ASHRAE Journal - September 2011 - 39

Figure 1: Classification of air-distribution strategies (from 2009 ASHRAE Handbook—Fundamentals). generated by the outlets and the spacing of the outlets. It is fairly common to see outlets spaced too close together and the downward draft that is generated. Also, it is common to see outlets placed too close to an architectural feature that protrudes below the ceiling, resulting in the horizontal jet being forced down into the space and causing drafts for the occupants. Several methods described below can be used to avoid high velocity jets and the resulting drafts when selecting outlets for an air-distribution system. These methods can be implemented without adding a significant amount of design time for a project. Designing to control room air motion will lead to higher levels of comfort and a more productive environment. The latest version of ASHRAE Standard 55-2010 allows for elevated air speeds “to be used to increase the maximum operative temperature for acceptability under certain conditions.” These temperatures and velocities are shown in the standard’s Figure 5.2.3.1. This article covers air outlet selection where the desired temperature range is 73°F to 77°F (23°C to 25°C).

Effect of Velocity on Comfort
Figure 2 shows the effect of room air velocity on the perception of comfort. The figure also shows the neck region is more sensitive to air motion then the ankle region. As you can see from the graph, in the neck region, the percentage of complaints are significant at and above 50 fpm (0.25 m/s) velocity. In many cases, the perception of a draft is not because of air temperature, but rather the velocity of air around the occupants. This can be demonstrated by the use of a ceiling fan that is turned on to provide cooling, even though the air being circulated is at room temperature.

Defining a Fully Mixed System
ASHRAE Handbook—Fundamentals, Chapter 20, Space Air Diffusion, defines a fully mixed system as the following: “In mixed-air systems, high-velocity supply jets from air outlets maintain comfort by mixing room air with supply air. This air mixing, heat transfer and resultant velocity reduction should occur outside the occupied zone. Occupant comfort is maintained not directly by motion of air from the outlets, but from secondary air motion that results from mixing in the unoccupied zone. Comfort is maximized when uniform temperature distribution and room air velocities of less than 50 fpm (0.25 m/s) are maintained in the occupied zone.” A fully mixed system ideally maintains a constant temperature gradient from the floor to the top of the occupied zone. Unlike displacement systems or underfloor systems, the fully mixed system maintains a uniform temperature through dilution of the space air with the air supplied into the space (Figure 1). Typically, a forced air system is designed to meet the following performance parameters: 73°F to 77°F (23°C to 25°C) with humidity at 25% to 60% and air velocity in the occupied zone of less than 50 fpm (0.25 m/s).
September 2011

Outlet Selection
When designing a fully mixed system for comfort, it is important to define the occupied and unoccupied zone for a space. The occupied zone is the space we live and work in. It is typically the volume from the floor up to a height of 6 ft (1.8 m) and 6 in. to 1 ft (152 mm to 305 mm) from the walls. The occupied zone depends on the specific space and use of that space. We can use the unoccupied zone to deliver all the high velocity supply air and do all the mixing. The tool that can be used by a designer to predict the location of a discharge jet is listed as “throw” in a manufacturer’s catalog. The current standard for outlet testing is ASHRAE Standard 70-2006 (RA 2011), Method of Testing the Performance of Air Outlets and Air Inlets, which defines how throw data is obtained and allows the testing of both isothermal and non-isothermal air. Most of the manufacturer’s throw data is, in fact, isothermal air. The reason for this is the data is easier to obtain and is reASHRAE Journal 39

ASHRAE Journal - September 2011

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