Air and Contaminant Movement between Building Zones

To evaluate air and contaminant movement within the building, the following classification of building zones is used:

• Building areas separated by physical walls (e. g., halls, rooms, booths) located on the same level. The wall has either intentional apertures or leaks (Fig. 7.107a).

• Building areas separated by physical walls located on different levels (floors). Air movement between these zones may occur (e. g., through stairways or ducts) (Fig. 7.107ft).

• Building areas separated by air jets located on the same level (e. g., jet — assisted hoods, air curtains) (Fig. 7.107c).

• Building areas within the same room (with no physical partitions) having different requirements for air cleanliness (“clean” and “dirty" areas) located on the same level (Fig. 7.107^).

(a)

Air and Contaminant Movement between Building Zones

Air and Contaminant Movement between Building Zones Air and Contaminant Movement between Building Zones(r)

Air and Contaminant Movement between Building Zones

Id)

Air and Contaminant Movement between Building Zones

FIGURE 7.107 Zones within a building: (a) located on the same level and separated by physical walls with apercures; (b) located on different levels and separated by physical walls with apertures; (c) located on the same level and separated by air jets; (d) located on the same level with no physical separation; (e) located in the same room on different levels without physical separation.

• Static pressure difference between two zones resulting from the unbalanced air supply and exhaust in each zone. Air and contaminants move from the zone with higher static pressure to the zone with lower static pressure (Fig. 7.108(3).

• Static pressure difference between two zones resulting from the wind effect on the building envelope (Fig. 7.1086).

• Buoyancy forces creating vertical air movement along the passage between two rooms located on different levels, or thermal plumes creating temperature and contaminant differences between two zones located on different levels of the same room (Fig. 7.108c).

• Turbulent exchange between air in different zones due to energy introduced by supply air jets, convective currents, or moving objects. In this case the resulting mass transferred between the zones equals 0 (Fig.

7.108[5]/).

To control air and contaminant movement between zones, different con­struction, process-related, and ventilation techniques are used. “Clean” and “dirty” areas can be separated using solid walls, curtains, or partitions (Fig. 7.109a). Ventilation techniques used to separate zones include

• Pressure management in different zones (Fig. 7.1096)

• Air oasis with specially organized local air supply and exhaust (Fig.

7.109c) "

• Natural or displacement ventilation systems creating temperature stratification (Fig. 7A09d)

• Air curtains and jet-assisted hoods separating “dirty” zones from “clean” zones (Fig. 7.109c)

Combined construction, process-related, and ventilation measures include “air locks” between two zones (Fig. 7.109/) and process equipment enclosures with air exhaust from the enclosures (Fig. 7.109g).

For pressure management between two zones, refer to the information provided in Section 7.8.2. It is important to mention, that the airflow created by pressure difference between the “clean” and “dirty” zones does not com­pletely prevent contaminant movement from the dirty to the clean zone. Let’s consider an example of a two-bay building with an air supply into the high “clean” bay and an air exhaust from the low “dirty” bay (Fig. 7.110a). The high bay has a higher air temperature than the low bay. The stack effect cre­ated between the two bays creates contaminant movement from the dirty zone to the clean zone through leaks and other openings in the walls separating these zones. In order to prevent contaminant movement between the clean and dirty zones, these zones can be separated by an air lock (Fig. 7.1106).

Another factor influencing contaminant and heat transfer from dirty to clean zones against the stable airflow is a turbulent exchange between these zones. This process should be considered in the design of displacement or nat­ural ventilation systems and evaluation of the emission rate of contaminants from the encapsulated process equipment (Fig. 7.111a).

The mechanism of turbulent heat exchange between the upper and lower zones in the case of ventilation system design with temperature stratification is described in Section 7.3.

(«>

Qcxhl

I +

Qsupl QcxJil

‘t

Air and Contaminant Movement between Building Zones

Qexhi

Qьup2 < Qexh2

 

6sup2

подпись: 6sup2

Qsupl

подпись: qsuplI

Air and Contaminant Movement between Building Zones

(b)

Upwind

,<P:„

Air and Contaminant Movement between Building Zones

Air and Contaminant Movement between Building ZonesFIGURE 7.108 Mechanisms of contaminant movement between zones: (o) difference in static pressure resulting from unbalanced air supply and return; (b) difference in static pressure resulting from wind effect; (c) buoyancy forces create vertical air movement along the passage between zones located on different levels; id) turbulent exchange between air in different zones due to energy introduced by supply air jets, moving objects, etc.

Air and Contaminant Movement between Building Zones

—————————— 1

Tп

N

(a)

C 2

* G.-P ■

C. T■ ^

— -1

C1>C1

C,.,

подпись: c,.,

R,<r2

подпись: r,<r2

G„

подпись: g„»hi

T,

b)

подпись: b)C,> c2

Jcxhl

C. T,

&

W

VJj * |

Giupl

1

■ r-

■ G, upi

Air lock

T, <T,

подпись: t, <t,

Gin(

подпись: gin(

FIGURE 7.110 (o) Contaminant movement between two building zones with different ceiling heights due

To the combined effect of unbalanced mechanical systems and the "stack" effect, (b) Prevention of contaminant movement between two zones using an "air lock."

The effect of turbulent exchange between the contaminated air in the pro­cess equipment enclosure and the room air (Fig. 7.11 lb) is described by Elter — man. According to Elterman, the air velocity in the process equipment enclosure opening assuring contaminant concentration C; at the distance / from the opening can be calculated from

Vcxh = 4in^-c~

Air and Contaminant Movement between Building Zones

(a)

1

< ———- —

K, c C1

► c f:i

1 c,

Air and Contaminant Movement between Building Zones

FIGURE 7.1 II Contaminant and heat transfer due to turbulent exchange between building zones: (a) contaminant movement against the airflow near the vicinity of local exhaust; (b) heat and contaminant transfer between the lower and upper zones of the building with displacement ventilation

Where CQ = contaminant concentration under the enclosure, C„ = contami­nant concentration in the room at the distant point, C( = contaminant con­centration at the point located distance L from the enclosure opening, and A = turbulent exchange coefficient.

Among the most important factors affecting the coefficient A value (see Section 7.3.) are the characteristic enclosure opening size and the air turbu­lence level under the enclosure. To decrease turbulent exchange with the room air, one can decrease the characteristic opening size (e. g., by inserting a flow equalizer) or install a spigot into the enclosure opening


Posted in INDUSTRIAL VENTILATION DESIGN GUIDEBOOK