area of cabinet dryers is limited by two conditions need for direct radiation on the drying material and small airflow rate. To dry larger quantities of material, the basic area of the dryer has to be increased. To avoid this problem it is preferable to place the material in several independent layers the necessary heat transfer is thus accomplished by convection. The increase in the mass flow rate of air can be achieved by increasing the effects that prodnce natural convection. These effects mnst also be increased if the air is to be circulated through a material laid in several layers one over the other, or through a thick layer, as in the case of the static bed type. To keep up the natural pressure difference without using a ventilator (for instance, in a field), the chimney effect must be exploited. For this purpose the vertical flow of hot air in the dryer must be increased. 

Evans, C.G.T., Harris-Smith, R., Stratton, J.E.D. and Melling, J. (1974). Design and construction of a ventilated cabinet for a continuous flow centrifuge, Biotech Bioeng., 16, 1681-1687. 

It cannot be stated that a normal down flow cabinet, with open front and without sleeves to the work top (so not a safety cabinet), or a down flow unit is safe for the operator if working with half-open or open systems. It may very much depend on airflow patterns if any contamination with a substance will be actively blown in the direction of the operator. If so, the exposure would be higher, in which case no ventilation or exhaust is preferred. 

Local safety cabinets of any type will influence the air pattern, temperature and HVAC system in the preparation area. (HVAC means heating, ventilation, air-conditioning see Sect. 27.5.1). The influence will vary. Sometimes all doors of the room must be closed in order to insure a correct function of a fume cupboard. Air expelled by the local safety flow systems will also influence the ventilation and the pressure in the room. Local devices produce energy in the form of heat and, unless exhausted, will change the room temperature. This can be uncomfortable for the operator. 

In general, the air from the production area enters the front of the LAF cabinet in a controlled way, passes at first a set of pre-filters in the cabinet that separates coarse dust. Sometimes more HEPA filters placed in series after the pre-filters act as supplementary pre-filters (in certain types of safety cabinets). After pre-filtration the air is forced via a ventilator box through a set of framed HEPA filters and finally enters the aseptic process area as sterile filtered air in a unidirectional flow. The speed of the unidirectional flow is kept between limits. Finally the exhaust air re-enters the room (crossflow LAF units), or can be exhausted to the outside with or without extra HEPA filtration (safety cabinets). 

A laminar flow cabinet will continuously produce heat from the ventilators. If the exhaust airflow from the LAF unit is returned into the room (recirculation), the temperature in the room will rise and more room ventilation and cooling is necessary. Air exhaust to outside the room will give less heating of the room. 

In safety cabinets of the type partial or total exhaust, the air eventually is collected in a box on top of the bench, coimected via a HEPA-filter with the air in the room (see Fig. 28.If). The box has underpressure due to the exhaust air velocity. In case the safety cabinet has a breakdown, the box construction prevents the ventilator of the exhaust channel to continue while the down flow ventilator in the cabinet stops. Otherwise contaminated air from the room would be sucked under the sash, contaminating the clean side of the HEPA filter in the work area of the safety cabinet. 

Safety cabinets placed in a room with underpressure (for the preparation of radiopharmaceuticals for example) must have an extra exhaust ventilator, discharging the exhaust air outside the building. This exhaust ventilator must be tested also in daily practice and at periodical electricity break tests. Safety cabinets have visual and acoustic alarms that warn for deviations in airflow (down flow and in flow alarms). 

The gases which supply these systems must be conveyed to them safely. Most process gases are mounted in ventilated gas cabinets and sensors and process interlocks prevent high levels of the gases from being released outside the process. High flow preventers such as critical orifices, flow sensors and auto shutoff valves maintain the gases in controlled circumstances. 

This delivery module is an enclosed ventilated cabinet that contains the reagent flow control valves and reagent reservoirs. There are four glass reservoirs of 220-mL capacity for normal active nucleoside... 

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