Ventilation System Design - A Fluid Dynamical Study with Focus on Demand Control

Doctoral thesis, 2008

Ventilation corresponds to a large part of the total energy use of a building and a lot of attention has been paid to how to reduce this energy use. However, much of this work on saving energy has been focused on reduction of the airflows and how this in turn reduces the energy use. In this study it is shown that lowered airflows does not automatically result in reduced fan energy. Instead this dissertation is focused on design of large and complex ventilation systems to obtain good function and low energy use, both the prerequisites for the design and actual solutions. In this work the energy use has been mostly limited to fan energy and how to reduce the pressure levels and flow resistances in the ventilation system. Another aspect that has not been widely studied is the interaction between demands and ventilation system design. This becomes important when working to optimise ventilation systems, and especially so for demand controlled ventilation systems. Demand control ventilation, DCV, is a concept that normally refers to a ventilation system with variable airflows. In such a system the airflows are reduced when the demands so permits in order to save energy. This study discusses the concept of DCV from a broader perspective and shows how the demands affect the ventilation system design. In systems with variable airflows pressure levels and pressure losses becomes even more important than in conventional systems with constant airflows. This dissertation is based on measurement studies combined with experiences gathered from building services engineers, manufacturers and so on, literature survey and basic fluid dynamics. Based on these results from the measurements simple models were made and used for simulations. Measurement results and calculations were than analyzed for both the air-conditioning and air distribution systems, using all this information some alternative ventilations system designs are presented and discussed. To show the saving potential for the fan energy with these new designs, calculations were done for an existing ventilation system in a hospital. Based on measurements and design data a basic model for this system was created. This showed that for this representative existing ventilation system it would be possible to save 14 % of the presently used fan energy. With improved motor technology of small fans these savings would increase to 32 %.