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or equal to 6 ft (1.8 m) can be sized such that their throw to a terminal velocity of 150 fpm (0.75 m/s) does not exceed half the distance to a beam with an opposing discharge. Figure 3 can be used to determine the maximum collision velocity that limits velocities within the occupied zone to 50 fpm (0.25 m/s) or less for any given distance H1 using the following equation: V C = 50 / (V H1 / V C ) (4) where V H1 / V C is the value from Figure 3 that corresponds to the distance H1. For example, if H1 is equal to 4 ft, V C = 50 / 0.43 = 116 fpm Room Humidity Design Considerations The sensible cooling contribution with chilled beams affords the designer an opportunity to significantly reduce the primary airflow rate compared to all air systems. As 50% to 75% of the sensible heat gains are typically removed by the chilled water coil, proportional reductions in the primary airflow rates within the system may be achievable. However, this should be done with caution as the beam must also deliver sufficient ventilation air and maintain acceptable space humidity levels. The primary airflow rate to the room must be the greater of that required to (1) ventilate the space in conformance to ASHRAE Standard 62.1-2007 6 (or other applicable ventilation codes); (2) offset space latent gains to control the room humidity level within ASHRAE Standard 55-2004 recommendations; and (3) provide sufficient sensible cooling to complement the sensible heat removed by the chilled water coil. In most common interior space applications, the primary airflow rate required to offset space latent gains will exceed both the ventilation airflow rate and the airflow rate required to complement the coil’s sensible cooling. The space airflow rate will be determined by the latent gains and the design room humidity ratio (W ROOM ). In perimeter spaces, the primary airflow rate to the beams will be driven by the sensible load (laboratories may be exceptions due to their high mandated ventilation rates). The use of beams whose water side cooling capacity contributes to more than about 65% of the total space load may be impractical due to architectural constraints that limit the installed beam quantities and lengths in these spaces. The goal of the design should be to reduce the primary airflow rate to as close to the required ventilation rate as possible. Designing chilled beam systems to maintain room air humidity levels lower than necessary can result in considerably higher primary air requirements. Figure 4 is presented in Standard 55-2004 7 and prescribes acceptable ranges of room temperatures and humidity ratios. Assuming a clothing level of 1.0 clo (0.15 m 2 · k/W), this diagram defines the thermal comfort window for a dry-bulb temperature of 75°F (24°C) to include room dew-point temperatures as high as 62°F Distance H1 (ft) 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 0.3 0.4 0.5 0.6 0.7 V H1 /V C Figure 3: Velocities entering the occupied zone. Figure 4: ASHRAE Summer and Winter Comfort Zones. Acceptable ranges of operative temperature and humidity with air speed ≤ 40 fpm (≤0.20 m/s) for people wearing 1.0 and 0.5 clo (0.15 m 2 · k/W and 0.08 m 2 · k/W) clothing during primarily sedentary activity (≤1.1 met [≤63.9 W/m 2 ]). 8 (16.7°C). Where chilled beams are applied, the room dewpoint temperature must not exceed the chilled water supply temperature, so design for dew points above about 57°F (14°C) is not recommended. Most conventional HVAC systems condition delivered air to about a 52°F (11°C) dew-point temperature, which coincides with a humidity ratio (W PRIMARY ) of 58 grains (3.8 g). The airflow rate (Q PRIMARY ) to offset the space latent heat gains (q LATENT ) can be determined by Equation 5. Q PRIMARY = q LATENT / [0.68 × (W ROOM – W PRIMARY )] (5) Using this equation and assuming the humidity ratio of the primary air is 58 grains (3.8 g), the primary airflow requirement 6 2 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
for a space whose latent gains total 400 Btu/h (117 W) can be calculated for various design humidity ratios: If W R OOM = 65 grains (50% RH) → Q PRIMARY = 84 cfm If W R OOM = 68 grains (52% RH) → Q PRIMARY = 59 cfm If W R OOM = 69 grains (53% RH) → Q PRIMARY = 53 cfm In this case, designing for 75°F (24°C) and 50% relative humidity would result in a primary airflow rate that is 58% higher than that required to maintain 53% RH in the space. As the 53% RH is within Standard 55-2004 recommendations and results in a space dew-point temperature of 57°F (14°C), it is probably a reasonable design goal. Chilled beams are often used with central HVAC equipment that includes heat recovery and enthalpy wheels. Lower space dew-point temperatures can be achieved when the dew-point temperature is further suppressed by these processes. In cases where room dew-point temperatures below 55°F (13°C) are desired, using such equipment is recommended. Summary Active chilled beams can be selected to remove large amounts of sensible heat while substantially reducing primary airflow requirements. However, this must be done with consideration Advertisement formerly in this space. of the occupant thermal comfort and space dehumidification. Because producing high levels of thermal comfort is the primary objective of any comfort cooling application, beams should be selected, sized and located with that in mind. While primary airflow reduction opportunities are an inherent characteristic of chilled beams, the reduction of such should be limited to that required to provide adequate space humidity control. All-air systems almost always deliver a sufficient amount of dry air to satisfy the space sensible load, therefore, engineers often do not consider space latent loads in their selection. Individual space latent loads should be considered when designing chilled beam systems. In conclusion, the following design guidelines should be observed when selecting, sizing and locating active chilled beams: • Chilled beams should not be used in low ceiling height applications where the distance between the ceiling and the top of the occupied zone is less than 3 ft (0.9 m). • When applied in lobbies, atriums or other areas with high and/or uncontrollable infiltration rates, provide adequate condensation prevention strategies. • To maintain high levels of thermal comfort (velocities within the occupied zone no greater than 50 fpm or [0.25 m/s]), active chilled beams were mounted at least 3.5 ft (1.1 m) above the designated occupied zone should be sized and located such that their throw to a terminal velocity of 100 fpm (0.5 m/s) does not exceed half the distance between them and another beam with an opposing blow. Active beams mounted 6 ft (2 m)or more above the designated occupied zone may be located such that their throw to a terminal velocity of 150 fpm (0.75 m/s) is as much as half the distance between the beam and an adjacent beam with an opposing discharge. • Smaller nozzles result in higher induction ratios and higher sensible cooling capacities per cfm (m 3 /h) of primary air. However, the use of smaller nozzles generally results in higher noise levels and inlet pressure requirements for a given primary airflow rate that increases the number of beams required. • Designing for space humidity levels lower than that actually required may result in significantly higher primary airflow rates. References 1. ANSI/ASHRAE Standard 55-2004, Thermal Environmental Conditions for Human Occupancy. 2. Standard 55-2004, p. 3. 3. 2009 ASHRAE Handbook—Fundamentals, p. 20.13. 4. Koestal, A. 1954. “Computing temperature and velocities in vertical jets of hot or cold air.” ASHVE Transactions 60:385. 5. 2007 ASHRAE Handbook—HVAC Applications, p. 56.4. 6. ANSI/ASHRAE Standard 62.1-2004, Ventilation for Acceptable Indoor Air Quality, Table 6-1. 7. Standard 55-2004, p. 5.2.1.1. 8. 2009 ASHRAE Handbook—Fundamentals, p. 9.12. 6 4 A S H R A E J o u r n a l O c t o b e r 2 0 0 9
Page 1 and 2: This article was published in ASHRA
Page 3: primary airflow rate or 350 cfm (59

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