Trane Tracer UC400 - Page 64

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BAS-SVX48H-EN
CO
2outdoors
= concentration of CO
2
in the outdoor air, ppm
N = rate at which CO
2
is generated by the zone occupants, cfm/person
The rate at which the occupants produce carbon dioxide (N) varies with diet and health, as
well as with the duration and intensity of physical activity. The following table suggests
typical values for various activity levels:
Occupant Activity Level
(a)
Met
CO2 Generation Rate (N), cfm/
Person
Sleeping
0.7 0.0059
Seated, Quiet
1 0.0084
Seated, Typing
1.1 0.0092
Seated, Filing
1.2 0.0101
Walking (2 mph)
2 0.0168
Lifting/Packing
2.1 0.0176
Light Machine Work
2.2 0.0185
Heavy Machine Work
4 0.0336
(a)
A more complete table of Met levels for various activities can be found in Chapter 9 (Table 4) of the 2013 ASHRAE
Handbook-Fundamentals. According to the Standard 62.1 User's Manual (Appendix A), the CO
2
generation rate (N)
averages about 0.0084 cfm/met/person over the general adult population.
Unless the outdoor air intake is located very close to an area with heavy traffic, it is fairly
common for design engineers to assume the outdoor concentration (CO
2outdoors
) to be 400
ppm.
EExxaammppllee:: For this example K-12 classroom, the CO
2
generation rate (N) of the occupants is
assumed to be 0.0101 cfm/person, and the outdoor CO
2
concentration (CO
2outdoors
) is
assumed to be 400 ppm. As calculated in Step 1, when this zone is fully occupied (P
z
= 35), its
design outdoor airflow rate (V
bz-Design
) is 470 cfm.
CO
2space-Design
= CO
2outdoors
+ [1,000,000 x N / (V
bz-Design
/ P
z-Design
)]
CO
2space-Design
= 400 ppm + [1,000,000 x 0.0101 cfm/p / (470 cfm / 35 people)] = 1200 ppm
• The steady-state indoor CO
2
concentration (CO
2space-DCVmin
) when the zone has no occupants
(P
z
= 0) will be equal to the outdoor CO
2
concentration.
EExxaammppllee:: For this example K-12 classroom, the outdoor CO
2
concentration is assumed to be
400 ppm.
CO
2space-DCVmin
= CO
2outdoors
= 400 ppm
If the information is not known to complete the calculations in Steps 1 through 4 above, then
as long as the Occupancy Category and zone floor area (ft
2
) are known, these values can be
calculated using the default occupant density from ASHRAE Standard 62.1 and an assumed
CO
2
generation rate (N). Refer to the following table for common occupancy categories.
SSeettttiinnggss ffoorr CCOO
22
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