Stratification Strategy
A similar temperature and contaminant distribution throughout the room is reached with stratification as with a piston. The driving forces of the two strategies are, however, completely different and the distribution of parameters is in practice different. Typical schemes for the vertical distribution of temperature and contaminants are presented in Fig. 8.11.10 While in the piston strategy the uniform flow pattern is created by the supply air, in stratification it is caused only by the density differences inside the room, i. e., the room airflows are controlled by the buoyancy’ forces. As a result, the contaminant removal and temperature effectiveness are more modest than with the piston air conditioning strategy.
8.6.4.2 Advantages and Disadvantages
Advantages include: low concentration in the ventilated zone can be achieved and relatively high contaminant removal and temperature effectiveness. However, the stratification strategy is sensitive to disturbances and stagnant areas with high
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FIGURE 8.1 I Vertical temperature and contaminant distribution within the stratification strategy, typical schemes.3 TQ is supply air temperature, Tr is the temperature at the floor level, TK is room temperature, C0 and CR are contaminant concentrations in supply and room air above the stratification height, and yst is the stratification height
Local concentrations are possible. It also only functions properly when conditions are favorable.
In the stratification strategy the supply air is used to substitute the outgoing air from the ventilated (in most cases occupied) zone, thus preventing circulation patterns between the zones. The supply air has to be distributed in such a way that the buoyancy flows are not disturbed. Exhaust air openings are to be located “downstream” in order to avoid reverse currents within the room. The location of the contaminant sources and the heat sources causing density differences must be the same in order to carry out the contaminants with equal or higher density than air.
8.6.4.4 Application
Stratification is a desirable strategy to provide efficient room air conditioning with much less effort than using the piston strategy. Its main application in room air conditioning is the thermal replacement method. However, it can also be applied for contaminants without any thermal buoyancy sources that have different density from the room air. Examples of different air distribution methods to create thermal replacement are presented in Fig. 8.12.
However, because of its physical nature—ventilating air having very little authority over the room airflow—the stratification strategy is very dependent on the stability of the density differences and the airflow balances and thus very sensitive to disturbances within the room. Also, the selection ot the room cooling and heating method can either aid or prevent the creation of the stratification strategy as described earlier in the chapter.
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