ASHRAE Journal: Designing Chilled Beams for Thermal ... - TROX

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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. V L ’ T L 1 Beam A V O ’ T O Q S Q A Q A 2 4 5 3 A Beam B V C V O ’ T O Q S H1 V H1 ’ T H1 Designing for Occupant Thermal Comfort Standard 55-2004 2 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 Applications 3 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 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 1 m (0.3 ft) Occupied Zone (Height Determined By Designer) Figure 1: Application of active chilled beams. 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 (ΔT O ) 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 (Q P ) and discharge (Q S ) 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 m 3 /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 O c t o b e r 2 0 0 9 A S H R A E J o u r n a l 5 9
primary airflow rate or 350 cfm (595 m 3 /h). The temperature (T OC ) of the air leaving the beam’s integral cooling coil can be conservatively estimated as 1°F (0.6°C) warmer than its mean chilled water temperature, which is the average of the entering and leaving chilled water temperatures. In fact, the leaving air temperature would typically be at least 2°F to 4°F (1°C to 2°C) higher than the coil mean water temperature. The beam manufacturer has this information as well as the beam’s IR. Upon identifying the primary air temperature (T PA ), the temperature (T Z ) leaving the beam as well as the initial room to supply air temperature difference (ΔT Z ) can be estimated using Equations 1 and 2. T Z = [T PA + (T OC × IR)] / (IR+1) (1) or, for this example, ΔT Z = T ROOM – T Z (2) T Z = [55°F + (60°F × 2.5)] / (2.5 + 1) = 58.6°F and ΔT Z = 75°F – 58.6°F = 16.4°F The initial velocity (V O ) of a supply airstream leaving the discharge slot can be calculated by dividing the supply airflow rate leaving each slot (for two-way beams this is 0.5 x Q S ) by the effective area of that slot. If the effective area is not available, V O can be conservatively estimated as 450 fpm (2.3 m/s) for the purposes of this calculation. The local temperature difference (ΔT X ) between the room and the supply airstream at any point along its travel can be estimated by Equation 3. 4 ΔT X = 0.8 × ΔT Z × (V X / V O ) (3) For a beam with an initial discharge velocity (V O ) of 450 fpm (2.3 m/s) and an initial supply to room temperature differential (ΔT Z ) of 16.4°F (9.1°C), the local temperature differential (ΔT X ) at the point coincident with a 50 fpm (0.25 m/s) terminal velocity is about 1.4°F (0.9°C). Referring to Figure 2, this predicts that less than 20% of the occupants would be dissatisfied with these local velocity/temperature conditions. As the region near the outside walls is not defined as part of the occupied zone, local velocities and temperatures do not generally affect occupant thermal comfort. Care should still be taken that they are not so high that they can affect processes (e.g., fume hoods) along the outer wall and that they are sufficient to provide adequate heating where applicable. Chapter 56 of the 2007 ASHRAE Handbook—HVAC Applications 5 recommends that outlets used for perimeter heating be selected and located such that their isothermal throw to 150 fpm (0.75 m/s) extends at least halfway down the outside surface or to a level 5 ft (1.5 m) above the floor, whichever is greater. The area of greatest draft risk usually occurs directly below the point where two opposing airstreams collide. Figure 1 illustrates such a point and defines the collision velocity as V C . If the velocity (V C ) of the colliding airstreams is of sufficient velocity (greater than 100 fpm or 0.5 m/s), some of the velocity and temperature differential of the airstreams is dissipated by the collision and the velocity (V H1 ) at the point where the airstream Figure 2: Percentage of occupants objecting to drafts in air. 3 enters the occupied zone is reduced accordingly. Figure 3 can be used to estimate the velocity at the entry point for various collision velocities and vertical distances (H1) between the point of collision and the top of the occupied zone. This figure illustrates that the velocity (V H1 ) entering the occupied zone directly below the point of collision of two airstreams will be less than half the collision velocity (V C ) if distance H1 (the distance between the ceiling and the top of the designated occupied zone) is greater than or equal to 3.5 ft (1.1 m). In cases where the collision velocity (V C ) is 100 fpm (0.5 m/s) V H1 would be less than or equal to 50 fpm provided H1 is greater than 3.5 ft (1.1 m). In cases where H1 is greater than 3.5 ft (1.1 m), active chilled beams should be sized and located so that their throw to a terminal velocity of 100 fpm (0.5 m/s) does not exceed half the distance to another beam with an opposing blow. In cases where the collision velocity (V C ) is 150 fpm (0.75 m/s), V H1 is less than or equal to 50 fpm provided H1 is greater than 6 ft (1.8 m). Active beams for which H1 is greater than 6 0 A S H R A E J o u r n a l a s h r a e . o r g O c t o b e r 2 0 0 9
Page 1: This article was published in ASHRA
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ASHRAE Journal: Designing Chilled Beams for Thermal ... - TROX

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