Supply inlets

It can sometimes be easier, inside a specific volume, to use supply than to use exhaust air to control contaminants because of the much longer range of influence air from a supply opening has when the openings have the same size and the same airflow rate. For some contaminants and in some processes it may also be necessary to supply additional breathing air to the workers and this air could then also be used to control and transport contaminants. For spot cooling of workers or work pieces it is possible to use point jets originating from specific supply inlets. 

To choose a supply inlet as the local ventilation system is not common because it is difficult to design for the specific spreading of contaminants. This is usually easier with an exhaust hood. However, there are moments when large flow rates or specific flow fields are necessary to transport contaminants or for shielding from contaminants. 

When choosing a supply inlet it is always necessary to be very careful, because the influence of the supply air could reach very far. This is most important when using air jets or similar devices to direct the contaminants in an intended direction, since very small changes in direction or momentum could totally destroy the intended flow field and transport the contaminants to, instead of from, the workers. 

A supply jet may have several different forms point jet, swirl jet, line jet, radial jet, etc. The choice depends on available volume and existing demands. A radial jet could be used when it is possible to utilize a wall or a ceiling for distributing the air a point jet could be used when it is advantageous to have a high air velocity in the volume (room). 

Supply inlets are also used when there is no room for an exhaust hood or when the contaminant-generation process has a form such that an exhaust 

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Given a number Nr of waste (rich) streams and a number Ns of lean streams (physical and reactive MSAs), it is desired to synthesize a cost-effective network of physical and/or reactive mass exchangers which can preferentially transfer a certain undesirable species. A, from the waste streams to the MSAs whereby it may be reacted into other species. Given also are the flowrate of each waste stream, G/, its supply (inlet) composition, yf, and target (outlet) composition, yj, where i = 1,2,..., Nr. In addition, the supply and target compositions, Xj and x j, are given for each MSA, where j = 1,2, Ns. TTie flowrate of any lean stream, Ly, is unknown but is bounded by a given maximum available flowrate of that stream, i.e.. 

There are many possible ways to classify local ventilation systems. When local ventilation is used to describe exhaust hoods only, one classification is hoods that totally surround the contaminant source (enclosing hoods), hoods that partially surround the contaminant source (partially enclosing hoods), and hoods where the contaminant source is outside the hood (exterior hoods). A similar classification is used here for the exhaust hoods. Since local ventila tion, as described in this chapter, includes more than exhaust hoods, the following three main categories are used exhaust hoods, supply inlets, and combinations of exhaust hoods and supply inlets. (See Fig. 10.1.)... 

FIGURE 10.1 Principles for the three different ways of protecting a volume by using an exhaust hood (above), a supply inlet (middle), and a combined exhaust hood and supply inlet (below). 

For supply inlets in rooms some performance measurements exist, such as air exchange and ventilation efficiencies (see Chapter 8). It is usually not possible to use these for local ventilation supply inlets, and for the moment there are no specific measurements to evaluate the influence of an inlet on contaminants. Some trials with comparison indices, which compare inhaled concentrations (or exposures) with and without a supply inlet, have been done. 

Mostly the use of a supply inlet as a local ventilation system presumes that the supply device (with air from outside the room) is located inside a large room, which also has an adequate exhaust airflow rate or has convenient ex-haust/transfer openings for the airflow. It is also necessary that the exhaust flow rate is maintained (or pressure difference kept). Otherwise the air supply could change in rate or direction. Instead of using air from a ventilation system, the supply air could be taken from the room (volume) it is situated in. In this case, the room must also have a supply and an exhaust flow rate. It is often necessary to clean the air before it is used in the supply inlet. 

A. supply inlet could be designed with or without an exhaust. There should naturally be an exhaust opening in the room. If the influence of the exhaust on the supply inlet is low, then it requires no special design, but if the influence of the exhaust is large it must be taken into account. The inlets could be combined with specific exhaust hoods to enhance their efficiency (see Section 10.4). There are many ways to design supply inlets and also to combine different supply inlets just a few inlets are described in this section. 

All supply inlets without a specific exhaust have the same problem, the spreading of contaminants in the room outside the supply air zone. When the inlets are used to create a cleaner zone in a normal workroom this is usually no problem. When a supply inlet is used to blow away some hazardous contaminant it is necessary to combine it with a specific exhaust, or the rest of the room will be contaminated. 

These systems can be inside large halls and may have no fixed limits for their influence, except for some parts of the system (inlet device surface, etc.) They can also be situated inside small rooms, where walls, floors, and ceilings are the natural boundaries. The systems usually consist of one exhaust hood and one supply inlet, which interact. There are also special combinations, as two or more inlets and one exhaust hood, or one supply inlet and two or more exhausts. All of these combinations need careful design and an accurate relation between supply and exhaust flow rates and velocities. Some systems also need stable temperature conditions to function properly. All combinations are dependent on having a defined contaminant concentration in the inlet air. This usually implies clean supply air, but some systems may use recirculated air with or without cleaning. 

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. 

Earlier descriptions for exhaust hoods and supply inlets will not be reproduced in this section. 

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