ASHRAE Journal - November 2020 - 15
TECHNICAL FEATURE
and cooling and heating zone setpoints are
assumed to be 75°F (23.9°C).
Five different cases are presented in
Table 1, using 8,760 hourly data obtained
from eQuest. Parameters included are time,
outdoor air temperatures (OAT), outdoor air
dew-point temperature (OA DP) and zone sensible loads. Negative numbers indicate heating
loads. Latent loads are calculated based on the
number of people in each zone (200 Btu/h per
person, or 59 W per person).
West (Zone 4) Area = 2,147 ft2 N# People = 11
Design Airflow = 5,000 cfm Minimum Airflow = 1,000 cfm*
North (Zone 5) Area = 2,147 ft2 N# People = 11
Design Airflow = 4,000 cfm Minimum Airflow = 800 cfm*
Typical Single-Duct VAV System
Core (Zone 3)
Area = 16,410 ft2, N# People = 82
Design Airflow = 8,000 cfm
Minimum Airflow = 1,600 cfm*
South (Zone 1) Area = 2,147 ft2 N# People = 11
East (Zone 2) Area = 2,147 ft2 N# People = 11
Design Airflow = 5,000 cfm Minimum Airflow = 1,000 cfm*
FIGURE 1 Floor plan for the example 25,000 ft2 (2323 m2) office building.
Design Airflow = 5,000 cfm Minimum Airflow = 1,000 cfm*
The VAV system typically consists of a single
AHU fan to supply conditioned air (with ade- *Minimum Airflow is 20% of Maximum Design Airflow
quate fresh air) to multiple zones through a
single-duct air distribution system. Let us
The five zones are served by single-duct VAV boxes
examine the previous office building example. The
and a single AHU, called AHU1. Table 3 shows the
zone airflow rates and reheat requirements† for the
system-level results,‡ which include system airflow
same five cases are shown in Table 2. These are calcurates, ventilation requirements calculated based on the
lated based on two supply air temperature (SAT) reset ASHRAE Standard 62.2-2016 multizone Ventilation Rate
strategies: Table 2a shows keeping SAT constant at 55°F Procedure,2 actual outdoor airflow rate (OA), outdoor air
fraction (OAF), mixing air temperature (MAT), supply
(12.7°C) and varying the SAT as a function of outdoor
air temperature (SAT), total cooling load, sensible coolair temperature (OAT). In the latter strategy (shown
ing load, system heating load and total reheat use.
in Table 2b), the SAT is 65°F (18.3°C) if the OAT is less
Case 3 is illustrated in Figure 2. In Table 3a, when the
than 50°F (10°C); the SAT is 55°F (12.7°C) if the OAT is
OAT is 58°F (14.4°C) (the dew-point temperature is
greater than 65°F (18.3°C). Otherwise, the SAT varies
55°F [12.8°C]), the economizer is "on," and the outdoor
linearly with the OAT.
TABLE 1 Sensible cooling and heating loads obtained from the building energy simulation software eQuest for an example office building in Cincinnati.
CASE
CONDITIONS (°F)
TIME
SENSIBLE LOADS (BTU/H)
Month
Day
Hour
OAT
OA DP
South (Zone 1
East (Zone 2)
Core (Zone 3)
West (Zone 4)
North (Zone 5)
1
1
3
11
38
27
25,342.7
16,454.5
79,975.0
-3,180.1
-1,103.3
2
3
21
10
41
27
11,173.0
29,825.0
82,544.8
-494.8
2,572.2
3
4
12
10
58
55
14,284.8
14,438.4
101,311.0
14,200.7
14,437.0
4
10
22
10
67
52
40,114.9
45,255.8
102,882.0
18,205.1
18,750.2
5
5
21
16
82
53
41,040.8
35,878.4
133,838.0
63,516.0
35,829.4
†The
airflow rate and reheat are calculated based on the sensible heat equation: a multiplication of mass flow rate, specific heat, and
temperature difference. For the airflow rate calculations, the temperature difference is equal to the supply air temperature (SAT) minus
zone air temperature. If the calculated airflow rate is below the minimum limit, it will be kept at the minimum value, and if the zone
temperature drops below the heating temperature setpoint, reheat will be activated. The amount of reheat is calculated using the same
sensible heat equation.
‡The system airflow rate is calculated as the sum of zone airflow rates. The return air humidity ratio is calculated based on the latent heat
equation: a multiplication of latent heat of water evaporation, mass flow rate and humidity ratio change. The initial supply humidity ratio is
assumed. The following iteration process is then applied: the mixing air conditions are calculated using energy and mass equations for
two streams of return and outdoor air. Ventilation is calculated based on the ASHRAE Standard 62.1-2016 multizone Ventilation Rate
Procedure. The dual-temperature economizer control algorithm is used. The cooling load is calculated as a multiplication of mass flow
rate and the mixing and supply air enthalpy difference.
N OVEM BER 2020
ashrae.org
ASHRAE JOURNAL
15
https://www.ashrae.org/
ASHRAE Journal - November 2020
Table of Contents for the Digital Edition of ASHRAE Journal - November 2020
Contents
ASHRAE Journal - November 2020 - Intro
ASHRAE Journal - November 2020 - Cover1
ASHRAE Journal - November 2020 - Cover2
ASHRAE Journal - November 2020 - 1
ASHRAE Journal - November 2020 - Contents
ASHRAE Journal - November 2020 - 3
ASHRAE Journal - November 2020 - 4
ASHRAE Journal - November 2020 - 5
ASHRAE Journal - November 2020 - 6
ASHRAE Journal - November 2020 - 7
ASHRAE Journal - November 2020 - 8
ASHRAE Journal - November 2020 - 9
ASHRAE Journal - November 2020 - 10
ASHRAE Journal - November 2020 - 11
ASHRAE Journal - November 2020 - 12
ASHRAE Journal - November 2020 - 13
ASHRAE Journal - November 2020 - 14
ASHRAE Journal - November 2020 - 15
ASHRAE Journal - November 2020 - 16
ASHRAE Journal - November 2020 - 17
ASHRAE Journal - November 2020 - 18
ASHRAE Journal - November 2020 - 19
ASHRAE Journal - November 2020 - 20
ASHRAE Journal - November 2020 - 21
ASHRAE Journal - November 2020 - 22
ASHRAE Journal - November 2020 - 23
ASHRAE Journal - November 2020 - 24
ASHRAE Journal - November 2020 - 25
ASHRAE Journal - November 2020 - 26
ASHRAE Journal - November 2020 - 27
ASHRAE Journal - November 2020 - 28
ASHRAE Journal - November 2020 - 29
ASHRAE Journal - November 2020 - 30
ASHRAE Journal - November 2020 - 31
ASHRAE Journal - November 2020 - 32
ASHRAE Journal - November 2020 - 33
ASHRAE Journal - November 2020 - 34
ASHRAE Journal - November 2020 - 35
ASHRAE Journal - November 2020 - 36
ASHRAE Journal - November 2020 - 37
ASHRAE Journal - November 2020 - 38
ASHRAE Journal - November 2020 - 39
ASHRAE Journal - November 2020 - 40
ASHRAE Journal - November 2020 - 41
ASHRAE Journal - November 2020 - 42
ASHRAE Journal - November 2020 - 43
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