There are many different combinations of supply inlets and exhaust hoods, where these have to be designed together. Some of these are described in some detail. Two examples of using air jets for special purposes are described and the use of wide air curtains is also described.
Many other combinations exist but will not be described here. In small cabins, for storage or work, it is possible to supply and exhaust air in a controlled way to have a defined climate. There are also special sluices, where air is used to rinse the clothes from settled contaminants before a person proceeds to the next, cleaner room. In this case, very high air velocities are used, which could cause discomfort to the person. The residence time for the person usually is less than a couple of minutes and the main objective is to clean the clothes (and sometimes the skin) and therefore the high velocities do not matter.
Glove boxes have been described in Section 10.2.3.6 as systems with only exhaust air. There are glove boxes with both supply and exhaust air openings inside the closed volume (see Section 10.4.6.4) that look and function like the boxes described earlier. The use of supply air directly to the closed volume means that the control of the flow rates must be very accurate. Otherwise the box may be at a higher pressure than the surrounding areas, resulting in leakage of contaminants.
Other examples of combinations include air supply along the center of the ceiling in a room and exhaust along the walls or along the corner between the walls and floor. These are quite similar to abrasive blasting rooms (Section 10.4.7). They have an air supply in the form of a half cylinder, through which the air is evenly distributed over the surface both lengthwise and radially. An alternative is to have the inlets as two or more plane air jets blowing in at some angle downward. The exhaust openings are the same length as the inlet, situated along the room on two sides in the lower part of the walls or in the floor. This combination makes it possible to have quite a large flow rate, at low velocity, through the room.
Combined systems could be designed by using information available in Sections 10.2 and 10.3 and taking into account the mutual influence of the supply and the exhaust system. Some combined systems are described in the literature.2’3’6
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 mav he directed sideways or downward.6 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).
Two or more plane jets can be placed above and outside the rim (all sides) of a canopy hood and directed downward. The exhaust flow into the hood makes the down-directed jets turn inward and upward when the jet velocity has slowed down enough to be influenced by the exhaust flow7. In many cases, the aim is to diminish the general supply airflow rate into the room and sometimes to use the jets as separators. This method is quite often used on large kitchen hoods to increase their capture efficiency. If the jet is directed toward the front of the fireplace and just reaches the front before turning inward, a high capture efficiency can be achieved.
Another variation is to use a thin air jet around a process or an opening to a process. For welding or soldering on a table, a circular tube with supply holes in the upper side could be used. The circular tube is placed around a fixed welding (soldering) place and blows upward around the welding point into a basic opening hood.
[ | Canopy hood
Airflow — ■ * — *► Contaminant flow
M Contaminant source / Air jets directed into hood
FIGURE 10.105 Two principal ways of using air jets to increase efficiency of a canopy hood.
Such a combination demands careful analysis of distances, air velocities, air directions, and contaminant generation direction and rate. It is necessary to direct the jet into the hood with the correct velocity and flow rate. High velocities or flow’ rates could result in more spreading of contaminants than without the jet, but: the velocity must be large enough to prevent the jet from collapsing into itself due to suction.
For a kitchen hood with a fixed air curtain, good results are possible if the exhaust flow rate is large enough to exhaust the large amounts of con taminants and induced air from the oven. The circular jet around a welding or soldering point is usually too cumbersome to use when working at more than one spot.
Plane jets could be used to create a closed volume in which a contaminant source could be placed. In some ways, these systems are similar to Aaberg exhaust hoods (Section 10.4.4). The objective is to use plane jets instead of walls around an exhaust opening to create a vortex which enhances the capture efficiency of the exhaust.
Usually a circular exhaust opening is placed in a horizontal surface and directed either downward (airflow upward) or upward (airflow downward), When the exhaust opening is placed in a table and directed upward, it is surrounded by four vertical tubes, which are covered by another horizontal plane. A vertical jet is blown from holes or a slot in each tube. The jets are directed nearly tangential to the circular exhaust and at a certain distance from the opening. This distance depends on the size of the opening and the distance between the two planes. One recommendation is to blow’ the jet air at 20° from tangential. The four jets blowing at each other create a vortex around the exhaust opening and contain the contaminant inside the vortex.
The exhaust flow rate influences the flow of the jets and some reports recommend a ratio of supply airflow rate to exhaust airflow rate of approximately
0. 3. A ratio of 0.2 is unsteady and ratios larger than 0.4 have not been studied. In the cases that have been studied, the exhaust opening was 80 mm in diameter, the distance between the horizontal planes was 750 mm, the tubes were placed in a square with side length equal to 670 mm, and the inward angles of the jets were 10 degrees. This configuration resulted in better capture of hot gases than use of an exhaust system alone.69
Figure 10.106 illustrates the vortex jets in combination with a plane exhaust.
Line jets are used as curtains both in bench hoods such as laboratory fume hoods and in large openings such as doors and gates. These jets have to be designed carefully, usually together with an exhaust opening. They must withstand the pressure difference across the opening and their velocities should be of the correct magnitude for the intended purpose. This means that the jet velocity should not be too large, which results in contaminant spreading or high energy consumption. The jet velocity should not be so small that no curtain is achieved. One way to make it easier to enter a department store or pass through an industrial door is to use a wide air curtain instead of a line jet.
From one side
“ " “ “ " Airflow • • ► Contaminant flow
S Supply tube for air jet Ј Exhaust opening (in floor)
M Contaminant source / Air jets
FIGURE 10.106 Two principal ways of using air jets to increase efficiency of a canopy hood.
TABLE 10.16 Airflow Rates for Heated Wide Air Curtains (Pressure Difference between Inside and Outside Is 15 Pa)
A wide air curtain is usually directed downward from above. The air is blown along the width of the entire door and the jet has a large depth. The ex haust, which could be connected to the inlet (recirculation), is then situated in the floor. The supply air is usually heated, by a heat regenerator, to increase the comfort of people passing through the opening.
The airflow’ rate should be 4.2 to 5.5 m3 s"! m-1 width of opening, and the exit velocity from the supply side should be 10 to 12 m s_1. With an opening height of 6 m, this will result in high-velocity airflow at the head height of passing persons, but this velocity is usually tolerated. The return air opening should be a floor grill with a surface area equal to 1.5 to 2 times the surface area of the supply opening. The recommended size of the floor grill is a depth of 1.5 to 2 m and a width of 3 to 4 m. One recommendation is to operate the system with a supply air temperature that is approximately 5 °C higher than the air temperature of the surrounding spaces.70
Another suggestion is to use a supply air temperature equal to 25-30 °C. S With an entrance depth of 1.1 m and a pressure difference from outside to inside of 15 Pa (which is equal to a wind velocity of 5 m s_1 at a right angle to the door), the values in Table 10.16 are suggested.
These combinations are possible without too detailed design of the combined jet and exhaust, since the wide jet itself will act as a separator between inside and outside and the consequences of an imbalance of supply and exhaust airflow are small. The relative mixing of outside and inside air into the jet is less than for a thin air jet, which also makes it easier to design fan(s) and ducts. The high velocity needed requires a high airflow rate and, with heating, can be expensive to operate.
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