Local recirculation

Local recirculation systems differ from central systems in that all exhausted air is passed back to the room after cleaning and that the flow rate could be larger than the flow rate through the room. 

FIGURE 8.2 Model of a local recirculating system (room air cleaner) used for calculating the connections between contaminant concentrations, airflow rates, contaminant source strength, and air cleaner efficiency, rj. c, p is the concentration in the supply (outside) air c (equal to c ) Is the concentration In the room c j is the concentration in the returned air qjirtm throu the 

The room air cleaner consists of a fan and some kind of air cleaner for particles or gases or both, usually mounted together as one unit. This is a local recirculating system and the equation for the contaminant concentration in the room, derived with the same assumptions and in the same way as for central systems, is the following  

JCj is the local recirculation ratio equal to q airrec/ fairexh Jairrec rate through the unit (cleaner), m s  

By manipulating this equation for the steady state, in the same way as for central systems, the following could be achieved  

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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. 

One of the most common systems for cleaning air in homes, offices, schools, etc. is the room air cleaner. Figure 8.2 outlines a model of a local recirculating system. Usually these units are situated inside the room if they are small and movable (see Chapter 10). For the model it does not matter if the unit is placed inside or outside the room with the contaminant source, as long as the exhaust and return air openings are inside. 

This equation makes it quite easy to calculate necessary flow rate and cleaning efficiency for a local recirculation system (room air cleaner). 

Control is by filtration in the plant but smoke can be removed by local recirculation through fan filter units. 

In the actual bed, solid exchange between the ascending and descending zone is significant, and the circulation is seen as a series combination of localized recirculation. If this scheme of solid mixing is more realistic, the temperature profile may be suitably expressed in terms of an effective thermal diffusivity, which will be used later for discussion of instability in the dilute phase. 

Experience in air separation plant operations and other ciyogenic processing plants has shown that local freeze-out of impurities such as carbon dioxide can occur at concentrations well below the solubihty limit. For this reason, the carbon dioxide content of the feed gas sub-jec t to the minimum operating temperature is usually kept below 50 ppm. The amine process and the molecular sieve adsorption process are the most widely used methods for carbon dioxide removal. The amine process involves adsorption of the impurity by a lean aqueous organic amine solution. With sufficient amine recirculation rate, the carbon dioxide in the treated gas can be reduced to less than 25 ppm. Oxygen is removed by a catalytic reaction with hydrogen to form water. 

It is possible to have a separate recirculating system in addition to the general ventilation system then there is no restriction on the flow rate. This case is the same as a recirculating local exhaust system (see below)... 

Local ventilation in industry usually differs from the description above in that it is connected to a local exhaust hood (Chapter 10), which has a capture efficiency less than 100%. The capture efficiency is defined as the amount of contaminants captured by the exhaust hood per time divided by the amount of contaminants generated per each time (see Section 10.5). Figure 8.3 outlines a model for a recirculation system with a specific exhaust hood. Here, the whole system could be situated inside the workroom as one unit or made up of separate units connected with tubes, with some parts outside the workroom. For the calculation model it makes no difference as long as the exhaust hood and the return air supply are inside the room. 

This latter equation can also be used for systems without a local exhaust hood by setting the capture efficiency to zero. It could also be used to show the result of recirculation from, e.g., a laboratory fume hood with immediate recirculation. In such a hood all contaminants are generated within the hood and usually also all generated contaminants are captured, so the capture efficiency is 1. The equation demonstrates that if the... 

In this burner configuration, fuel is injected directly into the combustion chamber and hence, one would initially categorize it as a nonpremixed burner. However, the overall combustion process is quite complex and involves features of nonpremixed, partially premixed, and stratified combustion, as well as the possibility that the autoignition of hot mixtures of fuel, air, and recirculated combushon products may play a role in stabilizing the flame. Thus, while one may start from simple concepts of nonpremixed turbulent flames, the inclusion of local exhnchon or flame lift-off quickly increases the physical and computational complexity of flames that begin with nonpremixed streams of fuel and oxidizer. 

This technology, with only small modifications to conform to local plant conditions, could have immediate application in any viscose rayon plant with soluble zinc in the plant wastestream. The techniques of initially precipitating the impurities, which would prohibit zinc recycle as well as the use of a sludge recirculation process to obtain a dense sludge, are excellent examples of good process engineering being applied to a waste problem. 

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