ASHRAE Journal - October 2009 - 59

The sensible cooling coil within the beam is supplied with chilled water whose supply temperature is maintained at, or above, the space dew-point temperature to prevent condensation. The sensible heat removed by the coil typically constitutes 50% to 75% of the required space sensible heat removal. As a result, the primary airflow rate required to accomplish the space sensible cooling can be reduced accordingly. Although primary (ducted) airflow rates associated with chilled beams are considerably lower than those in all-air systems, their discharge airflow rate to the room is always greater. Since the chilled water supplied to the beam is maintained above the space dew point, the beam’s off-coil temperature will be higher than the primary air temperatures used in all-air systems. The resultant temperature of the beam’s discharge mixture is typically 3°F to 6°F (2°C to 3.3°C) warmer than that of all-air systems. Therefore, a proportionally higher (20% to 30%) discharge airflow rate to the space must be provided. This higher discharge flow rate often contributes to greater draft risks, which may compromise occupant thermal comfort levels. Designing for Occupant Thermal Comfort 1 2 4 5 3 A Beam A QA QA Beam B VO TO ’ QS VC VO TO ’ QS H1 VL TL ’ VH1 TH1 ’ Standard 55-20042 defines the occupied zone as the portion of a space where occupants normally reside. It is further quantified as the volume of the room that is (1) no closer than 3.3 ft (1 m) from any outside walls or windows nor within 1 ft (0.3 m) of any internal wall and (2) is vertically bounded by the floor and the head level of the predominant space occupants. Although the head level is often accepted to be 67 in. (1.7 m) for standing occupants, the standard allows the designer to define that height according to the space occupancy. For example, if a space is predominantly occupied by seated persons, the occupied zone height could be considered as 42 in. (1.1 m). Chapter 20 of the 2007 ASHRAE Handbook— HVAC Applications3 predicts the percentage of occupants who might express thermal dissatisfaction for various combinations of local air speed and temperatures. Figure 2 (from that chapter) can be used to predict the percentage of occupants that will object to various air speeds and temperatures at the neck and ankle regions. As active chilled beams are normally mounted overhead, the neck region is usually the most critical. Comfort cooling applications should strive to minimize dissatisfaction levels, and in all cases limit the percentage of occupants objecting to these local conditions to 20% or less. Room Air Distribution 1m (0.3 ft) Occupied Zone (Height Determined By Designer) Figure 1: Application of active chilled beams. Active chilled beams distribute air within the room in a manner consistent with that of linear slot diffusers. As such, relationships between airstream terminal velocities and thermal decay of the supply airstream that apply to linear slot diffusers also apply to active chilled beams. Upon discharge to the open space, velocity and temperature differentials between the supply air mixture and the room begin to diminish due to room air entrainment. Linear slot diffusers exhibit relatively long throw characteristics and their velocity and temperature differentials October 2009 diminish at a rate that is proportional to the distance the air has traveled within the space. Manufacturers publish throw values that allow designers to estimate the travel distance of the airstream before it reaches a given terminal velocity. Most manufacturers present such data using isothermal air for terminal velocities of 150, 100 and 50 fpm (0.75, 0.5 and 0.25 m/s). These data can be used to map the airstream and predict the local velocity at the point where it enters the occupied zone. As the room-to-supply-air-differential decays at a similar rate, its temperature also can be predicted at the entry point based on the initial temperature difference (ΔTO) between the beam discharge temperature and that of the room into which it is introduced. Manufacturers supply selection software that can be used predict the value of local velocities and temperatures at critical locations such as that where the airstream enters the occupied zone. Figure 1 illustrates a space being served by two active beams with two-way discharge patterns delivering identical primary (QP) and discharge (QS) airflow rates. The discharge airflow rate is a function of the induction ratio of the nozzles chosen and is calculated by multiplying the primary airflow rate by the induction ratio. Assume a beam produces an induction ratio of 2.5 and is sized to deliver 100 cfm (170 m3/h) of 55°F (13°C) primary air to a 75°F (24°C) room. Also, assume that chilled water enters the beam at 57°F (14°C) and leaves at 61°F (16°C). The discharge airflow rate to the space will be 3.5 times the ASHRAE Journal 59

ASHRAE Journal - October 2009

Table of Contents for the Digital Edition of ASHRAE Journal - October 2009