Thermal Comfort and Cold Air Distribution

Cold air distribution systems supply air at temperatures between 38 deg. F and 51 deg. F. Cold air distribution systems are increasingly attractive when used in conjunction with ice storage systems to shave peak load by shifting the demand to off-peak hours. They also require less operating and capital costs because they use smaller fans, ducts, piping, and pumps. However, an important issue indesign and application of cold air systems is the effect on occupant comfort. There are several techniques and methodologies that practitioners use for evaluation of conventional air distribution systems. Among these is the Air Diffusion Performance Index (ADPI). It is widely used in the U.S. and is referenced in the 1993 ASHRAE Handbook--Fundamentals. However, this technique is based onempirical correlations obtained from tests conducted with conventional systems and it cannot be guaranteed that they will be equally applicable to cold air systems. This study was undertaken, therefore, to extend the existing techniques (especially the ADPI) to applications where cold air distribution systems are utilized. This work presents a critical review of the evolution of the ASPItechnique and offers several recommendations for developing a firm foundation for future room air distribution research. For this work, no new comfort tests were conducted. However, the experimental data of the research conducted by Nevins and Miller were employed for extending the existing ADPI methodology to cold air systems. The tests conducted by Nevins and Miller covered a wide range ofdischarge flow rates and temperature differences for different types of diffusers. A large number of those test conditions could be categorized as cold air conditions. As a first step in extending the existing ADPI to cold air systems, the local velocities in the occupies zone are correlated to the total momentum of the inlet air jet. These correlations can be used for directly relating the ADPIto the supply air momentum. An approach for determining the ADPI of cold air systems is introduced. In this approach, a one-step procedure is adopted where the ADPI is directly linked to the momentum number of the supply air. A set of curves correlating the ADPI to the momentum number of the room/diffuser combination under different loads are presented.