Controlled Airflow through an Envelope: Principles of Natural Ventilation

Natural ventilation is the controlled flow of air through doors, windows, vents, and other purposely provided openings caused by stack effect and wind pressure. Natural ventilation is used in spaces with a significant heat release, when process and hygienic requirements for indoor air quality allow outdoor air supply without filtration and treatment. Natural ventilation cannot be used when incoming out­door air causes mist or condensation. Natural ventilation allows significant air change rates (20 to 50 ach) for heat removal with minimal operation costs.

Though airflow through the building with natural ventilation is caused by both wind effect and buoyancy forces, design principles typically do not in­clude a wind pressure component. Wind speed and direction can change over wide ranges and thus wind does not provide a stable force to move air through the building. Thus, the wind effect is frequently considered as a re­serve. In general, maximum air change in the building is required in the sum­mer, with a maximum heat load and a minimal temperature difference between outdoor and indoor air. With a change in the heat load, natural venti­lation allows for self-adjusting airflow through the building.

Airflow through an opening used for natural building ventilation is ap­proximately turbulent. In this case the flow rate, Q (m3/s), can be calculated from an equation similar to Eq. (7.237):

Q=CdA(Ap/p)°s, (7.253)

Where

Cd — pressure loss coefficient of opening A — area of the opening, m2 Ap = pressure difference across the opening. Pa p = air density, kg/m3

Air outlets are designed such that their pressure characteristics are negative, which improves their performance in the presence of wind. Different types of ridge vents and roof ventilators have been designed; their sizes and pressure loss coeffi­cients are available from the manufacturers. Examples of some air inlets and their pressure loss coefficients are listed in Table 7.34. All inlets and outlets for natural ventilation must be supplied with controls for easy opening and closing. Ventilated interior halls can obtain outside air through ridge vents in adjacent “’cold” halls.

In summer, air inlets for natural ventilation are located in exterior walls; the lower level of the openings is 0.3 to 1.8 m above the floor. Air inlets can be arranged in one, two, or more rows in the longitudinal exterior walls. Win­dows, doorways, and other types of openings in the exterior walls, or aper­tures in floors over basements (with air transportation along special channels) also can be used as air inlets. During periods of the year other than summer, air inlets must located higher than 4 m. These inlets must be supplied with baffles to direct air at an upward angle. Air is evacuated from naturally venti­lated spaces through wind-protected continuous ridge vents, skylights, or round roof ventilators.

TABLE 7.34 Pressure Loss Coefficients for Inlets35

Pressure loss coefficient

Baffle angle, a

Schematic Baffle type

Mb

IS°

30°

45°

60°

90°

1

^ Single .

1’*■■■■■"’ top-hinged

0

30.8

9.2

5.2

3.5

2.6

)

0.5

20.6

6.9

4.0

3.2

2.6

Hxb

1

16.0

5,7

3.7

.3.1

2.6

Wr

0

59.0

13.6

6.6

3.2

2.7

W Sm8le’ ,

1.0

45.3

11.1

5.2

3.2

2.4

>

J^bxb

M.

0.5

30.8

9,8

5.2

3.5

2.4

7/.1* Double,

1.0

14.8

4.9

3.8

3.0

2.4

/a

…….. lop-hinged

h x b

.

W~

Natural ventilation design allows one to size the inlets, Ainl, and outlets, Aoup based on their pressure loss characteristics, Cp, and on the airflow rate, G0, required to maintain the occupied zone within desired limits. The reverse design procedure is commonly used to evaluate the airflow rate through the building given the sizes, characteristics, and locations of inlets and outlets and the heat load and characteristics of heat sources.

The use of a natural ventilation system assumes temperature stratification throughout the room height. Air close to heat sources is heated and rises as a thermal plume (Fig. 7.105). Part of this heated air is evacuated through air outlets in the upper zone, and part of it remains in the upper zone, in the so — called heat cushion. The separation level between the upper and lower zones is

Defined in terms of the equality of Gconv and G„, which are the airflow rate in

Thermal plumes above heat sources and the airflow supplied to the occupied zone, respectively. It is assumed that the air temperature in the lower zone is equal to that in the occupied zone, tm, and that the air temperature in the up­per zone is equal to that of the evacuated air, ?e! ch.

The air exchange rate, G0, required for temperature control in the occu­pied zone can be calculated from the room heat balance equation:

G0 = W[C,/<e(0UI-0(Jj, (7.254)

Where

G0 = air exchange rate, kg/s W = total surplus heat released in the space, kW Cp — specific heat of air, kj/(kg K)

Controlled Airflow through an Envelope: Principles of Natural Ventilation

K9 = coefficient of heat removal efficiency, calculated from the air

Temperatures of the occupied zone, 0O2, the air removed from the upper zone, 0exh, and the outdoor air, fl0:

^9 = (0eXh-eoV(0oz-0o),

Coefficient Kd can be evaluated through measurements in field or on a scaled model. Also, it is possible to predict the JCe value using the method of zone-by-zone heat balances.36 According to this method,

K() = l/{[<p(l — 40 — M/2{[<p(l — 40 — M2/4 + x}1/2}, (7.255)

Where

+= 5Z(x (7-256)

<P = y [(W0 x (1 — 1|0<P)(]/]T [W0( 1 — 4/)] (7.257)

X = aradAOT(0oz-eoul)/Јwo (7.258)

Where 0OZ = occupied zone air temperature, °C; 0out = outside air tempera­ture, °C; Aoz = occupied zone area, m2. A graphical interpretation of Eq, (7.255) is presented in Fig. 7.106.

Typically inlet and outlet locations and heights can be obtained prior to ventilation system design from construction drawings. The static pressure dif­ference across inlets and outlets can be calculated based on the height of the location (Fig. 7.104) and the air density at the respective height:

A P = g(Z-Z1)(p0-pm)-g(Z2-Z)(p0-pexh), (7.259)

Where Zj and Z2 = heights of inlet and outlet centers, m; and Z = separation zone height (typically between 0.4H and 0.7H), with H = ventilated space height, m. For a more detailed calculation of Kt and Z, see Chapter 8 and Stroiizdat.3-5

The temperature of exhausted air can be derived from Eq. (7.246):

Eexh = 0o+W/(CpGo). (7.260)

Based on the static pressure across the inlets, Apinl = (3 Ap, one can calcu­late the required inlet area, j4lnl:

AM = G0/(2p0 Apml/Cp lnl)’/2. (7.261)

Typically, the share of the static pressure across the inlets, (3, is selected to be between 0.1 and 0.4. This allows one to keep a low velocity of airflow through inlets so as not to disturb thermal plumes above heat sources.

The residual static pressure available for outlets, Apexh, and the required outlet area, Aout, can be calculated as

Apexh = AP — APini (7.262)

(7.26.3)

подпись: (7.26.3)^exh//(2pexh Apexh/Cp exh) “•

Y

R

^4

R~

A = 0.3 ^

0.1^ 0.05 ’’V/

I fl A")

W

F

* S —

0

L

0.9 0.8 0.7 0.6 1/K, 0.5 0.4 0.3 0.2

0.2 0.4 0.6

0.8

FIGURE 7.106 Supporting graph for the K, calculation (Eq. 7.255). X. = AOI Atot / (W0 — VVlosses); 0 < I.

In the case where the area of inlets and outlets is given prior to ventilation design, the static pressure across them can be calculated using the following equations:

(7.264)

(7.265)

подпись: (7.264)
(7.265)
Apexh ( Cp exh/2 Pt-xhM ^~JA e^h) Api„i = Ap — Ap, xh.

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