TABLE OF CONTENTS An acceptable thermal environment is achieved through proper selection of cold-air diffusion components. The main cold-air diffusion components to be chosen in a cold-air system are the diffusers and the mixing boxes. In this chapter, information about selection and sizing of diffusers and mixing boxes for cold-air systems is given. The chapter provides a description of the different types of diffusers used to produce cold-air jets followed by a introductory model of jet mixing. Next are guidelines for selecting various candidate cold-air diffusers. The selection of mixing boxes for cold-air systems is also addressed.
Proper selection of cold-air diffusers is accomplished by consideration of the primary characteristics of the cold-air jet. Jet diffusion and mixing is a complex topic in fluid mechanics. Simple models of jet mixing predict the mixing and entrainment of jets. These models have been incorporated into design procedures for diffuser selection. In this chapter a step-by-step design example is given to show how the current diffuser selection procedure given in chapter 31 of the 1993 ASHRAE Handbook Fundamentals (ASHRAE 1993) is extended to cold-air systems.
The three main design parameters for any type of diffuser, including cold-air diffusers, are the room characteristic length, the throw, and the separation distance. These design parameters are discussed in sections 5.3, 5.4, and 5.5. For use in diffuser selection, the length, throw, and separation design parameters are cast into nondimensional ratios, such as the ratio of the throw to the room characteristic length. The design procedure detailed in...
