Consulting-Specifying Engineer - October 2007 - (Page 24)

How To of air. Suppose we choose to design the air distribution system for the example gymnasium for 25,000 cfm (114,000 lbs. per hour). The 1,200 people generate 227 lbs. of water vapor each hour. Removing 227 lbs. of water vapor with 114,000 lbs. of air requires that the specific humidity of the supply air be 0.0020 lb. of water/lb. of air drier than the space. The specific humidity at 75°F and 50% RH is 0.0092 lb. of water/lb. of air. So the specific humidity of the supply air must be 0.0072 lb. of water/lb. of air to offset the latent load of the people. This corresponds to a supply air dew point of about 48°F. cooling coil capacity, but increases fan power because of the higher airflow and additional static pressure from the desiccant wheel. Both are viable options. It is noteworthy that the desiccant wheel may be a means to achieve low supply air dew points with conventional direct expansion equipment. Don’t forget part load situations Places of assembly often experience diverse loads. It is wise to evaluate the performance of these systems at part load. There are two part-load conditions we should evaluate. The first occurs when most of the people leave the gym. constant volume design rather than VAV? The reduced sensible cooling load requires a warmer supply air temperature, about 63°F for this example. At this supply-air temperature, the 20,000 cfm of supply air will remove 256,000 Btuh of sensible heat, but less than 7.6 pounds of water vapor. Space humidity rises to 65% RH, well above our target of 50% RH. Not only does a constant volume system use more fan energy at part load, but it is less adept at removing moisture. By comparison, a single-zone VAV system reduces fan energy while adequately removing moisture. Single-zone VAV with cold air provides humidity control at most load conditions, while simultaneously saving fan energy. Light-sensible loads The second part-load situation may be even more sinister: reduced buildingrelated sensible loads while the space is fully occupied. What happens with full occupancy when there are no envelope or glass loads? If the only loads in the space are due to lighting and people, the sensible heat ratio drops to 0.63. If we dim the lights, the situation gets even worse. This reduction in the spacesensible cooling load creates a sensible heat ratio more severe than what the system originally was designed to accommodate. Increasing the supply air temperature or reducing supply airflow in response to the reduced sensible load will hinder the ability to remove moisture. Reheat can help when the sensible heat ratio is lower than design. Both cold air and the desiccant wheel have the abilities to reduce sensible cooling capacity while maintaining a lower supply air dew point. These two systems perform well at this part-load condition. Single-zone VAV results in a slightly elevated space relative humidity, but still well within the comfort zone. This comfortable condition is achieved without reheat and uses less fan energy. Some reheat and additional fan energy may be needed if more precise humidity control is desired. The reduction in space-sensible cooling load creates a sensible heat ratio more severe than what the system was designed to accommodate. Cold air and desiccant wheel systems perform well at this part-load condition. So how do we create this 48°F dew point supply air? One common method is to cool all of the supply air to a drybulb temperature of about 49°F to 50°F. This should dehumidify the supply air to the 48°F dew point required to offset the latent load due to people. Desiccant approach But do we need colder air, or do we just need drier air? The truth is, we need air that is drier. Recent research in desiccants show that a Type III desiccant wheel is able to regenerate at low temperatures, often without the need to add heat. This allows the wheel to be configured in series with a cooling coil. A desiccant wheel delivers cooler supply air than needed with the cooling coil. Use of a desiccant wheel eliminates the need to design a cold-air distribution system, and the required capacity of the cooling load is substantially reduced. With the cold-air system, the required cooling coil capacity is about 150 tons (based on 1,200 people and 18,000 cfm of outdoor air) and supply fan power is 10 kW. The desiccant wheel reduces Perhaps the remaining occupancy is only 40 people instead of 1,200. This is an easy part-load condition to accommodate. The sensible loads drop to 256,000 Btuh and the latent load due to people drops to only 8000 Btuh. The resulting space sensible heat ratio increases to 0.97. If we supply air at 50°F with the cold air system, the required supply airflow is only 9,400 cfm. This system is called singlezone variable-air-volume (VAV). Supply airflow is reduced to match the reduced sensible cooling load in the space. Singlezone VAV is easy to control. The supply fan airflow is modulated based on space temperature. The 9,400 cfm of 50°F air will remove the 256,000 Btuh of sensible heat and has the potential to remove 116 lbs. of water vapor. However, at this reduced occupancy, people add only 7.6 lbs. of water vapor. The result is that space humidity is lowered to 40% RH. At this condition, the supply-air temperature could be reset upward to save some compressor energy. Constant volume system problems What happens if the cold air system is a 24 Consulting-Specifying Engineer • OCTOBER, 2007

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Consulting-Specifying Engineer - October 2007

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