Room air flow characteristics for different configurations

The supply air temperature and position of the supply air terminal determine the general room air flow pattern. Supply air that is warmer than the room air and that enters the room horizontally, close to the ceiling, tends to remain in a layer close to the ceiling, even after having reached the opposite side of the room. If the supply air is cooler than the room air, it tends to detach from the ceiling and to form a downward plume. Once arrived at the floor, it spreads over the floor. Heat sources such as occupants, floor heated or sun-warmed surfaces then cause the air to rise. The throw and spread of the supply air terminal have only a minor influence, at least in relatively small rooms.

'Warm' or 'cool' air entering the room horizontally close to the floor shows the opposite behav-

If the supply air enters the room with a vertical impulse, mixing is enhanced in cases of 'warm' air entering at ceiling level or 'cool' air entering from floor level.

Where air is extracted, it is of secondary importance. The forming of two air flow regimes is somewhat enhanced when air is extracted at approximately the same height as air is supplied, especially if each occurs on an opposite wall.

Thermal comfort characteristics in the occupant zone

• Vertical temperature gradient. Values above 3 K/m between 0.1 m and 1.1 m are regarded as critical according to EN ISO 7730 (1994).

• Draft risk according to EN ISO 7730 (1994) gives the percentage of dissatisfied persons as a function of room air temperature, air speed and turbulence. Values above 15 per cent are regarded as critical. A draft risk resulting primarily from the air heating is only expected if 'cool' air enters close to the ceiling. The grid shows higher values than the supply air valve (see Figure 11.1.5).

Notes: Supply air temperature = 16°C; room air temperature = 25°C. Source: Dorer and Haas (2003)

Notes: Supply air temperature = 16°C; room air temperature = 25°C. Source: Dorer and Haas (2003)

Figure 11.1.5 Draft risk characteristics: (left) supply air valve, adjustable; (right)

simple grid, not adjustable

Air exchange efficiency

The ideal case for air change efficiency can be described as 'piston flow'. In this case, the air exchange efficiency is equal to one, and the air change time is equal to the nominal time constant. For complete mixing of incoming air and room air, the air exchange efficiency is 0.5 and the air change time is twice the nominal time constant. With displacement ventilation, air exchange efficiencies of 0.6 to 0.7 can be reached.

Measurements at the EMPA as well as in one of the houses of the CEPHEUS project (Schnieders, 2001) show air exchange efficiencies of around 0.5. As expected, the configuration where the room air is extracted via a gap above the door shows the lowest efficiencies in case of heating. There are no significant differences whether the air is supplied via a valve (nozzle) or grid (see Table 11.1.1).

Table 11.1.1 Air exchange efficiency

Supply air valve

Supply

air grid

Supply air temperature (°C)

40

16

40

16

Air exchange efficiency

Gap below door, with internal heat source

0.52

0.51

Gap above door, with internal heat source

0.49

0.51

0.45

0.52

Gap above door, no internal heat source

0.51

0.54

Source: Dorer and Haas (2003)

Source: Dorer and Haas (2003)

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