Consulting-Specifying Engineer - October 2007 - (Page 23)

How To B E AT T H E C O D E S Maintaining a comfortable environment in places of assembly BY DON EPPELHEIMER, PE, Applications Engineer,Trane, Piscataway, N.J. esigning comfort systems for places of assembly—such as auditoriums, gymnasiums, arenas or houses of worship—presents some vexing challenges. Such facilities often have acoustical requirements that place limits on equipment location and air distribution design. Many places of assembly experience diverse loads and occupancy schedules, complicating part-load system control. But the biggest challenge is occupancy itself and its impact on ventilation and humidity control. Design guidelines that commonly are applied in commercial office space are troublesome here. A simple example—a school gymnasium during a band concert—can illustrate some of these issues. As this is a talented high school band, both the bleachers and the floor are full. Occupancy is at the fire marshal’s rated seating capacity. The 18,000sq.-ft. gymnasium is designed for 1,200 people, including use of the gym floor. A load calculation reveals the following space loads: Roof 69,600 Btuh Wall 43,000 Btuh Glass 10,500 Btuh Lights 122,900 Btuh People 300,000 Btuh (sensible) Totals: 546,000 Btuh (sensible) 240,000 Btuh (latent) Occupancy is major factor People constitute a significant portion of the space sensible cooling load, more than 50%. However, it is the impact on humidity that makes occupancy a difficult load to manage. The space sensible heat ratio for this example is 0.69. If the target space comfort condition is 75°F and 50% relative humidity (RH), and the air distribution system is designed for 55°F supply air, the required supply airflow is more than 4 cfm/sq. ft. of floor area—a huge amount. How did that happen? There are several ways to affect supply airflow. D Humidity ratio Humidity ratio is grains of water vapor per pound of air. The humidity ratio at 75°F dry bulb and 50% RH results in the desired space condition of 64.7 grains/lb. The 55°F supply air has a humidity ratio of 60.4 grains/lb. At these conditions, each pound of supply air removes 4.3 grains of water vapor. If each occupant contributes 200 Btuh to the latent load, 1,200 people add 230 lbs. of water vapor—1.61 million grains (7,000 grains are in 1 lb.) to the air in the gymnasium. If each pound of supply air removes 4.3 grains of water vapor, it will take 374,000 lbs. of supply air per hour. This equates to approximately 83,000 cfm, or 4.6 cfm/sq. ft. That’s a lot of air. In this example, 83,000 cfm is required to handle humidity, but only 18,000 cfm of this measurement is outdoor air (OA) for ventilation (assuming 15 cfm of OA per person, following the most recent version of ASHRAE Standard 62.1). With a space sensible cooling load of 546,000 Btuh and a supply-air temperature of 55°F; approximately 25,000 cfm is required to maintain the space temperature at 75°F. In this case, 72% of the supply air must be outdoor air. While this is a high fraction of outdoor air, it is manageable. Ventilation air is not the culprit. This 25,000 cfm of supply air equates to 1.4 cfm/sq. ft. There still needs to be 83,000 cfm of supply air to control humidity. How do we better equip the supply air to handle the high latent load associated with this many people? Obviously, the supply air needs to be drier. The drier the supply air, which lowers the dew point, the more water vapor it removes from the space. What supply air dew point is required to handle the space latent load? Calculating specific humidity The key is another humidity measurement called specific humidity. Specific humidity is expressed as pounds of water vapor per pound Consulting-Specifying Engineer • OCTOBER, 2007 23

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Consulting-Specifying Engineer - October 2007 Contents On the Web In the News M/E Roundtable How To Beat the Energy Codes Professional Practices Codes & Standards A Capital Job on Engineered Buiding Systems Going Green in Data Centers Overcurrent Protection: Fuses or Breakers? Third-Party Liability Management Report New Products Product Spotlight Jobs/Classifieds Specifier's Notebook

Consulting-Specifying Engineer - October 2007

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