ASHRAE Journal - October 2009 - 64

for a space whose latent gains total 400 Btu/h (117 W) can be calculated for various design humidity ratios: If WROOM = 65 grains (50% RH) → QPRIMARY = 84 cfm If WROOM = 68 grains (52% RH) → QPRIMARY = 59 cfm If WROOM = 69 grains (53% RH) → QPRIMARY = 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 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 (m3/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. October 2009 www.info.hotims.com/25209-36 ASHRAE Journal

ASHRAE Journal - October 2009

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