ROOM AIR CONDITIONING STRATEGIES
Traditionally the room air conditioning classification has been based on room air distribution methods. The most used division has been the division into mixing and displacement. ASHRAE classifies air distribution (diffusion) methods into mixing systems, displacement ventilation, unidirectional airflow ventilation, and localized ventilation methods.1 Tapola et al. made a division between mixing and displacement and additionally divided displacement into submethods: thermal displacement, piston displacement, and mixing displacement.2 Etheridge and Sandberg suggested that air distribution methods be classified as jet controlled or thermally controlled, which raises the important question of how well the room airflow patterns are controlled by the air distri bution method.3
The direct application of air distribution methods to describe the strategies has led to the wild usage of different terms with unclear definitions. Additionally, in some cases the same term has been used to describe both the air distribution method and air supply devices or in some cases even the whole air conditioning system. Using a wrong term can also lead to a complete misunderstanding of the physical phenomenon in the room. For example, the term “displacement” is currently used for the room air distribution method in which room airflows are primarily driven by the buoyancy of the thermal sources inside the room and not by the supply air that is introduced to replace (substitute) the air removed by the sources in order to prevent the return flow back to the occupied zone. In this case, the term “replace” (substitute) gives a correct picture of the phenomenon, whereas “displace” may mislead the user to believe that the flow field is created primarily by the air distribution method. The results of this inconsistency can also be seen in the presentations of experts in scientific conferences, but much more confusion is caused in the everyday construction business, where the customer often doesn’t have any expertise in our technological field.
German guidelines base the division on the resulting airflow pattern within the room rather than distribution methods.4 They suggest that airflow patterns be divided into four categories: hall-filling mixed flow; zonewise mixed flow; low-momentum, low-turbulence flow for the air supply in the work region; and zonewise displacement ventilation.
This chapter describes a strategy approach for room air conditioning based on the classification and terminology presented by Hagstrom et al.5 The basis
Of the classification is different aims or ideas of the temperature, gas, particle or humidity distributions, and airflow patterns that can be created within the room. The distributions are often described by using contaminant removal and temperature effectiveness coefficients, which are defined in Section 8.5.
The aim of this classification is not to value one strategy over another. They all have their advantages and disadvantages and it is up to the designer to select the most desirable strategy for each case. In practice a certain ty pe of room air conditioning strategy can be applied by using different kinds of air distribution installations and air supply devices. How well the real situation will fulfill the aim of the ideal strategy depends not only on the physical installation itself, but also on the operating parameters as well as the characteristics of other internal sources that influence the supply airflow patterns and the room airflows, such as heat and contaminant sources, cold drafts, and room heating and cooling methods. It is therefore important to separate ideal strategies from practical room air conditioning solutions.
A clear classification of the ideal strategies will help the evaluation of the present room air distribution methods in different operating conditions. It also creates a solid base for the development and promotion of new innovations in the field.
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