ASHRAE Journal - March 2010 - 30

to changes in primary airflow, as well as zone discharge airflow. Step 10 Find actual system ventilation efficiency, which equals the lowest efficiency among all zones. At this example condition, efficiency rises compared to the worst-case design condition (0.902 versus 0.650). Step 11 Find the outdoor air intake flow (Vot = Vou/Ev) required. It’s lower than the worst-case value at design conditions. When designed for worst-case, the outdoor air intake flow required at any part-load condition is always lower than the design value. As Table 4 shows, lowering system primary airflow alone (with no change in system population) increases efficiency, so in this example, the minimum required outdoor air intake flow drops from 4,300 cfm at design to only 3,100 cfm at this 80% airflow condition. Ventilation During Operation—80% of Design Population Procedural Step 1 cfm/p 2 Ez 1.0 1.0 1.0 1.0 1.0 1.0 0.8 1.0 cfm Rp 5 5 5 5 5 5 5 5 Person Pz cfm/ft2 Ra Az ft2 cfm Vbz Voz South Offices Ventilation Zones West Offices South Conference Room East Offices Southwest Interior Offices Northeast Interior Offices North Offices North Conference Room 16 18 11 16 32 22 6 10 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 2,000 2,000 3,000 2,000 10,000 10,000 2,000 2,000 200 210 235 200 760 710 150 170 200 210 235 200 760 710 187 170 Table 5: Zone calculations: 80% of design system population. Procedural Step 1–3 cfm 4 Zdz 0.125 0.117 0.094 0.286 0.289 0.203 0.188 0.213 Variable D Vou Vps Xs – Ev Vot 5 – 8 – – – – – – – – – 1.000 2,630 14,900 0.177 – 9 – 11 Evz 1.052 1.060 1.083 0.891 0.924 0.974 0.989 0.964 – – – – – 0.891 2,960 Vdz cfm Voz So, reduced airflow alone increases efficiency and reduces required intake airflow. But, what happens if both system primary airflow and system population drop to 80% of design values? Assume that at some occupied time, some occupants leave the building and zone population matches the values shown in Table 5, resulting in a system population of 131 people, 80% of the design population. Following Steps 1 through 3 above, we can find the current zone outdoor airflow required (Voz = [Rp × Pz + Ra × Az]/ Ez) in each zone. Note that we assumed that the North Offices require reheat, and are configured so that Ez = 0.8 in heating, so zone outdoor airflow rises in this zone. As shown in Table 6, solving Steps 4 through 11 using lower-than-design system population and system primary airflow VAV Design Errors Incorrect ventilation design seems to be very common in VAV systems. Some designers simply find the zone outdoor airflow values (Table 1) and add these values together to determine “outdoor air intake flow;” with no correction for efficiency. This approach results in inadequate ventilation (in the example, 3,100 cfm versus the 4,300 cfm actually needed) at many conditions. Other designers follow all of the design calculation 30 ASHRAE Journal Ventilation Zones South Offices West Offices South Conference Room East Offices Southwest Interior Offices Northeast Interior Offices North Offices North Conference Room 1,600 1,800 2,500 700 3,000 3,500 1,000 800 200 210 235 200 760 710 187 170 Step 5 6 7 8 – 10 11 Table 6: System calculations: 80% of design primary airflow (80% of design system population). results in a further increase in efficiency, and a further drop in required outdoor air intake flow (from 4,300 cfm at design to 3,100 cfm at 80% airflow and to 2,960 cfm at 80% system population). In this illustrative example, resetting intake airflow steps (Tables 1 and 2) to find design outdoor air intake flow, but then convert this airflow to a fraction of the primary design airflow (4,300/18,600 = 0.23, in our example) and set the outdoor air damper minimum position to correspond to this fraction. This approach results in adequate ventilation at high thermal loads, but as primary airflow drops, mixing box pressure rises and outdoor airflow drops to a less-than-adequate value. Depending upon the relationship of damper position ashrae.org based on airflows alone has much more impact than resetting based on both zone airflow and system population, but this outcome will vary from one system to the next. Also, note that the reduced outdoor air intake flow rates determined are the to flow, at a 60% load condition with the intake damper fixed at 23%, intake airflow in the example system may drop to 0.23×0.60×18,600 = 2,570 cfm, even though 3,300 cfm is actually needed. So, ignoring the multiple-zone system calculations and/or maintaining a fixed fraction of outdoor air (rather than modulating the damper to ensure outdoor air intake flow) can lead to underventilated VAV systems and associated IAQ problems. March 2010
ASHRAE Journal - March 2010

Table of Contents for the Digital Edition of ASHRAE Journal - March 2010
