By Aeration one understands the natural lift of warmed up air masses, an effect, which is used for the removal of also large amounts of heat from buildings and which natural ventilation or nature course ventilation is then called.

The lift (difference of pressure) is calculated in principle by the density variation of the lift height h resulting from the temperature difference the available.

[kg/ms

= density of cold air [kg/m

= density of warm air [kg/m

h = lift height [m]

g = acceleration due to gravity [9.81 m/s

This pressure resulting from the temperature difference can be technically used to an appropriate air agitation without mechanical air delivery by fans. The difference of pressure must be thereby always equivalent that necessarily to overcoming resistances. Resistances result from flow losses in the case of flowing through the necessary and exhaust air devices (shutters or the like). Resistances exist also by constrictions and turbulences when flowing through perfectly openly (free) winding and roof break-throughs.

Computation of the erforderderlichen and exhaust air surfaces:

= withdrawal surface

AE = entrance surface

= coefficient of drag withdrawal surface

= coefficient of drag entrance surface

t2 = temperature zone 2

T1 = temperature zone 1

TA = outside temperature

h = effective lift height

z = boundary layer height

Q = heat source

Lift by difference of the Luftemperaturen:

[N/m

without of influences by wind pressure

necessary flow rate: V is calculated by the supplied heat energy and the permissible exhaust air temperature

V =

V [m /s] (flow rate)

Q [W] (supplied amount of heat, d.s. mostly the internal calorific losses)

cp [kJ/kg"°K] (specific warmth of air ~ 1)

[kg/m (rho, density of air)

t ["°K]

Resistances when flowing through the ventilation shutter: = and =

the coefficients of drag are dependent on the respective equipment and by the manufacturer are measured and as cv-value communicated. Typical attainable cv-values lie between 0,40 and 0,80

wA the flow rate is in the diverting cross section undwE the flow rate in the entrance cross section

Hereunder applies:

=

Seepage pressure losses

With stationary conditions the seepage pressure losses must be equal to the available lift.

A so-called equivalent cross section and of the exhaust air surfaces is calculated:

Agl =

and thus the boundary layer height:

z =

The boundary layer height z is the substantial characteristic in the practical interpretation of a plant and must be always so highly selected that the jobs are clearly below this layer. If that is not reached, and/or the exhaust air surfaces must be The situation of the boundary layer (neutral zone) can be affected by an adapted dimensioning and of the exhaust air surfaces.

Above the boundary layer (opposite the building external pressure) positive pressure exists, below the boundary layer Unterdruck.Es flows thus by all openings below the boundary layer air into the building, while this escapes above the boundary layer.

In well laid out plants a temperature distribution adjusts itself in resounding, which from the soil to the boundary layer in approximately linear rises (if one of the temperature distributions in the direct flowing in zones foresees), and then above the boundary layer an air cushion with almost same temperature forms.

Outside temperature = + 11 "°C

cloudily, weakly windy

An exception form thereby for a resounding, in which very hot surfaces amount of heat over radiation deliver a high, which warms up the highest air layer again clearly then as ceiling heating, without thereby the lift substantially to increase (the effective lift height for this air layer is naturally only more very small).

Ideal anyhow the situation of the boundary layer is in 2/3 of the resounding height, whereby in approximately equal large and exhaust air surfaces are calculated.

Types of device:

In practice several different types of device are used, which differ constructionally particularly by the different overall heights. The cultivation of sound damming mechanisms will consider the devices usually with electrical or pneumatic drives regulated, often from the authorities often prescribed and is (cv-value) to become range shiftings, use both and heat exhausting installation and for removal of smoke in the case of fire (RWA plant). The devices must have test certificates after DIN 18232.

Labyrinth exhaust:

Advantage: small overall height, cultivation of muffler easily realizable

Disadvantage: bad efficiency, cv-values < 0,50

Equipment is suitable for and exhaust air

Shutter:

Advantage: small overall height (- deep)

Disadvantage: middle efficiency, cv-values > 0,60

Equipment is suitable for supply air, exhaust air conditionally because of missing rain security

often in combination with other types of device one uses

Execution variants with sound damming insert with smaller effect degree

Wind conducting surface exhaust:

Advantage: good efficiency, cv-values 0.60 to 0.80, rainsafe exhaust possible

Disadvantage: large overall height

Exhaust air equipment, which is used into many building variants, the efficiency of this equipment often rises with correct arrangement and wind over 1,00

There are further numerous mixing variants o.a. of the Grundtypen, usually with the goal of the overall height reduction with simultaneous rain security.

Reference work

Dr. - engineer Lothar the TZE, building information, GDR, 1979, natural ventilation of warm-intensive enterprises

To Dr.W.Schneider, Dipl.Ing.E.Zauner, DO Vienna, 1981, computation of the resounding ventilation by natural convection

Calculation sheet, down load

• Kunicky/Matscheko, Linz 1980, computation of the natural ventilation

Left to manufacturers

• Rogal

• RWA

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