ASHRAE Journal - September 2011 - 40

Ankle Region

Neck Region

Air Velocity (fpm)

Temperature Difference (°F)

Air Velocity (fpm)

Temperature Difference (°F)

Figure 2: Difference in perception of drafts between the ankle (left) and neck (right) region and percentage of occupants objecting to drafts in air-conditioned rooms (from 2009 ASHRAE Handbook—Fundamentals). Temperature difference is between local temperature and average room temperature. peatable. Non-isothermal air requires a balanced room, which does require a significant amount of time to conduct testing. areas. Placing returns in stagnant areas will not create air motion to pull room air to the return.

Discharge Jets
Outlets generate a discharge jet. The designer should select the type of outlet and space the outlets so that the maximum velocity in the occupied space does not exceed 50 fpm (0.25 m/s). There are several principles of jet behavior that if used along with the manufacturer’s catalog throw data, the designer can better predict the resulting velocities in the occupied zone. These principles include: • A free jet expands at approximately 22°. The expansion of the jet is due to the induction of air around the jet. • If a jet is directed toward a surface, the jet will want to stay on that surface regardless if the surface is a ceiling, wall or floor. • At some point, a cooled jet projected along a horizontal ceiling service will separate from the surface and drop downwards. The point where the jet leaves the ceiling is defined as the separation point. In a mixed air system, the outlet is an “engine” that produces a discharge jet into the unoccupied zone causing air in the occupied zone to be induced into the supply jet. A typical diffuser induces 20 to 30 times the amount of supply air discharged. This rate of induction is how we maintain comfort in a mixed air system. By discharging jets of air into the unoccupied zone, we create air motion and circulation in the occupied zone. Because of the high rate of induction generated by outlets, the temperature of the supply jet reaches near space temperature within a very short distance from the outlet. Most of the exchange of load from the supply air to the room air occurs within several feet from the outlet. The outlets should be spaced to ensure that the entire occupied zone has some induction and air motion to avoid stagnant
40 ASHRAE Journal

Selection by Noise Criteria
The most common method of outlet selection is by using noise criteria (NC) for a device at the required cfm. The designer determines the outlet type and size by using the required cfm for an outlet and the maximum NC levels desired for a space. The NC targeted values most commonly used are from Table 1 of the 2011 ASHRAE Handbook—HVAC Applications, Chapter 48, Noise and Vibration Control (examples of recommended NC values from the Handbook are shown in Table 1) . Although this method will select devices that meet required sound levels, it does not address the primary requirements of maximizing thermal comfort in the occupied zone. Using this method, the diffusers can be spaced too close together creating a draft with a velocity of more than 50 fpm (0.25 m/s) in the occupied zone. This method can also lead to stagnant zones where there is little or no induction from the space into the supply outlet.

Selection Using Throw Data
Using the published throw data obtained from a manufacturer’s catalog is one method of outlet selection that can be used to predict room air velocity. This information will indicate the performance of the discharge jet once it leaves the outlet and can be used to fairly accurately predict resulting space velocities (Figures 3a and 3b). Throw data obtained per ASHRAE Standard 70-2006 is typically given in a manufacturer’s catalog at 50, 100 and 150 fpm (0.25, 0.51 and 0.76 m/s) (Figure 4). Throw is defined as the distance, in ft (m), from the center of the outlet perpendicular to a point in the mixed airstream where the velocity has been reduced to a specified terminal velocity. In most cases, throw data is based on isothermal air. If either cooled or heated air is
ashrae.org September 2011

ASHRAE Journal - September 2011

Table of Contents for the Digital Edition of ASHRAE Journal - September 2011