Basic exhaust opening

Basic exhaust openings are not recommended for use when the distance between source and hood is great, since it is easy for contaminants to spread outside the reach of the exhaust due to the sharp decrease in velocity with increasing distance. It is usually better to use partially closed systems. 

It should be noted that when there is no jet reinforcement of the flow, i.e., the exhaust hood is used in its conventional mode, then in the two-dimensional form of the Aaberg principle the fluid flow velocity due to the exhaust decays approximately inversely proportionally to the distance from the exhaust opening. However, for three-dimensional exhaust hoods the fluid velocity outside the hood decays approximately inversely as the square of the distance from the exhaust hood. Thus in the three-dimensional conventional hood operating conditions the hood has to be placed much closer to the contaminant in order to exhaust the contaminant than is the situation for the two-dimensional hood (see section on Basic Exhaust Openings). Thus for ease of operation it is even more vital to develop hoods with a larger range of operation in the three-dimensional situation in comparison with two-dimensional hoods. 

BEOs consisting of open tube or duct ends can be connected directly to a source. In principle, these could be called closed systems, since their main function is to exhaust contaminants directly from the source, which is enclosed in the duct. However, they are usually regarded as basic exhausts since they function as such when not connected to the source. 

When it is necessary to confine an air volume from the ambient environment and simultaneously have access for operators or machinery, plane air jets offer a possible and simple solution. Air jets (plane and round) are described in Chapter 7. This section describes plane air jets combined with exhaust openings. In principle, they are similar to the air jets described in Chapter 7 and Section 10.3, but the combination with an exhaust opening makes it necessary to consider the influence of the exhaust on the jet. Usually these curtains are used in large doors to shield the interior from the exterior when the door is open. For example, experimental results have shown that from the moment a door is opened, a short time interval, less than 1 minute, is sufficient to get complete development of the airflow through the door. An air curtain allows a reduction of the overall flow through the door. The principles and use of air curtains are described in many textbooks.Some basics of air curtains are described here. 

Specitic hoods Basic openings, nm exhausts, [.VHV, Booths, laboratory fume... 

The exterior hoods described here are divided into basic openings, rim exhausts, low-volume high-velocity (LVHV) hoods, receptor hoods (canopy hoods), and downdraft ventilation tables. Many varieties of these types of hoods exist. Some of these have been described and investigated more thoroughly than others because they are used more often or they are of more general use and applicability than the more specialized hoods. 

Most BEOs are situated at the end of a tube, but there are also basic openings situated in walls. BEOs can be used for nearly all kinds of sources, but are usually used for point sources. Use for line or area sources usually demands flexible or movable exhausts, or a slot placed along the line source or along the sides of an area source, or a very large (circular or rectangular) opening placed close to tlie source. A high flow rate is needed to get efficient exhaust in many cases. 

There are many possible combinations of supply and exhaust air. For example, a line jet could be used as a shield in an opening, as a stripping system on surfaces, for blowing contaminants into an exhaust, etc. An enclosure could be designed with a line jet in the opening, with a wall jet inside to increase efficiency, or with a low-momentum jet inside or outside the opening to replace the room air supply. In this section, only some basic combinations are described. 

One common combination is a jet and an exhaust hood. The jet can be circular or plane and situated around or in front of a (hot) contaminant source. The intention is to direct the contaminant into a basic opening or a receptor hood. Mostly these jets are directed upward into hoods, but may be directed sideways or downward. There is a difference to jets covering openings. When directed into a hood the jet is intended to help the natural flow into the hood and not to act as a shield, even though it sometimes also has this function. Figure 10.105 illustrates two principal ways that air jets could be used to direct contaminants into a hood (see also Section 10.4.5). 

Fig. 6.2 shows a simplified diagram of the basic STIG plant with steam injection S per unit air flow into the combustion chamber the state points are numbered. Lloyd 2 presented a simple analysis for such a STIG plant based on heat input, work output and heat rejected (as though it were a closed cycle air and water/steam plant, with external heat supplied instead of combustion and the exhaust steam and air restored to their entry conditions by heat rejection). His analysis is adapted here to deal with an open cycle plant with a fuel input/to the combustion chamber per unit air flow, at ambient temperature To, i.e. a fuel enthalpy flux of/7i,o. For the combustion chamber, we may write... 

The first fume hoods were simply boxes that were open on one side and connected to an exhaust duct. Since they were first introduced, many variations on this basic design have been made. Six of the major variants in fume hood airflow design are listed below with their characteristics. Conventional hoods are the most common and include benchtop, distillation, and walk-in hoods of the constant air volume (CAV), variable air volume (VAV), bypass and non-bypass variety, with or without airfoils. Auxihary air hoods and ductless fume hoods are not considered "conventional" and are used less often. Laboratory workers should know what kind of hood they are using and what its advantages and limitations are. 

A basic open-cycle gas turbine, as used for aircraft propulsion, has a combustion chamber as the heater , and gases from the turbine are exhausted to the atmosphere without recycling. The compressor is used to raise the pressure of the combustion air. 

The basic airflow system for flotation dryers is shown in Figure 40.10. It consists of three basic components (1) supply fan (2) exhaust fan and (3) heater. The supply fan is sized for the air volume required by the open area of the dryer nozzles and the maximum nozzle outlet velocity. It blows hot air through the air-bar nozzles onto the coated web surface. The spent air is exhausted by an exhaust fan. The heater can be a direct-fired gas burner for air temperatures up to 400°C or high-steam coils for air temperatures up to 200°C. When drying aqueous coatings, the exhaust is led to a recirculation system where part is bled to the atmosphere and the remainder reheated for recirculation. When solvents are involved, this spent air may be incinerated or led to a solvent-recovery unit. 

Besides the analysis of basic patterns, there are many problems and openings for the future. It is beyond the scope of this chapter to give an exhaustive view of these questions. We shall only give an overview of a few selected topics which are the subject of current research. 

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