Ventilation System Ducting

Chapter 2 contains detailed descriptions of the Zone 1 and 2A portions of the ventilation system. Ventilation system ducting provides the necessary confinement pathway during normal ventilation system operation to ensure filtration of exhaust gases prior to atmospheric release. The Zone 1 ventilation exhaust system contains redundant series-parallel charcoal filter banks located in the MER. The Zone 2A ventilation exhaust system contains charcoal filter banks located in the MER. In addition, ventilation system hot exhaust from Zone 1 and Zone 2A is routed through a HEPA filter before entering the HCF stack. These ventilation systems are described in more detail in Chapter 2. 

The ventilation system ducting between the MER and the stack is conservatively assumed to fail in all scenarios. The potential states of the balance of the ventilation system have been assessed to occur with the following likelihood ... 

Report the total of all releases to the air that are not released through stacks, vents, ducts, pipes, or any other confined air stream. You must include (1) fugitive equipment leaks from valves, pump seals, flanges, compressors, sampling connections, open-ended lines, etc. (2) evaporative losses from surface impoundments and spills (3) releases from building ventilation systems and (4) any other fugitive or non-point air emissions. 

FIGURE 5.59 Schemack View o( a typical cf ntraJ 5cation ventilation system, mdtiding a fen ducts, and difYusers/... 

Efforts to reduce the noise from a ventilation system may be concentrated on measures concerning the fan, the fan room, the fan ducts, and the supply and exhaust air terminals. - ... 

The following equations separately outline calculating contaminant concentration inside a room with central and local recirculation. The assumptions for the room are that it has one main ventilation system with supply and exhaust air and that the contaminant concentration is the same in the whole volume (except very close to the contaminant source or in the ducts, etc.). The contaminant source is steady and continuous. The model for local ventilation assumes also one main ventilation system to which is added one local exhaust hood connected to a local ventilation system (see Chapter 10) from which all the air is recirculated. In the central system the number of inlets and outlets could vary. The flow rates are continuous and steady. 

It there is no large duct after the fan which needs a large static pressure, then there is no need for spiral casing. The air then leaves steadily from the impeller circumference, and the air is not collected into one point. At the exit side in ventilating systems of buildings, the need for static pressure is small if the leaving air is directly sent to the atmosphere. 

Factory-constructed ventilation systems are constructed in a compact manner. Frequently, due to the smaller-space installation, inspection and maintenance of the sensors can be difficult. In ducts, deviations from the real... 

The parts of local ventilation systems, situated inside rooms, that influence the flow field are described here. This presumes that the inlet and outlet openings are properly connected to duct systems either directly or through flexible connections (tubes). These ducts and tubes and other parts of importance for the function of these systems are described in other chapters. 

The primary components of LVHV ventilation systems are the exhaust nozzles, flexible hose, fixed duct, air cleaner, and the exhauster and motor. Each of these is discussed below. Figures 10.23,10.27, and 10.28 illustrate LVHV nozzles. Figures 10.29 and 10.30 illustrate other system components and installations. 

The range of the airflow in ventilation systems is wide. The flow rate in an individual supply or exhaust terminal may only be a few liters per second, while the flow in a main duct or supply chamber of a large sy.stem may be in excess of 100 cubic meters per second. No general method to deal with the whole range exists. Each case requires individual consideration for the most suitable methods and instrumentation to be selected. 

Because all measurement methods and instruments are sensitive to the velocity profile, the choice of the measurement cross-section is of vital importance. In most ventilation systems there is seldom enough straight duct to allow a fully developed velocity profile to develop, which is the most favorable for flow measurement. Thus, the principle in selecting the measurement cross-section is to find the place where the velocity profile is as near to the fully developed profile as possible. In practice the distance from the nearest source of disturbance upstream is maximized, ensuring that the distance to the nearest downstream disturbance is at least 3 to 5 duct diameters. 

Air type Classification of air at a specific point in its passage through an air conditioning or ventilation system, either in the duct or the space, e.g., outdoor air, supply air, treated air, recirculated air, extract air, etc. 

Larger ducted ventilation systems, as used in offices and commercial premises using a central air-handling unit and fabricated distribution ductwork, are akin to air-conditioning systems but with less treatment to the air at the AHU (see Chapter 29). 

Many processes involve open operations (e.g. filters, drum handling, etc.) that create VOC emissions. If this is the case, it is often not practical to enclose all such operations, in which case, a ventilation system needs to draw a continuous flow across the operation into a duct and then a vapor recovery system, before being released to the atmosphere. 

An air ventilating system must be designed to deliver air at 20°F and atmospheric pressure at a rate of 150 ft3/s, through 4000 ft of square duct. If the air blower is 60% efficient and is driven by a 30 hp motor, what size duct is required if it is made of sheet metal ... 

Ventilation systems are composed of fans and ducts. The fans produce a small pressure drop (less than 0.1 psi) that moves the air. The best system is a negative pressure system, with the fans located at the exhaust end of the system, pulling air out. This ensures that leaks in the system draw air in from the workplace rather than expel contaminated air from the ducts into the workplace. This is shown in Figure 3-5. 

Wet or dry chemical fixed suppression systems are typically provided over the kitchen cooking appliances and in exhaust plenums and ducts. Activation means is afforded by fusible links located in the exhaust ducts/plenums usually rated at 232°C (450°F). Manual activation means should not be provided near the cooking area, but in the exit routes from the facility. The facility fire alarm should sound upon activation of the fixed suppression system and power or gas to the cooking appliances should be automatically shut off. The ventilation system should also be shut down by the activation of the fire alarm system. Protective caps should be provided on the suppression nozzles to prevent plugging from grease or cooking particulates. 

The filter unit was located entirely inside the respective buildings and was independent of the ventilation system. The unit consisted of a suction box (usually floor standing) to which the filter material was attached usually as a flat vertical surface of an area to give a face velocity of about 0.75 m/s. Dust collected on this surface and was removed with a vacuum cleaner. Cleaned air was directed into an overhead discharge duct with a permeable lower surface. The capacity of the fan in the filter unit was made the same order as that of the main ventilating fan. The overhead duct was located above and as near to the stock as possible so that the clean air flowed downwards towards the stock. This air did not cause a draught because it was discharged at a low speed (0.3 m/s) and as it was recirculated it was at the same temperature as the air in the room. 

Recirculating ventilation systems should be designed for shutdown in the event of a fire or chemical spill. Exhaust systems should continue to run during the incident to facilitate the removal/ treatment of potentially toxic materials. Caution must be exercised when considering the placement of automatic sprinklers inside exhaust ducting. The flowing sprinklers have been shown to greatly reduce the capacity of the exhaust system. 

On October 30, 1998, a flammable gas release led to gas migration to a non-electrically classified Electrical and Instrumentation (E l) Room. The flammable gas was ingested into the exterior heating and ventilation inlet duct. When the 480 volt primary power switch, within the E l room was remotely shut off, an explosion resulted. The E l module explosion led to a subsequent fire in adjacent production buildings. There were no injuries to personnel. The root cause of the incident was determined to be the failure of management systems... 

Examples of local exhaust ventilation system are drawdown exhaust tables, slot hoods, dust extraction systems, and portable vapor and dust collectors. Care must be taken that the ventilation systems are utilized and maintained as designed. The condition of the filters and air flow rate should be checked periodically as well as the condition of the duct work, motors, belts, etc. 

When designing a local exhaust ventilation system for a process that generates dust particles, it is important to consider the minimum air velocity. The minimum air velocity is the velocity required to prevent settling of dust particles in the air ducts. The minimum velocity is a function of dust particle size and particle density. Listed in the table below are the minimum air velocities recommended for the transport of various types of particulate contaminants. 

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