ASHRAE Journal - June 2009 - 25

1 Occupancy Category 2 cfm/Person 3 4 People/1,000 ft2 5 Combined Outdoor Air cfm/Person 6 Per Person Latent Load, Btu/h 7 Space Enthalpy at 75°F 50% RH 8 Required Supply Air Enthalpy, Btu/lbm 9 Supply Air Dew-Point Temperature,°F 10 30% More Combined Outdoor Air/Person 11 Required Supply Air Enthalpy, Btu/lbm With 30% Surplus Air 12 Supply Air Dew-Point Temperature,°F With 30% Surplus Air 41.63 46.08 34.75 49.20 38.56 Lecture Classroom Classroom >9 Years Old Conference Room Office Museums 7.5 0.06 cfm/ft2 65 8.42 200 28.14 22.86 35.96 10.96 24.08 10.0 0.06 35 11.71 200 28.14 24.36 42.75 15.23 25.22 5.0 5.0 7.5 0.06 0.06 0.06 50 5.0 40 6.20 17.0 9.0 200 200 250 28.14 28.14 28.14 20.97 25.53 21.97 24.84 47.18 31.05 8.06 22.10 11.70 22.63 26.13 23.39 Note: Columns 1 through 4 are taken from ASHRAE Standard 62.1-2007, Table 6-1. Table 1: Ventilation data for different categories of occupancies. pumps, small packaged units, variable-air-volume terminals, chilled ceiling panels, chilled beams, etc.) in or near each space maintain space temperature. Treating the outdoor air separately from recirculated air can make it easy to verify that sufficient ventilation airflow reaches each occupied space and can help limit indoor humidity levels. The latter is accomplished by dehumidifying the ventilation air to remove all of the outdoor air and most, or all, of the space latent cooling load, leaving the local HVAC units to primarily handle space sensible cooling loads. (Some types of local HVAC equipment must operate dry to avoid problems associated with condensation, limiting their duty to sensible loads only.) DOAS Issues Related to 90.1 as well as temper the outdoor air (OA), but only about 20% as much as would occur if TER equipment were not used. The second hypothesis is that increasing the ventilation airflow rate will result in a reduction in the winter cooling plant operation, saving operating costs. Consequently, any surplus air that can be brought in during the free cooling periods should prove beneficial in most locations where buildings are served by a DOAS. The third hypothesis is that the extra free winter cooling will more than offset the increased cooling energy use during the summer months. The analysis to follow compares, on an annual basis, the first and operating cost for DOASs with and without TER in four different geographic locations. Exploring Typical Flow Rates Specified by 62.1 • Air- or water-side economizers are in the prescriptive requirement for many locations. An exception is that economizers are not required for most locations east of 97°W longitude and south of 40°N latitude, i.e., Atlanta and New Orleans. An all air system with air-side economizer is generally able to meet the entire space cooling loads when the outdoor air temperature drops below 55°F (13°C). On the other hand, with the DOAS flow rate as low as 20% that of an all air system, the outdoor air temperature may need to fall below 0°F (–18°C) for it to meet the full cooling load.4,5 • Total energy recovery equipment (TER) with an energy recovery effectiveness (sensible and latent) of at least 50% shall be used when individual fan systems with capacities of 5,000 cfm (2360 L/s) or greater, consisting of at least 70% outdoor air, subject to exceptions. DOAS 30% Surplus Ventilation Air Energy Hypotheses The first hypothesis is that increasing the ventilation airflow rate will increase the energy required to cool and dehumidify, June 2009 Two important issues to be explored are the required supply air (SA) dew-point temperature (DPT) and the steady-state space carbon dioxide (CO2) concentration (a measure of contaminant dilution) as a function of combined OA per person. For that purpose, a limited number of occupancy categories (OC) found in Standard 62.1, Table 6.16 was selected. For those selections, the minimum combined (occupant and floor components considered) OA flow rate per person range from 6 scfm (3 L/s) per person for conference rooms to 17 scfm (8 L/s) per person for offices. Information for the various OCs using minimal and 30% surplus ventilation air is presented in Table 1. The computed (solving the equation: QL=0.68×scfm×DW) SA DPT ranges from 24.8°F to 47°F (–4°C to 8°C) using the minimum OA per person. When the SA flow rate is increased by 30%, the SA DPTs are elevated, to some extent, to the range of 35°F to 49°F (2°C to 9°C). SA DPTs below about 44°F (7°C) are difficult to obtain with mechanical refrigeration alone, and generally require the ASHRAE Journal 25
ASHRAE Journal - June 2009

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