ASHRAE Journal - March 2010 - 28

Ventilation Zones have been required for compliance with Standard 62 since 1989. They result in worst-case outdoor air intake flow, which is a key value when selecting cooling and heating coils. Incorporated into the model codes5,6 and included as a prerequisite for LEED-certified projects, these rates and equations lend themselves to spreadsheet analysis and have been incorporated into some of the major system energy/economic performance programs. Ventilation During Operation—80% of Design Primary Airflow Procedural Step 1 cfm/p 2 Ez 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 cfm Rp 5 5 5 5 5 5 5 5 Person Pz cfm/ft2 Ra Az ft2 cfm Vbz Voz South Offices West Offices South Conference Room East Offices Southwest Interior Offices Northeast Interior Offices North Offices North Conference Room 16 18 16 16 42 32 14 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 2,000 200 210 260 200 810 810 190 170 200 210 260 200 810 760 190 170 Properly designed single-path VAV systems must sense outdoor air intake flow (or in some other way compensate for changes in system primary airflow and mixing box pressure) and maintain it at the design value by modulating the position of the outdoor air damper as conditions change. But systems do not need this worst-case design value during all operating conditions to ensure proper ventilation. This section presents eleven calculation steps, very similar to those described previously for design, but using actual population for each zone along with actual airflow values at 80% system airflow and at two populations, to show that ventilation airflow can be reduced at non-design conditions. Assuming that at some occupied time, zone population matches those values shown in Table 3, the sum of which equals the design population for the system. Following Steps 1 through 3 using the actual zone population (Pz) for each zone, we can find the current zone outdoor airflow required (Voz = [Rp × Pz + Ra × Az]/Ez) in each zone. Note that lower-than-design population reduces some of these outdoor airflow values compared with Table 1. At the same occupied time, cooling loads could result in the actual zone discharge airflow (Vdz) values shown in Table 4. All zones need less than coolingdesign airflow. Step 4 Find the zone outdoor air fraction for each zone (Zdz = Voz /Vdz) using the zone outdoor airflow (Voz) values based on actual population (Table 3) and actual zone discharge airflow values (Vdz) shown in Table 4. 28 ASHRAE Journal Table 3: Zone calculations: Design system population. Procedural Step 1–3 cfm 4 Zdz 0.125 0.117 0.104 0.286 0.270 0.217 0.190 0.213 Variable D Vou Vps Xs – Ev Vot 5 – 8 – – – – – – – – – 1.000 2,800 14,900 0.188 – 9 – 11 Evz 1.063 1.071 1.084 0.902 0.918 0.971 0.998 0.975 – – – – – 0.902 3,100 Vdz cfm Voz 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 1,000 800 200 210 260 200 810 760 170 220 Step 5 6 7 8 – 10 11 Table 4: System calculations: 80% of design (with design system population). Step 5 Since this example focuses on actual zone and system population, system population equals the sum of zone population, so population diversity has no impact on these calculations (D = Ps /SPz = 1.0). Step 6 Find the uncorrected outdoor airflow (Vou = D × S[Rp × Pz] + S[Ra × Az]) using actual population in each zone and a population diversity of D = 1.0. Step 7 In a single-path VAV system, zone primary airflow (Vpz) equals zone discharge airflow at each VAV box. Consequently, determine the system primary airflow (Vps) during operation as the sum of actual zone discharge airflow values (Vdz) (plus any supply duct leakage). In the exashrae.org ample, assuming no duct leakage, system primary airflow equals the 80% of the design value (Vps = 0.80 × SVdz = 14,900 cfm). Step 8 Determine the actual average outdoor air fraction based on current uncorrected outdoor airflow and the actual system primary airflow (Xs = Vou / Vps). In the example, this fraction is a little higher than the design value (0.188 versus 0.150) because system primary airflow is reduced. Step 9 For each zone, find the current zone ventilation efficiency using the actual values for average outdoor air fraction and zone discharge outdoor air fraction (Evz = 1 + Xs – Zdz). These values differ from the design values due March 2010
ASHRAE Journal - March 2010

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