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Positioning cases for the exhaust device
According to the agreement reached by the University of Gävle, Swedish Standards Institue (SIS), Swedish Work Environment Authority (SWEA) and three interested manufacturers of local exhaust devices, some specific positions were considered to be tested.
Observing Fig. 26, two different positions were considered for the study in which the vertical axis is concerned, h, and another two referred to the horizontal axis, d. By varying the distance between the EH centre and the table surface in the vertical direction, h, two cases of interest were tested, Case A and Case B. Additionally, by modifying the distances between EH and SA, d, another two cases were tested, Case 2D and Case 3D, corresponding to d being equal to two and three diameters of the exhaust nozzle, respectively.
Movable cylinder – turbulence generation
For the purpose of inducing room air turbulence for challenging the contaminant capture efficiency, a vertical cylinder was set into motion in front of the EH table; see in Fig. 31. The movable cylinder (MC) had a diameter of 400 mm and a height of 1900 mm. These dimensions were chosen in order to simulate the performance of a grown-up human and to compare the obtained results with the movable plate of same dimensions (1900 mm high and 400 mm wide) of the report of Mattsson [14]. The movable cylinder was placed at a distance of 400 mm from the table and 700 mm from the 3-D sonic anemometer. The MC was upright on a carriage with its lower edge 200 mm above the floor level (same construction as specified for use in tests of fume cupboards, SS-EN 14175-3 2004, but there with a flat plate). The carriage was connected to a timing belt, driven by a computer controlled stepping motor. The movements executed by the cylinder were perpendicular to the capture direction of the EH. The velocity of the cylinder when passing in front of the EH was 1.0 m/s, according to the movement of a human being on a relaxed walking pace.
Percentage of Negative Velocities
Above was observed that the movement of the movable cylinder occasionally generates negative velocities in the direction of x axis, u velocity, which represents an opposite effect to the desired one. Thus the extraction efficiency of the exhaust system can be expected to be reduced, with the consequent risk of polluting the air of the working area. The Percentage of Negatives Velocities, PNV, is the measure that represents the percentage of the time when the air flow has the opposite direction to the exhaust flow along the x axis. The studied periods of time are shown in the Table 4, which represent not only the time spent by the MC to go from one side to the other, but also the resting period between the movement intervals.
To summarize the results of the recorded PNV values, the diagrams for the different cases have been compiled in order to make the results comparable; see in Fig. 39.
As it can be seen in Fig. 39, the diagrams show some general tendencies to take into consideration. (1) At lower exhaust flow rates, Q, the PNV value increases. (2) By increasing the distance between the exhaust hood, EH, and the testing point where the 3-D sonic anemometer, SA, was placed, the PNV values experience a rise. (3) When the exhaust hood, EH, is placed at a larger distance from the supporting surface, in this case the table, the PNV value increases. (4) By reducing the movement interval time, Δt, the PNV value gets higher. Note that some values do not follow this last trend at low exhaust flow rates, Q.
Comparison of the PNV between the use of a movable cylinder and a movable plate
One of the main objectives of this study was to see how a movable cylinder affects the exhaust capture efficiency as compared to the previous study performed by Mattsson [14], where instead a movable plate was used to simulate the movement of a walking person. For this purpose the cases studied by Mattsson were reproduced in order to make the results comparable, modifying only the moving object, see in Fig. 41.
In Fig. 42 is shown a comparison between the recorded PNV values for the studied cases using a movable cylinder and the same cases using a movable plate. The results of the movable plate PNV were directly obtained from the report of Mattsson [14].
For a clearer understanding of the results, the PNV values due to the movement of the movable cylinder cases are placed on the left column, whereas the PNV values when using the movable plate are the ones on the right column, maintaining the same order to facilitate the comprehension of the results.
Exhaust device
For the tests performed it was used a local exhaust ventilation (LEV) device as the main component for extracting the supposed contaminated air of the room. The LEV device consisted of a 400 mm long straight aluminium cylinder with an inner diameter of 75 mm, connected to the exhaust fan of the roof through a tube of the same diameter. The exhaust inlet opening was equipped with a 300 mm wide and 200 mm high flat plate, centred onto the inlet, thus forming a simple, exterior, flanged exhaust hood (EH). The EH was placed on a wooden table whose dimensions were 900 mm high, 1640 mm wide and 1220 mm deep; see in Fig. 25.
The test room
To perform the relevant trials a test room placed in the building number 45 of the University of Gävle was used. The test room, shown in the Fig. 18, was one of the three rooms in the test building situated in the lab hall; Fig 19. It was the same room as used by Mattsson in his research [13], in order to do a comparative analysis. A drawing of the test room and its surroundings is shown in Fig. 20. The ceiling height in the test room was 3.00 m and its room volume was 54.7 m3. The building envelope of the test building, including inner walls, consisted mainly of wooden boards with 5-10 cm mineral wool insulation in between. The temperature of test room was maintained at about 22 ºC, with a variation of 2ºC due to the heat stemming from lighting, measuring equipment and body heat of the researchers. Except ceiling lighting and measuring equipment there was no heating in the test building.
Table of contents :
1. INTRODUCTION
Local exhaust ventilation (LEV)
Literature review
Scope of the thesis
2. THEORY
Interaction between fluid flow and a cylinder
Percentage of Negative Velocities
3. METHOD
Air velocity measurements
Calibration
The test room
Exhaust device
Positioning cases for the exhaust device
Movable cylinder – turbulence generation
4. RESULTS
Comparison of air turbulence generated by the movable cylinder and a real walking person
Percentage of Negative Velocities
Air stabilization time
Comparison of the PNV between the use of a movable cylinder and a movable plate
5. DISCUSSION
6. CONCLUSIONS
Reference
