ASHRAE Journal - October 2021 - 60

COLUMN ENGINEER'S NOTEBOOK
FIGURE 4 The impact of different operating variables on the shape of the perfect load lines and cloud.
A. Economizer-equipped constant volume system and B. 100% OA constant volume
system operating as intended.
* A. vs. B. illustrates the impact of moisture for an economizer-equipped system
vs. a 100% OA system operating at a constant volume, 24/7.
* A. vs. C. illustrates the impact of setting the economizer high limit too high.
* A. vs. D. illustrates the impact of VAV operation vs. 24/7 constant volume
operation.
* A. vs. E. illustrates the impact of VAV operation plus a schedule vs. 24/7
constant volume operation.
A.
120
110
100
90
80
70
60
50
40
30
20
10
-10
B.
120
110
100
90
80
70
60
50
40
30
20
10
-10
D.
120
110
100
90
80
70
60
50
40
30
20
10
-10
10 20
30 40
The Perfect Chilled Water Load
Cooling Coil Leaving Air Setpoint: 51°F, No Reset
Variable Volume System, 24/7 Operation
Outdoor Air Temperature Dry-Bulb Economizer High Limit: 65.7°F
Design Cooling
Condition - Economizer
System, 25% MOA
Economizer High Limit
Design Heating
Condition
50 60 70
Design Cooling Coil LAT
80
Outdoor Temperature (°F)
scheduling or flow management opportunities.
* If there are economizers but no " bend " in the data
cloud in warm weather, then the economizer high limits
60
ASHRAE JOURNAL ashrae.o rg O CTO B E R 2021
90 100 110 120
10 20 30 40 50
60 70 80 90 100 110 120
Outdoor Temperature (°F)
E.
120
110
100
90
80
70
60
50
40
30
20
10
-10
10
20
The Perfect Chilled Water Load
Cooling Coil Leaving Air Setpoint: 51°F, No Reset
100% Outdoor Air
Design Cooling
Condition - 100%
OA System
C.
Design Heating
Condition
Design Cooling Coil LAT
120
110
100
90
80
70
60
50
40
30
20
10
-10
10
The Perfect Chilled Water Load
Cooling Coil Leaving Air Setpoint: 51°F, No Reset
Outdoor Air Temperature Dry-Bulb Economizer High Limit: 75°F
Design Cooling
Condition - Economizer
System, 25% MOA
Economizer High Limit
Design Heating
Condition
20 30 40
Outdoor Temperature (°F)
Design Cooling Coil LAT
50
60 70 80
90 100 110 120
The Perfect Chilled Water Load
Cooling Coil Leaving Air Setpoint: 51°F, No Reset
Variable Volume System, Scheduled Operation
Outdoor Air Temperature Dry-Bulb Economizer High Limit: 65.7°F
Design Cooling
Condition - Economizer
System, 25% MOA
Economizer High Limit
Design Heating
Condition
30 40
Outdoor Temperature (°F)
Design Cooling Coil LAT
50
60 70 80
90 100 110 120
are suspect.
* A flat (vs. inclined) cloud above the economizer high
limit setpoint can point to under-ventilating or incor10
20
30
The Perfect Chilled Water Load
Cooling Coil Leaving Air Setpoint: 51°F, No Reset
Constant Volume System, 24/7 Operation
Outdoor Air Temperature Dry-Bulb Economizer High Limit: 65.7°F
Economizer
High Limit
Design
Heating Condition
Outdoor Temperature (°F)
Design Cooling Coil LAT
40
50 60
70 80
90 100 110 120
Design Cooling
Condition -
Economizer
System,
25% MOA
Percent Full Load
Percent Full Load
Percent Full Load
Percent Full Load
Percent Full Load

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ASHRAE Journal - October 2021

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