Room Width
The width of the room, B, should be less than or equal to 3H in order to be properly ventilated by one jet. Wider rooms require several jets beside each other, as indicated in Fig. 8.18.
When a jet is confined in a short room, the jet will be deflected by the opposite wall, and the flow pattern will be as indicated in Fig. 8.19. Outside the
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Jet will be a recirculation zone which consists of the air that is entrained into the jet and the air that is exhausted through the outlet. The amount of entrained air is far greater than the exhaust air. The maximum airflow rate in the reverse flow in Fig. 8.19 equals the airflow rate in the jet.
The velocity distribution in the reverse flow is assumed to be uniform in the case of isothermal conditions. In most practical cases, however, there is a
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TABLE 8.10 Decay Coefficients Kf
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Heat surplus in the room, and the supply air is colder than the room air. In this case the velocity will be higher at floor level, as indicated in Fig. 8.19.
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8 7 AIR DISTRIBUTION METHODS AND DIMENSIONING |
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FIGURE 8.18 Wide room. |
Long Rooms
If the room is longer than the length stated in Eq. (8.22), the air jet reaches a point where it decelerates more than in free air, and the reverse flow becomes more dominant. See Fig. 8.20. We define a critical length, xcri, as the
Velocity profile
FIGURE 8.19 Reverse flow in 3 short room.
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T I » — — ‘ i 1 ‘ t — П ‘ ’ t, Ј |
I ‘ > |
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T I V * V |
U — > |
** A [* |
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> |
/ — |
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K . |
-m |
« |
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, |
: |
FIGURE 8.20 Maximum penetrating distance for an air jet
Length where the jet starts getting narrower instead of continuing to widen. Th is critical length is less than the maximum room length defined in Eq. (8.22). Stensaas gives the following estimate for the critical length:-
*8-24)
Equation (8.24) is about 75-90% of the maximum room length for a compound free jet (decay coefficient of 7.0-5.3 in a decay coefficient equation (8.23).
If the room is longer there will be rotating cells as indicated in Fig. 8.21 in isothermal conditions.
If the room has a certain amount of heat surplus, this will lead to thermal stratification. The thermal stratification will attenuate the rotation, and eventually lead to a flow pattern as shown in Fig. 8.22.
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I *— |
‘ ‘ ? *V. |
" V |
‘ f |
R |
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H |
* * rz ‘ V ‘ , J |
> V |
4 * |
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I |
R f P |
T |
/ K |
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I |
■*- |
-• |
— J |
— — |
~ — _ |
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>«< -1-2 H ~H FIGURE 8.21 Multiple rotating cells in a long room: Isothermal conditions. |
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~H |
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II |
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8.7.4.1
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A |
Warm Contaminants
A simple example of displacement ventilation is shown in Fig. 8.29.
Note that the supply air should be at least 1-2 °C colder than the air in the lower part of the room in order to make the supply air layer at floor level. Thus, the ventilation air cannot be used for heating the room.
The dimensioning of displacement ventilation is shown in the example in Section 7.5.4. That given above is an example of displacement ventilation with weak thermal stratification. Even though the stratification is weak, the contamination in the lower, cleaner zone is normally on the order of one-third of the contamination in the upper zone.
Figure 8.30 shows an example of displacement ventilation in a silicon carbide furnace room. The thermal stratification is very strong, as indicated in the graph on the right-hand side of the figure.
8.7.4.2Cold Contaminants
An example of displacement ventilation with cold contaminants is shown in Fig. 8.31. The contaminants, i. e., the fumes from the sewer, are colder than the room air, and thus tend to stay low due to the negative buoyancy.
A good arrangement in this case is to locate the air supply below the ceiling. The supply air should not be colder than the room air, in order to layer below the ceiling. The fresh air will fill the room from above.
In this case, hearing is possible by means of the supply air.
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