ABSTRACT
This chapter deals with all-air systems, which are the most popular secondary system type in the United States. We first describe the working of single-duct single-zone HVAC systems along with how to size and analyze such systems for cooling and heating design conditions and how to draw the process diagrams on a psychrometric chart. Subsequently, we discuss system layout, working principles, reasons for evolution, advantages, and areas of applicability of the following generic all-air systems for single-zone and multizone buildings: single-duct constant air volume (CAV) and variable air volume (VAV) systems, dual-duct CAV systems, and fan-powered VAV systems. The operating conditions and the reasons why simultaneous heating and cooling (called coil bucking) occur in CAV and VAV systems are discussed. We then use solved examples to analyze part-load performance of the various all-air systems and illustrate the use of the bin method to estimate annual heating, cooling, and fan energy of secondary systems coupled with primary systems (such as chillers and boilers). Next, the energy efficiency benefits of using economizer cycles and energy recovery devices as well as the implementation of different operating and control strategies (such as deck reset, discriminating temperature controls, and critical zone reset) are described. Finally, we discuss two efficiency metrics that quantify energy penalties due to coil bucking.
